Guidelines for Design of Turbines

A publication from DNV/Risø

Second Edition

Guidelines for Design of Wind Turbines 2nd Edition  Det Norske Veritas, Copenhagen ([email protected]) and Wind Energy Department, Risø National Laboratory ([email protected]) 2002.

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Printed by Jydsk Centraltrykkeri, Denmark 2002

ISBN 87-550-2870-5

Guidelines for Design of Wind Turbines − DNV/Risø

Preface The guidelines can be used by manufacturers, certifying authorities, and wind turbine owners. The guidelines will The guidelines for design of wind turbines also be useful as an introduction and tutorial have been developed with an aim to compile for new technical personnel and as a refer- into one book much of the knowledge about ence for experienced engineers. design and construction of wind turbines that has been gained over the past few years. The guidelines are available as a printed This applies to knowledge achieved from book in a handy format as well as electroni- research projects as well as to knowledge cally in pdf format on a CD-ROM. resulting from practical design experience. In addition, the various rules and methods The development of the guidelines is the required for type approval within the major result of a joint effort between Det Norske markets for the wind turbine industry form a Veritas and Risø National Laboratory. The basis for the guidelines, with emphasis on development has been founded by Danish the international standards for wind turbines Energy Agency, Det Norske Veritas and given by the International Electrotechnical Risø National Laboratory. Commission, IEC. These guidelines for design of wind turbines The objective is to provide guidelines, have been thoroughly reviewed by internal which can be used for design of different and external experts. However, no warranty, types of wind turbines in the future. The expressed or implied, is made by Det Norske guidelines provide recommendations and Veritas and Risø National Laboratory, as to guidance for design together with applica- the accuracy or functionality of the guide- tion-oriented solutions to commonly en- lines, and no responsibility is assumed in countered design problems. connection therewith.

Guidelines for Design of Wind Turbines © 2002 Det Norske Veritas and Risø National Laboratory

Preface i Guidelines for Design of Wind Turbines − DNV/Risø

Contents

1. WIND TURBINE CONCEPTS ...... 1 2.5.1 Transportation, installation and commissioning...... 28 1.1 INTRODUCTION...... 1 2.5.2 Normal operation...... 29 1.2 CONCEPTUAL ASPECTS...... 1 2.5.3 Service, maintenance and repair. 29 1.2.1 Vertical axis turbines...... 2 2.6 CODES AND STANDARDS ...... 30 1.2.2 Horizontal axis turbines...... 2 REFERENCES ...... 30 1.2.3 Number of rotor blades...... 3

1.2.4 Power control aspects ...... 3

1.3 ECONOMICAL ASPECTS...... 5 3. EXTERNAL CONDITIONS...... 32 1.4 POWER PRODUCTION ...... 5 1.4.1 Power curve...... 6 3.1 WIND CONDITIONS ...... 32 1.4.2 Annual energy production ...... 7 3.1.1 10-minute mean wind speed ...... 32 1.5 CONFIGURATIONS AND SIZES ...... 7 3.1.2 Standard deviation of 1.6 FUTURE CONCEPTS ...... 8 wind speed...... 34 REFERENCES ...... 9 3.1.3 Turbulence intensity ...... 36 3.1.4 Lateral and vertical turbulence ... 37 3.1.5 Stochastic turbulence models ..... 37 2. SAFETY AND RELIABILITY...... 10 3.1.6 Wind shear...... 40 3.1.7 Wind direction...... 42 2.1 SAFETY PHILOSOPHY...... 10 3.1.8 Transient wind conditions ...... 43 2.2 SYSTEM SAFETY AND 3.1.9 Extreme – gusts...... 43 OPERATIONAL RELIABILITY ...... 11 3.1.10 Site assessment ...... 46 2.2.1 Control system...... 12 3.2 OTHER EXTERNAL CONDITIONS...... 48 2.2.2 Protection system...... 13 3.2.1 Temperatures ...... 48 2.2.3 system ...... 14 3.2.2 Density of air ...... 49 2.2.4 Failure mode and effects 3.2.3 Humidity...... 50 analysis ...... 15 3.2.4 Radiation and ultraviolet light .... 50 2.2.5 Fault tree analysis ...... 16 3.2.5 Ice ...... 50 2.3 STRUCTURAL SAFETY...... 18 3.2.6 Rain, snow and hail ...... 50 2.3.1 Limit states ...... 18 3.2.7 Atmospheric corrosion 2.3.2 Failure probability and other and abrasion...... 51 measures of structural reliability. 18 3.2.8 Earthquake...... 51 2.3.3 Structural reliability methods ..... 19 3.2.9 Lightning ...... 53 2.3.4 Code format, characteristic REFERENCES ...... 53 values, and partial safety factors. 19

2.3.5 Code calibration...... 20

2.3.6 Example – axially loaded 4. LOADS...... 55 steel tower...... 22 2.3.7 Example – fatigue of FRP 4.1 LOAD CASES...... 55 blade root in bending ...... 24 4.1.1 Design situations ...... 55 2.3.8 Tests and calculations for 4.1.2 Wind events...... 55 verification...... 26 4.1.3 Design load cases ...... 55 2.3.9 Inspection and 4.2 LOAD TYPES ...... 58 inspection intervals ...... 26 4.2.1 Inertia and gravity loads ...... 58 2.4 MECHANICAL SAFETY ...... 27 4.2.2 Aerodynamic loads...... 59 2.5 LABOUR SAFETY ...... 28 4.2.3 Functional loads...... 60 ii Contents Guidelines for Design of Wind Turbines − DNV/Risø

4.2.4 Other loads...... 60 5.2.5 Materials...... 118 4.3 AEROELASTIC LOAD 5.2.6 Standards ...... 119 CALCULATIONS ...... 60 REFERENCES ...... 119 4.3.1 Model elements...... 61 4.3.2 Aeroelastic models for load prediction...... 70 6. ...... 120 4.3.3 Aerodynamic data assessment .... 70 6.1 MAIN SHAFT...... 120 4.3.4 Special considerations ...... 72 6.1.1 Determination of design loads.. 120 4.4 LOAD ANALYSIS AND SYNTHESIS ... 76 6.1.2 Strength analysis...... 120 4.4.1 Fatigue loads...... 76 6.1.3 Fatigue strength ...... 121 4.4.2 Ultimate loads...... 82 6.1.4 Ultimate strength ...... 125 4.5 SIMPLIFIED LOAD CALCULATIONS .. 86 6.1.5 Main shaft- connection ...... 126 4.5.1 Parametrised empirical models... 86 6.1.6 Materials...... 126 4.5.2 The simple load basis ...... 86 6.1.7 Standards ...... 127 4.5.3 Quasi-static method ...... 87 6.2 MAIN BEARING ...... 127 4.5.4 Peak factor approach for 6.2.1 Determination of design loads.. 129 extreme loads...... 88 6.2.2 Selection of bearing types...... 130 4.5.5 Parametrised load spectra ...... 89 6.2.3 Operational and 4.6 SITE-SPECIFIC DESIGN LOADS ...... 92 environmental conditions...... 130 4.7 LOADS FROM OTHER SOURCES 6.2.4 Seals, lubrication THAN WIND...... 93 and temperatures...... 130 4.7.1 Wave loads ...... 93 6.2.5 Rating life calculations ...... 132 4.7.2 Current loads ...... 99 6.2.6 Connection to main shaft...... 133 4.7.3 Ice loads...... 99 6.2.7 Bearing housing...... 133 4.7.4 Earthquake loads...... 99 6.2.8 Connection to machine frame... 133 4.8 LOAD COMBINATION ...... 99 6.2.9 Standards ...... 133 REFERENCES ...... 101 6.3 MAIN GEAR ...... 133

6.3.1 Gear types...... 134

6.3.2 Loads and capacity ...... 137 5. ROTOR...... 104 6.3.3 Codes and standards ...... 141 5.1 BLADES...... 104 6.3.4 Lubrication ...... 141 5.1.1 Blade geometry...... 104 6.3.5 Materials and testing...... 142 5.1.2 Design loads ...... 105 6.4 COUPLINGS ...... 145 5.1.3 Blade materials ...... 105 6.4.1 Flange couplings...... 145 5.1.4 Manufacturing techniques ...... 108 6.4.2 Shrink fit couplings ...... 146 5.1.5 Quality assurance for blade 6.4.3 Key connections ...... 146 design and manufacture ...... 109 6.4.4 Torsionally elastic couplings. ... 146 5.1.6 Strength analyses ...... 110 6.4.5 Tooth couplings...... 146 5.1.7 Tip deflections ...... 113 6.5 MECHANICAL BRAKE ...... 146 5.1.8 Lightning protection ...... 113 6.5.1 Types of ...... 146 5.1.9 Blade testing ...... 114 6.5.2 Brake discs and brake pads...... 148 5.1.10 Maintenance ...... 116 6.5.3 Brake sequence...... 148 5.2 HUB ...... 116 6.6 HYDRAULIC SYSTEMS ...... 149 5.2.1 Determination of design loads.. 117 6.6.1 Arrangement...... 149 5.2.2 Strength Analysis...... 117 6.6.2 Accumulators...... 149 5.2.3 Analysis of bolt connections..... 118 6.6.3 Valves...... 149 5.2.4 Hub enclosure...... 118

Contents iii Guidelines for Design of Wind Turbines − DNV/Risø

6.6.4 Application in 7.4.7 Stability analysis...... 177 protection systems ...... 150 7.4.8 Flange connections ...... 178 6.6.5 Additional provisions ...... 151 7.4.9 Corrosion protection...... 179 6.6.6 Codes and standards ...... 151 7.4.10 Tolerances and specifications... 179 6.7 GENERATOR ...... 151 7.5 ACCESS AND WORKING 6.7.1 Types of generators ...... 151 ENVIRONMENT ...... 180 6.7.2 Climate aspects...... 153 7.6 EXAMPLE OF TOWER LOAD 6.7.3 Safety aspects ...... 153 CALCULATION ...... 180 6.7.4 Cooling and degree of sealing .. 155 7.6.1 Loads and responses ...... 180 6.7.5 ...... 155 7.6.2 Occurrence of extreme loads during 6.7.6 Overspeed...... 155 normal power production...... 181 6.7.7 Overloading ...... 155 7.6.3 Extreme loads – parked turbine 182 6.7.8 Materials...... 156 7.6.4 Fatigue loading ...... 183 6.7.9 Generator braking...... 156 REFERENCES ...... 186 6.7.10 Lifetime ...... 157 6.7.11 Testing of generators ...... 157 6.8 MACHINE SUPPORT FRAME...... 157 8. FOUNDATIONS...... 187 6.9 NACELLE ENCLOSURE ...... 158 8.1 SOIL INVESTIGATIONS ...... 187 6.10 ...... 158 8.1.1 General ...... 187 6.10.1 Determination of design loads.. 160 8.1.2 Recommendations for gravity 6.10.2 ...... 161 based foundations ...... 188 6.10.3 Yaw ring ...... 162 8.1.3 Recommendations for pile 6.10.4 Yaw brake...... 162 foundations ...... 189 6.10.5 ...... 163 8.2 GRAVITY-BASED FOUNDATIONS... 189 6.10.6 Yaw error and control...... 166 8.2.1 Bearing capacity formulas ...... 190 6.10.7 Cable twist...... 166 8.3 PILE-SUPPORTED FOUNDATIONS... 193 6.10.8 Special design considerations... 166 8.3.1 Pile groups...... 194 REFERENCES ...... 167 8.3.2 Axial pile resistance...... 195

8.3.3 Laterally loaded piles...... 197

8.3.4 Soil resistance for embedded 7. TOWER ...... 169 pile caps...... 200 7.1 LOAD CASES...... 170 8.4 FOUNDATION STIFFNESS...... 201 7.2 DESIGN LOADS ...... 170 8.5 PROPERTIES OF REINFORCED 7.3 GENERAL VERIFICATIONS FOR CONCRETE ...... 206 TOWERS...... 171 8.5.1 Fatigue...... 206 7.3.1 Dynamic response 8.5.2 Crack-width ...... 207 and resonance ...... 171 8.5.3 Execution...... 208 7.3.2 Critical blade 8.6 SELECTED FOUNDATION deflection analysis ...... 172 STRUCTURE CONCEPTS FOR 7.4 TUBULAR TOWERS ...... 173 OFFSHORE APPLICATIONS ...... 208 7.4.1 Loads and responses ...... 173 8.6.1 Introduction to concepts ...... 208 7.4.2 Extreme loads ...... 174 8.6.2 Monopile ...... 209 7.4.3 Fatigue loads...... 174 8.6.3 Tripod ...... 215 7.4.4 Vortex induced vibrations...... 174 REFERENCES ...... 221 7.4.5 Welded joints...... 175 7.4.6 Stress concentrations near hatches and doors...... 176 iv Contents Guidelines for Design of Wind Turbines − DNV/Risø

9. ELECTRICAL SYSTEM...... 223 A.7 MINIMUM DEPTH OF THREADED HOLES...... 244 9.1 ELECTRICAL COMPONENTS...... 223 A.8 BOLT FORCE ANALYSIS ...... 245 9.1.1 Generators...... 223 A.8.1 Stiffness of bolts ...... 245 9.1.2 Softstarter ...... 225 A.8.2 Stiffness of the mating parts ..... 246 9.1.3 Capacitor bank...... 225 A.8.3 Force triangle...... 246 9.1.4 Frequency converter ...... 226 A.9 CONNECTIONS SUBJECTED 9.2 WIND TURBINE CONFIGURATIONS 227 TO SHEAR ...... 247 9.3 POWER QUALITY AND GRID A.10 BOLTS SUBJECTED CONNECTION ...... 229 TO TENSILE LOAD ...... 248 9.4 ELECTRICAL SAFETY ...... 230 A.11 BOLTS SUBJECTED TO TENSILE 9.5 INTEGRATION ...... 231 LOAD AND SHEAR ...... 249 REFERENCES ...... 232 A.12 EXECUTION OF BOLT

CONNECTIONS ...... 249

A.13 CODES AND STANDARDS...... 249 10. MANUALS ...... 233 REFERENCES ...... 249 10.1 USER MANUAL ...... 233 10.2 SERVICE AND MAINTENANCE MANUAL...... 233 B. RULES OF THUMB...... 250 10.3 INSTALLATION MANUAL...... 233 B.1 LOADS...... 250 REFERENCE ...... 233 B.1.1 Rotor loads...... 250

B.1.2 Fatigue loads...... 250

B.2 ROTOR ...... 250 11. TESTS AND MEASUREMENTS.. 234 B.3 NACELLE...... 251 11.1 POWER PERFORMANCE B.3.1 Main shaft...... 251 MEASUREMENTS...... 234 B.4 NOISE...... 251 11.2 LOAD MEASUREMENTS...... 236 REFERENCES ...... 251 11.3 TEST OF CONTROL AND PROTECTION SYSTEM...... 237 11.4 POWER QUALITY MEASUREMENT .237 C. FATIGUE CALCULATIONS ...... 252 11.5 BLADE TESTING...... 237 C.1 STRESS RANGES...... 252 11.6 NOISE MEASUREMENTS ...... 237 C.2 FRACTURE MECHANICS ...... 252 REFERENCES ...... 237 C.3 S-N CURVES ...... 253

C.4 THE PALMGREN-MINER RULE ...... 254

C.5 FATIGUE IN WELDED STRUCTURES 255 A. BOLT CONNECTIONS...... 239 C.6 CHARACTERISTIC S-N CURVES A.1 BOLT STANDARDIZATION ...... 239 FOR STRUCTURAL STEEL...... 256 A.2 STRENGTH...... 239 C.7 CHARACTERISTIC S-N CURVES A.3 IMPACT STRENGTH ...... 239 FOR FORGED OR ROLLED STEEL .... 256 A.4 SURFACE TREATMENT ...... 239 C.8 S-N CURVES FOR COMPOSITES ...... 257 A.5 S-N CURVES...... 240 C.9 OTHER TYPES OF FATIGUE A.5.1 S-N curves in structural ASSESSMENT ...... 258 steel codes...... 241 REFERENCES ...... 258 A.5.2 Allowable surface pressure...... 242 A.6 PRETENSION ...... 242 A.6.1 Safety against loosening ...... 244

Contents v Guidelines for Design of Wind Turbines − DNV/Risø

D. FEM CALCULATIONS...... 260 D.1 TYPES OF ANALYSIS ...... 260 D.2 MODELLING ...... 261 D.2.1 Model...... 261 D.2.2 Elements ...... 262 D.2.3 Boundary conditions...... 264 D.2.4 Loads ...... 265 D.3 DOCUMENTATION ...... 265 D.3.1 Model...... 265 D.3.2 Results ...... 267

E. MATERIAL PROPERTIES ...... 268 E.1 STEEL...... 268 E.1.1 Structural steel...... 268 E.1.2 Alloy steel...... 269 E.2 CAST IRON...... 269 E.3 FIBRE REINFORCED PLASTICS ...... 269 E.3.1 Glass fibre reinforced plastics .. 269 E.4 CONCRETE ...... 270 E.4.1 Mechanical properties...... 270 REFERENCES ...... 270

F. TERMS AND DEFINITIONS ...... 271 REFERENCES ...... 276

G. TABLES AND CONVERSIONS... 277 G.1 ENGLISH/METRIC CONVERSION .... 277 G.2 AIR DENSITY VS. TEMPERATURE .. 277 G.3 AIR DENSITY VS. HEIGHT...... 277 G.4 RAYLEIGH WIND DISTRIBUTION.... 277

vi Contents Guidelines for Design of Wind Turbines − DNV/Risø

1. Wind Turbine Concepts turbines operating at constant rotor speed have been dominating up to now, turbines with variable rotor speed are becoming 1.1 Introduction increasingly more common in an attempt to Wind-powered ships, grain mills, water optimise the energy capture, lower the loads, , and threshing machines all obtain better power quality, and enable more exemplify that extraction of power from advanced power control aspects. wind is an ancient endeavour. With the evolution of mechanical insight and technology, the last decades of the 20th 1.2 Conceptual aspects century, in particular, saw the development Some early wind turbine designs include of machines which efficiently extract power multiple-bladed concepts. These turbines are from wind. ”Wind turbines” is now being all characterised by rotors with high solidity, used as a generic term for machines with i.e. the exposed area of the blades is rotating blades that convert the kinetic relatively large compared to the swept area energy of wind into useful power. of the rotor.

In the 20th century, early wind turbine designs were driven by three basic philosophies for handling loads: (1) withstanding loads, (2) shedding or avoiding loads, and (3) managing loads mechanically, electrically, or both. In the midst of this evolution, many wind turbine designs saw the light of day, including horizontal axis and vertical axis turbines. Turbines that spin about horizontal and vertical axes, respectively, and are equipped with one, two, three or multiple blades.

Modern turbines evolved from the early designs and can be classified as two or three-bladed turbines with horizontal axes and upwind rotors. Today, the choice between two or three-bladed wind turbines is merely a matter of a trade-off between aerodynamic efficiency, complexity, cost, noise and aesthetics. Figure 1-1. Multiple-bladed wind turbines of various designs. Additional key turbine design considerations include wind climate, rotor type, generator A disadvantage of such a high-solidity rotor type, load and noise minimisation, and is the excessive forces that it will attract control approach. Moreover, current trends, during extreme wind speeds such as in driven by the operating regime and the hurricanes. To limit this undesirable effect market environment, involve development of extreme winds and to increase efficiency, of low-cost, megawatt-scale turbines and modern wind turbines are built with fewer, lightweight turbine concepts. Whereas longer, and more slender blades, i.e. with a

1 – Wind Turbine Concepts 1 Guidelines for Design of Wind Turbines − DNV/Risø much smaller solidity. To compensate for the one illustrated in Figure 1-2, are that the the slenderness of the blades, modern generator and gearbox are placed on the turbines operate at high tip speeds. ground and are thus easily accessible, and that no yaw mechanism is needed. Among 1.2.1 Vertical axis turbines the disadvantages are an overall much lower level of efficiency, the fact that the turbine Vertical axis wind turbines (VAWTs), such needs total dismantling just to replace the as the one shown in Figure 1-2 with C- main bearing, and that the rotor is placed shaped blades, are among the types of relatively close to the ground where there is turbine that have seen the light of day in the not much wind. past century.

1.2.2 Horizontal axis turbines Horizontal axis wind turbines (HAWTs), such as the ones shown in Figure 1-3, constitute the most common type of wind turbine in use today. In fact all grid- connected commercial wind turbines are today designed with propeller-type rotors mounted on a horizontal axis on top of a vertical tower. In contrast to the mode of operation of the vertical axis turbines, the horizontal axis turbines need to be aligned with the direction of the wind, thereby allowing the wind to flow parallel to the axis of rotation.

Figure 1-2. Eole C, a 4200 kW vertical axis with 100 m rotor diameter at Cap Chat, Québec, Canada. The machine, which is the world's largest wind turbine, is no longer operational. From www.windpower.org (2000), © Danish Wind Turbine Manufacturers Association.

Classical water wheels allow the water to Figure 1-3. Three-bladed upwind turbines being tested arrive tangentially to the water wheel at a at Risø, August 1986. right angle to the rotational axis of the wheel. Vertical axis wind turbines are Insofar as concerns horizontal axis wind designed to act correspondingly towards air. turbines, a distinction is made between Though, such a design would, in principle, upwind and downwind rotors. Upwind work with a horizontal axis as well, it would rotors face the wind in front of the vertical require a more complex design, which tower and have the advantage of somewhat would hardly be able to beat the efficiency avoiding the wind shade effect from the of a propeller-type turbine. The major presence of the tower. Upwind rotors need a advantages of a vertical axis wind turbine, as yaw mechanism to keep the rotor axis

2 1 – Wind Turbine Concepts Guidelines for Design of Wind Turbines − DNV/Risø aligned with the direction of the wind. more fluctuating loads because of the Downwind rotors are placed on the lee side variation of the inertia, depending on the of the tower. A great disadvantage in this blades being in horizontal or vertical design is the fluctuations in the position and on the variation of wind speed due to the rotor passing through the wind when the blade is pointing upward and shade of the tower which gives rise to more downward. Therefore, the two and one- fatigue loads. Theoretically, downwind bladed concepts usually have so-called rotors can be built without a yaw teetering hubs, implying that they have the mechanism, provided that the rotor and rotor hinged to the main shaft. This design nacelle can be designed in such a way that allows the rotor to teeter in order to the nacelle will follow the wind passively. eliminate some of the unbalanced loads. This may, however, induce gyroscopic loads One-bladed wind turbines are less and hamper the possibility of unwinding the widespread than two-bladed turbines. This is cables when the rotor has been yawing due to the fact that they, in addition to a passively in the same direction for a long higher rotational speed, more noise and time, thereby causing the power cables to visual intrusion problems, need a counter- twist. As regards large wind turbines, it is weight to balance the rotor blade. rather difficult to use slip rings or mechanical collectors to circumvent this problem. Whereas, upwind rotors need to be rather inflexible to keep the rotor blades clear of the tower, downwind rotors can be made more flexible. The latter implies possible savings with respect to weight and may contribute to reducing the loads on the tower. The vast majority of wind turbines in Figure 1-4. Three, two and one-bladed wind turbine operation today have upwind rotors. concepts. From www.windpower.org (2000), © Danish Wind Turbine Manufacturers Association. 1.2.3 Number of rotor blades 1.2.4 Power control aspects The three-bladed concept is the most common concept for modern wind turbines. Wind turbines are designed to produce A turbine with an upwind rotor, an as cheap as possible. For this asynchronous generator and an active yaw purpose, wind turbines are in general system is usually referred to as the Danish designed to yield a maximum power output concept. This is a concept, which tends to be at wind speeds around 15 m/s. It would not a standard against which other concepts are pay to design turbines to maximise their evaluated. power output at stronger winds, because such strong winds are usually too rare. Relative to the three-bladed concept, the two However, in case of stronger winds, it is and one-bladed concepts have the advantage necessary to waste part of the excess energy of representing a possible saving in relation to avoid damage on the wind turbine. Thus, to the cost and weight of the rotor. However, the wind turbine needs some sort of power their use of fewer rotor blades implies that a control. The power control is divided into higher rotational speed or a larger chord is two regimes with different concepts: needed to yield the same energy output as a • power optimisation for low wind speeds three-bladed turbine of a similar size. The • power limitation for high wind speeds use of one or two blades will also result in

1 – Wind Turbine Concepts 3 Guidelines for Design of Wind Turbines − DNV/Risø

These regimes are separated by the wind rated power of the generator at high wind speed at which the maximum power output speeds. Disadvantages encompass extra is achieved, typically about 15 m/sec. complexity due to the pitch mechanism and high power fluctuations at high wind speeds. Basically, there are three approaches to power control: Active stall-controlled turbines resemble • stall control pitch-controlled turbines by having pitchable • pitch control blades. At low wind speeds, active stall • active stall control turbines will operate like pitch-controlled turbines. At high wind speeds, they will Stall-controlled wind turbines have their pitch the blades in the opposite direction of rotor blades bolted to the hub at a fixed what a pitch-controlled turbine would do angle. The stall phenomenon is used to limit and force the blades into stall. This enables a the power output when the wind speed rather accurate control of the power output, becomes too high. This is achieved by and makes it possible to run the turbine at designing the geometry of the rotor blade in the rated power at all high wind speeds. This such a way that flow separation is created on control type has the advantage of having the the downwind side of the blade when the ability to compensate for the variations in wind speed exceeds some chosen critical the air density. value. Stall control of wind turbines requires correct trimming of the rotor blades and Figure 1-5 shows iso-power curves for a correct setting of the blade angle relative to wind turbine as a function of the blade angle the rotor plane. Some drawbacks of this and the mean wind speed. The ranges for method are: lower efficiency at low wind pitch control and active stall control are speeds, no assisted start and variations in the separated at a blade angle of 0° with the maximum steady state power due to rotor plane. At low wind speeds, the optimal variation in the air density and grid operation of the wind turbine is achieved at frequencies. a blade angle close to 0°. At higher wind speeds, the turbine will overproduce if the Pitch-controlled wind turbines have blades blade angle is not adjusted accordingly. that can be pitched out of the wind to an With pitch control, the blade is pitched angle where the blade chord is parallel to the positively with its leading edge being turned wind direction. The power output is towards the wind. With active stall control, monitored and whenever it becomes too the blade is pitched negatively with its high, the blades will be pitched slightly out trailing edge turned towards the wind. The of the wind to reduce the produced power. power control and, in particular, the power The blades will be pitched back again once limitation at higher wind speeds are the wind speed drops. Pitch control of wind indicated in an idealised manner for both turbines requires a design that ensures that control approaches by the dashed curves in the blades are pitched at the exact angle Figure 1-5. The dashed curves illustrate how required in order to optimise the power the transition between operation with 0° output at all wind speeds. Nowadays, pitch blade angle at low wind speeds and power- control of wind turbines is only used in limiting operation along an iso-power curve conjunction with variable rotor speed. An at high wind speeds can be achieved for a advantage of this type of control is that it three-bladed rotor at a rated power of has a good power control, i.e. that the mean 400 kW. In this example, the rated power is value of the power output is kept close to the reached at a wind speed of about 12 m/sec.

4 1 – Wind Turbine Concepts Guidelines for Design of Wind Turbines − DNV/Risø

1.3 Economical aspects The choice of rotor size and generator size depends heavily on the distribution of the The ideal is not dictated wind speed and the wind energy potential at by technology alone, but by a combination a prospective location. A large rotor fitted of technology and economy. Wind turbine with a small generator will produce manufacturers wish to optimise their electricity during many hours of the year, machines, so that they deliver electricity at but it will only capture a small part of the the lowest possible cost per unit of energy. wind energy potential. A large generator In this context, it is not necessarily optimal will be very efficient at high wind speeds, to maximise the annual energy production, if but inefficient at low wind speeds. that would require a very expensive wind Sometimes it will be beneficial to fit a wind turbine. Since the energy input (the wind) is turbine with two generators with different free, the optimal turbine design is one with rated powers. low production costs per produced kWh.

Figure 1-5. Iso-power curves for a wind turbine at 26.88 rpm vs. blade angle and mean wind speed

1.4 Power production almost independent of the rotor size. See Petersen, 1998. A study of different Danish wind turbine designs shows that the specific power Hence, the main consideration in the performance in terms of produced energy evaluation of the cost of the turbine is the per m2 rotor area per year (kWh/m2/year) is specific rotor power (kW/m2) and the specific cost (cost/m2 rotor) together with

1 – Wind Turbine Concepts 5 Guidelines for Design of Wind Turbines − DNV/Risø expected service life and cost and availability. An availability factor, i.e. the amount of time that the turbine is producing energy, or is ready for production, of 98 % is common for commercial turbines.

Thus the primary factor affecting the power performance is the rotor size. Secondary Power [kW] Efficiency [%] [%] Efficiency factors are the control principle such as stall- or pitch control and single- dual- or variable speed. Wind speed 1.4.1 Power curve Figure 1-6. Power- and efficiency curve

The power being produced by any type of The produced power varies with the wind wind turbine can be expressed as speed as can be seen from the blue graph in Figure 1-6. The form of the graph varies 3 P = ½⋅ρ⋅V ⋅A⋅CP slightly from different concepts. Assuming constant efficiency (e.g. constant tip speed P output power ratio) the graph basically consists of a third ρ air density degree polynomial up to the rated wind V free wind speed speed at which the nominal power is A rotor area reached. At this point the power regulation CP efficiency factor sets in, either by the blades stalling or by pitching the blades to attain an The power coefficient CP is a product of the approximately constant power. The power mechanical efficiency ηm, the electrical curve and the power efficiency curve are efficiency ηe, and of the aerodynamic often presented in the same graph, with the efficiency. All three factors are dependent power and the efficiency scales on each side on the wind speed and the produced power, of the graph as shown in Figure 1-6. respectively. The mechanical efficiency ηm is mainly determined by losses in the Figure 1-7 illustrates the controlled power gearbox and is typically 0.95 to 0.97 at full curve of a wind turbine, in the case of 1) load. The electrical efficiency covers losses stall controlled, fixed speed configuration, in the generator and electrical circuits. At and 2) pitch controlled, variable speed configuration. full load ηe = 0.97 - 0.98 is common for configurations with an induction generator.

It can be shown that the maximum possible Power Rated power [Kw] value of the aerodynamic efficiency is 16/27 = 0.59, which is achieved when the turbine reduces the wind speed to one-third Wind power resource Stall regulatedregulation & & fixed fixed speed speed of the free wind speed (Betz' law). Pitch control & variable speed Wind speed 5 10 15 20 25 [m/s] Figure 1-7. Examples of power curves for two types of wind turbines.

6 1 – Wind Turbine Concepts Guidelines for Design of Wind Turbines − DNV/Risø

The efficiency factor CP typically reaches a For a particular wind turbine, the power maximum at a wind speed of 7-9 m/sec and, production is defined by the power curve as normally, it does not exceed 50%. The described above. By combining the power electric power typically reaches the rated curve with the wind distribution the actual power of the turbine at a wind speed of 14- energy production is yielded, often 16 m/sec. expressed in terms of the annual energy production Eyear. In the Danish approval scheme as well as in the IEC wind turbine classification system, Vstop the power curve is required to be determined = E year N 0 P(u) f (u) du from measurements. ∫ V start

1.4.2 Annual energy production P(u) power curve function The wind speed distribution is often f(u) wind distribution function represented by a Weibull distribution. The Vstart cut-in wind speed density function of the Weibull probability Vstop cut-out wind speed can be expressed as shown in Figure 1-8. N0 = 8765 hours/year

Dividing Eyear by the rotor area yields the k − u f (u) = uk 1 exp(−( )k ) specific power performance, which is U10 k A A another common way to express the turbine efficiency.

Yet another way of expressing the efficiency of a particular turbine is given by using the , which is defined as the ratio of actual average power to the rated power measured over a period of time (average Figure 1-8. Weibull density function. kW/rated kW). The total energy that would be produced by a wind turbine during a one- Hence, for a specific site the wind climate year period, assuming a certain distribution can be described in terms of the parameters of wind speed probability density and A and k, and from these the average wind assuming 100 per cent availability, is speed Vave, as defined in IEC 61400-1, can referred to as the potential annual energy be calculated output.

Γ + A (1 1/ k) V = ave  1.5 Configurations and sizes A π / 2, if k = 2 Wind turbines are erected as stand-alone in which Γ is the gamma function. turbines, in clusters of multiple turbines, or – on a larger scale – in park configurations. In the Danish standard it is common to Before the 1980’s, wind energy describe the wind climate in terms of the development focused on the individual wind roughness class, and an explicit correlation turbine. By the late 1980’s, this perspective exists between the roughness class and the began to change as attention shifted to average wind speed. collective generation of electric power from

1 – Wind Turbine Concepts 7 Guidelines for Design of Wind Turbines − DNV/Risø an array of wind turbines located in the high population density and thus with vicinity of each other and commonly difficulties in finding suitable sites on land. referred to as wind parks or wind farms. In Construction costs are much higher at sea, the early 1980’s, the typical size of a wind but energy production is also much higher. turbine was about 55 kW in terms of rated Currently, wind energy from turbines power, whereas turbine sizes today have erected on fixed foundations in up to 15 m exceeded 2 MW. Table 1-1 gives examples water depth is considered economically of typical combinations of rotor diameter feasible. Figure 1-9 shows an example of an and rated power for a number of different early offshore wind farm. tower heights.

Table 1-1. Typical wind turbine sizes 1.6 Future concepts Tower Rotor Rated height (m) diameter (m) power (kW) 22 21 55 31 30 225 35 35 450 35-40 41-44 500 44 43 600 50 48 750 50 54 1000 60 58 1500 64-80 72-76 2000 85 115 5000

Figure 1-10. A conception of a flexible wind turbine. During stand-still, the blades deflect as a rush in the wind.

Until now the development of large wind turbines has mainly been prompted by upscaling the dominating concepts described in this section. Still, new concepts for wind turbine designs and components are being developed in an attempt to anticipate the demands for the continuing growth of wind Figure 1-9. Vindeby Offshore Wind Farm. From turbines. www.windpower.org (1997) © Bonus. One proposal for a future concept is While wind energy is already economic in characterised by more flexible wind turbine terms of good onshore locations, it is concepts. One element in this is an expected currently about to cross the economic increase in the structural flexibility of wind frontier set by shorelines: offshore wind turbines. Figure 1-10 exemplifies how the energy is becoming competitive with other latter can be conceived. Another element is power-generating technologies. Offshore an expected increase in the flexibility of the wind energy is a promising application of drive train, e.g. in terms of gearless designs wind power, in particular, in countries with with variable rotational speeds, and a more

8 1 – Wind Turbine Concepts Guidelines for Design of Wind Turbines − DNV/Risø extensive use of power electronics can also be expected.

Also, more flexible control systems can be foreseen as an increasing number of computers and sensors are incorporated to allow for adaptive operation. In this context, it is possible that a shift may take place from focusing on wind turbine control to focusing on wind farm control. A development which will inevitably, put other requirements on the individual wind turbine in addition to the ones we are experiencing today.

REFERENCES

Danish Energy Agency, Technical Criteria for the Danish Approval Scheme for Wind Turbines, 2000.

IEC 61400-12 Wind turbine generator systems, part 12: Wind turbine power performance testing, 1st edition, 1999.

IEC WT0, IEC System for Conformity Testing and Certification of Wind turbines, Rules and Procedures, International Electrotechnical Commission, 1st edition, 2001-04.

Petersen, H., Comparison of wind turbines based on power curve analysis, Helge Petersen Consult, Darup Associates Ltd., 1998.

1 – Wind Turbine Concepts 9 Guidelines for Design of Wind Turbines − DNV/Risø

2. Safety and Reliability high level of reliability to render the joint probability negligible that a failure should occur during an extreme event and that the 2.1 Safety philosophy protection system should be unable to fulfil A wind turbine should be designed, its task. Usually, non-redundant structural dimensioned and manufactured in such a parts of the protection system are therefore way that it, if correctly used and maintained designed to high safety class. over its anticipated service life, can withstand the assumed loads within the The prescribed level of safety or the choice prescribed level of safety and possess a of safety class may differ for different parts sufficient degree of durability and of the wind turbine. The rotor is usually robustness. Calculation, or a combination of designed to at least normal safety class. calculation and testing, can be used to Other structural parts such as tower and demonstrate that the structural elements of a foundation are usually assigned to safety wind turbine meet the prescribed level of classes according to the possible safety. consequences of a failure. Since failure of the foundation will have consequences for The prescribed level of safety for a the tower and the rotor, while failure of the structural element can be expressed in terms rotor or the tower may not necessarily have of a requirement for the probability of consequences for the foundation, an failure and be determined on the basis of so- attractive approach to the choice of safety called risk acceptance criteria. It depends on classes for various structural parts could be the type and consequence of failure. The to attempt a so-called fail-grace sequence, type of failure can be characterised by the i.e. a sequence of failures in which the degree of ductility and the amount of reserve foundation will be the last structural part to capacity or structural redundancy. The fail. This sequence is based on a feasibility consequence of failure can be characterised study of the consequences of failure for the in terms of the fatalities and societal wind turbine structure and its foundation consequences involved. The more severe the only. However, usually the rotor is designed consequence is, and the more limited the to normal or high safety class, which is not reserve capacity is, the smaller is the necessarily in accordance with the above acceptable failure probability. The fail-grace philosophy. Requirements for prescribed safety is standardised in terms of designing a wind turbine rotor to normal or safety classes as described in more detail in high safety class derive from the hazards Section 2.3. A distinction is made between that the rotor poses on its surroundings, low, normal and high safety class. The when the turbine runs away, or when the higher the safety class is, the heavier is the rotor fails. In such events, parts of the rotor requirement for the level of safety, i.e. the may be shed in distances up to one kilometre smaller is the acceptable failure probability. or even farther away from the turbine location. A wind turbine is equipped with a control and protection system, which defines an It should be noted that the level of safety is envelope of possible design situations that usually the result of a trade-off with the wind turbine will experience. To keep economy. In DS472, emphasis is placed on the wind turbine within this envelope, it is safety. However, in terms of offshore part of the safety philosophy that the turbines it is inevitable that economical protection system shall possess a sufficiently aspects will become more predominant than

10 2 – Safety and Reliability Guidelines for Design of Wind Turbines − DNV/Risø they are onshore, and that more emphasis consider a 20-year design lifetime for wind will eventually be placed on financial factors turbines. Characteristic capacities are when it comes to safety issues and usually chosen as low quantiles in the determination of acceptable safety levels. associated capacity distributions. The partial Limit state design is used to achieve the safety factors that are applied in the design prescribed level of safety. It is common to account for the possible more unfavourable verify the safety of a wind turbine with realisation of the loads and capacities than respect to the following limit states: those assumed by the choice of • ultimate limit state characteristic values. Note in this context • serviceability limit state that some of the partial safety factors, which • accidental limit state are specified in standards, are not safety For this purpose, design loads are derived factors in the true sense, but rather reduction from multiplying characteristic loads by one factors which account for degradation or more partial safety factors, and design effects, scale effects, temperature effects, capacities are derived from dividing etc. and which happen to appear in the characteristic capacities by one or several design expressions in exactly the same other partial safety factors. Partial safety manner as true partial safety factors. factors are applied to loads and material strengths to account for uncertainties in the With structural safety being a major goal of characteristic values. Verification of the the design, it is important to make sure that structural safety is achieved by ensuring that the characteristic values of load and material the design load, or the combination of a set quantities, which have been assumed for the of design loads, does not exceed the design design, are achieved in practice. Non- capacity. In case a combination of loads is destructive testing of completed structural used, it should be noted that it is common parts plays a role in this context, and control always to combine one extreme load with of workmanship another. Material one or several “normal” loads. Two or certificates also come in handy in this several extreme loads are usually not context. In general, one may say that combined, unless they have some inspection is an important part of the safety correlation. philosophy. Not least, as it will allow for verification of assumptions made during the Characteristic loads and characteristic design and for taking remedial actions if capacities constitute important parameters in averse conditions are detected during the the design process. Characteristic loads for service life of the wind turbine. assessment of the ultimate limit state are usually determined as load values with a 50- For details about structural safety and limit year recurrence period, and they are state design, reference is made to Section therefore often interpreted as the 98% 2.3. For details about combinations of quantile in the distribution of the annual design situations and external conditions, as maximum load. This choice does not well as a definition of load cases, reference necessarily imply that a design lifetime of is made to Chapter 4. exactly 50 years is considered. It is more a matter of tradition and convenience. Nor should it be taken as a 50-year guarantee 2.2 System safety and operational within which failures will not occur. For reliability assessment of fatigue, a design lifetime is A wind turbine is to be equipped with needed, and in this context it is common to control and protection systems which are

2 – Safety and Reliability 11 Guidelines for Design of Wind Turbines − DNV/Risø meant to govern the safe operation of the • voltage and frequency at mains connec- wind turbine and to protect the wind turbine tion from ill conditions. Some components will • connection of the electrical load act in both the control and the protection • power output function, but distinction is made in that the • cable twist control system monitors and regulates the • yaw error essential operating parameters to keep the • brake wear turbine within defined operating range whereas the protection system ensures that The control system is meant to keep the the turbine is kept within the design limits. wind turbine within its normal operating The protection system must takes range. As a minimum, the normal operating precedence over the control system. range should be characterised by the following properties and requirements: 2.2.1 Control system • a maximum 10-minute mean wind Controls are used for the following speed at hub height, Vmax, i.e. the stop functions: wind speed below which the wind • to enable automatic operation turbine may be in operation • to keep the turbine in alignment with • a maximum long-term mean nominal the wind power Pnom, interpreted as the highest • to engage and disengage the generator power on the power curve of the wind • to govern the rotor speed turbine in the wind speed interval [ ] • to protect the turbine from overspeed or Vmin;Vmax , where Vmin denotes the start damage caused by very strong winds wind speed for the turbine • • to sense malfunctions and warn a maximum nominal power Pmax, which operators of the need for maintenance on average over 10 minutes may not be or repair exceeded for a wind speed at hub height of V10min,hub < Vmax The control system is meant to control the • a maximum operating frequency of operation of the wind turbine by active or rotation nr,max for the wind turbine passive means and to keep operating • a maximum transient frequency of parameters within their normal limits. rotation nmax for the wind turbine Passive controls use their own sensing and • a wind speed below which the wind are exercised by use of natural forces, e.g. turbine may be stopped centrifugal stalling or centrifugal feathering. The wind turbine is kept within its normal Active controls use electrical, mechanical, operating range by means of the control hydraulic or pneumatic means and require system, which activates and/or deactivates transducers to sense the variables that will the necessary controls, e.g.: determine the control action needed. Typical • yaw (alignment with the wind) variables and features to be monitored in • blade angle regulation this respect include: • activation of the brake system • rotor speed • power network connection • wind speed • power limitation • • shutdown at loss of electrical network • external temperature or electrical load • generator temperature

12 2 – Safety and Reliability Guidelines for Design of Wind Turbines − DNV/Risø

In addition, it must be possible to stop the turbine to a safe condition and maintain the wind turbine, e.g. for the purpose of turbine in this condition. It is usually inspection and repairs, or in case of required that the protection system shall be emergencies. Monitoring of the control capable of bringing the rotor to rest or to an system and its functions must be adapted to idling state from any operating condition. In the actual design of the wind turbine. Design the IEC standard an additional requirement of a wind turbine control system requires a is that means shall be provided for bringing background in servo theory, i.e. theory the rotor to a complete stop from a dealing with control of continuous systems. hazardous idling state in any wind speed less than the annual extreme wind speed. The The control system is of particular activation levels for the protection system importance in areas where weak grids are have to be set in such a way that design encountered. Weak grids can, for example, limits are not exceeded. be found in sparsely populated areas where the capacity of the grids can often be a Situations which call for activation of the limiting factor for the exploitation of the protection system include, but are not wind resource in question. Two problems necessarily limited to: are identified in this context: • overspeed • increase of the steady-state voltage level • generator overload or fault of the grid above the limit where power • excessive vibration consumption is low and wind power • failure to shut down following network input is high loss, disconnection from the network, or • voltage fluctuations above the flicker loss of electrical load limit may result from fluctuating wind • abnormal cable twist owing to nacelle power input caused by fluctuating wind rotation by yawing and wind turbine cut-ins. The protection system should therefore as a The solution to the above problems is to use minimum cover monitoring of the a so-called power control as part of the following: control system. The power control concept • rotational speed or rotational frequency implies buffering the wind turbine power in • periods where the voltage limits may be overload of a generator or other energy violated and releasing it when the voltage is conversion system/load • lower. This method is combined with a extreme vibrations in the nacelle • smoothening of the power output, such that safety-related functioning of the control fluctuations are removed, in particular those system that would exceed the flicker limit. As overspeed is by far the most critical error, rotational speed monitors form a 2.2.2 Protection system crucial element of the protection system.

The protection system is sometimes referred A protection system consists of: to as the safety system. Mechanical, • a registering unit electrical and aerodynamic protection • an activating unit systems are available. The protection system • a brake unit is to be activated when, as a result of control The protection system shall include one or system failure or of the effects of some other more systems (mechanical, electrical or failure event, the wind turbine is not kept aerodynamic) capable of bringing the rotor within its normal operation range. The to rest or to an idling state. In the Danish protection system shall then bring the wind

2 – Safety and Reliability 13 Guidelines for Design of Wind Turbines − DNV/Risø standard at least two brake systems must be which risk assessment is used to determine included, and at least one of these must have the interval between inspections. an aerodynamically operated brake unit. See Section 2.3.3. Fail-safe is a design philosophy, which through redundancy or adequacy of design To ensure immediate machine shutdown in in the structure, ensures that in the event of a case of personal risk, an emergency stop failure of a component or power source, the button, which will overrule both the control wind turbine will remain in a non-hazardous and normal protection system, shall be condition. provided at all work places. 2.2.3 Brake system In addition to what is stated above, the The brake system is the active part of the protection system is, as a minimum, to be protection system. Examples of brake subjected to the following requirements: systems are: • the protection system must take • mechanical brake precedence over the control system • aerodynamic brake • the protection system must be fail-safe • generator brake in the event that the power supply fails An aerodynamic brake system usually • structural components in mechanisms of consists of turning the blade tip or, as the protection system shall be designed commonly seen on active stall- and pitch- to high safety class controlled turbines, of turning the entire • the protection system must be able to blade 90° about the longitudinal axis of the register a fault and to bring the wind blade. This results in aerodynamic forces turbine to a standstill or to controlled that counteract the rotor torque. Also, freewheeling in all situations in which spoilers and parachutes have been used as the rotor speed is less than nmax. aerodynamic brakes. • the protection system must be tolerant towards a single fault in a sensor, in the electronic and electrical as well as the hydraulic systems or in active Figure 2-1. Tip brake, from www.windpower.org mechanical devices, i.e. an undetected (2000), © Danish Wind Turbine Manufacturers' fault in the system must not prevent the Association.

system from detecting a fault condition The reliability of a brake system is of the and carrying out its function • utmost importance to ensure that the system the reliability of the protection system will serve its purpose adequately. In this must ensure that situations caused by respect, it is important to be aware of failures in the protection system, possible dependencies between different whereby the extreme operating range is brakes or different brake components. For exceeded, can be neglected example, if all three blades are equipped The reliability of the protection system may with tip brakes, some dependency between be ensured by means of either (1) the entire the three tip brakes can be expected, cf. the protection system being of a fail-safe design, common cause failures that can be foreseen or (2) redundancy of the parts of the for these brakes. This will influence the protection system where it cannot be made overall reliability against failure of the fail-safe, or (3) frequent inspections of the system of the three tip brakes and needs to functioning of the protection system, in

14 2 – Safety and Reliability Guidelines for Design of Wind Turbines − DNV/Risø be taken into account if the failure of the system for determination of possible probability exceeds 0.0002 per year. failure modes and identification of their effects on the system. The analysis is based Brakes or components of brake systems will on a worksheet that systematically lists all be subject to wear. Thus, current monitoring components in the system, including: and maintenance are required. • component name • function of component IEC61400-1 requires that the protection • possible failure modes system shall include one or more systems, • causes of failure i.e. mechanical, electrical, or aerodynamic • how failures are detected brakes, capable of bringing the rotor to rest • effects of failure on primary system or to an idling state from any operating function condition. At least one of these systems shall • effects of failure on other components act on the low-speed shaft or on the rotor of • necessary preventative/repair measures the wind turbine. The idea behind this is to The failure mode and effects analysis can be have a brake system which ensures that a supplemented by a criticality analysis, which fault will not lead to a complete failure of is a procedure that rates the failure modes the wind turbine. according to their frequency or probability

of occurrence and according to their DS472 is more strict by requiring at least consequences. The assigned ratings can be two fail-safe brake systems. If the two used to rank the components with respect to systems are not independent, i.e. if they have their criticality for the safety of the system. some parts in common, then the turbine shall An example of a worksheet is given in Table automatically be brought to a complete stop 2-1. or to controlled idling in the event of a failure in the common parts. At least one A failure mode and effects analysis can be brake system is required to have an conducted at various levels. Before aerodynamic brake unit. commencing, it is thus important to decide

what level should be adopted as some areas 2.2.4 Failure mode and effects may otherwise be examined in great detail, analysis while others will be examined at the system A failure mode and effects analysis is a level only without examination of the qualitative reliability technique for individual components. If conducted at too systematic analysis of mechanical or detailed a level, the analysis can be rather electrical systems, such as a wind turbine time-consuming and tedious, but will protection system. The analysis includes undoubtedly lead to a thorough under- examination of each individual component standing of the system.

Table 2-1. Example of worksheet. COMPONENT FAILURE FAILURE FAILURE FAILURE FREQUENCY SEVERITY MODE CAUSE EFFECT DETECTION RATING RATING Valve Leak past Deteriorate Oil leak Visual by Low Low stem d seal ROV Fails to Control Valve will Flow does not Medium Low close on system not shut off shut off command failure flow

2 – Safety and Reliability 15 Guidelines for Design of Wind Turbines − DNV/Risø

The failure mode and effects analysis is at the next level. If one of several events primarily a risk management tool. The causes the higher event, it is joined with an strength of the failure mode and effects OR gate. If two or more events must occur analysis is that, if carried out correctly, it in combination, they are joined with an identifies safety-critical components where a AND gate. single failure will be critical for the entire system. It is a weakness, however, that it depends on the experience of the analyst and that it cannot easily be applied to cover multiple failures.

2.2.5 Fault tree analysis A fault tree is a logical representation of the many events and component failures that may combine to cause one critical event such as a system failure. It uses “logic gates” (mainly AND and OR gates) to show how “basic events” may combine to cause the critical “top event”.

Application Fault tree analysis has several potential uses in relation to wind turbine protection systems: • In frequency analysis, it is common to quantify the probability of the top event occurring based on estimates of the failure rates of each component. The top event may comprise an individual Figure 2-2. Fault tree symbols. failure case, or a branch probability in an event tree. If quantification of the fault tree is the • In risk presentation, it may also be used objective, downward development should to show how the various risk stop once all branches have been reduced to contributors combine to produce the events that can be quantified in terms of overall risk. probabilities or frequencies of occurrence. − • In hazard identification, it may be used Various standards for symbols are used qualitatively to identify combinations of the most typical ones are shown in Figure basic events that are sufficient to cause 2-2. An example of a fault tree is shown in the top event, also known as “cut sets”. Figure 2-3.

Construction of a fault tree Some types of events, for example a fire or Construction of a fault tree usually power failure, may affect many components commences with the top event and then in the system at once. These are known as works its way downwards to the basic “common-cause failures” and may be events. For each event, it considers what represented by having the same basic event conditions are necessary to produce the occurring at each appropriate place in the event, and it then represents these as events fault tree.

16 2 – Safety and Reliability Guidelines for Design of Wind Turbines − DNV/Risø

Combination of frequencies and proba- computer programs are used to identify bilities minimal cut sets. Both frequencies and probabilities can be combined in a fault tree, providing the rules Minimal cut sets can be used in hazard in Table 2-2 are followed. identification to describe combinations of events necessary to cause the top event. Table 2-2. Rules for Combining Frequencies and Probabilities Minimal cut sets can, moreover, be used to Gate Inputs Outputs OR Probability + Probability Probability rank and screen hazards according to the AND Frequency + Frequency Frequency number of events that must occur Frequency + Probability Not permitted simultaneously. In principle, single event cut Probability × Probability Probability sets are of concern because only one failure Frequency × Frequency Not permitted can lead to the top event. In reality, larger Frequency × Probability Frequency cut sets may have a higher frequency of occurrence. Nevertheless, the method can be useful for hazard screening and for suggesting where additional safeguards may be needed.

Quantification of a fault tree Simple fault trees may be analysed by using a gate-by-gate approach to determine the top event probability, provided that all events are independent and that there are no common cause failures. This gate-by-gate approach is useful for QRA (Quantitative Risk Analysis), because it quantifies all intermediate events in the fault tree and provides a good insight into the main contributors to the top event and the effectiveness of safeguards represented in the tree. However, because it cannot represent repeated events or dependencies Figure 2-3. Example of fault tree. correctly, it is normally not used for formal

reliability analysis. Reliability analysis of Minimal cut set analysis more complex fault trees requires minimal Cut sets comprise combinations of events cut set analysis to remove repeated events. that are sufficient to cause the top event.

Minimal cut sets contain the minimum sets Strengths and weaknesses of events necessary to cause the top event, Preparation and execution of a fault tree i.e. after eliminating any events that occur analysis is advantageous in the sense that it more than once in the fault tree. In case of forces the wind turbine manufacturer to simple fault trees, with each basic event only examine the protection system of his wind occurring once, the minimal cut sets can be turbine systematically. When probabilities identified by means of inspection. For more can be assigned to events, the fault tree complex trees, formal methods such as the methodology can be applied to assess the Boolean analysis are required. Most often, overall reliability of the wind turbine

2 – Safety and Reliability 17 Guidelines for Design of Wind Turbines − DNV/Risø protection system. Furthermore, it can be how likely it is that the structure will reach a used to determine the most critical events limit state and enter a state of failure. and parts of the protection system. There are several types of limit states. Two Beware, however, that it is often hard to find types are common, ultimate limit states and out whether a fault tree analysis has been serviceability limit states. Ultimate limit carried out properly. Fault tree analyses states correspond to the limit of the load- become complicated, time-consuming and carrying capacity of a structure or structural difficult to follow for large systems, and it component, e.g. plastic yield, brittle fracture, becomes easy to overlook failure modes and fatigue fracture, instability, buckling, and common cause failures. It is a weakness that overturning. Serviceability limit states imply the diagrammatic format discourages that there are deformations in excess of analysts from stating assumptions and tolerance without exceeding the load- conditional probabilities for each gate carrying capacity. Examples are cracks, explicitly. This can be overcome by careful wear, corrosion, permanent deflections and back-up text documentation. It is a vibrations. Fatigue is sometimes treated as a limitation that all events are assumed to be separate type of limit state. Other types of independent. Fault tree analyses lose their limit states are possible, e.g. accidental limit clarity when applied to systems that do not states and progressive limit states. fall into simple failed or working states such as human error, adverse weather conditions, 2.3.2 Failure probability and other etc. measures of structural reliability

In structural design, the reliability of a

structural component is evaluated with 2.3 Structural safety respect to one or more failure modes. One 2.3.1 Limit states such failure mode is assumed in the following. The structural component is During the lifetime of a structure, the described by a set of stochastic basic structure is subjected to loads or actions. variables grouped into one vector X, The loads may cause a change of the including, e.g. its strength, stiffness, condition or state of the structure from an geometry, and loading. Each of these undamaged or intact state to a state of variables are stochastic in the sense that they deterioration, damage, or failure. Structural − owing to natural variability and other malfunction can occur in a number of modes − covering all failure possibilities that can be possible uncertainties may take on random imagined for the structure. Although the degrees of realisation according to some transition from an intact state to a state of probability distribution. For the considered malfunction can indeed be continuous, it is failure mode, the possible realisation of X common to assume that all states with can be separated into two sets:1) the set for respect to a particular mode of malfunction which the structural component will be safe, can be divided into two sets: 1) states that and 2) the set for which it will fail. The have failed, and 2) states that have not failed surface between the safe set and the failure or are safe. The boundary between the safe set in the space of basic variables is denoted states and the failed states is referred to as the limit state surface, and the reliability the set of limit states. The safety or problem is conveniently described by a so- reliability of a structure is concerned with called limit state function g(X), which is defined such that

18 2 – Safety and Reliability Guidelines for Design of Wind Turbines − DNV/Risø

 > 0 for X in safe set P = P[]R − L < 0 = f (r) f (l)drdl  F ∫ R L g(X) = 0 for X on limit state surface R−L<0  ∞  l < 0 for X in failure set = f (r) f (l)drdl  ∫∫ R L −∞∞ − ∞ The limit state function is usually based on = F (l) f (l)dl ∫ R L some mathematical engineering model for −∞ the considered limit state, based on the underlying physics, and expressed in terms where fR and fL are the probability density of the governing load and resistance functions of R and L, respectively, and variables. fR(r) = dFR(r)/dr, where FR is the cumulative distribution function of R. The failure probability is the probability content in the failure set 2.3.3 Structural reliability methods

= []≤ = The reliability index β can be solved in a PF P g(X) 0 ∫ f X (x)dx g(X)≤0 structural reliability analysis by means of a reliability method which can be any amongst where fX(x) is the joint probability density several available methods, including function for X and represents the uncertainty numerical integration, analytical first- and and natural variability in the governing second-order reliability methods, and variables X. The complement PS = 1−PF is simulation methods. Reference is made to referred to as the reliability and is Madsen et al. (1986). Some of these sometimes also denoted the probability of methods are approximate methods, which survival. The reliability may be expressed in will lead to approximate results for the terms of the reliability index, reliability index. Numerical integration is usually only feasible when X consists of −1 β = −Φ (PF), very few stochastic variables such as in the example above. Analytical first- and second- where Φ is the standardised normal order solutions to the failure probability are distribution function. The failure probability, often sufficiently accurate, and they are the reliability, and the reliability index are advantageous to simulation results when all suitable measures of structural safety. failure probabilities are small.

For the simple example that X consists of A useful by-product of a structural reliability two variables, the load L and the resistance analysis by these methods is the so-called R, and the limit state function can be design point x*. This is a point on the limit specified as g(X) = R−L, the failure state surface and is the most likely probability becomes a simple convolution realisation of the stochastic variables X at integral failure.

2.3.4 Code format, characteristic values, and partial safety factors A structural design code specifies design rules that are to be fulfilled during the design of a structure or structural component. The general layout of the design

2 – Safety and Reliability 19 Guidelines for Design of Wind Turbines − DNV/Risø rules in a design code is known as the code The characteristic values are usually taken format. as specific quantiles in the load and resistance distributions, respectively. The The code format most frequently used in characteristic values are often the mean design codes today is a format which is value for dead load, the 98% quantile in the expressed in terms of design values of distribution of the annual maxima for governing load and resistance variables. variable (environmental) load, and the 2% or These design values are defined as 5% quantile for strength. Reference is made characteristic values of the load and to Figure 2-4. The code also specifies values resistance variables, factored by partial to be used for the partial safety factors γf and safety factors. Such a form of code format γm . results from requirements for an easy and yet economical design and is known as a design value format. Design according to a fL, fR design value format is sometimes referred to as load and resistance factor design (LRFD). L R In its simplest form, a code requirement can be expressed as a design rule in terms of an L, R inequality LC RC LD < RD Figure 2-4. Probability density functions and characteristic values for load L and resistance R. in which LD is the design load effect and RD is the design resistance. The design load effect is calculated as The design equation is a special case of the design rule obtained by turning the inequality into an equality, i.e., RD = LD in LD = γf LC the example. where LC is the characteristic load effect and 2.3.5 Code calibration γf is a load factor. Similarly, the design resistance is calculated as Fulfilment of the design rules, as required by a structural design code, is meant to ensure R that a particular prescribed structural safety R = C D γ level is achieved. The purpose of a code m calibration is to determine the set of partial

safety factors to be used with the chosen where R is the characteristic resistance and C code format, such that structural designs γ is a material factor. m according to the code will meet this

prescribed level of safety. Usually, a number of different load effects have to be combined into a resulting load Structural reliability analysis results, effect, and often the largest of several obtained as outlined above, play an different such load combinations is used for important role in codified practice and the load effect in the design rule. An design. Their application to calibration of example of such a load combination is the partial safety factors for use in structural combination of gravitational loads and wind design codes is of particular interest. loads.

20 2 – Safety and Reliability Guidelines for Design of Wind Turbines − DNV/Risø

Structural reliability analysis directly In the case of a prescribed reliability index, capitalises on the variability and which is different from the one that results uncertainties in load and strength and from an actually executed reliability analysis therefore produces a reliability estimate, of a structural component, the geometrical which is a direct measure of the structural quantities of this component must be safety. Because of this, structural reliability adjusted. The adjustment is made in such a analysis forms the rational basis for way that the required reliability index will calibration of partial safety factors, which result from a new reliability analysis of the are used in code checks during conventional modified component. The geometrical deterministic design. With structural quantities, which can be adjusted to achieve reliability method available, it is possible to a specified reliability index, are sometimes determine sets of equivalent partial safety denoted design parameters. It is most factors which, when applied with design practicable to operate on just one such rules in structural design codes, will lead to design parameter when adjusting the design designs with the prescribed reliability. in order to reach the specified reliability index.

As a first step, a target reliability index βT must be selected. The choice of the target A design case is formed as a specific reliability index can be derived from a combination of environmental loading utility-based feasibility study in a decision regime, type of material, and type and shape analysis, or by requiring that the safety level of structure. For a particular design case, as resulting from the design by a structural which can be analysed on the basis of a reliability analysis shall be the same as the structural reliability method, a set of partial one resulting from current deterministic safety factors can thus be determined that design practice. The latter approach is based will lead to a design which exactly meets the on the assumption that established design prescribed level of reliability. Different sets practice is optimal with respect to safety and of partial safety factors may result from economy or, at least, leads to a safety level different design cases. A simple example of acceptable by society. a calibration of partial safety factors is given below for an axially loaded steel truss. A

When the target reliability index βT cannot structural design code usually has a scope be established by calibration against that covers an entire class of design cases, established design practice, or otherwise, formed by combinations among multiple then its value may be taken from Table 2-3, environmental loading regimes, different depending on the type and consequence of structural materials, and several types and failure. Note that the numbers given in Table shapes of structures. The design code will 2-3 are given for a reference period of one usually specify one common set of partial year, i.e. they refer to annual probabilities of safety factors, which is to be applied failure and corresponding reliability indices. regardless of which design case is being Reference is made to NKB (1978). analysed. This practical simplification implies that the prescribed level of reliability will usually not be met in full, but only

2 – Safety and Reliability 21 Guidelines for Design of Wind Turbines − DNV/Risø

Table 2-3. Target annual failure probabilities PFT and corresponding reliability indices βT. Failure consequence Failure type Less serious Serious Very serious LOW SAFETY CLASS NORMAL SAFETY CLASS HIGH SAFETY CLASS (small possibility for (possibilities for (large possibilities for personal injuries and personal injuries, personal injuries, pollution, small fatalities, pollution, fatalities, significant economic and significant pollution, and very consequences, economic large economic negligible risk to life) consequences) consequences) Ductile failure −3 −4 −5 with reserve PF = 10 PF = 10 PF = 10 capacity βT = 3.09 βT = 3.72 βT = 4.26 (redundant structure) Ductile failure −4 −5 −6 with no reserve PF = 10 PF = 10 PF = 10 capacity βT = 3.72 βT = 4.26 βT = 4.75 (significant warning before occurrence of failure in non- redundant structure) Brittle failure −5 −6 −7 (no warning PF = 10 PF = 10 PF = 10 before occurrence βT = 4.26 βT = 4.75 βT = 5.20 of failure in non- redundant structure) approximately, when designs are carried out Hauge et al. (1992), Ronold (1999), and according to the code. Hence, the goal of a Ronold and Christensen (2001). reliability-based code calibration is to determine the particular common set of 2.3.6 Example – axially loaded steel partial safety factors that reduces the scatter tower of the reliabilities, achieved by designs The example given below deals with design according to the code, to a minimum over of an axially loaded steel tower against the scope. This can be accomplished by failure in ultimate loading. The probabilistic means of an optimisation technique, once a modelling required for representation of closeness measure for the achieved load and capacity is presented. A structural reliability has been defined, e.g. expressed reliability analysis of the tower is carried in terms of a penalty function that penalises out, and a simple calibration of partial safety deviations from the prescribed target factors is performed. reliability. For principles and examples of such code optimisation, reference is made to The design of the axially loaded tower is governed by the maximum axial force Q in a

22 2 – Safety and Reliability Guidelines for Design of Wind Turbines − DNV/Risø one-year reference period. The maximum which gives a requirement to the ratio of the axial force Q follows a Gumbel distribution partial safety factors γ1/γ2 = 1.252. There is thus an infinite number of pairs (γ1,γ2) that FQ(q) = exp(−exp(−a(q−b))) will lead to the required reliability. This implies an arbitrariness in selecting the in which a = 0.4275 and b = 48.65 partial safety factor set (γ1,γ2) for the code. correspond to a mean value E[Q] = 50 MN The reliability analysis gives the design and a standard deviation D[Q] = 3 MN. The point values q* = 70.89 MN for the force yield strength of steel σF follows a normal and σF* = 353.2 MPa for the strength. These distribution with the mean value E[σF] = are the most likely values of the governing 400 MPa and the standard deviation D[σF] = variables at failure. A robust choice for the 24 MPa. The cross-sectional area of the partial safety factors (γ1,γ2) can be achieved tower is A. Failure occurs when the axial by designing to the design point values from force Q exceeds the capacity σFA, thus a the reliability analysis. The partial safety natural format of the design rule is σF,DA ≥ factors are therefore selected as qD. The subscript D denotes design value. The limit state function is correspondingly q * σ * γ = = 1.227 and γ = F = 0.980 chosen as 1 2 σ qC F ,C

σ − g = FA Q According to current design practice, a load

factor is used as a factor on the characteristic and the area A is used as the design load to give the design load, and a material parameter. factor is used as a divisor on the

characteristic resistance to give the design Analysis by a first-order reliability method 2 resistance. Hence, these factors become leads to determination of A = 0.2007 m in order to meet a target reliability index β γ = γ = γ = 1 = = 4.2648, which corresponds to an annual f 1 1.227 and m 1.021 , − γ 5 2 failure probability PF = 10 . The characteristic value of the axial force is taken as the 98% quantile in the distribution respectively. of the annual maximum force, qC = q98% = 57.78 MN. The characteristic value of the Note that the example is purely tutorial to yield strength is taken as the 5% quantile in explain a principle. In reality, the capacity may be more uncertain than assumed here, the strength distribution, σF,C = σF,5% = 360.5 e.g. owing to model uncertainty not MPa. One partial safety factor, γ1, is introduced as a factor on the characteristic accounted for. Moreover, a larger degree of variability in the axial force may also be force, and another one, γ2, is introduced as a factor on the characteristic capacity. expected, depending on the source and type Substitution of the expressions for the of loading and the amount of data available. design force and the design capacity in the The resulting partial safety factors may then design equation yields become larger than the ones found here.

γ2σF,CA−γ1qC = γ2⋅360.5⋅0.2007−γ1⋅57.78 = 0

2 – Safety and Reliability 23 Guidelines for Design of Wind Turbines − DNV/Risø

2.3.7 Example – fatigue of FRP blade deviation σε = 0.86. The cumulative damage root in bending is calculated as the Miner’s sum

The example given here deals with design of ∆n(s ) an FRP blade root against fatigue failure D = ∑ i during its design life of 20 years. The i N(si ) probabilistic modelling required for dFS representation of load and resistance is ∞ ntot ds presented. A structural reliability analysis of = ds ∫ ε −m the blade root is carried out, and a simple 0 K exp( )s safety factor calibration is performed. n x = tot Γ(m +1)( 0 ) m K exp(ε) W The design of the blade root is governed by the long-term distribution of the bending Γ range X. In the long term, the where denotes the gamma function. bending moment ranges are assumed to follow an exponential distribution According to Miner’s rule, fatigue failure occurs when the cumulative damage exceeds a threshold of 1.0. A natural format of the = − − x ≤ FX (x) 1 exp( ) design rule is DD 1, where DD is the design x0 damage. The limit state function is chosen as in which x0 = 50 kNm is recognised as a g = 1−D Weibull scale parameter. The total number of bending moment ranges over the design and the section modulus W is used as the 9 life TL = 20 years is ntot = 0.9⋅10 . The design parameter. bending moment ranges X give rise to bending stress ranges S = X/W, where W Analysis by a first-order reliability method denotes the section modulus of the blade leads to determination of W = 0.00209 m3 in root. Hence, the bending stress range order to meet a target reliability index distribution becomes β = 3.29, which corresponds to a target −3 failure probability PF = 0.5⋅10 over the = − − sW design life of 20 years. FS (s) 1 exp( ) x0 For design against fatigue failure it has For a given stress range S, the number of hardly any meaning to choose the 98% bending stress cycles N to failure is quantile of the annual maximum load, or any generally expressed through an S−N curve, other quantile for that matter, as the which on logarithmic form reads characteristic load value. A characteristic load distribution is needed rather than a ln N = ln K − mln S + ε characteristic load value. The long-term stress range distribution, F (s), is chosen as S the characteristic stress range distribution, where the pair (lnK,m) = (114.7,8.0) γ describes the expected behaviour. The zero- and a load factor f = 1.0 on all stress ranges according to this distribution is prescribed. mean term ε represents the natural variability about the expectation and follows a normal distribution with a standard

24 2 – Safety and Reliability Guidelines for Design of Wind Turbines − DNV/Risø

This is based on the assumption that the Note also that with γf = 1.0 prescribed, all long-term stress range distribution is known. levels of uncertainty and variability The validity of this assumption in the associated with a fatigue problem such as context of wind turbines is discussed later. It the present, are accounted for by one single is also assumed that variability in the safety factor, γm. This factor is thus applied individual damage contributions from the as a safety factor on resistance, regardless of individual stress ranges averages out over whether some of the uncertainty is the many contributing stress ranges in the associated with load rather than with long-term distribution. This assumption resistance. This is in accordance with most would not hold if the cumulative damage standards. Note, however, that in the new was dominated by damage contributions Danish standard DS409/DS410, a partial from only one or a very few large stress safety factor, γf, on load is introduced which, ranges, which could be the case for very under certain conditions, is to be taken as a large m values, say m>10. value greater than 1.0. This applies to situations where the loads causing fatigue − The characteristic S N curve is taken as the damage are encumbered with uncertainty or − expected S N curve minus two standard ambiguity, such as if they are traffic loads, γ deviations. A partial safety factor, m, is or if the various quantiles of the long-term applied as a divisor on all stress range values stress distribution over the design life are of the characteristic S−N curve. statistically uncertain. The design damage DD is then obtained as As regards design of wind turbines against n γ x fatigue, the loads causing fatigue damage D = tot Γ(m +1)( m 0 ) m D are dominated by loads generated by the K C W wind. Whereas the distribution of the 10-

minute mean wind speed on a location may in which KC = Kexp(−2σε) reflects the − be well-known, the distribution of the chosen characteristic S N curve. turbulence intensity is usually not well- Substitution of numbers into the design determined, owing to local conditions and equation DD = 1.0 leads to the following influence from the presence of the turbine. requirement to the material factor Nor is the transfer function to stress response in the wind turbine always clear. γ m = 1.149 The distribution of wind-generated loads in a wind turbine can therefore be expected to Discussion be known only with some uncertainty, and a Note that this example is purely tutorial to load factor γf greater than 1.0 would thus be explain a principle. In reality, the resistance required. However, in practice, one would may be more uncertain than assumed here, account for such uncertainty or ambiguity in e.g. when there is a limited amount of the load distribution by choosing a load material data available, such that the distribution “on the safe side”, a estimated values of K and m are uncertain. conservative “envelope load spectrum”, so Moreover, model uncertainty may be to speak. In the presented example, this associated with the application of Miner’s would imply the choice of a conservatively rule. With such uncertainties properly high value of the Weibull scale parameter x0, accounted for, the value of the resulting which could then be used in conjunction material factor, γm, will become larger than with γf = 1.0. the one found here.

2 – Safety and Reliability 25 Guidelines for Design of Wind Turbines − DNV/Risø

2.3.8 Tests and calculations for 2.3.9 Inspection and inspection verification intervals It is important to demonstrate that the The design process is only one element in structural strengths or capacities of the ensuring safe and reliable structures. In various components that constitute the wind fabrication and service, other safety turbine structure are sufficient. This can be elements can be introduced such as quality done by undertaking calculations according control, alignment control, visual inspection, to some theory or calculation method, and it instrumented monitoring, and proof loading. can be supported by carrying out full-scale Each of these items provide information tests of the component in question. Note that about the structure, additional to the a full-scale test of a structural component information present at the design stage, and may give a more accurate estimate of the may hence reduce the overall uncertainty component strength than theoretical associated with the structure. The calculations, because model uncertainty and probabilistic model used in design can then bias, owing to simplifications and be updated and calibrated against reality by limitations associated with the applied including the additional information. calculation method, will be reduced or removed. The additional information obtained during fabrication and in service may be obtained When carrying out a full-scale test of a either directly as information about some of structural component such as a blade, it is the governing variables themselves, e.g. important to acknowledge that the loads strength, or indirectly by observing used in the test are generated and controlled substitute variables, which are functions of in a completely different manner than the the governing variables, e.g. cracks or variable natural loads (such as wind loads) deformations. which the component will experience in reality and which it should be designed for. It is of interest to update the failure For selection of loads to be used in full-scale probability from its value in the design stage tests for verification, it is therefore not to a value which reflects the additional relevant to apply partial safety factors for information gained by inspection. loads that are prescribed for design in Probability updating by inspection is based accordance with particular codes and on the definition of conditional probability. standards. The loads to be used in tests need Let F denote the event of structural failure. to be chosen after thorough consideration of In the design process the probability of the variability and uncertainty in the failure PF = P[F] is solved according to the strength, given the degree of knowledge procedures described above. Let I denote an about the strength or capacity available prior event such as the observation of a governing to the test. variable or the observation of a function of one or more of the governing variables, In this context it should be noted that the obtained by inspection when the structure is effect of proof loading represents an in service. The updated probability of failure increase in confidence in the structural is the probability of failure conditioned on strength or capacity, resulting from prior the inspection event I, successful loading of the component. P[]F ∩ I P[]F | I = P[]I

26 2 – Safety and Reliability Guidelines for Design of Wind Turbines − DNV/Risø

The probability in the numerator can be • : hub, shaft, gear, solved by a reliability analysis of a parallel couplings, brakes, bearings and system, for which solutions are available. generator The probability in the denominator can be • mechanical control systems: pitch solved by a reliability analysis of a structural system, teeter mechanism, yaw system, component as described above, once a hydraulic system and pneumatic system suitable limit state function has been defined, and with due account for The safety of mechanical components will measurement uncertainty and probability of usually be determined by their structural detection, which are two contributing safety as described in previous sections. uncertainty sources of importance in this However, as the components are part of context. mechanical systems, there are several aspects to be considered in addition to the For some limit states, e.g. crack growth and structural safety when the safety of fatigue failure, the failure probability PF mechanical systems is to be evaluated. increases as a function of time. For such limit states, prediction of the failure The structural strength will in many cases probability as a function of time can be used become limited by surface damages due to to predict the time when the failure wear such as fretting corrosion in probability will exceed some critical connections due to micro movements, or threshold, e.g. a maximum acceptable failure gray staining of gear teeth due to poor probability. This predicted point in time is a lubrication conditions. Hence, aspects like natural choice for execution of an and lubrication conditions as well as inspection. The failure probability can then surface treatment are essential for the be updated as outlined above, depending on mechanical safety. the findings from the inspection and including improvements from a possible Mechanical components are often made of repair following the inspection. The time rather brittle high-strength material such as until the failure probability will again case-hardened steel for and induction- exceed the critical threshold and trigger a hardened roller bearing steel. Furthermore, new inspection can be predicted and thus the strength of mechanical components often forms an inspection interval. This can be relies on extremely fine tolerances, e.g. used to establish an inspection plan. correction grinding of gear teeth or mating surfaces in connections. Note in this context that for some limit states such as the fatigue limit state, it may Non-metallic materials such as rubber are actually be a prerequisite for maintaining the often used in mechanical components to required safety level over the design life that achieve damping, or they are used as sealing inspections are carried out at specified in hydraulic components. Ageing properties intervals. and temperature dependence are of importance for such materials.

2.4 Mechanical safety Mechanical components are in many cases There are several mechanical systems in a subjected to internal forces such as pressure wind turbine: in hydraulic systems, pretension in bolts and shrinkage stress in shrink fit connections. In other cases, strength requirements rely on

2 – Safety and Reliability 27 Guidelines for Design of Wind Turbines − DNV/Risø the rotational speed as is the case with an an alternative part with low impedance, and internal gear shaft, which will experience a reduction of current through the movable one complete bending cycle for each components, e.g. by means of electrical revolution. insulation.

Mechanical safety will thus depend on several more or less well-defined 2.5 Labour safety parameters, often in a rather complex The safety of personnel working on a wind combination. It may be difficult to define all turbine or nearby shall be considered when governing phenomena adequately, and designing a wind turbine, not least when assumptions may not always hold in issuing instructions and procedures for practice. In terms of failure probability, it transportation and assembly, operation, may therefore not always be feasible to take maintenance and repair. a probabilistic approach to assess mechanical safety. It shall be possible to operate the control

levers and buttons of the wind turbine with Due to the complexity of mechanical ease and without danger. These levers and systems, the required level of safety needs to buttons should be placed and arranged in rely on experience together with the such a manner that unintentional or consequences of a given failure. Typically, erroneous operation, which can lead to codes and standards used for mechanical dangerous situations, is prevented. The wind design do not define requirements for safety turbine shall in general be designed in such a factors. This also applies to the most manner that dangerous situations do not commonly used standards, which are the occur. If a wind turbine has more than one gear standards ISO 6336 and DIN 3990, and control panel or control unit, it shall only be the bearing dynamic load rating standard possible to operate it from one panel or unit ISO 281. Hence, minimum safety at a time. requirements need to be determined on the basis of the manufacturers’ experience and, The Danish “National Working if available, on requirements from Environment Authority Regulation No. 561” authorities, certification bodies and wind of June 24, 1994 as later amended, cf. the turbine developers. As a general guideline, “National Working Environment Authority the requirement for structural safety shall at Regulation No. 669” of August 7, 1995, the same time constitute the lower limit for regarding design of technical facilities, the requirement for mechanical safety. commonly referred to as “Maskin-

direktivet”, shall be complied with at all Machine components may be vulnerable to times. lightning and may fail as a result. They should therefore be bonded to local ground. Reference is also made to prEN 50308, DEFU (1999) may be consulted for Wind turbines - Labour safety - December requirements for cross sections of 18, 1998. equipotential bonding connections. Due to their small contact areas, rotating and 2.5.1 Transportation, installation and movable components such as roller bearings commissioning may burn if struck by lightning. They can be protected by a protection system consisting Requirements concerning personnel safety of two parts: a diversion of the current via shall be described in the instructions in the

28 2 – Safety and Reliability Guidelines for Design of Wind Turbines − DNV/Risø wind turbine manuals and in the procedures contact with any rotating, moving or for assembly, installation and commission- conducting parts. ing. The light in access routes shall have an 2.5.2 Normal operation intensity of at least 25 lux. This shall also apply when the main switch of the wind During normal operation of the wind turbine is turned off. turbine, the safety of personnel inside and outside the wind turbine shall be considered. Working conditions Normal operation of the wind turbine shall The wind turbine shall be constructed in be possible without accessing the nacelle. such a way that replacing components

subject to service does not entail working Operational procedures and operation of the postures or movements which are hazardous wind turbine shall be described in the user to health or otherwise dangerous. It shall be manual, which is furnished to the turbine possible to block the rotor and yaw system owner or to the person responsible for the of the wind turbine in a safe and simple operation of the wind turbine. It shall appear manner other than by using the ordinary from any instructions how personnel safety brake and yaw system of the turbine. For has been accounted for. pitch-controlled turbines, fixation of the

pitch setting shall be possible. Blocking of 2.5.3 Service, maintenance and repair the rotor shall be done by mechanical The manufacturer or supplier of the wind fixation of the rotor and shall be capable of turbine shall provide instructions and keeping the rotor fixed at all wind speeds procedures, which consider wind speeds and below the defined normal stop wind speed. other external conditions in such a manner Blocking of the yaw and pitch systems shall that service, maintenance and repair work on keep the yaw and pitch systems, the wind turbine can be performed safely. respectively, fixed at all wind speeds below The wind turbine shall be designed with a the defined normal stop wind speed. view to ensuring safe access to and safe re- placement of all components to be serviced. Operation of the blocking mechanisms and of their area of application shall be Access described in the user manual of the wind It shall be made clear by means of locks turbine in order to avoid incorrect use. and/or signs that it can be dangerous to ascend the wind turbine. It shall be It shall be possible to illuminate working prevented that unauthorised persons get areas with a light intensity of at least 50 lux. access to the control panel and the In addition, the lighting must be designed machinery of the wind turbine. Operation of such that glare, stroboscopic influences and the wind turbine and access to its local other disadvantageous lighting conditions control system shall not require access to are avoided. electrical circuits with a higher voltage than 50V. It shall be possible to initiate emergency shutdown close to the working areas in the Wherever screens and shields are used for wind turbine. As a minimum, it shall be protection, it shall be ensured that, during possible to initiate emergency shutdown at normal operation, personnel cannot get in the bottom of the tower, i.e. at the control panel, and in the nacelle.

2 – Safety and Reliability 29 Guidelines for Design of Wind Turbines − DNV/Risø

2.6 Codes and standards Nederlands Normalisatie-institut, The Netherlands, 1999. DS472

“Load and Safety for Wind Turbine This is the Dutch standard for safety-based Structures”, DS472, 1st edition, Dansk design of wind turbine structures. It is valid Ingeniørforening, Copenhagen, Den-mark, for wind turbines with a swept rotor area of 1992. at least 40 m2. To a great extent, it is based

on IEC61400-1, however, since it is to be This is the Danish standard for design of used also for type certification of wind wind turbine structures. This standard, with turbines in the Netherlands, it covers its two annexes A and B, and with the requirements for additional aspects such as standards DS409 and DS410 and relevant type testing. structural codes for materials, forms the

Danish safety basis for structural design of DIBt RICHTLINIEN horizontal axis wind turbines. The standard “Richtlinie. Windkraftanlagen. is valid for the environmental conditions of Einwirkungen und Standsicherheits- Denmark and for turbines with rotor nachweise für Turm und Gründung” (in diameters in excess of 5 m. German). Guidelines for loads on wind

turbine towers and foundations. Deutsche IEC61400-1 Institut für Bautechnik (DIBt), Berlin, “Wind turbine generator systems – Part 1: Germany, 1993. Safety requirements”, 2nd edition,

International Electrotechnical Commission, GL REGULATIONS Geneva, Switzerland, 1999. “Regulation for the Certification of Wind

Energy Conversion Systems,” Vol. IV – This is an international standard that deals Non-Marine Technology, Part 1 – Wind with safety philosophy, quality assurance Energy, in “Germanischer Lloyd Rules and and engineering integrity. Moreover, it Regulations,” Hamburg, Germany, 1993. specifies requirements for the safety of wind turbine generator systems. It covers design, installation, maintenance, and operation REFERENCES under specified environmental conditions. Its purpose is to provide an appropriate level Danske Elværkers Forenings Undersøgelser of protection against damage from all (DEFU), Lightning protection of wind hazards during the design life. The standard turbines, Recommendation 25, Edition 1, is concerned with control and protection 1999. mechanisms, internal electrical systems, mechanical systems, support structures, and Hauge, L.H., R. Løseth, and R. Skjong, electrical interconnection equipment. The “Optimal Code Calibration and Probabilistic standard applies to wind turbines with a Design”, Proceedings, 11th International swept area equal to or greater than 40 m2. Conference on Offshore Mechanics and The standard shall be used together with a Arctic Engineering (OMAE), Calgary, number of other specified IEC standards and Alberta, Canada, Vol. 2, pp. 191-199, 1992. together with ISO2394.

Madsen, H.O., S. Krenk, and N.C. Lind, NVN11400-0 Methods of Structural Safety, Prentice-Hall “Wind turbines – Part 0: Criteria for type Inc., Englewood Cliffs, N.J., 1986. certification – technical criteria,” 1st edition,

30 2 – Safety and Reliability Guidelines for Design of Wind Turbines − DNV/Risø

Nordic Committee on Building Regulations (NKB), Recommendations for Loading and Safety Regulations for Structural Design, NKB Report No. 36, Copenhagen, Denmark, 1978.

Ronold, K.O., “Reliability-Based Optimization of Design Code for Tension Piles,” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 125, No. 8, August 1999.

Ronold, K.O., and C.J. Christensen, “Optimization of a Design Code for Wind- Turbine Rotor Blades in Fatigue,” accepted for publication in Engineering Structures, Elsevier, 2001.

2 – Safety and Reliability 31 Guidelines for Design of Wind Turbines − DNV/Risø

3. External Conditions logarithmic variation with height. AH is the scale parameter at a reference height H. A common choice for the reference height is 3.1 Wind conditions H = 10 m. However, in the context of wind The wind climate that governs the loading of turbines, the hub height is a natural choice a wind turbine is usually represented by the for H. The expression for A is based on a logarithmic wind speed profile above the 10-minute mean wind speed U10 at the site in conjunction with the standard deviation ground

σU of the wind speed. Over a 10-minute u * z period, stationary wind climate conditions u(z) = ln σ κ are assumed to prevail, i.e. U10 and U are z 0 assumed to remain constant during this short period of time. Only when special where u* is the frictional velocity, κ = 0.4 is conditions are present, such as tornadoes von Karman’s constant, and neutral and cyclones, representation of the wind atmospheric conditions are assumed. The σ 1/2 climate in terms of U10 and U will be frictional velocity is defined as u* = (τ/ρ) , insufficient. in which τ is the surface shear stress, and ρ is the air density. 3.1.1 10-minute mean wind speed The 10-minute mean wind speed will vary For engineering calculations it may from one 10-minute period to the next. This sometimes prove useful to apply the variability is a natural variability and can be following empirical approximation for the represented in terms of a probability scale parameter A distribution function. In the long run, the α distribution of the 10-minute mean wind  z  A = A speed can for most sites be taken as a 10    H  Weibull distribution

α u where the exponent depends on the terrain F (u) =1 − exp(−( ) k ) U10 roughness. Note that if the logarithmic and A exponential expressions for A given above are combined, a height-dependent expres- in which the shape parameter k and the scale sion for the exponent α results parameter A are site and height-dependent coefficients. The scale parameter A at height  z  z can be calculated as follows  ln  z ln 0  z  H  ln  ln  z  z  = 0  0  A AH α = H  z  ln ln z   0  H  where z0 is the terrain roughness parameter Note also that the interpretation of the which is defined as the extrapolated height limiting value α = 1/ln(z/z0) is similar to that at which the mean wind speed becomes of a turbulence intensity as z approaches the zero, if the vertical wind profile has a reference height H, cf. the definitions given

32 3 – External Conditions Guidelines for Design of Wind Turbines − DNV/Risø in Sections 3.1.2 and 3.1.3. As an alternative τ α α u = 0 to the quoted expression for , values for * ρ tabulated in Table 3-1 may be used.

A homogeneous terrain is characterised by a is the frictional velocity expressed as a function of the shear stress τ at the sea constant z0 over the terrain. Typical values 0 ρ for z0 are given in Table 3-1 for various surface and the density of the air. Ac = types of terrain. For offshore locations, 0.011 is recommended for open sea. As an where the terrain consists of the sea surface, approximation, Charnock’s formula can also the roughness parameter is not constant, but be applied to near-coastal locations provided depends on: that Ac = 0.034 is used. Expressions for Ac, • wind speed which include the dependency on the wave • upstream distance to land velocity and the available water fetch, are • water depth available in the literature, see Astrup et al. • wave field (1999). Based on a logarithmic wind speed profile, Charnock’s formula leads to the Table 3-1. Wind speed parameters for various types of following expression for the roughness terrain. parameter for a water surface Terrain type Roughness Expon 2 parameter ent α A  κU  = c 10 z (m) z 0   0 g  ln(z z )  Plane ice 0.00001  0  Open sea without 0.0001 waves from which z0 can be determined implicitly, Open sea with 0.0001- 0.12 and from which the dependency on the wind waves 0.003 speed in terms of U10 is evident. For Coastal areas with 0.001 offshore locations, this implies that onshore wind determination of z0 and of the distribution of Open country 0.01 U10, respectively, involves an iterative pro- κ without significant cedure. = 0.4 is von Karman’s constant. build-ings and vegetation The basic wind speed vB, used in the Danish Cultivated land with 0.05 0.16 design code, is the 50-year return value of scattered buildings the 10-minute mean wind speed at 10 m Forests and suburbs 0.3 0.30 height above land with terrain roughness City centres 1-10 0.40 z0 = 0.05. The 10-minute mean wind speed with a 50-year recurrence period at other

heights and with another terrain roughness A widely used expression for the roughness can be found as parameter of the open, deep sea far from land is given by Charnock’s formula = z v10 min,50 yr v B k t ln 2 z u 0 = * z 0 Ac g 0.078 in which kt = 0.19(z0/0.05) in which g is the acceleration of gravity, and

3 – External Conditions 33 Guidelines for Design of Wind Turbines − DNV/Risø

3.1.2 Standard deviation of wind be the case for example if a house is located speed nearby. Houses and other “disturbing” elements will, in general, lead to more For a given value of U , the standard 10 turbulence, i.e. larger values of E[σ ] and deviation σ of the wind speed exhibits a U U D[σ ], than will normally be found in natural variability from one 10-minute U smoother terrain. Figure 3-1 and Figure 3-2 period to another. This variability of the [σ ] wind speed is known as the turbulence, and give examples of the variation of E U and D[σU] with U10 for onshore and offshore σU is therefore often referred to as the standard deviation of the turbulence locations, respectively. The difference components. Measurements from several between the two Figures is mainly attributable to the different shape of the locations show that σ conditioned by U U 10 mean curve. This reflects the effect of the can often be well-represented by a increasing roughness length for increasing lognormal distribution U on the offshore location. 10

lnσ − b σ = Φ 0 Fσ ( ) ( ) U |U10 b1 3

2,5 mean value in which Φ() denotes the standard Gaussian st. dev. cumulative distribution function. The 2 coefficients b0 and b1 are site-dependent ] (m/sec) coefficients conditioned by U10. See Ronold U 1,5 σ and Larsen (1999). [ E 1 ] The coefficient b0 can be interpreted as the U σ σ [ 0,5 mean value of ln U, and b1 can be D interpreted as the standard deviation of lnσU. The following relationships can be used to 0 0 5 10 15 20 calculate the mean value E[σU] and the standard deviation D[σ ] of σ from the U 10 (m/sec) U U values of b0 and b1 Figure 3-1. Mean value and standard deviation of σU as functions of U10 – onshore location.

[]σ = + 1 2 E U exp(b0 b1 ) In some cases, a lognormal distribution for 2 σ U conditioned by U10 will underestimate the higher values of σ . A Frechet []σ = []σ 2 − U D U E U exp(b1 ) 1 distribution may form an attractive

distribution model for σU in such cases, These quantities will, in addition to their hence dependency on U10, also depend on local conditions, first of all the terrain roughness σ 0 k Fσ (σ ) = exp(−( ) ) z0, which is also known as the roughness U |U10 σ length. When different terrain roughness prevails in different directions, i.e. the terrain is not homogeneous, E[σU] and D[σU] may vary with the direction. This will

34 3 – External Conditions Guidelines for Design of Wind Turbines − DNV/Risø

remove data, which belong to 10-minute 2,5 series for which the stationarity assumption for U10 is not fulfilled. If this is not done, 2 mean value such data may confuse the determination of st. dev. an appropriate distribution model for σU 1,5 conditioned by U10. ] (m/sec) U σ [

E 1 Based on boundary-layer theory, the following expression for the mean value of ] U

σ σ [ 0,5 the standard deviation U, conditioned by D U10, can be derived 0 0 5 10 15 20 25 []σ = κ 1 E U U 10 Ax U (m/sec) z 10 ln z 0 σ Figure 3-2. Mean value and standard deviation of U as functions of U10 – offshore location. κ for homogeneous terrain, in which = 0.4 is von Karman’s constant, z is the height above The distribution parameter k can be solved implicitly from terrain, z0 is the terrain roughness, also known as the roughness length, and A is a x constant which depends on z . Measure- 2 0 Γ(1− ) ments from a number of locations with D[]σ ( U ) 2 = k −1 uniform and flat terrain indicate an average E[]σ 1 U Γ 2 (1− ) value of Ax equal to 2.4, see Panofsky and k Dutton (1984). Dyrbye and Hansen (1997) suggest Ax = 2.5 for z0 = 0.05m and Ax = 1.8 and the distribution parameter σ0 then for z0 = 0.3m. A conservative fixed choice results in for σU is desirable for design purposes, i.e. a characteristic value, and DS472 suggests E[]σ σ = U 0 1 Γ − 1 σ = U (1 ) U ,c 10 z k ln z0 where Γ denotes the gamma function. Note that this value, although higher than Caution must be exercised when fitting a the mean value of σU, may not always be distribution model to data. Normally, the sufficiently conservative for design pur- lognormal distribution provides a good fit to poses. data, but utilisation of a normal distribution, a Weibull distribution or a Frechet The IEC61400-1 standard requires distribution is also seen. The choice of utilisation of a characteristic standard distribution model may depend on the deviation for the wind speed application, i.e. whether a good fit to data is required for the entire distribution, only in U + aU σ = I 10,15 10 the body, or in the upper tail of the U ,c 15 a +1 distribution. It is important to identify and

3 – External Conditions 35 Guidelines for Design of Wind Turbines − DNV/Risø

in which U10,15 = 15 m/s is a reference wind N speed, I is the characteristic value of the = − ⋅ m + m ⋅ T,15 IT ,total m (1 N pw )IT pw ∑ IT ,w si turbulence intensity at 15 m/s, and a is a i=1 slope parameter. IT,15 = 0.18 and a = 2 are to be used in the category for higher turbulence pw = 0.06 characteristics, while IT,15 = 0.16 and a = 3 are to be used in the category for lower si = xi / D turbulence characteristics. The expression for the characteristic value σ is based on a U,c 1 definition of the characteristic value as the I = + I 2 T ,w + ⋅ ⋅ 2 T mean value of σU plus one standard (1.5 0.3 si v) deviation of σU. N number of closest neighbouring wind 3.1.3 Turbulence intensity turbines m Wöhler curve exponent corresponding The turbulence intensity I is defined as the T to the material of the considered ratio between the standard deviation σ of U structural component the wind speed, and the 10-minute mean σ v free flow mean wind speed at hub wind speed U10, i.e. IT = U/U10. height p probability of wake condition Note that the presence of a wind turbine will w xi distance to the i'th wind turbine influence the wind flow locally, and that the D rotor diameter turbulence in the wake behind the turbine I free flow turbulence intensity will be different from that in front of the T IT,w maximum turbulence intensity at hub turbine. This phenomenon of a wind turbine height in the centre of the wake influenced turbulence is known as a wake effect. Typically, the presence of the wind The number of closest neighbouring wind turbine will lead to increased turbulence turbines N can be chosen as follows: intensity in the wake. Wake effects need to be considered for wind turbines installed 2 wind turbines: N = 1 behind other turbines with a distance of less 1 row: N = 2 than 20 rotor diameters. This is of particular 2 rows: N = 5 interest wherever wind farms with many in a farm with more than 2 rows: N = 8 turbines in several rows are to be installed.

The following method, Frandsen 2001, can be used to take wake effects into account. By this method, the free flow turbulence intensity IT is modified by the wake turbulence intensity IT,w to give the total turbulence intensity IT,total. In the evaluation of the wake effect, a uniform distribution of the wind direction is assumed. The formulas can be adjusted if the distribution of wind direction is not uniform. Reference is made Figure 3-3. Example of determination of neighbouring to Frandsen, 2001. wind turbines.

36 3 – External Conditions Guidelines for Design of Wind Turbines − DNV/Risø

If the wind farm consists of more than five σU can be expected. Beware that rows with more than five turbines in each calculations for changes in the direction of row, or if the distance between the turbines the wind in complex terrain may come out in the rows that are located perpendicular to very wrongly, if values for σUy and σUz, the predominant wind direction is less than which are valid for homogeneous terrain, are 3D, the increase in mean turbulence applied. intensity shall be taken into account. This is done by substituting the free flow turbulence Very often, the wind climate at a particular * IT with IT : location cannot be documented by site- specific measurements. In such situations, I * = ½ I 2 + I 2 + I the distribution of U10 can usually be well- T w T T represented, for example on the basis of wind speed measurements from a nearby 0.36 σ I = location. However, the distribution of U w + 1 0.08 sr s f v will usually be harder to obtain as it is highly dependent on the local roughness conditions. Thus, it cannot be inferred sr = xr / D automatically from known wind speed conditions at nearby locations. On a location sf = xf / D where wind speed measurements are not available, determination of the distribution where x is the distance within a row, and x r f of the standard deviation σ of the wind is the distance between rows. U speed is often encumbered with ambiguity.

It is thus common practice to account for 3.1.4 Lateral and vertical turbulence this ambiguity by using conservatively high

The 10-minute mean wind speed, the values for σU for design purposes, viz. the standard deviation of the wind speed, and characteristic values for σU given in DS472 the turbulence intensity presented above all and IEC61400-1 and referenced above. refer to the wind speed in the constant direction of the mean wind during the 3.1.5 Stochastic turbulence models considered 10-minute period of stationary conditions. During this period, in addition to Wind in one direction is considered, i.e. in the turbulence in the direction of the mean the direction of the 10-minute mean wind wind, there will be turbulence also laterally speed. The wind speed process U(t) within a σ and vertically. The mean lateral wind speed 10-minute period of constant U10 and U is will be zero, while the lateral standard considered and can be assumed to be deviation of the wind speed can be taken as stationary. The spectral density of the wind σ = 0.75σ according to Dyrbye and speed process expresses how the energy of Uy U the wind turbulence is distributed between Hansen (1997) and as σ = 0.80σ Uy U various frequencies. Several models for the according to Panofsky and Dutton (1984). spectral density exist. A commonly used The mean vertical wind speed will be zero, model for the spectral density is the Harris while the vertical standard deviation of the spectrum wind speed can be taken as σUz = 0.5σU. These values all refer to homogeneous terrain. For complex terrain, the wind speed field will be much more isotropic, and values for σUy and σUz very near the value of

3 – External Conditions 37 Guidelines for Design of Wind Turbines − DNV/Risø

L  0.7z for z < 30 m 3.66 λ =  2 U 10 21 m for z > 30 m S ( f ) = σ  U U 3 2πfL (1+ ( ) 2 ) 5 / 6 2 U 10 The turbulence scale parameter is by definition the wavelength where the non- in which f denotes the frequency, and L is a dimensional, longitudinal power spectral characteristic length, which relates to the density is equal to 0.05. integral length scale Lu by L = 1.09Lu. A calibration to full scale data indicates values 10 for L in the range 66-440 m with L ≅ 200 m used to match the high frequency portion of 1 the spectrum. Based on experience, the m C, m C Harris spectrum is not recommended for use 0,1 in the low frequency range, i.e. for f < 0.01 Hz. 0,01 0,001 0,01 0,1 1 10 Another frequently used model for the z0 (m) power spectral density is the Kaimal spectrum Figure 3-4. Coefficients C and m for the integral length L scale of the Kaimal spectrum. 6.8 u U S ( f ) = σ 2 10 For design purposes it is common to relate U U fL calculations to wind conditions at the hub, (1+10.2 u ) 5 / 3 i.e. U and σ refer to the wind speed at the U 10 10 U hub height. Note that the turbulence scale λ in which the integral length scale is given by parameter relates to the integral length scale L through L = 4.76λ. This gives the m following expression for the Kaimal Lu = 100Cz spectrum, which is well-known from where z is the height and C and m depend on IEC61400-1 the roughness length z0 as given in Figure

3-4. This spectrum is used in Eurocode 1. Lk 4 U S ( f ) = σ 2 10 Towards the high frequency end of the U U fL inertial subrange, IEC61400-1 requires that (1+ 6 k ) 5 / 3 U the power spectral density used for design 10 shall approach the form with Lk = 8.1λ −2 / 3  λ  S ( f ) = 0.05⋅σ 2   f −5 / 3 For calculation of spectral densities for U U ,c U   10  lateral and vertical wind speeds, the above

formulas can be used with σUy and σUz, λ in which the turbulence scale parameter respectively, substituted for σU, and with depends on the height z above the terrain λy = 0.3λ and λz = 0.1λ, respectively, substituted for λ.

38 3 – External Conditions Guidelines for Design of Wind Turbines − DNV/Risø

Note that there is some arbitrariness in the a homogeneous terrain. However, for models for power spectral density. Each turbulence in a complex terrain it is not model implies idealization and uncommon to see a skewness of −0.1, which simplification and is usually calibrated to implies that the Gaussian assumption has not provide a good fit to data within a limited been fulfilled. frequency range. At low frequencies, in particular, the models show significant Note that although the short-term wind differences. In Figure 3-5 three models for speed process will be Gaussian for power spectral densities are plotted on homogeneous terrain, it will usually not be a dimensionless scales for comparison narrow-banded Gaussian process. This comes about as a result of the spectral

Harris density and is of importance for prediction 10 of extreme wind speed values. Such extreme Kaimal (Eurocode) values and their probability distributions can IEC limit be expressed in terms of spectral moments. 1 Reference is made to textbooks on stochastic process theory. 2

U σ

/ 0,1 u

fS At any point in time there will be a variability in the wind speed from one point to another. The closer together the two 0,01 points are, the higher is the correlation between their respective wind speeds. The wind speed will form a random field in 0,001 0,01 0,1 1 10 100 space. A commonly used model for the fLu/U10 autocorrelation function of the wind speed field can be derived from the exponential Figure 3-5. Comparison between the Harris, Kaimal and IEC power spectral densities. Davenport coherence spectrum

r Spectral moments are useful for Coh(r, f ) = exp(−cf ) representation of the wind speed process u U(t). The jth spectral moment is defined by where r is the distance between the two points, u is the average wind speed over the ∞ distance r, f is a frequency, and c is the non- m = ω j S (ω)dω dimensional decay constant, which is j ∫ U 0 referred to as the coherence decrement, and which reflects the correlation length of the In the short term, such as within a 10-minute wind speed field. The auto-correlation period, the wind speed process U(t) can function can be found as usually be represented as a Gaussian process, conditioned by a particular 10- 1 ∞ ρ(r) = Coh(r, f )S ( f )df minute mean wind speed U10 and a given σ 2 ∫ U U 0 standard deviation σU. The arbitrary wind speed U at a considered point in time will then follow a normal distribution with the in which SU(f) is the power spectral density of the wind speed. The coherence model can mean value U10 and the standard deviation σ . This is usually the case for turbulence in be refined to account for different U correlation lengths, horizontally and

3 – External Conditions 39 Guidelines for Design of Wind Turbines − DNV/Risø vertically. Note that it is a shortcoming of are not considered important for small wind the Davenport model that it is not turbines, with rotor diameters in the order of differentiable for r = 0. Note also that, 10 m. Wind shear may be important for owing to separation, the limiting value ρ(0) large and/or flexible rotors. A number of will often take on a value somewhat less failures have been attributed to blade loads than 1.0, whereas the Davenport model induced by wind shear. always leads to ρ(0) = 1.0. The wind profile depends heavily on the Note that the integral length scale Lu, atmospheric stability conditions, see Figure referenced above as a parameter in the 3-6 for an example. Even within the course models for the power spectral density, is of 24 hours, the wind profile will change defined as between day and night, dawn and dusk.

∞ Wind shear profiles can be derived from the L = ρ(r)dr u ∫ logarithmic model presented in Section 0 3.1.1, modified by a stability correction. The stability-corrected logarithmic wind shear 3.1.6 Wind shear profile reads Wind shear is understood as the variation of u * z the wind speed with the height. Its effects u(z) = (ln −ψ ) κ z 0

10000 in which ψ is a stability-dependent function, Strongly stable which is positive for unstable conditions, Stable negative for stable conditions, and zero for Near neutral

Neutral neutral conditions. Unstable conditions

1000 Unstable

Strongly unstable 60 ) neutral 50 stable

100 unstable

40 height above ground (m

30 height (m)

10 20

10

0

1 6 7 8 9 10 11 12 13 0 5 10 15 wind speed (m/s) wind speed (m/s) Figure 3-7. Wind profiles for neutral, stable and Figure 3-6. Wind shear profiles for various stability unstable conditions. conditions on a location with roughness z0 = 0.02 m and geostrophic wind speed G = 12 m/s.

40 3 – External Conditions Guidelines for Design of Wind Turbines − DNV/Risø typically prevail when the surface is heated z L = in unstable air and the vertical mixing is increasing. Stable MO R conditions prevail when the surface is cooled, such as during the night, and vertical − = 1 5R mixing is suppressed. Figure 3-7 shows LMO z in stable air examples of stability-corrected logarithmic R wind shear profiles for various conditions on a particular location. In lieu of data, the Richardson number can be computed from averaged conditions as The stability function ψ depends on the non- follows dimensional stability measure ζ = z/L , MO g where z is the height, and LMO is the Monin- (γ −γ ) d 0.07 Obukhov length. The stability function can R = T (1+ ) be calculated from the expressions ∂ 2 ∂ 2 B  u  +  v       ∂z   ∂z  ψ = −4.8ζ for ζ ≥ 0

g acceleration of gravity ψ 2 − −1 ζ = 2ln(1+x)+ln(1+x ) 2tan (x) for < 0 T temperature

γ = −∂T/∂z lapse rate in which x = (1−19.3⋅ζ)1/4. γ ≈ 9.8°C/km dry adiabatic lapse rate d

The Monin-Obukhov length L depends on MO Further, ∂u / ∂z and ∂v / ∂z are the vertical the heat flux and on the frictional velocity u*. Its value reflects the relative influence of gradients of the two horizontal average wind mechanical and thermal forcing on the speed components u and v , and z denotes turbulence. Typical values for the Monin- the vertical height. Finally, the Bowen ratio B of sensible to latent heat flux at the Obukhov length LMO are given in Table 3-2. surface can near the ground be approximated Table 3-2. Monin-Obukhov length. by

Atmospheric conditions LMO(m) − Strongly convective days − c p (T T ) 10 B ≈ 2 1 − − Windy days with some solar 100 LMO (q 2 q1 ) heating Windy days with little sunshine − 150 in which cp is the specific heat, LMO is the No vertical turbulence 0 Monin-Obukhov length, T and T are the Purely mechanical turbulence ∞ 1 2 average temperatures at two levels denoted 1 Nights where temperature >0 and 2, respectively, and q and q are the stratification slightly dampens 1 2 mechanical turbulence generation average specific humidity at the same two Nights where temperature >>0 levels. The specific humidity q is in this stratification severely suppresses context calculated as the fraction of moisture mechanical turbulence generation by mass. Reference is made to Panofsky and Dutton (1984). If data for the Richardson number R are available, the following empirical relation- Topographic features such as hills, ridges ships can be used to obtain the Monin- and escarpments affect the wind speed. Obukhov length Certain layers of the flow will accelerate

3 – External Conditions 41 Guidelines for Design of Wind Turbines − DNV/Risø near such features, and the wind profiles will Though the yaw system of the wind turbine become altered. Theories exist for will hold the rotor in the direction of the calculation of such changed wind profiles, mean wind direction, the short-term see Jensen (1999). An example of effects of fluctuations in the wind direction give rise to a ridge is given in Figure 3-8. fatigue loading. At high wind speeds sudden changes in the wind direction during production can give rise to extreme loads. 100

downwind Wind rose The distribution of the wind direction is of upwind particular interest with respect to installation ridge crest of turbines in wind farms. As can be seen in Section 3.1.3, wind turbines installed behind 10 obstacles, such as for example other

height (m) turbines, cause a considerable increase in turbulence intensity.

The distribution of the wind direction is often represented by a wind rose as the one

1 seen in Figure 3-10. 0 5 10 15 20 wind speed (m/s)

Figure 3-8. Wind profiles observed upwind, at the crest, and at the foot downwind of a two-dimensional ridge.

3.1.7 Wind direction The wind direction and changes in the wind direction are determined by geography, global and local climatic conditions and by the rotation of earth. Locally, the wind direction will vary with the lateral turbulence intensity and for coast near locations, in particular, the wind direction can vary between day and night.

Figure 3-10. Wind rose for Kastrup, Denmark. Lundtang et al. 1989.

The 360° around the site is typically divided into 12 sectors of 30° each. The radius of the outer wedge in each sector represents the relative frequencies of wind from that direction. The middle wedge shows the

Figure 3-9. Example of fluctuations in the wind contribution from each sector to the total direction. mean wind speed, and the inner wedge shows the contribution to the total mean cube of the wind speed. The scale for each

42 3 – External Conditions Guidelines for Design of Wind Turbines − DNV/Risø quantity is normalised, thereby allowing the pass and is usually not covered by maximum to reach the outer circle. The commonly available analysis tools for corresponding frequency in % for each of simulation of turbulence. Figure 3-11 to 12 the three quantities is given in the small box show examples of the most import transient below the wind rose. The inner dotted circle wind events. corresponds to half of the value of the outer circle.

3.1.8 Transient wind conditions It is important to be aware of transient wind conditions which occur when the wind speed or the direction of the wind changes. As these events are rare, usually, there are not many data available. The most important transient wind conditions to consider are listed below: Figure 3-13. Simultaneous change in wind direction and • speed. From www.winddata.com. extreme of wind speed gradient, i.e. extreme of rise time of gust 3.1.9 Extreme winds – gusts • strong wind shear • simultaneous change in wind direction Extreme winds and gusts consist of extremes and wind speed of the wind speed or of extremes of the • extreme changes in wind direction short-term mean of the wind speed, e.g. over a 10-second period. Extreme wind speeds can be treated in a traditional manner as extremes of the wind speed process during stationary 10-minute conditions. An example of a wind speed record containing extreme gust is given in Figure 3-14.

Figure 3-11. Example of extreme rise time for gust. From www.winddata.com.

Figure 3-14. Example of extreme gust. From www.winddata.com.

Figure 3-12. Example of strong wind shear. Notice that Extreme value analysis the wind speed is inversely proportional to the height, Extreme winds are usually given in terms of i.e. so-called negative wind shear. Courtesy . 10-minute mean wind speeds, which occur with some prescribed recurrence period, e.g. These are all wind events, which by nature the 50-year wind speed. The 50-year wind fall outside of what can normally be speed is the 10-minute mean wind speed, represented by stationary wind conditions. which, on average, is exceeded once every Note that the simultaneous change in wind fifty years. Determination of the 50-year direction and wind speed occurs when fronts wind speed requires an extreme value

3 – External Conditions 43 Guidelines for Design of Wind Turbines − DNV/Risø analysis of available wind speed data. It has formula is used to implicitly solve a new proven useful to carry out such an extreme value of u* that corresponds to the desired value analysis on the friction velocity new reference roughness z0. pressures derived from the wind speed data, rather than on the wind speed data When the friction velocities u* have been themselves. For this purpose, the observed determined from the wind speed data as wind speeds u are transformed to friction described above, the corresponding velocity velocities u* by pressure q is calculated from

κu 1 u* = q = ρu *2 ln(z z 0 ) 2

where ρ is the density of air. κ = 0.4 von Karman’s constant

Z height over the terrain The original wind speed data u are now z roughness parameter 0 transformed into a set of velocity pressure

data q. The q data are grouped into n More complex formulas for the subrecords of a specified duration, e.g. one transformation from u to u* are available in year, and the maximum value of q in each of the literature, by which effects of different the n subrecords is extracted. When the terrain roughness in different directions are duration of a subrecord is one year, these n taken into account, viz. “WASP cleaning”. maximum values of q constitute an

empirical distribution of the annual The friction velocities u* derived from the maximum velocity pressure. The annual above transformation of the original data maximum of the velocity pressure is refer to the prevailing local roughness z . It 0 expected to follow a Type 1 extreme value is often desirable to transform the friction distribution, i.e. a Gumbel distribution velocities to data that relate to a reference roughness z = 0.05 m, which is different 0 q − b from the true local roughness parameter. F(q) = exp(− exp(− )) This can be done by geostrophic mapping, a thus utilising the fact that the geostrophic wind speed is constant and equal to which has two distribution parameters a and b. The values of a and b are determined by u * u * fitting to the n observations of the annual G = (ln( ) − A) 2 + B 2 maximum velocity pressure. The shift κ fz 0 parameter b is the mode of the distribution and is interpreted as the value of q, which for every roughness z0. Here, the Coriolis has a recurrence period of one year. The parameter is f = 2×(earth’s rate of rotation value of q, which has a recurrence period of in radians per second)×sin(latitude) ≈ T years, can be found as 1.2⋅10−4 rad/s at latitude 55.5°, and the coefficients A and B take on values A = 1.8 T q = b + a ln and B = 4.5. The procedure is as follows: the T T geostrophic wind speed G is calculated 0 explicitly by the given formula when the true roughness z0 and the corresponding u* are given. For this value of G, the same

44 3 – External Conditions Guidelines for Design of Wind Turbines − DNV/Risø

where T0 = 1 year. This, in particular, can be When the wind speed with a 50-year used to find the 50-year velocity pressure for recurrence period is given, i.e. U10,50-yr, the T = 50 years. wind speed with the recurrence period T years can be found as The corresponding 10-minute mean wind speed with the recurrence period T at height = + ⋅ T z and terrain roughness z can be found as U 10,T U 10,50− yr 0.57 0.11 ln 0 ln(1 p)

1 2q z U = T ln in which p = exp(−nT) is the probability of 10,T κ ρ 0 z 0 no exceedance in T years, and n is the number of exceedances per year. For T = 50 κ = 0.4 Von Karman’s constant years, n = 0.02. Reference is made to 3 ρ0 = 1.225 kg/m density of air DS410.

The above method of extreme value analysis Hurricanes focuses on the maximum wind speed within Saffir-Simpson’s hurricane scale, see Table a specified period of time such as one year, 3-3, groups and ranks hurricanes according and it is useful for estimation of wind speeds to the wind speeds involved and gives of specified recurrence periods. Note, consequences in terms of implied storm however, that other methods of extreme surges and associated resulting damage. value analysis are available, which may also prove useful, the most important of which is the peak-over-threshold method.

Table 3-3. Saffir-Simpson Hurricane Scale. Wind Storm Damage description speed surge (m/s) (m) * 1 33-42 1.0-1.7 Some damage to trees, shrubbery, and unanchored mobile homes 2 43-49 1.8-2.6 Considerable damage to shrubbery and tree foliage; some trees blown down. Damage to poorly constructed signs and roofs on buildings. Major damage to mobile homes. 3 50-58 2.7-3.8 Foliage torn from trees; large trees and poorly constructed signs blown down. Some damage to roofing materials and structures of buildings. Mobile homes destroyed. 4 59-69 3.9-5.6 Shrubs and trees blown down; destruction of all signs and mobile homes. Extensive damage to roofing materials, windows and doors. Complete failure of roofs on many small houses. 5 70+ 5.7 Shrubs and trees blown down. Severe damage to windows and doors. Complete failure of roofs on many houses and industrial buildings. Some complete building failures. Small buildings overturned or blown away. * maximum 1-minute average

3 – External Conditions 45 Guidelines for Design of Wind Turbines − DNV/Risø

Some parts of the world are characterised by intervals of another duration than 10 having other weather phenomena than those minutes, the mean wind speed and standard considered here. One such weather deviation over these other periods need to be phenomenon is cyclones. As this is a transformed to values referring to a duration weather phenomenon totally different from of 10 minutes. The following approximate the storms dealt with above, one cannot just formula applies to transformation of the extrapolate results for such storms. mean wind speed UT in the measurement period T to the 10-minute mean wind speed 3.1.10 Site assessment U10

Before a wind turbine is installed, various U conditions of the specific site have to be U ≈ T 10 T evaluated. It shall be assessed that the 1− 0.047 ln environmental, electrical and soil properties 10 are more benign than those assumed for the design of a turbine. If the site conditions are in which T is to be given in units of minutes. more severe than those assumed, the Reference is made to Gran (1992) and DNV engineering integrity shall be demonstrated. (1998). The environmental conditions include: temperature, icing, humidity, solar radiation, When the measurement period T is less than corrosion conditions and possible 10 minutes, the standard deviation of the earthquakes. wind speed measurements will come out smaller than the sought-after value for σU. The wind conditions shall be assessed from monitoring measurements made on the site, The following approximate formula applies long-term records from a nearby to transformation of the standard deviation meteorological station or from local codes or σU,T obtained from wind speed standards. Where appropriate, the site measurements in the measurement period T conditions shall be correlated with long-term to the 10-minute standard deviation σU data from local meteorological stations. The monitoring period shall be sufficient to σ ≈ σ 2 + 2 T 2 obtain a minimum of six months of reliable U U ,T U 10 (0.047 ln ) data. Where seasonal variations contribute 10 significantly to the wind conditions, the monitoring period shall include these in which T is to be given in units of minutes. effects. The formula is valid for T < 10 minutes. In cases where T > 10 minutes, the formula to be used reads The two variables U10 and σU are essential as parameters in the models available for representation of the wind speed. Estimation 2 2 T σ ≈ σ −U (0.047 ln ) 2 of wind speed measurements constitutes the U U ,T 10 10 most common method for determination of σ these two parameters. Both U10 and U refer As described in Section 3.1.1, the long-term to a 10-minute reference period, U10 being distribution of the 10-minute mean wind the 10-minute mean wind speed and σ U speed U10 can be represented by a Weibull being the standard deviation of the wind distribution speed over the 10 minutes. Note that if wind speed measurements are obtained over

46 3 – External Conditions Guidelines for Design of Wind Turbines − DNV/Risø

u complications in the form of terrain F (u) =1 − exp(−( ) k ) U10 A inhomogenenity, sheltering obstacles, and terrain height differences. The method is For a specific location, the distribution based on theory for attached flow and allows parameters A and k can be estimated from for site-specific prediction of: wind speed measurements. • wind shear and wind profiles • extreme wind speeds The characteristic value of the turbulence • turbulence intensity is determined by adding the measured standard deviation of the When data for orography and roughness turbulence intensity to the measured or variations are given for the site, and when estimated mean value. If several turbines are reference wind data are available from a to be installed in a wind farm, the influence reference site subject to the same overall from the surrounding turbines must be taken weather regime as the prediction site. The into account. mean wind speed can increase considerably on a hill relative to that in a flat terrain. The Insofar as regards data processing from extreme wind speeds on the hill will be climate measurements, reference is made correspondingly higher. The hill will also to European Wind Atlas. affect the turbulence structure. Typically, the turbulence in the direction of the mean Some locations have a topography, which wind will be attenuated, while the vertical leads to different wind regimes in different turbulence will be amplified. Separation will directions, and which thus warrants use of usually occur when the hill slope exceeds about 30°, and the assumption of attached different distribution models for U10 and σU in different directions. A location on the flow will then no longer be valid. coast is a good example of this phenomenon as wind from the ocean will adhere to one In case of very complex, mountainous distribution model and wind from land will terrain, the prediction accuracy of WASP adhere to another. When two such can be assessed by means of the so-called distributions, conditional on the direction of RIX number. The RIX number is a site- specific ruggedness index, defined as the for example U10, are combined to one unconditional long-term distribution, this fractional extent of the terrain which is may well come out as a bimodal steeper than a critical slope. The critical distribution, e.g. represented by parts of two slope is typically chosen as the separation Weibull distributions. slope of 30°. The RIX number can be estimated from measurements on a WASP Engineering topographical map with a sufficiently fine Site-specific loads can be derived from site- representation of height contours. The RIX specific wind conditions, which can be number is a coarse measure of the extent of modelled by WASP engineering. WASP − flow separation, and thereby of the extent to the Wind Atlas Analysis and Application which the terrain violates the requirement of Program − is a method for prediction of WASP, i.e. that the surrounding terrain wind properties in moderately complex should be sufficiently gentle and smooth to terrain with relevance for loads on wind ensure mostly attached flows. The operation turbines and other large structures, envelope of WASP thus corresponds to RIX ≈ www.wasp.dk. For detailed wind turbine 0%. Reference is made to Mortensen and siting, WASP allows for modelling Petersen (1997).

3 – External Conditions 47 Guidelines for Design of Wind Turbines − DNV/Risø

When the prediction site is more rugged is very limited. Insofar as regards wind than the reference site, the wind speeds at turbine structures, DS472 gives a the prediction site will be overpredicted by temperature interval (−10°C, 30°C) for WASP. Similarly, the wind speeds will be operation as an example. However, the underpredicted when the prediction site is temperature interval for normal operation less rugged. The difference in RIX numbers should always be chosen in accordance with between the two sites is a fairly coarse the specified recommendations for the measure of the significance of the problem respective materials which are being used and provides estimates of the magnitude and for the construction of the wind turbine. In sign of the prediction error. As a rule of the this context, the following issues may be thumb, when there is a difference ∆RIX in critical: RIX numbers between the prediction site • components and connections, which and the reference site, the approximate involve two or more materials with magnitude of the relative prediction error in different coefficients of expansion. The the wind speed will be 2∆RIX. For example, level of expansion in relation to glass, if RIXprediction = 20% and RIXreference = 10%, concrete and steel is more or less the then ∆RIX = 20%−10% = 10%, and the same, whereas plastic expands more. overall relative error in the predicted wind • choice of fluids for lubrication and speeds will be about 2×10 = 20%. hydraulic systems. • materials whose mechanical properties Note that WASP engineering can give change when the temperature changes, accurate results outside its operation limits, e.g. rubber for seals, gaskets and provided that the difference in RIX numbers dampers becomes brittle at low between the actual site and the reference site temperatures, and polyester behaves is small and that the topographical data are like glass when the temperature falls adequate and reliable. below the so-called glass transition temperature. It is important to consider the temperature 3.2 Other external conditions interval for normal operation when materials are selected for the construction of the wind 3.2.1 Temperatures turbine and when other conditions of Operational temperatures importance for the safety of the turbine are A temperature interval for normal operation to be evaluated. of the wind turbine is to be chosen. Under normal functional conditions, the wind Extreme temperatures turbine is considered to be in operation For structural parts which can be damaged when the temperature of the air is within this by extreme temperatures, extreme values of interval. temperatures need to be considered. Extreme values of high and low temperatures should Different structural design codes offer be expressed in terms of the most probable different approaches to how temperatures upper or lower values with their corre- can be handled. Insofar as regards steel sponding recurrence periods. structures, a rather general instruction can be found in DS412 according to which one may Table 3-4 gives values of the daily minimum set the lowest temperature of operation at – temperature and the daily maximum 10°C for Danish locations. This is due to the temperature for various recurrence periods fact that the frequency of low temperatures at two Danish locations. These values are

48 3 – External Conditions Guidelines for Design of Wind Turbines − DNV/Risø based on 123 years of temperature expected and considered in the context of observations, reported by Laursen et al. wind turbine design is an event with (1999a). Note that the temperature values extremely low temperature in conjunction tabulated for recurrence periods of 1000 and with power failure, in particular, power 10000 years are achieved by extrapolation failure of some duration. For Swedish and should therefore be used with caution. locations, Statens Planverks Författnings- samling (1980) gives acceptable design For many materials, peak temperatures such values for the daily mean temperature for as a daily maximum or a daily minimum structural design against low temperatures. temperature have too short a duration to Advice is given on how to transform these pose any problems for the materials and values into acceptable one-hour mean their behaviour. The duration is too short for temperatures. the materials to be cooled down or heated up. For such materials it will be more apt to Table 3-5. Monthly mean temperatures at Danish consider extreme values of the one-hour locations. Recur- Kastrup Fanø mean temperature, of the daily mean rence Monthly Monthly Monthly Monthly temperature, or of the monthly mean period mean mean mean mean temperature. (years) tempera- tempera- tempera- tempera- ture (°C) ture (°C) ture (°C) ture (°C) Table 3-4. Daily minimum and maximum low high low high temperatures at Danish locations. 1 −0.4 16.4 0.2 16.0 Recur- Landbohøjskolen Fanø 10 −5.0 18.4 −4.2 18.3 rence Daily Daily Daily Daily 50 −6.0 19.3 −6.0 19.5 period min. max. min. max. 100 −7.7 19.6 −6.7 19.9 (years) tempera- tempera- tempera- tempera- 1000 −9.7 20.6 −8.4 21.1 ture (°C) ture (°C) ture (°C) ture (°C) 10000 −11.2 21.3 −9.8 22.1 1 −13.6 29.3 −13.3 29.2 − − 10 18.7 31.7 18.2 32.0 3.2.2 Density of air 50 −22.3 33.1 −21.5 33.7 100 −23.7 33.7 −23.0 34.3 The density of air is dependent on 1000 −28.8 35.3 −27.7 36.3 temperature and atmospheric pressure. 10000 −33.9 36.8 −32.4 38.0 Depending on which standard is used as reference, the standard density of air is set to Table 3-5 gives low and high values of the monthly mean temperature for various 3 ρIEC 61400-1 = 1.225 kg/m recurrence periods at two Danish locations. This is based on 30 years of temperature 3 ρ = 1.25 kg/m observations, reported by Laursen et al. DS 472

(1999b). Note that the temperature values, The air density averaged over 10 minutes tabulated for recurrence periods of 100, can be determined from the measured 1000 and 10000 years, are achieved by absolute air temperature T (in units of °K) extrapolation and should therefore be used averaged over 10 minutes and from the with caution. measured air pressure B averaged over 10

minutes by Note that for a coastal climate like the climate of Denmark, extreme temperatures such as 35°C and –20°C will usually not ρ = B 10 min occur simultaneously with strong winds. The RT most severe temperature event to be

3 – External Conditions 49 Guidelines for Design of Wind Turbines − DNV/Risø in which R = 287.05 J/kg/°K is the gas 3.2.5 Ice constant. At high temperatures, it is When the turbine is located in an area where recommended also to measure the humidity ice may develop, ice conditions should be and make corrections for it. determined. The density of ice can be set

equal to ρ = 700 kg/m3. For non-rotating High wind speeds usually occur at low parts, an ice formation of 30 mm thickness pressure, for which the density of air takes on all exposed surfaces can be considered on relatively low values. It is important to be for Denmark, the Netherlands, and Northern aware that the density of air attains high Germany. Insofar as concerns the wind values in arctic regions, whereas it attains turbine at stand-still, it is, furthermore, low values at high altitudes in tropical relevant to consider the rotor blades with an regions. ice cover of this thickness. For the rotating

wind turbine, it is relevant to consider a It appears that − depending on temperature situation with all blades covered by ice, and and pressure conditions – the density of air a situation with all blades but one covered will actually follow some probability by ice. distribution. This may be important to consider when fatigue assessments are For wind turbines on offshore locations, sea carried out. ice may develop and expose the foundation

of the turbine to ice loads. Loads from 3.2.3 Humidity laterally moving ice should be based on Humidity is a function of temperature and relevant full scale measurements, model atmospheric pressure. If the wind turbine is experiments, which can be reliably scaled, to be located in humid areas or in areas with or on recognised theoretical methods. When high humidity part of the day and/or part of determining the magnitude and direction of the year, components sensitive to such ice loads, considerations should be given to conditions have to be sufficiently protected. the nature of ice, mechanical properties of A relative humidity of up to 95% should the ice, ice structure contact area, shape of usually be considered for the design. structure, direction of ice movements, etc. The oscillating nature of ice loads due to Increase of corrosion rates due to humidity, build-up and fracture of moving ice should and especially cyclic exposure to humidity, also be considered. should be estimated. Bacterial and fungal growth should be considered for Where relevant, ice loads other than those components sensitive to such activity. caused by laterally moving ice, such as loads due to masses of ice frozen to the structure 3.2.4 Radiation and ultraviolet light and possible impact loads during thaw of the ice, should be taken into account. The intensity of solar radiation should be considered for components, which are A possible increase in the area due to icing sensitive to ultraviolet radiation and/or should be considered when determining temperature. It is common to consider a 2 wind or wave loads on such areas. solar radiation intensity of 1000 W/m for design. Further details about solar radiation 3.2.6 Rain, snow and hail can be found in NASA (1978). Rain storms should be considered in terms of components that may be subject to water

50 3 – External Conditions Guidelines for Design of Wind Turbines − DNV/Risø damage. If no special provisions are made, be such that a sufficient corrosion protection leakage through covers should be system may be established. considered. If there is a possibility for accumulation of snow on the wind turbine, Industrial environments may be rather harsh the increase of weight due to such to wind turbine structures. accumulation should be considered. The density of heavy snow is in the range 100- Offshore wind turbines will be exposed to 150 kg/m3. corrosion owing to their saline and marine environment. Their foundations will have In general, caution should be exhibited with structural parts whose locations in the wave regard to installation of wind turbines in splash zone will be particularly exposed to areas where hail storms are known to corrosion. prevail. When designing wind turbines for areas where hail storms are common, The expected corrosion rate depends on the possible damage due to impact from hail environment. stones should be considered. The nacelle and the coating on the blades may be 3.2.8 Earthquake especially vulnerable to such damage, in The effects of earthquakes should be particular, if the hail stones fall during considered for wind turbines to be located in strong winds, i.e. they hit the turbine at an areas that are considered seismically active angle different from vertical. The velocity based on previous records of earthquake by which a hail stone falls towards the activity. ground is governed by drag and is not influenced by the weight of the hail stone. For areas where detailed information on When the speed of the hail stone is known seismic activity is available, the seismicity together with the wind speed, the angle by of the area may be determined from such which the hail stone will hit a wind turbine information. structure can be determined.

For areas where detailed information about Extreme hail conditions may exist on some seismic activity is not generally available, locations. Such conditions can be critical for the seismicity should preferably be a wind turbine and may force the turbine to determined on the basis of detailed stop. Extreme hail events may in some cases investigations, including a study of the govern the design of the leading edges of the geological history and the seismic events of rotor blades. The density of hail is usually the region. higher than that of snow. NASA (1978) reports a density of hail of about 240 kg/m3. If the area is determined to be seismically

active and the wind turbine is deemed to be 3.2.7 Atmospheric corrosion and affected by a possible earthquake, an abrasion evaluation should be made of the regional Abrasive action by particles transported by and local geology in order to determine the the wind should be considered for exposed location relative to the alignment of faults, surfaces. the epicentral and focal distances, the source mechanism for energy release, and the The corrosive environment should be re- source-to-site attenuation characteristics. presented by generally recognised methods. Local soil conditions need to be taken into The classification of the environment should account to the extent they may affect the

3 – External Conditions 51 Guidelines for Design of Wind Turbines − DNV/Risø

ground motion. The evaluation should • SD, response spectral displacement consider both the design earthquake and the • SV, response spectral velocity maximum credible earthquake. • SA, response spectral acceleration For a lightly damped structure, the following When a wind turbine is to be designed for approximate relationships apply installation on a site which may be subject to earthquakes, the wind turbine has to be 2 SA ≈ ω SD and SV ≈ ωSD designed so as to withstand the earthquake loads. Response spectra in terms of so-called such that it suffices to establish the pseudo response spectra can be used for this acceleration spectrum and to use this to purpose. compute the other two spectra. The three spectra can be plotted together for various γ as shown in the example in Figure 3-15. In this example, the period T = 2π/ω is used as the spectral parameter instead of ω.

It is important to analyse the wind turbine structure for the earthquake-induced accelerations in one vertical and two horizontal directions. It usually suffices to reduce the analysis of two horizontal directions to an analysis in one horizontal direction, due to the symmetry of the dynamic system. The vertical acceleration may lead to buckling in the tower. Since there is not expected to be much dynamics involved with the vertical motion, the tower may be analszed with respect to buckling for the load induced by the maximum vertical acceleration caused by the earthquake. However, normally the only apparent buckling is that associated with the ground motion in the two horizontal directions, and

Figure 3-15. Example of pseudo response spectrum. the buckling analysis for the vertical motion may then not be relevant. For analysis of the Pseudo response spectra for a structure are horizontal motions and accelerations, the defined for displacement, velocity and wind turbine can be represented by a acceleration. For a given damping ratio γ concentrated mass on top of a vertical rod, and an angular frequency ω, the pseudo and the response spectra can be used directly response spectrum S gives the maximum to determine the horizontal loads set up by value of the response in question over the the ground motions. An example of a pseudo duration of the response. This can be response spectrum is shown in Figure 3-15. calculated from the ground acceleration For a typical wind turbine, the concentrated history by means of Duhamel’s integral. The mass can be taken as the mass of the nacelle, following pseudo response spectra are including the rotor mass, plus ¼ of the tower considered: mass.

52 3 – External Conditions Guidelines for Design of Wind Turbines − DNV/Risø

3.2.9 Lightning Statistically, as many as 40-50% of all lightning stroke events for wind turbines Lightning strokes imply discharging of cause damage to the control system, whose current to the earth following a separation of availability is of vital importance for the charge in thunderstorm clouds. Median operation of a wind turbine. values of lightning current, charge transfer, and specific energy produced by a single Ω For detailed information on lightning, stroke are 30 kA, 10 C, and 50 kJ/ , reference is made to DEFU (1999). respectively. Maximum recorded values of these quantities are 300 kA, 400 C, and 20 Ω MJ/ , respectively. REFERENCES

When lightning strokes hit wind turbines, Astrup, P., S.E. Larsen, O. Rathmann, P.H. lightning current has to be conducted Madsen, and J. Højstrup, “WAsP through the wind turbine structure to the Engineering – Wind Flow Modelling over ground, passing through or near to Land and Sea”, in Wind Engineering into the practically all wind turbine components and 21st Century, eds. A.L.G.L. Larose and F.M. exposing them to possible damage. Large Livesey, Balkema, Rotterdam, the wind turbine components, such as blades Netherlands, 1999. and nacelle cover, are often made of composite materials incapable of sustaining DEFU, Lightning protection of wind direct lightning stroke or conducting turbines, Recommendation No. 25, 1st lightning current, and are therefore edition 1, DEFU, 1999. vulnerable to lightning damage. DNV, Environmental Conditions and In areas with a high probability of damage to Environmental Loads, Classification Notes wind turbines due to lightning, a protection No. 30.5, Det Norske Veritas, Høvik, system should be installed. The purpose of a Norway, 1998. lightning protection system is to reduce the damage caused by lightning to a tolerable DS410, Norm for last på konstruktioner, in level. Danish, Copenhagen, Denmark, 1999.

The design of any lightning protection DS412, Norm for stålkonstruktioner, in system should take into account the risk of Danish, 3rd edition, Copenhagen, Denmark, lightning striking and/or damaging the wind 1999. turbine structure. The risk of lightning striking the wind turbine structure is a DS472, Last og sikkerhed for function of the height of the structure, the vindmøllekonstruktioner, in Danish, 1st local topography, and the local level of edition, Copenhagen, Denmark, 1992. lightning activity. The lightning risk is usually higher in low mountainous areas Dyrbye, C., and S.O. Hansen, Wind Loads than in coastal areas. For structures taller on Structures, John Wiley and Sons, than 60 m, side flashes occur, i.e. some of Chichester, England, 1997. the lightning flashes strike the side of the structure rather than the tip. Such side Frandsen, S., Turbulence and flashes are a cause for concern in connection turbulencegenerated fatigue loading in wind with wind turbines as blades struck on the turbine clusters, Report No. Risø-R- sides may be severely damaged.

3 – External Conditions 53 Guidelines for Design of Wind Turbines − DNV/Risø

1188(EN), Risø National Laboratory, Wind Mortensen, N.G., and E.L. Petersen, Energy Department, May 2001. “Influence of topographical input data on the accuracy of wind flow modelling in complex Gran, S., A Course in Ocean Engineering, terrain,” Proceedings, European Wind Elsevier, Amsterdam, the Netherlands, 1992. Energy Conference, Dublin, Ireland, 1997. The Internet version, located at www.dnv.com/ocean/, provides on-line National Aeronautics and Space calculation facilities within the fields of Administration (NASA), Engineering ocean waves, wave loads, fatigue analysis, Handbook on the Atmospheric and statistics. Environmental Guidelines for Use in Wind Turbine Generator Development, NASA IEC61400-1, Wind turbine generator Technical Paper 1359, 1978. systems – Part 1: Safety requirements, International Standard, 2nd edition, 1999. Norwegian Petroleum Directorate, Acts, regulations and provisions for the petroleum Jensen, N.O., “Atmospheric boundary layers activity, Stavanger, Norway, 1994. and turbulence”, in: Wind engineering into the 21st century. Proceedings of Tenth Panofsky, H.A., and J.A. Dutton, Atmo- International Conference on Wind spheric Turbulence, Models and Methods Engineering, Copenhagen, Denmark, 21-24 for Engineering Applications, John Wiley June 1999. Larsen, A.; Larose, G.L.; and Sons, New York, N.Y., 1984. Liversey, F.M. (eds.), A.A. Balkema, Rotterdam/ Brookfield, Vol. 1, pp. 29-42, Statens Planverks Författningssamling, 1999. “Svensk Byggnorm, SBN avd. 2A, Bärande konstruktioner, med kommentarer,” in Laursen, E.V., J. Larsen, K. Rajakumar, J. Swedish, LiberFörlag, Stockholm, Sweden, Cappelen, and T. Schmith, Observed daily 1980. precipitation and temperature from six Danish sites, 1874-1998, Technical Report No. 99-20, Danish Meteorological Institute, Copenhagen, Denmark, 1999a.

Laursen, E.V., R.S. Thomsen, and J. Cappelen, Observed Air Temperature, Humidity, Pressure, Cloud Cover and Weather in Denmark – with Climatological Standard Normals, 1961-90, Technical Report No. 99-5, Danish Meteorological Institute, Copenhagen, Denmark, 1999b.

Lundtang, E., Troen, I. , European Wind Atlas, The Handbook of European Wind Resources, Risø, Denmark, 1989.

Madsen, H.O, S. Krenk, and N.C. Lind, Methods of Structural Safety, Prentice-Hall Inc., Englewood Cliffs, N.J., 1986.

54 3 – External Conditions Guidelines for Design of Wind Turbines − DNV/Risø

4. Loads • maintenance and repair • testing This list of conditions can be used as a tool 4.1 Load cases to identify design situations, which can be As part of the design process, a wind turbine expected to be encountered, and to assess must be analysed for the various loads it will whether listed load cases are relevant, and experience during its design life. A prime whether relevant load cases are missing. purpose in this respect is to verify that the turbine will be able to withstand these loads 4.1.2 Wind events with a sufficient safety margin. This task is The wind conditions can be divided into systematised by analysing the wind turbine normal and extreme conditions. The IEC- for a number of relevant load cases. 61400-1 standard makes a further distinction and defines the following conditions: Load cases can be constructed by combining • normal wind profile, see Section 3.1.1 relevant design situations for the wind • normal turbulence, see Section 3.1.2 turbine with various external conditions. The • extreme coherent gust design situations mainly consist of various • operational conditions of the wind turbine. extreme direction change • Fort the most part, the external conditions extreme operating gust • consist of various wind conditions or wind extreme wind speed, see Section 3.1.9 events. • extreme wind shear, see Section 3.1.9

As guidance, lists of design situations, wind 4.1.3 Design load cases events, and design load cases are provided in The design load cases to be analysed during the following subsections. These lists are the design process of a wind turbine are meant as an aid and are not necessarily constructed by a combination of relevant exhaustive. In the design of a wind turbine, design situations and external conditions. it is important to identify all load cases The following combinations constitute a which are relevant for the wind turbine. minimum number of relevant combinations: Failure mode and effects analysis is a useful • normal operation and normal external tool in assessing which load cases are conditions relevant and which are not, see Chapter 2. • normal operation and extreme external

conditions 4.1.1 Design situations • fault situations and appropriate external The relevant design situations consist of the conditions, which may include extreme most significant conditions that a wind external conditions. Examples of fault turbine is likely to experience. The design situations are generator short circuit or situations can be divided into operational network failure and fault in braking conditions and temporary conditions. The system. operational conditions include: • transportation, installation and • normal operation and power production maintenance situations and appropriate • cut-in, cut-out, idling, and standstill external conditions The temporary conditions include: Table 4-1 lists a number of design load cases • transportation to consider according to DS472. Table 4-2 • installation, assembly presents a similar list extracted from IEC • faults, such as control system fault 61400-1. These lists serve as examples, and

4 – Loads 55 Guidelines for Design of Wind Turbines − DNV/Risø it is the duty of the designer to validate their • wake effects wherever the wind turbine adequacy. When calculating loads it is is to be located behind other turbines, important to keep in mind a few factors e.g. in wind farms which may influence the involved wind • misalignment of wind flow relative to conditions and the magnitude of the loads: the rotor axis, e.g. owing to a yaw error • tower shadow and tower stemming, i.e. the disturbances of the wind flow owing Note that some of the load cases that are to the presence of the tower listed in Table 4-1 and Table 4-2 are load cases which refer to site-specific wind turbines only.

Table 4-1. Design load cases, cf. DS472. Limit state Load Design situation Wind condition Fati- Ulti- Acci- case type gue mate dent Normal Power production Normal x x Start and switch Normal x x Stop and transition to Normal x x idling Standstill and idling Normal x Extra- Power production Extreme stationary wind (x) x ordinary with U10 = v10min,50yr Power production Transient wind (x) x conditions for direction change 0-90° and wind speed change 10-25 m/sec in 30 sec. Transport, assembly Largest allowable U10 to x and erection be specified Manual operation of To be defined according x x wind turbine to relevance Emergency stop 1.3 times cut-out wind x x speed Activation of air brakes 1.3 times cut-out wind x x speed Idling at yaw error 0.5v10min,50yr x x Fault conditions Normal x x Acci- Serious failure Cut-out wind speed (x) x dental x to be included (x) to be included unless it can be established as unnecessary for the current design of the wind turbine

56 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

Table 4-2. Design load cases, cf. IEC 61400-1. Design situation Wind condition Other conditions Type of analysis Power production Normal turbulence Ultimate Normal turbulence Fatigue Extreme coherent gust with Ultimate direction change Normal wind profile External electrical Ultimate fault Extreme operating gust, one- Loss of electrical Ultimate year recurrence period connection Extreme operating gust, Ultimate fifty-year recurrence period Extreme wind shear Ultimate Extreme direction change, Ultimate fifty-year recurrence period Extreme coherent gust Ultimate Power production plus Normal wind profile Control system fault Ultimate occurrence of fault Normal wind profile Protection system Ultimate fault or preceding internal electrical fault Normal turbulence Control or protection Fatigue system fault Start-up Normal wind profile Fatigue Extreme operating gust, one- Ultimate year recurrence period Extreme direction change, Ultimate one-year recurrence period Normal shutdown Normal wind profile Fatigue Extreme operating gust, one- Ultimate year recurrence period Emergency shutdown Normal wind profile Ultimate Parked (standing still Extreme wind speed, fifty- Possible loss of Ultimate or idling) year recurrence period electric power network Normal turbulence Fatigue Parked and fault Extreme wind speed, one- Ultimate conditions year recurrence period Transport, assembly, To be stated by Ultimate maintenance and repair manufacturer

4 – Loads 57 Guidelines for Design of Wind Turbines − DNV/Risø

4.2 Load types mean load, i.e. the mean flapwise bending moment, and in a reduction of the stiffness. The external loads acting on a wind turbine are mainly wind loads. As a wind turbine The load response in a rotor blade is very consists of slender elements such as blades dependent on the damping. The total and tower, inertia loads will be generated in damping is a combination of aerodynamic addition to the gravity loads that act on these damping and structural damping. The elements. Loads due to operation such as aerodynamic damping depends on: centrifugal forces, Coriolis forces and • choice of blade aerodynamic profile in gyroscopic forces must also be considered. conjunction with chosen blade twist • In most cases, the loads on a wind turbine operational condition • can thus be classified as follows: wind speed • • aerodynamic blade loads rotor frequency • • gravity loads on the rotor blades vibration direction of blade cross- • centrifugal forces and Coriolis forces section • due to rotation motion of blade section relative to • gyroscopic loads due to yawing incoming flow The structural damping depends much on • aerodynamic drag forces on tower and the blade material. The aerodynamic load nacelle response is very much a result of the lift and • gravity loads on tower and nacelle drag forces in conjunction with the blade

profile properties and damping, and with Gravity loads on the rotor blades cause effects of the motion of the rotor structure bending moments in the blades in the included. edgewise direction. For a pitch-controlled turbine, gravity loads will also cause The following subsections give a brief bending moments in the flapwise direction. introduction to the most important load Due to the rotation of the blades, the gravity types encountered for a wind turbine with load effects in the blades will be cyclically emphasis on the physics behind them. More varying bending moments. The larger the details about loads and how to predict them rotor diameter is, the greater are the gravity are given in Sections 4.3 through 4.5. load effects. Typically, the bending moment at the blade root will follow a fourth-power 4.2.1 Inertia and gravity loads law in the rotor diameter. Considering that the rotor area follows a quadratic power law The inertia and gravity loads on the rotor are in the rotor diameter, this forms one of the mass dependent loads. The cross-sectional greater challenges in making wind turbines centrifugal force Fc depends on the angular larger. rotor speed, the radial position, and the mass of each blade element. At the blade root, this Centrifugal forces induced by the rotation of force is the blades can be utilised in conjunction with rearwards coning of the blades to n F = m r ω 2 compensate for the effects of some of the c ∑ i i i =1 wind loads and also to provide more stiffness. Forwards coning of the blades in which mi [kg] is the mass of the ith blade would, in contrast, lead to an increase in the element, ω [rad/s] is the angular rotor speed, and ri [m] is the radial position of the ith

58 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø blade element in a discretisation of the blade respectively. into n elements. The gravity force is simply given as In many cases it is possible to neglect gyroscopic effects, because the angular n velocity of the yaw system is usually rather F = m g or F = m g g ∑ i g blade small. However, flexible rotor-bearing i=1 supports can lead to significant gyroscopic g = 9.82 m/s2 acceleration of gravity forces (rotor whirl), and gyroscopic forces m mass of the ith blade element should never be neglected for present MW i turbines. mblade total blade mass

In general, gyroscopic loads on the rotor will Note that the quoted formulas for MK and occur for any flexible rotor support. In MG need to be adjusted for a possible rotor particular, gyroscopic loads on the rotor will tilt. occur whenever the turbine is yawing during operation. This will happen regardless of the 4.2.2 Aerodynamic loads structural flexibility and will lead to a yaw Blades moment MK about the vertical axis and a tilt The true wind flow in the vicinity of the moment MG about a horizontal axis in the wind turbine rotor is rather complex, rotor plane. because the rotor induces velocities. Hence it is common practice to use a simplified For a three-bladed rotor, the net resulting method for calculating rotor loads to be used yaw moment due to the gyroscopic load for a wind turbine design. effects is zero, MK = 0, whereas a non-zero constant tilt moment is produced, MG = The wind velocity conditions at a blade 3M0/2, in which cross-section are illustrated in Figure 4-1.

n = ω ω 2 M 0 2 K ∑ mi ri i=1

ω plane Rotor where is the angular velocity of the rotor, Rotor axis ωK is the angular yaw velocity, and mi is the ith mass located at radius ri in a discretisation of the rotor blade into n discrete masses.

For a two-bladed rotor, the gyroscopic load effects lead to a cyclic yaw moment and a cyclic tilt moment

MK = 2 M0 cos(ωt) sin(ωt) Figure 4-1. Diagram of air velocities at a blade cross- and section.

2 The wind velocity perpendicular to the rotor M = 2 M cos (ωt), G 0 plane is V . This is the inflow wind velocity. 0 When the wind passes through the rotor

4 – Loads 59 Guidelines for Design of Wind Turbines − DNV/Risø plane, this wind speed becomes reduced by 4.2.3 Functional loads an amount aV due to axial interference. The 0 Functional loads on a wind turbine occur rotor rotates with angular rotor speed ω. when the turbine is subject to transient Thus, a blade element at a distance r from ω operational conditions such as braking and the rotor axis will be moving at a speed r yawing, or when generators are connected to in the rotor plane. When the wind passes the grid. Further, functional loads from the through the rotor plane and interacts with yaw system may also be present. The most the moving rotor, a tangential slipstream important functional loads can be ω wind velocity a’ r is introduced. The categorised as follows: resulting relative inflow wind velocity that • brake loads from mechanical and the rotor blade will experience comes out as aerodynamic brakes shown in Figure 4-1 and is denoted W. This • transient loads in the transmission resulting relative wind velocity gives rise to system, e.g. caused by engagement of aerodynamic forces on the blade, viz. a lift the generator force • yawing loads, i.e. loads produced

directly by yawing = 1 ρ 2 • FL CL cW loads caused by pitching the blades or 2 engaging the air brakes, initiated by the control system and a drag force 4.2.4 Other loads = 1 ρ 2 FD CD cW Other loads or load effects to consider are 2 those induced by tower shadow, vortex shedding by the tower, damping, and CL lift coefficients C drag coefficients instabilities like stall-induced blade D vibrations. Blade vibrations might be both ρ density of air flapwise and edgewise. Either mode shape c chord length of the blade (and possibly others) might be excited due

to negative aerodynamic damping. For more details about how to calculate aerodynamic forces, see Sections 4.3 For wind turbine structures installed in through 4.5. water, it is necessary also to consider loads

set up by the marine environment such as Tower and nacelle wave loads, current loads, and ice loads. The aerodynamic drag force, F on the tower d and the nacelle can be calculated on the A general reference is made to Andersen et basis of the projected area perpendicular to al. (1980). the flow

F = 0.5 ρ A V 2 C d 0 D 4.3 Aeroelastic load calculations

CD aerodynamic drag coefficient Load calculations for a wind turbine A projected area perpendicular to the flow structure are usually performed by means of a computer program based on an aeroelastic calculation procedure. The purpose of an aeroelastic wind turbine analysis is to solve the equations of motion for a given arbitrary

60 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø set of forces acting on the structure and for The loads that are derived from an forces generated by the structure itself. aeroelastic model are used for design of the Often, such a code applies a geometrically various components that constitute the wind non-linear finite element approach or a turbine structure. In this context, most modified modal analysis approach. components are subjected to investigations Reference is made to Figure 4-2. concerning both extreme loading and fatigue loading. In any case, it is required that the code must be able to include and simplify the complex It is inevitable that some engineering mechanical structure of the wind turbine as judgment will be involved when preparing well as being able to model arbitrary for aeroelastic load calculations. It is, deterministic and stochastic forces acting on therefore, important that the modelling as far the turbine. The general formulation of the as possible is supported by measured data, differential equations of motion is which may be available from various sources. These include type testing of the M&x& + Cx& + Kx = F rotor blades and prototype testing of an actual turbine, and they are both important M mass matrix to consider when an aeroelastic model is to C damping matrix be validated. K stiffness matrix F force vector acting on the structure and 4.3.1 Model elements typically varying with time Wind field modelling x and the derivatives of x are unknown The wind field contains three wind velocity vectors containing translations and/or components: rotations and their derivative • longitudinal wind velocity

• transversal wind velocity • vertical wind velocity The wind field is usually divided into: • a mean wind field with shear and slope • a fluctuating wind field, i.e. turbulence Wind field simulation is an important part of a structural wind turbine analysis. For wind turbines, spatial variations in the turbulence must be considered, and three-dimensional wind simulation is required. A prime purpose of wind field simulation is to predict time series of the wind speed in a number of points in space, e.g. a number of points across the rotor disc of a wind turbine. Such time series of the wind speed form useful input to structural analysis models for wind turbines.

The parameters used to describe the mean Figure 4-2. Example showing representation of a wind wind field are usually those describing the turbine structure by finite element model (HawC). wind profile and the vertical component of

4 – Loads 61 Guidelines for Design of Wind Turbines − DNV/Risø the wind vector. For flat homogeneous r Coh (r, f ) = exp(−c f ) terrain, the vertical component of the wind i i U vector can usually be taken as zero. index i identifies the component The mean wind field is superimposed by a F frequency fluctuating wind field, also referred to as the R Distance or spatial separation turbulence field, with wind velocity L length scale components in three directions, i.e. the σ Standard deviation of the wind speed longitudinal, transversal and vertical c coherence decay factor turbulence components, respectively. The parameters used to describe the fluctuating The original Veers model is extended from a part of the wind field depend on which one-component longitudinal turbulence model is used for its representation. model to a full-field three-dimensional and

three-component model by using the In order to predict the wind field in a calculation method for the longitudinal number of points in space, the spatial direction in the transversal and vertical coherence of the wind field must be properly directions as well. No cross-correlation accounted for. There are two models between the three components is modelled. available for generating a synthetic wind The parameters used in the Veers model are: field over a rotor disc: • • mean wind speed U the Veers model by Sandia (Veers, 1988) • • standard deviation of wind speed the Mann model by Risø (Mann, 1994) σ σ σ The Veers model uses a circular grid in the components u, v, w • rotor plane, while the Mann model applies a integral length scales Lu, Lv, Lw of the quadratic grid. Both models generate a turbulence components • synthetic set of time series of turbulent wind coherence decay factors cu, cv, cw by an inverse Fourier transformation in which the indices u, v and w refer to the together with a “defactorisation” of the longitudinal, transversal and vertical coherence. components, respectively. Reference is made to Section 3.1.5 for details about the The Veers model is based on a method integral length scales. developed by Shinozuka. It is based on a single point spectral representation of the The Mann model is based on a spectral turbulence and a coherence function. A tensor formulation of the atmospheric Kaimal formulation is chosen as the spectral surface layer turbulence. The model is model developed for homogeneous terrain and the parameters used are: • L mean wind speed U 6.8 i • height above terrain z S ( f ) = σ 2 U • i i fL roughness length z0 (1+10.2 i )5 / 3 The Mann model is capable of representing U the cross-correlation between the three wind

velocity components. and an exponential Davenport coherence model is used Measurements are used to establish the

parameters that are used in the models. The turbulence intensity, defined from the

62 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø standard deviation of the longitudinal wind turbine blades stall and thereby limit the velocity by IT = σu/U, is usually measured power output and the loads. The overall by means of a cup anemometer. This three-dimensional flow on a rotor is a very measurement corresponds to vectorial complex, unsteady flow depending on a summation of the longitudinal and number of variables, which include: transversal wind velocity components. • wind speed Hence, the turbulence intensity calculated on • wind shear this basis usually comes out somewhat • atmospheric turbulence higher than the sought-after turbulence • yaw angle intensity for the longitudinal component • rotational speed alone. However, it is often used to represent • rotor radius the longitudinal intensity when no other • overall layout of the rotor blade estimate is available. • twist

• taper Data for the relationship between σ , σ , and u v • thickness distribution σ are given in Section 3.1. The longitudinal w • airfoil properties length scale L is in general dependent on u • thickness the height and the terrain roughness, see • Section 3.1 for details. The transverse and camber • vertical length scales can be assumed to smoothness of surface • relate to the longitudinal length scale by L = leading edge thickness v • 0.3Lu and Lw = 0.1Lu. Typical values of the roughness insensitivity • longitudinal decay factors ci are in the range blunt/sharp trailing edge 2-27. Sum of pressure forces Both the Veers model and the Mann model can be used to simulate time series of the Sum of friction forces wind speed in a series of points in a plane perpendicular to the mean direction of the wind, e.g. a rotor plane. For details about the models, reference is made to Veers (1988) and Mann (1994, 1998). Figure 4-3. Distributions of aerodynamic forces on an Aerodynamic modelling airfoil, and their resultants. Aerodynamics deals with the motion of air and the forces acting on bodies in motion A brief introduction to aerodynamic relative to the air. Aerodynamic theory modelling is given in the following with a makes it possible to perform quantitative special view to application to rotor blades. predictions of the forces set up by the air The most important concepts are introduced, flow on the rotor. and the so-called blade element momentum theory for calculation of aerodynamic forces On stall-regulated wind turbines, large on a rotor blade is presented. By this regions of separated flow will occur during method, momentum balance is used for operation at high wind speeds. The stall calculation of blade element forces. As with regulation controls the power output from any other method, this method implies a the turbine by exploiting the decrease in the number of simplifications and idealisations, lift force that takes place when the wind which make it practical for use. Other

4 – Loads 63 Guidelines for Design of Wind Turbines − DNV/Risø

methods do exist, but they are time- αstall has been reached. The stall angle αstall consuming and not considered any better. is somewhat arbitrary.

Consider first a two-dimensional flow past a Lift and drag coefficients CL and CD are profile. Two-dimensional flow is confined to defined as a plane. The out-of-plane velocity is thus zero. In order to obtain a two-dimensional F C = L flow, it is necessary to extrude the profile L 1 into a blade of infinite span. For a true ρW 2 c blade, the shape, the twist and the profile 2 change over the span, and the blade starts at a hub and ends in a tip. However, for long and slender blades, the spanwise velocity F component is usually small relative to the = D C D streamwise component, such that 1 2 ρW c aerodynamic solutions for two-dimensional 2 flow have a practical interest for wind- turbine rotor blades. The resultant force F ρ density of air from the flow on the blade comes about as c Chord length of the airfoil the integral of pressure and frictional forces, FL lift forces per unit length see Figure 4-3. The resultant force F is FD drag forces per unit length decomposed into one component FL perpendicular to the direction of the The lift and drag coefficients are functions resulting relative wind velocity W and one of the inflow angle α, as shown in Figure component FD parallel to this direction. FL is 4-5, of the airfoil shape, and of the Reynolds the lift and FD is the drag, see Figure 4-4. number Re = cW/ν, in which ν is the Both the lift FL and the drag FD depend on kinematic viscosity. C increases linearly α L the inflow angle . with α up to the stall angle, α , where the stall profile stalls. For greater values of α, CL reaches a maximum value, followed by a

decrease in CL for further increases in α. For small α, CD is almost constant, but increases rapidly after stall. The stall phenomenon is closely related to the separation of the boundary layer from the upper side of the airfoil. The manner in which the profile stalls is dependent on the geometry of the profile. Thin profiles with a sharp nose, i.e. a Figure 4-4. Definition of lift and drag on airfoil in 2-D flow. high curvature around the leading edge, tend to stall more abruptly than thicker airfoils. c/4 in Figure 4-4 is referred to as the quarter This is a result of differences in the way the chord point. boundary layer separates. Values of the coefficients CL and CD can be looked up in Stall is a nonlinear phenomenon that results tables, calibrated to wind tunnel in a dramatic loss of flow attachment and measurements, see Abbott and von airfoil lift when some limiting inflow angle Doenhoff (1959).

64 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

The lift coefficient is given by

C = A sin 2 (α) + A (cos 2 (α) / sin(α)), L 1 2 15 o ≤ α ≤ 90 o

= A1 B1 / 2 A = (C − C sin(α ) cos(α ))⋅ 2 Ls D max s s α 2 α (sin( s ) / cos ( s ))

Figure 4-5. Lift and drag curve for an airfoil (Bak et al., o 1999a). αs inflow angle at stall onset (usually 15 ) CDs drag coefficient at stall onset An example of the variation of CL and CD CLs lift coefficient at onset of stall with the inflow angle α is given in Figure 4-5. It appears that two-dimensional wind To describe the physics in a mathematical tunnel measurements are used to obtain the way, the wind turbine rotor can be coefficient values in the pre-stall region. considered as a disc, which is able to absorb Computational fluid dynamics with energy from the wind by reduction of the corrections for three-dimensional effects is wind speed. The wind speed and the used to obtain these values in the stall pressure conditions around the disc are region. In the post-stall region, illustrated in Figure 4-6. measurements are typically used to determine the coefficient values. Otherwise, a method by Viterna and Corrigan (1981) can be used for their prediction. This method assumes rotors with zero twist angle, and results by the method therefore need modification when this assumption is not fulfilled. By this method, the maximum drag coefficient at inflow angle α = 90o is

= + ⋅ c D,max 1.11 0.018 AR Speed in which AR denotes the aspect ratio of the airfoil. The aspect ratio AR is defined for a blade as the ratio between the length of the blade and a representative chord. The drag Pressure Pressure coefficient in the post-stall region is given by

C = B sin 2 (α) + B cos(α), 15 o ≤ α ≤ 90 o Figure 4-6. Influence of a wind turbine on wind speed D 1 2 and air pressure.

= B1 C D max The three-dimensional flow will produce a B = (1/ cos(α ))⋅(C − C sin 2 (α )) continuous sheet of tangential vorticity 2 s Ds D,max s behind the trailing edge of the blade, and the

4 – Loads 65 Guidelines for Design of Wind Turbines − DNV/Risø wake behind the rotor will be rotating. The The torque on the ring element is wake is illustrated in Figure 4-7. 2 dQ = 2π r ρuCθ dr

when the tangential wind speed at radius r is zero upstream of the rotor and uw in the wake. By introducing the axial induction

factor a = 1−u/V0 and the tangential induction factor a’ = ½uw/(ωr), where ω denotes the angular velocity of the rotor, the expressions for the thrust and the torque can be rewritten as Figure 4-7. Wake illustrated by means of smoke. = π ρ 2 − dT 4 r V0 a(1 a)dr Calculation procedure for the blade element momentum method and By the blade element momentum method, the flow area swept by the rotor is divided = π 3 ρ ω − into a number of concentric ring elements. dQ 4 r V0 (1 a)a'dr These elements are considered separately under the assumption that there is no radial At this point, it is necessary to make an dependency between them, i.e. what initial choice for a and a’. It is suggested to happens at one element cannot be felt by any assume a = a’ = 0 as an initial guess. other elements. Usually, each ring element is divided into a number of tubes, which are The flow angle φ is the angle between the also assumed to be independent. This rotor plane and the direction of the relative approach allows for nonsymmetric wind velocity Vrel on the rotating blade. The induction. The wind speed is assumed to be flow angle can be calculated from uniformly distributed over each ring element. The forces from the blades on the (1− a)V tanφ = 0 flow through each ring element are assumed (1+ a')ωr constant. This assumption corresponds to assuming that the rotor has an infinite α φ−θ number of blades. This assumption is The local inflow angle is = , where the θ corrected for later. pitch angle is the local pitch of the blade relative to the rotor plane. See Figure 4-8. Consider now the ring element of radius r and thickness dr. The thrust on this element Determine the lift and drag coefficients CL from the disc defined by the rotor is and CD for the blade from adequate tables. α They are functions of and of the blade dT = 2πrρu(V − u )dr thickness relative to the chord length. 0 1 Transform these coefficients to normal and

tangential coefficients CN and CT by V0 wind speed before the rotor u wind speed in the wake behind the rotor 1 C = C cosφ + C sinφ u = ½(V +u ) is the wind speed through the N L D 0 1 rotor plane. CT = CLsinφ − CDcosφ

66 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

The solidity σ is defined as the fraction of 1 a' = the cross-sectional area of the annular  4F sin φ cosφ  element which is covered by the blades. The  −1  σC  solidity depends on the radius r of the  T  annular element and can be found as in which c(r)B σ (r) = π 2 B R − r 2 r F = arccos(exp(− )) π 2 r sin φ in which B denotes the number of blades. where R is the rotor radius. Note that F is known as Prandtl’s tip loss factor. This is a reduction factor, which corrects for the finite length of the blades, implying that it is difficult to fully keep the pressure difference between the top and bottom of the blade profile close to the blade tip. Recalling the assumption of an infinite number of blades sweeping the rotor area, Prandtl’s tip loss factor – by its expression – also implies a correction for the actual finite number of blades of the rotor.

Examine these values of a and a’. If they deviate significantly, i.e. by more than some acceptable tolerance, from the values of a and a’ assumed for the calculations, go back to where the values for a and a’ were initially assumed and use the new values of a and a’ to recalculate the flow angle φ. Repeat the calculation procedure from there on to obtain a new updated set of a and a’. This iterative procedure needs to be repeated until a convergent set of values for a and a’ Figure 4-8. Velocity components. results. Note that simple momentum theory Calculate the influence factors a and a’ by breaks down when a becomes greater than about 0.3. Formulas exist for correction of a when this situation occurs. Whenever a > a , 1 c a = where ac ≈ 0.2, Glauert’s correction can be  4F sin 2 φ   +1 applied, and implies that a is replaced by  σ   C N  = 1 + − a (2 K(1 2ac ) and 2 − − + 2 + 2 − (K(1 2ac ) 2) 4(Ka c 1))

4 – Loads 67 Guidelines for Design of Wind Turbines − DNV/Risø in which theory, a number of definitions relating to the blade profile are useful. Consider a 4F sin 2 φ section of the blade. The elastic axis is K = σC perpendicular to the section and intersects N the section in a point where a normal force (out of the plane of the section) will not give Note that it is of particular importance to rise to bending. The shear centre is the point apply Glauert’s correction in order to where an in-plane force will not rotate the compute the induced velocities correctly for profile in the plane of section. The two in- small wind speeds. plane principal axes are mutually perpendicular and both cross the elastic axis. Once a convergent set of a and a’ has been The principal axes are defined by the determined, it can be used to calculate the phenomenon that whenever a bending local forces on a rotor blade at distance r moment is applied about one of them, the from the axis of rotation, i.e. the force beam will only bend about this axis. normal to the rotor plane and the force Applying a bending moment about any other tangential to the rotor plane. The normal axis will induce bending, also about another force per length unit of the blade is axis than the one corresponding to the applied moment. 2 − 2 1 V0 (1 a) FN = ρ cC 2 sin 2 φ N Moments of stiffness inertia about the various axes defined in Figure 4-9 can be and the tangential force per length unit of calculated by means of standard formulas, the blade is which can be found in structural engineering α textbooks. The angle between the reference axis X’ and the principal axis X 1 V (1− a)ωr(1+ a') F = ρ 0 cC can be calculated as T φ φ T 2 sin cos 1 2[]ED α = arctan X 'Y ' The procedure is repeated for all ring 2 [][]EI − EI elements modelled, i.e. for all radius values Y ' X ' r, and the result thus consists of distributions along the rotor blade of the normal and where tangential forces per unit length. These []ED = EX 'Y ' dA distributions form the basis for calculating X 'Y ' ∫ stresses, forces and moments in any cross- A section along the blade. In particular, rFT can be integrated along the blade to give the is the deviation , and contribution from one blade to the total shaft torque. []EI = EX ' 2 dA and []EI = EY' 2 dA X ' ∫ Y ' ∫ A A Structural modelling Rotor blades are slender such that they from are the bending stiffness properties about the a structural point of view will act like X’ and Y’ references axes, respectively. The beams, and beam theory can be applied. bending stiffness about the principal axes Reference is made to Section 5.1. For can now be computed as analysis of a rotor blade by means of beam

68 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

[][][]= − α EI X EI X ' ED X 'Y ' tan parameters are usually controlled by monitoring their current values and/or their and first or second derivatives, a regulation algorithm can always be set up and coded [][][EI = EI − ED ]tanα for use together with an aeroelastic code for Y Y X 'Y ' load prediction. respectively. For a pitch-regulated turbine with variable speed, a method exists for representation of the control system. The description in the following pertains to the control system of a wind turbine with variable speed.

The mechanical effect of the wind turbine is

1 P = ρAu 3C (β , λ) 2 P

ρ density of air A rotor area u wind speed

Figure 4-9. Section of blade showing principal axes, CP rotor efficiency chord line, tip chord line, and angles to chord line. CP which is a function of the pitch angle β The most important structural stiffness data and of the tip speed ratio λ = ωRR/u. ωR is for a rotor blade have been dealt with here. the angular frequency of the rotor, and R is Note that since present wind turbine blades the rotor radius. are usually relatively stiff in torsion, the torsional stiffness has usually not been The pitch angle β and the rotor speed ωR are considered. Note also that this may change the two parameters which can be used for in relation to future wind turbine designs. the control of the turbine. The control of the turbine is usually based on one of the In structural modelling and analysis, it is following two approaches: important to be aware of the flutter • Optimisation of the power below the phenomenon, which may result from nominal power, i.e. the rotor is kept as coupled torsional and flapping motion. Low close as possible to the maximum value ratios of torsional and flapwise frequencies of CP, which occurs for a particular and high tip speeds indicate rise of flutter, optimal set (βopt, λopt) of β and λ. This is which may be destructive for the rotor blade. achieved by choosing β equal to its Reference is made to Section 4.3.4. optimal value βopt, and keeping λ constant at its optimal value λ . Control system modelling opt However, λ cannot be controlled The control system is to keep the operating directly, since u is hard to measure with parameters of the wind turbine within sufficient accuracy. Therefore, instead, specified ”normal” limits. This, in turn, is to information about which power keep the loads on the wind turbine within production is optimal for a given value certain limits. Because the operating

4 – Loads 69 Guidelines for Design of Wind Turbines − DNV/Risø

of ωR is utilised, and ωR is used as the an aeroelastic code based mainly on parameter to control the turbine and methods related to modal analysis. optimise the power rather than λ. This information can be established from the A number of computer programs for aerodynamic data for the rotor, first of aeroelastic analysis with load and all profile data, and computer software deformation prediction for wind turbines is available for this purpose. exist. Some of these programs are • Limitation of the power by keeping it as commercially available and can be close as possible to the nominal power. purchased, while others are developed by wind turbine manufacturers and are not The aerodynamic driving torque varies available to the public. Most of the programs continuously due to the turbulence of the provide solutions in the time domain. wind. This moment is transferred to electric However, a few programs exist which offer power by the transmission system and is solutions in the frequency domain. used as a primary indicator for the loading on the transmission system. By means of the Regardless of which aeroelastic code is pitch and speed regulation of the turbine, applied to prediction of wind turbine loads, changes in the aerodynamic power are it is essential that it is subject to validation. absorbed as changes in the angular velocity The IEC requires that the load model used to of the rotor instead of inducing changes in predict loads for design verification of wind the torque which is transferred to the turbines be validated for each design load gearbox. This implies that relative to a case. This validation of the load prediction is conventional fixed-speed turbine, a lower to be based on a representative comparison level of gearbox forces can be kept. between measured and predicted loads on a similar wind turbine. This comparison and 4.3.2 Aeroelastic models for load the subsequent calibration shall include both prediction the peak loads and the fatigue loads.

Aeroelasticity is a discipline where mutual 4.3.3 Aerodynamic data assessment interaction between aerodynamic and elastic deflections is investigated. Aeroelastic Choice of aerodynamic coefficients for use models for load prediction are usually based as input to any aerodynamic analysis method on the blade element momentum method for requires careful consideration. In this transformation of the wind flow field to context, it is important to give special loads on the wind turbine structure. attention to which regulation strategy is adopted for the wind turbine, which is Two important and commonly applied subject to design. methods used for discretisation in connection with structural modelling of When the turbine is pitch-controlled, the wind turbines are the finite element method major task is to program as correctly as (FEM) and the modal analysis method (or possible the regulation routine to be used methods strongly related to modal analysis). with the aeroelastic computer code. The Both methods are implemented in computer aerodynamic coefficients are fairly easy to codes for load prediction. The computer obtain when the turbine is pitch-controlled, code HawC is an example of an aeroelastic since the turbine will only operate in the code based on the finite element method. linear part of the lift curve and three- The computer code FLEX4 is an example of dimensional effects are of little importance. However, the initial part of the lift curve in

70 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø the stall region should still be correctly • the lift coefficients are lower at the modelled, because no pitch regulation blade tip in the stall region when 3-D mechanism can be considered so perfect that effects are included the blades will never experience a stalled • the lift coefficients are unchanged at a condition. distance approximately two thirds of the rotor radius from the rotor axis When the turbine is stall-regulated or active- • the lift coefficients are higher on the stall-regulated, an appropriate choice of the inner part of the blade, i.e. closer to the aerodynamic coefficients is more difficult to rotor axis, when 3-D effects are achieve. A first step in deriving included representative values for the aerodynamic • the drag coefficients are unchanged on coefficients consists of identifying the the outer part of the blade profile series used for the rotor blade. When • the drag coefficients are slightly lower the profiles are well-known, it is easy to find on the inner part of the blade for inflow two-dimensional wind tunnel data to support angles up to approximately 20° when 3- the choice of coefficient values. Data for so- D effects are included. For higher called NACA profiles can readily be found inflow angles, the drag coefficients are in Abbott and von Doenhoff (1959). When higher than indicated by 2-D data. the profiles are not well-known, it is recommended to inspect a visualisation of Beware that these guidelines are based on the profile shape and to find a representative calibration of computational results for one and well-known profile series for which particular blade, viz. the LM19.1 blade, and measured aerodynamic two-dimensional cannot necessarily be projected to apply to data exist. other blades without validation.

The next step is to assign values to the aerodynamic coefficients with due It is usually not possible to find consideration to possible three-dimensional aerodynamic data for inflow angle outside ° ° effects. It is recommended to evaluate or the range –20 -20 . As high inflow angles calibrate the aerodynamic coefficients from outside this range do occur on wind turbines, power curve data or thrust curve data. A in particular, during extreme conditions representative thrust curve can easily be formed by extreme yaw errors or extreme derived from the tower bottom bending. wind speeds, it is often necessary to When reliable measured data are available, extrapolate the available aerodynamic data this will provide a good basis for deriving to values for these high angles. A method ”correct” data. for such extrapolation can be found in Eggleston and Stoddard (1987). In most cases, measurements are not Alternatively, the method by Viterna and available. The assignment of values to the Corigan (1981), described in Section 4.3.1, aerodynamic coefficients must be based on a can be used. general impression and on experience. Alternatively, it can be considered to apply Note that proper selection of values for the guidelines for three-dimensional corrections aerodynamic coefficients is a very important of two-dimensional data according to Bak et step in the design analyses of a wind turbine, al. (1999b). These guidelines can be since a reliable prediction of the dynamic summarised as follows: response of the wind turbine is very dependent on correct choices of aero- dynamic coefficients.

4 – Loads 71 Guidelines for Design of Wind Turbines − DNV/Risø

4.3.4 Special considerations the equations of motion whenever a modal formulation is used. The damping model has Structural damping the form In order to achieve a realistic response from an analysis by an aeroelastic code, C = α M + β K specification of the structural damping must be made with caution. The structural damping model is included in the equations C damping matrix of motion in order to assure dissipation of M mass matrix energy from the structural system. Quite K stiffness matrix often, measurements of the structural α, β model constants damping are only obtained for a limited number of mode shapes, if available at all. Reference is made to Bathe (1982). The Rayleigh damping model enables an Mode Freq. Damp. [% log.] accurate fit of two measured damping ratios α β shape [Hz] Mean Std.dev. only. Here, and are determined from 1st flap 1.636 1.782 0.080 two damping ratios that correspond to 2nd flap 4.914 2.021 0.011 unequal frequencies of vibration. 3rd flap 9.734 2.468 0.026 4th flap 16.22 3.227 0.033 A disadvantage of the use of the Rayleigh 1st edge 2.943 3.603 0.011 model is that it is known to overpredict the 2nd edge 10.62 5.571 0.041 damping at high frequencies of vibration. 1st torsion 23.16 5.807 0.062 Table 4-3. Modal damping in logarithmic decrement for In a more general damping model, it is a 19m blade. assumed that p damping ratios have been determined. Based on this, the damping Experience shows that the structural matrix is represented by a Caughey series: damping in terms of the logarithmic decrement is usually of the order of 3% for p−1 k the blades and of 5% for the shaft and the = []−1 C M∑ ak M K tower. An example of logarithmic k =0 decrements found for a 19m rotor blade is shown in Table 4-3. (Baumgart et al, 2000). C damping matrix In order to ensure dissipation of energy, it is M mass matrix required that the part of the damping matrix K stiffness matrix C in the equations of motion that originates P number of damping ratios from structural damping is positive definite ak model constant or at least semi-positive definite

The modelling constants ak are determined xT Cx ≥ 0 from the p simultaneous equations

Positive definiteness of the structural   ξ = 1 a0 + ω + ω 3 + + ω 2 p−3 damping matrices ensures dissipation of i  a1 i a2 i L a p−1 i  2 ω energy at all velocities.  i 

ξ A commonly used model for representation i = 1,p, in which i is the ith damping ratio of damping is the Rayleigh damping model and ωi is the angular frequency for the whose major advantage is a decoupling of vibration mode that ξi was obtained for.

72 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

The disadvantage of using this generalised and the damping coefficient is easily damping model is that the damping matrix C determined from a curve fit of the equation – in the general case – is a full matrix. This = ⋅ −δ ⋅ ⋅ may cause the simulations in an aeroelastic f (x) C exp( f n t) analysis to become rather time-consuming. For p = 2 this model is reduced to a t time Rayleigh damping model. fn excitation frequency δ logarithmic decrement For appreciation of the results presented C calibration constant from the fit herein, recall that the ith damping ratio is defined as An example of such a fit is shown in Figure 4-10. The dynamic coefficients in the model c ξ = must be adjusted until the correct i δ 2 mi k i logarithmic decrement results from this exercise. mi ith mass ci ith damping ki ith stiffness

The corresponding logarithmic decrement is

2πξ δ = i ≈ πξ i 2 i −ξ 2 1 i

The logarithmic decrement is the natural logarithm of the amplitude decay ratio, i.e. Figure 4-10. Damping test of rotor blade in edgewise the natural logarithm of the ratio of the vibration. amplitudes in two consecutive displacement cycles. Upon application of a Rayleigh damping model, it is only possible to fit the correct Verification of the structural damping damping properties at two frequencies. If the When some initial guesses of the damping damping properties are known at more coefficients have been chosen for the frequencies, such as in the example in Table aeroelastic model, it must be verified that 4-3, one is restricted to select some average the model leads to a correct representation representation of the damping. In this of the damping. This is done by fixing all context, it is important to be aware of the degrees of freedom of the turbine, except the overprediction of the damping by the one for which the damping model is to be Rayleigh model at high frequencies, e.g. checked. An external periodic force is when analysing high-frequency impulse applied to the structural component in forces on fibreglass materials. question (e.g. a blade) using the natural frequency of that component as the Stall-induced vibrations of blades excitation frequency. The structure is only Examples exist of wind turbines, which have excited for a few seconds. The subsequent developed significant edgewise vibrations of decay of the induced vibration is measured, the blades, even during relatively moderate wind situations such as those characterised

4 – Loads 73 Guidelines for Design of Wind Turbines − DNV/Risø by 10-minute mean wind speeds of about 15 configuration of the turbine, whereas Figure m/sec. Such edgewise vibrations are 4-12 shows the vibrations for the new undesirable from a structural point of view configuration, with increased shaft stiffness. and need to be avoided. Edgewise vibrations Both figures are extracted from Petersen et occur whenever the negative aerodynamic al. (1998). Note that flapwise vibrations are damping exceeds the structural damping. equally important to consider. The aerodynamic forces supply more energy to the vibrations than the structural damping Recommendations concerning the design of can absorb. An example of such vibrations, new blades and modification of old blades, simulated for a 500 kW turbine, are shown when edgewise vibrations pose a problem, in Figure 4-11 and Figure 4-12. Figure 4-11 are given by Petersen et al. (1998) and are shows the vibrations for the basic reproduced in the following.

Figure 4-11. Edgewise blade root moment for the 500 Figure 4-12. Edgewise blade root moment for the 500 kW turbine in basic configuration. kW turbine with an increased shaft stiffness.

For a new turbine design, where a great deal 3. Use a quasi-steady approach (Petersen of freedom is at hand, the following et al., 1998) to calculate the basic procedure is recommended: aerodynamic damping characteristics 1. Base the airfoil on well-documented for a single blade, considering both the airfoil data. fundamental flapwise and edgewise 2. Choose airfoils, blade planform (chord, mode shapes. If not satisfactory, repeat aerodynamic twist) and blade structural from Step 2. properties (stiffness, mass, mode shapes 4. Extend the quasi-steady analysis and – i.e. amplitude and direction of include verified models for dynamic deformation, structural damping, modal stall on both the airfoil lift and drag, mass, modal stiffness and modal natural still considering a single blade. If not frequency), considering not only the satisfactory repeat from Step 2. rotor performance, but also the 5. When the single blade analysis results aerodynamic damping characteristics. in appropriate aerodynamic damping, Especially, the blade twist should be continue the analysis by use of the full considered, not only in relation to aeroelastic model, including verified power performance, but also in relation models for stall hysteresis on both to mode shape vibration direction. airfoil lift and drag. The structural models must have a sufficiently detailed

74 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

representation of the blade bending 4. Mechanical damping devices may be modes and the rotor whirling modes. used either on the blades or on the 6. Carry out full aeroelastic calculations supporting structure. It should be for the wind speeds of interest. These mentioned that these devices do not wind speeds can, to a large extent, be remove the cause of the vibrations, identified through a single blade which is the supply of energy by the analysis. aerodynamic forces. 7. Investigate the damping characteristics by looking at the responses at the Flutter flapwise and edgewise natural Flutter is an aeroelastic instability that may frequencies of the blade, for instance result in large amplitude vibrations of a based on the power spectral densities of blade, and possibly in its failure. Flutter the blade root bending moments. vibrations consist of a coupling of flapwise 8. Repeat from Step 2 if the results are and torsional blade vibrations, which are unsatisfactory. sustained by the airflow around the blade. There are several undesirable conditions that When problems with flapwise and edgewise must exist for flutter to occur. The two most vibrations have been recorded on a critical conditions occur when the particular turbine, the turbine must be re- frequencies of a flapwise bending mode and designed. In this case, only a limited number the first torsional mode are not sufficiently of possibilities remain: separated, and when the centre of mass for 1. In general, the scheme above should be the blade cross-sections is positioned aft of applied to the existing design. the aerodynamic centre, which is 2. The aerodynamic properties of the approximately at the quarter chord point, blades may be modified by using which is marked in Figure 4-4 at a distance different aerodynamic devices, e.g. stall “c/4” from the leading edge. In the design of strips and/or vortex generators. blades, it is therefore important to ensure a 3. The structural properties of the turbine high torsional stiffness (e.g. first torsional may be changed by adding local masses frequency larger than ten times the first or increasing local stiffness properties. flapwise frequency), and – if applicable – to These changes may alter the relevant ensure that the centre of mass for the blade mode shapes and natural frequencies in cross-sections is located between the leading a favourable manner. edge and the quarter chord point.

Figure 4-13. Airfoil lift data (unmodified). Figure 4-14. Airfoil drag data (unmodified).

4 – Loads 75 Guidelines for Design of Wind Turbines − DNV/Risø

Figure 4-15. Airfoil data with stall strips. Figure 4-16. Airfoil data with vortex generators.

Use of stall strips and vortex generators lift and drag coefficients. It is important to Stall strips can be used on the outer section account for dynamic stall when it occurs, (typically the far one-third) of a rotor blade since lift and drag coefficients are usually as an ‘emergency’ action when problems derived from wind tunnel tests with constant with heavy vibrations of the blades occur. In inflow angle. Various models for dynamic general, stall strips increase the damping on stall can be found in Petersen et al. (1998). local sections of the blade by increasing the drag after the onset of stall.

Vortex generators are often used to increase the lift and thus to improve the aerodynamic properties of the blade. The vortex generators are used on the inner section of the blade and improve the maximum lift of the section. An example of an airfoil with stall strip and vortex generator modifications is shown in Figure 4-13 to 16, (Petersen et al., 1998).

Figure 4-17. CL and CD loops, dynamic stall. Dynamic stall The onset of stall becomes delayed when the An example of dynamic stall measured in a inflow angle is varying periodically, e.g. wind tunnel for β = 2° is given in Figure according to 4-17.

α = α + β ⋅ ω ⋅ (t) 0 sin( t) 4.4 Load analysis and synthesis α 0 mean inflow angle 4.4.1 Fatigue loads β amplitude t time Fatigue loads are cyclic loads, which cause ω angular frequency cumulative damage in the materials of the structural components, and which eventually The phenomenon of delayed onset of stall is lead to structural failure. Fatigue loads are denoted dynamic stall, and it influences the usually loads well below the load level that

76 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø will cause static failure, and many load defined according to the following rules cycles are required before a fatigue failure (Wirsching and Shehata, 1977): will take place. This is commonly referred to 1. A rain flow is started at each peak and as high-cycle fatigue. However, for some trough. materials with particularly high S-N curve 2. When a rain-flow path started at a slopes, such as some epoxy materials, the trough comes to the tip of the roof, the loads of importance for fatigue are close to flow stops if the opposite trough is more those that will cause static failure. For such negative than that at the start of the path materials fatigue becomes an extreme value under consideration (e.g., path [1-8], problem as far as the loads are concerned path [9-10], etc.). A path started at a with only a few load cycles required to peak is stopped by a peak which is more cause fatigue failure. This is commonly positive than that at the start of the rain referred to as low-cycle fatigue. path (e.g. path [2-3], path [4-5], path [6- 7], etc.). The cumulative damage is commonly 3. If the rain flowing down a roof determined using the Palmgren-Miner rule, intercepts flow from a previous path, as explained in Appendix C. This method the present path is stopped (e.g. path [3- requires knowledge of the distribution of the 3a], path [5-5a], etc.). stress ranges, which is dealt with in the 4. A new path is not started until the path following subsections. under consideration is stopped.

Rain-flow counting Half-cycles of trough-originated stress range Cycle counting methods are used to magnitudes Si are projected distances on the establish distributions of stress ranges from stress axis (e.g. [1-8], [3-3a], [5-5a], etc.). It a stress history. Several methods of cycle should be noted that for sufficiently long counting exist, for example: time series, any trough-originated half-cycle • peak counting will be followed by another peak-originated • range counting half-cycle of the same range. This is also the • rain-flow counting case for short stress histories if the stress These three counting methods give the same history starts and ends at the same stress result for a pure sinusoidal stress history and value. for an ideal narrow-banded stress history. Rain-flow counting will be considered here The rain-flow method is not restricted to and is described for stationary processes. high-cycle fatigue but can also be used for For simplicity, the mean level is taken as low-cycle fatigue where strain range is the zero in the description. important parameter. Figure 4-18 shows a simple example. In this sequence, four In the rain-flow counting method, the stress events that resemble constant-amplitude history is first converted into a series of cycling are recognised, 1-6-9, 2-3-3a, 4-5-6, peaks and troughs as shown in Figure 4-18 and 7-8-8a. These events are closed with the peaks evenly numbered. The time hysteresis loops, and each event is axis is oriented vertically with the positive associated with a strain range and a mean direction downward. The time series is then strain. Each closed hysteresis loop can viewed as a sequence of roofs with rain therefore be compared with constant- falling on them. The rain-flow paths are amplitude data in order to calculate the accumulated damage.

4 – Loads 77 Guidelines for Design of Wind Turbines − DNV/Risø

Figure 4-18. Illustration of the rain-flow counting method: (a), (b) application to a stress history, from Wirsching and Shehata (1977); (c) application to a low-cycle strain history, from Madsen et al. (1986).

In the rain-flow method small reversals are three that identifies both slowly varying treated as interruptions of the larger ranges, stress cycles and more rapid stress reversals and the rain-flow method also identifies a on top of these. The peak counting method mean stress for each stress cycle. A will in general assign larger probabilities to comparison between test results and results larger stress ranges, while the range predicted by the three mentioned cycle counting method will assign larger counting methods shows that the rain-flow probabilities to smaller stress ranges. counting method generally gives the best Compared to the rain-flow counting method, results. This method is the only one of the the peak counting will therefore result in

78 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø larger estimates for the accumulated Stress range damage, while the range counting method ) will predict smaller values of the damage. j Analytical results for the stress range distribution obtained through rain-flow (col. counting are very difficult to obtain, and the (row i) nij method is generally used with measured or stress Mean simulated stress histories only. Figure 4-19. Matrix representation of rain-flow counted The rain-flow method identifies a mean stress range distribution. stress for each stress cycle. An attractive representation of the resulting stress range This approach is useful for damage distribution from cycle counting by the rain- predictions for structural materials, such as flow method is therefore to form a matrix blade materials, for which a non-zero mean with one row for each mean stress level and stress is of importance. For such materials, one column for each stress range. Each the dependency on the non-zero mean stress element of this matrix will then contain the will not be adequately accounted for by a number of stress cycles associated with a Miner’s sum model, which is based on only particular stress range and a particular mean one single S-N curve associated with full stress. Each row of the matrix will contain a stress reversals about a zero mean stress. discretised stress range distribution conditioned on a particular mean stress. Stress range distributions When S-N curves are available for various To establish the stress range distributions for ratios R between the compressive stress a structural component in a wind turbine, it amplitude and the tensile stress amplitude, is important to consider the various con- this representation will allow for prediction ditions that the turbine can be in. There are of the partial damage in each element of the various operational conditions, including: matrix by applying the appropriate S-N • production curve for that element. The total fatigue • start at cut-in and start at cut-out damage D can subsequently be determined • stop at cut-in and stop at cut-out by summing up the partial damage over all • idling and standstill elements in the matrix, • yaw misalignment

Start and stop are transient conditions, for n (S ) D = ∑∑ ij j which the stress distributions in the ijN ij (S j ) considered structural component are not easily determined. During production, stationary conditions can be assumed to in which nij denotes the number of stress cycles in the matrix element corresponding prevail in the short term, e.g. during 10- to the jth stress range, S , and the ith mean minute periods. During a 10-minute period, j the wind climate parameters such as the 10- stress, and Nij is the number of stress cycles to failure in this element. Reference is made minute mean wind speed U10 and the to Figure 4-19. turbulence intensity IT at the hub height can be assumed to be constant. During such short periods of stationary conditions, the load response processes that give rise to the stress ranges in the considered structural

4 – Loads 79 Guidelines for Design of Wind Turbines − DNV/Risø component can be taken as stationary logarithmic diagram in Figure 4-20 had been processes. Under stationary conditions, a straight line, the distribution would have stress ranges are often seen to have been an exponential distribution, which is a distributions which are equal to or close to a special case of the Weibull distribution. Weibull distribution,

B FS(s) = 1−exp(−(s/SA) ) 250

Note that when the exponent B equals 1, this 200 distribution turns into an exponential distribution, and when it equals 2 it becomes 150 S a Rayleigh distribution. For representation 100 of stress range distributions, which are not 50 quite Weibull distributions, it will often suf- fice to apply a somewhat distorted Weibull 0 distribution. The simplest distribution model 0246810 among this family of distributions is a three- log10n parameter Weibull distribution, Figure 4-20. Example of compound stress distribution, a so-called load spectrum. B FS(s) = 1−exp(−((s−a)/SA) ) To obtain the distribution of all fatigue loads Other models for moderate distortions of a in the design life, this compound distribution parent Weibull distribution to form a para- has to be supplemented by the stress ranges metric distribution model, which fits data owing to start, stop, standstill, idling, and well, exist (Ronold et al., 1999). yaw misalignment, to the extent that these are considered to contribute to the The coefficients a, B and SA are distribution cumulative fatigue damage. parameters, and they can often be expressed as functions of the wind climate parameters For this purpose, information about the duty U10 and IT. When the short-term stress range cycle of the turbine is essential. The duty distributions, conditional on U10 and IT have cycle is a repetitive period of operation, been established, when the long-term which is characterised by a typical distributions of U10 and IT are known as succession and duration of different modes outlined in Section 3.1, and when the total of operation. The duty cycle can be specified number of stress cycles during the by a sequence of parameters, which define production life of the turbine is assessed, the mode of operation for consecutive 10- then the compound distribution of all stress minute periods for a representative time ranges during this production life can be span. Information about the duty cycle established. This compound distribution can should as a minimum contain the number of itself often be represented by a Weibull starts and stops during the time span, which distribution, and it can be represented as the duty cycle covers, together with the wind shown in Figure 4-20, where n denotes the climate parameters in this time span. number of stress cycles which exceed a stress range S during the production life. Starting the wind turbine is considered to be This distribution forms a significant potentially critical with respect to fatigue. contribution to the loads that cause fatigue One reason for this is that connection of an damage. Note that if the curve in the semi- electric generator to the grid can cause high

80 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

transient loads in the drive train and the neq. This is a constant load range S0, which rotor blades. It is recommended to in neq cycles will lead to the same distinguish between start at the cut-in wind accumulated damage as the true load speed and start slightly below the cut-out spectrum that consists of many different wind speed. load ranges Si and their corresponding cycle numbers ni. When the equivalent number of When stopping the wind turbine, it is cycles neq is chosen or specified, the recommended to distinguish between equivalent load range S0 can be found as stopping slightly below the cut-in wind 1 m speed and stopping at the cut-out wind  n S m  speed, as the latter is associated with rather  ∑ i i  S = i large aerodynamic loads. 0    neq    For standstill and idling below the cut-in wind speed, loads are considered in which m denotes the S-N curve slope of insignificant with respect to fatigue. the material in question. This definition of a damage-equivalent load range is well-known Yaw misalignment can be critical with in fatigue analysis and is often used in wind respect to fatigue. turbine load analysis. An example of application can be found in Stiesdal (1991). Note that stresses in a particular location of Note that the equivalent load concept only the wind turbine structure, considered applies to materials whose S-N curves are critical with respect to fatigue, may be due described by one slope m, i.e. it can-not be to a combination of stresses arising from used for materials with bilinear S-N curves. different sources or load processes. When the stress levels are sufficiently low, elastic Uncertainties material behaviour can be assumed for the Load spectra are usually not known with combination of the stresses before fatigue certainty, but are predicted, e.g. from a damage calculations are carried out. It can limited number of time series of load further – somewhat conservatively – be response, obtained by simulation according assumed that load peaks of two load to some aeroelastic analysis scheme. Time processes occur simultaneously, and that series of 10-minute duration are usually used high-amplitude cycles can be combined with for this purpose, and one estimate of the high-amplitude cycles. Mean values of two equivalent load range can be obtained from load processes should be combined in the each simulated time series. For a given wind most unfavourable manner to produce the climate (U10,IT), the uncertainty in the largest mean stress value. For more details estimated equivalent load range can be on how to combine load spectra, see IEA expressed in terms of the coefficient of (1990). variance (COV) of the estimate. COV can be expected to be proportional to 1/ NT , Equivalent loads Once the load spectrum has been established where N is the number of simulated time with contributions from all operational series for the given wind climate, and T is the duration of the simulated time series, e.g. modes over the design life of the wind turbine, it is often convenient to define a so- 10 minutes. called damage-equivalent load range S to be 0 Consider now lifetime load spectra and used with an equivalent number of cycles corresponding equivalent life time load

4 – Loads 81 Guidelines for Design of Wind Turbines − DNV/Risø ranges. Time series of load response are Table 4-4. Quantiles of the Student’s t distribution simulated for various wind climates. t α 1− ,n−1 Compound lifetime load spectra are 2 established on this basis by appropriate Degrees of Confidence 1−α weighting according to the long-term freedom, n−1 0.90 0.95 distribution of the wind climate parameters. 1 6.31 12.71 In this way, a total of n simulated lifetime 2 2.92 4.30 load spectra are established, and one 3 2.35 3.18 equivalent load range S0i is interpreted from 5 2.02 2.57 each such simulated lifetime load spectrum. 10 1.81 2.23 The central estimate of the equivalent 20 1.72 2.09 lifetime load range is taken as the arithmetic 50 1.68 2.01 mean ∞ 1.64 1.96

n = 1 4.4.2 Ultimate loads S 0 ∑ S 0i n i=1 Extreme value distributions are of interest

when extreme load responses are needed The standard deviation of the S ’s is 0i such as for design against failure in ultimate estimated by loading. Reference is usually made to a

particular load case, e.g.: n = 1 − 2 • normal operation at a 10-minute mean s ∑ (S 0i S 0 ) n −1 i=1 wind speed near the cut-out wind speed • standstill at a rare 10-minute mean wind A Gaussian assumption can usually be made speed such as the one with a 50-year for the simulated equivalent load ranges S0i. recurrence period On this background, a two-sided confidence • faulty operation at a high wind speed interval for the estimated equivalent load due to error in the protection system range with confidence 1−α can be established as For the considered load case, it is assumed that a total of n 10-minute time series of the load response X has been generated by ± s S 0 t α 1− ,n−1 aeroelastic simulations. The following n 2 quantities associated with the load response X can be interpreted from each of the n time −α in which t1−α/2,n−1 is the 1 /2 quantile in the series: Student’s t distribution with n−1 degrees of • mean value µ freedom. Table 4-4 tabulates these quantiles • standard deviation σ for selected degree-of-freedom values for a • skewness α3 couple of common choices for the • kurtosis α confidence 1−α. Quantiles of the Student’s t 4 • rate νµ of upcrossings of level µ distribution can, in general, be found in most • maximum x in 10 minutes published tables of statistical distributions. m

The maximum value Xm of the load response X in the 10 minutes is of interest. The maximum value will not be a fixed value, but will have a natural variability, which can

82 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

be represented by a probability distribution. n n − = 1 = 1 r 1 The natural variability is reflected in terms b0 ∑ x r and b1 ∑ xr n r =1 n r =1 n −1 of different values for xm in the n simulated time series. The mean value of the and α and β are estimated by maximum load response in 10 minutes is denoted µ , and the standard deviation is m ln 2 γ denoted σ . For design purposes, the αˆ = and βˆ = b − E m 2b − b 0 αˆ characteristic load response is usually taken 1 0 as some quantile of the distribution of the maximum load response in 10 minutes. in which γE = 0.57722 is Euler’s constant.

There are two fundamentally different The mean value and standard deviation of approaches to predicting the maximum load Xm are estimated by response and particular quantiles of its distribution: γ π µˆ = βˆ + E σˆ = • m and m statistical model, which utilises the αˆ αˆ 6 information about the maximum load

response obtained from the n simulated respectively. time series in terms of n simulated

maximum values x m The θ-quantile in the distribution of X can • semi-analytical model, which − based m − be estimated by on stochastic process theory utilises the information about the underlying xˆ = µˆ + k σˆ load response process in terms of the m,θ m θ m

four statistical moments µ, σ, α3 and α4, in which and the crossing rate νµ.

The two approaches are presented in the 6 1 k = (− ln(ln( )) −γ ) following sections, and their levels of θ π θ E accuracy is discussed. The standard error in the estimate of the θ- Statistical model quantile is estimated by The maximum load response Xm in 10 minutes can be assumed asymptotically to σˆ = m + + 2 follow a Gumbel distribution se(xˆ m,θ ) 1 1.14kθ 1.1kθ n F (x ) = exp(− exp(−α(x − β ))) X m m m This reduces to in which α is a scale parameter and β is a σˆ µ = m location parameter. From the n simulated se( ˆ m ) time series there are n observations of the n maximum load response X . For estimation m θ of α and β, the n values of X are ranked in for the special case that the -quantile is m µ increasing order, xm,1,...xm,n. Two coeffi- replaced by the mean value m. cients b0 and b1 are calculated from the data Assuming a normal distribution for the estimate of the θ-quantile, the two-sided

4 – Loads 83 Guidelines for Design of Wind Turbines − DNV/Risø

confidence interval for the θ-quantile with sided confidence interval for xm,95% becomes confidence 1−α becomes 4.673 ±2.78⋅0.413 = 4.673 ±1.148. This indicates a rather wide interval about the ± ⋅ xˆ m,θ t α se(xˆ m,θ ) central estimate. If n is changed from 5 to 1− ,n−1 2 100, the interval is narrowed considerably to 4.673 ±0.183. −α in which t1−α/2,n−1 is the 1 /2 quantile in the Student’s t distribution with n−1 degrees of Semi-analytical model freedom. When a characteristic value with a The semi-analytical model owing to specified confidence is aimed for, it is Davenport (1961) utilises more information usually taken as the upper confidence limit, about the n 10-minute time series of the load i.e. a one-sided confidence interval is response than just the n maximum response considered. The characteristic value with values xm. The load response X can be confidence 1−α then becomes viewed as a stochastic process during the 10-minute series. The process X can be + ⋅ xˆ m,θ t1−α ,n−1 se(xˆ m,θ ) considered as a quadratic transformation of a parent standard Gaussian process U, Note that a high number of simulations n may be necessary to achieve a sufficiently X = ζ +η(U + εU 2 ), ε << 1 accurate estimate of xm,θ or µm. A first-order approximation gives the Example following expressions for the coefficients Consider a load response process X whose extreme value Xmax in 10 minutes has been α ε = 3 estimated on the basis of n = 5 simulated 10- 6 minute time series. The following estimates η = σ pertaining to the extreme value distribution ζ = µ −εσ have resulted:

from which it appears that use is made of the αˆ = 3.69, βˆ = 3.87, µˆ = 4.02, σˆ = 0.35. m m mean value µ, the standard deviation σ and the skewness α of the load response process θ 3 An estimate of the = 95% quantile of Xmax X. with 1−α = 95% confidence is sought. This gives kθ = 1.866. The central estimate of the The mean value and standard deviation of 95% quantile of Xmax is Xm are estimated by

= + ⋅ = xˆ m,95% 4.02 1.866 0.35 4.673 . µ = η +ζ ν ˆ m ( 2 ln( µ T)

+ ε 2 ln(ν T )) The standard error in this estimate is µ γ ζ (1+ 2ε 2 ln(ν T)) + E µ = 0.35 = ν se(xˆ m,95% ) 2.64 0.413 . 2 ln( µ T ) 5

and The pertinent quantile in the Student’s t distribution is t1−α/2,n−1 = 2.78, and the two-

84 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

+ ε ν accuracy when semi-analytical results are πη 1 2 2 ln( µ T ) σˆ = used for the extreme value estimates than m ν 6 2 ln( µ T) when only statistical results for the extreme values are used. It is recommended always respectively, where T denotes duration and to predict extreme loads by means of the is usually the length of a simulated time semi-analytical method, based on the series, i.e. T = 10 minutes. Corresponding statistics of the simulated load response estimates of the Gumbel distribution process, rather than using the observed maximum values only. parameters α and β can be found as

π Two or more arbitrarily selected simulated αˆ = 10-minute time series may give considerably σ ˆ m 6 different extreme values. This implies that γ the practice of performing a few simulations βˆ = µˆ − E and selecting the average extreme load or m αˆ the largest extreme load as the ultimate load without proper consideration of the The standard error in the mean value stochastic nature of the extremes will not µˆ estimate m is give reproducible results. Further, the results cannot be extrapolated to a characteristic σˆ value defined by a quantile or to a different se(µˆ ) = m m duration of the load case than 10 minutes. n The semi-analytical approach takes the stochastic nature of the extremes into where n is the sample size, i.e. the number account and provides a rationale for analysis of 10-minute series available for the of extreme loads from simulated time series µ estimation of m by the above formula. of load responses. σ When m can be considered well-determined σˆ by the above formula for m , the two-sided As an alternative to the presented quadratic confidence interval for µm with confidence transformation of the parent Gaussian 1−α becomes process U to the physical load response process X, a cubic transformation can be σˆ applied. This can be made according to a µ ± ⋅ m ˆ m u α fourth-moment Hermite polynomial 1− 2 n expansion as described by Winterstein (1988). This will also allow for −α in which u1−α/2 is the 1 /2 quantile of the representation of the load response process standard normal distribution function. in terms of the kurtosis α4.

Comparison and recommendations It is not recommended to consider The semi-analytical results will usually transformations, which involve higher-order provide a higher accuracy of the extreme statistical moments of the load response X. value estimates than the statistical results. This is due to the fact that the lengths of This comes about because the semi- available time series of the load response are analytical results utilise much more of the usually much too short to allow for a available information than the statistical sufficiently accurate estimation of such results do. In other words, a smaller sample higher-order statistical moments. In other size n is required to achieve the same

4 – Loads 85 Guidelines for Design of Wind Turbines − DNV/Risø

words, higher-order moment estimates based The upper bounds of the mean value µm of on available simulated time series of load the largest extreme Xmax of X in all sectors response can usually not be trusted. during the time T are

Correction for periodic loads  γ  µˆ = max µ +σ (β + E ) The presented semi-analytical model m,upper  i i α  provides good accuracy for prediction of   extreme values for wind turbines, which are not in operation. For the operating load More details about this method can be found cases, the periodic nature of the response in Madsen et al. (1999). A recommended mean and standard deviation for some loads value for discretisation of the rotor disc into must be accounted for. A method, based on sectors is M = 36. azimuthal binning, can be used for this purpose. By such a method, the rotor disc is divided into a number of sectors, each 4.5 Simplified load calculations identified by its azimuth angle. When the 4.5.1 Parametrised empirical models rotor disc is discretised into M sectors of an equal angle of aperture, sector-specific mean Various simplified models for load values and standard deviations of the load calculation exist and are presented in the response process can be established as following subsections. Today, where available computer resources usually do not π prohibit execution of computer-intensive µ = µ 2 − 1 i ( (i )) calculations, most load calculations for wind M 2 turbines are carried out by means of and aeroelastic codes. The simplified models are therefore included mainly for historical π reasons and to provide tools for preliminary σ = σ 2 − 1 i ( (i )) calculations and quick verification of results. M 2

4.5.2 The simple load basis in which µ and σ are the mean value and standard deviation, respectively, of the load Based on a systematisation of measurements response X. Let α and β denote the and experience from typical stall-regulated distribution parameters in the Gumbel wind turbines with active yaw and distribution of the maximum value during approximately constant rotor speed, a the time T of the normalised process simplified method for calculation of wind (X−µ)/σ. They can be determined by the turbine loads has been established. The analytical model as described above. The method is useful for preliminary design of wind turbines with rotor diameters between lower bounds of the mean value µm of the 5 and 25 m and with rotor speeds between largest extreme Xmax of X in all sectors during the time T are 35 and 50 m/s.

 ln M −γ  The method is based on the following three µˆ = µ +σ β − E m,lower max i i ( ) load quantities:  α  • a static horizontal airflow load F0 = 2 300A, where F0 is in units of N, A = πR is the swept area of the rotor, and R is the rotor radius in units of m.

86 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

• a driving torque Me,nom = Pnom/(2πnrη), reduced by 33%. For design of the main where Pnom is the nominal power of the shaft, a torque from the mechanical brake of wind turbine, nr is the rotor frequency, two times the value of MY is to be assumed. and η is the nominal efficiency, usually η ≤ 0.9. The design loads given here are compatible • the weight of the rotor mg, where m is with the Danish codes. Thus, when applied the rotor mass and g is the acceleration for design purposes, they need to be checked of gravity against design capacities calculated according to the Danish codes. The rotor loads are expressed in terms of these three quantities as summarised in 4.5.3 Quasi-static method Table 4-5. Moments are based on the The quasi-static method presented in the assumption that the airflow load F0 is following gives simple expressions for four applied with an eccentricity e = R/6. different design loads as derived for a still- standing wind turbine in rather severe wind For calculation of blade loads, it is assumed conditions: the blade load is calculated for a that the airflow load is distributed evenly vertical blade above the hub. The root between the NV blades, such that the moment is used as the design tilt moment. flapwise airflow load on one blade becomes The axial force is determined by summing the wind load over the blades. The yaw Table 4-5. Design rotor loads by simplified method. moment is determined as the root moment Load component Sym- Static Dynamic bol load load from a horizontal blade. amplitude Horizontal force FX 0 0 By the quasi-static method, the load per unit in rotor plane length of the blade is calculated by Moment about MX eF0 0.25eF0 horizontal axis in rotor plane 1 2 p(r) = ψρU D(r)C Horizontal force FY F0 0.25F0 2 10 along rotor axis

Moment about MY 1.3Me,nom 0.25⋅1.3Me,nom rotor axis in which C is the maximum value of the lift Vertical force FZ −mg 0 coefficient CL or the drag coefficient CD. Moment about M eF 0.25eF Z 0 0 Typical maximum values of CL and CD are vertical axis in the range 1.3-1.5. U10 is the 10-minute mean wind speed with a recurrence period PV = F0/NV of fifty years at a height h. The height h and the coefficient C are given in Table 4-6, This load is assumed to be the resultant of a depending on which design load is to be triangular-shaped flapwise line load along calculated. ρ is the density of air, and D(r) is the blade with the maximum value occurring the chord length of the blade at distance r at the blade tip. from the hub.

For calculation of blade loads in the rotor The quasi-static gust factor ψ in the load plane, i.e. edgewise blade loads for the expression is to be calculated as individual blades, the gravity loads need to be considered. For calculation of effects on machinery and tower, the dynamic load amplitudes given in Table 4-5 may be

4 – Loads 87 Guidelines for Design of Wind Turbines − DNV/Risø

2   z    ln( ) + 3.1 z hub height  z 0  n0 L  for > n* z0 roughness length   z  U L rotor radius   ln( )  10 ψ =   n* given in Table 4-6   z 0   n eigenfrequency of the vibration mode 2 k + k n L 0 1+ 3.9 b r for 0 ≤ n* associated with the particular design  z U 10 load which is to be calculated  ln( )  z 0

Table 4-6. Quasi-static load specification parameters. Load Eigenfrequency n0 Threshold C Height h corresponding to n* Blade load Blade flapwise bending 1.7 CL,max 2 z + L 3

Axial force Tower bending 0.45 CD,max z Tilt moment Blade flapwise bending 1.7 CL,max 2 z + L 3

Yaw moment N/A 0 CD,max z

Moreover, the background turbulence effect whose value depends on which design load kb in the expression for ψ is approximated is being considered by  − L  0.9 2.5 for blade load  k = l b  L   0.75 − 3 for axial force  1 for blade load  l  nL 1+ 3  U and the resonance effect k in the same 10 r  1 expression is given by F(n) =  for axial force nL  1+12 n  U 10 0 l  U π 2 nL = 10  2.7 k r F(n0 ) U  for rotor n 2δ  10 (1+1.5 0 l ) 5 3   + nL + nL 2 moments U 10 1 4.4 21.8( )  U U  10 10 Here, ℓ = 6.8Lu, in which Lu is the integral length scale of the Kaimal spectrum, see 4.5.4 Peak factor approach for

Section 3.1.5. δ = 2π(ζ0+ζa) is the extreme loads logarithmic increment of damping expressed Extreme loads are often specified in terms of ζ in terms of the structural damping ratio 0 the load which has a certain recurrence ζ and the aerodynamic damping ratio a. F(n) period, such as fifty years, i.e. the load that is the aerodynamic admittance function on average will be exceeded once every fifty years. Two approaches are presented here.

88 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

1 p = ρW 2 cC Peak-over-threshold method 0 2 L By the so-called peak-over-threshold method, the load with a recurrence period T 4π 2 2 = 2 + can be estimated by W ( nr R) V0 3 qT = q0+αln(λT) ρ density of air in which q0 is some chosen threshold for the c characteristic chord length of the blade load q, λ is the mean rate of exceedances of at a distance r = 2/3R the threshold q by the load q, and α is the CL lift coefficient at a distance r = 2/3R 0 W resulting wind speed mean value of the exceedances ∆q = q−q0. nr rotor frequency Periodical maximum method R rotor radius The distribution of the maximum load in a V0 nominal stall wind speed at the height period of specified duration such as one year of the hub can be assumed to be a Gumbel distribution V0 is defined as the minor of the following − −α −β two wind speeds: F(q) = exp( exp( (q ))) • the nominal 10-minute mean wind speed V at which the turbine reaches When the distribution parameters α and β nom its nominal power Pnom refer to the distribution of the maximum • load in one year, the load with the the 10-minute mean wind speed at recurrence period T years can be estimated which stall just extends to the entire by blade for airflow parallel to the rotor shaft

1 T 1 qT = β − ln(ln ) ≈ β + lnT The load distribution along the blade is α T −1 α represented as a triangular line load whose value is 0 at the hub and p0 at the blade tip a 4.5.5 Parametrised load spectra distance R away from the hub, such that the Parametrised load spectra are useful for value of the line load at a distance r from the calculations where not only the extreme load hub can be calculated as is of interest, but also the entire load p r distribution, such as for design against p = 0 fatigue failure. Parametrised load spectra are R simplified and idealised load distributions expressed in terms of a number of and such that the resulting bending moment characteristic parameters. at the blade root becomes

A useful parametrised load spectrum is the R 2 M = p one which is presented in DS472. This load root 0 3 spectrum is meant for calculation of blade loads and is based on the characteristic For fatigue calculations, loads are, in aerodynamic line load p0. The value of p0 general, represented by some mean load, can be calculated as which is superimposed by some cyclically varying load. The cyclically varying load is

4 – Loads 89 Guidelines for Design of Wind Turbines − DNV/Risø considered in the following. The load ranges in which F∆ of the cyclically varying load are represented by a probability distribution. β = 0.11kβ(IT+0.1)(A+4.4) This probability distribution is expressed such that F∆(N) is the load range which is IT is the characteristic turbulence intensity at exceeded N times during the design life of the hub height according to the formula in the wind turbine. DS472, see Sections 3.1.2 and 3.1.3. Note that this generic standardised load range The cyclically varying load consists of a distribution is valid for rotor diameters less deterministic part owing to gravity loads and than 25 m. Application to larger rotor a stochastic part owing to aerodynamic diameters may lead to overconservative loads. The cyclically varying deterministic results. load appears for example as a cyclically varying edgewise bending moment at the In general, kβ = 1 such as for loads on blade root and results from the rotation of individual blades, however kβ = 2.5 for the blades about the hub. The corresponding calculation of rotor pressure from all three load range distribution consists of load blades. ranges of constant magnitude, and their k k number is equal to the total number of NF = nCTL(exp(−(Vmin/A) )−exp(−(Vmax/A) ) rotations NR of the rotor during the design life. The constant magnitude load range of is the number of load ranges in the design this deterministic load will typically be a life TL of the turbine corresponding to a function of the mass m per unit length of the characteristic load frequency nC. rotor blade and of the acceleration g of gravity. A and k are scale and shape parameters, respectively, in the long-term Weibull The cyclically varying stochastic load distribution of the 10-minute mean wind appears for example as a cyclically varying speed U10, see Section 3.1.1. flapwise bending moment at the blade root and results from the airflow forces set up on Vmin and Vmax are cut-in and cut-out wind the blades by the wind. The corresponding speeds, respectively, for the operation of the load range distribution is represented by a wind turbine, see Section 2.2. generic standardised distribution, whose unitless range values F∆* are to be For combination of deterministic loads, a multiplied by a design constant, which harmonic variation with time can be depends on load type and direction, to give assumed, and the following formula can be the sought-after load range F∆. used, based on the assumption that the blade is vertical at time t = 0: According to DS472, the generic standardised load range distribution to be = + 1 π used for the stochastic load ranges is given p(t) p p∆C cos(2 nC t) 2 by the following expression + 1 π p∆S sin(2 nC t) F∆*(N) = β⋅(log10(NF)−log10(N))+0.18 2 with the additional condition F∆*(N)≤2kβ where p may denote, for example, line load for blades. p is then the mean line load,

90 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

and p∆C and p∆S are load ranges for cosinus- The characteristic frequency nC to be used is oidal and sinusoidal load components, also indicated and expressed in terms of the respectively. rotor frequency nR.

For combination of deterministic load F∆*(N) ranges and stochastic load ranges, it is recommended, conservatively, to simply add (sum) the load range values from the two distributions. The largest deterministic load (deterministic) range value is then added to the largest stochastic load range value, the second- largest deterministic load range value is (stochastic) added to the second largest stochastic load range value, and so on, until all load range log10(N) values of the two distributions have been combined to form the combined distribution. 0 log (N )log(3N ) 10 R 10 R An example of such a combination of a Figure 4-21. Standardised distribution. deterministic and stochastic load range distribution is given in Figure 4-21 for the Rotor loads edgewise line load on a rotor blade. Rotor loads are expressed as the sum of a deterministic mean load and a harmonically Table 4-7 gives expressions for varying load with stochastic range. deterministic and stochastic load components for line loads for rotor blades.

Table 4-7. Line load on rotor blade. Mean Load range distributions Direction Deterministic Stochastic

p p ∆C p ∆S p ∆,stochastic 2 Edgewise 2Mnom/(3R ) 0 +2mg 0.3F∆*(N)p0 Flapwise 1.5p0r/R 0 0 F∆*(N)p0r/R 2 Along blade (2πnR) mr −2mg 0 0 Frequency nC nR nR 3nR

4 – Loads 91 Guidelines for Design of Wind Turbines − DNV/Risø

Table 4-8. Rotor loads. Load Sym- Determi- Stochastic load Fre- Lowest resonance component bol nistic range F∆ quen- frequency n0 mean F cy nC (oscillation form) Horizontal force FX 0 0 in rotor plane 2 Moment about MX 0 0.33F∆*(N)kRp0R 3nR nROTOR (rotor, tilt, horizontal axis asymm.) in rotor plane

Horizontal force FY 1.5p0R 0. 5F∆*(N)p0R 3nR nTOWER (tower, along rotor axis bending) 2 Moment about MY 0.5Mnom 0. 45F∆*(N)p0R 3nR rotor axis Vertical force FZ −Mg 0 2 Moment about MZ 0 0. 33F∆*(N)kRp0R 3nR nROTOR (rotor, yaw, vertical axis asymm.)

1 range distribution given in terms of F∆*(N) F = F + F∆ cos(2π n t) 2 C as outlined above. Some of the expressions include a correction factor kR that accounts t time for amplification effects, which are a function of the ratio between the resonance nC frequency frequency n0 and the rotor frequency nR. The correction factor kR is given in Figure 4-22. 1,8 The resonance frequency n0 is the lowest

1,6 resonance frequency for the associated oscillation form, i.e. nTOWER for towers in 1,4 bending and nROTOR for the collective 1,2 asymmetric rotor oscillation at standstill, 1 where one blade oscillates out of phase with k R 0,8 the two other blades.

0,6 illicit 0,4 frequency 4.6 Site-specific design loads 0,2 range As part of a scheme to reduce the cost of 0 012345678 produced energy, site-specific characteristics can be included directly in the design n 0/n R process of wind turbines. The idea is to optimise the wind turbine design by Figure 4-22. Amplification factor kR vs. frequency ratio n0/nR. minimising the cost of produced energy, given the characteristics at a particular site. Table 4-8 gives expressions for determin- Extreme loads and fatigue loads on a wind istic and stochastic load components for turbine are site-dependent. Site-specific rotor loads. The expressions are based on the design loads will therefore form part of such blade line load p0, the nominal torque Mnom, a cost-optimal design process. the rotor mass M, and the standardised load

92 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

Two approaches to site-specific designs are foundation structure will be exposed to wave envisaged: loads, current loads and ice loads. The • adaptation of existing wind turbine prediction of wave loads is dealt with in the designs to specific site by minor following. A brief introduction to current adjustments loads and ice loads is also given. • design from scratch 4.7.1 Wave loads Site-specific design has a potential Wave climate whenever wind turbines are to be installed in Usually, the wave climate at a location can complex terrain such as in mountainous be considered stationary within periods of areas, where special conditions may prevail, typically three hours duration. The wave or for large offshore wind farms where a climate is represented by the significant large number of identical turbines are to be wave height H and the peak period T . The installed at the same site. It can be used to S P significant wave height HS is a measure of verify that all relevant load cases have been the intensity of the wave climate and is considered. defined as four times the standard deviation

of the sea elevation process η. Some sources Using site-specific design loads and carrying define the significant wave height as the out site-specific wind turbine designs is average of the highest one third of the wave somewhat in contrast with the current trend heights. For a narrow-banded Gaussian sea within the wind turbine industry. In order to elevation process, the two definitions keep down manufacturing costs, the current converge. The peak period T is related to trend is not to site-optimise wind turbines, P the mean zero-crossing period of the sea but rather to produce a selection of standard elevation process. The significant wave wind turbines. The task is then to choose a height and the peak period can be taken as standard wind turbine from this selection constant within each three-hour period. and verify that it is suitable for a given location. The tower and the foundation may Long-term distributions of H and T are still be site-optimised if desirable, and site- S P site-dependent. The long-term distribution specific loads will be required for this of H can often be represented well by a purpose. The foundation design will always S Weibull distribution, whereas the have to be site-specific in that it needs to be distribution of T conditioned on H is designed for the prevailing local soil P S usually well-represented by a lognormal conditions. distribution whose distribution parameters

are functions of H . Examples can be found Reference is made to Section 3.1.10 on the S in Bitner-Gregersen and Hagen (2000). subject of site assessment.

Wave spectrum

The frequency content of the sea elevation 4.7 Loads from other sources than process can be represented by the power wind spectral density. The spectral density of the Installation of wind turbines in shallow sea elevation process can be represented by waters of up to 15 m water depth can be the JONSWAP spectrum foreseen. A support structure or foundation structure is used for transfer of loads from the wind turbine and its tower to the supporting soils at the seabed. The

4 – Loads 93 Guidelines for Design of Wind Turbines − DNV/Risø

−5 ω The maximum wave height Hmax during 5 2  T  S (ω) = H T  P  η π S P  π  some time span TL is often of interest for 32  2  design. Let N denote the number of zero- −4 5  ωT  upcrossings of the sea elevation process in ⋅ − P exp(   ) this period of time, i.e. N = T /T . The 4  2π  L Z distribution of Hmax can then be 2  ωT   − 1  P −   exp  1  approximated by  2σ 2  2π   ⋅C(γ )γ   2h 2 F (h) ≈ exp(−N exp(− )) C(γ) = 1−0.287⋅lnγ Hmax (1 −ν 2 )H 2 S

σ = 0.07 for 0 < ω < 2π/TP and the expected value of the maximum σ = 0.09 for ω > 2π/TP wave height can be approximated by The peak-enhancement factor γ can be taken as (1 −ν 2 ) ln N E[]H ≈ H max S 2 T T γ = exp(5.75 −1.15 P ) ; 3.6 ≤ P ≤ 5 A first-order approximation yields the H S H S following value for the spectral width

parameter, ν = 0.43. With this value of ν, in which HS is in metres and TP in seconds. The following approximate relationship the following relationship between the maximum wave height Hmax and the exists between the peak period TP and the maximum wave crest Zmax holds zero-crossing period TZ

H ≈ 1.8⋅Z +γ max max = 5 TZ TP 11+γ In shallow waters, shoaling effects imply that wave crests become more peaked while Reference is made to Gran (1992) and DNV wave troughs become flatter and not quite as (2000). deep. The sea elevation process has become somewhat ”skewed” and will not quite be Wave heights Gaussian, and the individual wave heights In deep waters, the sea elevation process η will not quite be Rayleigh distributed. is a Gaussian process, and the individual Techniques are available to account for the wave heights H, measured form trough to skewness introduced by the shoaling. crest, will follow a Rayleigh distribution Reference is made to Winterstein et al. (1991) and to U.S. Army Coastal when HS is given Engineering Research Center (1973).

2h 2 F (h) =1 − exp(− ) Note that in shallow waters, the wave H −ν 2 2 (1 )H S heights will be limited by the water depth, d. The maximum possible wave height at a ν spectral width parameter water depth d is approximately equal to the water depth

94 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

≈ H max,lim d 4 2π 2kd h = H (1+ ) tanh[]kd and the Rayleigh distribution of the wave 0 max [] H S sinh 2kd heights will become distorted in the upper tail to approach this limit asymptotically. in which k is the wave number, which results implicitly as the solution to Wave periods Once the significant wave height H is S ω 2 = gk tanh[]kd given, the zero-upcrossing period TZ is usually well-represented by a shifted lognormal distribution where L = 2π/k and ω = 2π/T

ln(t −δ ) − a g acceleration of gravity = Φ 1 ≥δ FT (t) ( ) ; t L wave length Z a 2 k wave number ω angular velocity in which Φ denotes the standardised normal T wave period distribution function, the distribution para- meters a1 and a2 are functions of HS, and the Wave forces by Morison’s equation shift parameter δ can be approximated by Wave forces on slender structural members, such as a cylinder submerged in water, can δ ≈ be predicted by Morison’s equation. By this 2.2 H S equation, the horizontal force on a vertical

element dz of the structure at level z is when H is given in metres and δ is given in S expressed as seconds. This is based on braking con- siderations, see Haver (1990). dF = dF + dF M D 2 The mean zero-upcrossing period TZ is an = ρπ D + ρ D C M &x&dz C D x& x&dz average wave period associated with a sea 4 2 state of a given significant wave height HS. The wave period T associated with the CM inertia coefficients maximum wave height Hmax in this sea state CD drag coefficients can be represented by a Longuet-Higgins D diameter of the cylinder distribution ρ density of water x& horizontal wave-induced velocity of T h Φ((1− ( Z ) 2 (ν 2 +1)) 0 ) water ν x horizontal wave-induced acceleration F (t) = t 2 && T h of water Φ( 0 ) 2ν The first term in the formula is an inertia Φ standardised normal distribution force and the second term is a drag force. function The level z is measured from stillwater level, ν spectral width parameter as referenced and the z axis points upwards. Thus, at − above seabed z = d, when the water depth is d. The movement of the structure is considered h0 normalised maximum wave height in deep water to be very small.

4 – Loads 95 Guidelines for Design of Wind Turbines − DNV/Risø

According to first-order linear wave theory, when a nodal line at the stillwater level the horizontal wave-induced velocity is passes the structure. The drag force FD, on the other hand, reaches its maximum when []+ the crest passes the structure, and if this = ω cosh k(z d) ω x& AW sin( t) sinh[]kd force is dominating, a significant error can be introduced by ignoring the contribution

from the wave crest. and the acceleration is

Note also that Morison’s equation is only cosh[]k(z + d ) = ω 2 ω valid when the dimension of the structure is &x& AW cos( t) sinh[]kd small relative to the wave length, i.e. when D < 0.2L, and that it is only valid for AW wave amplitude nonbreaking waves. In deep water, waves break when H/L exceeds about 0.14. The resulting horizontal force F on the The inertia coefficient depends on the cross- cylinder can be found by integration of sectional shape of the structure and of the Morison’s equation for values of z from –d orientation of the body. Typically, CM is in to 0 the range 1.6-2.5. For a vertical cylinder, CM = 2.0. For a cylinder with in-service = + marine roughness, e.g. owing to marine F FM FD 0 2 growth, CM should normally not be less than D H 2 = (C ρπ ω cos(ωt) 1.8. The drag coefficient CD is never less ∫ M 4 2 −d than 0.6, and for a smooth cylinder CD = 1.0. cosh[]k(z + d ) ⋅ )dz sinh[]kd An example of application of Morison’s equation is given in Figure 4-24 for a 0 D H 2 + (C ρ ω 2 sin(ωt) sin(ωt) cylinder with 4 m diameter in 10 m water ∫ D −d 2 4 depth. The upper part of this figure shows cosh 2 []k(z + d) the sea elevation process, the middle part ⋅ )dz shows the horizontal water particle velocity sinh 2 []kd and acceleration at the stillwater level, and the lower part of the figure shows the Note that the integration from –d to 0 resulting horizontal wave force and ignores contributions to the force from the overturning moment at the seabed. The wave crest above the stillwater level at z = 0. example is based on CM = 2 and CD = 1.2. However, this is a minor problem for the inertia force FM, since this has its maximum

96 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

Figure 4-23. Relative magnitude of inertia and drag forces.

The inertia force can be expressed as

= ω FM AM cos( t)

and the drag force as

= ω ω FD AD sin( t) sin( t)

AM amplitudes of inertia force AD amplitudes of drag force

Let A denote the ratio between the amplitudes, A = AM/AD. The following relationship can be established

H C sinh 2 []kd = π M []+ D C D ((sinh 2kd / 4 kd / 2)A

and can be represented as shown in Figure 4-23. For a given structure and location,

Figure 4-24. Example of sea elevation process, Figure 4-23 can be used to quickly establish associated water particle velocities and accelerations, whether the inertia force or the drag force is and resulting Morison forces at the seabed for a the dominating force, once the ratios H/D cylindrical example structure. and d/L have been calculated.

4 – Loads 97 Guidelines for Design of Wind Turbines − DNV/Risø

With reference to Figure 4-23, structures amplitude A, this theory gives the following located above the curve for A = 1 experience maximum horizontal wave force loads which are dominated by the drag term in Morison’s equation. Structures located 4ρgA sinh[]k(d + Asinα) F = ξ below the curve for A = 1 are dominated by X ,max k 2 tanh[]kd inertia loads. The two dashed asymptotes in

Figure 4-23 are valid for shallow water whose vertical arm measured from the sea waves (d/L < 1/20) and deep water waves floor is (d/L > 0.5), respectively. The asymptotes are derived from asymptotic results for small- []− []+ amplitude wave kinematics. = kd sinh kd cosh kd 1 hF d kd sinh[]kd Example A cylinder with diameter D = 5 m is The coefficients ξ and α are given in Table considered subjected to the wave climates at 4-10, extracted from Gran (1992). the and Rødsand sites in Danish waters. Water depths and wave data Table 4-10. Coefficients ξ and α. for a number of cases at these two locations are tabulated in Table 4-9 with the resulting ratios H/D, d/L and A. It appears that for H/D, d/L and A. It appears that for both sites A values occur, which indicate that the inertia force dominates the loading.

Table 4-9. Examples of ratio between inertia force and drag force. Site Wave Depth Wave H/D d/L A height d length H (m) L (m) (m) MG 3.8 5.5 40 0.76 .138 7.1 MG 2.6 5.5 28 0.46 .196 16.2 RS 3.5 8.0 45 0.70 .178 9.8 RS 3.5 9.5 47 0.70 .202 10.9 RS 3.5 11.0 50 0.70 .220 11.7 RS 6.2 8.0 64 1.24 .125 4.0 RS 6.7 9.5 76 1.34 .125 3.7 RS 6.7 11.0 88 1.34 .125 3.7 MG = Middelgrunden RS = Rødsand

Wave forces by diffraction theory When the dimension of the structure in question is large compared with the wave length, typically when D > 0.2L, Morison’s equation is no longer valid. The inertia force will then be dominating and can be predicted by means of diffraction theory. For a cylinder of radius R = D/2 installed in This can be used for prediction of wave water of depth d and subjected to a wave of forces on foundation structures shaped like cylinders, such as monopiles and some

98 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø gravity-based structures. Note, however, that For ice loads in general, reference is made to the formulas may lead to erroneous results if API (1995). the structural geometry deviates much from the assumed cylindrical shape, such as when 4.7.4 Earthquake loads a conical structural component is present in For prediction of earthquake loads, reference the wavesplash zone to absorb or reduce ice is made to Section 3.2.8. loads.

Note also that in shallow waters waves may 4.8 Load combination break locally over a sloping seabed, if they are large enough, and thereby violate the Principles for how to combine loads arising kinematic assumptions behind the presented from different concurrent load processes are formulas. For a horizontal seabed, however, outlined in this section. such local breaking is not expected as too large waves will have broken prior to When several load processes are acting arriving at the particular location. The concurrently, their combined load response assumed wave kinematics of these waves in the structure needs to be considered for will regenerate before the waves arrive at design. For example, the foundation the location, while the heights of waves will structure of an offshore wind turbine will be conform to the limit given by the water subject to the combined action from wind depth. and wave loads, and the resulting structural response from this action governs the Wave loads are strongly dependent on the design. Another load combination, which is water depth. For this reason it is important possible, is the combination of wind and ice to consider effects of local variations in the loads, and current may combine with any of water depths, including astronomical tides the other load types mentioned. and storm surges. Consider the combination of wind and wave 4.7.2 Current loads loads. The short-term wind climate is usually represented by the 10-minute mean Morison’s equation can be used to predict wind speed U , and the short-term wave current loads. Note in this context that the 10 climate is usually represented by the velocity x and the acceleration x in & && significant wave height H . U and H may Morison’s equation need to be taken as the S 10 S be interpreted as intensities of the resulting combined current and wave corresponding wind speed and sea elevation velocity and acceleration, respectively. As in processes, respectively. The wind and the the case of pure wave load prediction, waves at a particular location often have a Morison’s equation is only applicable as common cause such as a low pressure. The long as the wave length is longer than five waves are driven by the wind and often times the diameter of the cylindrical generated locally. At the same time, the structure. roughness implied by the wave-affected sea

surface influences the wind. A high wave 4.7.3 Ice loads intensity will imply a high wind intensity, It is current practice to distinguish between and vice versa. static ice loads and dynamic ice loads. For conical structures, Ralston’s formula for ice It is important to consider this usually strong loads can be applied, see Ralston (1977). dependency – the simultaneous occurrence

4 – Loads 99 Guidelines for Design of Wind Turbines − DNV/Risø of wind and wave climates of high that occurs for this climate over its duration. intensities – in design. In a probabilistic Practically, one may for example consider analysis, this can practically be done by the wave climate for the significant wave modelling one of the climate variables as a height with a 50-year recurrence period in so-called independent variable by means of combination with a wind climate its marginal cumulative distribution conditioned on this wave climate, e.g. the function, and then model the other variable expected value of U10 conditional on the 50- as a dependent variable by means of a year significant wave height, or some higher distribution conditioned on the independent quantile of U10. With such a rare variable. For the considered wind and wave characteristic wave and wind climate in example, one could represent the significant mind, the following paragraphs outline how wave height HS by its marginal long-term the combined load response, e.g. the distribution, typically a Weibull distribution, horizontal force in some section of the and then model the 10-minute mean wind foundation structure, during this climate can speed U10 conditional on HS. The be found. distribution of U10 conditioned on HS will typically be a lognormal distribution For linear load combinations, Turkstra’s rule plays a central role. The rule states that the ln u − b maximum value of the sum of two F (u) = Φ( 1 ) U10 |H S b independent random processes occurs when 2 one of the processes has its maximum value. Application of Turkstra’s rule to the Φ standard normal distribution combination of two load processes, e.g. function wave load and wind load, implies that the b1, b2 functions of the significant wave combined load will have its maximum either height HS, i.e. b1 = b1(HS) and b2 = when the wave load has its maximum or b2(HS when the wind load has its maximum. Let

Q1 and Q2 denote the two load processes. In some cases, other generic distribution Mathematically, the maximum combined types than the lognormal distribution may load Qmax over a time span T will be provide the best representation of U10 conditional on H , e.g. a Weibull S maxQ (t) + Q (t)  ≤ ≤ 1 2 distribution. Q = max 0 t T max  + Q1 (t) maxQ2 (t)  0≤t≤T Once the wind and wave climates are given in terms of a specific set of concurrent U 10 Turkstra’s rule indicates that a natural code and HS values, the wind speed process format for a combination of two loads for conditioned on U10 and the wave process use in deterministic design is conditioned on H can be considered S independent. It is therefore, for example, not γ q + γ ψ q reasonable to expect that the maximum wind q = max 1 1k 2 2 2k design γ ψ + γ speed will occur at the same time as the  1 1q1k 2 q2k maximum wave height.

in which q1k and q2k are characteristic values For design, it is reasonable to consider some of Q and Q , γ and γ are associated partial relatively rare combination of wave and 1 2 1 2 safety factors, and the ψ-factors are load wind climate as the characteristic climate combination factors. and then to find the maximum load response

100 4 – Loads Guidelines for Design of Wind Turbines − DNV/Risø

For a wind turbine structure, loads do not Risø R-1065(EN), Risø National always combine linearly to give the sought- Laboratory, 1999a. after maximum load response. Aeroelastic wind load calculations may be nonlinear, Bak, C., H.A. Madsen, and N.N. Sørensen, and the combined wave and wind load Profilkoefficienter til LM19.1 vingen response may not necessarily come out as bestemt ud fra 3D CFD, 1999b. the linear combination of the separately calculated wave load response and the Bathe, K.J., Finite Element Procedures in separately calculated wind load response. Engineering Analysis, Prentice-Hall, 1982 For such nonlinear cases, the combined response is to be calculated from some Baumgart, A., I. Carlén, M. Hansen, G. appropriate structural analysis for the Larsen, S.M. Petersen, Experimental Modal concurrent characteristic wave and wind Analysis of a LM 19.1 m blade, unpublished load processes without applying any partial work. load factors. The resulting maximum load response from this analysis can be Bitner-Gregersen, E., and Ø. Hagen, Aspects interpreted as a characteristic load response, of Joint Distribution for Metocean which reflects the combined wave and wind Phenomena at the Norwegian Continental loading. A common partial safety factor is Shelf, ASME Paper No. OMAE-2000-6021, then to be applied to this characteristic load Proceedings, International Conference on response to give the design value qdesign, i.e. Offshore Mechanics and Arctic Engineering, it will no longer be possible to distinguish 2000. between different partial safety factors for wave loads and wind loads. Davenport, A.G., The Application of Statistical Concepts to the Wind Loading of Structures, Proc. Inst. of Civil Engineers, REFERENCES Vol. 19, 1961.

Andersen, P.S., U. Krabbe, P. Lundsager, DNV, Environmental Conditions and and H. Petersen, Basismateriale for Environmental Loads, Classification Notes beregning af propelvindmøller, Report No. No. 30.5, Det Norske Veritas, Høvik, Risø-M-2153 (in Danish), Risø National Norway, 2000. Laboratory, revised version, 1980. Eggleston, D.M, and F.S. Stoddard, Wind Abbott, I.H., and A.E. von Doenhoff, Turbine Engineering Design, Van Nostrand Theory of Wing Sections, Dover Publications Reinhold Co. Inc., New York, N.Y., 1987. Inc., New York, N.Y., 1959. Gran, S., A Course in Ocean Engineering, API, Recommended practice for planning, Elsevier, Amsterdam, the Netherlands, 1992. designing and constructing structures and The Internet version, located at pipelines for arctic conditions, RP2N, 2nd http://www.dnv.com/ocean/, provides on- edition, American Petroleum Institute, 1995. line calculation facilities within the fields of ocean waves, wave loads, fatigue analysis, Bak, C., P. Fuglsang, N.N. Sørensen, H.A. and statistics. Madsen, Wen Zhong Shen, J.N. Sørensen, Airfoil Characteristics for Wind Turbines,

4 – Loads 101 Guidelines for Design of Wind Turbines − DNV/Risø

Hallam, M.G., N.J. Heaf, and L.R. Petersen, J.T., Geometric nonlinear finite Whootton, Dynamics of Marine Structures: element model for a horizontal axis wind Methods of Calculating the Dynamic turbine, Risø National Laboratory, 1990. Response of Fixed Structures Subject to Waves and Current Action, CIRIA Petersen, J.T., H.A. Madsen, A. Björk, P. Underwater Engineering Group, 6 Storey’s Enevoldsen, S. Øye, H. Ganander, D. Gate, London SW1P 3AU, Report UR8, Winkelaar, Prediction of Dynamic Loads 1978. and Induced Vibration in Stall, Risø-R- 1045, Risø National Laboratory, 1998 Haver, S., On a Possible Lower Limit for the Spectral Peak Period, Statoil Report No. Ralston, T.D., “Ice Force Design F&U-MT 90009, Stavanger, Norway, 1990. Considerations for Conical Offshore Structures,” Proceedings, Fourth POAC IEA, Expert Group Study on Recommended Conference, Vol. 2, pp. 741-752, St. John’s, Practices for Wind Turbine Testing and Nfld., Canada, 1977. Evaluation, 3. Fatigue Loads, 2nd edition, 1990. Ronold, K.O, J. Wedel-Heinen, and C.J. Christensen, Reliability-based fatigue design Larsen, G., and P. Sørensen, “Design Basis of wind-turbine rotor blades, Engineering 2,” Proceedings, IEA Symposium “State-of- Structures, Elsevier Science Ltd., Vol. 21, the-Art of Aeroelastic Codes for Wind No. 12, pp. 1101-1114, 1999. Turbine Calculations,” pp. 137-145, Lyngby, Denmark, 1996. Stiesdal, H., “Rotor Loadings on the BONUS 450 kW Turbine,” Proceedings, Madsen, H.O., S. Krenk, and N.C. Lind, EWEC’91, Amsterdam, the Netherlands, Methods of Structural Safety, Prentice-Hall 1991. Inc., Englewood Cliffs, N.J., 1986. U.S. Army Coastal Engineering Research Madsen, P.H., K. Pierce, and M. Buhl, “The Center, Shore Protection Manual, Vols. I- use of aeroelastic wind turbine response III, Washington, D.C., 1973. simulations for prediction of ultimate design loads,” Proceedings, 3rd ASME/JSME Joint Veers, P.S., Three-Dimensional Wind Fluids Engineering Conference, Paper No. Simulation, Report No. SAND88-0152, FEDSM99-S295-10, San Francisco, Cal., Sandia National Laboratories, Albuquerque, 1999. N.M., 1988.

Mann, J., “Wind field simulation,” Viterna, L.A. and R.D. Corrigan, Fixed pitch Probabilistic Engineering Mechanics, rotor performance of large horizontal axis Elsevier Science Ltd., Vol. 13, No. 4, pp. wind turbines, DOE/NASA Workshop on 269-282, 1998. Large Horizontal Axis Wind Turbines, Cleveland, Ohio, July 28-30, 1984. Mann, J., “The spatial structure of neutral atmospheric surface-layer turbulence,” Winterstein, S.R., “Nonlinear Vibration Journal of Fluid Mechanics, No. 273, pp. Modes for Extremes and Fatigue,” Journal 141-168, 1994. of Engineering Mechanics, ASCE, Vol. 114, No. 10, pp. 1772-1790, 1988.

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Winterstein, S.R., E. Bitner-Gregersen, and K.O. Ronold, “Statistical and Physical Models of Nonlinear Random Waves,” Proceedings, International Conference on Offshore Mechanics and Arctic Engineering (OMAE), Vol. II , pp. 23-31, Stavanger, Norway, 1991.

Wirsching, P.H., and A.M. Shehata, “Fatigue under Wide Band Random Stresses Using the Rain-Flow Method,” Journal of Engineering Materials and Technology, ASME, July 1977, pp. 205-211.

4 – Loads 103 Guidelines for Design of Wind Turbines − DNV/Risø

5. Rotor It is important that the blade sections near the hub are able to resist forces and stresses from the rest of the blade. Therefore, the 5.1 Blades blade profile near the root is both thick and Rotor blades are usually made of a matrix of wide. Further, along the blade, the blade fibreglass mats, which are impregnated with profile becomes thinner so as to obtain a material such as polyester, hence the term acceptable aerodynamic properties. As the glass fibre reinforced polyester, GRP. The blade speed increases towards the tip, also polyester is hardened after it has impreg- the lift force will increase towards the tip. nated the fibre-glass. Epoxy is sometimes Decreasing the chord width towards the tip used instead of polyester. Likewise, the will contribute to counteract this effect. In basic matrix is sometimes made wholly or other words, the blade tapers from a point partly of carbon fibres, which form a lighter, somewhere near the root towards the tip as but more expensive material with a high seen in Figure 5-2. In general, the blade strength. Wood-epoxy laminates are some- profile constitutes a compromise between times used in large rotor blades. the desire for strength and the desire for good aerodynamic properties. At the root, 5.1.1 Blade geometry the blade profile is usually narrower and tubular to fit the hub. The design of the outer contour of a wind turbine rotor blade is based on aerodynamic considerations. The cross-section of the blade has a streamlined asymmetrical shape, with the flattest side facing the wind. Once Figure 5-2. Side view of a blade. the aerodynamic outer contour is given, the blade is to be designed to be sufficiently strong and stiff. The blade profile is a hollow profile usually formed by two shell structures glued together, one upper shell on the suction side, and one lower shell on the pressure side. To make the blade sufficiently strong and stiff, so-called webs are glued onto the shells in the interior of the blade, thus forming a boxlike structure and cross- section, see Figure 5-1. From a structural point of view, this web will act like a beam, and simple beam theory can be applied to Figure 5-3. View of a blade from the tip, illustrating the model the blade for structural analysis in twist of the blade: the wind comes in from the left, and the pitch is 0° at the tip and about 25° at the hub. order to determine the overall strength of the blade. The blade is twisted along its axis so as to

enable it to follow the change in the direction of the resulting wind along the blade, which the blade will experience when it rotates. Hence, the pitch will vary along Figure 5-1. Section of a blade showing upper and lower the blade. The pitch is the angle between the shells and two webs, respectively. chord of the blade profile and the rotor plane, see Chapter 4. Figure 5-3 illustrates

104 5 – Rotor Guidelines for Design of Wind Turbines − DNV/Risø the twist of a blade. Note in this context that direction of rotation at the root than at the the pitch angle, which is referred to tip. Relative to the rotor axis, the force at the throughout this document, usually refers to root has a smaller torque arm than the force the collective rotation of the entire blade at the tip and will therefore give about the relative to the rotor plane. same contribution to the starting torque as the force at the tip. For a blade, four non-coincident trajectories can be defined: During operation, the wind approaching the • mass axis, the spanwise locus of section blade profile constitutes the vectorial sum of mass centres the farfield wind speed perpendicular to the • elastic axis, the spanwise locus of points rotor plane and the head wind due to the about which no section is exposed to rotational movement of the blade through bending deflection the air. Smaller aerodynamic forces are • control axis, the axis of mechanical produced near the root than at the tip. feathering, which is determined by the However, the forces produced near the root blade retention and pitching mechanism are more aligned with the direction of • aerodynamic axis, the blade section rotation than the forces near the tip. The quarterchord for a conventional airfoil change in magnitude and direction of the shape within linear performance limits forces along the blade contributes to These four trajectories are not axes in the determine the shape and design of the blade, true sense, since they – owing to the including the width, thickness and twist of geometry of the blade – are not straight the blade. lines. However, for calculations for a particular section of the blade, four axes, The design loads for a blade can be perpendicular to the section, can be defined determined by means of blade momentum as the tangents to the trajectories at their theory and aeroelasticity. For details, respective intersections with the section. reference is made to Chapter 4.

5.1.2 Design loads 5.1.3 Blade materials In principle, the same airflow conditions Wind turbine blades are made of lightweight would apply at all sections along a blade as materials to minimise the loads from long as the profile stays the same, while the rotating mass. magnitude and direction of the forces would change depending on the distance to the tip. A rotor blade is built up of the following However, in practice, the profile and the elements: blade thickness vary along the blade and • external panels - form the aerodynamic thereby make the airflow conditions more shape and carry a part of the bending complex. load • internal longitudinal spars/webs – carry At stand-still, the wind pressure force will – shear load and a part of the bending depending on the load case – be somewhat load, restrain the cross section against larger at the root than at the tip. This is due deformation and the panels against to the fact that the blade is wider at the root. buckling. The force is acting roughly at a right angle • inserts like bushings - transfer the loads to the flat side of the blade profile. As the from the panels and spars into the steel blade is more twisted at the root, a larger hub. component of the force will act in the

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• lightning protection – carries a lightning Carbon fibre is used for aerodynamic brake hitting the blade tip to the root shafts. • aerodynamic brake – for some types of turbines with fixed pitch an Glass and carbon fibres come in different aerodynamic brake is part of the types having different chemical com- protection system. The aerodynamic positions. The most important type of glass brake is typically the tip turning on a is E-glass. shaft. The mechanical properties, modulus and Fibre reinforced plastics (FRP) is the tensile strength, may vary significantly favoured group of materials for external between different types of fibres and can panels, internal spars/webs and shafts for also vary considerably within each type of aerodynamic brakes. fibre. There are for example low, medium and high strength carbon fibres. The FRP is materials where fibres are used for selection and qualification of fibres is part of transferring the global loads and a polymeric the blade design. resin is used to distribute the load between the fibres and to restrain the fibres against It is important to realise that the final local relative displacements. properties of a FRP laminate not only depend on the properties of the fibres. The FRP is often used on both sides of a cellular properties of the matrix, the structural core in a sandwich structure. The core in a interaction between fibres and matrix and sandwich structure is used for increasing the the volume fraction of fibres are also local bending stiffness of the panel or web. important. An increased local bending stiffness may be required to avoid buckling when subjected Sizing is deposited on the surface of the to compressive or shear loading. fibre during production. The purpose of the sizing is to increase the strength of the bond Gelcoats and topcoats are used to protect the between the fibre and the resin. Sizing can FRP against abrasion, UV radiation and also be applied to enable easy handling moisture and to give the right colour. without inducing defects in the fibres during blade manufacturing. A particular sizing that Wood is a natural material with the same has been developed for one type of type of structure as FRP. Wood is used as an application or one type of resin is not alternative to FRP in design of blade panels necessarily adequate for another application and spars/webs. or another resin. It is important that the supplier of the fibre material provides Further information about FRP can be found adequate information about this and that the in Mayer (1992), in DNV (1999) or on supplier's recommendations are followed www.marinecomposites.com. during blade manufacturing.

FRP Reinforcements The reinforcement fibres for FRP panels are Both glass fibre and carbon fibre is used in supplied in fabrics/mats. manufacturing the blade panels and internal spars. The basic building blocks of reinforcement fabrics are:

106 5 – Rotor Guidelines for Design of Wind Turbines − DNV/Risø

• roving where long fibre strands are Resins arranged in bundles with little or no Polyester, vinylester and epoxy are the most twist. commonly used resins in wind turbine • Chopped Strand Mat (CSM) where the blades. Polyester is dominating with epoxy orientation of relatively short fibre being used in structurally more demanding strands is random. Only glass fibres are applications. supplied as CSM. The resin consists of a base into which From these the following basic types of hardener is mixed to initiate the cross- fabrics are pre-fabricated: unidirectional linking process (sometimes aided by the (UD), woven roving, angle-ply and addition of a catalyst). In addition multiaxial fabrics. Woven rovings and accelerators or inhibitors may be added to multiaxial fabrics often have a light (100 adjust the gel-time and cure time to the g/m2 or 300 g/m2) CSM bonded to one or working temperature and the characteristics both sides to enhance the interlaminar bond of the operation to be carried out. The to adjacent plies in which case they are possibility for adjusting the gel-time and called combination mats. Fabrics may be cure time is relatively large for polyester. bonded together by a bonding agent (which For epoxy variation of these parameters will later is dissolved in the polyester) or normally have an effect on the properties of mechanically by stitching a yarn through the the cured resin. Waxes may be added to thickness. polyester to control evaporation of styrene and exposure to oxygen during curing. Different types of fibres, e.g. glass and carbon, may be combined in one fabric. In addition other compounds may be added Such fabrics are referred to as hybrid for specific reasons, e.g. low cost fillers, fabrics. pigments and thixotropic agents which can stabilize the resin on a vertical surface. The A complete set of reinforcement or fabrics effect of such additives on the properties of in one laminate is defined by its stacking the resin can be significant and shall be sequence. The stacking sequence specifies evaluated. the sequence of plies (starting from one side of the laminate) and their respective Core materials orientation, with respect to a reference The most common core materials are direction. structural foams and wood products.

Fabrics are defined, apart form the type and Foams are based on thermoplastics, e.g. stacking sequence, also by the total weight PVC, and are delivered in a range of of reinforcement per unit area, usually densities. The most important mechanical expressed as g/m2. properties are the shear modulus, shear strength and ductility or yielding behaviour. Note that carbon fibres used as Stiffness and strength increases with the reinforcement of the rotor blades will cause density. Different types and grades of foam galvanic corrosion of any steel parts they get can have significantly different properties. in touch with, unless stainless steel is used Detailed information on the properties is for the making of these parts. given in the manufacturer's product specification. PVC foams may be supplied in a "heat stabilised" condition, which

5 – Rotor 107 Guidelines for Design of Wind Turbines − DNV/Risø provides better dimensional stability and cleaning by solvents may be necessary to reduces the chance of outgassing. meet this requirement for bushings. Outgassing is the release of volatile gasses from the core when the panel has been The thickness of adhesive joints shall be completed. controlled as it has an impact on the strength. Wood core materials include balsa, conventional wood and plywood. Balsa is by 5.1.4 Manufacturing techniques far the most commonly used and is supplied Several manufacturing techniques are in different densities, stiffness and strength applied in manufacturing of wind turbine increasing with density. blades.

Core/Sandwich adhesives are used to bond Wet hand lay-up the sheets/pieces together and to fill the A layer of resin is applied in a mould. (The voids between pieces/sheets of core. A good mould has previously been covered with a bond is necessary to maintain the shear release agent and a layer of gelcoat). The capacity of the core. The bond should have first ply of reinforcement fabric is then at least the same shear strength and fracture applied in the wet resin. Subsequently the elongation as the core material. A high remaining plies are applied (according to the ductility of the adhesive normally increases laminate schedule) alternating with appli- the strength of the joint. cation of resin.

Wood During the course of application of all plies Several species of wood can be used for the fabrics are worked using metallic or wind turbine blades. bristle rollers to ensure a thorough wetting

out of the fibres, to compact/consolidate the Wood is applied as plywood or in lamellas laminate and to make sure that all entrapped to minimize the impact of imperfections like air is removed. In addition superfluous resin knots on the strength. is removed using an elastic scraper. For

glass the colour indicates whether It is important for the resistance of the wood satisfactory wet-out has been achieved. This that the water content is low. A high water colour/opacity change is not apparent when content will result in low mechanical values, carbon fibre is used. rot and fungus. The humidity shall be con- trolled during storage and manufacturing. Since the quality of the material is entirely Coating and sealing shall be qualified as part dependent on the system for laying, any of design to govern the long-term water form of automation is beneficial to quality content in the wood. control. Systems involving automatic resin /

catalyst mixing pumps, wet out machines Adhesives (where cloth is weighed dipped in resin and Adhesives are used for joining blades that squeezed through rollers and then reweighed are manufactured in parts and to bond to verify resin ratio before being laid), gel metallic inserts like steel bushings at the time measuring machines, etc. will be of root. It is important for adhesive joints that great benefit. the surfaces are clean and absolutely free from wax, dust and grease. Sandblasting and If the preceding ply has cured completely the operation is no longer a wet-in-wet

108 5 – Rotor Guidelines for Design of Wind Turbines − DNV/Risø lamination and the new bond shall be treated The advantage of this technique is the good as a new bond. The surface preparation for a control of, and the relatively high, volume new bond is critical since there will be no fraction of fibres one achieves and thus effective crosslinking between the consistency in the properties of the laminates. The new applied resin will in this laminates and a lower laminate weight. case act as an adhesive. It is important that the gel time is Wet lay-up may be carried out with vacuum sufficiently long such that the resin has time bagging where an airtight (plastic) mem- to infuse the whole mould before it gels. brane is applied over the hand laid-up lami- nate or core. The air under the membrane is Filament winding sucked out by means of a such that an Shafts for aerodynamic brakes are typically overpressure (from the atmosphere) acts manufactured by filament winding where over the entire laminate. fibres or tapes are pre-impregnated with a resin and wound on a rotating mandrel. The vacuum bagging technique is used to improve the compacting of the laminate and Internal spars can also be fabricated by thus to increase the volume fraction of filament winding reinforcement and to improve the consistency of the finished laminates. 5.1.5 Quality assurance for blade Normally a porous mat is applied on top of design and manufacture the laminate and under the membrane to The blade manufacturer, not the supplier of facilitate even resin distribution and the the raw materials, controls the final escape of the air. Vent holes may be properties of the laminates. required through the core for the same purpose. The blade designer shall consider the

qualification of FRP materials. Properties to Pre-pregs be considered in this context include, but are Plies are supplied as fabric saturated with not limited to: resin that will cure when heated. The plies • are stored cold. The complete stack of plies stiffness, ultimate and fatigue strength is laid-up in the mould. The laminates in the at relevant temperatures • mould is vacuum bagged and cured by toughness (at low temperatures if heating the mould. The applied vacuum appropriate) • consolidates the laminate and removes creep entrapped air. • ageing characteristics (considering humidity and temperature) Vacuum-assisted resin transfer moulding • resistance of wood to rot and fungus (VARTM) The technique has similarities to vacuum- Note that for some materials, some of these bagging. The complete set of reinforcement properties may not be relevant. Guaranteed plies is applied to the mould, but without property values may be given in terms of: adding the resin. The airtight membrane is • manufacturer’s nominal value, put on top and vacuum drawn. The resin is • manufacturer’s specified value, or then transferred via piping/hoses from a • manufacturer’s specified minimum container of premixed resin by the suction value. created by the vacuum.

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A type approval of a given material, e.g. in For each load case, a set of design loads is accordance with DNV standards, can be established by multiplying the relevant used as part of the quality control and design characteristic loads by partial safety factors documentation, but is in itself not sufficient for load. The standards applied for this for approval of the material for its intended purpose should be quoted when the design use. loads are documented. In principle, each load case can be defined in terms of six load Full-scale tests of a sample blade are components and their variation over the required to verify the strength of the blade, blade span. The resolution used to specify statically as well as in fatigue as not all this variation must be fine enough to allow aspects can be covered by the material for sufficiently accurate calculations in all qualification. points of interest along the blade, especially in all critical areas, e.g. wherever changes in FRP material specimens for testing shall be geometry or material occur. manufactured with a curing cycle that is representative for the blade and have a Environmental conditions, which affect the representative fibre to resin ratio. material behaviour, should be considered and taken into account. In particular, such Test specimens should be wide enough to conditions include humidity and tempera- cover at least four repetitions of the structure ture, which may both lead to degradation of of the weave/fabric/mat. strength and stiffness, and their design effects calculated by multiplying charac- Ultimate strength of FRP and wood shall be teristic effects by appropriate partial safety investigated in both tension and factors should be applied in the strength compression. Fatigue testing shall cover analysis. both the effects of stress width and the mean stress. Loads on critical components such as tip brakes are often different in character from The fibre alignment is important for the load the general loads on the blades and may thus carrying capacity of FRP in compression. need extra attention. The work instructions for blade manu- facturing shall control the fibre alignment. As blades are getting larger, large unsupported panels will be present between 5.1.6 Strength analyses the webs and the leading and trailing edges. This may have an impact on the stability of Structural analyses of the rotor blades must the blades. Therefore, the buckling capacity be carried out for all relevant load cases in of a blade must be verified by a separate order to verify that the strength of the blades calculation, in addition to the full-scale test. is sufficient to withstand the loads that these For this calculation, a FEM analysis will load cases exert on the blades. By the normally be required. Furthermore, buckling strength calculations in these analyses, it of the webs may also have to be considered. must be verified that both the ultimate strength and the fatigue strength, for a given As rotor blades become longer, evaluation design life, are sufficient. For structural of the stability against buckling becomes parts in compression, stability against still more important because of the large buckling must also be considered. unstiffened panel segments, which are

usually involved.

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Standards factor, i.e. a partial safety factor for materials, The following standards are normally used cf. Section 2.3. for verification of wind turbine blades: • DS472, “Last og sikkerhed for In general, each section of interest along the vindmøllekonstruktioner” (“Load and blade should be checked, as well as all six Safety for Wind Turbine Structures”, in load components in that section, to ensure Danish), DS472, 1st edition, the Danish that the calculated design load does not Society of Chemical, Civil, Electrical exceed the corresponding calculated design and Mechanical Engineers (Dansk capacity. Ingeniørforening), Copenhagen, Den- mark, 1992. In this context, the tensile strength σF,T in the • DS456 “Konstruktioner af direction of the fibres is one of the strengths, glasfiberarmeret umættet polyester” which is important to consider. This strength (“Structural Use of Glass Fibre is dominated by the strength σF,B of the fibre Reinforced Unsaturated Polyester”, in bundles. The strength σF,B of a fibre bundle is Danish), DS456, 1st edition, the Danish proportional to the mean failure stress σ of Society of Chemical, Civil, Electrical the individual fibres and Mechanical Engineers (Dansk Ingeniørforening), Copenhagen, Den- exp(1/ m) σ = σ mark, 1985. F ,B 1 m1 mΓ(1+ ) m Ultimate strength

When the direction of the load is time- m material constant dependent, information about phase and Γ the gamma function frequency should be given. For each section of interest along the blade, the design loads have to be calculated. In principle, all six load Reference is made to Beaumont and Schultz σ components need to be calculated. Normally, (1990). The individual fibre strengths very the bending moments and the shear forces are often follow a Weibull distribution, most critical, but also the torsional moment however, their mean will under the central and the axial force can, in some cases, be limit theorem follow a normal σ σ important for the design. distribution. and hence also F,T can then be deduced to be normally distributed. The σ Once the necessary information regarding normal distribution prerequisite for F,T is material strength and stiffness, geometry of important when a particular lower-tail blade and lay-up of laminate is established, quantile of the distribution of σF,T is to be the capacity of the blade can be calculated interpreted and used as the characteristic section by section. This can be done more or value for σF,T. Note that very erroneous less by hand, or by some calibrated finite numbers may result for the characteristic element programs. In principle, all six values of σF,T, if they are interpreted on the capacity components need to be calculated, basis of a Weibull distribution assumption e.g. flapwise and edgewise bending moment for σF,T. capacities. When characteristic values for material strength are used as input, The characteristic value for a strength characteristic capacity values result. The property is usually defined as a particular design capacities are then found by dividing quantile in the probability distribution of the the characteristic capacities by a materials property. This is often a quantile in the

5 – Rotor 111 Guidelines for Design of Wind Turbines − DNV/Risø lower tail of the distribution, e.g. the 2% or components should be given in all points of 5% quantile. Note that different standards relevance along the blade, including phase may define the characteristic value and frequency information. This differently, i.e. it is not always defined as requirement is automatically fulfilled when the same percentile in the different the six load components are given as time standards. Accordingly, different standards series. From the time series of the six load may prescribe different partial safety factors components, long-term stress distributions to be used with their respective can be established in all points of relevance. characteristic values for design. As an In principle, this includes distributions of the example, DS472 defines the characteristic mean stress as well as distributions of the value of an FRP material as the 5% quantile stress range that represents the variation and requires a materials factor of 1.7, while about the mean stress. Rain-flow counting is DS456 defines the characteristic value as the a commonly used method for this purpose. 10% quantile and requires a materials factor The total number of stress cycles in the of 1.8. In design, it is essential that the design life can also be extracted from the characteristic value of a strength-property in time series and can be used to transform the question is combined with the correct partial stress range distribution into a design safety factor. It is not licit to combine the lifetime histogram of stress ranges. A characteristic value of one standard with the sufficiently fine discretisation of the stress partial safety factor of another as this can range axis needs to be chosen for this lead to erroneous results and unsafe designs. purpose.

Stability In each section of interest along the blade it The longer the blades are, the more likely it must be verified that the fatigue strength is is that stability against buckling will govern not exceeded. This is in practice done by the design of the blades instead of ultimate checking that the Miner’s sum, calculated strength. The stability against buckling can for the design stress range histogram in most easily be verified by calculations by conjunction with the design S-N curve, does means of a properly calibrated finite element not exceed a critical value, usually equal to program. Performing such a calibration by 1.0. See Appendix C for further reference. hand is more difficult owing to the complex geometry of a rotor blade, and well-proven Fatigue in edgewise bending is dominated tools for this purpose, such as closed-form by gravity loads and depends, to a large solutions for the buckling capacity, are not extent, on the weight of the blade and on the available. actual number of rotations of the rotor during the design life. Fatigue in flapwise When designing for a sufficient stability bending is dominated by the blade response against buckling, it is a standard approach to to aerodynamic loads exerted by the wind. apply an extra design margin to take effects Note that transient loads during start/stop of geometrical imperfection, fibre mis- and loads due to yaw errors may give alignment, workmanship, etc. into account. significant contributions to the cumulative fatigue damage and need to be given Fatigue strength thorough consideration in addition to the Sufficient fatigue strength must be loads that occur during normal operation. documented. This applies to all sections For an example of fatigue calculations and along the blades and to all directions in each fatigue design, see Section 2.3.7. location. For this purpose, all load

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Frequency computer code, but it can also be calculated As a minimum, the two lowest eigen- by hand or by some finite element program. frequencies of the rotor blade, both in Knowing the initial distance from the blade flapwise and edgewise oscillation, should be tip to the tower in the no-load condition, this calculated. These eigenfrequencies should allows for determination of the clearance be compared to the rotational frequencies of between the rotor blade and the tower. The the wind turbine. A sufficient margin to clearance should be determined by using the these frequencies must be available to avoid most unfavourable combination of geo- resonance of the blade. It is recommended to metrical tolerances and characteristic keep the eigenfrequencies outside a range stiffness properties of the rotor blade and its defined as the rotational frequency ±12%. supports. The effect of damping may be important and should be considered. Further, Calibration of design tools if the rotor or its supports is subject to creep, It is essential only to use well-proven and shrinkage, temperature deformations or properly calibrated design tools. Especially degradation with time, this must be allowed in case of advanced computer programs, for in the clearance measure. such as finite element programs, it is important to tune or calibrate the results When comparing the resulting available from use of the computer program models clearance between the rotor blade tip and the against results achieved from full-scale tests tower, a specified minimum clearance must on rotor blades, thereby obtaining the be met. Danish rules require that the models which the best possible reflect clearance shall be calculated for the reality. As calibration results obtained from characteristic extreme load on the blade a one bladed model cannot automatically be times a safety factor of 1.3, and that this transported and rendered valid for a model clearance shall at least be 0, i.e. the blade of another type of blade, caution must be shall not hit the tower when subjected to the exercised when it comes to generalisation of extreme design load. Dutch rules have a model calibration results. similar requirement, but this is stricter in the sense that the safety factor on the load is to Delamination be taken as 1.5 rather than 1.3. Delamination may occur if the shear strength of the fibre reinforced laminate is In order to increase the distance between the insufficient to withstand the shear loads that blade tip and the tower during operation, the occur in the blades. It is in this context rotor can be coned, the blade can be important to consider also a sufficient fibre produced with a predeflection, and the rotor strength perpendicular to the blade axis. plane can be tilted. A tilt angle of about 5° between the rotor axis and the horizontal 5.1.7 Tip deflections plane is quite common.

A deflection analysis of a blade must be Normally, the stiffness and mass properties carried out. As part of the deflection analysis of the blade, as used in the tip deflection it must be proven, for all load cases, that the calculations, have to be verified through the ultimate tip deflection (caused by a static test of the blade. load or by an interaction of dynamic loading and structural response) is acceptable. 5.1.8 Lightning protection

The tip deflection can normally be Lightning striking a rotor blade may cause calculated by means of an aeroelastic damage to the blade such as – in the extreme

5 – Rotor 113 Guidelines for Design of Wind Turbines − DNV/Risø case – peeling at the back edge. Lightning blades, i.e. blades longer than about 20 m, it protection of rotor blades can, in principle, may be necessary to secure the blades be designed in two different ways: against damage from lightning that strikes • the lightning can be prevented from the blade in other locations than the blade penetrating into the blade by diverting tip. Note also that it is necessary to account the current along a prepared current for the limited conductivity of the carbon path, such as a conductive tape, on or in fibre materials that are used in tip shafts. the surface of the blade. From the aircraft industry, methods exist for • the lightning can be directed from the lightning protection of glass fibre and point of stroke on the blade through the carbon fibre materials by which these blade by way of a conductor cable. materials are made electrically conductive by means of metallic sheets, nets, or threads, rather than by mounting metallic conductors on the material surfaces. Reference is made to DEFU (1999). Examples are shown in Figure 5-4.

5.1.9 Blade testing The purpose of rotor blade testing is to verify that the laminates in the blade are structurally safe, i.e. that the plies of the rotor blade do not separate by delamination in static or cyclic loading. The purpose is also to verify that the fibres do not fail under repeated loading. Full-scale blades are used for the rotor blade tests, and tests are carried out for verification of fatigue strength as well as static strength.

Figure 5-4. Examples of how lightning protection of For every new type of blade to be manu- blades can be arranged From DEFU (1999). factured, one blade is to be tested statically and dynamically to verify the strength of the The latter approach is the most common blade design. approach. By this approach, a lightning conductor is mounted in the blade tip, and The production of a test blade is to be through a commutator lug in the interior of inspected in order to verify that the blade is the blade, the current is directed to the blade representative of the blade design. root. The effectiveness of this method is Alternatively, the blade may be sampled at expected to be highly dependent on the size random from the blade production. How- of the blade and on the amount of metals or ever, this alternative approach is usually not carbon fibres used in the blade. Note that a feasible, because it is often desirable to carry lightning protection like the ones presented out the blade test in the early phases of a here cannot be expected to work in all cases. production. Therefore, the test blade is often A blade may be hit by many lightning selected on a deterministic basis as one of strokes over its design life, and there may be the first blades in the production series. cases for which the protection will fail, e.g. if the conductor cable fuses. For large

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The test blade is to be equipped with strain Furthermore, the lowest natural frequencies gauges and displacement transducers. The and corresponding damping ratios of the measurement results from the strain gauges blade are measured as these are important are continuously monitored on computers. input parameters for the load calculations. Nonlinear variations in the pattern of This is achieved by excitation of the blade at bending may reveal a damage in the rotor different frequencies and in different blade structure. directions. It is essential that the natural frequencies of the blade do not coincide The blade is to be tested statically by a static with the rotational frequencies of the wind load in two opposite directions flapwise and turbine. The purpose of this part of the test in two opposite directions edgewise. The is to make sure that the natural frequencies static load must at least equal the extreme of the blade differ from the rotational design load. Tests in two opposite directions frequency with sufficient margin. Note that instead of just in one direction are necessary the aerodynamic damping is also measured, due to lack of symmetry in the blade. in particular, in the flapwise direction. Measurements by means of the strain gauges and displacement transducers are used to In case the natural damping of the blade is monitor and verify that the strains and insufficient to avoid vibrations, e.g. displacements stay within the design limits edgewise vibrations, a damper may be built and design calculations during the entire into the blade. The damper may consist of test. Nonlinearity in the measured strains materials with high internal damping, which may indicate buckling or damage to the is built into the blade, or it may be a blade. mechanical damper. It is essential that this damper is also included in the test blade, Dynamic testing is also carried out for since the design of the connection between loading in both flapwise and edgewise the damper and the blade may introduce directions. Normally, the blade is tested for weak areas or large stiffness changes. loading in one direction at a time, but a load with simultaneous components in both A rotor blade can also be tested for its directions may also be applied. The dynamic residual static strength (and thus its ability to testing is carried out as an accelerated test, withstand extreme loads in the long term) by i.e. at a load level, which causes the same being bent once by a very large force. The damage to the blade as the true load magnitude of this force is usually spectrum, including a test factor of 1.3. determined by the load level of the static test Normally, the test is carried out at a fixed described above. The residual strength test is load level and for a number of load cycles of usually performed after the blade has been between 2⋅106 and 5⋅106 cycles. subject to fatigue testing, and the purpose is to verify that the static strength of a blade, Infrared cameras can be used to reveal local which has been in operation for a substantial build-up of heat in the blade. This may amount of time, is sufficient and that the either indicate an area with structural stiffness – although it may have changed – damping, i.e. an area where the blade is still acceptable. designer has deliberately laid out fibres which convert the bending energy into heat A test specification for the blade test should in order to stabilise the blade, or it may be worked out. The specification should indicate a zone of delamination, or a zone specify the loads and the measurement set- where the fibres are close to failure. up, including means of avoiding overload of

5 – Rotor 115 Guidelines for Design of Wind Turbines − DNV/Risø the blade. This is an important aspect, as the dynamic test is normally carried out at a frequency close to the lowest flapwise/ edgewise natural frequency, at which even small variations in the energy input may significantly alter the load amplitude. The test loads must be determined in such a way that it is ensured that major parts of the blade are tested to the required load level. Note that in case the test is running outdoor, weather conditions such as sun and rain may influence the resulting blade deflections.

During testing the stiffness of the test rig must be considered. Ideally, the stiffness of the test rig should be comparable to that of Figure 5-5. View of wind turbine hub. the blade hub connection when the blade is The moments and forces transmitted to the installed on the turbine. As a minimum, the hub and tower depend on the type of hub. stiffness of the test rig must be measured to Three types of hub are common: allow for adjustment of the measured • deflections. hingeless rigid hub, has cantilevered blades and transmits all moments to the Guidance on blade testing is given in IEC tower. • 61400-23 TS Ed. 1: “Wind turbine generator teetering rotor, has two rigidly systems - Part 23. Full-scale structural connected blades supported by a teeter- testing of rotor blades for WTGS's”. pin joint, which can only transmit in- plane moments to the hub. Flapwise 5.1.10 Maintenance moments are not transmitted. • articulated hub, has free hinges in The blade should be kept clean to ensure flapping and lead-lag, so there is no that the aerodynamic properties of the blade mechanical restraint moment on the remain unchanged. blades in either flapping or lead-lag. The hingeless hub is the most common configuration for wind turbine hubs. 5.2 Hub The hub is the fixture for attaching the Figure 5-5 shows an example of such a hub, blades to the rotor shaft. It usually consists and Figure 5-6 and Figure 5-7 show the hub of nodular cast iron components for in the context of the transmission system, in distribution of the blade loads to the wind which it forms part of the link between the support structure, i.e. ultimately to the rotor blades and the generator. Figure 5-6 tower. A major reason for using cast iron is and Figure 5-7 provide examples of two the complex shape of the hub, which makes different bearing arrangements with one and it hard to produce in any other way. In two main bearings, respectively. For details, addition hereto, it must be highly resistant to see Section 6.2. metal fatigue. Thus, any welded hub structure is regarded as less feasible.

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Flexible blades have the advantage of relieving some of the load into centrifugal terms. Since yaw stability and fatigue life are both significantly affected by blade flexibility, other considerations apply in this context.

The hub should be constructed in such a Figure 5-6. Transmission system consisting of hub, one way that it will be possible to adjust the tip main bearing, main shaft, gear and coupling. Courtesy angle of the blade and tighten up the bolt Bonus Energy A/S. connections.

5.2.2 Strength Analysis Cast components should be designed with smooth transitions and fillets in order to limit geometrical stress concentrations.

The characteristic resistance for cast iron components can be taken as the 5th or 95th percentile of test results, whichever is the most unfavourable.

Figure 5-7. Transmission system consisting of hub, two main bearings, main shaft and gear, and with the The structural resistance in the ultimate limit generator marked out. state is to be determined by elastic theory. When design is carried out according to the 5.2.1 Determination of design loads allowable stress method, the maximum The loads at the blade-hub interfaces to be allowable stress should be taken as the considered at the blade root for design of the characteristic resistance divided by a safety hub consist of the following: factor. • full flapping moment • flapping shear, resulting thrust on one Fatigue design can be carried out by blade methods based on fatigue tests and • lead-lag moment, power torque of one cumulative damage analysis. When design is blade, and gravity loads carried out according to the allowable stress method, the allowable fatigue stress range is • lead-lag shear, in-plane force that to be taken as the characteristic resistance produces power torque divided by a safety factor. The characteristic • centrifugal forces • resistance for a given number of cycles to pitching moments of one blade failure is defined as the stress range that Design loads can be calculated from the corresponds to 95% survival probability. blade loads in accordance with this list. The characteristic resistance should be modified to account for size effects, surface Obviously, it is structurally beneficial to conditions and mean stress. When limited reduce the loads at the blade-hub interfaces, test data are available for estimation of the in particular, the large blade-flapping characteristic resistance, the characteristic moment. The blade-root flapping moment resistance should be given with 95% can be traded off against tip deflection. confidence.

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The layout of a wind turbine hub often • treatment of treads, e.g. waxing makes it difficult to determine which section • torque to be applied is the structurally most critical section of the The relation between bolt torque and bolt hub. In this context, the Finite Element tension may be subject to test. Method (FEM) forms a suitable tool for strength analysis of the hub and can be used 5.2.4 Hub enclosure in conjunction with state-of-the-art fatigue analysis techniques to determine the fatigue The hub enclosure, which is sometimes life and to optimise the design with respect referred to as the nose cone, is usually made to strength and cost. It is advisable to qualify of glass fibre reinforced polyester. In cases the loading assumptions for such FEM where the hub enclosure is large, it is analyses by measurements. recommended to consider the wind load it will be exposed to. The FEM analyses can, in particular, be used to document that a satisfactory strength 5.2.5 Materials is available in critical sections, such as at Spheroidal graphite cast iron, also known as stress concentrations and stiffness nodular cast iron, is the preferred material transitions, as well as at the shaft-hub for the hub. Cast iron is classified according connection and the interfaces to the blades. to its mechanical properties, such as strength and hardness, in EN1563. Cast hubs are 5.2.3 Analysis of bolt connections usually tested by non-destructive testing The blades are usually bolted to the hub. (NDT) for verification of the mechanical There are two techniques for mounting the properties and for detection of possible bolts in the blades: defects and internal discontinuities. The • a flange is established at the blade root following NDT methods are available: • by moulding the glass fibre reinforced ultrasonic inspection • plastics to form a ring, in which steel magnetic particle inspection bushes for the bolts are embedded. • visual inspection • treaded steel bushes are mounted • hardness measurements directly into the blade root and fixed to Ultrasonic inspection can be carried out as the blade by glue. point testing or, more thoroughly, as In both cases, the bolts from the blade pass complete scanning. For ultrasonic through a flange on the cast hub. The bolt inspection, it is common to assign different holes in this flange can be made somewhat acceptance criteria, e.g. in terms of different elongated to enable adjustment of the tip allowable defect sizes, to different areas of angle. the cast hub. These areas and the assigned acceptance criteria should be indicated on For these bolt connections, a bolt tension the drawing of the casting. Usually, a strict procedure is usually required. Such a acceptance criterion is assigned to an area procedure should usually specify: with high stresses, whereas a more lax • bolt, nut, washer type, dimension and acceptance criterion is assigned to an area quality with low stresses. The stricter the • flatness tolerances for surfaces acceptance criterion is in a particular area, • roughness of surface the more thorough is the ultrasonic inspection in that area, and the stricter are • surface treatment and protection the requirements to the allowable size of • bolt tensioning sequence and method detected discontinuities.

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It is important to consider whether the Hück, M., W. Schütz, and H. Walter, chosen structural material possesses the “Modern Fatigue Data for Dimensioning necessary ductility. Low temperatures can Vehicle Components of Ductile and be critical for cast hubs in this respect, and Malleable Cast Iron,” first published in the choice of hub material should, therefore, Automobiltechnische Zeitschrift 86, Nos. 7, be made with due consideration to the 8 and 9, 1984. temperatures of the surroundings. Mayer, R.M. “Design with reinforced Note that repair of cast hubs by means of plastics”, The Design Council, London, welding is not permitted. 1992.

5.2.6 Standards EN1563 Founding – Spheroidal graphite cast irons. CEN, 1997.

EN1369 Founding – Magnetic particle inspection. European Standard. CEN, 1996.

REFERENCES

Beaumont, P.W.R., and J.M. Schultz, “Statistical Aspects of Fracture,” Chapter 4.6 in Failure Analysis of Composite Materials, Volume 4, ed. by P.W.R. Beaumont, J.M. Schultz, and K. Friedrich, Technomic Publishing Co., Inc., Lancaster, Penn., 1990.

Danske Elværkers Forenings Undersøgelser (DEFU), Lynbeskyttelse af vindmøller. Del 7: Vinger DEFU Report No. TR394-7 (in Danish), 1999.

DNV, Rules for Classification of High Speed and Light Craft, Materials and Welding, Part 2, Chapter 4, Composite Materials, Det Norske Veritas, Høvik, Norway, 1999.

Hück, M., Berechnung von Wöhlerlinien für Bauteile aus Stahl, Stahlguss und Grauguss, Verein Deutscher Eisenhüttenleute Bericht Nr. ABF 11 (in German), July 1983.

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6. Nacelle Figure 6-1 illustrates the loads and reactions that the main shaft is subjected to. Under normal circumstances, reactions in the main 6.1 Main shaft shaft can be calculated from equilibrium The main shaft transmits the rotational considerations. energy from the rotor hub to the gearbox or directly to the generator. Moreover, the mgear My purpose of the main shaft is to transfer loads to the fixed system of the nacelle. In addition to the aerodynamic loads from the rotor, the main shaft is exposed to gravi- Mz tational loads and reactions from bearings R2v and gear. mhub R1h Fy R1v The main shaft is also subjected to torsional Fx My vibrations in the drive train. Such vibrations Mx will usually be of importance to possible Figure 6-1. Loads and reactions on the main shaft. frictional couplings like shrink fit couplings between shaft and gear. For each load case, a set of design loads are established by multiplying the relevant A wind turbine can be exposed to large characteristic loads by a partial safety factor transient loads. Therefore, it has to be for loads. The loads can be combined by considered whether the chosen structural simple superposition. material possesses the necessary ductility. This is particularly important if the turbine Fatigue loads consist of histories of stress is to be operated at low temperatures. Since amplitudes. One history per load component corrosion may imply a considerable exists. Hence, a combination of fatigue loads reduction of the assumed fatigue capacity, it implies a combination of stress histories. should be ensured that the shaft is protected Unless the phase differences between the against corrosion. Suitable quality assurance individual load components are known, the should be implemented to make sure that the largest stress amplitude of one load geometrical and mechanical assumptions for component is to be added to the largest the design are fulfilled, e.g. surface stress amplitude of each of the other load roughness, that the specified values of components, the second largest amplitude is material parameters are met, and that the to be added to the second largest amplitude imperfections of the material do not exceed of each of the other load components, and so any critical level. forth.

6.1.1 Determination of design loads 6.1.2 Strength analysis In the following, it is assumed that all rele- Structural analysis of the main shaft shall be vant load cases are taken into consideration. carried out for all relevant load cases in Selection of the relevant load cases and order to verify that the strength of the shaft determination of the characteristic values of is sufficient to withstand the loads to which the individual load components on the rotor it is subjected. can be made according to procedures given in Sections 4.4 - 4.8 for both extreme loads and fatigue loads.

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It shall be verified that both the ultimate Geometrical size effect strength and the fatigue strength are The reduction due to the geometrical size sufficient for the actual design life. effect is based on the fact that specimens of different dimensions exhibit differences in 6.1.3 Fatigue strength their fatigue properties, despite the fact that they are made of materials of identical initial The fatigue strength can be expressed in dimensions. This can be accounted for by terms of the particular stress amplitude that means of the influence factor K , applied to leads to failure after a specified number of 2 the fatigue strength. K can be determined constant-amplitude stress cycles. This can be 2 from Figure 6-3. If fatigue strength in pure expressed in terms of an S-N curve, also tension-compression is considered as a basis known as a Wöhler curve, which gives the for the calculations, K can be set equal to 1. number of cycles N to failure at stress 2 amplitude S. Effects of surface roughness

Fatigue cracks are often initiated from Various factors influence the strength unevenness in the surface and from small relative to reference values determined from surface cracks. An ideal “reference surface” small specimens in laboratory tests, and may for the test specimens corresponds to a lead to a reduction of this strength. In this polished surface with a surface roughness of respect, the following elements of import- R = 0.05-0.1µm. The surface roughness that ance should, as a minimum, be considered: a can be expected from a careful machining is • technological size effect R = 0.4-1.6µm. A reduction factor K to • geometrical size effect a r • account for the effect of surface conditions surface roughness deviating from the reference surface can be • stress concentrations determined from Figure 6-4. • stress ratio R, defined as the ratio between the minimum stress and the Technological size effect maximum stress with tensile stresses defined as positive and compressive 1 stresses as negative. 0,95 structural Technological size effect steel 0,9 Reduction due to the technological size effect is based on the fact that specimens of Alloy steels K1 0,85 identical dimensions, made of materials of the same kind but of different dimensions, 0,8 have different fatigue properties. Test specimens are made with relatively small 0,75 dimensions (typical diameter d = 5-10 mm) and have had their mechanical properties 0,7 improved as a result of the reduction of the 1 10 100 1000 d [mm] cross-section by forging or rolling. The technological size effect can be accounted Figure 6-2. Technological size effect (d denotes for by means of the influence factor K1, diameter of considered part of shaft), from Roloff and Matek (1994). applied to the fatigue strength. K1 can be determined from Figure 6-2.

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fit couplings and tight bearing fits. A Geometrical size effect considerably amount of stress concentration factors for various common shapes are given 1 in Peterson (1974). The sensitivity of the 0,95 Tension- material to such stress concentrations compression depends primarily on the ratio between the 0,9 yield strength and ultimate strength of the Bending and material, and on the stress gradient in the torsion K2 0,85 considered part of the structure. This so- called notch sensitivity can be accounted for 0,8 by means of the notch sensitivity factor q 0,75 1 0,7 q = + A 1 10 100 1000 1 ρ d [mm]

Figure 6-3. Geometrical size effect (d denotes diameter ρ notch radius of considered part of shaft), from Roloff and Matek (1994). A Neuber factor, which can be determined from Figure 6-5 Note that Ra denotes an average surface roughness. In some literature on the subject, Note that the notch radius appears as “rc” in the peak surface roughness Rz is given rather the drawing in Figure 6-6. than Ra. There is no unambiguous relation between Ra and Rz, but for preliminary calculations one may consider the peak surface roughness Rz equal to approximately 6 times Ra.

Figure 6-5. Notch sensitivity factor q, from Sundström (1998).

Figure 6-4. Reduction factor KR vs. surface roughness Ra, from Sundström (1998).

Stress concentrations Figure 6-6. Shaft lay-out in vicinity of bearings, from Stress concentrations will occur at local SKF (1989). changes in geometry, e.g. where cross- sections alter. This is also the case at shrink

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Whenever a notch is encountered, the notch The geometrical stress concentration factor sensitivity factor q can be used to convert is denoted α and can be determined from the stress concentration factor α into a notch Figure 6-7 and Figure 6-8. factor β to be used instead

Stress concentration factor, torsion β = 1+q(α-1)

2,1 A commonly encountered notch is found by h/r=4 the recess, which is to transfer the axial load 2 from a main bearing. A common lay-out of a h/r=3 shaft in the vicinity of bearings is shown in 1,9 Figure 6-6, and data for the involved α geometrical quantities are given in Table 1,8 h/r=2 6-1. 1,7 Table 6-1. rs ba ha rc 1,6 Mm Mm mm mm

1 2 0.2 1.3 1,5 1.1 2.4 0.3 1.5 0,005 0,01 0,015 0,02 1.5 3.2 0.4 2 r/D 2 4 0.5 2.5 2.1 4 0.5 2.5 Figure 6-8. Stress concentration factor, from Roloff and 3 4.7 0.5 3 Matek (1994). 4 5.9 0.5 4 5 7.4 0.6 5 6 8.6 0.6 6 The following symbols are used in the 7.5 10 0.6 7 figures: D Larger diameter of shaft d Smaller diameter of shaft Stress concentration factor, bending h (D-d)/2 r radius of notch 3 h/r=5 2,9 Stress concentrations due to tight bearing 2,8 h/r=4 fits, corresponding to the ISO 286 shaft 2,7 tolerances from m to r, can be accounted for 2,6 h/r=3 by a stress concentration factor α = 1.1-1.2. α 2,5 2,4 For shrink fit couplings a stress h/r=2 2,3 concentration factor α = 1.7-2.0 can be used. 2,2 2,1 Influence of mean stress Most material data are obtained from tests 2 0,005 0,01 0,015 0,02 carried out as either fully reversed bending r/D tests or as pulsating tension-compression tests, i.e. the stress ratio R = σmin/σmax is −1. Figure 6-7. Stress concentration factor, from Roloff and For tests in torsion, R is usually equal to −1. Matek (1994). When other stress ratios prevail than those represented in available tests, it may be

6 – Nacelle 123 Guidelines for Design of Wind Turbines – DNV/Risø necessary to reduce the maximum allowable use the following estimate, which refers to stress range. However, for main shafts the the mean S-N curve with 50% failure stress ratio often assumes values near to –1 probability such that it may not be necessary to consider any such reduction. In more accurate σD50% = (0.436×Re+77) calculations, one may take into account the influence of the mean stress by means of Re yield strength in MPa methods available for this purpose, e.g. a

Haig diagram. Reference is made to σD50% in units of MPa applies to polished Gudehus and Zenner (1999), Bergmann and test specimens of small dimensions, 7-10 Thumser (1999) and VDI. mm, made of mild steel or low-alloy steel.

Fatigue resistance and characteristic S-N To achieve results corresponding to 2.3% curve failure probability rather than 50% failure

In lack of endurance tests of the actual probability, σD50% has to be reduced as material, a number of different methods to follows establish a synthetic characteristic S-N curve are available. Each method has its own σD2.3% = σD50% −2·s advantages and disadvantages. Some of the methods can be found in Gudehus and in which s represents a standard deviation. Zenner (1999), Bergmann and Thumser For mild steel and low-alloy steel, s can be (1999) and Sundström (1998). General taken as 6% of σD50%, see Sundström (1998). aspects of fatigue calculation can be found in Appendix C. In general, it is not Determination of design S-N curve recommended to combine two different On a logS−logN scale, the S-N curve can be methods because this may cause the considered linearly decreasing from (10, R ) assessment of the overall safety level to m to (106, σ ), where R denotes the ultimate become nontransparent. D m strength. For N > 106, the curve may be

taken as a horizontal line at S = σ . In the following, one of several methods for D establishment of a characteristic S-N curve Stresses exceeding the yield stress R are will be referenced, which is simple and e usually not allowed in a structure subjected suitable for preliminary calculations. to fatigue loading. Consequently, the S-N

curve is cut off at R . It may be difficult to establish S-N curves e for alloy steel. If test data are available, the Due to the variation of the influence factors test assumptions are often not documented. from point to point within the shaft, the S-N If reliable S-N curves for the applied steel curve to be used for the actual shaft is quality are not available, a synthetic S-N unique for each individual location curve may be used, based on static strength considered within the shaft. data for the material in conjunction with fatigue strength σ under rotational bending D One may, conservatively, use the following and torsion (and possibly tension- design rules compression). Such data are always available for standard reference materials. σ⋅γ < σ ⋅K ⋅K /( K ⋅β⋅γ ) When σ is not available for the actual f D2.3% 1 2 r m D material, one may for tension-compression τ ⋅γf < τD2.3%⋅ K1⋅K2/(Kr⋅β⋅γm)

124 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø in which σ and τ are the actual characteristic For main shafts, both bending and torsion normal stress and torsional stress, occur at the same time. One can take this respectively, in the observed cross-section. into account by using the reference stress

σD2.3% is the characteristic value for the σ 2 2 endurance limit, calculated from D50% as σ = σ + 3τ specified above. i,ref i i

in which σ is the normal stress, and τ is the For the notch factor β, a somewhat better i i torsional stress. As the bending stress is approximation can be achieved by oscillating with a stress ratio near –1, while accounting for the smaller sensitivity of the the shear stress is oscillating with a positive material to notches at lower numbers of stress ratio between 0 and approximately cycles. At N = 10 one can set β = 1 and then 0.8, use of the reference stress σ implies let β increase linearly with logN to β = i,ref α− 6 an approximation. A more accurate 1+q( 1) at N = 10 as shown in Figure 6-9. approach can be found in Gudehus and Zenner (1999). Notch factor 6.1.4 Ultimate strength

βµαξ = 1 + q ( α - 1 ) Stress calculation is to be carried out according to standard mechanical engineering methods and is usually straight- forward. For materials with a high ultimate

β strength, it may be necessary to account for stress concentration in extreme load cases. The influence of the ultimate strength can be accounted for by the following factor

2 βextreme = 1+(α−1)⋅(Rm/1000) 1 012345678 Rm ultimate strength in Mpa Log (N) 10 α stress concentration factor taken from

Figure 6-9. Notch factor. Figure 6-7 and Figure 6-8

Fatigue loads are usually specified in terms The following criterion is to be fulfilled in of a distribution of stress amplitudes, i.e. a design so-called load spectrum, which on discretised form gives the number of stress σ⋅γf < Re/(γm⋅βextreme) cycles in each interval of a suitably discretised stress amplitude axis. The stress σ actual stress in the observed section cycles in each stress amplitude interval Re yield strength contribute to the total fatigue damage. The total fatigue damage can be calculated by For stress ratios > 0 and dissimilar stress Palmgren-Miner’s method and has to be less distribution, a plastic strain up to 0.2% may than 1.0 in order to ensure a sufficiently low be allowed for single extreme load cycles. failure probability in the design life of the Reference is made to Gudehus and Zenner shaft. Reference is also made to Appendix (1999). The amount of plastic strain may be C. estimated from Neuber’s correction, see

6 – Nacelle 125 Guidelines for Design of Wind Turbines – DNV/Risø

Gudehus and Zenner (1999) and Sundström can also be difficult. Usually, some kind of (1998). For repetitive extreme load cycles shrink fit coupling is used, e.g. in with stress ratios < 0, a low-cycle fatigue conjunction with a shrink disk. calculation should be conducted, see Gudehus and Zenner (1999) and Sundström Shrink disks (1998). A correct functioning of clamp couplings requires careful dimensioning and assembly. Partial safety factors If the coupling, in addition to torque, also is The requested safety level is normally to transfer bending moments, it is important achieved by multiplying the characteristic to include this in the calculations. When loads by a load safety factor γf and reducing applying catalogue values for transferred the characteristic material properties with a torque, it is recommended to apply a safety material safety factor γm. factor of 1.5 to the peak torque in order to be on the safe side. The load safety factor accounts for the random nature of the load components (e.g. To achieve the prescribed level of safety, it fatigue loads, extreme loads, gravitational is important to make sure that the roughness loads, etc.) and the uncertainty in the conditions of the surface and the tolerances calculation method. The material safety for both parts are in accordance with the factor accounts for the scatter in the material recommendations of the manufacturer. properties and for the level of quality control During assembly, correct bolt pretensions of the material. shall be applied, and the required cleanness of the frictional surfaces shall be accounted The values of these safety factors are for. normally given in the relevant wind turbine regulations, e.g. DS472, and in the Couplings associated material standards. For machine In certain cases where the gear is rigidly components, the certifying bodies or the mounted on the machine foundation, a owner sometimes define their own values coupling is applied which can only transfer for γm. torque and which is flexible with respect to bending moments. In these cases, the hub 6.1.5 Main shaft-gear connection parts of the couplings are usually shrunk both on main shaft and on gear shaft. Insofar In cases where the main shaft does not as regards the design of the coupling itself, constitute an integral part of the gear and in reference is made to special literature. cases where it forms the generator shaft in multi-polar generators, the main shaft should 6.1.6 Materials somehow be connected to the transmission input shaft. Formerly, hollow-axis gears In most cases, non- or low-alloyed were used by which the torsional moment machinery steels are used, i.e. steel with a was transferred from the main shaft to the carbon content of 0.3-0.7% and with less shaft bushing of the gear by means of a than 5% alloy of metals such as Mn, Cr, Mo, keyway connection. This principle for Ni and V. torsional moment transfer has now been abandoned, except for small simple wind This classification covers steel with rather turbines, as keyway connections are only different mechanical properties, ranging poorly suited to transfer varying and from non-alloyed steel with an ultimate reversing loads. Assembly and dismantling strength of about 500 MPa and failure

126 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø elongation of 15% to low-alloyed steel with an ultimate strength of up to 1500 MPa and Technical terms of delivery and testing: failure elongation less than 10%. DIN 1690, Parts 1 and 2, DIN 54111, Part 2. Magnetic particle inspection SIS 114401; In addition to obtaining an adequate ultimate Capillary Liquid Testing: Non-destructive strength, the material should exhibit a high Testing, PI-4-2 Liquid Penetrant testing. fracture toughness and a low brittle General Dynamics. transition temperature. Steel within this category is standardised in DIN 17200. Certificates: EN 10 204. Such steel is suitable also as a basis for forged materials for main shafts. This is a very common design approach, which 6.2 Main Bearing implies an improvement of the structure and The main bearing of a wind turbine supports a good transition to the flange for the rotor the main shaft and transmits the reactions hub. from the rotor loads to the machine frame.

On account of the relatively large In some cases, cast main shafts are used. deformations in the main shaft and its The casting provides a great degree of supports, the spherical roller bearing type is freedom as far as the shaping of the shafts is often used, see Figure 6-10 for an example. concerned, whereas it poses some limits in terms of relatively low ultimate strength and failure elongation.

Usually, nodular iron in the qualities GGG.40 or GGG.50, according to DIN 1693, is used.

The main shaft is one of the most critical components in a wind turbine structure. Thus, it is important to make sure, by means of appropriate quality assurance, that the assumed material quality is met, and that the manufacturing has not caused development Figure 6-10. Spherical roller bearing, from Bonus (1999). of surface cracks or other imperfections. This implies that the material is checked for Spherical roller bearings have two rows of imperfections by some suitable non- rollers with a common sphered raceway in destructive testing procedure, such as the outer ring. The two inner ring raceways ultrasonic testing, and that the shaft is are inclined at an angel to the bearing axis. delivered with a materials certificate. The bearings are self-aligning and consequently insensitive to errors in respect 6.1.7 Standards of alignment of the shaft relative to the Materials: housing and to shaft bending. In addition to Structural steels EN 10 025 high radial load capacity, the bearings can Tempered steels DIN 17200 accommodate axial loads in both directions. Nodular irons DIN 1693, Parts 1 and 2.

6 – Nacelle 127 Guidelines for Design of Wind Turbines – DNV/Risø

Figure 6-11. Bearing frame with main shaft and bearing arrangement for wind turbine with two main bearings; courtesy Vestas.

The allowable angular misalignment is rotor loads and, subsequently, to prevent normally 1-2.5 degrees depending on the excessive edge loads, which would result in bearing series. This is sufficient to possible damage to the bearing. compensate for deformations in shafts, housing and machine frame caused by the

128 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø

arrangement makes it easy to replace the gearbox. Courtesy NEG Micon.

yg ys

yr

MzR

Fs FzR Fa

FyR Frh Figure 6-12. Nacelle with transmission system for FxR Frv which the main bearing is integrated in the gearbox. MyR Courtesy NEG Micon. MxR

Figure 6-14. Main shaft. The main bearings are mounted in bearing housings bolted to the main frame. The F side force on rotor and nacelle quantity of bearings vary among the xR FyR thrust on rotor different types of wind turbines. Many wind F weight of rotor turbines have two bearings, each with its zR MxR tilting moment at rotor own flanged bearing housing. Some turbines M driving torque at rotor with two bearings use the hub as a housing. yR MzR yaw moment at rotor Some turbines have only one main bearing, F shaft mass given that the gearbox functions as a second S yr distance rotor to bearing centre main bearing. Each bearing arrangement has y distance bearing centre to gear stay its own advantages and disadvantages. g ys distance shaft centre of gravity to Figure 6-11 shows an example of the main gear stay shaft and bearing arrangement for a turbine with two main bearings in the same housing. 6.2.1 Determination of design loads Two other examples are given in Figure 6-12 and Figure 6-13. A couple of additional Figure 6-14 shows the conventional design examples appear from illustrations of the of a rotor shaft. The shaft is supported by a hub and transmission system in Section 5.2. bearing placed adjacent to the rotor. Another bearing at the opposite end of the rotor shaft is integrated in the main gearbox.

The loads in Figure 6-14 are drawn in directions according to a conventional coordinate system, and not in the direction that they will normally have.

From the example in Figure 6-14, the front main bearing loads can be calculated from simple beam theory as

Figure 6-13. Nacelle with transmission system with two − main bearings. The second bearing on the main shaft Fa = FyR forms the foremost bearing in the gearbox. This

6 – Nacelle 129 Guidelines for Design of Wind Turbines – DNV/Risø

1 2 2 chemically active substances and abrasive Fr = M + M y 1 2 particles have to be considered. g Lightening currents passing through the with bearings may have to be considered for some sites. = − − + M 1 M xR FS y S FzR (y r y g ) = + + 6.2.4 Seals, lubrication and M 2 FxR (y r y g ) M zR temperatures

The loads are usually specified in terms of a Seals load spectrum or distribution of loads, which Bearing seals are needed, partly to hold back on discretised form gives the number of bearing lubrication, and partly to keep out hours of operation within each defined load contaminants. As the main bearing is often interval in the discretisation. For each placed relatively unprotected and quite close interval, the associated operational and to the outside, this is of particular environmental conditions have to be taken importance because dirt and rainwater can into account. All relevant load cases are to easily come in contact with the bearing. be included in this load spectrum. Non-rubbing seals (labyrinth seals) are 6.2.2 Selection of bearing types appropriate since they exhibit practically no friction and no wear. A labyrinth seal forms The main bearing must be able to a good supplement to other seals. accommodate axial as well as radial forces from the rotor. Further, the bearing must When using rubbing seals it should be aimed allow misalignment from deflection of the at mounting the seal on the shaft and to let shaft and the support. These requirements the bearing housing form the sealing are fulfilled by means of spherical roller surface. This is done to reduce the risk of bearings, see Figure 6-10. scratches in the shaft. The compatibility of

the grease with the seal material has to be 6.2.3 Operational and environmental checked. conditions

The main shaft speed range of a Lubrication conventional wind turbine of rated power in The main purpose of lubrication is to create the range 500-2500 kW is about 10 to 30 a lubricant film between the rolling elements rpm at nominal load. Depending on the to prevent metal-to-metal contact. This is to operational strategy, the wind turbine will avoid wear and premature rolling bearing experience all speeds from standstill to fatigue. In addition, lubrication reduces the nominal speed for varying periods of time. development of noise and friction, thus These conditions have to be included in the improving the operating characteristics of a load spectrum for the bearing. The bearing. Additional functions may include temperature range can vary considerably and protection against corrosion and enhance- has to be evaluated with due consideration ment of the sealing effect of the bearing to the actual site. In the IEC 61400-1 seals. standard, a normal ambient temperature range from −10º C to +40º C is specified. When selecting lubrication, some of the Environmental conditions such as salinity, matters that need to be considered are vis- cosity, consistency, operating temperature

130 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø range, the ability of protection against Lithium soap base grease has the quality of corrosion and the load carrying ability. being waterproof and usable at a wide of range temperatures –35 °C to +130 °C. Water in the lubrication leads to corrosion, degradation of the lubrication, formation of Other possibilities are sodium grease or aggressive substances together with oil calcium soap base grease. additives, and it affects the formation of a load carrying lubricating film. Sodium grease absorbs large quantities of water and is useful in environments with Since the rotational speed varies from zero condensation. It may, however, soften to to the nominal speed, a boundary lubrication such an extent that it flows out of the condition will consist a considerable part of bearing. the operational time. This will normally call for a lubricant with EP (extreme pressure) Calcium soap base greases of penetration additives and as high a viscosity as class 3 do not absorb any water. This is practically possible. In this respect, it is advantageous in situations where bearing essential to consider possible load cases seals are exposed to splash water. where the radial load is low or even zero combined with low temperatures (high The stiffness of the grease is determined by viscosity), because it may cause sliding in the consistency class. A stiff grease, which the bearing. belongs to consistency class 3 or higher, can contribute to the sealing of the bearing and Grease lubrication keep out contaminants by lying in a The most commonly used lubrication in the labyrinth seal or in the contact area of a main bearings is grease. rubbing seal.

Grease has the advantage of being easily However, for high P/C load ratios, greases retained in the bearing arrangement, it of consistency class 1-2 should be selected. contributes to sealing the bearing In a dusty environment, a stiff grease of arrangement from contamination, and the penetration class 3 should be used. use of an expensive circulation system is avoided. Rolling bearing greases are The relubrication interval corresponds to the standardised in DIN 51825. minimum grease life F10 of standard greases in accordance with DIN 51 825. The grease Grease consists of a base oil with thickeners service life is dependent on the type and and possibly additives added. The following amount of grease, bearing type and size, grease types are distinguished: loading, speed, temperature and mounting • mineral oil with metal soaps as conditions. For fairly large bearings (> 300 thickener. mm) the relubrication interval is • mineral oil with non-soap thickener recommended to be more frequent than F10. • synthetic oils with non-soap thickener In some cases, continuous lubrication is established. A possible choice of grease lubrication would be a lithium soap base grease of Great care should be exercised if the grease penetration class 2-3 with EP additives and type is to be changed. If incompatible maybe corrosion and oxidation inhibitors. greases are mixed, their structure can change

6 – Nacelle 131 Guidelines for Design of Wind Turbines – DNV/Risø drastically, and the greases may even soften Before going into operation, oil must be considerably. supplied to the bearing. In case of circulating oil lubrication, the oil pump As regards the amount of grease to be used, should be started before the turbine goes a rule of the thumb is to fill the bearing into operation. In other cases, the bearing completely with lubrication while the must be manually lubricated before first housing is half filled (SKF, 1989). start-up, and total drain of the bearing during service must be avoided. Oil lubrication Since temperatures are normally relatively 6.2.5 Rating life calculations low, the lubricant does not need to function The fatigue load carrying capacity of the as heat dissipator. In that case, there is no bearing is characterised by the basic need for circulation of the lubricant, which dynamic load rating C. This quantity can be simplifies the design. On the other hand, calculated according to ISO 281 (1990). when the lubrication is not circulated, there is no possibility of filtering, and The static load carrying capacity of the relubrication is therefore necessary. bearing is characterised by the static load

rating C . This quantity can be calculated Oil lubrication makes it necessary to 0 according to ISO 76 (1990). monitor the lubrication system because of the risk of leakage. A disadvantage The general methodology for selecting and compared to grease lubrication is that it calculating rolling bearings is given in demands a better sealing and a circulation Section 6.3 for both dynamic and static system. loads. In general, for a design life of 20

years, the required basic rating life L Straight oils and preferably corrosion- and 10h should equal or exceed 300,000 hours. The deterioration-inhibited oils can be used. If modified rating life L should at the same the recommended viscosity values are not 10mh time reach 175,000 hours. In the context of maintained, oils with suitable EP additives grease lubrication and oil lubrication and anti-wear additives should be selected. without filter, the contamination factor η If the bearings are heavily loaded (i.e. the C should not be chosen higher than 0.2 if load ratio P/C > 0.1), or if the operating special precautions are not taken to obtain viscosity ν is smaller than the rated viscosity ν and verify higher values. For oil lubrication 1, oils with anti-wear additives should be with off-line filtering, a contamination factor used. EP additives reduce the harmful ηC of 0.5-0.7 may be obtained. The C/P ratio effects of metal-to-metal contact, which should not exceed 2.5 for the largest load occurs in some places. The suitability of EP interval. additives varies and usually depends largely on the temperature. Their effectiveness can The above-mentioned figures apply to only be evaluated by means of tests in medium size wind turbines (600-1000 kW). rolling bearings. (FAG WL-81 115/4.) It should be borne in mind that large bearings are relatively less sensitive to The intervals between oil changes depend on contamination particles of a certain size than the specific lubrication system and smaller bearings. circulation, contamination and ageing of the oil.

132 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø

The static safety factor C0/P0 based on the 6.2.7 Bearing housing extreme load cases should not be lower than The bearing must be firmly supported by the 4. whole circumference to achieve a proper

load transmission. 6.2.6 Connection to main shaft

The thrust from the rotor should be For bearings with normal tolerances, the considered to be taken up by shoulder or by dimensional accuracy of the cylindrical friction, or a combination of both. One seating in the housing should be at least IT should then be aware of the stress concen- grade 7, and on the shaft at least IT grade 6. tration in the main shaft at the shoulder. The tolerance for cylindrical form should be In cases where there is a possibility of the at least one IT grade better than the wind turbine being in a situation where the dimensional tolerance. wind is coming from the back, one should consider a stop ring as shown in Figure Since cast bearing houses will often have a 6-15. complex geometry, an obvious method to be used for verification would be a finite back wind element analysis, see Appendix D. secure ring 6.2.8 Connection to machine frame The connection between the bearing housing and the machine frame will most likely be a bolt connection. The connection should be capable of transferring the combination of Figure 6-15. Back wind stop ring. axial and radial forces from the bearing to the main frame by friction or by shear in A tight fit, i.e. complete support of the bolts by tight fit, depending on the geometry bearing ring over its entire circumference, of the connection (see Appendix A). makes a full utilisation of the bearing's load carrying capacity possible. 6.2.9 Standards

The bearing clearance should be as small as ISO 76 Roller bearings – Static load ratings. possible to ensure accurate guidance, however, large enough in order not to get ISO 281 Roller bearings – Dynamic load stuck in any situation. When considering the ratings and rating life + Amendment 1 and bearing clearance things to take into account 2. will be a possible difference in temperature and expansion of inner ring and compression IEC 61400-1 Wind generator systems, Part of outer ring during mounting, respectively. 1. Safety requirements.

The bearing clearance should always be checked after mounting on the shaft. 6.3 Main gear The purpose of the main gear is to act as a speed increaser and to transmit energy between the rotor and the generator.

6 – Nacelle 133 Guidelines for Design of Wind Turbines – DNV/Risø

6.3.1 Gear types wheel. The axes are nonparallel and nonintersecting, see Figure 6-19. Worm The most common gear types used in main gears are not used for main gears in gears for wind turbines can be identified and wind turbines, but are sometimes used classified as follows, based on their as yaw gears in yaw systems. geometrical design: • hypoid gears constitute a cross between • spur and helical gears consist of a pair worm gears and bevel gears in that they of gear wheels with parallel axes. Spur are similar to bevel gears but have gears have cylindrical gear wheels with nonintersecting axes. Hypoid gears are radial teeth parallel to the axes. In turned into spiral bevel gears in the helical gears, the teeth are helical, i.e. limiting case when the offset between they are aligned at an angle with the the axes approaches zero. Reference is shaft axes. Double-helical gears have made to Figure 6-20. two sets of helical teeth on each wheel.

Helical gears are sometimes referred to as spiral gears or oblique gears. See Figure 6-16. • epicyclic or planetary gears consist of epicyclic trains of gear wheels, i.e. gears where one or more parts – so- called planets – travel around the circumference of another fixed or revolving part. See Figure 6-17. Figure 6-16. Examples of spur and helical gears, from Niemann and Winter (1985). Planetary gears in combination with one or more parallel axis gears form the most commonly applied gear type for the main gear in wind turbines. Gears in which the power is transferred from one wheel to two or more meshing wheels are referred to as gears with a split power path.

Other types of gears, rarely or never used in Figure 6-17. Planetary gear principle, outer fixed wind turbine main gears, include the annulus with three revolving planets and one rotating following, which are listed here merely for planet carrier in the middle, from Niemann and Winter completeness: (1985).

• bevel gears consist of a pair of toothed Tooth form conical wheels whose working surfaces The tooth forms used practically universally are inclined to nonparallel intersecting in spur, helical, bevel and worm gears are axes. Bevel gear wheels can be so-called involute teeth. This tooth form designed with spur teeth, helical teeth, implies that rotation of the base circle at a and curved spiral teeth. Spiral bevel uniform rate is associated with uniform gears result when the teeth are helical or displacement. The path of contact is a curved. See Figure 6-18. • straight line, which coincides with the line worm gears consist of a worm-thread of action. wheel, or “endless screw”, working in conjunction with a cylindrical toothed

134 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø

• tapered roller bearings

Examples of bearings are shown in Figure 6-21. Two bearings should be used to support each gear shaft, one for support of both radial and thrust forces, the other for support of only radial forces and free to allow for axial growth under thermal changes. Bearing fits should be tight to prevent damage to the bearing or the housing and to prevent spinning of inner and outer bearing races.

Figure 6-18. Examples of bevel gears, from Niemann and Winter (1985).

Figure 6-21. Examples of roller bearings: Spherical bearing (left) and cylindrical bearing (right). From SKF (1997).

Figure 6-19. Worm gear. The bearing fits should be carefully selected in order to avoid spinning bearing rings as well as squeezing of the rollers.

For roller bearings in gears, it is normally sufficient to have a predicted L10h lifetime of at least 100,000 hrs referring to the cubic mean load, or 30,000 hrs referring to the nominal power.

The loads should be specified in terms of a Figure 6-20. Hypoid gear, from Niemann and Winter (1985). load duration spectrum, which on discretised form gives the number of hours of operation Bearing types within each of a number of suitably chosen Bearings for wind turbine gears should all load intervals. The equivalent power, which be rolling element, anti-friction type is defined as the cubic mean power, can be bearings. Different bearing types applied in calculated as follows gears include: 1 3 • ball bearings  h  P =  (P 3 i  • cylindrical roller bearings eq ∑ i   i ht  • spherical roller bearings

6 – Nacelle 135 Guidelines for Design of Wind Turbines – DNV/Risø

Pi power in the ith defined load interval Table 6-2. Reliability factor a1. hi number of hours of operation in the ith Failure Reliability Reliability interval probability 1−n factor a1 ht total number of hours of operation n (%) (%) 10 90 1.00 Rolling bearings 5 95 0.62 The design of roller bearings for a gear is 4 96 0.53 made using a load duration spectrum. The 3 97 0.44 load duration spectrum may be transformed 2 98 0.33 to an equivalent cubic mean load, Peq. 1 99 0.21 According to ISO281/1, the basic rating life is defined as Bearing manufacturers recommend values

for the factor product a23 = a2 a3. These p  C  values may vary from manufacturer to =  dyn  ⋅ 6 L10 10 revolutions manufacturer, and a will usually depend  P  23  eq  on the viscosity ratio

C basic dynamic capacity of the bearing ν dyn κ = 10/3 for roller bearings ν p = 1 3 for ball bearings in which ν is the operational viscosity of the The bearing manufacturers usually quote lubrication, and ν1 is the viscosity required values for Cdyn. The index 10 refers to a 10% for an adequate lubrication, i.e. a lubrication failure probability associated with the which is sufficient to avoid metallic contact bearing life L10. ISO 281/1 suggests the between the rolling elements and the race following formula for calculation of ways. adjusted rating life

p New methods for bearing life prediction are  C dyn  being developed, e.g. by SKF and INA. SKF L = a a a   ⋅10 6 revolutions na 1 2 3  P  defines a modified rating life  eq 

p in which the suffix n denotes the associated  C  L = a a  dyn  ⋅10 6 revolutions probability of failure. The corresponding naa 1 SKF    P  survival probability is 1−n, which is also referred to as the reliability. The coefficient in which the adjustment factor a depends a is a reliability factor, which depends on SKF 1 on the cleanliness of the lubricant and the the actual reliability, see Table 6-2. The load ratio P /P, where P denotes the coefficients a and a are factors of the u u 2 3 endurance load limit, and P is the actual bearing material and service conditions, load. Since the a factor will vary from respectively. For a commonly applied SKF one load level to another, use of this reliability of 90%, a conventional bearing approach to lifetime predictions requires material and normal operating conditions, application of the entire load spectrum of a = a = a = 1.0. 1 2 3 loads P , not only an equivalent load P . i eq The total rating life can accordingly be obtained as follows

136 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø

h The major function of the material selection L = t 10mh h and subsequent heat treatment of steel ∑ i materials to be used for gears is to achieve i L 10mh,i the desired microstructure at the critical locations so that, in particular, the teeth will in which the numerator ht denotes the total have the desired contact and tooth root number of hours of operation, and the strength capacity. Common heat treatments denominator denotes the sum of all relative for steel include: life consumptions over a suitable discreti- • preheat treatments (anneal, normalise, sation of the load spectrum. temper) • heat treatments Other methods exist which require advanced • through-hardening (anneal, calculation programs developed by the normalise, normalise and temper, major bearing manufacturers. quench and temper) • surface hardening (flame and Lubrication conditions are essential for the induction harden, carburise, bearing design. The oil viscosity should carbonitride, and nitride) κ preferably result in a viscosity ratio > 1.0. • subsequent heat treatment (stress relief) κ κ In the region from = 0.4 to = 1.0, the bearing manufacturers recommend lubri- Case-hardening implies that carburisation is cation oils with approved EP (extreme carried out after a prior heat treatment and is pressure) additives. followed by hardening and tempering. The case-hardening implies that the surface layer The oil supplied to bearings should have a becomes harder than the interior. cleanliness and a temperature corresponding to the design assumptions. The cleanliness Requirements to materials and hardening of the lubricant as defined in ISO 4406 treatments for gear transmissions can be consists of three numbers, for example found in DNV Rules for Classification of 17/14/12, which corresponds to certain Ships, Part 4, Chapter 2. numbers of particles per 100 ml sample greater than 2µm, 5µm and 15µm, 6.3.2 Loads and capacity respectively. Insofar as regards gears, the load levels are The static capacity of roller bearings is in focus rather than the load ranges. This is defined in ISO 76 and is the load for which in contrast with what is otherwise the case the total permanent deflection of the bearing for many structural details. Procedures for is 1/10000 times the rolling element calculation of loads and for prediction of diameter. The margin to this capacity should load capacities for gears are outlined in the not be less than 4.0 for extreme design following and include prediction of surface loads. durability, tooth strength, and scuffing load capacity. Other damage such as wear, gray Materials staining (micropitting) and fractures starting Gears should preferably be made from from flanks may also limit the gear capacity, separate steel forging. Heat treatment and although limited or no calculation case-hardening can be used to harden the procedures are given for the associated steel. capacities.

6 – Nacelle 137 Guidelines for Design of Wind Turbines – DNV/Risø

The nominal tangential load is given as load, and accounts for the maldistribution of load in multiple- = 2T path transmissions FT d KV Internal dynamic factor to account for internally generated dynamic loads in T applied torque the gear d reference diameter KHβ face load factor of contact stress and scuffing Surface durability KHα transverse load distribution factor of contact stress and scuffing Contact stress. The design contact stress σH is derived from the nominal tangential load as follows The factor Z is defined as the product

Z = Z Z Z ZεZβ F (u +1) BD H E σ = T H Z K d1bu ZBD zone factor for inner point of single pair contact for pinion or wheel u gear ratio per stage ZH zone factor for pitch point b face width ZE elasticity factor that accounts for d1 reference diameter of the pinion influence of modulus of elasticity and Young’s modulus. This is a factor The factors K and Z are compound influence whose squared value is in units of factors to account for various effects. The stresses factor K is defined as the product Zε contact ratio factor that accounts for influence of transverse contact ratio

K = KAKγKVKHβKHα and overlap ratio Zβ helix angle factor KA application factor, defined as the ratio between maximum repetitive torque Note that all factors, except ZE, are and nominal torque, and accounts for dimensionless. Formulas and details for dynamic overloads external to the calculation of the various factors can be gearing found in ISO6336, DIN3990, and DNV Kγ load-sharing factor, defined as the ratio CN41.2 between the maximum load through the actual path and the evenly shared

Table 6-3. Endurance limits. 2 Steel grade σHlim (N/mm ) Alloyed case-hardened steel of special approved high grade 1650 Alloyed case-hardened steel of normal grade 1500 Nitrided steel of approved grade 1250 Alloyed quenched and tempered steel, bath or gas nitrided 1000 Alloyed, flame or induction hardened steel (HV = 500-650 N/mm2) 0.75HV+750 Alloyed quenched and tempered steel 1.4HV+350 Carbon steel 1.5HV+250 Note that these values refer to forged or hot-rolled steel. For cast steel, values need to be reduced by 15%. HV denotes surface hardness.

138 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø

Capacity. The characteristic endurance limit Tooth root strength σHlim for contact stresses is the stress that Tooth stress. The following section deals can be sustained for 5⋅107 cycles without the with the tooth root strength as limited by occurrence of progressive pitting. This limit tooth root cracking. The local tooth stress σF usually defines the beginning of the for pinion and wheel can be predicted as endurance stress range, i.e. it defines the lower knee of the σ−N curve. Values of F σ = T F YF YSYβ K A KV K Fβ K Fα σHlim are given in Table 6-3, but should only bmn be used for materials which are subjected to relevant quality control. Results of suitable Y tooth form factor fatigue tests may also be used to establish F YS stress concentration factor values of σHlim. Yβ helix angle factor b face width at the tooth root, either for The design endurance strength, also known pinion or for wheel as the permissible contact stress, is mn normal module

σ σ = H lim Other coefficients and variables are defined HP Z N Z L Z V Z R Z W Z X S H elsewhere, see Section 6.3.1. Expressions for the tooth form factor, the stress SH required safety factor concentration factor and the helix angle ZN life factor for endurance strength, re- factor can be found in ISO6336, DIN3990 duces the endurance strength when the and DNV CN41.2. design life of the gear is greater than 5⋅107 cycles, and increases it when the Capacity. The permissible local tooth root 7 σ design life is less than 5⋅10 cycles stress FP for pinion and wheel can be predicted as ZL lubricant factor referring to various aspects of oil film influence σ Y Y ZV speed factor referring to various σ = FE d N Yδ Y Y Y aspects of oil film influence FP relT RrelT X C S F ZR roughness factor referring to various aspects of oil film influence ZW work hardening factor σFE local tooth root bending endurance ZX size factor limit of the reference test gear. The stress concentration factor for Formulas and details for calculation of the the test gear is normally 2.0 various factors can be found in ISO6336, Yd design factor, which accounts for DIN3990 and DNV CN41.2. other loads than constant load direction, e.g. idler gears, Design rule. The design rule to be fulfilled temporary change of load is direction, prestress due to shrinkage, etc. σ ≤ σ H HP YN life factor for tooth root stresses related to the reference test gear dimensions. It is used to take into account the higher load bearing

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capacity for a limited number of weighted average of contact temperatures load cycles, and a reduced load along the path of contact. Usually, the flash bearing capacity when the number temperature criterion will govern the design of load cycles is large against scuffing failure. SF required safety factor YδrelT relative sensitivity factor of the Two inequalities should be fulfilled to meet gear, related to the reference test the flash temperature criterion gear YRrelT relative surface condition factor of θ −θ θ ≤ S oil +θ and θ ≤ θ − 50° the gear, related to the reference B S oil B S test gear S

YX size factor θS scuffing temperature as determined YC case depth factor considering subsurface fatigue from FZG tests θ oil oil temperature before it reaches the Formulas and details for calculation of the mesh, i.e. the normal alarm various factors can be found in ISO6336, temperature θ DIN 3990 and DNV CN41.2. B maximum contact temperature along the path of contact, calculated as the θ Design rule. The design rule to be fulfilled sum of the bulk temperature MB and θ is the maximum flash temperature flamax along the path of contact σ ≤ σ SS required safety factor, usually taken as F FP 1.50.

Scuffing load capacity The integral temperature criterion reads High surface temperatures due to high loads and sliding velocities can cause lubricant θ films to break down in the gear. This will θ ≤ S int lead to seizure or welding-together of areas S S of tooth surfaces between the wheel and the pinion. This phenomenon is known as θint = θMC+1.5⋅θflaint scuffing and may lead to failure. In contrast to pitting and fatigue failure, which both θint integral temperature exhibit a distinct incubation period, a single θMC bulk temperature short overloading can lead to scuffing θflaint mean flash temperature along the path failure. of contact

Two criteria are to be fulfilled to ensure a Formulas for calculation of θS, θMB, θMB, sufficient level of safety against scuffing θflamax, and θflaint can be found in DNV failure. Both criteria are formulated in terms CN41.2. of criteria on temperature, i.e. the local contact temperature may not exceed some Note that gray staining may occur under the permissible temperature. The one criterion is same conditions that may lead to scuffing, a so-called flash temperature criterion, based and that this may even happen without or on contact temperatures, which vary along before the occurrence of a literal scuffing the path of contact. The other criterion is an failure. integral temperature criterion, based on the

140 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø

Capacity of shaft/wheel connections stability, resistance to foaming, separation Several types of couplings between shaft from water, and prevention of corrosion. and wheel appear in the context of gears. However, the principal function of the oil is The major types are listed in Section 6.4 the protection of the rubbing surfaces of the dealing with issues of importance for their gear teeth. There are many requirements for design. the lubrication system. The following items are recommended as a minimum. Other capacities Other capacities to be considered in the The choice of a lubricant and lubrication context of gears include: system should be the joint responsibility of • bearing calculations the gearbox manufacturer and the gear box • shaft capacities for torsional loads and purchaser. tooth loads • gearbox housing and suspension. The Wind turbine gears have a relatively low capacity of the gearbox is highly pitchline velocity and high gear tooth loads. dependent on the type of gearbox, e.g. These conditions require either synthetic or whether it is cast or welded, whether it mineral gear oil with antiscuff additives and is small or large, and whether it is one- the highest viscosity that is practical. The or two-pieced. The gearbox housing choice of lubricant depends on many factors, shall have sufficient strength and including viscosity, viscosity index, pour stiffness to avoid mesh misalignment point, additives and costs. due to deflection of the gearbox housing. It is recommended to evaluate the ability of a lubricant to resist micropitting. 6.3.3 Codes and standards Viscosity classes for lubricants according to DIN 3990, “Tragfähigkeitsberechnung von ISO are given in Table 6-4, and Stirnrädern”, Part 1-5, Deutches Institut für recommended criteria for acceptance of gear Normung e.V., 1987. lubricants are given in Table 6-5.

ISO 6336-1 to 6336-5 Monitoring Oil level indication by means of a sight glass AGMA, “Recommended Practices for or dipstick or equivalent is to be provided. Design and Specification of Gearboxes for The lubrication oil temperature is to be Wind Turbine Generator Systems,” monitored. A temperature in excess of the American Gear Manufacturers Association, approved maximum should result in Alexandria, Virginia, 1997. automatic shutdown. The monitoring system should be arranged so as to imply shutdown 6.3.4 Lubrication in case of malfunction. The functioning of a gear involves relative motion of surfaces in contact under load. For gears with a forced lubrication system, Separation of the surfaces by a thin film of the pressure is to be monitored. Oil pressure oil is a key factor in achieving smooth below minimum, with a running gear, operation and a good service life. should result in automatic shutdown. This Lubrication is used for this purpose. Oils for requirement does not apply if the forced the lubrication of gearing have to meet systemis mainly arranged for oil cooling, diverse performance requirements, including and if the gear can work satisfactorily with

6 – Nacelle 141 Guidelines for Design of Wind Turbines – DNV/Risø splash lubrication. However, an indication Assembly and testing in the workshop of the insufficient oil pressure is to be The accuracy of meshing is to be verified for provided. all meshes. The journals should be in their expected working positions in the bearings. Table 6-4. Viscosity classes according to ISO. The mesh contact should be consistent with Viscosity Average Limits of that which would result in the required load class viscosity kinematic distribution at full load. at 40°C viscosity at 40°C (mm2/s) (mm2/s) The gear transmission is to be spin tested in min. max. the workshop and checked with regard to oil VG 2 2.2 1.98 2.42 tightness. The spin test can also be used for VG 3 3.2 2.88 3.52 gear mesh verification. As regards the proto- VG 5 4.6 4.14 5.06 type gear as well as selected gears from serial VG 7 6.8 6.12 7.48 production, special testing will be necessary, VG 10 10 9.0 11.0 in particular, with respect to face load VG 15 15 13.5 16.5 distribution, lubrication/temperatures and VG 22 22 19.8 24.2 vibrations. VG 32 32 28.8 35.2 VG 46 46 41.4 50.6 Assembly and testing in the nacelle VG 68 68 61.2 74.8 The functioning of the lubrication oil system VG 100 100 90 110 and monitoring system is to be tested.

VG 150 150 135 165 If the arrangement is made so that external VG 220 220 198 242 bending moments (e.g. due to the rotor or VG 320 320 288 352 torque reaction) can influence the gear mesh VG 460 460 414 506 alignment, the contact pattern has to be VG 680 680 612 748 verified under real or simulated conditions VG 1000 1000 900 1100 for some gears in a series. This can be VG 1500 1500 1350 1650 achieved by applying a thin suitable lacquer VG 2200 2200 1980 2420 to the teeth before the test. The tooth contact VG 3200 3200 2880 3520 pattern at the actual test load is to be analysed with respect to load distribution at Installation of gearing the rated load. This requirement also applies The gear is to be installed so that appropriate to gears where no part load or full load alignment and running conditions for the testing has been made in the workshop. gear are maintained under all operating conditions. In case of flexible mounting, 6.3.5 Materials and testing harmful vibrations are to be avoided. Excessive movements of flexibly mounted The quality requirements for materials and gears are to be limited by stopper heat treatment for gears are divided into arrangements. Design of the oil systems and three levels according to DIN3990 T5 and maintenance methods with respect to ISO6336-5.The three levels are denoted ME, changing the oil should be developed to MQ and ML, respectively, and ME is the minimise oil leaks and spills. highest quality level.

Specific requirements for quality control and material requirements and testing are given for each material type and quality level. The

142 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø strength values for both pitting and bending N41.2 is based partly on ISO6336. The fatigue are dependent on the quality level. material and testing requirements that the Values are given in DIN3990 T5 and Classification Note 41.2 is based on are ISO6336-5. The DNV Classification Note given below.

Table 6-5. Recommended criteria for acceptance of gear lubricants. Parameter Methodology Recommended criteria for acceptance Viscosity (mm2/s) ISO 3104 ±10% Viscosity indices ISO 2909 Min. 90 Oxidation stability ASTM D 2893 Increase in viscosity of a test sample oxidised at 121°C should not exceed 6% of reference value Corrosion properties, iron ISO 7120 No rust after 24 hours with synthetic sea water Corrosion properties, copper ISO 2160 #1b strip after 3 hours at 100°C Foaming properties ASTM 892 Sequence 1: max. 75/10 10:00 Sequence 2: max. 75/10 10:00 Sequence 3: max. 75/10 10:00 Load carrying property DIN 51 354 Load stage min. 12 Micropitting resistance test FVA. No. 54 Stage 10 Filterability ISO/DIS 13357-1,2 As stated in standard

Testing and inspection of gearing • mechanical properties (including Requirements to testing and inspection of Charpy-V) gearing depend on which type of heat • ultrasonic test treatment is applied. The following types of heat treatment are dealt with in separate For steel that will be case-hardened, the subsections: mechanical properties need not be docu- • case-hardened mented before the heat treatment process, as • alloyed through-hardened (quenched they are generally documented after the final and tempered) heat treatment. • nitrided The impact energy (KV) at ambient Case-hardened gears temperature in tangential direction to For wind turbine gears, the test requirements pinion/wheel is not to be less than specified for case-hardened steel normally correspond for the approved material type and in no to ordinary and intermediate grades of case less than 30 J. quality as defined in DNV Classification Note 41.2. A higher level of material and For case-hardening the following is required: heat treatment quality control may result in • core hardenability (Jominy) is randomly acceptance of increased endurance values checked (see Classification Note 41.2). • suitable heat treatment is made prior to machining in order to avoid excessive Certificates are required for: distortions during quenching • chemical composition • carburisation is made by gas in a controlled atmosphere furnace. The furnace shall be equipped with carbon

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potential controls and continuously It is required that the depths to 550 HV, 400 recorded HV and 300 HV and the core hardness are documented to be within the approved The entire case-hardening process is specification. Further, the hardness at any checked at regular intervals with regard to: point below the surface is not to exceed the • The case microstructure: to be surface hardness (before grinding) with martensite with allowance for up to more than 30 HV. The core impact energy is 15% retained austenite and fine not to be less than the approved dispersed individual carbides. (Higher specification, and in no case less than 30 J. percentage of retained austenite may be accepted provided increased safety As mentioned above, the case-hardening factor against scuffing). process (the coupons) is normally to be • Decarburisation: not to be visible at a documented for each hardening batch and magnification of 500. each material charge. The use of coupons • The core microstructure: to be marten- made of the same material type but not same sitic/bainitic with no free ferrite in charge may be accepted provided that the critical tooth root area. manufacturer has a quality assurance system which ensures sufficient reproducibility. In The case-hardening process is normally to particular, the limits of elements in the be documented for each hardening batch and chemical composition combined with the each material charge with a certificate, respective heat treatment processes must which includes the following: ensure that the required core properties are • hardness profile obtained. This part of the quality assurance • core impact energy (KV) system has to undergo special evaluation. A If no alternative procedure is approved, the reduced extent of impact testing may also be certificate is to be based on a coupon test. considered. The coupon is normally to be made of ma- terial from the same charge as the actual Case-hardened gears are to have a minimum gear and heat-treated along with this charge. tooth root space hardness of 58 HRC over The coupon is to be sampled in the the entire face width. Otherwise, a reduction tangential direction and is not to be of permissible tooth root stresses applies, separately forged. If it is not possible to see Classification Note 41.2 Part 3, Section sample coupons tangentially, longitudinal or 7. radial samples may be accepted. Depending on the specific material type, this The coupon diameter is not to be less than 2 may be difficult to obtain for large gears and times the normal modulus, minimum 20 control testing may be required. Therefore, mm. Further, the size is to be sufficient for manufacturers may carry out special making 2 test pieces for impact energy (KV) procedure tests in order to document the of the core. permissible sizes for their various material types. Components of smaller sizes than The hardness profile (hardness as a function those tested need no documentation of tooth of depth) is to be determined by hardness root hardness. measurements with a load of 10-50 N. The measurements are to be made from the sur- If a component exceeds the tested size, or if face to the core. The expected amount of the manufacturer has not carried out a grinding is to be subtracted. procedure test, the tooth root space hardness

144 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø is to be checked at mid face. If this hardness Inspection is less than the specified minimum (58 HRC 100% surface crack detection by means of if nothing else is specified), this wet fluorescent magnetic particle method is measurement is to be carried out over the required for the toothed area, including the entire face width. ends of the teeth. Upon request, liquid penetrant may be considered. Nitrided gears For nitrided gears the process is to be docu- The tooth accuracy of pinions and wheels is mented for each nitriding batch and material to be documented with reference to ISO charge with a certificate containing: 1328-1975 or to a corresponding national • hardness profile standard. • white layer thickness Visual inspection is to be carried out with If no alternative procedure is specified, the respect to: certificate is to be based on a coupon test. • surface roughness of flanks The coupon is to be made of material from • surface roughness of root fillets the same charge as the actual gear and heat- • toot fillet radius treated along with this charge. • possible grinding notches of root fillet. Any grinding in the root fillet area, and The coupon diameter is not to be less than 2 in particular when leaving notches will times the normal modulus. The hardness result in reduction of permissible profile (hardness as a function of depth) is to stresses. See Classification Note 41.2 be determined by means of hardness Part. 3, Section 7. measurements with a load of 10-50 N. The measurements are to be made from the surface to the core. If further grinding is 6.4 Couplings intended (and is approved), the expected amount is to be subtracted. The major types of couplings are listed in the following along with issues of It is required that the depth to 400 HV and the importance for their design. core hardness are documented to be within the approved specification. 6.4.1 Flange couplings The flange thickness just outside the flange The white layer thickness is not to exceed fillet is normally to be at least 20% of the 10µm. required shaft diameter.

As mentioned above, the nitriding process Coupling bolts are to be prestressed so that a (the coupons) is normally to be documented suitable amount of prestress remains even for each hardening batch and each material under the most severe running conditions, in charge. The use of coupons made of the particular, with regard to bending moments. same material type but not from the same The level of safety is to be demonstrated in charge may be accepted provided that the both the ultimate limit state and the fatigue manufacturer has a quality assurance limit state. The same minimum safety system, which ensures sufficient reproduci- factors as those for shaft design apply, see bility. Section 6.1.

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If the torque transmission is based only on The yield strength to be applied in checks friction between the mating surfaces of according to these two criteria is not to flange couplings, the friction torque exceed 2/3 of the tensile strength of the key, (including the influence of axial forces and and it is not to exceed twice the yield strength bending moments) is not to be less than 1.5 of the shaft or the hub, whichever is involved. times the characteristic peak torque. In principle, there is to be no clearance The torque transmission may also be based between the hub and the shaft, however, a on a combination of shear bolts and friction certain amount of minimum interference fit is between the mating flange surfaces. A basic required, e.g. approximately 0.02% of the principle is that both the friction alone shaft diameter. (including the influence of axial forces and bending moments) and the shear bolts alone 6.4.4 Torsionally elastic couplings. should be able to transmit the characteristic peak torque. Rubber couplings are to be designed such that a failure of a rubber element does not 6.4.2 Shrink fit couplings cause loss of the connection between the rotor and the brake. The friction connection is to be able to transmit at least 1.5 times the characteristic 6.4.5 Tooth couplings peak torque without slipping. Bending moment influence is to be considered. Tooth couplings are to have a reasonable degree of safety with respect to surface For tapered mating surfaces where a durability and tooth strength. This is subject slippage due to torque and/or axial force to special consideration. may cause a relative axial movement between the tapered members, the axial movement is to be prevented by a nut or 6.5 Mechanical brake similar. When a nut is required, the pre- Mechanical brakes are usually used as a stress is to be of the same magnitude as the backup system for the aerodynamic braking axial force component from the tape. system of the wind turbine and/or as a parking brake, once the turbine is stopped, The permissible material stress depends on e.g. for service purposes. Mechanical brakes the relative wall thickness, material type, are sometimes also used as part of the yaw and whether the coupling is demountable or system. In a mechanical brake, brake not, and the usual range of permissible callipers, brake discs and brake pads form equivalent stress (von Mises) is 70% to crucial parts. A hydraulic system is usually 110% of the yield strength of the hub. used for the actuation and release of the brake. 6.4.3 Key connections The connection is to be able to transmit the 6.5.1 Types of brakes characteristic peak torque. Mechanical brakes can be active or passive, depending on how the hydraulic system of The shear stress in the key is not to exceed the brake is applied: 50% of the yield strength in shear. The • active brake: the pressure of the pressure on the side of the keyway is not to hydraulic system actively pushes the exceed 85% of the yield strength of the key. brake pads against the brake disc.

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1 cover for cardan 19 washer 2 suspension for screw pump 20 set screw 3 foundation plate for brake arrange- 21 washer ment 22 set screw 4 brake disc for hub 23 washer 5 middle plate for distance pipe 24 hexagonal socket head cap screw 6 fitting for cover 25 set screw 7 lock screw for brake disc 26 set screw 8 bushing 27 set screw 9 bushing 28 washer 10 cover for brake 29 hexagonal head bolt 11 side cover for brake, top 30 slotted raised countersunk head 12 side cover for brake, bottom screw 13 sensor pick-up plate at cardan 31 brake calliper 14 fitting for sensor 32 blade calliper air bleed 15 angle iron for cover 33 screw pump 16 washer 34 brake lining 17 hexagonal head bolt 35 fork sensor 18 hexagonal head bolt 36 shims

Figure 6-22. Example of brake arrangement. Courtesy Vestas.

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• passive brake: the pressure of the In general, the thicker the disc is, the better hydraulic system keeps a spring tight. is its ability to absorb temperature loading. Once the pressure is released, the spring is also released and will push the brake The possible variation in the frictional pads against the brake disc. coefficient poses a problem, which must be In either case, the hydraulic pressure of the given due consideration when the brake hydraulic system is crucial in order to be system is being dimensioned. If the able to operate the brake as intended. The frictional coefficient is too big, the brake hydraulic pressure is usually provided by force will become too large. If the frictional means of an accumulator. For active coefficient is too small, the brake system systems, it is particularly important to make will be unable to brake. sure that the pressure in the accumulator is always available, and it is important to have 6.5.3 Brake torque sequence redundancy in this respect, i.e. an extra The rise time from zero to maximum brake pressure source is necessary for backup. torque will influence the dynamic response

of the turbine heavily. Since the turbine is The type of spring used in a mechanical rotating when the brake is actuated, full brake to keep up a pressure is often a coil brake force will be mobilised immediately spring of the disc spring type. This type of upon the actuation of the brake. spring is nonlinear and has the advantage that it is capable of maintaining an approximately constant spring pressure over a considerable range of deflection.

An example of a brake arrangement is given in Figure 6-22.

6.5.2 Brake discs and brake pads Brake discs and brake pads must be able to withstand temperature loading, since the friction during braking leads to dissipation of energy in terms of heat and causes high temperatures to develop locally. Figure 6-23. Temporal evolution of torque at and after grid loss with subsequent actuation of the brake. Brake pads may be made of different kind of materials. Ceramic brake pads do not In the design of the brake system, it is withstand high temperatures (temperatures important to consider the maximum torque during the course of the braking. Depending in excess of 300-400°C) very well, in the on the dynamics, the maximum torque may sense that they lose their frictional well occur towards the end of this course, resistance. For high temperatures, brake and transient vibrations may sometimes pads made of sinter bronze can be used. follow. An example of the temporal

evolution of the torque during the course of Brake discs must be subject to temperature braking is given in Figure 6-23. The turbine calculations or temperature measurements. is rotating when a grid loss occurs, and the They must meet requirements to planarity in torque drops to zero. When the connection order to work properly, i.e. they must not to the grid is lost, the brake is actuated, and warp when subjected to temperature loading.

148 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø the torque raises and develops as shown in is not to have a flash point the figure. lower than 150°C and is to be suitable for operation at all temperatures that the system may normally be exposed to. Means for 6.6 Hydraulic systems filtration and cooling of the fluid are to be incorporated in the system wherever In a hydraulic system, power is transmitted necessary. and controlled through a liquid under pressure within an enclosed circuit. 6.6.2 Accumulators Hydraulic systems are used in wind turbines, for example in terms of a hydraulic For gas and hydraulic fluid type accumulator for pitching of the rotor blades. accumulators, the two media are to be A hydraulic system must be protected suitably separated if their mixture would be against exceeding the maximum admissible dangerous or would result in the pressure. A pressure release valve can be contamination of the hydraulic fluid and/or used for this purpose and will prevent loss of gas through absorption. explosion in the event of fire. All components of the hydraulic system must be Each accumulator is to be protected on both easily accessible for assembly, adjustment its gas side and its hydraulic fluid side by a and maintenance. safety device such as a relief valve, a fuse plug or a rupture disc to prevent excessive Pressure shocks should be kept to a pressure if overheated. When the minimum. Pressure shocks or a large accumulator forms an integral part of a pressure drop must not lead to a dangerous system with such a safety device, the condition. A safe condition must be accumulator itself need not be supplied with guaranteed in the event of power supply such a safety device. failure and in the subsequent event of restoration of the power supply. 6.6.3 Valves

The following external factors must not Valves are used in hydraulic systems to affect the operation of a hydraulic system: control the hydraulic effect between a pump and an engine, cylinder, or actuator. The • salt and other corrosive substances purpose of valves is to govern the direction • sand and dust • and amount of the volume flow rate or to moisture block the volume flow rate, and it is also to • external magnetic, electromagnetic and limit or control the pressure of the fluid. A electric fields distinction can be made between four major • sunlight types of valves, viz. • vibrations • shut-off valves, which block flow in one direction and allow for partly or full When a hydraulic system forms part of the flow in the opposite direction protection system, grid failures and extreme • directional control valves, which control temperatures must not compromise the the direction of the volume flow rate, operation of the system. and which can block the volume flow rate or adjust the amount of volume 6.6.1 Arrangement flow rate Hydraulic systems should have no • pressure valves, which limit or control connections with other piping systems. The the hydraulic pressure

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• flow control valves, which adjust the • the return pipes must be made volume flow rate to the hydraulic sufficiently strong by their design actuator such that the desired speed of and construction, or by protection, the hydraulic actuator is achieved. such that they cannot be closed off by external damage. 6.6.4 Application in protection • filters must be installed on the systems pressure side of the pump and should be avoided in the return If a hydraulic system forms part of a pipe. If filters are installed in the protection system, the design and return pipe, a bypass is required. construction of the hydraulic system must • comply with the requirements to the no components must be installed in protection system, and it must be designed, the return pipe which can lead to constructed and used as a fail-safe or blockage of the pipe as the result of redundant system. Hydraulic systems, which maloperation. form parts of protection systems, can be 2. For actively released brakes, which are divided into three categories as follows: being actuated hydraulically or pneumatically, the actuation must be 1. Systems in which the brake is actively accomplished by means of a pressure released by a hydraulic or pneumatic accumulator, and the following pressure medium. additional requirements apply: • 2. Systems in which the brake is actively the connecting pipe between the released (mechanically, hydraulically, accumulator and the actuated pneumatically or electrically), but component, e.g. the blade adjust- actuated hydraulically or pneumatically. ment mechanism, must be as short The active release thus takes place as possible. • against a ”passive” oil or air pressure. no other components, such as 3. Systems in which the brake is released valves, couplings and rotating in the neutral state (passively released) elements, are allowed in this and is actuated hydraulically or connection. pneumatically. • the pressure in the accumulator must be monitored at a level which It is recommended to comply with the is sufficiently high to guarantee following requirements to these three types independent braking action. of hydraulic systems: • the other protection system of the 1. When brakes (mechanically or wind turbine must be actuated aerodynamically) are actively released, mechanically, and the actuating the pressure medium shall be able to element shall be designed and flow away in a reliable manner during a constructed ”safe-life”, e.g. a braking action. In the hydraulic system, mechanical spring. the following features shall be present 3. The use of a passively released braking as a minimum: system, by which the braking action • two valves must be placed parallel takes place by means of build-up of to one another as a switching pressure, is only permissible when the element. An incorrect switch following conditions are met: position of a valve must lead to a • the monitoring, control and safe situation. It must be possible to actuation systems are designed and test each valve individually. constructed with redundancy, i.e.

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they are designed and constructed 6.7 Generator at least in duplicate, and function 6.7.1 Types of generators independently of the electrical grid. • the pressure build-up is supported The generator is the unit of the wind turbine by a pressure accumulator to which that transforms mechanical energy into the same requirements apply as electric power. While the blades transfer the those stated above. of the wind to rotational • the other protection system of the energy in the transmission system, the wind turbine must be equipped generator provides the next step in the with a ”safe-life” actuation supply of energy from the wind turbine to element. the electrical grid.

In addition, for Categories 2 and 3, an This section deals with the safety aspects of actively operated hydraulic or pneumatic the generator and its interaction with the installation shall not be used to keep a wind rotor and the transmission system. The turbine in a safe state for a long period after functioning of the generator and its a protection system has been actuated. interaction with the grid is dealt with in Chapter 9. An example of a generator is 6.6.5 Additional provisions shown in Figure 6-24.

It is recommended to take the necessary steps to ensure that failure of a redundant system can be detected. Long-term standby redundancy should be avoided. In the case of oil leaks in hydraulic systems, other wind turbine components or other systems must not be affected.

6.6.6 Codes and standards The following guidelines and standards deal with design of hydraulic systems.

Teknisk Forlag, Hydraulik Ståbi, (in Danish), ed. T. Rump, Teknisk Forlag a.s., Figure 6-24. Six-pole generator © Danish Wind Copenhagen, Denmark, 1996. Turbine Manufacturers Association

ISO, ”Hydraulic fluid power – General rules The rotational speed of the generator is for the application of equipment to dependant on the grid frequency and the transmission and control systems,” number of poles International Standard, ISO4413, 1st edition, 1979. = ⋅ f ns 60 p ISO, ”Hydraulic fluid power – Gas-loaded accumulators with separators – Range of f grid frequency in Hz pressures and volumes, characteristic p number of pole pairs quantities and identification,” International ns synchronous rotational speed in rpm. Standard, ISO5596, 1st edition, 1982.

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The produced alternating current, which is insignificant. When a larger slip is allowed, transmitted to the electrical grid, must match say within 10%, and can be varied the frequency of the grid. The required electronically, e.g. by a rotor current rotational speed of the generator’s rotor is controller, it is referred to as variable slip. achieved by means of the gearbox of the The pitch or stall control of the wind turbine wind turbine, since the wind turbine rotor is meant to ensure that the allowable slip of itself is not allowed to rotate at this high the generator is not exceeded. speed for physical reasons. Rotation of the wind turbine rotor at the high rotational The advantage of the variable slip comes speed of the generator rotor would cause about when the wind turbine is operated at aerodynamic problems and supersonic its rated power. At the rated power, power speeds. Noise would also be a problem, and fluctuations caused by changes in the wind excessive centrifugal forces would be speed are undesirable. When a wind gust generated. A multi-pole generator has the hits the wind turbine rotor, the slip enables advantage of reducing the mechanical the generator speed to increase a little in complexity of the generator and makes it response to the gust without causing a possible to reduce the gearbox and corresponding increase in the generated sometimes even to omit it. power output. Thus, the slip ensures a smooth power output and at the same time There are two major types of generators: contributes to keeping the loads on blades, • synchronous generators main shaft and gearbox down. • asynchronous generators The variation of the operating speed with the A synchronous generator operates at a applied torque for an asynchronous constant speed, dictated by the frequency of generator is beneficial because it implies a the connected grid, regardless of the smaller peak torque and less wear and tear magnitude of the applied torque. The speed on the gearbox than for a synchronous dictated by the frequency of the grid is also generator. This is one of the most important known as the synchronous speed. reasons for using an asynchronous generator rather than a synchronous generator in a An asynchronous generator is a generator, wind turbine, which is connected directly to which allows slip, i.e. deviations from the the electrical grid. Another reason for using rotational speed dictated by the frequency of asynchronous generators is that the involved the connected grid. In other words, the slip is beneficial when there is flexibility in rotational speed is allowed to vary the structural system. somewhat with the applied torque. This is the most common generator type used in Traditionally, the active materials in a wind turbines. A variant with coiled rotors is generator consist of magnetically conducting prevalent. The slip is defined as the iron and electrically conducting thread difference between the rotational speed of arranged in a coil. Permanent magnets are the generator and the rotational speed becoming increasingly common, and dictated by the frequency of the grid. The electrical components, such as temperature slip is sometimes expressed in percent of the sensors, are becoming integral parts of the latter. When a slip of up to about 1% is generator. possible, the operation mode for the asynchronous generator is referred to as With a view to switches and power failure, constant speed, given that this slip is rather the generator must be able to withstand

152 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø switching on in phase opposition at 100% humidity in the external ambient air of 95% residual voltage, or it must be secured within the entire temperature range as against the occurrence of this situation by specified above. means of a special arrangement in the control system. The influence of the humidity of air on the electrical components is always dependent Generators are to be constructed in such a on other climatic parameters, in particular, way that when running at any working on the temperature and on changes in the speed, all revolving parts are well-balanced. temperature. Creation of condensation can Suitable fixed terminal connectors are to be be remedied by means of heating when the provided in an accessible position with generator is not running, and by heating of sufficient space for convenient connection closets when the self-heating is insufficient of the external cables. to avoid damaging condensation of water.

6.7.2 Climate aspects Resistance toward saline atmospheres When the wind turbine is to be located near Temperature a coastline or offshore, the electrical The generator shall be designed to be fully equipment shall be constructed in such a functional at the temperatures that are likely way that it will not be damaged by the to occur locally, when the external ambient [− ]° impact from the saline and moist temperature is within the range 10,30 C environment. The encapsulation, the cooling for normal operation. These limits refer to and the insulation of the generator are all to Danish conditions. For other countries, this be designed in such a manner that the wind [− ]° range is typically expanded to 20,30 C. turbine can withstand these impacts. As The ambient temperature is the regards equipment in the turbine, this can be instantaneous value of the temperature of the achieved by applying climate control in air outside the wind turbine. The terms of desalination and dehumidification temperature range for the location of the systems or by heating to avoid condensation generator should be documented, and the and saline deposit. self-heating of the generator should also be documented. When the generator is Electrical immission and emission assembled at its intended location, it needs Electric and electronic equipment, whose to be ensured that it is not placed near heat functions can be affected by electrical dissipating components, or it has to be immission, shall meet the requirements designed to withstand the associated given in the EMC directive as described in temperatures. DS/EN50082-2, Generic Immunity Standard, Industrial Environment. Electric Relative humidity of air and electronic equipment, from which To avoid leak currents, i.e. low insulation electrical emission can occur, shall meet the resistance, corrosion and other damaging requirements laid down in the EMC influences on the components, these directive as described in DS/EN50081-2, − components should either by their design Generic Emission Standard, Industrial or by climate control in the wind turbine − Environment. be secured so that damaging condensation cannot occur. 6.7.3 Safety aspects

The electrical components of the turbine The generator forms one of several links in shall be fully functional at a relative the transmission between the rotating system

6 – Nacelle 153 Guidelines for Design of Wind Turbines – DNV/Risø and the electrical system of a wind turbine, interaction between these features and the i.e. between the blades and the grid. Failure generator which ensures that the turbine is of any link in this chain implies a risk. This kept in a safe condition. If the generator is risk is absorbed by the protection system, disconnected due to an error or due to which brings the wind turbine to a safe intervention from the protection system, or condition in which it remains until normal if an error occurs in the blade pitch system operation can be resumed. The number of or the speed control, at least one of the two failures that demand activation of the fail-safe brake systems, which form part of protection system should be minimised in the protection system, shall begin working. order to reduce the burden on the protection These are usually pitchable blades and system. The probability of breakdown mechanical brakes. Note that for turbines results from the probability of failure of the with pitchable blades, one of the two brake protection system combined with the systems – the pitchable blades – forms part probability of a critical error that requires of both the protection system and the control the intervention of the protection system. system. Errors which have common root The probability of breakdown shall be less causes need to be given special attention. than 0.0002 per machine year. The target for Neither blade pitch nor mechanical brake the reliability of the protection system is a will have the required effect if the blades are number, which is large enough to keep the locked at a position of, for example, +15°. probability of breakdown below this level. For such concepts, the target reliability The grid-connected asynchronous generator quoted above still applies. Components with short-circuited cage winding in the which form part of both control and rotor has been the basis for the safety protection functions are to be fail-safe considerations behind the Danish Approval designed, or their probability of failure is to Scheme and for the associated recommen- be minimised. dations. The generator is to be designed such that it For a stall-controlled wind turbine with can produce a sufficiently large torque to asynchronous generator, it is the generator keep the turbine within its defined range of and its relatively simple control that most of operation, see Section 2.2.1. Disconnection the time constitute the system that keeps the (switching off) of the generator should be turbine in a safe condition. If the generator based on reverse power, on zero-power, or is disconnected due to an error or due to on a signal from the relay protection against intervention by the protection system, at electrical errors. The purpose is to utilise the least one of the two fail-safe brake systems, braking power of the generator in all which are part of the protection system, shall situations. begin working. These are usually pitchable blade tips and mechanical brakes. “Fail- When a frequency converter is used, it needs safe” is defined as a design philosophy by to be considered together with the generator which the safety of the turbine is maintained with respect to safety and frequency of even during component failure or grid errors. failure. The generator is to be designed for the Insofar as regards a turbine with variable mechanical impacts it will be exposed to. At speed and pitchable blades, and thus a start-up of a turbine with fixed unpitchable relatively complex control, it is the blades in high wind speeds, the cut-in of the

154 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø generator should be performed at an 60034-3. For stall-controlled turbines with undersynchronous number of revolutions to pitchable blade tips, a typical upper limit for limit the acceleration until the full number overspeed in the context of grid failure and of revolutions is achieved. At pitch control activation of the blade tips will be about 1.4 of the blades, a control strategy is to be times the operational number of rotations. applied which ensures limitation of the acceleration. Synchronous generators may produce overvoltage, which may be damaging to the 6.7.4 Cooling and degree of sealing frequency converter. To prevent overvoltage, the voltage should be adjusted The cooling system of the generator should downward. When permanent magnets are as a minimum correspond to IC41 for jacket used, the generator terminals may be short- cooling according to DS/EN60034-6. circuited or loaded by brake resistances.

Generators are to be capable of withstanding The generator, including its possible the following overspeed during two minutes: external encapsulation and its external 1.25 times the rated maximum speed. cooling system with cooling agents such as air or water, should as a minimum be 6.7.7 Overloading protected against external impacts corresponding to degree of sealing IP54 in The wind turbine is to be automatically accordance with DS/EN60034-5. controlled in such a way that it will be brought to either standstill or idling at a low The machine cabin of the wind turbine is not rotational speed if the average produced considered as a sufficient enclosure of the power in 10 minutes exceeds 115% of the generator for protection against unintended nominal (rated) power Pnom. For a stall- intrusion of objects such as tools, dust from controlled wind turbine with a conventional brakes, and hydraulic liquids. The internal asynchronous generator, it is in addition shielding in the cabin is to provide safety for required that it shall be brought to a personnel and protection against unintended standstill or idling if the produced one- objects. The temperature can be higher in second mean power exceeds 140% of the the cabin than outside the turbine, in which nominal (rated) power. The corresponding case the generator is to be designed for the limiting value for a pitch-controlled turbine temperature within the cabin, unless some with a conventional asynchronous generator other external cooling system is in place. is 200% of the nominal (rated) power Pnom.

6.7.5 Vibrations For a passively controlled generator, the instantaneous value of the torque shall as a The generator shall be balanced such that it minimum be 1.35 times the one-second as a minimum fulfils the requirements to value. For an actively controlled generator, Class N according to DS/IEC60034-14. The which forms part of the protection system, generator shall be capable of withstanding the overload capacity of the generator shall vibrations from other parts of the wind be documented for each individual concept. turbine. At short-circuiting of the grid and at short-

term grid failure, the generator shall be 6.7.6 Overspeed capable of absorbing the thermal and The generator is to be constructed such that dynamic forces. it fulfils the requirements to overspeed according to DS/EN60034-1 and DS/EN

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6.7.8 Materials exposed to. A frequently applied maximum value for dU/dt is 1 kV/µsec. However, in Permanent magnets µ Permanent magnets shall be designed in some cases up to 5 kV/ sec is used. As a such a manner that the minimum induction minimum, the insulation shall be capable of locally in the magnet during an electrical resisting impulse voltages of 1300 V as fault will not fall short of the break point measured on the generator clamps. value, at which irreversible demagnetisation Alternatively, a different wire can be used to sets in at some extreme temperature. It is obtain extra insulation. recommended to keep a margin to the break point value of at least 0.1 Tesla for NeBFe If there is a risk that the internal heating is and 0.05 Tesla for ferrite materials. insufficient to avoid damaging condensation Mechanical stability of NeBFe magnets can of water in the generator, in particular at be protected against corrosion by means of standstill, the generator shall be furnished coating by tin, zinc or similar. with at-rest heating (heating system for use at standstill). In case of grid failure and Coils longer periods of standstill, it must be Coils are to be constructed with an ensured that the coil has dried out before the insulation, which as a minimum meets the turbine is restarted. Note that for a generator requirements to Class F according to in protection class IP54, at-rest heating is IEC60085. The temperature rise at normally not considered necessary. maximum load must not exceed the limits set forth for the chosen class of insulation in Bearings IEC60034-1. This refers to coils in air- When frequency converters are used, cooled generators. The allowable tempera- capacitive couplings might produce flow ture rise values are reproduced in Table 6-6 paths through the bearings. This might together with acceptable values for the total imply a reduced lifetime for the bearings. temperature. The impact shall be reduced to a level that the bearings can withstand, e.g. by Table 6-6. Allowable temperatures and temperature insulation of the bearings, or by dU/dt rise values. filtering. Insu- Temperature Total lation rise (°C) temperature Bearings are to be efficiently and class (°C) automatically lubricated at all running A 50 105 speeds and within the service intervals E 65 120 specified by the manufacturer. Provisions B 70 130 are to be made to prevent the lubricant from F 90 155 gaining access to windings or other insulated H 115 180 or exposed conducting parts.

When a frequency converter is used, the coil 6.7.9 Generator braking insulation becomes exposed to a large By controlling a frequency converter, the impact due to large voltages U and large generator can be used to reduce the time derivatives dU/dt of the voltage. The rotational speed to the level that corresponds impact can be reduced by application of to the minimum frequency of the converter. filters for smoothing of the voltage. It shall This is referred to as generator braking. By always be ensured that the insulation of the means of external brake resistances, braking generator can resist the impacts that it is by means of the generator can form part of

156 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø the protection system brakes as well as of • measurement of excitation current at the operational brake system, provided that rated current, voltage and power factor the generator remains magnetised during the (if possible also at cosφ = 0 lag, where φ braking and that braking to bring the turbine denotes phase angle) to a safe condition is fail-safe. Generator • short-circuit test braking is suitable for synchronous generator types, including permanently Generators should withstand terminal short- magnetised generators and multiple-poled circuits at least during one second without generators without gearbox, for which damage to the generator itself or to its mechanical braking is made difficult by excitation equipment. This can be verified large dimensions. by means of type tests or random tests.

6.7.10 Lifetime Generators should have sufficient The generator shall be designed for the same momentary and steady short-circuit current lifetime as the rest of the wind turbine. The in relation to the release characteristics of design lifetime shall be at least 20 years. switch and fuse gear on the installation, This requirement does not apply to thereby ensuring a reliable release by short- components subject to wear, for which circuits anywhere in the grid. This can be replacement intervals are specified in the verified by random tests. user’s manual for the actual type of wind turbine. It is recommended that the steady short- circuit current should normally not be less 6.7.11 Testing of generators than 3 times the full-load current.

For new generators, it is recommended that the manufacturer carries out tests as 6.8 Machine support frame specified in the following. The tests may be arranged as type tests, as production sample The machine support frame is located on top tests, or as routine tests. of the tower and supports the machinery, including the gear box. Usually, it also The manufacturer’s test reports should provides support for the nacelle cover. In provide information about make, type, serial contrast to the tower, the machine support number, insulation class, all technical data frame is usually a very turbine-specific necessary for the application of the structure, which can be constructed in many generator, and the results of the tests. different ways and according to many different layouts. It can be a welded, bolted Recommended tests: or cast steel structure. Sometimes it is • temperature test at full load (minimum formed as an integral part of the gear box, 1.15 times rated load) i.e. the gear box itself acts as the machine frame. Sometimes it is integrated with the • overload test yaw system. Sometimes it is as simple as a • overspeed test • big plate. It is, in general, a much less high-voltage test standardised product than a tower. • measurement of insulation resistance • measurement of resistance of windings For design of the machine support frame, the • measurement of air gap following issues are important to consider: • open-circuit voltage characteristics • a sufficient stiffness of the frame must • short-circuit current resistance be ensured in order to meet the stiffness

6 – Nacelle 157 Guidelines for Design of Wind Turbines – DNV/Risø

requirements for the machinery. The load. In this context, it is important to gear box should not be able to move choose correct lift and drag coefficients. relative to the bearings, and the yaw system will not get sufficient mesh with The nacelle enclosure is usually used also as the gear rim if the frame is not a walkway and needs to be designed with sufficiently stiff. sufficient dimensions for this purpose. In • sufficiently fine tolerances must be met this context, it is particularly important to during the manufacturing of the frame consider the design of the fixtures of the in order to facilitate the proper enclosure. positioning and assembly of the yaw system and the transmissions. Moreover, it is important that the nacelle • the frame must be designed against enclosure can be opened to allow for fatigue owing to its exposure to the removal of damaged components for rotor forces. replacement, for example, by means of a • access to the nacelle is obtained through helicopter. the tower and the machine support frame, which implies that the machine Artificial light has to be installed to ensure support frame must include a hole of safe access and working conditions. convenient size for personnel to pass.

The machine support frame is exposed to 6.10 Yaw system rotor loads consisting of thrust, yaw moment Yaw denotes the rotation of the nacelle and and tilt moment. These load components are the rotor about the vertical tower axis. By not necessarily in phase. In addition hereto yawing the wind turbine, the rotor can be comes weight. Also, local forces wherever positioned such that the wind hits the rotor forces are being transmitted should be plane at a right angle. The yaw system considered, i.e. forces at bearing housings, provides a mechanism to yaw the turbine gear shafts, and yaw system. It is and to keep the rotor axis aligned with the recommended to use finite element methods direction of the wind. If situations occur for structural analysis of the machine where this alignment is not achieved, yaw support frame. errors are produced. The yaw error, or the yaw angle, is defined as the angle between the horizontal projections of the wind 6.9 Nacelle enclosure direction and the rotor axis. The purpose of the nacelle enclosure is to protect the machinery and the control system The yaw system can be either passive or of the wind turbine against rain and moisture active. A passive yaw system implies that and against salt and solid particles, such as the rotor plane is kept perpendicular to the sand grains in the air. The nacelle enclosure direction of the wind by utilisation of the is also meant to protect against noise and is surface pressure, which is set up by the wind therefore often covered with some noise- and which produces a restoring moment reducing material. The enclosure needs to be about the yaw axis. For upwind turbines, tight to fulfil its purpose. However, it also this usually requires a tail vane in order to needs to have ventilation to allow for work properly. Also, coning of the rotor can adequate cooling of the gear system. The help keeping the nacelle in place. Note that a nacelle enclosure is to be designed for wind passive yaw system may pose a problem in

158 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø terms of cable twisting if the turbine keeps positioning of the turbine relative to the yawing in the same direction for a long time. wind is also referred to as forced yaw. Most An active yaw system employs a mechanism large horizontal axis wind turbines use of hydraulic or electrically driven motors forced yaw to align the rotor axis with the and gearboxes to yaw the turbine and keep it wind. An example of an active yaw system turned against the wind. Such active is given in Figure 6-25.

Yaw drive Mashin frame (nacelle)

Slewing bearing Yaw gear wheel

Yaw brake

Tower top

Figure 6-25. Typical active yaw system involving a slewing bearing

The mechanism used for an active yaw system, including the yaw ring, the yaw system usually consists of a number of drive with the yaw motors, the yaw bearing, electrically operated motors in conjunction and the yaw brakes. with a gear that actuates a large toothed yaw ring in the tower circumference. Together The yaw error is usually measured by means with yaw brakes and yaw bearings, these of direction sensors such as one or more components are most often delivered as wind vanes. The wind vanes are usually standard components from a supplier, who placed on top of the nacelle. Whenever the also provides the pertaining design wind turbine is operating, an electronic documentation. When used in a yaw system, controller checks the orientation of the wind it should be noted that these components vanes and activates the yaw mechanism may be exposed to conditions which have accordingly. In addition to this automated not been taken into account by the supplier. yaw of the wind turbine, it should be The following sections deal with the possible to yaw the nacelle manually. different components of an active yaw Manual yaw is needed during start-up,

6 – Nacelle 159 Guidelines for Design of Wind Turbines – DNV/Risø service of the yaw system, and testing of the place. When the duration of yaw is not turbine. known for design, DS472 specifies that yaw can be assumed to take place during 10% of 6.10.1 Determination of design loads the time for all wind speeds that occur. Assuming yaw in 10% of the time is, Yaw is characterised by the maximum however, quite conservative for most sites. angular yaw velocity ωk and the fraction of the design life during which yaw takes

MzR FN yN FzR MyR FyR yR z Fx N R zR Yaw bearing MxR Rotor centre

Figure 6-26. Loads acting on the rotor and the yaw bearing.

The static load consists of the weight of the M = M 2 + M 2 nacelle and rotor, acting as an axial force on tilt 1 2 the bearing. This load is superimposed by the wind load on the rotor as illustrated in with Figure 6-26. = + M 1 M yR FxR z R In Figure 6-26, the rotor loads are drawn in = − − + M 2 M xR FzR y R FyR z R FN y N directions according to a conventional co- ordinate system and not in the direction that they will normally have. 2 + 2 Fr = FyR FxR

Fa = FzR + FN FxR side force on rotor and nacelle

FyR thrust on rotor The yaw moment depends on the magnitude FzR weight of rotor of the yaw error and the wind speed. The MxR tilting moment at rotor direction of the yaw moment depends on the MyR driving torque at rotor direction of the rotation of the rotor in MzR yaw moment at rotor conjunction with the direction of the yaw FN weight of nacelle error. yN horizontal distance to nacelle c.o.g. yR horizontal distance to rotor c.o.g. The extreme design yaw moment, which is zN vertical distance to nacelle c.o.g. the design basis for yaw drives and yaw zR vertical distance to rotor c.o.g. brakes, is likely to appear during operation From these quantities, the static loading on at a maximum wind speed with a maximum yaw error. Dynamically, the yaw moment the yaw bearing, tilt moment Mtilt, yaw will have a tendency to oscillate with a moment Myaw, radial force Fr and axial force frequency of x times the rotor frequency, Fa, can be calculated as where x denotes the number of blades.

Myaw = MzR + FxR ⋅ yR + Mbrake + Mfriction

160 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø

Figure 6-27. Relation between wind speed, yaw error and yaw moment.

Figure 6-27 shows a computer simulation of for gyro effects to become negligible. the yaw moment as a function of the yaw Reference is made to Section 4.2.1. error and wind speed for a three-bladed wind turbine. It appears that the yaw error A passive yaw system will normally require oscillates with frequencies equal to the rotor some sort of damping arrangement to reduce frequency and three times the rotor the yaw speed. frequency. It also appears that the sign of the yaw moment changes when the yaw For large turbines with active yaw systems, direction changes, and – not surprisingly – the yaw speed is usually lower than 1°/sec, the yaw moment increases when the yaw which is small enough for the gyro effects to error and the wind speed increase. be ignored. To achieve such a low yaw speed, the yaw motors needs to be connected 6.10.2 Yaw drive through a gearbox.

The yaw drive is the system of components Such drives can be delivered as standard used to cause the yaw motion. A large yaw equipment from manufacturers of speed will produce gyro effects that will electrically operated motors. result in large loads on the wind turbine. The yaw speed must therefore be small enough The yaw drives must have sufficient power to overcome the largest mean yaw moment

6 – Nacelle 161 Guidelines for Design of Wind Turbines – DNV/Risø occurring plus friction in the yaw bearing. yaw moment might be transmitted through Installation, lubrication and service should the yaw drive. be undertaken in accordance with the specification from the manufacturer. 6.10.4 Yaw brake

During power production, turbulence, wind Pinion shaft, pinion and mounting must be shear and fairly small inevitable yaw errors designed to withstand the maximum yaw will give rise to a torque moment about the moment, including partial safety factors. tower axis. To keep the nacelle in position

and to spare the gears, it is common to 6.10.3 Yaw ring mount a brake disc in connection with the The yaw drive is normally mounted on the yaw bearing. Braking can then be performed nacelle in such a way that it is in gear with a by means of hydraulic activated callipers. toothed yaw ring mounted on the top of the Passive hydraulic callipers, i.e. spring- tower. Yaw rings are normally toothed on applied brake callipers, are preferable the inner surface as this provides a better because they can ensure braking also in the protection from the surroundings and case of a leakage in the hydraulic system. implies a slightly better mesh. The brake calliper manufacturer will provide The gears connected to the drive and the surface tolerances and geometrical tolerances yaw ring must be designed in such a manner for the brake disc. These tolerances must be that tooth failure for the maximum peak yaw complied with. moment is prevented. The yaw brake callipers, discs and mounting Bending stress in a tooth can be calculated bolts shall be designed to withstand the according to DIN 3990, Part 3 from maximum occurring yaw moments. For turbines with yaw brake systems designed F σ with limited safety (S = 1.15), it shall be σ = ⋅ ⋅ ≤ Fl F b ⋅ m Y f Y ε proven that the turbine will withstand free S F yaw operation, i.e. inertia forces due to acceleration of the nacelle. Yf tooth shape factor

Yε load reduction factor F tooth force The brake is to be protected against dust, b tooth width corrosion, oil and any other influence that m module might alter the friction. The brake and σ tooth yield stress control system shall be designed such that a Fl situation where pads are worn out is avoided. SF safety factor

Scuffing might occur in gears with hardened In addition to using the ordinary yaw brake, pinions whose tip edges act as scrapers. it must be possible to block the yaw Damage due to wear and fatigue should be mechanism to enable service and considered. maintenance adjacent to the yaw mechanism without any personal risk. It should be noted that the yaw ring is not only loaded during yawing. Even though Blocking of the yaw system can be done by yaw brakes might be applied a part of the mechanical fixation of the yaw ring or the yaw motor shaft. Blocking of the yaw system must not be dependent on external

162 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø power supply. Only in the case of negative Tolerances for the mounting surfaces should callipers, the motor is allowed to be be in accordance with specifications from electrically braked. A fixation of the ring is the bearing manufacturer. To prevent the preferable because a possible backlash in the bearing from becoming distorted, the contact yaw gear can be large enough to cause the surfaces must be carefully machined and nacelle to jerk during service. attention must be drawn to the stiffness of the surrounding structure. Application of a The locking mechanism must be designed plastic grouting may compensate for for wind speeds up to the defined stop wind irregularities in the contact surfaces. speed, above which maintenance, which demands locking of the yaw mechanism, is Slewing bearings used in yaw systems are not permissible. different from normal bearings in that they are exposed to oscillatory motion. This 6.10.5 Yaw bearing oscillatory motion and the low rotational speed imply that slewing bearings have a The yaw bearing is the bearing that supports tendency to exhibit a relatively low ratio the nacelle in a horizontal axis wind turbine. between the lubrication film thickness and It is located between the rotating nacelle and the surface roughness. the stationary tower and transmits wind loads from the nacelle to the tower. As Calculation of the load rating for bearings in regards the yaw bearing, it has been oscillatory motion is described in NWTC. It common in the past to choose between two is quite complex and involves the variation different solutions – slide plates or rolling of load and the bearing rotational position as bearings. a function of time. In practice, it is common

that the bearing manufacturer provides the Rolling bearings will often be designed as design calculation or means for how to slewing bearings, which are capable of verify the design. accommodating combinations of axial, radial and moment loads. Yaw motion is Unlike regular rolling bearings, whose generated by gears mounted in the nacelle strengths are represented by the load rating and being in gear with the toothed bearing. C, the strength of a slewing bearing will Slewing bearings, as seen in Figure 6-28, are normally be represented in terms of a curve mounted by means of bolting to the seating that gives the relation between the allowable surfaces. Usually, bolts of quality 10.9 are equivalent tilt moment and the equivalent recommended, see Appendix A. axial load. Reference is made to Figure 6-29

for an example.

The equivalent axial load Feq is calculated from the radial load Fr and the axial load Fa in the same way as for ordinary rolling bearings,

Feq = (X⋅Fa + Y⋅Fr)⋅ KA ⋅ KS

in which X and Y are combination factors which depend on the bearing type and the Figure 6-28. Four Point Ball Bearing. ratio between Fa and Fr. Values of X and Y

6 – Nacelle 163 Guidelines for Design of Wind Turbines – DNV/Risø

will be provided by the manufacturer. KA is As for any other component in the wind an application factor and is recommended to turbine, the dynamic loading of the bearing be in the range 1.7-2.0 for yaw bearings. can be represented by one or more load The safety factor KS equals the partial safety spectra, or – on a simpler form – by a factor for the load of the relevant load case. number of load cases that are assumed to adequately represent the loading that the The tilt moment as calculated in Section bearing will experience over the design life 6.10.1 is denoted Mt, and the equivalent tilt of the turbine. In the latter case, at least four moment Meq is calculated as load cases should be modelled. The equivalent load for a specified load spectrum

Meq = Mt ⋅ KA ⋅ KS or a set of representative load cases can be calculated according to the formula in which KA and KS are to be taken as for the axial load. p ⋅ ∑ Pi Oi i P = p , F eq ∑Oi

in which P denotes either the equivalent L axial force Feq or the equivalent tilt moment M , depending on whether axial force or tilt P eq moment is considered. Oi denotes the duration of the load Pi corresponding to the O Meq ith load block in a discretisation of the load Figure 6-29. Bearing life curve. spectrum or the ith load case when a load case representation is adopted. The exponent The service life curve, which is exemplified p is referred to as the bearing exponent and in Figure 6-29, is usually provided by the is to be taken as 3 for ball bearings and as manufacturer of the bearing. The service life 10/3 for roller bearings. curve represents the combinations of equivalent tilt moment and equivalent axial A curve similar to the one shown in Figure load for which the failure probability at a 6-29 is used to calculate the stress reserve specified number of revolutions is 10%. factor during extreme loading of the bearing. Note that different manufacturers specify Calculation of equivalent loads Feq and Meq different numbers of revolutions for their is also analogue. The different application respective service life curves. The number factors, X, Y, etc. will, however, differ. of revolutions is a measure of the bearing life. When Meq and Feq have been calculated, When slewing bearings are provided with the corresponding point P can be plotted as spur gears, the manufacturer will normally shown in Figure 6-29. The extension of the specify the allowable tangential force with line OP intersects with the service life curve reference to the bending stress at the root of in the point L. The bearing life, specified by the tooth in order to prevent tooth failure. the manufacturer in terms of a number of revolutions, can now be increased by The friction torque moment in a slewing multiplication by a factor which is to be bearing can be calculated as calculated as the line length ratio |OL|/|OP|.

164 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø

Myaw = µ⋅(a⋅Mt + b⋅Fa⋅Dr + c⋅Fr⋅Dr), number of plates and claws that engage the nacelle to the tower, see Figure 6-30. Myaw friction moment µ friction coefficient The materials used for slide bearings are Mt tilting moment cast polyamide plates or similar materials Fa axial force (gravity) which are relatively strong, with good Fr radial force (thrust + side) sliding and hard-wearing properties. Dr raceway diameter Table 6-7 gives properties for a few types of For a four-point ball bearing the coefficients relevant materials. The mechanical can be set to properties listed apply to the unreinforced material. The data given are only to be µ = 0.006, a = 2.2, b = 0.5 and c = 1.9 considered as guidance, since data from different manufacturers vary considerably. Proper lubrication of the yaw bearing is of great importance. It is advised to follow the Polyurethane (PUR) has a high ultimate bearing manufacturer’s instructions strength combined with a large elongation at regarding type and amount of grease, and failure, i.e. it exhibits a ductile behaviour. It relubrication intervals. has good wearing properties and exhibits constancy towards oil and grease. Different One way of improving the lubrication in a types of PUR are categorised according to bearing is periodically to let the bearing their “Shore hardness”. The Young’s rotate through an angle equal to at least one modulus of elasticity of polyurethanes bearing segment. grows exponentially with the Shore hardness. Bearing seals should be checked every six months. Polyamide (PA) is characterised by a combination of mechanical strength and DNV (1992) recommends that the safety chemical resistance, good sliding properties against failure for the slew ring toothing is to and high fatigue strength. PA is categorised be 1.6. according to the number of C-atoms in its molecules.

Acetal (POM) offers excellent inherent lubricity, fatigue resistance, and chemical resistance. Acetals suffer from outgassing problems at elevated temperatures, and they are brittle at low temperatures.

Polyethylene Terephtalate (PET) exhibits good creep constancy, which means that it can absorb large static loads. It also exhibits good dimensional stability, wear properties

Figure 6-30. Schematic diagram of slide bearing. and low friction. On the negative side, it is sensitive to notches. An alternative to a slewing bearing is a slide bearing, which is often designed as a

6 – Nacelle 165 Guidelines for Design of Wind Turbines – DNV/Risø

Plates used for slide bearings will normally A major difference between rolling bearings need to be greased to obtain a sufficiently and slide bearings is that slide bearings low friction torque and for corrosion involve larger frictional resistance. Slide protection of the steel parts. bearings therefore require larger yaw motors but less brake capacity than rolling bearings.

Table 6-7. Material properties for materials used in slide bearings. PA 66 PUR PET POM Tensile strength 52 MPa 83 MPa 46 MPa 61 MPa Compressive strength 60 MPa NA 97 MPa 31 MPa Flexural module 1379 MPa 3447 MPa 2758 MPa 2620 MPa Hardness Rockwell R 100 119 120 107 Temperature Maximum 121 °C 110 °C 100 °C N/A Minimum −79 °C −40 °C −15 °C N/A Source: www.plasticsusa.com

6.10.6 Yaw error and control for a long time. The wind turbine must, therefore, be equipped with a cable twist Yaw errors have a significant impact on the sensor, which monitors the number of blade loads, the rotor loads and the power revolutions and informs the controller when production. Hence, it is important that the it is time to untwist the cables, usually after yaw mechanism is efficient and reliable. 2 to 4 revolutions. The sensor can be Even small yaw errors give rise to increased designed quite simply as a switch on the fatigue loads in the blades due to the yaw drive which is activated once per variation in the inflow angle during the revolution. rotation of the rotor that they cause.

With regard to redundancy in the cable twist If the yaw error becomes too big, i.e. if it monitoring system, the wind turbine can falls outside the specified or assumed range also be equipped with a pull switch which of operation, the wind turbine must be becomes activated when the cables become stopped. too twisted.

Special arrangements to prevent long-term To emphasise the importance of preventing yaw errors can be made. One such cable twisting, it suffices to note that the arrangement is formed by using a set of turbine can easily experience 50-100 independent wind vanes. In case one of the rotations in the same direction during one vanes gets stuck, the vanes will misalign. An year, if it is allowed to rotate unrestricted. error message will then be produced, and the wind turbine will be stopped. 6.10.8 Special design considerations

6.10.7 Cable twist Due to yaw moments being cyclic as mentioned in Section 6.10.1, loose fits In large power producing turbines, cables should be avoided. are needed to conduct the current from the wind turbine generator down through the tower. The cables will become twisted if the turbine keeps yawing in the same direction

166 6 – Nacelle Guidelines for Design of Wind Turbines – DNV/Risø

REFERENCES FAG OEM und Handel AG, Rolling Bearing Lubrication, Publ. No. WL 81 115/4 EA. AGMA, Recommended Practices for Design and Specification of Gearboxes for Wind Gudehus, H., and H. Zenner, Leitfaden für Turbine Generator Systems, American Gear eine Betriebsfestigkeitsrechnung, 4. Auflage, Manufacturers Association, Alexandria, Vir- Verein Deutscher Eisenhüttenleute, ginia, 1997. Düsseldorf, Germany, 1999.

Bergmann, J., and R. Thumser, Forschung IEC, Wind turbine generator systems – Part für die Praxis P 249, Synthetische 1: Safety requirements, International Wöhlerlinien für Eisenwerkstoffe, Standard, IEC61400-1, 2nd edition, 1999. Studiengesellschaft Stahlanwendung e.V., Verlag und Vertriebsgesellschaft mbH, ISO, Rolling bearings – Dynamic load Düsseldorf, Germany, 1999. ratings and rating life, International Standard, ISO281, 1st edition, 1990. Bonus, Bonus Info, Special Issue, the Wind Turbine Components and Operation, ISO, Rolling bearings - Static load ratings, Autumn, Brande, Denmark, 1999. International Standard, ISO 76, 2nd edition, 1987. The Danish Energy Agency, Technical Criteria for the Danish Approval Scheme for Niemann, G., and H. Winter, Maschinenele- Wind Turbines, Copenhagen, Denmark, mente, Band II, Getriebe allgemein, Zahn- April 2000. radgetriebe – Grundlagen, Stirnradgetriebe, Springer-Verlag, Berlin, Germany, 1985. Dansk Ingeniørforening, Last og sikkerhed for vindmøllekonstruktioner, (in Danish), NWTC, Guideline DG03, Wind Turbine DS472, 1st edition, Copenhagen, Denmark, Design, Yaw & Pitch Rolling Bearing life, 1992. National Laboratory, NWTC – Certification Team. Det Norske Veritas, Calculation of Gear Rating for Marine Transmissions, DNV Peterson, R.E., Stress Concentration Classification Note No. 41.2, Høvik, Factors, John Wiley and Sons, New York, Norway, 1993. N.Y., 1974.

Det Norske Veritas, Guidelines for Pilkey, W.D., Peterson’s Stress Certification of Wind Turbine Power Plants, Concentration Factors, 2nd edition, John Copenhagen, Denmark, 1992. Wiley and Sons, New York, N.Y., 1997.

DIN 743, Tragfähigkeitsberechnung von Roloff, H., and W. Matek, Maschinen- Wellen und Achsen, 1998. elemente. Formelsammlung, Vieweg Verlag, Braunschweig/Wiesbaden, Germany, 1994. DIN 743, Part 1-3, Tragfähigkeits- berechnung von Welle und Achsen. SKF, General catalogue, SKF, Denmark, 1989. FAG Technical Information, FAG Rolling Bearings, TI No. WL 43-1190 EA.

6 – Nacelle 167 Guidelines for Design of Wind Turbines – DNV/Risø

SKF, Roller Bearings in Industrial Gearboxes, Handbook for the gearbox designer, SKF, Denmark, 1997.

Sundström, B., Handbok och formelsamling i Hållfasthetslära, Institutionen for hållfasthetsläre, KTH, Stockholm, Sweden, 1998.

VDI Berichte 1442, Festigkeitsberechung Metallischer Bauteile.

168 6 – Nacelle Guidelines for Design of Wind Turbines − DNV/Risø

7. Tower Figure 7-1 shows an array of the most common tower structures. The main features of the different tower types are briefly dealt The tower of a wind turbine supports the with below. nacelle and the rotor and provides the necessary elevation of the rotor to keep it Tubular towers clear off the ground and bring it up to the Most large wind turbines are delivered with level where the wind resources are. The tubular steel towers, which are manufactured towers for large wind turbines are typically in sections of 20-30 m length with flanges at made of steel, but concrete towers are either end. The sections are bolted together sometimes used. Nowadays, most towers are on the site. The towers are conical, i.e. their tubular towers, however, lattice towers are diameters increase towards the base, thereby also in use. Guyed towers are used for increasing their strength towards the tower relatively small wind turbines only. The base, where it is needed the most, because tower is usually connected to its supporting this is where the load response owing to the foundation by means of a bolted flange wind loading is largest. Since the necessary connection or a weld. shell thickness is reduced when the diameter increases, the conical shape allows for In the context of wind turbines, the tower saving on the material consumption. constitutes a low-technology component whose design is easy to optimise, and which The maximum length of the tower sections therefore – during the design process – lends is usually governed by requirements to allow itself easily as an object for possible cost for transportation. Also, the upper limit for reduction. This may come in useful as the the outer diameter of the tower is usually cost of a tower usually forms a significant governed by such requirements, at least for part of the total cost of a wind turbine. land-based turbines – the maximum clearance under highway bridges in

Tubular steel tower Tubular concrete Lattice tower Three-legged tower Guy-wired pole tower tower Figure 7-1. Various tower structures. From www.windpower.org, © Danish Wind Turbine Manufacturers Association.

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Denmark is 4.2 m. An advantage of tubular critical loads. Such special cases should be towers compared to other towers is that they investigated as appropriate. are safer and more comfortable for service personnel and others that have to enter and climb the towers. 7.2 Design loads

As described in Section 4.3, the design loads Lattice towers are most often determined on the basis of an Lattice towers are manufactured by means − of using welded steel profiles or L-section aeroelastic analysis or – less frequently by steel profiles. Since a lattice tower requires a simplified calculation, see Section 4.5. only about half as much material as a freely Regardless of which analysis approach is standing tubular tower with a similar used for these calculations, the loads should stiffness, the basic advantage of lattice be calculated from a model, in which the towers is reduced cost. It also gives less tower properties (geometry, materials, wind shade than a massive tower. The major stiffness) are in agreement with the ones disadvantage of lattice towers is their visual used in the final design. Thus, the design of appearance, although this is a debatable the tower may demand an iterative issue. Nevertheless, for aesthetic reasons procedure to get from an initial design to the lattice towers have almost disappeared from final design with the correct stiffness. use for large, modern wind turbines. The design loads for fatigue are to be Guyed pole towers determined by calculations, which are to be Many small wind turbines are built with supplemented and verified by actual narrow pole towers supported by guy wires. measurements from a prototype turbine. The advantage is weight savings and thereby Since load measurements cannot be made reduced costs. The disadvantages include until the turbine has been designed and difficult access around the towers, which constructed, it is recommended to apply an make them less suitable in farm areas. additional partial safety factor of 1.2 on Finally, this type of tower is more prone to fatigue loads until measurements are carried vandalism, thus compromising the overall out and become available. The use of an safety. additional safety factor as an extra precaution is meant to avoid a major Other types redesign in the event of increased design Some towers are designed as hybrids of the loads. above types, for example the three-legged tower for a 95 kW turbine in Figure 7-1. The extreme design loads can only be determined by calculations, because these loads cannot be measured due to the long 7.1 Load cases recurrence period between events.

The load cases to be investigated for tower When designing a turbine with fixed speed, design are described in Section 4.1. the frequency of the rotor revolution is of However, some special load cases may the utmost importance. This frequency, apply particularly to the tower, for example often referred to as ‘1P’, may induce those that need to be considered when a long increased dynamic loads, e.g. due to rotor tower is to be transported. In addition, the unbalances, wind shear and tower shadow. erection of the tower on site may involve In addition, the higher ‘P’s’ are of importance, e.g. the ‘2P’ and the ‘3P’, which

170 7 – Tower Guidelines for Design of Wind Turbines − DNV/Risø are the frequencies of blades passing the i.e. the 1P and 3P frequencies, respectively. tower on a two- and three-bladed turbine, If it is confirmed that the tower frequency is respectively. When designing a turbine with kept outside ranges defined as the rotor variable speed, one must verify that the rotor frequency ±10% and the blade passing speed of the turbine does not operate in or frequency ±10%, respectively, then there near the first natural frequency of the tower, will normally not be any problems due to see Section 7.3.1. load amplification arising from vibrations at or near the natural frequency.

7.3 General verifications for towers For turbines with two generators or two 7.3.1 Dynamic response and generator speeds, this investigation should resonance be performed for both corresponding blade- passing frequencies. Special attention should The first natural frequency of the tower be given to variable-speed turbines, in which should always be measured in connection cases the turbine should not be allowed to with the erection of a prototype turbine. operate in a frequency interval defined as the eigenfrequency of the tower ±10%. It should be verified that the first natural frequency of the tower does not coincide with the rotor and blade-passing frequencies,

Figure 7-2: Time series: response at tower bottom

Figure 7-3: Power spectrum: response at tower bottom

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When turbines are operating in an over- synchronous mode, i.e. where the 1P d0 distance between the tower and the frequency is larger than the first tower blade tip in the unloaded/undeflected natural frequency, one must include a proper condition analysis of the start-up and stopping umax maximum deflection of the blade sequences. This should be done in order to considering all relevant load cases and account for the increased dynamic loading based on characteristic load values and that occurs when the rotor frequency is characteristic material properties passing through the first natural frequency γ partial safety factor on the maximum of the tower. deflection of the blade, to be chosen according to the relevant load case An example for a 1.8 MW turbine is shown F requirement to the residual clearance in Figure 7-2 and Figure 7-3. From the between the tower and blade tip, power spectrum it appears that the major usually 0.0 part of the energy is found at the natural frequency of the tower at 0.418 Hz. The 1P The definition of the safety factor γ depends to 4P passing frequencies are not close to the on which set of standards is used for the first natural frequency. Furthermore, only a critical blade deflection analysis. small quantity of energy is found on the passing frequencies in the response According to IEC61400-1, the safety factor spectrum. on the maximum deflection is taken as equal It should be noted that the natural frequency to the corresponding partial safety factor for of the tower is very dependent on the load γf times the material factor γm. The ‘efficiency’ of the presumably fixed support value of γm to be used for this purpose at the foundation level. If the calculation depends on the coefficient of variation of the model assumes that the tower is completely strength and on the definition of the fixed, then the error in the natural frequency characteristic strength. For example, when of the tower may be up to 20%. Guidelines the coefficient of variation is 10% and the for selection of elastic springs as supports characteristic strength is defined as the 5% for a tower whose foundation is not rigid can quantile with 95% confidence, then γm = 1.1. be found in Chapter 8. Note that it is The value of γf to be used depends on which becoming practice to use tower dampers to load case governs the design. compensate for low damping perpendicular to the direction of the wind. When considering the blade-to-tower distance, one should be aware that not only When large variations in the foundation the blades but also the tower will deflect stiffness are encountered within a particular when exposed to wind loading, and the project, such as the development of a wind deflection of the tower may be out of phase farm, the natural frequency in bending with the deflection of the blades. should be measured at all installations. Note that the clearance between blade and 7.3.2 Critical blade deflection analysis tower is not only governed by the structural The distance between tower and the blade deflections, but also by a possible slip at the tip should comply with the following yaw bearing, by the perpendicularity of the condition tower flange, and by the tolerances on the tilt and on the rotor plane. −γ ⋅ > d 0 umax F

172 7 – Tower Guidelines for Design of Wind Turbines − DNV/Risø

Large blade deflections are typically H encountered in extreme wind situations, but = + Fz (h) FzT ρt ∫ A(z) dz also in the operating condition when the yaw h angle is large or when the terrain is sloping. Atypical wind profiles may also give rise to M (h) = M large blade deflections. Special attention z zT should therefore be given to wind turbine = + designs that are sensitive to special load F y ( h) F yT F w (h) cases such as negative wind shear in = + ⋅ − + complex terrain. M x (h) M xT FyT (H h) M w (h)

+ F ⋅(δ (H )-δ (h)) The current Danish practice uses validated zT stiffness data in the aeroelastic calculations for analysis of deflections. Validated MzT stiffness data are blade stiffness data, which comply with the experimental data from FzT static tests of the blades. This implies that FyT one may have to produce two sets of MxT aeroelastic calculations. The first is a load wind calculation, in which the model is tuned to the correct natural frequencies and damping properties of the structural system. The second is a special deflection model where the deflections are tuned to be in agreement H with the static experiments. Ideally, the two models would be the same, but in practice, this is not always the case.

7.4 Tubular towers

7.4.1 Loads and responses h For the purpose of calculating section loads in the tower, the tower can be viewed as a cantilever beam as shown in Figure 7-4. External loads, denoted by index T in this Figure 7-4. Cantilever beam model of a tubular tower figure, are applied at the tower top flange, subject to loading at the level of the hub. which is located at a height H above the tower base. Note that this height may Fy thrust from wind load deviate somewhat from the hub height. Mx bending moment from wind load Fz gravity force Section loads in the tower at height h can be Mz torsional moment ρ calculated from the loads applied at the top t density of tower including of the tower: appurtenances A(z) cross-sectional area as a function of height z δ deflection of tower due to thrust from wind

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External loads, here denoted by index T, are Alternatively, loads must be combined by assumed to include the dynamic effect or taking the maximum of each load gust factor (referring to a quasi-static component from the particular load case approach). where the most dominant load has its maximum, or more conservatively they can In MxT, it is particularly important to include be combined by combining the maxima of the contribution from the possible eccen- the various load components regardless of tricity of the nacelle. which load case they actually appear in.

The section force Fw(h) and the moment 7.4.3 Fatigue loads M (h) from the wind load on the tower can w Combining fatigue loads is an even more be calculated as complicated task. When using the rain-flow

method as described in Section 4.4.1, the H = ρ 2 ϕ load spectra for the different load Fw (h) ½ ∫ V ( z) D ( z)C ( z)dz components are normally not directly h combinable.

H Therefore it might be a good idea, if = ρ − − 2ϕ M w (h) ½ ∫(H h z)V (z) D(z)C(z)dz possible, to combine the time series of the h various load components resulting from the aeroelastic simulation. For example, the ρ air density resulting bending moment in the direction θ V(z) wind speed relative to the y-axis can be calculated as D(z) outer tower diameter = θ + θ C(z) form factor M res M x sin M y cos ϕ gust factor in which Mx and My are the bending C(z) depends on the Reynold’s number, moments associated with the load Re = VD/ν, in which ν denotes the kinematic components in the x and y directions, − viscosity of air. At 20°C, ν = 15.09⋅10 6 respectively. m2/sec. For painted steel towers, C(z) can be set to 0.6. This could be taken even further to calculate the stress at relevant sections in the tower 7.4.2 Extreme loads for every time step during the simulation and subsequently rain-flow count the For identification of the loads that govern resulting time series of the stress. the design, the specific combination of load components that produces the highest stress 7.4.4 Vortex induced vibrations must be found. This can be quite a task when an aeroelastic analysis program is used The turbine must be checked for vortex that simulates a large number of load cases induced vibration. The vortex excitation in 10-minute time series. Further, may occur during mounting of the turbine, determination of which load case actually i.e. in a situation where the rotor and nacelle governs the design will most likely vary have not yet been mounted on the tower. A from different sections of the tower. suggested procedure to be followed for this purpose can be found in Eurocode 1, Section 2.4, Annex C or alternatively in the Danish

174 7 – Tower Guidelines for Design of Wind Turbines − DNV/Risø

DS410, according to which the critical wind vibrations, is the aerodynamic damping of speed vr can be calculated as the blades and the nacelle.

⋅ 7.4.5 Welded joints = n D v r St Welds are, in general, treated in the same manner as the rest of the structure when a n tower natural frequency proper reduction factor for the weld quality D tower diameter and base material is included. St Strouhal number Table 7-1 shows the recommended detail For conical towers, D should be set equal to categories for bolts with rolled threads after the top diameter. heat treatment and common welds in tubular towers according to the standards Eurocode St 0.21 3 and DS412. The given detail categories 0.20 assume 100% controlled full penetration 0.19 butt welds of quality level B according to 0.18 0.17 DS/ISO 25817. 0.16 0.15 H/b Weld Categories 1 10 100 Figure 7-5. The Strouhal number vs. ratio between plate to plate 80 tower height H and tower diameter b. plate to flange 71 plate to door frame 80 The analysis might prove that certain wind axially loaded bolts 71 velocities should be avoided when erecting Table 7-1. Detail categories for bolts and common the tower. However, the sensitivity to vortex welds in tubular towers. vibrations may be changed by temporary guy wiring of the tower or by mounting a Figure 7-6 shows typical welds in a tubular temporary mass near the top of the tower. tower.

Normally, vortex-induced vibrations do not pose any problems after installation of the tower and the wind turbine. Once the nacelle is in place, its weight will lower the critical wind speed for vortex-induced vibrations to a low level − typically below 10 m/s − a. b. c. which is within the interval of power Typical weld Typical weld Weld production. at door frame at flange between two shells of When the blades rotate and pass the tower, different they will reduce the wind speed and create thickness turbulence in the wind that passes the tower Figure 7-6. Typical weld details in tubular tower behind the blades, thereby obstructing the generation of vortices. Note that for the weld between the tower shell and the flange in Figure 7-6b, the given Another aspect, which contributes to detail category assumes a small shell reducing the effect of vortex-induced thickness relative to the flange thickness.

7 – Tower 175 Guidelines for Design of Wind Turbines − DNV/Risø

Note also that a weld between two shells of The fatigue damage can be calculated using different thickness as shown in Figure 7-6c the Palmgren-Miner’s rule as described in is symmetrically tapered to avoid stress Appendix C. concentrations. The slope of the tapering should not be greater than 1:4. Whenever the weld is perpendicular to the direction of loading and the material In case of single-sided tapering, as shown in thickness t is greater than 25 mm, the fatigue

Figure 7-7, a stress concentration is strength σfatd should be reduced in introduced. accordance with the following formula

0.25 σred,fatd = σfatd⋅(25/t)

t 2 In cases where it is not possible to achieve the required design lifetime for a detail in an analysis that includes partial safety factors, inspection of the detail is required. e However, it is still necessary to meet the requirements to the nominal lifetime when calculations are made without partial safety t1 factors on the material properties included. Note that this approach is only allowed in Danish standards. Figure 7-7. Single-sided plate tapering in tubular tower The period of time until the first inspection According to DNV (1987), the stress is carried out should at the most be set equal concentration factor for the single-sided to the calculated design lifetime in the event plate tapering can be calculated as that partial safety factors are included. Once one or more inspections have been carried e , SCF = 1+ 6 out, subsequent inspection intervals should taper 2.5   t   be chosen depending on the results of the t 1+  1   previous inspections. 2   t     2   7.4.6 Stress concentrations near in which t1 and t2 are the plate thickness of hatches and doors the lower and upper part of the tower shell, The doors and hatches induce stress respectively, and the eccentricity e is given concentrations near the openings. This stress by concentration is traditionally represented by

= ()− a stress concentration factor (SCF), which e ½ t1 t2 expresses the stress ratio between a sample with and a sample without the opening. The When using quality levels poorer than B SCF could be determined from parametric according to DS/ISO 25817, it is equations, from a finite element analysis or recommended to apply detail categories a by model experiments. A study by corresponding number of levels lower. Jørgensen (1990) found a SCF of about 1.8 for a door opening. However, it is re- commended that an individual SCF analysis

176 7 – Tower Guidelines for Design of Wind Turbines − DNV/Risø should be performed for each detail in the approach described in Annex D of question. DS449 combined with DS412, DIN 18800 or other recognised standards. It is important to consider stress concentrations at the tower door. Especially, In the following, the method suggested in as they depend heavily on how the door the Danish standard is presented. flange is carried out, i.e. whether it is straight, or whether it is curved in order to Stresses owing to the axial force, σad, and follow the curvature of the tower wall at the owing to the bending moment, σbd, are given top and bottom of the door. Moreover, they by depend on how the flange is aligned with the tower wall, and whether it is placed N σ = d externally or internally with respect to the ad 2πRt tower wall.

M Further, it must be considered to what extent σ = d the door frame replaces the missing tower bd πR 2 t shell regarding the cross-sectional area, ε moment of inertia and centre of gravity. A reduction factor is calculated as Finally, it might be relevant to consider local stability of the door region. ε = 0.83 a R 1+ 0.01 For unstrengthened circular holes, like t inspection holes and port holes, the stress concentration factor for bending will ε = 0.1887 + 0.8113ε normally be about 3. b a

ε σ + ε σ For small holes like bolt holes, detail a ad b bd ε = categories can be found in structural steel σ +σ ad bd codes. According to theory of elasticity, the critical Nowadays, the fatigue loading often governs compressive stress is the design near openings. In this case, a suitable fatigue strength curve is to be E selected. This should be done in agreement σ = d el R with Section 9.6.3 of Eurocode 3, which 3(1−ν 2 ) specifies the use of a “Category 71 curve” t for a full penetration butt with permitted welds defects acceptance criteria satisfied. The relative slenderness ratio for local The fatigue damage is calculated using a buckling is Palmgren-Miner’s approach. f λ = yd a εσ 7.4.7 Stability analysis el

The buckling strength of the tower usually λ ≤ σ governs the tower design as far as the shell If a 0.3, the critical compressive stress cr is given by thickness is concerned. The buckling strength of the tower can be analysed using

7 – Tower 177 Guidelines for Design of Wind Turbines − DNV/Risø

σ = cr f yd If

λ ≤ 2 If 0.3 < a 1, the critical compressive e > H stress σcr is given by 1000

σ = (1.5 − 0.913 λ ) f then an additional increment cr a yd 2 However, if the tower height H does not ∆e = (e − H ) 1000 exceed 1.42 R R / t , then is to be added to e. = σ cr f yd Finally, the following inequality must be From theory of elasticity, the Euler force for fulfilled a cantilever beam is given by + N d + Nel ⋅ M d N d e < σ 1 π − π 2 cr π 2 π 3 2 Rt N el N d R t E d R t = 4 N el 2 H Nd design axial force Md design bending moment The relative slenderness ratio for global R tower radius stability is t tower shell thickness H tower height σ Ed design modulus of elasticity λ = cr r ν Poisson's ratio  N   el  f design yield stress π yd  2 Rt  7.4.8 Flange connections The core radius k of a tube is given by The tower sections and the connection to the

foundation are often adjoined with L or T R k = flange connections. 2 Figure 7-8 shows an L flange connection For cold-formed welded towers, the along with the deformed shape of the upper equivalent geometrical imperfection can side of the connection. Using the model in now be calculated as Figure 7-8, the tension force Z in the tower shell and the bolt force F are calculated from = λ − e 0.49( r 0.2)k 4⋅ M Z = For welded towers, it can be calculated as D ⋅ n e = 0.34(λ − 0.2)k r and

However, if λr≤0.2 then e = 0.

178 7 – Tower Guidelines for Design of Wind Turbines − DNV/Risø

a + b common type of corrosion protection used F = Z + R = Z a for tubular towers is paint. respectively. 7.4.10 Tolerances and specifications Whatever standard is used for the stability analysis, the calculations assume certain maximum imperfections. These tolerances Z Z are concerned with maximum deviations D D from nominal longitudinal and circum- b a b a ferential dimensions and must be respected. Otherwise, when a tolerance is exceeded, the excess must be included in the factor e in the stability calculations.

F

R

Figure 7-8. L flange connections. Figure 7-9. Tower shell dent tolerances. The bolt stress, disregarding pretension, in the L flange connection is given by When using the buckling strength analysis approach, as described in Section 7.4.7, the assumed maximum dent tolerances are as = F σ L follows with reference to Figure 7-9: A st Between circumferential seams Md design bending moment • measured along an arbitrary production D diameter of tower shell with a straight bar of length l = 4 , n number of bolts r rt however, not longer than 95 % of the Ast bolt stress area distance between adjoining Since tower flanges often have a circumferential seams, the maximum considerable thickness, the risk of brittle deviation from w should fulfil: w/lr < fracture should be considered. This can be 0.01 (Figure 7-9 a). • done according to Eurocode 3, Annex C or measured along an arbitrary DS412, Annex A. circumference with a shape with curvature equal to the nominal outer 7.4.9 Corrosion protection radius of the cylinder and length lr = 4 rt , the maximum deviation from w Corrosion protection of the tower should be should fulfil w/l < 0.01 (Figure 7-9 b). in accordance with DS/R 454 and for r offshore turbines according to DS/R 464 or similar recognised standards. The most

7 – Tower 179 Guidelines for Design of Wind Turbines − DNV/Risø

At circumferential seams the tower side must be dealt with. Turbines • measured along an arbitrary production erected in Denmark should comply with the across a weld with a straight bar of requirements specified by the Danish length lr = 25t, the maximum deviation w Working Environment Service. should fulfil w/lr < 0.01 (Figure 7-9 c).

Elements of large thickness, such as tower 7.6 Example of tower load cal- flanges, should be checked for stratification. culation Further, the flanges must satisfy certain 7.6.1 Loads and responses tolerances regarding straightness and circularity to enable correct tightening of the The behaviour of the tower during extreme connection. The stresses in the flange should loading is illustrated through an aeroelastic be based on the smallest flange thickness, cf. calculation for a reference turbine with a the tolerance specifications. rated power of 1800 kW. The main characte- ristics of this turbine are given in Table 7-2, and its dynamics are given in Table 7-3. 7.5 Access and working environ- ment No. of blades 3 When designing the tower it must be Hub height 71.5 m verified that proper access to the nacelle is Rotor diameter 66 m possible. It must be assured that the access Rotational speed 15.9 rpm complies with the requirements for Rated power 1800 kW personnel safety and maintenance. In Tower tubular addition, the working environment during Table 7-2. Main characteristics of reference wind blade inspection using an inspection hatch in turbine.

Figure 7-10. Tower response from analysis of a 1.8 MW turbine (left = alongwind response, right = transverse wind response). Operational loads for 24 m/s wind speed.

180 7 – Tower Guidelines for Design of Wind Turbines − DNV/Risø

An aeroelastic response calculation for an tions. These processes constitute the wind operating turbine – showing the bending speed process, the electric power and the moments – is shown as a function of time in horizontal force and moment at the Figure 7-10. The response is calculated for a foundation level. Considering the variation 10-minute mean wind speed U10 = 24 m/s, of the processes and the time of operation, it which is near the cut-out wind speed, and a is obvious that a proper selection of the turbulence intensity IT = 11%. From Figure characteristic value must be rooted in 7-10 it appears that the response is highly probabilistic methods as described in fluctuating with time, thus the design Section 4.3.1. responses are to be determined from the pro- bability distribution of the extreme response, Table 7-4 provides simulated estimates of see Section 4.4. This fluctuation is due to the the first four central moments of the moment dynamics of the turbine in conjunction with and horizontal force responses, viz. the the stochastic turbulence field. mean, the standard deviation, the skewness and the kurtosis of these two responses. The 7.6.2 Occurrence of extreme loads bottom lines in the table give estimates of during normal power production the maximum moment and the simultaneous value of the force, and of the maximum The results of a response analysis based on force and the simultaneous value of the 49 realisations (calculations with different moment. The central estimates are denoted seeds) of the turbulence field are shown in by µ in the table. Statistical uncertainty in Table 7-4. The load case is analysed for the estimates is also included in terms of the U = 24 m/s and I = 11%, and the 10 T standard deviations of the estimates, denoted simulation time for each realisation is 10 σ minutes. by in the table. It appears that the maximum of the moment response process Table 7-4 shows results for four important and the maximum of the force response processes as interpreted from the 49 calcula- process do not occur at the same time.

Configuration of turbine Blades: Blades: In normal position 90o pitched Mode shape Freq. [Hz] Damp. (%) Freq. [Hz] 1st tower transversal 0.418 6.0 0.417 1st tower longitudinal 0.419 6.0 0.420 1st rotor torsion 0.805 5.0 0.704 1st rotor torsion 0.979 1.002 1st asymmetric rotor (yaw) 1.000 1.064 1st symmetric rotor (flap) 1.067 3.1 1.769 1st edgewise mode 1.857 3.1 1.032 2nd edgewise mode 1.045 Table 7-3. Dynamic properties of reference wind turbine.

The characteristic extreme response and the 1. Simulate/calculate the extreme design extreme response for an operating responses for the codified load cases turbine should be selected according to the and, if necessary, include other load following scheme: cases that might be critical for the considered concept (e.g. variable speed,

7 – Tower 181 Guidelines for Design of Wind Turbines − DNV/Risø

etc.). For each load case at least five configuration. The wind is acting different seeds should be used. perpendicular to the nacelle, i.e. with a full drag on the nacelle, and the blades are Run Wind Power Moment Horis. pitched to give a full drag also on the blades. statistics M force, F Mean µ 24.09 1810 14323 210 The extreme loads calculated from 60 σ 0.00 5.5 28.8 0.413 simulations of the response processes are Std. dev. µ 2.63 119.6 2046 30.9 shown in Table 7-5. The mean wind speed is 34 m/s and the turbulence intensity is 10%. σ 0.01 4.2 152 2.3 The central estimates are denoted by µ in the Skewness µ -0.052 -0.187 -0.028 -0.055 table. Statistical uncertainty in the estimates σ 0.218 0.085 0.127 0.239 is also included in terms of the standard Kurtosis µ 2.953 6.650 2.950 2.953 deviations of the estimates, denoted by σ in σ 0.377 0.550 0.256 0.239 the table. From the data it appears that the Upcross. µ 6.703 1.714 2.520 1.678 statistical variations of the various responses period σ 0.881 0.052 0.748 0.896 under consideration are significant also in max[M] µ 28.2 1986.7 21018 312 the parked condition. and simul. σ 2.9 90.2 777 12 max[H] µ 27.4 1978.0 20907 314 Run Wind Moment Horis. and simul. σ 1.9 95.9 812 12 statistics M force, F Table 7-4. Extreme loads during power production Mean µ 33.51 12293.7 234.2 (simulation time: 10 minutes). σ 0.001 23.3 0.45

Std. dev. µ 3.333 2555.2 45.79 2. Calculate the statistics (as exemplified σ 0.005 79.3 1.09 in Table 7-4) for each executed simu- µ lation (at least 10 minutes). Skewness -0.027 0.208 0.206 σ 3. Use the procedures in Section 4.4.2 to 0.205 0.203 0.210 project the responses calculated from a Kurtosis µ 2.939 3.091 3.055 10-minute simulation to a longer period σ 0.248 0.266 0.233 of operation (e.g. more days – max[U] µ 43.74 17515.3 331.5 calculated from the mean wind speed and simul. σ 1.03 2362.6 26.4 distribution – see Chapter 3). max[M] µ 40.30 21490.8 395.9 4. Choose the characteristic value of the and simul. σ 1.72 1052.0 17.4 response as described in the code. (Not max[H] µ 40.38 21449.9 396.5 many codes deal with this issue, and and simul. σ 1.67 1056.4 17.0 those that do, do not necessarily define Table 7-5. Extreme loads for parked turbine (simulation the characteristic value in the same time: 10 minutes) manner). 5. Apply a safety factor as described in the The characteristic extreme response should codes (the revised edition of the Danish be chosen equal to the expected maximum code of practice DS472 specifies γf = response in a 10-minute period. This 1.5) to calculate the design value of the approach is justified only if the coefficient response. of variation on the maximum value is sufficiently low, see Mørk. An approach by 7.6.3 Extreme loads – parked turbine Davenport can be used for this purpose, see Section 4.4.2. The corresponding design An extreme load analysis is also performed value is found by applying the appropriate with the reference turbine in a parked

182 7 – Tower Guidelines for Design of Wind Turbines − DNV/Risø partial safety factor (the current edition of given in Table 7-7, calculated for 107 DS410 and the revised edition of DS472 equivalent load cycles. Note that the specify γf = 1.5). uncertainty in the lifetime fatigue loading is still significant with a coefficient of variance 7.6.4 Fatigue loading of 6%. This may call for a safety factor for fatigue loading different from 1.0. The fatigue loads are also fluctuating loads, thus one must assure that a sufficient Wind Statistical Equivalent number of simulations are performed before speed parameter load the design of the tower can be carried out. A [m/s] [kNm] 1.5 MW stall-controlled turbine with a µ tubular tower as reported in Thomsen (1998) 7 828.7 is considered as an example, and the fatigue COV 17% loads are as shown in Table 7-6. The table is 10 µ 1511.1 calculated with the Wöhler curve exponent COV 10% m = 3, which is the value traditionally 15 µ 3151.1 selected for steel, and the equivalent number COV 13% of cycles is 600. The turbulence intensity is 20 µ 6059.4 set to 15%. COV 15% 24 µ 7703.1 Table 7-6 shows the variation in the COV 15% equivalent load as a function of the mean Table 7-6. Equivalent load for tower bending in a 10- wind speed. The table is based on a large minute simulation. number of simulations for each wind speed, and both the mean and the coefficient of Quantity Stat. Eq. load [kNm] variation of the load estimates are included, Par. denoted by µ and COV, respectively. The Req µ 10292.42 coefficient of variation appears to be rather COV 6% 7 large (greater than 10%) in this example, neq, L 10 despite the large number of simulations used Table 7-7. Equivalent load for tower bending in a 10- for the estimations. This indicates the minute simulation. importance of carrying out a sufficient number of simulations. It is recommended The dependency of the damage-equivalent not to use less than five simulations, see load on the wind speed and on the Danish Energy Agency (2000), however, turbulence intensity is shown in Figure 7-11. five simulations may in many cases be The upper half of the figure shows the insufficient. equivalent load in a 10-minute simulation period as a function of the wind speed. Also, The equivalent load as shown in Table 7-6 three levels of turbulence intensity are must be corrected with respect to the proper analysed. From the figure, it is clearly seen operation time at different wind speeds that the fatigue damage increases with the (adjusting for the distribution of the mean wind speed and turbulence intensity. wind speed – resulting in the lifetime Moreover, the uncertainty in the damage equivalent load). The mean wind velocity is increases with increasing wind speed and assumed to follow a Weibull distribution turbulence. The error bars in Figure 7-11 with scale parameter A = 10 m/s and slope illustrate the mean of the fatigue load parameter k = 2.0. The lifetime fatigue plus/minus one standard deviation. equivalent load for a 20-year lifetime is

7 – Tower 183 Guidelines for Design of Wind Turbines − DNV/Risø

The lower half of Figure 7-11 gives the lifetime damage and the lifetime equivalent load as functions of the site-specific design turbulence intensity. The figure shows that the lifetime equivalent load increases from about 6000 kNm to about 14000 kNm when the turbulence intensity increases from 10% to 20%. The fatigue damage is corre- spondingly – due to the nonlinearity in the fatigue Wöhler curve – increased by a factor of 8.

An analysis of a 500 kW stall-controlled turbine gives the relative contribution to the fatigue damage from different load cases and is reported in Thomsen et al. (1997). The results of this investigation are reproduced in Table 7-8, from which it appears that the major fatigue contribution arises from the ‘normal production’ load combinations. Load combinations involving failures or errors do hardly influence the fa- Figure 7-11. Sensitivity of 10-minute equivalent load to tigue damage at all. The load combinations wind speed (above) and lifetime equivalent fatigue load are identical to those specified in DS472. to turbulence intensity (below). Error bars correspond to ±one standard deviation in estimates.

184 7 – Tower Guidelines for Design of Wind Turbines − DNV/Risø

LC U 10 Yaw I T Cont. Comment no. [m/s] [deg] [%] [%] 1 6 -10 18 0,9 2 8 -10 18 1,7 7,0 3 10 -10 18 3,1 4 12 -10 18 4,1 6,0 5 14 -10 18 5,0 6 16 -10 18 6,5 5,0 7 18 -10 18 6,4 8 20 -10 18 6,1 9 22 -10 18 5,9 4,0 10 24 -10 18 5,6 11 6 +10 18 0,9 3,0 12 8 +10 18 2,3 13 10 +10 18 4,1 2,0 14 12 +10 18 5,0 15 14 +10 18 5,3

LC's: normal power production 1,0 16 16 +10 18 6,6 17 18 +10 18 6,6 18 20 +10 18 6,1 0,0 1 5 9 13 17 21 25 29 33 19 22 +10 18 5,9 Load combination number 20 24 +10 18 5,3 21 Low Start 0,0 22 Nom Start 0,1 23 High Start 0,1 24 Low Stop 0,0 25 Nom Stop 0,1 26 High Stop 0,1 LC's: starts/stops 27 5Idle0,1 28 0.5 Umax -40 18 0,1 Tip brakes activated 29 0.5 Umax 0 18 0,0 Tip brakes activated 30 0.5 Umax +40 18 0,2 Tip brakes activated 31 25 -40 18 0,4 Large yaw error 32 25 40 18 1,0 Large yaw error 33 25 -10 18 2,1 Only one tip brake 25 10 18 2,6 Only one tip brake

LC's: With errors 34 35 25 -10 18 0,0 Only two tip brakes Table 7-8. Relative contribution to the total fatigue damage from different load combinations (load cases according to DS472, 500 kW turbine, tower subjected to bending).

7 – Tower 185 Guidelines for Design of Wind Turbines − DNV/Risø

REFERENCES Krohn, S., www.windpower.org, Danish Wind Turbine Manufacturers Association, Arbejdstilsynet, anvisning nr. 2.2.0.1. Mørk, K., Vindlast på svingningsfølsomme Danish Energy Agency, “Rekommandation konstruktioner, AUC, Aalborg, Denmark. for Teknisk Godkendelse af Vindmøller på Havet,” Bilag til Teknisk Grundlag for Thomsen, K., The Statistical Variation of Typegodkendelse og Certificering af Wind Turbine Fatigue Loads, Risø National Vindmøller i Danmark, Copenhagen, Laboratory, Risø-R-1063(EN), 1998. Denmark, 2000. Thomsen, K., P.P. Madsen, E. Jørgensen,

Status og perspektiv for forskning i DS 449, The Danish Code of Practice for aeroelasticitet – Lastgrundlag og sikkerhed, the Design and Construction of Pile Risø-R-964(DA), Forskningscenter Risø, Supported Offshore Steel Structures, Dansk Roskilde, Denmark, February, 1997. Ingeniørforening, 1983.

DS410, The Danish Code of Practice for Loads for the Design of Structures, Dansk Ingeniørforening, 1998.

DS412, The Danish Code of Practice for Loads for the structural use of steel, Dansk Ingeniørforening, 1998.

Det Norske Veritas, Column Stabilized Units (Semisubmersible Platforms), Classification Notes No. 31.4, Høvik, Norway, 1987.

Eurocode 1, Basis of design and actions on structures, Section 2-4: Wind actions on structures.

Eurocode 3. Design of steel structures-Part 1-1: General rules and rules for buildings DS/ENV1993-1-1

Forskningscenter Risø, Rekommandation til opfyldelse af krav i teknisk grundlag, Energistyrelsens regeludvalg for god- kendelse af vindmøller, Godkendelses- sekretariatet, Prøvestationen for Vindmøller, Roskilde, Denmark.

Jørgensen, E., Notat om undersøgelse af spændingsforhold ved luge på rørtårn, Prøvestationen for vindmøller, Risø, Denmark, July 1990.

186 7 – Tower Guidelines for Design of Wind Turbines − DNV/Risø

8. Foundations olivine inside the cavities of the steel structure. The tripod foundation is a steel frame structure with three legs. The wind Onshore wind turbines are usually supported turbine tower is mounted on top of the by either a slab foundation or a pile tripod, while each leg is supported by either foundation. Soil conditions at the specific a driven pile or a suction bucket for transfer site usually govern whether a slab of loads to the supporting soils. foundation or a pile foundation is chosen. A slab foundation is normally preferred when The choice of foundation type is much the top soil is strong enough to support the dependent on the soil conditions prevailing loads from the wind turbine, while a pile- at the planned site of a wind turbine. Once a supported foundation is attractive when the foundation concept has been selected and a top soil is of a softer quality and the loads foundation design is to be carried out, the need to be transferred to larger depths where following geotechnical issues need to be stronger soils are present to absorb the loads. addressed: When assessing whether the top soil is • bearing capacity, i.e. geotechnical strong enough to carry the foundation loads, stability, e.g. against sliding and over- it is important to consider how far below the turning foundation base the water table is located. • degradation of soil strength in cyclic loading As regards offshore wind turbines, the foun- • consolidation settlements dation is a more comprehensive structure in • differential settlements that it includes a separate structure to • scour and erosion transfer loads from the bottom of the wind turbine tower through the water to the supporting soils. In addition to the loads 8.1 Soil investigations from the wind turbine, such a foundation structure will experience loads from current, 8.1.1 General waves and ice owing to its placement in a marine environment. Three basically diffe- Soil investigations should provide all rent foundation structure concepts exist for necessary soil data for detailed design of a offshore wind turbines: specific foundation structure at a specific location. Soil investigations may be divided • monopile into the following parts: • gravity base • geological studies • tripod • geophysical surveys The monopile is in principle a vertical pipe, • driven or bored into the soil like any other geotechnical investigations pile, onto which the wind turbine tower is which are briefly dealt with in the following. mounted. The gravity base foundation rests on the sea floor or on an excavated bottom A geological study should be based on by means of its own weight and is usually information about the geological history of constructed from reinforced concrete. The the area where the wind turbine is to be wind turbine tower is mounted on top of this installed. The purpose of the study is to concrete structure. The gravity base foun- establish a basis for selection of methods dation can also be constructed from steel in and extent of the site investigation. which case the necessary weight is achieved by placing a heavy ballast such as crushed A geophysical survey can be used to extend the localised information from single

8 – Foundations 187 Guidelines for Design of Wind Turbines − DNV/Risø borings and in-situ testing in order to get an interpret parameters such as the undrained understanding of the soil stratification with- shear strength of clay. The extent to which in a given area, and – for offshore locations the various types of in-situ tests and – of the seabed topography within that area. laboratory tests are required depends much Such a survey can provide guidelines for on the foundation type in question, for selection of a suitable foundation site within example, whether it is a piled foundation or the area, if not already decided. Geophysical a gravity-based foundation. surveys are carried out by means of seismic methods. The recommendations of the Danish Technical Criteria for Type Approval of A geotechnical investigation consists of: Wind Turbines (Danish Energy Agency, • soil sampling for laboratory testing 1998) specify that a geotechnical report • in-situ testing of soil from the geological and geotechnical Soil investigations should be tailored to the surveys shall be prepared. This geotechnical geotechnical design methods used. The field report should contain sufficient information and laboratory investigations should about the site and its soils, e.g. in terms of establish the detailed soil stratigraphy across soil strength and deformation properties, to the site, thus providing the following types allow for design of the foundation with of geotechnical data for all important soil respect to: layers: • bearing capacity • data for classification and description of • stability against sliding the soil, such as • settlements • unit weight of sample • foundation stiffness • unit weight of solid particle • need for and possibility of drainage • water content • static and dynamic coefficients of • liquid and plastic limits compressibility • grain size distribution • sensitivity to dynamic loading • parameters required for a detailed and The geotechnical report is further required to complete foundation design, such as contain identification of soil type at foun- • permeability tests dation level, classification of environment, • consolidation tests and estimation of highest possible water • static tests for determination of shear table. For further details, reference is made strength parameters such as friction to Danish Energy Agency (1998). angle φ for sand and undrained shear 8.1.2 Recommendations for gravity strength cu for clay (triaxial tests and direct simple shear tests) based foundations • cyclic tests for determination of strength Insofar as regards a gravity-based and stiffness parameters (triaxial tests, foundation, an extensive investigation of the direct simple shear tests and resonant shallow soil deposits should be undertaken. column tests) This investigation should cover soil deposits to a depth, which is deeper than the depth of Sampling can be carried out with and any possible critical shear surface. Further, without drilling. The cone penetrometer test all soil layers influenced by the structure (CPT) and various vane tests form the most from a settlement point of view should be commonly used in-situ testing methods. thoroughly investigated. This also holds for Results from such in-situ tests can be used to all soil layers contributing to the foundation

188 8 – Foundations Guidelines for Design of Wind Turbines − DNV/Risø stiffness. The foundation stiffness is of recommended that the sampling interval is importance for the design of the structure not in excess of 1.0-1.5m. supported by the foundation. The depth to be covered by the thorough investigation As regards axial pile analysis, at least one should at least equal the largest base down-the-hole CPT boring should be carried dimension of the structure. out to give a continuous CPT profile. Moreover, one nearby boring with sampling The extent of shallow borings with sampling should be carried out for the axial pile capa- should be determined on the basis of the city analysis. The minimum depth should be type and size of structure as well as on the anticipated penetration of the pile plus a general knowledge about the soil conditions zone of influence sufficient for evaluation of in the area considered for installation. the risk of punch-through failure. The sam- Emphasis should be given to the upper pling interval should be determined from the layers and potentially weaker layers further CPT results, but is recommended not to down. It is recommended that the sampling exceed 3 m. interval is not in excess of 1.0-1.5 m. A number of seabed samples (gravity cores or For offshore installations, a number of the equivalent) evenly distributed over the seabed samples (gravity cores or equivalent) area should be taken for evaluation of the evenly distributed over the area considered scour potential. for installation of the foundation should be taken for evaluation of the scour potential. Shallow CPTs distributed across the installation area should be carried out in Special attention should be paid when addition to the borings. The number of CPTs potential end bearing layers or other dense depend on the soil conditions and on the layers are found. Here, additional CPTs and type and size of structure. If the soil sampling should be carried out in order to conditions are very irregular across the foun- determine the thickness and lateral extension dation site, the number of CPTs will have to of such layers within the area considered for be increased. The shallow CPTs should the foundation. provide continuous graphs from the soil surface to the maximum depth of interest. 8.2 Gravity-based foundations Special tests such as plate loading tests, Requirements for foundation stability often pressuremeter tests and shear wave velocity constitute the most decisive factors as measurements should be considered where regards determination of foundation area, relevant. foundation embedment and necessary

weight for a structure with a gravity-based 8.1.3 Recommendations for pile foundation. It is therefore essential in an foundations optimal design process to give high For lateral pile analysis, shallow cone emphasis to foundation stability calcu- penetration tests should be carried out from lations. the surface to 20-30m depth. In addition, shallow borings with sampling should be The question of foundation stability is most considered for better determination of commonly solved by limiting equilibrium characteristics of the individual layers methods, i.e. by ensuring equilibrium identified by the cone penetration tests. It is between driving and resisting forces. When using limiting equilibrium methods, several

8 – Foundations 189 Guidelines for Design of Wind Turbines − DNV/Risø trial failure surfaces will have to be analysed vertical force V relative to the centre line of in order to find the most critical one with the foundation. Reference is made to Figure respect to stability. 8-1, and the eccentricity is calculated as

However, as foundations of wind turbines M e = d usually have relatively small areas, bearing V capacity formulas for idealised conditions d will normally suffice and be acceptable for design. Bearing capacity formulas are given where Md denotes the resulting design in the following. overturning moment about the foundation- soil interface. 8.2.1 Bearing capacity formulas Correction for torque Forces When a torque MZ is applied to the All forces acting on the foundation, foundation in addition to the forces H and V, including forces transferred from the wind the interaction between the torque and these turbine, are transferred to the foundation forces can be accounted for by replacing H base and combined into resultant forces H and MZ with an equivalent horizontal force and V in the horizontal and vertical H’. The bearing capacity of the foundation is direction, respectively, at the foundation-soil then to be evaluated for the force set (H’,V) interface. instead of the force set (H,V). According to a method by Hansen (1978), the equivalent horizontal force can be calculated as

2 2 ⋅ M  2 ⋅ M  H '= z + H 2 +  z    LC leff  leff  V

] in which leff is the length of the effective H 2 area as determined in the following.

f [kN/m Effective foundation area e [m] For use in bearing capacity analysis an

rupture 2 rupture 1 effective foundation area Aeff is needed. The effective foundation area is constructed such that its geometrical centre coincides with the Figure 8-1. Loading under idealised conditions. load centre, and such that it follows as closely as possible the nearest contour of the In the following, it is assumed that H and V true area of the foundation base. For a are design forces, i.e. they are characteristic quadratic area of width b, the effective area forces that have been multiplied by their Aeff can be defined as relevant partial load factor γf. This is indicated by index d in the bearing capacity = ⋅ Aeff beff leff formulas, hence H and V . The load centre, d d denoted LC, is the point where the resultant in which the effective dimensions b and l of H and V intersects the foundation-soil eff eff depend on which of two idealised loading interface, and implies an eccentricity e of the

190 8 – Foundations Guidelines for Design of Wind Turbines − DNV/Risø scenarios leads to the most critical bearing Reference is made to Figure 8-2. The capacity for the actual foundation. effective area representation that leads to the poorest or most critical result for the bearing

e Aeff capacity of the foundation is the effective area representation to be chosen.

For a circular foundation area with radius R, an elliptical effective foundation area Aeff LC1 can be defined as

eff l

 e  A = 2 R2 arccos( ) − e R2 − e2 eff    R 

with major axes beff

= ()− be 2 R e Aeff e and

2 = −  − b  le 2R 1 1   2R 

LC2 eff l e

Aeff

beff R Figure 8-2. Quadratic footing with two approaches to LC how to make up the effective foundation area. e eff l l Scenario 1 corresponds to load eccentricity with respect to one of the two symmetry axes of the foundation. By this scenario, the following effective dimensions are used: beff b = b − 2 ⋅ e , l = b be eff eff Figure 8-3. Circular and octangular footings with Scenario 2 corresponds to load eccentricity effective foundation area marked out. with respect to both symmetry axes of the foundation. By this scenario, the following The effective foundation area Aeff can now effective dimensions are used: be represented by a rectangle with the following dimensions

= = − beff leff b e 2

8 – Foundations 191 Guidelines for Design of Wind Turbines − DNV/Risø

l l In principle, the quoted formulas apply to = e = eff leff Aeff and beff be foundations, which are not embedded. b l e e However, the formulas may also be applied to embedded foundations, for which they For an area shaped as a double symmetrical will lead to results, which will be on the polygon (octagonal or more), the above conservative side. Alternatively, depth formulas for the circular foundation area can effects associated with embedded be used provided that a radius equal to the foundations can be calculated according to radius of the inscribed circle of the polygon formulas given in DNV (1992). is used for the calculations. The calculations are to be based on design Bearing capacity shear strength parameters: For fully drained conditions and failure according to Rupture 1 as indicated in c tan(φ) c = and φ = arctan( ) Figure 8-1, the following general formula ud γ d γ can be applied for the bearing capacity of a c φ foundation with a horizontal base, resting on the soil surface The material factors γf an γφ must be those associated with the actual design code and the type of analysis, i.e. whether drained or = 1 γ + ' + q d 'beff N γ sγ iγ p 0 N q s q iq c d N c s c ic undrained conditions apply. 2

The dimensionless factors N, s and i can be For undrained conditions, which imply determined by means of formulas given in φ = 0, the following formula for the bearing the following. capacity applies

Drained conditions: q =c ⋅N 0 ⋅ s 0 ⋅ i 0 + p d ud c c c 0 Bearing capacity factors N: 2 qd design bearing capacity [kN/m ] γ effective (submerged ) unit weight ' π φ 1+ sinφ 3 N = e tan d ⋅ d of soil [kN/m ] q − φ 1 sin d p'0 effective overburden pressure at the level of the foundation-soil 2 = − ⋅ φ interface [kN/m ] N c (N q 1) cot d cd design cohesion or design un- drained shear strength assessed on 1 3 = ⋅ − ⋅ φ 2 the basis of the actual shear N γ ((N q 1) cos d ) 4 strength profile, load configu-

ration and estimated depth of po- According to Hansen (1970), Nγ may tential failure surface [kN/m2] alternatively be calculated according to Nγ N bearing capacity factors, dimen- q Nc sionless = 3 ⋅ − ⋅ φ sγ sq sc shape factors, dimensionless N γ (N 1) tan 2 q d iγ iq ic inclination factors, dimensionless

Reference is made to DS415 (DS 415, 1998).

192 8 – Foundations Guidelines for Design of Wind Turbines − DNV/Risø

Shape factors s: H i = i = 1+ q c + ⋅ ⋅ φ V Aeff c cot beff sγ = 1− 0.4 ⋅ 2 leff = iγ iq b s = s = 1+ 0.2 ⋅ eff q c l eff H i 0 = 0.5 + 0.5⋅ 1+ c A ⋅ c Inclination factors i: eff ud

2 The bearing capacity is to be taken as the  H  i = i = 1− d  smallest of the values for qd resulting from q c  V + A ⋅ c ⋅ cotφ   d eff d d  the calculations for Rupture 1 and Rupture 2 2. i = i γ q Sliding resistance of soil Undrained conditions, φ = 0: Foundations subjected to horizontal loading must also be investigated for sufficient 0 = π + N c 2 sliding resistance. The following criterion applies in the case of drained conditions: 0 = sc s c H < Aeff ⋅c + V ⋅ tanφ

H φ i 0 = 0.5 + 0.5⋅ 1− For undrained conditions in clay, = 0, the c ⋅ following criterion applies: Aeff cud

H < A ⋅c Extremely eccentric loading eff ud

In the case of extremely eccentric loading, and it must in addition be verified that i.e. an eccentricity in excess of 0.3 times the foundation width, e > 0.3b, an additional H bearing capacity calculation needs to be < 0.4 carried out, corresponding to the possibility V of a failure according to Rupture 2 in Figure 8-1. This failure mode involves failure of the soil also under the unloaded part of the 8.3 Pile-supported foundations foundation area, i.e. under the heel of the A pile foundation consists of one or more foundation. For this failure mode, the piles that transfer loads from a following formula for the bearing capacity superstructure, such as a wind turbine tower applies or a distinctive structure supporting the tower, to the supporting soils. The loads = γ + + 3 φ q d 'beff N γ sγ iγ c d N c s c ic (1.05 tan ) applied to a pile at its head are transferred down the pile and absorbed by the soil with inclination factors through axial and lateral pile resistance. The axial and lateral pile resistance arises from soil resistance mobilised against the pile

8 – Foundations 193 Guidelines for Design of Wind Turbines − DNV/Risø when the pile, subjected to its loading, is resistance. Note that the local axial displaced relative to the soil. resistance and the torsional resistance, respectively, are interdependent, since they For design of piles, it is common to both arise from the skin friction against the disregard a possible interaction between the pile surface. The axial, lateral and torsional axial pile resistance and the lateral pile pile displacements and corresponding soil resistance locally at any point along the pile reactions for a single pile subjected to and to treat these two forms of resistance as external loading at its head are illustrated in being independent of each other. The Figure 8-4. argument for this is that the soil near the soil surface principally determines the lateral Note that for design of piles, it is important resistance without contributing much to the to consider effects of the installation axial resistance, while the soil further down procedure. For example, the stress history along the pile toward the pile tip principally during pile driving contributes significantly determines the axial resistance without to fatigue loading and needs to be con- contributing much to the lateral capacity. sidered for design against fatigue failure in The axial and lateral resistance models the pile wall. presented in the following conform to this assumption of independence between local 8.3.1 Pile groups axial resistance and local lateral resistance. For foundations consisting of pile groups,

i.e. clusters of two or more piles spaced closely together, pile group effects need to be considered when the axial and lateral resistance of the piles is to be evaluated. There are two types of group effects: • the total capacity of the pile group is less than the sum of the capacities of the individual piles in the group, because of overlap between plastified soil zones around the individual piles. A lower limit for the axial pile group capacity is the axial capacity of the envelope “pier” that encloses all the piles in the group and the soil between them. • larger pile displacements of a given Figure 8-4. Single loaded pile with displacements and load result for an individual pile when it reactions, from Reese et al. (1996) is located in a pile group than when it is

an only pile, because its supporting soils Note, however, that on a global level the will have displacements caused by loads lateral pile capacity and the axial pile transferred to the soil from adjacent capacity, resulting from local resistance piles in the group. This type of group integrated along the length of the pile, may effect is also known as pile-soil-pile interact, because second-order effects may interaction. For practical purposes, such cause the axial loading to influence the pile-soil-pile interaction can often be lateral behaviour of the pile. When the pile reasonably well represented by means is subjected to torque, torsional resistance of Mindlin’s point force solutions for an will be set up in addition to axial and lateral elastic halfspace.

194 8 – Foundations Guidelines for Design of Wind Turbines − DNV/Risø

The knowledge of the behaviour of a pile fSi = αcu group relative to the behaviour of individual piles in the same group is limited, and in which conservative assumptions are therefore recommended for the prediction of pile  1 for c / p '≤ 1.0 group resistance. Note, when dealing with  u 0 2 c p ' pile groups, that in addition to the above pile α =  u 0 group effects there is also an effect of the 1 >  for cu / p0 ' 1.0 presence of the superstructure and the way 24 c p '  u 0 in which it is connected to the pile heads.

The interconnection of the piles through where c is the undrained shear strength of their attachment to the common super- u the soil, and p ’ is the effective overburden structure influences the distribution of the 0 pressure at the point in question. loads between the piles in the group. The piles can often be assumed to be fixed to the β superstructure, and the superstructure can (2) effective stress methods, e.g. the often be assumed as a rigid structure or a method, which yields rigid cap when the responses in the piles are = β to be determined. f Si p0 '

8.3.2 Axial pile resistance in which β values in the range 0.1-0.25 are suggested for pile lengths exceeding 15 m. Axial pile resistance is composed of two parts (3) semi-empirical λ method, by which the • accumulated skin resistance • soil deposit is taken as one single layer, for tip resistance which the average skin friction is calculated as For a pile in a stratified soil deposit of N soil layers, the pile resistance R can be expressed f = λ( p '+2c ) as S 0m um

N p0m' average effective overburden pressure = + = + R RS RT ∑ f Si ASi qT AT between the pile head and the pile tip i=1 cum average undrained shear strength along the pile shaft fSi average unit skin friction along the pile λ dimensionless coefficient, which shaft in layer i depends on the pile length as shown in ASi shaft area of the pile in layer i Figure 8-5 qT unit end resistance AT gross tip area of the pile Hence, by this method, the total shaft

resistance becomes RS = fS AS, where AS is Clay. For piles in mainly cohesive soils, the the pile shaft area. average unit skin friction fS may be calculated according to For long flexible piles, failure between pile and soil may occur close to the seabed even α (1) total stress methods, e.g. the method, before the soil resistance near the pile tip has which yields been mobilized at all. This is a result of the flexibility of the pile and the associated

8 – Foundations 195 Guidelines for Design of Wind Turbines − DNV/Risø differences in relative pile-soil displacement given in Table 8-1 along the length of the pile. This is a length fl limiting unit skin friction, see Table effect, which for a strain-softening soil will 8-1 for guidance imply that the static capacity of the pile will be less than that of a rigid pile. Tip resistance The unit tip resistance of plugged piles in cohesionless soils can be calculated as 0,5 0,45 qp = Nq p0’≤ ql 0,4 0,35 Nq can be taken from Table 8-1 0,3 ql limiting tip resistance, see Table 8-1 λ 0,25 for guidance 0,2

0,15

0,1 The unit tip resistance of piles in cohesive

0,05 soils can be calculated as

0 0 10203040506070 qp = Nc cu Pile length (m) Nc = 9 c undrained shear strength of the soil at Figure 8-5. Coefficient λ vs. pile length u the pile tip For deformation and stress analysis of an axially loaded flexible pile, the pile can be modelled as a number of consecutive column elements supported by nonlinear springs applied at the nodal points between the elements. The nonlinear springs are denoted t-z curves and represent the axial load-displacement relationship between the pile and the soil. The stress t is the inte- grated axial skin friction per unit area of pile surface and z is the relative axial pile-soil displacement necessary to mobilize this skin friction.

Sand. For piles in mainly cohesionless soils (sand), the average unit skin friction may be calculated according to

fS = K p0’ tanδ ≤ fl Table 8-1. Design parameters for axial resistance of driven piles in cohesionless siliceous soil (extracted from API (1987)).  = 0.8 for open - ended piles K  = 1.0 for closed - ended piles t-z curves The t-z curves can be generated according to p0’ effective overburden pressure δ angle of soil friction on the pile wall as a method by Kraft et al. (1981). By this

196 8 – Foundations Guidelines for Design of Wind Turbines − DNV/Risø method, a nonlinear relation exists between For clay, the initial shear modulus of the soil the point of origin and the point where the to be used for generation of t-z curves can be maximum skin resistance tmax is reached, taken as

t z − r G0 = 2600 cu R IF f t z = t ln max for 0≤t≤t t max However, Eide and Andersen (1984) suggest G0 − 1 r f a somewhat softer value according to the t max formula

R radius of the pile = − − G0 600cu 170cu OCR 1 G0 initial shear modulus of the soil ZIF dimensionless zone of influence, defined as the radius of the zone of cu undrained shear strength of the clay influence around the pile divided by R OCR overconsolidation ratio rf curve fitting factor For sand, the initial shear modulus of the For displacements z beyond the soil to be used for generation of t-z curves is displacement where tmax is reached, the skin to be taken as resistance t decreases in linear manner with z until a residual skin resistance tres is m σ σ = a v ⋅ φ reached. For further displacements beyond G0 with m = 1000 tan 2(1+ν ) this point, the skin resistance t stays constant. An example of t-z curves σ generated according to this method is given a = 100 kPa, atmospheric pressure in Figure 8-6. The maximum skin resistance σv vertical effective stress can be calculated according to one of the ν Poisson’s ratio of the soil methods for prediction of unit skin friction φ friction angle of the soil given above. 8.3.3 Laterally loaded piles The most common method for analysis of laterally loaded piles is based on the use of so-called p-y curves. The p-y curves give the relation between the integral value p of the mobilized resistance from the surrounding soil when the pile deflects a distance y laterally. The pile is modelled as a number of consecutive beam-column elements, supported by nonlinear springs applied at the nodal points between the elements. The nonlinear support springs are characterized by one p-y curve at each nodal point, see Figure 8-7.

The solution of pile displacements and pile stresses in any point along the pile for any Figure 8-6. Example of t-z curves generated by model. applied load at the pile head results from the

8 – Foundations 197 Guidelines for Design of Wind Turbines − DNV/Risø solution to the differential equation of the pile

d 4 y d 2 y EI + Q − p(y) + q = 0 dx 4 A dx 2 with

d 3 y dy d 2 y EI + Q = Q and EI = M dx 3 A dx L dx 2

x position along the pile axis y lateral displacement of the pile EI flexural rigidity of the pile QA axial force in the pile QL lateral force in the pile p(y) lateral soil reaction q distributed load along the pile M bending moment in the pile, all at the position x. Reference is made to Figure 8-8. Figure 8-7. p-y curves applied at nodal points in beam A finite difference method usually forms the column representation of pile, from Reese et al. (1996). most feasible approach to achieve the sought-after solution of the differential equation of the pile. A number of commer- cial computer programs are available for this purpose. These programs usually provide full solutions of pile stresses and displace- ments for a combination of axial force, late- ral force and bending moment at the pile head, i.e. also the gradual transfer of axial load to the soil along the pile according to the t-z curve approach presented above is included. Some of the available programs can be used to analyse not only single piles but also pile groups, including possible pile- soil-pile interaction and allowing for proper representation of a superstructure attached at the pile heads, either as a rigid cap or as a structure of finite stiffness.

Figure 8-8. Beam column model with details, from Reese et al. (1996).

198 8 – Foundations Guidelines for Design of Wind Turbines − DNV/Risø

Several methods are available for For cyclic loading and X > XR, the p-y curve representation of the p-y curves that are can be generated according to essential in solving the differential equation for a laterally loaded pile. For construction  pu y 1/3 ≤ of p-y curves, the type of soil, the type of =  ( ) for y 3yc p  2 yc loading, the remoulding due to pile 0.72 p for y > 3y installation and the effect of scour should be  u c considered. The most commonly applied ≤ procedures for construction of p-y curves are For cyclic loading and X XR, the p-y curve those given by DNV (1992) and API (1987). can be generated according to For piles in clay, the procedure according to  p y DNV is presented below. For sand, the u ( )1 / 3 for y ≤ 3y  2 y c procedures according to DNV and API are  c X y − 3y identical and are presented below. =  − − c < ≤ p 0.72 pu (1 (1 ) ) for 3y c y 15y c X 12y  R c  X The lateral resistance per unit length of pile 0.72 p for y > 15y  u X c for a lateral pile deflection y is denoted p.  R The static ultimate lateral resistance per unit ε length is denoted pu. This is the maximum Here, yc = 2.5 cD, in which D is the pile value that p can take on when the pile is diameter and εc is the strain which occurs at deflected laterally. one-half the maximum stress in laboratory undrained compression tests of undisturbed Clay. For piles in cohesive soils, the static soil samples. For further details, reference is ultimate lateral resistance is recommended made to DNV (1992). to be calculated as Sand. For piles in cohesionless soils, the (3c +γ ' X )D + Jc X for 0 < X ≤ X static ultimate lateral resistance is p = u u R recommended to be calculated as u  > 9cu D for X X R (C X + C D)γ ' X for 0 < X ≤ X p = 1 2 R where X is the depth below soil surface and u  γ > C3 D ' X for X X R XR is a transition depth, below which the γ value of (3cu+ ’X)D+JcuX exceeds 9cuD. where the coefficients C , C and C depend Further, D is the pile diameter, c is the 1 2 3 u on the friction angle φ as shown in Figure undrained shear strength of the soil, γ’ is the 8-9, where X is the depth below soil surface effective unit weight of soil, and J is a and X is a transition depth, below which the dimensionless empirical constant whose R value of (C1X+C2D)γ’X exceeds C3Dγ’X. value is in the range 0.25-0.50 with 0.50 γ recommended for soft normally consolidated Further, D is the pile diameter, and ’ is the clay. submerged unit weight of soil.

For static loading, the p-y curve can be The p-y curve can be generated according to generated according to = kX p Apu tanh( y) Apu  pu y 1/3 ≤ =  ( ) for y 8yc p  2 yc  >  pu for y 8yc

8 – Foundations 199 Guidelines for Design of Wind Turbines − DNV/Risø in which k is the initial modulus of subgrade reaction and depends on the friction angle φ as given in Figure 8-10, and A is a factor to account for static or cyclic loading conditions as follows

Figure 8-10. Initial modulus of subgrade reaction k as function of friction angle φ, from DNV Class. Notes 30.4. Figure 8-9. Coefficients as functions of friction angle, from DNV Class. Notes 30.4. 8.3.4 Soil resistance for embedded pile caps 0.9 for cyclic loading  Some piled foundations include an A = H  − ≥ embedded pile cap as indicated in Figure 8- (3 0.8 ) 0.9 for static loading  D 11. Whereas the rotational capacity and stiffness of a pile cap is primarily governed For further details, reference is made to by the axial capacity and stiffness of the DNV (1992). piles, it is important to consider the effect on the lateral capacity and stiffness of the soil acting against the embedded part of the cap. This effect is similar to the soil resistance, represented by p-y curves, on piles under lateral loading. When assessing such an effect of soil in front of the pile cap, it is important also to assess the possibility that this soil may be removed due to erosion or other natural actions.

200 8 – Foundations Guidelines for Design of Wind Turbines − DNV/Risø

structure cannot be assumed to have a fixed support. In any analysis of a foundation structure and of the wind turbine structure that it supports, it is therefore important to model the actual boundary conditions formed by the supporting soils properly.

To represent the finite stiffness of the sup- porting soils in such analyses, it is common to model a set of so-called foundation springs to be applied in one or more modelled support points on the structure to be analysed. The set of foundation springs may include the following springs associ- ated with various modes of motion: Figure 8-11. Distribution of soil resistance on the front • side of pile cap, from Reese et al (1996). vertical spring stiffness • horizontal spring stiffness The pile cap as dealt with here is essentially • rocking spring stiffness considered and modelled as a rigid structure. • torsional spring stiffness Sometimes, however, the pile heads are not interconnected by such a rigid super- Soil behaves in nonlinear manner. structure, but are connected to a super- Foundation springs are therefore nonlinear. structure with some finite stiffness, such as a It is common to apply linear spring stiff- frame or a lattice tower. This will influence nesses, in which case the stiffness values are the distribution of forces between the piles chosen dependent on the strain level that the relative to that of the rigid cap structure and soil will experience for the load case under will in turn have an impact on the overall consideration. It is common to deal with the resulting foundation stiffness. shear modulus of the soil, G. This equivalent shear modulus relates to the initial shear modulus G0 as a function of the shear strain 8.4 Foundation stiffness γ, as indicated in Figure 8-12. Figure 8-12 also gives damping ratios ξ for the soil as a The overall foundation stiffness is depen- γ dent on the strength and stiffness of the soil function of the shear strain . as well as on the structural foundation ele- ments. The foundation stiffness needs to be The following shear strain levels can be determined as a basis for predicting the dy- expected for the three most important namic structural response to wind, wave and sources of dynamic loading of soils: • earthquakes: large strains up to 10−2 to earthquake loading. The foundation stiffness −1 is in general frequency dependent. This is 10 particularly important when predicting dy- • rotating machines: small strains usually −5 namic response to earthquake. less than 10 • wind and ocean waves: moderate strains − − The soil that supports a foundation structure up to 10 2, typically 10 3 usually has finite stiffness. It can therefore usually not be justified to model the soil as a rigid mass. In other words, the foundation

8 – Foundations 201 Guidelines for Design of Wind Turbines − DNV/Risø

Note that in geotechnics, the effective stress is defined as the total stress minus the pore pressure.

Alternatively, the following relation for sand can be applied

= σ G0 1000K 0 '

in which σ0’ and G0 are both to be given in units of kPa, and K takes on values Figure 8-12. Shear modulus and damping ratio vs. according to Table 8-3. For clay, one can strain level. use the following relation as an alternative

The following empirical relation can be used to the above formula to establish the initial shear modulus G0 in a soil G0 = 2600su

(3 − e) 2 in which su is the undrained shear strength of G = A σ '(OCR) k 0 + 0 the soil. 1 e

σ Table 8-3. Factor K. in which 0’ and G0 are both to be given in Soil type K ± units of kPa and A = 3000 1000 depending Loose sand 8 on material (size, angularity of grains, etc.). Dense sand 12 OCR denotes the overconsolidation ratio for Very dense sand 16 clay and has to be set equal to 1.0 for sand, Very dense sand and gravel 30-40 the exponent k is a function of the plasticity index I as given in Table 8-2, and e is the P The steps in establishing the shear modulus void ratio. and the damping ratio can be listed as Table 8-2. Exponent k. follows: Plasticity k 1. determine source of dynamic loading index IP (earthquake, wind, waves and machine 0 0 vibrations) 20 0.18 2. find expected strain level γ for loading 40 0.30 from list below Figure 8-12 60 0.41 3. Find damping ratio ξ from Figure 8-12 80 0.48 4. Find shear modulus ratio G/G0 from >100 0.50 Figure 8-12 5. calculate G0 from one of the formulas The confining effective stress σ0’ is defined given above, chosen dependent on soil as the average of the three principal type effective stresses 6. calculate G as the product of G0 and 1 G/G0 σ ' = (σ '+σ '+σ ') 0 3 1 2 3

202 8 – Foundations Guidelines for Design of Wind Turbines − DNV/Risø

Example: wind loading on a foundation in • circular footing on stratum over bedrock clay with undrained shear strength su = 200 • circular footing on stratum over kPa: halfspace 1. the source of loading is wind • circular footing embedded in stratum 2. the expected strain level is typically over bedrock −3 γ = 10 • piled foundations 3. the damping ratio is about ξ = 0.10-0.15 For the footings it is assumed that they are 4. the shear modulus ratio is G/G0 = 0.35. rigid relative to the soil and always in full 5. the initial shear modulus is G0 = contact with the soil. The piled foundations 2600su = 520 MPa are assumed to be flexible as dealt with 6. the shear modulus becomes G = below. 0.35⋅520 = 180 MPa The spring stiffnesses given in Table 8-5 to Table 8-4 provides guidance for assessment Table 8-7 are all static stiffnesses, i.e. they of Poisson’s ratio. are stiffnesses for frequencies approaching zero. The dynamic stiffnesses may deviate Table 8-4. Poisson’s ratio ν. from the static stiffnesses in particular in Soil type ν case of high-frequent vibrations. However, Dense sands 0.25-0.30 for wind and wave loading of wind turbine Loose sands, stiff clays 0.35-0.45 foundations, onshore as well as offshore, the Saturated clays ≈ 0.50 induced vibrations will be of such a nature that the static stiffnesses will be Once the equivalent shear modulus G has representative for the dynamic stiffnesses been established from G0 and Figure 8-12, that are required in structural analyses. For and the Poisson’s ratio ν has been assessed, earthquake loading, however, frequency- the foundation stiffnesses can be derived. dependent reductions of the static stiffnesses The following four stiffnesses are to get appropriate dynamic stiffness values considered: may be necessary and should be considered.

• vertical stiffness, KV = V/δV, which expresses the ratio between the vertical The slenderness ratio for a pile is defined as L/D where L is the length and D is the force V and the vertical displacement δV diameter of the pile. For L/D > 10, most • horizontal stiffness, KH = H/δH, which expresses the ratio between the piles are flexible, i.e. the active length of the horizontal force H and the horizontal pile is less than L, and the pile head response is independent of the length of the pile. displacement δ H Spring stiffnesses at the pile head of flexible • rotational stiffness, K = M/θ, which R piles are given in Table 8-7 for three expresses the ratio between the idealised soil profiles and the following overturning moment M and the rotation modes of motion: angle θ in rocking • horizontal • torsional stiffness, K = M /θ , which T T T • rocking expresses the ratio between the torque • coupled horizontal-rocking M and the twist θ T T Reference is made to the Young’s modulus

of the soil E, which relates to the shear Formulas for spring stiffnesses are given in modulus G through Table 8-5 to Table 8-7 for various types of foundations which cover: E = 2G(1+ν)

8 – Foundations 203 Guidelines for Design of Wind Turbines − DNV/Risø

ES is the value of E at a depth z equal to the analyses. As a rule of thumb, the natural pile diameter D, and EP is the Young’s frequency of the tower will be reduced by modulus of the pile material. 0% to 5%, when the assumption of a rigid foundation (fixed-ended tower) is replaced The formulas given for foundation by a realistic finite foundation stiffness. stiffnesses in the tables in this section can be Under special conditions this error may used to calculate spring stiffnesses to however be up to 20%. support the tower in aeroelastic wind turbine

Table 8-5. Circular footing on stratum over bedrock or on stratum over half-space. On stratum over bedrock On stratum over half-space

Mode of motion Foundation stiffness Foundation stiffness R 1+1.28 4G R 4GR R = 1 H ≤ ≤ Vertical K = (1+1.28 ) K V ; 1 H/R 5 V −ν 1−ν R G 1 H 1 1+1.28 1 H G2 R 1+ 8G R 4GR R = 1 2H ≤ ≤ Horizontal K = (1+1.28 ) K H ; 1 H/R 4 H −ν 1−ν R G 1 H 1 1+ 1 2H G2 + R 3 3 1 8GR R 8G1 R 6H K = (1+ ) K = ; 0.75≤ H/R ≤2 Rocking R R −ν 3(1−ν ) 6H 3(1 ) R G1 1 1+ 6H G2 16GR 3 Torsion K = Not given T 3

204 8 – Foundations Guidelines for Design of Wind Turbines − DNV/Risø

Table 8-6. Circular footing embedded in stratum over bedrock

Mode of motion Foundation stiffness 4GR R D D D / H Vertical K = (1+1.28 )(1+ )(1+ (0.85 − 0.28 ) ) V 1−ν H 2R R 1− D / H 8GR R 2 D 5 D Horizontal K = (1+ )(1+ )(1+ ) H 1−ν 2H 3 R 4 H 3 = 8GR + R + D + D Rocking K R (1 )(1 2 )(1 0.7 ) 3(1−ν ) 6H R H 16GR 3 8D Torsion K = (1+ ) T 3 3R

Table 8-7. Flexible pile. Standardised springs at pile head Soil profile K K K Horizontal H Rocking R H ,R 3 Coupled 2 DE S D E S D E S 0.35 0.80 0.60 Linear increase  E   E   E  with depth E = 0.6 P  0.14 P  − 0.17 P   E   E   E  ESz/D  S   S   S  Increase with 0.28 0.77 0.53 square-root of  E   E   E   P   P  −  P  depth E = 0.8  0.15  0.24   E S   E S   E S  ES z / D 0.21 0.75 0.50 Homogeneous  E   E   E  P P − P = 1.08  0.16  0.22  E E S        E S   E S   E S 

8 – Foundations 205 Guidelines for Design of Wind Turbines − DNV/Risø

8.5 Properties of Reinforced σ σ c,max ≤ + ⋅ c,min ≤ Concrete 0.5 0.45 0.9 f cd fcd In the following attention will be paid to σ concrete with reinforcing steel (RFC). σ c,max If c,min < 0 (tension) then: ≤ 0.5 Furthermore, emphasis is given to f conditions, which are of importance to the cd durability. Or, as expressed in the CEB-FIP σ maximum compressive stress at a Model Code 1990 (MC 90): “Concrete c,max fibre under the frequent combi- structures shall be designed, constructed and nation of actions operated in such a way that, under the σc,min minimum compressive stress at the expected environmental influences, they σ maintain their safety, serviceability and same fibre where c,max occurs acceptable appearance during an explicit or fcd design compression strength of the implicit period of time without requiring concrete, see Appendix E unforeseen high costs for maintenance and repair.” In structures without shear reinforcement, adequate fatigue resistance of concrete under shear may be assumed if either of the Durability of RFC strongly depends on the exposure, which usually is expressed in two equations are satisfied terms of environmental classes. As an τ τ τ example the basis for both material and min ≥ 0 : max ≤ 0.5 + 0.45⋅ min ≤ 0.9 τ τ τ structural specifications in DS411 (DS 411, max Rd1 Rd1 1999) distinguishes between different environmental classes, namely passive (P), τ τ τ moderate (M), aggressive (A) and extra min < 0 : max ≤ 0.5 − min τ τ τ aggressive (E). Class P is rarely used in this max Rd1 Rd1 context, class M is used for onshore foundations when covered by earth, class A τmax maximum nominal shear stress is used offshore where class E is also used in under the frequent combination of the splash zone. actions

τmin minimum nominal shear stress at 8.5.1 Fatigue the section where σc,max occurs Fatigue analysis of concrete structures for V wind turbines is important and must not be τ = Rd1 RD1 ⋅ omitted. Verification must be performed b hef both for the concrete and for the reinforce- ment in separate analyses. For verification VRd1 design shear resistance, according to of the fatigue-life of the foundation only the Equation (4.18) in EC 2, Part 1-1. largest of the base load components Mtt b, hef see Figure 8-13 (tower tilt moment) or Ftn (normal shear force) may be taken into consideration. As For unwelded reinforcement bars subjected regards concrete, adequate fatigue may be to tension, adequate fatigue resistance may assumed according to Eurocode 2, Part 2-2. be assumed if, under the frequent Otherwise, more refined fatigue verification combination of actions, the stress variation may be necessary. Under compression, the does not exceed following expression shall be satisfied

206 8 – Foundations Guidelines for Design of Wind Turbines − DNV/Risø

∆σ < 70 N/mm2 s A =2b(h-h ) cef ef If the stress variation does not fulfil this equation, more refined fatigue verification must be carried out, i.e. using the Palmgren- Miner cumulative damage law. The relation-

ef

ship between the characteristic fatigue h strength σfat and the characteristic fatigue h value nfat is given by the expression

1 ⋅ 12  ⋅ 12  m = k 10 =  k 10  n fat ⇒ σ fat (σ )m  n  fat  fat  b

The Wöhler curves are characteristic values Figure 8-13 Active concrete area in tension zone for calculation of crack-width and should be reduced by partial safety factors according to the chosen safety level. The external loads at which the crack widths The Wöhler constants m and k, according to are determined are usually 60% of the wind DEA (1998) can be used. turbines maximum operational loads.

8.5.2 Crack-width 2 The stress in the reinforcement σs [N/mm ] Crack widths appear in RFC from the due to external load can be found by means influence of the loads and as a result of of the elasticity theory assuming that the deformations due to temperature or surrounding concrete stress (in tension) is 0. shrinkage. The size of the crack width w can The external force is the sectional moment be found acc. to DS411 (DS 411, 1999) as a M only: function of the reinforcement stress as follows σ = M Concrete stress c max ϕ ⋅ ⋅ 2 b b hef = ⋅ −5 ⋅σ ⋅ , [mm] w 5 10 s aw σ = α ⋅γ ⋅σ Reinforcement stress s max c max The crack parameter aw [mm] is calculated as the ratio between the active concrete area M sectional bending moment Acef and the sum of the crack width decisive As reinforcement area diameters dw of the reinforcement bars in the b and hef see Figure 8-13 tension zone

Parameters α ϕb and γ to be extracted by A a = cef iteration from formulas below: w Σ d w A α ⋅ϕ = α ⋅ s ⋅ Acef is to be calculated as the maximum b hef concrete area of which the centre of gravity COG is coincident with the COG of the 2 β = α ⋅ϕ ⋅( +1 −1) tension reinforcement. α ⋅ϕ

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1 • maximum aggregate size, d < 32mm ϕ = ⋅ β ⋅(3− β ) max b 6 or minimum distance between reinforcement bars − β • γ = 1 maximum distance between non- β prestressed reinforcement bars is 150- 200 mm • Adding the contribution from temperature application of reinforcement with differences shrinkage (and creep) one can do relatively insignificant reinforcement more accurate evaluation of the size of the diameters (D = 12-20 mm) to the extent crack widths. E.g. by determination of the possible corresponding reinforcement stress expressed by strains as found in FEM- When choosing materials for parts of the analyses. structure, reinforcement and adhesion for the structure, it shall be ensured that alloys are Maximum calculated crack-width must in not applied which will function as cathodes general be within the interval 0.2-0.3 mm. for the additional structure. A distinct risk of More specific for offshore wind turbine in corrosion will be present for the metals zinc the splash zone the interval is 0.1-0.2 mm. aluminium and lead in uncured concrete. During corrosion hydrogen develops and as Cracks from shrinkage of the concrete are the corrosion products are more voluminous, distributed through the reinforcement. In spalling might happen. The corrosion attack order to minimise the development of these will decrease when the concrete dries out. cracks the reinforcement degree However, if the concrete remains wet even A in hardened condition the attack will persist. ϕ = s , where As: Reinforcement area A b Ab: Concrete area 8.6 Selected foundation structure concepts for offshore is therefor normally set to 0.25-0.50% applications

In order to minimise the risk of crack 8.6.1 Introduction to concepts formation the temperature must not exceed Three basically different foundation ° 70 C during curing, see DS 482 (1999). structure concepts exist for offshore wind Furthermore differences in temperature turbines: should be minimised; normally temperature • monopile ∆ ° differences T greater than 12-15 C • gravity base measured over the cross section is not • tripod allowed. These three foundation concepts are different in the manner by which they 8.5.3 Execution transfer the loads to the supporting soils. It must be ensured that the following The monopile and the tripod both transfer minimum requirements are fulfilled: vertical forces as axial shear forces at the • Water/cement ratio for the concrete is pile-soil interfaces. The gravity base determined in consideration of the foundation transfers such forces as a vertical environmental class, typically v/c < 0.55 contact pressure at the foundation-soil at all times. interface. Both the monopile and the tripod primarily transfer horizontal forces by way

208 8 – Foundations Guidelines for Design of Wind Turbines − DNV/Risø of horizontal earth pressures against the deeper below the sea floor, and it will piles at some depth. The gravity base contribute to prevent scour from occurring foundation primarily transfers such forces around and in particular underneath the directly to horizontal shear forces in the soil foundation, especially in sand. Gravity- at the foundation-soil interface. The based foundations may often be feasible monopile also transfers bending moments from a technical point of view, but may not about the seabed as horizontal earth always come out economically feasible. pressures against the pile. The tripod They form a well-tested concept, which is, primarily decomposes such overturning however, also a time and resource deman- moments into axial forces in the three legs, ding concept. With these pros and cons of and these forces are then transferred to the the various foundation types in mind, it soil as axial shear forces at the pile-soil should be noted that the type of foundation interfaces. The gravity base foundation to be chosen also depends heavily on the transfers overturning moments about the actual soil profile and soil properties on the seabed by a varying vertical contact pressure site. over the foundation-soil interface. During design, it is important to make sure that this Two of the three distinct foundation struc- contact pressure in extreme load situations tures for offshore wind turbine installations never takes on negative values. Any are dealt with in the following sections, viz. significant, negative contact pressure would the monopile and the tripod structures. correspond to a tension, which can hardly be transferred unless purely undrained 8.6.2 Monopile conditions with development of a suction General Description can be counted on, and which may lead to an Monopiles have traditionally been used as undesirable separation between foundation the preferred foundation solution for some and soil. However, a limited “lift” of the types of marine structures. Piles with foundation of limited duration during diameters up to about 1 m have been used to extreme loading is allowable. support lighthouses and moorings.

Equipment to install piles of diameters up to Each of the three offshore foundation 3-4 m to large penetration depths is avai- concepts have their pros and cons. The lable and makes monopiles a feasible foun- monopile is usually an attractive foundation dation alternative for offshore wind turbines. solution for wind turbines to be placed in An example of a monopile foundation is shallow waters and smooth seas. This depicted in Figure 8-14. concept is attractive also because it offers a fast installation. The tripod is preferred over Advantages of a monopile foundation the monopile in harsher climates and less include: shallow waters, however, the solution with • suction buckets under the three legs is only a fast and highly automated installation feasible for foundations in clay and will not with no prior preparation of the seabed • work in sand or other permeable soils. The simple fabrication tripod concept is a light-weight structure, The monopile foundation of a wind turbine which is usually cost-efficient. It is an consists of three basic parts: the bare pile, a attractive feature of gravity-based foun- conical transition to the tower that it dations that they can be equipped with more supports, and a boat landing. By alteration or less shallow skirts under their bottoms. of the conical transition, the foundation can This will move the depth of load transfer easily be adjusted to fit different tower diameters.

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• length of pile in soil, L • outer diameter of pile, D • wall thickness of pile, t The values of these geometrical parameters are partly interdependent and have to be selected in an iterative process, because some of the loads, in particular the ice loads and the wave loads, will increase with increasing pile diameter. At the same time, however, the bending stress ranges causing fatigue will decrease with increasing diameter.

The pile diameter D and the wall thickness t are determined from requirements expressed in terms of design rules for the pile as a steel structure. The design rules to be fulfilled consist of four criteria: • ultimate limit state criterion • fatigue limit state criterion • local buckling criterion Figure 8-14. Monopile Foundation, from LIC Engineering (1997) • hard driving criteria Once the pile diameter and the wall The boat landing is clamped to the pile and thickness have been determined by making provides a basis for the J-tube that carries sure that these criteria are fulfilled, the power cable from the seabed to the wind geotechnical bearing capacity considerations turbine. The conical transition is welded to for the supporting soil are used to determine the pile after completion of the pile driving. the necessary length L of the pile. The axial load is typically a compressive load. Usually Ice loads have a very large impact on the the axial bearing capacity of the pile will be necessary dimensions of a monopile much larger than required, and the require- foundation. If only small variations in the ments for the lateral capacity, needed to water level are expected, it is possible to counteract the horizontal shear force and the design an ice cone, which will reduce the ice overturning moment set up by wind, wave load significantly and contribute to eliminate and ice loads, will govern the design. The large oscillations of the wind turbine tower. most economical design comprises a combi- When large variations in water level are nation of D, t and L which will minimise the present, such an ice cone will not be total pile mass. Once D, t and L have been feasible. This may leave a very large determined, a final step in the design of the dynamic ice load acting on the monopile, monopile consists of a dynamic analysis of and large oscillations of the wind turbine the pile subjected to ice loading, and t is tower may result. adjusted if necessary. The design rules for determination of D and t are listed in the Determination of monopile dimensions following for the respective limit states (design approach) considered: Three basic dimensions characterise a monopile:

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Ultimate Limit State Criterion criterion is taken according to API (API, In the ULS situation, the maximum design 1993). von Mises stress in the pile wall shall be smaller than or equal to the design yield The loads to be considered are: stress. The design von Mises stress is the • loads due to wind acting on the wind characteristic von Mises stress multiplied by turbine. a load factor. The design yield stress is the • loads due to waves and current acting characteristic yield stress divided by a on the foundation. These loads are material factor. included in the Ultimate Limit State (ULS) as well as in the Fatigue Limit Fatigue Limit State Criterion State (FLS) and may amount to 30-40% The fatigue damage is calculated as a of the wind load acting on the wind Miner’s sum. For this purpose, a design turbine. long-term distribution of stress ranges shall • ice loads are very important. The result be applied together with a design S-N curve. of the analysis is a Dynamic The design stress range distribution is Amplification Factor (DAF factor), by derived from the characteristic stress range which the stresses found from a quasi- distribution by multiplication of all stress static analysis shall be multiplied. range values according to this distribution For load calculations, reference is made to by a load factor. At wall thickness Chapter 4. transitions and at the offshore on-site weld between the pile and the conical transition, stress concentration factors are to be included. The design S-N curve is derived from the characteristic S-N curve by division of all characteristic stress range values according to this curve by a material factor. The total fatigue damage from wind and wave load and from pile driving shall be below the acceptable level according to the Palmgren-Miner’s rule.

Local Buckling Criterion When a cylinder with a large diameter-to- wall-thickness ratio is exposed to bending or axial loading, a risk of local buckling is present. Local buckling can be checked according to DS 449 (DS 449, 1983). Figure 8-15. Lateral deflections of a pile exposed to horizontal loading. (Barltrop, 1991). Hard Driving Criterion During driving, the pile is exposed to very Analysis of laterally loaded piles large axial stresses, which may cause local The minimum length of the pile is the buckling of the pile during so-called hard smallest length for which there will be no driving. Hard driving is defined as more toe-kick when the pile head is subjected to a than 820 blows per meter penetration into lateral displacement at mudline. This is the the soil. Hard driving shall be avoided. The length, at which no further reduction of the lateral deflection at mudline can be obtained

8 – Foundations 211 Guidelines for Design of Wind Turbines − DNV/Risø when the length of the pile is increased. The design life of a wind turbine foundation Reference is made to Figure 8-15 and usually varies between 30 and 50 years. The Barltrop (1991). For details about methods maximum corrosion is located in the splash for analysis of laterally loaded piles, zone. The maximum bending stresses in the reference is made to Section 8.3. pile during extreme loading are found below this zone. Thus extreme stability will be Connection between monopile and tower maintained even if a rather large corrosion The monopile foundation concept includes a depth is found in the splash zone. conical connection piece between the monopile and the wind turbine tower. The However, a corroded surface generates areas connection piece comprises the access with very large stress concentrations and platform as an integral part. At the top of the cracks will propagate faster in corroded steel conical connection piece, a flange for con- than in uncorroded steel. Therefore, it is nection of the wind turbine tower is located. recommended to use corrosion protection in At the bottom of the conical connection corrosive environments, even if the structure piece, an in-situ connection to the monopile is capable of accommodating the stresses foundation is to be established. over the extent of the corroded cross-section (DS 412, 1998). The connection between the pile and the conical connection piece is an essential part With a view to the above numbers, this calls of the monopile foundation concept. The for implementation of a corrosion protection basic requirements to this connection are: system. • sufficient strength against extreme and fatigue loading Three primary options exist for corrosion • easy installation protection of a monopile: • low cost • coating (vulnerable to damage during Two alternative concepts are considered installation) feasible: • cathodic protection by sacrificial anodes • flange connection (fast installation) (a large number of anodes are required • welded connection (strong connection) for a 50-year design life) Reference is made to Lyngesen and • impressed current (facilitated by the Brendstrup (1997) for more details. presence of electrical power supply) For a design life in excess of 30 years, the Corrosion protection preferred option for monopile foundations is As the surface of the pile is exposed to a protection by use of impressed current. harsh environment due to waves, abrasion from suspended sediments and a high The pile has a large surface, but normally salinity, a large corrosion rate is expected. only a part of this requires protection. The For unprotected steel, the extreme surface surface exposed to wave action, including corrosion of an exposed steel pile in ocean the splash zone, shall be protected. water can be estimated to (DS 464/R, 1988): Depending on the type of sediment, the 0 years after installation 0 mm potential for scour, etc., also a part of the 10 years after installation 8 mm pile below mudline may need protection. 20 years after installation 11 mm Typically, the area to be protected extends 30 years after installation 14 mm down to a level 2 to 10 m below mudline. Below this limiting level, protection of the pile surface is usually not required.

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Inside the hollow pile water will be present, shock wave amplitude, and the larger is the but there will only be little exchange of possibility for pile refusal. water. Therefore, corrosion on the inside of the pile is expected to be limited. However, Driving using vibrators it is recommended that internal zones are In frictional soils, the pile may be vibrated protected either with coating or with into the soil. Vibration is a very fast cathodic protection (DNV, 1998). If the pile installation method even compared to is filled with sand, corrosion will be driving by hammer. Furthermore, it does not minimised or even prevented (DS 464/R, generate very large shock waves in the soil 1988). that could otherwise have caused damage of adjacent structures. However, only a few Installation very large vibration hammers are available Driving of large diameter piles is well- world-wide. The vibrations generate a lique- known from the offshore industry. faction of the soil locally around the pile, Generally, at least three different pile- thus reducing the side adhesion to a mini- driving methods are available. These mum. Then the pile penetrates into the soil methods are described briefly in the mainly due to its own weight. following. Driving using drilling or excavation Driving using piling hammer For installation of hollow piles, drilling or Pile driving by means of a hammer is the excavation may be applied. The soil in the oldest technique. In its simplest version, it core and below the pile is excavated, consists of a ram dropped on the top of the generating a hole with a diameter usually pile from a certain height. Today, hydraulic slightly larger than that of the pile. Then the fluid is widely used for lifting the ram and pile penetrates into the soil due to the mass further accelerating it during the downward of the pile. If side adhesion or lateral resis- stroke. Hydraulic piling hammers are the tance is required, grout may be injected in most efficient compared to diesel and steam the annulus between the pile surface and the hammers. The technique capitalises on the soil. shock wave which is generated in the pile by the impact from the hammer. The transfer of Notes on pile refusal the shock wave to the soil generates plastic The pile has to be driven to a particular pen- deformations of the soil and thereby moves etration depth, which is required to enable the pile downwards. the pile to carry its design lateral load. However, pile refusal may be encountered So-called pile refusal occurs when the shock before the final penetration depth is reached. wave amplitude is not large enough to create This may happen if the piling hammer does plastic soil deformations. The penetration of not have sufficient excess capacity the pile then ceases. Damping of the shock compared to the theoretically calculated waves caused by soil adhesion along the pile driving resistance. surface may contribute to pile refusal, in particular, in cohesive soils. In this context, In case of pile refusal, a decrease of up to it is important to be aware that the smaller 50% in the driving resistance can be the ratio between the cross-sectional pile obtained by removing the soil plug inside area and ram area is, the longer is the shock the pile. In sandy and silty soils, this can be wave transmission time, the smaller is the achieved by jetting. In clay and tills, this can be achieved by using an auger. The driving

8 – Foundations 213 Guidelines for Design of Wind Turbines − DNV/Risø resistance can also be reduced by installation inspection on the wind turbine tower is of an external “driving shoe” on the pile tip, carried out. however, this may also lead to a reduced lateral capacity and may therefore not Inspection of Scour Protection always be recommended. Scour will occur around the monopile. To provide protection against local scour Maintenance around the pile, scour protection is installed. Inspection and maintenance are to be carried Throughout the lifetime of the wind turbine out to ensure a sufficient structural strength. the scour holes around the pile are to be measured from time to time. The holes can Crack Inspection be measured by means of a 3-D echo soun- Cracks may be initiated at the weld between der. The scour protection is not absolutely the monopile and the conical transition stable and needs to be maintained. When an section. Therefore, the weld is to be inspec- unacceptable degree of scour occurs, ted for fatigue cracks. The inspection will additional scour protection is to be installed. consist of: visual inspection and/or non-destructive Spring Inspection testing (NDT): When the foundation is located in an • magnetic powder inspection (MPI) environment where ice loading may occur, it • eddy current is recommended to perform an annual • ultrasonic examination inspection every spring to verify whether ice loading has caused damage to any structural Depending on the fabrication method of the members or coating during the past winter. monopile, welds may also be located in the In case of damage, repair is to be carried submerged zone above mudline. These out. welds also need to be inspected. The extent of each inspection will be determined as part Dismantling of the inspection planning. The interval In the following, two different methods are between each inspection is to be determined outlined for removing the monopile on the basis of requirements to the lifetime foundation from the seabed after termination and on the predicted accumulated fatigue of its service life. damage. 1. Cutting off the piles is a method that can Inspection of Cathodic Protection be used in general. By this method, the The monopile foundation is protected monopile is cut below the mudline. The against corrosion by means of an impressed method is based on the use of a cutting tool current system. A reference control point lowered down inside the pile. The subsea (the point with minimum potential) on the cutting is performed by a high pressure jet, monopile is selected. The electrical potential consisting of water mixed with a non-pollute of this point is measured and is reported grinding compound (sand). through the wind turbine monitoring system. In case the anode disappears or the area of The procedure consists of the following the anode becomes too small, the potential at steps: the control point will increase, and the anode • the soil core inside the tubular is needs to be replaced. Further, it is removed to the elevation of the cut line recommended to perform a visual check of • the cutting tool is lowered to the the cathodic protection system whenever an elevation of the cut line, inside the pile

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• the pile is cut at elevation of cut line • the upper part of the pile is lifted off and transported to the shore on a barge

The remaining part of the pile will be left below the seabed, and will not cause any damage to the environment.

2. Complete removal of the pile is another method of dismantling. A vibration hammer can be used for this purpose. The vibration hammer is typically used for removal of piles in non-cohesive soils. The hammer is placed on top of the pile, and the induced vibrations in the pile will cause local lique- faction in the soil around the outer surface of the pile. When the soil is liquefied, no skin friction between soil and pile will be Figure 8-16. Tripod with piles, from Rambøll (1997). present, and the pile can then be pulled up by means of using a crane vessel. This concept is advantageous for several reasons: The advantage of this method is that the • the steel volume applied as corrosion entire pile is removed. The disadvantage is appliance is reduced since the diameter that the method is not likely to work for is reduced. removal of piles in cohesive soils. • the layout of the joints connecting upper leg bracing to centre column is 8.6.3 Tripod improved with less difference in General description diameters. The punching shear capacity The tripod foundation concept consists of a of the connections between the upper steel frame that transfers the sectional forces braces and the centre column are from the tower to primarily tensile and increased, and the stiffness of the joint compressive loads in three hollow steel piles is improved. This is of paramount that are driven into the seabed. From below importance for achieving the desired the tie-in flange, a large diameter tubular, fatigue life. referred to as the centre column, extends • The conical section will decrease the downwards. On the upper section design governing ice loading somewhat, immediately below the flange, the dimen- although the cone would have to be sion of the centre column is identical to that higher to provide full reduction for all of the tower. Below this section, a transition sea levels depending on the section reduces the diameter and increases environmental conditions. the wall thickness. Reference is made to • The hydrodynamic loading is reduced. Figure 8-16. As the tripod concept is still under development, the presentation given Each leg frame consists of a pile, a pile here is relatively brief. sleeve and two braces. At the bottom of each pile sleeve, a mudmat is located to support the structure on the seabed prior to the pile driving. After completion of the pile driving,

8 – Foundations 215 Guidelines for Design of Wind Turbines − DNV/Risø the annulus between the pile and the sleeve is filled with grout to form a rigid connection. It is recommended to batter the pile from the vertical, such that the extension of the centre line of the pile intersects with the point of impact of the dominating load on the tripod structure. Note that excessive batter requires the hammer to be supported laterally during pile driving and should therefore be avoided.

As an alternative to using three piles to support the tripod structure and transfer the loads to the soil, so-called skirt foundations may be applied, e.g. in the form of suction buckets. One suction bucket would then have to support each of the three tripod legs. When suction buckets are used for the foundation, a variant of the tripod structure is feasible, in which the central column supporting the tower is extended down to Figure 8-17. Tripod with suction buckets, from the seabed and further supported by a Rambøll (1998). suction bucket. With this configuration of the central column with a bucket support, diameter than that of the tower, and the ice only two more supports with suction buckets load on the structure is correspondingly are needed. These are placed such that their smaller. A complete elimination of ice respective frames are perpendicular to each crushing is not feasible, since this would other. This implies that access of service pose requirements to the inclination of the vessels is possible within a 270 degrees conical transition far beyond what would be angle. By this design, the centre column will feasible considering the diameters of the participate more actively in the transfer of involved tubulars. The disadvantage forces from the tower to the seabed. All associated with the use of the conical supporting elements, including the centre transition as an ice cone is that it implies column, are designed as circular tubes acting that an upwards vertical ice load will be as strut members, i.e. transferring acting on the centre column. However, as compressive and tensile forces only. the vertical capacity is usually quite large, Reference is made to Figure 8-17. this disadvantage is usually not critical, yet it needs to be duly accounted for in the The tripod concept makes use of the conical design. transition between the tower and the supporting column in the tripod structure as The tripod structure is not compatible with an ice cone. When solid ice is forced against too shallow water depths. This is due to the the structure, the conical transition causes fact that: the ice to be pressed downwards, and • a sufficient water depth above all parts crushing takes place at a level with a smaller of the structure is required to allow ser- vice vessels to approach the structure.

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• at very low water depths, the distance MP(N) Design plastic moment capacity as between the upper and lower braces a function of the axial force becomes too short for the forces to be N Axial force in pile transferred in any reasonably sized σy Nominal steel yield stress cross-section. WP Plastic section modulus • a solution with the braces protruding the D Pile diameter water surface is not desirable due to the t Pile wall thickness possible interlocking or packing of ice A Steel area of pile and the possibility of collision with service vessels. As a minimum, the result of the analysis must contain the pile utilization ratio and a The tripod causes relatively little blocking verification that the penetration depth is for wave action and for current flows which sufficient. All piles are allowed to reach the is of importance for environmentally sensi- design plastic moment. The forces and tive areas. This even applies when a large moments at the pile heads shall be in number of foundations are installed as is the equilibrium with the design loading acting case in an offshore wind farm. upon the structure and shall be smaller or equal to the allowable design values. Geotechnical analysis – piles In the ultimate limit state, the foundation In both elastic and plastic analyses, the analysis shall consist of an elastic and a following features must be taken into plastic analysis. The two analysis checks can account: be described as follows: • linear-elastic superstructure that interconnects the piles In the elastic analysis, a check is carried out • non-linear behaviour of the piles of the stresses in the piles and the structure laterally and axially with design loads (ULS) and design material • pile group effects properties. One pile only is allowed to reach • second-order moments in the piles its design maximum allowable stress. The • soil plug length taken as 0.9 times allowable design value of the von Mises’ length of pile embedded in soil stress is the yield stress fy divided by the partial safety factor for steel strength. The The procedures for development of t-z and result of the analysis must as a minimum q-w curves for the axial bearing capacity and contain the utilisation ratio of the pile steel. p-y for the lateral bearing capacity can be In the plastic analysis, the entire tripod done by using the API approach (API, structure is checked using design loads 1993). (ULS) and design material properties. All the piles are permitted to yield or fail When characteristic values for the skin completely, provided that the tripod as a friction and tip resistance are given, the t-z whole can absorb the design loads. curves for axial pile-soil interaction can be established by means of any relevant The plastic design moment shall be recognised computer program. The shape of calculated by the curves is fully described by

()= σ ⋅ ⋅ [π ⋅ N ] M N γ W cos σ ⋅ , P P 2 Y A 2  z π G  = ⋅ ⋅ ⋅ ⋅ t t max arctan  3 − − ⋅ 3 π 3 D t = D ( D 2 t )  max  WP 6

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G Shear modulus inside the bucket. A sudden application of z Deflection pile/soil loading yields a remarkable resistance D Diameter of pile towards pull-out, gradually tapering off if tmax Maximum skin friction, calculated in the loading is sustained. This resistance is accordance with API procedure formed by suction in the porewater inside E Young’s Modulus the bucket, mobilised as the immediate ν Poisson’s ratio reaction to the suddenly applied load. The Reference is made to Clausen et al. (1982). bucket foundation is thus primarily designed to absorb load peaks. A static load in excess The shape of the q-w curves for the tip load- of a certain limit and acting over some displacement relationship can be assumed to length of time will gradually pull out the be bilinear with a required relative bucket as water is allowed to flow from the displacement between pile and soil of 5% of outside to the inside of the bucket and the pile diameter to cause yield. thereby neutralises and eventually eliminates the suction condition inside the bucket. The lateral bearing capacity can be based on p-y data developed by the API-RP2A-WSD The concept is new only in relation to the procedures using cycling soil strength. Soil application of suction technology to the parameters needed in the modelling of the design of offshore wind turbine foundations. curves are: The suction (or differential pressure) technology has been suggested and applied Clay: cu Undrained shear strength as both anchoring device for ships (and γ’ Submerged unit weight tension leg platforms) and as foundation for J Empirical constant fixed leg platforms as an alternative to piles.

ε50 Strain at one-half the max. However model testing and analyses are still stress in laboratory un- required in order to obtain a sufficient drained compression test design basis against hydraulic instability of Sand: γ’ Submerged unit weight skirted foundation of offshore wind turbines. Φ Angle of internal friction (plane) Structural analysis (limit states) The structure and all structural members The curves and the pile group effects can be shall be verified according to three limit modeled by means of a relevant recognised states. Different sets of partial safety factors computer program. for load and resistance apply to the different limit states. The normal safety class In a natural frequency and fatigue analysis according to DS449 (DS 449, 1983) can be linear behaviour of the piles laterally and adopted, as the consequences of failure are axially can be assumed and modeled as limited (loss of investment but no loss of spring constants. The true pile-soil inter- human life). action relationship is usually a smooth non- linear curve. The applied spring constants In the ultimate limit state (ULS), the can be taken as the initial slope of the structure is checked against extreme loads. smooth curve. The structure above seabed should be able to sustain these loads without collapse or Geotechnical analysis – Suction Buckets permanent deformation. For piles, a fully The buckets work by principle of restricting developed plastic failure mode is accepted. flow of water from outside the bucket to Horizontal ice load is based on static ice

218 8 – Foundations Guidelines for Design of Wind Turbines − DNV/Risø load, multiplied by a dynamic amplification lead to a maximum stress above the upper factor (DAF). Vertical ice load may destabi- joint at two points on a line perpendicular to lize the structure, because it is applied as an the axis of the applied moment. The stresses upward load. will vary linearly between these points, but as the wind direction changes, also the In the serviceability limit state (SLS), the locations of these points move. Thus, the maximum deflections are checked with loading (both in terms of magnitude and in special emphasis on the allowable tilt of the terms of number of stress cycles) shall be foundation due to differential settlements. In applied for a fixed but critical point in the this state, no hydrodynamic loads on the cross-section, i.e. the fatigue loads are to be foundation need to be included, since these applied from one particular direction only, loads will contribute only marginally to the and this direction is to be taken as the most overall moment on the tripod. All partial critical direction. The conical transition load factors are to be set equal to unity, i.e. induces an additional stress, which has to be characteristic loads are to be used in accounted for. The designer may find it calculations. Loads from the wind turbine feasible to locate internal ringstiffeners, shall be taken as the worst damaging load where the cone connects to the tubular, and case among all dynamic load cases. The thereby lower the overall dimensions of the maximum allowable tilt under this conical section. Several layouts of the characteristic loading is 0.5° from vertical. stiffener(s) are possible: • a number of internal ringstiffeners In the fatigue limit state (FLS), the structure • bulkheads is checked against failure due to fatigue • the leg framing may protrude into the damage. The cumulative fatigue damage for interior of the centre column to be all load situations during the design life joined at the centre line, thereby must be taken into account and the distributing the loads directly between integrated effect should be investigated, e.g. the leg frames by using the rain-flow counting scheme for The most advantageous option may be stress cycles. Fatigue loads from the wind chosen, taking into consideration the turbine shall be considered in conjunction requirements of internal clearance for risers, with wave-induced fatigue loading, and it is conductors, power cables, etc. to be verified whether it is acceptable to disregard contributions to the fatigue Natural frequency analysis damage from ice crushing. A complete natural frequency analysis shall be performed for the combined structure As fatigue is considered critical for the consisting of turbine, tower, tripod and piles. structure, it is of importance to verify that For this purpose, the non-linear soil must be the fatigue life does not fall short of the linearised. It is to be verified that the lowest design life, based on detailed information of frequencies differ from at least ±10% of the waves from different geographical 1P and 3P rotor frequencies at nominal directions. The areas which are most likely power. to be dimensioned by fatigue loads are the centre column transition to the ice cone and Grouted connections the upper joint at the centre column where Axially loaded pile-to-sleeve connections the upper leg framing attaches to the struc- are checked using the formula given by ture. Consider the distribution of stresses DNV (1977), including a safety factor of due to pure bending of the tower. This will 3.0. The connections can be established by a

8 – Foundations 219 Guidelines for Design of Wind Turbines − DNV/Risø system widely used within the oil/gas Installation industry. This system consists of flexible Besides the geophysical and geotechnical grout lines (OD 50 mm) attached to the surveys as described in Section 8.1, the only tubes and connecting a simple manifold preparation of the seabed necessary before above water level to an outlet in the upper installation can take place is the establish- part of the pile sleeves. ment of the mudmats. The purpose of the mudmats is to secure the on-bottom stability Corrosion protection prior to final driving of the three piles. They Two options are found advantageous as shall be designed to satisfy the following primary protection. They are both of the requirements: cathodic type: • axial and lateral capacity of the soil • sacrificial anodes of the ZN-Al-In type, • lateral resistance due to stabbing pile placed as slender anodes in “handles” • horizontal level adjusted installation attached to the structure below sea level tolerances • impressed current system The installation procedure is to a large The disadvantage of the sacrificial anode extent based on the choice made by the system is the need for inspection. A contractor and is therefore not taken into sacrificial anode system may have to be consideration here. renewed during a design life, depending on water salinity. Due to the on-line monitoring Maintenance system already present for the turbine, The maintenance of the steel structure measuring potential differences easily includes activities that are not specific to the monitors an impressed current system. The tripod but apply to all steel foundation current requirements may, however, pose a structures. problem for remote and isolated offshore steel structures. Prior to coating, the entire Crack monitoring structure must be sandblasted to comply Crack monitoring of welded joints will have with at least Sa 2½ according to ISO 8501- to be performed at regular intervals, at least 1n. The areas above the lower limit of the until general experience with the structure splash zone are to be coated with 2.5 mm of has been established. Crack monitoring solvent-free epoxy coating. The remaining consists of ultrasound or X-ray inspection at structure is to be coated with a shop primer critical time intervals. For the tripod, the to resist the environment at the construction critical details are considered to be the cone site. sections and the joint between the centre column and the upper leg bracing. To Scour protection facilitate NDT inspection of the tripod, the No scour protection is needed for the tripod centre column can be sealed off at some structure. The small overall dimensions of elevation below the joint and thereby allow all members close to the seabed yield rather for dry testing. The structure may be small increases in velocities, and thus only equipped with attach points for the scanning little local scour. However, the design shall equipment to facilitate positioning. take into account the presence of both local and global scour around the tripod. The By specifying the allowed fatigue utilisation, centre column must be sealed with a flange the designer can allow for a trade-off in order to avoid resonance between water between initial manufacturing costs and inside the column and the surrounding sea. subsequent maintenance costs.

220 8 – Foundations Guidelines for Design of Wind Turbines − DNV/Risø

Surface protection system Clausen, C.J.F., P.M. Aas, and I.B. Maintenance consists of repair of coating in Almeland, “Analysis of the Pile Foundation case of damage and monitoring/maintenance System for a North Sea Drilling Platform”, of the impressed current system. The Proceedings, BOSS, 1982. impressed current system is believed to have a lifetime of 15 to 25 years after which the DNV, Rules for the Design, Construction anode must be replaced. and Inspection of Offshore Structures, Det Norske Veritas, Høvik, Norway, 1977. Dismantling (Reprint 1978.) The tripod can be moved from the seabed by cutting off the piles at or preferably below DNV, Foundations, Classification Notes, mudline. The pile sections thus left behind No. 30.4, Det Norske Veritas, Høvik, pose no ecological threat. Total removal of Norway, 1992. the pile can be accomplished by an inverse driving procedure and/or by vibration DNV, Rules for Classification of Fixed equipment. Offshore Installations” Det Norske Veritas, If suction buckets are used for the Høvik, Norway, 1998. foundation, the tripod can be removed completely. DS 411, Code of practice for the structural use of concrete, 4. edition, Danish standard 1999 REFERENCES DS 412, Code of Practice for the structural Aage D. Herholdt et all. Beton-Bogen use of steel, 3. edition, Danish standard 2.udgave 1986, CtO, Cementfabrikkernes 1998. tekniske Oplysningskontor. DS 415, Code of Practice for foundation API, Recommended Practice for Planning, engineering, 4. edition, Danish standard Designing and Constructing Fixed Offshore 1998. Platforms – Working Stress Design, API Recommended Practice 2A-WSD(RP2A- DS449 Dansk Ingeniørforenings Norm for WSD) 20th Edition, American Petroleum Pælefunderede Offshore Stålkonstruktioner, Institute, July 1, 1993. (in Danish), Dansk Ingeniørforening, Teknisk Forlag, Normstyrelsens API, Recommended Practice for Planning, Publikationer, Copenhagen, Denmark, April Designing and Constructing Fixed Offshore 1983. Platforms, API RP2A, 17th Edition, American Petroleum Institute, 1987. DS 464/R, Dansk Ingeniørforenings Anvisning for Korrosionsbeskyttelse af Barltrop, N.D.P., and A.J. Adams, Dynamics Stålkonstruktioner i Marine Omgivelser, of Fixed Marine Structures, The Marine Dansk Ingeniørforening, Teknisk Forlag, Technology Directorate Limited, Normstyrelsens Publikationer, Copenhagen, Butterworth-Heinemann Ltd., Third Edition, Denmark, June 1988. 1991. DS 482, Execution of Concrete Structures, CEB-FIP Model Code 1990 (MC 90). 1. edition 1999.

8 – Foundations 221 Guidelines for Design of Wind Turbines − DNV/Risø

Eide, O., and K.H. Andersen, Foundation Rambøll, Havmøllefundamenter med sug, Engineering for Gravity Structures in the UVE-98 J.nr. 51171/97-0047, Fase 1- Northern North Sea, Norwegian rapport, SEAS, Niras, Rambøll, DGI, Risø, Geotechnical Institute, Publication No. 154, September 1998. Oslo, Norway, 1984. Recommendation to Comply with the Eurocode 2, Design of concrete structures – Requirements in the Technical Criteria for Part 1-1: General basis for buildings and Danish Approval Scheme for Wind Turbines, civil engineering works, ENV 1992-1- Foundations, Danish Energy Agency, 1:1991 August 1998. Part 2-2: Reinforced and Prestressed Concrete Bridges, ENV 1992-2:1996. Reese, L.C. and H. Matlock, “Numerical Analysis of Laterally Loaded Piles”, Hansen, B., Limit Design of Pile Proceedings, Second Structural Division Foundations, Bygningsstatiske Meddelser, Conference on Electronic Computation, No. 2, September, 1959. American Society of Civil Engineers, Pittsburgh, Pennsylvania, pp. 657, 1960. Hansen, B., ”Geoteknik og Fundering, Del II Forelæsningsnotater til Kursus 5821 − Reese, L.C. and S.-T. Wang, Documentation Geoteknik 2”, Notat nr. 16, Den private of Computer Program Group 4.0, Ensoft, ingeniørfond ved Danmarks tekniske Høj- Inc., Austin, Texas, 1996. skole, Lyngby, Denmark, 1978.

Hansen, J.B., A Revised and Extended Formula for Bearing Capacity, Danish Geotechnical Institute, Bulletin No. 28, pp. 5-11, Copenhagen, Denmark, 1970.

Kraft, L.M., R.P. Ray and T. Kagawa, “Theoretical t-z curves,” Journal of Geotechnical Engineering, ASCE, Vol. 107, No. 11, pp. 1543-1561, 1981.

Lyngesen, S., and C. Brendstrup Vindmøllefundamenter i Havet, EFP-96, J.nr. 1363/96-0006, Final Report Mono Pile Foundation, LIC Engineering A/S, February 1997.

Rambøll, Vindmøllefundamenter i Havet, EFP-96, J.nr.1363/96-0006, Tripod Foundation Volume 1 – Report, Rambøll, February 1997.

222 8 – Foundations Guidelines for Design of Wind Turbines − DNV/Risø

9. Electrical System integration of wind turbines are also discussed.

The electrical system of a grid connected wind turbine includes all components for 9.1 Electrical components converting mechanical power into electric power as well as the auxiliary electric This section does not seek to provide a aggregates (the yaw system, the cooling general introduction to electrical system system, the ventilation system), the control technology. Standard literature on the and supervisory systems. Figure 9-1 illu- subject is available for this purpose (Heier strates the main components of the electrical S., 1998), (Mohan N. et al., 1989). Thus, the system of a grid connected wind turbine. following sections will mainly summarise some of the essential properties of the most Electrical system important types of each electrical component in a wind turbine. Turbine Electrical interface Generator Soft starter Transformer Wind 9.1.1 Generators Gear Grid

Capacitor Insofar as concerns generation of electric bank power, a wind turbine can, basically, be equipped with any type of generator. However, two specific types of three-phase Control generators constitute the most frequently Figure 9-1. Block diagram of wind turbine connected to used generators in the industry: induction a grid with an electrical interface. (asynchronous) generators and synchronous generators. Today, the demand for The scheme comprises the wind turbine gridcompatible electric current can be met rotor, linked via a gearbox to a generator, by connecting frequency converters, even if which through an electrical interface is the generator supplies alternating current of connected to the grid. A control system is variable frequency or direct current. necessary to ensure proper operation of the wind turbine under all conditions. Different Induction generator configurations of the electrical interface The most common type of generator used in exist. A typical solution contains a wind turbines is the induction generator. softstarter, a capacitor bank and a transformer. The transformer transforms the In the induction generator, an electric field is generator voltage to a higher voltage of the induced between the rotor and the rotating grid. Another popular solution involves a stator field by a relative motion (slip), which frequency converter instead of the above- causes a current in the rotor windings. The mentioned softstarter and capacitor bank. interaction of the associated magnetic field of the rotor with the stator field results in the In the following sections, focus will first be torque acting on the rotor. on the most relevant electrical components of a wind turbine: generator, softstarter, The rotor of an induction generator can be capacitorbank and frequency converter. designed as a so-called short-circuit rotor Later on, the most utilised generator (squirrel-cage rotor) or as a wound rotor configurations and control strategies for (Heier S., 1998). The windings of the wound wind turbines will be presented. Issues rotor can be externally connected through related to power quality, grid connection and

9 – Electrical Installations 223 Guidelines for Design of Wind Turbines − DNV/Risø

slip rings. In this way, the electrical positive for a motor (nr < ns). If the slip is characteristics of the rotor can be controlled zero (the generator is in synchronism), the from the outside by means of electric generator is running idle (it does not equipment. produce torque). If the slip is 1, the rotor is blocked. The torque shows a maximum, the The induction generator has several so-called pull-out torque Tp, at the rotational advantages such as robustness and mechani- speed np, as shown in Figure 9-2. cal simplicity. Moreover, as it is produced in large series, it can be purchased at a Torque relatively low price. Motor mode Generator mode The major disadvantage is that the stator is dependent on a reactive magnetising current. As the asynchronous generator does not contain any permanent magnets and is not separately excited, it is bound to obtain its 0 -1 s n n n exciting current from somewhere else - and s p r thus to consume reactive power. The Operation point reactive power may be supplied by the grid or e.g. by the capacitor bank. Its magnetic T field is only established when the generator p is connected to the grid.

Figure 9-2. Torque characteristics of an induction The synchronous speed of the rotating stator motor/generator. field of an induction generator depends on the grid frequency and on the number of Synchronous generator pole pairs: Another type of generator used in wind turbines is the synchronous generator. It has f a wound rotor, which is excited with direct n = 60 ⋅ s p current via slip rings, thus constituting the basic difference between this generator and f grid frequency in Hz the above-mentioned induction generator. p number of pole pairs The rotor winding, through which direct current flows, generates the exciter field, ns synchronous rotational speed in rpm. which rotates with synchronous speed. The The torque of the induction generator – see speed of the synchronous generator is Figure 9-2, is a function of the slip s, which determined by the frequency of the rotating is defined as: field and of the number of pole pairs of the rotor.

n − n s = s r The synchronous generator has one clear ns advantage compared with the induction generator: it does not need a reactive nr rotational speed of the rotor in rpm magnetising current. However, compared to the induction generator it is much more The slip is expressed in percentages. It is expensive and mechanically more negative for a generator (nr > ns) and complicated.

224 9 – Electrical Installations Guidelines for Design of Wind Turbines − DNV/Risø

9.1.2 Softstarter After in-rush, the thyristors are bypassed in order to avoid conducting losses. The effects The generator should be gradually of the softstarter are thus: connected to the grid in order to limit the in- • reduction of the peak current at in-rush. rush current. Without a softstarter, the in- • rush current can be up to 7-8 times the rated reduction of the voltage dips in the grid current, which can cause severe voltage at in-rush. • disturbance in the grid. The use of a reduction of the transient torque in the softstarter, which contains thyristors, can generator and in the gearbox at in-rush. cause a limitation of the in-rush current. 9.1.3 Capacitor bank A thyristor is a semiconductor, which has As mentioned, an induction generator is a two states: a blocking and a conducting state reactive power consumer while generating (Mohan N. et al., 1989). The transition from active power. The active P and reactive Q blocking to conducting state is initiated by electrical power are expressed as: supplying the gate with a power impulse. This is called “firing of the thyristor”. The = φ [] P 3 U eff Ieff cos kW thyristor remains in the conducting state as = φ [] long as the current flows in the positive Q 3 U eff Ieff sin kVAr direction. As can be seen from Figure 9-3, the function of the softstarter is to slowly U r.m.s. line-to-line voltage “open up” for the voltage and thereby the eff I line current current by adjusting the firing angle θ. eff cos φ power factor.

In this way, the generator is gradually Assuming a perfect sinusoidal waveform, connected to the grid. the r.m.s. voltage and current can be

expressed in terms of the maximum voltage

and the maximum current as follows θ 9 π V = 10 I U I = max = max π 2 π U eff and I eff 2 2

V θ 5 π = 10 The maximum voltage Umax and the maximum current I are defined as the π 2 π max peak line-to-line voltage and the peak line current, respectively.

The amount of reactive power for the V θ 1 π generator varies depending on the wind = 10 conditions. This means that if the wind π π 2 speed is high, the wind turbine can produce more active power, but only if the generator Figure 9-3. Voltage waveforms for a softstarter during gets more reactive power. Without any start-up. electrical components to supply the reactive power, the reactive power for the generator must be taken directly from the grid. Reactive power supplied by the grid causes

9 – Electrical Installations 225 Guidelines for Design of Wind Turbines − DNV/Risø additional transmission losses and can, in power is zero, full compensation is a some situations, make the grid unstable. To dynamic compensation, where a certain avoid this, a capacitor bank can be used number of capacitors are connected or between the generator and grid – see Figure disconnected continuously, depending on 9-4. The capacitor bank is connected to the the varying reactive power demand of the wind turbine immediately after the generator generator. is coupled to the grid. By ensuring a power factor close to 1, the effects of a capacitor bank are: Generator QGen QGrid • improvement of voltage stability • reduction of network losses Gear 9.1.4 Frequency converter The frequency converter, located especially Capacitor in modern wind turbines, is a power bank electronic device which facilitates Figure 9-4. Capacitor bank coupled to the wind turbine interconnection of two electrical systems to compensate the reactive power. with independent frequencies (Mohan N. et al., 1989). The idea of the capacitor bank is thus to supply locally with reactive power Usually, a modern converter contains IGBT Q in such a way that the reactive (Insulated Gate Bipolar Transistors) based ∑ Ci i power devices, which are characterised by a power taken from the grid QGrid is high switching frequency of up to 10 kHz. A minimized. Thus, as the extracted current switching frequency of 10 kHz is the limit from the grid decreases, the losses in the for acceptable switching losses. grid also decrease. Figure 9-5 illustrates the reactive power consumption QGen of the Figure 9-6 illustrates two typical basic induction generator and how the use of a converter structures: a current source capacitor bank minimizes the absorbed converter and a voltage source converter. reactive power from the grid QGrid. The latter is today used in wind turbines (Heier S., 1998). Q ~ f1 ~ f1 Q C6 Q C5 Q C4 Q C3 Q Q C2 C1 P L C Q Grid

f ~f2 ~ 2 Q Gen Current source converter Voltage source convert Figure 9-5. Reactive power as a function of active power. Reactive power compensation using a capacitor Figure 9-6. Basic power converter structures for bank. frequency converter.

There is both a no-load compensation and a Frequency converters will become more and full compensation. Whereas, the no-load more significant in future control of wind compensation is only effected when active

226 9 – Electrical Installations Guidelines for Design of Wind Turbines − DNV/Risø turbines. Their most important properties possibility for wind farms to behave as and implications are highlighted in the active elements in the power system following: (Sørensen P. et al., 2000). A wind farm with power plant characteristics is able 1. Controllable frequency – is the property to: of the frequency converter which has • control the active/reactive power. the unique possibility to connect a • influence positively the stability of variable speed wind turbine to the grid, the network. thereby allowing the generator • improve the power quality. frequency to differ from grid frequency. Some of the implications of the controllable frequency property are: 9.2 Wind turbine configurations • the rotor behaves as an (as the variations in wind In this section the most commonly applied speeds are absorbed by the rotor generator and power electronic configu- speed changes). rations in wind turbines are presented. • the loads on the gear and drive train Further details can be found in (Hansen L.H. can be reduced. et al., 2001a).

• the power capture at low wind Wind turbines can be divided into two main speed can be improved. groups, i.e. fixed speed and variable speed • the acoustical noise emission can be wind turbines. reduced (as the wind turbines can

have a low rotational speed at low The fixed speed wind turbine, equipped with wind speeds). • a generator connected directly to the grid, a the frequency converter can replace softstarter and a capacitor bank, is the most the softstarter and the capacitor common type of wind turbine. In order to bank. increase power production, some of the • the frequency converter is a fixed speed wind turbines are equipped with practical necessity for the gearless two speed generators instead of with one wind turbine. generator only.

2. Controllable reactive power – is The advantages of the fixed speed wind another property of the frequency turbine are: converter, which makes it possible to • simplicity. improve the power quality. Some of the • low price of the electrical system. implications of the controllable reactive

power property are: • The disadvantages are: the voltage stability is improved. • • limited power quality control. the flicker level is reduced. • • mechanical stress on gear and drive the frequency converter can replace train. the capacitor bank. • the frequency converter can be used The variable speed wind turbine is today not as a local reactive power source. as common as the fixed speed wind turbine. However an increasing number of turbines 3. Power plant characteristics – is an have variable speed rotors. Two general important property of the frequency converter because it provides the

9 – Electrical Installations 227 Guidelines for Design of Wind Turbines − DNV/Risø configurations of variable speed wind This type of variable speed wind turbine is turbines exist: used by several large manufacturers. The 1) one where the generator stator is rotor is connected to the grid through a connected to the grid, while the frequency converter, which is not a full- rotor frequency is controlled. The scale power converter, since typically only a speed can typically be varied fraction (up to 70%) of the speed range will within a limited range only. be utilised. 2) one where the generator stator is connected to the grid through a frequency converter. The speed can ~ be varied to an unlimited extent. ~

The advantages of variable speed wind Gear turbines are: • improved power quality. • reduced mechanical stress. Figure 9-8. Schematics of a wind turbine equipped with The disadvantages are: a doubly-fed induction generator with a converter connected to the rotor circuit. • losses in power electronics. • price of frequency converter. Full variable speed systems are equipped with a frequency converter. Two different Two possible configurations exist for modes of implementation with full variable variable speed wind turbines with controlled speed can be found. rotor frequency (limited variable speed): One is the classical configuration, where a cage rotor induction generator is connected to the grid through a full power frequency converter – see Figure 9-9. The range of the variable speed operation is, in principle, Gear from zero to the maximum which can be handled by the wind turbine. Optislip

Figure 9-7: Schematics of a wind turbine with controllable rotor resistance.

1. Induction generator with variable rotor ~ resistance – see Figure 9-7 Gear ~

A Danish manufacturer produces a wind turbine with an Optislip generator, where the slip can differ with up to 10%, by varying Figure 9-9. Schematics of a wind turbine with the resistance of the rotor. frequency control.

2. Double-fed induction generator with Another configuration with full variable converter connected to the rotor circuit speed involves the connection of a multipole – see Figure 9-8 synchronous machine to the grid by a

228 9 – Electrical Installations Guidelines for Design of Wind Turbines − DNV/Risø frequency converter. The frequency characteristics of the wind turbine, converter thus converts the variable characteristics which can be used to frequency generator voltage into the grid characterise the influence of the wind frequency. A particular feature of this type turbines on the grid. Grid interference of configuration is that it avoids the use of a caused by wind turbines can be voltage gearbox – see Figure 9-10. distortions, frequency distortions and failure – see Figure 9-11.

Power quality Synchronous generator ~ ~

Wind Turbines Grid Figure 9-10. Schematics of a variable speed system with a synchronous generator.

Voltage Frequency Failures 9.3 Power quality and grid connection Figure 9-11. Power quality expressed in grid The term “power quality of a wind turbine” interference of the wind turbines. describes the electrical performance of the turbine’s electricity generating system in its The main interference of wind turbines with interaction with the grid. Methods which can the grid is caused by voltage distortions. The be used to measure and quantify the power frequency of large power systems is usually quality of wind turbines have earlier on been very stable, and therefore wind turbines do developed at a national level. However, the not typically influence the frequency. need for a common frame of reference However, this is not the case for small across national borders has called for autonomous grids, where wind turbines may international standardisation work in the cause frequency variations. Wind turbines field. do not normally cause any interruptions/ failures on a high voltage grid. Wind A perfect power quality means that the turbines, as they are designed today, are voltage and current are continuous and disconnected from the grid by the protection sinusoidal, implying that they have a system, if for example grid failures occur. constant amplitude and frequency. This aspect can generate serious problems in the future. For example in Denmark where it Grid connected wind turbines affect the is envisaged to cover 50% of the overall power quality of the grid. The wind turbines electricity consumption by wind power in and the grid are in continuous interaction, as 2030. Thus, grid failures can have severe illustrated in Figure 9-11. consequences for the stability of the whole power system, if large wind farms are Power quality depends on the interaction disconnected from the grid simultaneously. between the grid and wind turbines. IEC As a consequence hereof, research and 61400-21 defines power quality development are today focused on making

9 – Electrical Installations 229 Guidelines for Design of Wind Turbines − DNV/Risø wind farms more robust towards grid faults, distortions – see Table 9-1. In switching for example by using power electronic operation focus is on flicker and voltage devices. drop generated at the connection of the generator, and on voltage changes generated Voltage distortions can be described in at the connection of the capacitor bank. different time intervals: • steady-state voltage variations are Depending on the grid configuration and on changes in the r.m.s. value of the the specific type of wind turbine used, voltage, occurring with a frequency less different power quality problems may arise. than 0.01 Hz. For example, in the case of a fixed speed • flicker disturbances are voltage wind turbine, natural variations of the wind fluctuations between 0.01-35 Hz that and the tower shadow will result in can cause visible variations in domestic fluctuating power, which may cause flicker lighting. disturbances. In the case of a variable speed • harmonics are voltage fluctuations wind turbine, the presence of the frequency above 50 Hz, caused by the presence of converter will inject harmonic currents into power electronics (frequency the grid. The frequency of a large power converter). system is normally very stable. However, in • transients are random voltage the case of a weak or autonomous grid, fluctuations caused by e.g. connection where diesel engines are used, wind turbines of capacitor bank. may cause frequency fluctuations.

Power quality analysis is performed both for continuous and switching operation (Risø 9.4 Electrical safety and DEFU, 1996). Continuous operation The design of the electrical system for a means the continuous operation of the wind wind turbine shall ensure minimal hazards to turbine during permanent grid connection. people and livestock as well as minimal Switching operation conditions are mainly potential damage to the wind turbine and the switching operations, occurring only for external electrical system during operation short time intervals. and periods of maintenance of the wind turbine under all normal and extreme Continuous Switching conditions. It is usually required that the operation operation design of the electrical system shall take into Steady-state Connection of the account the fluctuating nature of the power voltage variation generator: generation from the wind turbine. It is - flicker usually also required that the electrical - voltage drop system shall include suitable devices that Flicker Connection of the ensure protection against malfunctioning of capacitor bank: both the wind turbine and the external - transients electrical system which may lead to an Harmonics unsafe condition. Table 9-1. Power quality analysis in different operation modes. A wind turbine should be equipped with an

earthing system, both for conducting In the case of continuous operation, the lightning currents and for earthing the analysis is focused on slow voltage electrical system of the turbine. Connection variations, on flicker and harmonic to the earthing network should be possible at

230 9 – Electrical Installations Guidelines for Design of Wind Turbines − DNV/Risø every location where electrical systems or In other words, wind farms must develop equipment are located. power plant characteristics. The two system responsible utilities in Denmark, Eltra It is recommended that disconnection of the (Eltra, 2000) and Elkraft System have laid electrical system from all electric sources, as down requirements for the influence of wind required in the case of maintenance or farms on grid stability, power quality and on testing, shall be possible. Semiconductor the control capabilities of wind farms. devices should not be used as disconnection devices alone. Where lighting or other Another consequence of the increased size electrical systems are necessary for reasons of future wind farms is that large wind farms of safety during maintenance, auxiliary will be connected directly to the circuits should be provided with separate transmission level. Until now, wind turbines disconnection devices, such that these and wind farms have been connected to the circuits may remain energised while all distribution system, typically on 10/20 kV other circuits are deenergized. It is grid or on 50/60 kV grid. Consequently, recommended that any electrical system main focus has been on the influence of operating above 1000 V AC or 1500 V DC wind farms on the power quality in the shall be able to be earthed for maintenance. distribution system. In Denmark, this has been regulated by the Danish Utilities The IEC 61400-1 requires that all electrical Research Institute’s (DEFU) requirement for components and systems shall meet the grid connection of wind turbines (DEFU requirements set forth in IEC 60204-1, and KR111, 1998) to the distribution system. compliance with the requirements of IEC However, stricter requirements for large 60364 for design of the electrical system of wind farms, connected directly to the a wind turbine is also required. transmission system (Eltra, 2000), are now issued by a.o. transmission system operators It is recommended to make provisions to in Denmark. As mentioned before, national enable the wind turbine to be isolated from standards for power quality of wind turbines the public grid in a safe manner. This have been supplemented by the new applies to normal situations, abnormal standard IEC 61400-21 (IEC 61400-21, and/or faulty conditions as well as during 2001) for measurement and assessment of maintenance and repair. the power quality of grid connected wind turbines.

9.5 Wind farm integration An important requirement made by the system responsible is that in future, wind In the last few years the trend has moved farm owners must provide models that can from installations with single or small simulate the dynamic interaction between a groups of wind turbines, to large wind farms given wind farm and a power system during with a capacity of hundreds of MW. transient grid fault events.

The increasing and concentrated penetration Such developed models will enable both of wind energy makes the power system wind farm investors and technical staff at more and more dependent on and vulnerable the respective utility grid to undertake the to wind energy production. Future wind necessary preliminary studies before farms must be able to replace the present connecting wind farms to the grid (Hansen power stations, and thus to act as active A.D. et al., 2001), (Sørensen P. et al., controllable elements in the power system.

9 – Electrical Installations 231 Guidelines for Design of Wind Turbines − DNV/Risø

2001a). Simulation of the wind farm Hansen L.H., Helle L., Blaabjerg F., Ritchie interaction with the grid may thus provide E., Munk-Nielsen S., Bindner H., & quite valuable information and may even Sørensen P, Conceptual survey of lower the overall grid connection costs. In generators and power electronics for wind addition, these models can be used to study turbines, Risø-R-1205 (EN), 2001. control strategies for wind farms. Hansen L.H., Madsen P.H., Blaabjerg F., Large research projects have been initiated Christensen H.C., Lindhard U., & Eskildsen to analyse different control strategies of K, Generators and Power Electronics large wind farms. There are many control Technology for Wind Turbines, In , IECON ' topologies, and they all have their particular 01, Denver, 2001. advantages and disadvantages (Hansen L.H. et al., 2001b). One option is a decentralised Heier S, Grid Integration of Wind Energy control structure with an internal AC grid Conversion Systems, 1998. connected to the main grid, where each wind turbine has its own control system with its IEC 61400-21, Final Draft International own frequency converter. Such a system has Standard - Wind turbine generator systems - the advantage of ensuring that each wind Part 21: Measurement and assessment of turbine can work optimally with respect to power quality characteristics of grid its local wind conditions. Another option is a connected wind turbines, 2001. centralised control structure where, for example, the wind farm is connected to the Mohan N., Undeland T.M., & Robbins grid via an HVDC connection. Here, the W.P., Power Electronics: converters, internal behaviour of wind turbines is applications and design, 1989. separated from the grid behaviour, thus enabling the wind farm to become Risø and DEFU. Power quality and grid sufficiently robust to withstand possible connection of wind turbines - summary failures on the grid. Another way to control report. Risø-R-853 (Summ.)(EN), 1996. wind farms is the combination of wind farms and energy storage systems, which Sørensen P., Hansen A.D., Janosi L., Bech makes it possible to buffer some of the J., & Bak-Jensen B., Simulation of energy. interaction between wind farm and power system, Risø-R-1281, Risø National Laboratory, 2001. REFERENCES Sørensen P., Unnikrishnan A.K., & Sajan DEFU KR111, Connection of wind turbines A.M. Wind farms connected to weak grids in to low and medium voltage networks, 1998. India, Wind Energy, 4. 2001.

Eltra, Tilslutningsbetingelser for vindmølle- Sørensen P., Bak-Jensen B., Kristiansen? J., parker tilsluttet transmissions-nettet, TP98- Hansen A.D., Janosi L., & Bech J., Power 328b, 2000. plant characteristics of wind farms, In Kassel, 2000. Hansen A.D., Sørensen P., Janosi L., & Bech J, Wind farm modelling for power quality, In Denver , IECON ' 01, (2001).

232 9 – Electrical Installations Guidelines for Design of Wind Turbines − DNV/Risø

10. Manuals • inspection for corrosion, wear and cracks, including remedial action • list of spare parts 10.1 User manual • trouble shooting instructions The user manual and/or operating • relevant drawings, diagrams, specifica- instructions are issued to assist the wind tions and descriptions turbine operator. Normally, the following information will be included: • general description of the wind turbine 10.3 Installation manual • personal safety instructions The installation manual will describe the • service/maintenance schedule various installation activities as well as the • instructions for use of the controller, limitation for these with respect to, for including settings and list describing example, wind speed. The activities should alarms be described specifying, inter alia: • trouble shooting, incl. instructions for • general description of the wind turbine dangerous events like runaway and fire • personal safety instructions • description of locking devices for rotor, • transportation and handling yaw and pitch system • procedures for bolt connections, • list of grease, lubrication oil, hydraulic including pretension procedures oil, etc. • tests and checks for various parts and • list defining bolt pretension for main systems of the turbine connections • commissioning test, normally including start, stop, yawing and test of protection In the EU, minimum requirements to the system manual are laid down in the Machinery Directive, see European Union (1998). REFERENCE

10.2 Service and maintenance The European Union, Machinery Directive, manual European Directive No. 98/37/EC, 1998. This manual shall assist qualified engineers in their work and will normally include the same information as that which is given in the user manual in addition to specific instructions for the service and maintenance work. The level of detail depends on the education of the service staff. The manual will normally prescribe in detail how to maintain: • hydraulic systems • main gear • yaw system • blade pitch or tip brake mechanism • sensors • lubrication and cooling systems

10 – Manuals 233 Guidelines for Design of Wind Turbines − DNV/Risø

11. Tests and Measurements meteorology mast is erected at the position of the wind turbine, and correction factors are found from different directions of the Measurements on wind turbines are made to measurement sector. verify power performance, design loads and function of control and protection systems as well as other types of operational behaviour 11.1 Power performance measure- under field conditions. ments

In most cases, the wind turbine parameters The purpose of power performance must be correlated with the wind conditions measurements is mostly economic as the at the site and for that purpose, a performance determines the overall meteorology mast is erected close to the economics of the wind turbine installation. wind turbine. In some cases, measurements Therefore, power performance measure- can be made without correlation with the ments are part of contractual matters wind speed on the site. A meteorology mast, between wind turbine manufacturers and which is to measure the given wind speed on developers. the site, should not be erected too close to the wind turbine. This is due to the fact that When power performance is measured, the the wind speed sensors are influenced by the smallest uncertainty of the measurement is wind turbine rotor. Neither should the mast found in flat and non-complex terrain. In be erected too far from the wind turbine, in most power performance measurement which case the correlation is decreased procedures, IEC, Danish Energy Agency substantially. In a flat and non-complex and MEASNET, requirements to the terrain terrain where the topography causes have been set up, so-called “ideal site”, that insignificant flow distortion, a good do not require a site calibration. If these correlation with the wind velocity at the requirements are not met, a site calibration wind turbine position is found. In a complex shall be made. A site calibration can be terrain, the correlation between the wind at made even in flat terrain to reduce the mast and the wind at the wind turbine uncertainty of the wind speed measurement, position is poorer. In such cases, a site which shall otherwise be stated as 2-3% due calibration must be made. Meanwhile, a

> 4 L and < 8 L Measurement sector slope < 10 % variations < 0,25 D 8 L

> 2 L and < 4 L 4 L slope < 5 % variations < 0,1 D 2 L

<2 L: meteorology mast at distance L slope < 3 % variations < 0,08 D

> 2 L and < 4 L slope < 10 % WTGS IEC 148/98 Figure 11-1. Requirements in IEC 61400-12 to topographical variations for an “ideal site”, top view.

234 11 – Measurements Guidelines for Design of Wind Turbines − DNV/Risø

Meteorology mast at 4 D

Distance of meteorology mast to WTGS between 2 D and 4 D; 2,5 D is 2,5 D recommended 2 D

Wind D

WTGS

Maximum measurement sector: Disturbed sector due 257° at 2 D to wake of of WTGS 267° at 2,5 D on meteorology 286° at 4 D mast; sector angle taken from annex A: 103° at 2 D 93° at 2,5 D 74° at 4 D IEC 145/98

Figure 11-2. Requirements in IEC 61400-12 as to distance of the meteorological mast and maximum allowed measurement sectors.

120

Undisturbed 100

) α = 2Arctan(2D /L + 0,25) or α = 2Arctan(2D /L + 0,25) ° e e n n

(

α 80

r

o

t

c

e

s

d

e 60

b

r

u

t

s

i

D 40

Disturbed 20

2684 10 12 14 16 18 20

Relative distance Le/De or Ln/Dn IEC 149/98 Figure 11-3. Requirements in IEC 61400-12 on the exclusion of particular sectors due to wakes of neighbouring and operating wind turbines and significant obstacles.

11 – Measurements 235 Guidelines for Design of Wind Turbines − DNV/Risø to arbitrary site effects, e.g. small In addition, for many purposes it is topographic variations, wake effects from advantageous to measure precipitation, distant wind turbines, obstacles like trees atmospheric stability, wind shear and other and houses, roughness changes due to crop wind turbine parameters (rotational speed, changes and climatologic changes over the pitch angle) to determine more specific measurement period. behaviour of performance under such parametric variations. The measured wind speed in power performance measurements shall be defined The measurement period should be long as the horizontal wind speed. This means enough to include variations of atmospheric that the vertical turbulence component shall conditions like varying stability and front not be taken into account, and when there is passages. inclined flow as in complex terrain, it is only the average horizontal component that is The power performance measurement considered. This definition of measured procedures should be followed in detail, and wind speed ensures a reasonable degree of specific concern should be paid to the consistency of calculated annual energy calculations of uncertainty, which should be production from the power curve measure- minimised in all aspects of the ment in flat and complex terrains. If a vector measurement, specifically the wind speed defined wind speed is used (the vertical measurement. wind speed component is included), the measured annual energy production at inclined flow in complex terrain is 11.2 Load measurements substantially lower. Load measurements are made to verify

design loads or to investigate loads under Application of a cup anemometer wind specific conditions. Design loads are speed sensor is required. This constitutes the required to be verified by certification most accurate wind speed sensor for average institutes. It is not possible to verify all wind speed measurements. Some characte- loads. Certain ranges of external conditions ristics of the sensors are very important in can be covered by field measurements, but terms of keeping uncertainties low in extreme conditions cannot be reached. The measurements: cosine angular characte- verification of loads is therefore limited to ristics, low distance constant, low maximum verification of a number of specific overspeeding level and low friction in measured load cases. These load cases are bearings. used to verify load calculation models, and

in this way to extrapolate the verification to The wind speed, wind direction, air extreme design loads. temperature, air pressure, active power of the wind turbine and the status of the wind Loads are measured on blades, shaft, nacelle turbine shall always be measured. The components, tower and foundation. In prin- power performance shall be normalised to a ciple, the loads being developed in the blade specific air density. In Denmark, this air can be followed through the construction. density is 1.225 kg/m3. At higher elevation, Strain gauges are mostly used in full and the specific air density is based on the local half bridges. The loads are measured over average air density. 10-minute periods, and equivalent load

spectra are generated through rain-flow counting procedures.

236 11 – Measurements Guidelines for Design of Wind Turbines − DNV/Risø

Load measurement procedures are found in more detailed measurement procedure, IEC the recommendations for compliance with 61400-11. the Technical Criteria, July 1992, and in IEC 61400-13. REFERENCES

11.3 Test of control and protection Danish Energy Agency, Recommendation system for measuring the power curves of a wind turbine for usage in type approvals of wind The purpose of these tests is to verify that turbines in relation to Technical Criteria, the wind turbine functions as predicted and September 18, 1992. to verify functionality and efficiency of www.vindmoellegodkendelse.dk protection systems. Procedures for function and safety tests are described in recommen- Danish Energy Agency, Requirements to dations for basic tests by the Danish Energy Cup Anemometers Applied for Power Curve Agency. The most important part of these Measurements under the Danish Approval tests is the verification of the overspeed Scheme for Wind Turbines, January 14,2002, protection systems e.g. aerodynamic and www.vindmoellegodkendelse.dk mechanical brake systems.

Danish Energy Agency, Recommendation

for Basic Tests, According to the Technical 11.4 Power quality measurement Criteria for Type Approval and Certification Measurement of power quality i.e. the of Wind Turbines in Denmark, January effects that the wind turbine has on the 1997. public electricity network, is standardised in IEC 61400-21. This standard describes Danish Energy Agency, Recommendations measurement procedures for quantifying the for fulfilling requirements in Technical power quality of a grid connected wind Criteria, July 1, 1992. turbine and the procedures for assessing compliance with power quality require- IEC 61400-11, Wind turbine generator ments. See also section 9.3 of this book. systems, part 11: Acoustic noise measurement techniques, 1998

11.5 Blade testing IEC 61400-12 Wind turbine generator systems, part 12: Wind turbine power Reference is made to IEC 61400-23 and performance testing, 1998 Section 5.1.9 of this book.

IEC 61400-13, Wind turbine generator

systems, part 13: Measurement of 11.6 Noise measurements mechanical loads, 1. ed, 2001 The objective of noise measurements is to verify that the wind turbine meets the IEC 61400-21, Wind turbine generator requirements of authorities to noise systems - Part 21: Measurement and regulation. Measurement procedures for assessment of power quality characteristics noise measurements are described in of grid connected wind turbines, Ed. 1.0, recommendations for basic tests by the 2001 Danish Energy Agency. IEC has made a

11 – Measurements 237 Guidelines for Design of Wind Turbines − DNV/Risø

IEC 61400-23, Wind turbine generator systems - Part 23: Full-scale structural testing of rotor blades, First edition, 2001- 04

MEASNET Power Performance Measure- ment Procedure, version 3, November 2000.

238 11 – Measurements Guidelines for Design of Wind Turbines − DNV/Risø

A. Bolt Connections associated with this quality, especially at low temperatures.

A.1 Bolt standardization Stainless steel bolts are standardised Bolts and nuts are highly standardized according to ISO 3506. Bolts are made from machine elements. The following standards three different types of stainless steel apply: namely: • • ISO 261 ISO general-purpose metric austenitic steel screw threads- General plan • ferritic steel • ISO 262 ISO general-purpose metric • martensitic steel screw threads–Selected sizes for screws, bolts and nuts Normally the austenitic type is used. The • ISO 724 ISO general-purpose metric quality designation is then A1, A2 and A4 screw threads corresponding the strength classes 50, 70 • ISO 3506 Corrosion-resistant stainless- and 80. The numbers are 1/10 of the steel fasteners-Specifications ultimate strength. The yield strength is • ISO 4762, Allen screws correspondingly 210, 450 and 600 MPa.

• ISO 4014; 4017, Cap screws

• ISO 4016; 4018, Bolt with nut A.3 Impact strength • ISO 4032, 4034, Nuts • ISO 1896, Tight-fitting bolts In general the elongation at fracture and • ISO 7089, 7090, Tempered washers 200 impact strength are inversely proportional to HV the strength of the steel. As an example, the • ISO 7091, Washers 100 HV impact strength of a grade 12.9 bolt is only the half of that of an 8.8 bolt. For pure static loads this is of no major importance, but in A.2 Strength case of dynamic and transient loads this features have to be carefully considered Bolt strength and mechanical properties are especially at low ambient temperatures. In standardised according to ISO898/1-2. The general the magnitude for impact strength different qualities are characterised, in terms shall at least reach 27 Joule at the lowest of quality classes, by two numbers separated temperature for operation. by a dot. The first number is 1/100 of the minimum ultimate strength measured in MPa. The product of the two numbers is A.4 Surface treatment 1/10 of the minimum yield strength, also measured in MPa. Different surface treatments are available to achieve specific properties such as corrosion For example, bolt quality 10.9 has a protection, and low friction. minimum ultimate strength of 100⋅10 = 1000 MPa and a minimum yield strength of The most commonly used bolts are black 10⋅10⋅9 = 900 MPa. (no surface treatment) and hot dip galvanized bolts. Hot dip galvanized bolts For pre-stressed bolts, only qualities 8.8, offers an excellent protection against 10.9 and 12.9 are relevant. Caution should corrosion in non acidic atmospheres. The be exercised when using quality 12.9 bolts are first treated in a grease solvent and because of the risk of brittle failure then in a pickling agent and finally in a bath

A – Bolt Connections 239 Guidelines for Design of Wind Turbines − DNV/Risø with melted zinc at 540º in few minutes • for special bolts threads may be cut on a forming a zinc layer of approximate 50- lathe or by hand cutting tools (dies). 60µm. The actual lifetime for hot dip galvanized bolts are dependent on the A widely used approach for lifetime environmental conditions (pollution in the prediction and strength assessment is the atmosphere). German guideline VDI-2230. VDI distinguishes between thread form-rolled Environment Average before heat-treated and thread form-rolled lifetime after heat-treated. [years] Open agricultural areas 45 In the latter case the thread rolling process Small towns 30 introduces compression stresses in the Large towns 12 surface, causing better fatigue properties but Heavy polluted industrial areas 5 also dependence of the mean-stress in the Coast-near areas 30 bolt. For finally heat-treated bolts the Table A-1. Average lifetime for hot dip galvanized following equation will apply bolts. (Arvid Nilsson). σASV = 0.85· (150/d + 45) For materials with hardness > HV 300, hot dip galvanizing implies a risk of hydrogen 50 embrittlement. Consequently such materials 50 shall be heat treated at approx. 200˚ for 2-4 48 hours. In general it is not recommended to 46 σASV()d hot dip galvanize bolt at higher grade than 44 8.8. Grade 10.9 may be hot dip galvanized 42 with great caution. It is not possible to hot 40 40 dip galvanize materials with higher strength 10 20 30 40 50 60 because of temper effects and zinc 10 d 60 depositions in the grain boundaries causing Figure A-1. Fatigue strength as function of bolt diameter for finally heat-treated bolts (VDI 2230) micro cracks in the structure. The (stress amplitudes). consequences in such cases may be catastrophic. For finally thread form-rolled bolts the following equation will apply

A.5 S-N curves σASG = (2 – FSm/F0.2)· σASV For design of bolt connections, the single most important issue is to consider the FSm = (FSao + FSau)/2 + FMzul proper choice of an S-N curve for fatigue predictions. This S-N curve is much Valid for: 0.3 < FSm/F0.2 < 1 dependent on how the bolt is manufactured. Several possibilities exist for this FSm/F0.2 = Rb manufacturing, including: • thread form-rolled and then heat-treated FSao higher force from external load • material of near nominal diameter fully FSau lower force from external load heat-treated before form-rolling of FMzul allowable pre-tensional force thread

240 A – Bolt Connections Guidelines for Design of Wind Turbines − DNV/Risø

80 These S-N curves are intended for use at 80 loads of constant amplitude and are not σASG16()Rb 70 unconditionally valid for loads with varying σASG24()Rb amplitudes (defined as a load spectrum) 60 σASG36()Rb (VDI 2230).

σASG48()Rb 50 A.5.1 S-N curves in structural steel codes 40 40 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.3 Rb 1 S-N curves in the structural steel codes DS412 and Eurocode 3 are given for 97.7% survival probability in the following general Figure A-2. Fatigue strength of bolts M16 (red), M24 form (blue), M36 (green) and M48 (brown) as a function of mean stress ratio FSm/F02 with bolt diameter as parameter for finally form-rolled threads (stress log10N = log10a − m⋅log10∆σr amplitudes). where N is the number of cycles to failure at

These values are valid for grade 8.8-12.9 stress amplitude ∆σr, and log10a and m are 6 bolts subjected to more than 2·10 cycles the parameters of the S-N curve. Depending and correspond to 99% survival probability. on the material quality, heat-treatment and pre-stressing different S-N curves can be 6 For a number of cycles lower than 2·10 the established. corresponding stress to failure can be found from Standard bolts without controlled pre- stressing 1/3 σAZSV = σASV·(ND/NZ) Standard bolt means mass produced bolts according to ISO 898/1-2. For finally heat 1/6 σAZSG = σASV·(ND/NZ) treated or cut bolts category *36 according to DS 412 / Eurocode 3 will apply (stress For finally heat-treated and finally thread amplitudes): 6 form-rolled ND = 2·10 and NZ is the actual 7 number of load cycles. N < 10 : log10a = 11.101, m = 3 7 N > 10 : log10a = 13.385, m = 5 3 1 .10 8 500 N > 10 : σfat = 7.5 MPa

For finally thread form-rolled bolts category 100 *50 will apply (stress amplitude): σAZSV() NZ σAZSG() NZ 7 N < 10 : log10a = 11.551, m = 3 7 10 N > 10 : log10a = 14.585, m = 5 8 N > 10 : σfat = 11.5 MPa

Standard bolts with controlled pre- 1 1 3 4 5 6 7 8 9 1 .10 1 .10 1 .10 1 .10 1 .10 1 .10 1 .10 3 9 stressing 110× NZ 110× For class 8.8 and 10.9 standard bolts with Figure A-3. S-N curve for finally heat-treated (red) and controlled pre-stressing category 71 finally thread form-rolled (blue) (VDI 2230) (stress according to DS 412/Eurocode 3 may be amplitudes).

A – Bolt Connections 241 Guidelines for Design of Wind Turbines − DNV/Risø applied based on the S-N curves reported in A.5.2 Allowable surface pressure VDI 2230 (stress amplitudes). The maximum surface pressure between

mating surfaces shall be evaluated. Be aware N < 5·106: log a = 11.851, m = 3 10 that the allowable surface pressure P is not N > 5·106: log a = 15.286, m = 5 G 10 proportional to the ultimate strength of the N > 108: σ = 14.5 MPa fat material. The maximum bolt force shall, of

course, always be adjusted to the material 3 1.10 with the lower PG in the actual bolt connection. Figures for some commonly used materials are shown in Table A-2. σ71N()100

σ 50N() Material Ultimate Allowable σ36N() 10 strength surface Rm,min stress PG [MPa] [MPa] 1 3 4 5 6 7 8 9 St 37-2 340 490 1.10 1.10 1.10 1.10 1.10 1.10 1.10 N St 50-2 470 710 St 52-3 510 760 Figure A-4. S-N curve (stress amplitudes) for bolts without pre-stressing *36 (black) and *50 (blue) and 34CrMo4 1000 870 with pre-stressing 71 (red) According DS412/ 34CrNiMo6 1200 1080 Eurocode 3. X5 CrNi 18 12 500 630

3 GG-25 250 900 1 .10 500 GGG-40 400 700 GGG-50 500 900 GGG-60 600 1000 σAZSV() NZ 100 AlMgSi 1 F28 260 230 σAZSG() NZ Table A-2. Allowable surface pressure PG (VDI 2230) σ36() NZ σ71() NZ 10 A.6 Pretension Depending on the bolt diameter and the 1 1 3 4 5 6 7 8 9 1 .10 1 .10 1 .10 1 .10 1 .10 1 .10 1 .10 required accuracy, several types of pre- 3 9 110× NZ 110× stressing methods are in use. The most Figure A-5. Comparison of S-N curves (stress commonly used methods are: amplitudes) from VDI 2230 (red, blue) and category 71 • torque wrench and *36 from DS412/Eurocode 3 (violet, black). VDI • curves are valid for M36 with a pre-tension of 0.7 times turning angel method • σ02 hydraulic tension device • measurement of bolt extension These S-N curves will apply for M12 – M36 untreated (black) bolts. For hot dip In this context only the two first methods galvanised bolts the figures will be 12-15% will be mentioned since a large majority of lower. bolts are pre-stressed with these methods. For selected bolt diameters, surface treatments and lubrication conditions, suitable pre-tension and pretension torque, can be taken from Table A-3 to A-6.

242 A – Bolt Connections Guidelines for Design of Wind Turbines − DNV/Risø

Bolt Area Bolt force at σ02 kN] combinations of surface and lubrication can dia. AS be calculated from: [mm] [mm2] 8.8 10.9 12.9 M12 84.3 54 76 91 FSm = Fs0 · GF M16 157 100 141 170 M20 245 157 220 265 Mv = MV0 · C

M24 353 226 318 381 Scatter on the average pre-tension: M27 459 294 413 496

M30 561 359 505 606 ∆FS = FSm ± SF/FSM M33 694 444 625 750 M36 817 523 735 882 SF is the scatter on average pre-tension force M42 976 624 878 1054 FSM is the average pre-tension force. Table A-3. Bolt forces at σ02. (Arvid Nilsson) For an M20 8.8 black oiled bolt the with C = Bolt Area Pre-tension torque MV0 1.0 and GF = 0.71 and MV0 = 385Nm it dia. AS [Nm] results in a average pre-stress of 0.71 times [mm] [mm2] 8.8 10.9 12.9 σ02. For the same bolt lubricated with wax M12 84.3 81 114 136 the average pre-stress will arrive at 0.83 M16 157 197 277 333 times σ02 with a corresponding pre-tension M20 245 385 541 649 torque of 0.63 times MV0.

M24 353 665 935 1120 Surface Lubri- S /F G C M27 459 961 1350 1620 F SM F cation M30 561 1310 1840 2210 Black Dry 0.29 0.62 0.96 M33 694 1770 2480 2980 Oil 0.16 0.71 1.00 M36 817 2280 3210 3850 MoS2 0.16 0.75 0.86 M42 976 3640 5110 6140 Hot dip Dry 0.29 0.55 1.17 Table A-4. Pre-tension torque for oil lubricated black galva- Oil 0.16 0.69 1.07 bolts. (Arvid Nilsson) nised Bolt Area Pre-tension torque All Wax 0.11 0.83 0.63 Stainless Oil 0.29 0.55 0.84 dia. AS MV0 [Nm] [mm] [mm2] 50 70 80 steel Wax 0.23 0.65 1.00 Table A-6. Modifying coefficients for pre-tension force M12 84.3 27 57 76 GF and pre-tension torque C as well as scatter at different surface treatments and lubrication conditions. M16 157 65 140 187 (Arvid Nilsson) M20 245 127 273 364 M24 353 220 472 629 For the turning angel method following M27 459 318 682 909 guidelines (for grade 8.8) can be given in M30 561 434 930 1240 which d is the bolt diameter: M33 694 585 1250 1670 M36 817 755 1620 2160 Table A-5. Pre-tension torque for stainless steel bolts lubricated with wax. (Arvid Nilsson)

The corresponding average pre-tension force FSm and pre-tension torque Mv for other

A – Bolt Connections 243 Guidelines for Design of Wind Turbines − DNV/Risø

Total clamping Total turning angel • hardness of components length LK after “tight • surface roughness contact” [ º ] • geometrical accuracy of joint faces LK < 2d 120 • accuracy of threads 2d ≤ LK ≤ 4d 150 • liners made from material with low 4d ≤ LK ≤ 6d 180 stiffness 6d ≤ LK ≤ 8d 210 • clamping ratio LK/d 8d ≤ LK ≤ 10d 240 • retightening sequence 10d > LK By test Table A-7. Turning angel for mean pretension to 0.7 Number Internal consolidation times the bolt tensile strength (grade 8.8 bolts) (ENV 1090-1). of joint For LK/d [µm] faces 1 2.5 5 10 For torque wrench and turning angle 2 - 3 1 1.5 2 2.5 method, the rotating part must be provided 4 - 5 0.75 1.0 1.25 1.5 with a hardened washer. This to prevent 6 -7 0.5 0.7 0.8 1.1 galling and the resulting increase in friction. Table A-8. Internal consolidation for each joint face. (Handbuch der Verschraubungstechnik) The pretension method should be calibrated regularly (ENV 1090-1). For torque wrench Especially bolt connections where more and the turning angel method this may be factors are involved e.g. bolt connections done by means of a bolt connection with the with a small clamping ratio, many joint same characteristics as the actual faces and hot dip galvanized surfaces may connection, in which the bolt pre-tension be critical. In such cases a number of force can be monitored. The Accuracy of the retightening (3-4) after initial assembly are torque wrench should be within ± 5%. (ENV necessary as well as spot-checks at regular 1090-1). A more detailed method for intervals. calculation of pre-tension force and pre- tension torque can be taken from e.g. VDI Main influence factors to consider for loose 2230 Chapter 5.4. turning of bolt or nut: • transverse load A.6.1 Safety against loosening • vibrations • possibility of transverse movements Many damages on machine elements are • clearance between bolt shaft and hole caused by loose bolt joints. Consequently all • clamping ratio L /d essential bolt connections should be K carefully evaluated. In principle, two In general pre-stressed bolt are self-locking mechanisms are responsible for this effect: if the conditions mentioned above are • loss of pre-tension caused by considered. For not pre-stressed bolts it is consolidations/deformation of the joint recommended to use nuts with a locking faces etc. • device or use a sort of locking compound. loose turning of bolt or nut

Main influence factors to consider for loss A.7 Minimum depth of threaded of pretension: holes • plastic deformations between bolt head/nut and contact face For standard bolts with nuts in same strength • number of joint faces class no further calculation of strength of the

244 A – Bolt Connections Guidelines for Design of Wind Turbines − DNV/Risø nut is required. For threaded holes in • clamping length material with lower strength than bolt • E-modulus of material material it is however required to calculate • surface properties (roughness, surface the min. length of the threaded part of the treatment, alignment etc.) hole / height of nut. The following equation • eccentricity will apply (VDI 2230) • level of force attack meff,min = (Rm·AS··P)/(C1·C3·τBM(P/2+ To deal with every case is beyond the scope (d-D2)tan30)·π·d) + 0.8·P of this book. Information can be taken from the literature e.g. VDI 2230 and handbooks meff,min min. length of the threaded part of of machine elements. However the simple the hole / height of nut. centric case is shown hereafter. Rm ultimate strength of bolt material AS stress area of bolt A.8.1 Stiffness of bolts P pitch of the threads τBM ultimate shear strength of nut material δS = δK +δ1+δ2+….δG+δM d outer diameter of bolt (nom. diameter) δS total stiffness of the bolt D2 flank diameter δK stiffness of the bolt head δ1 stiffness of the unthreaded part of the 2 C1 = 3.8·s/d-(s/d) -2.61 bolt δ2 stiffness of the threaded part of the bolt C1 = 1 for threaded holes δG stiffness of the threaded part of the bolt in the nut or threaded hole 2 3 C3 = 0.728+1.769·RS –2.896·RS +1.296·RS δM stiffness of the threaded part of the nut For 0.4

li length of part with area AN and Ad3 A.8 Bolt force analysis respectively Ad3 core area The internal forces in a bolt connection are unique for this actual detail and dependent δG = 0.5 · d / ES · Ad3 of a lot of details such as: • stiffness of mating components δM = 0.4 · d / ES · AN • stiffness of bolt

A – Bolt Connections 245 Guidelines for Design of Wind Turbines − DNV/Risø

A.8.2 Stiffness of the mating parts For the mating parts the effective stiffness of the so called “replacement area” Aers is depending of the amount of material around the bolt hole. The equation for the replace- ment area is defined for three cases:

For dw > DA: 2 2 Aers = π / 4·(DA – dh )

For dW ≤ DA ≤ dW + lK: 2 2 Aers = π / 4·(dW –dh ) + π / 8· 2 dW(DA – dW)·[(x + 1) – 1] where 2 1/3 x = ((lK·dW)/DA )

For DA ≥ dW + lK: Aers = Aers for DA= dW + lK dW diameter of rest area D with of the flange Figure A-6. Typical force-deformation triangles. A d diameter of the hole h The additional bolt force from the external l clamping length K force on the bolt connection:

Stiffness of mating parts F = Ф ·F SA K A

δ = l / A ·E P K ers P Reduction in pre-stressing force:

E modulus of elasticity for the mating P F = (1-Ф )·F parts PA K A

This magnitude of the force ratio Ф is only A.8.3 Force triangle K valid if the force is applied at a level just The force ratio is defined as below the head of the bolt. If the force is applied at another level in the connection the ФK = δP / δP+ δS ratio ФK will be reduced due to an increased relative flexibility of the bolt as compared to FV average pre-stressing force the mating parts. The reduction factor n is FA applied external force defined as the ratio between the distance FSA additional force from the external between force inlets l and the clamping applied force experienced by the bolt. length lK, see Figure A-7. FPA reduction of the pre-stressing force FV FKR residual pre-stressing force n = l / lK δP stiffness of mating parts δS stiffness of bolt ФK, red = ФK · n

246 A – Bolt Connections Guidelines for Design of Wind Turbines − DNV/Risø

For bolts with finally thread form-rolled following equation for shear capacity is defined in Eurocode 3

FV,R = c3·A·fub/γm Figure A-7. Illustration of the force inlet ratio n. FV,R design carrying capacity in shear Calculation of more complicated bolt connections e.g. eccentric loaded connec- c3 = 0.6 for quality class 4.6, 5.6, and 8.8 tions is beyond the scope of this book but with section through thread. more information’s can be taken from e.g. c3 = 0.5 for quality class 5.8, 6.8 and VDI 2230. 10.9 with section through thread. c3 = 0.6 for all quality classes with A.9 Connections subjected to section through shaft shear A either the stress area of the thread or area of bolt shaft Three categories of shear connections in f characteristic ultimate strength structural steel constructions are defined in bu for the bolt material DS412/Eurocode 3: γ partial safety factor for material m

A. Bearing type For bolts with finally cut threads c shall be B. Combination of bearing/friction type 3 multiplied by 0.85. C. Friction type

Bearing resistance: Category A:

• it shall be shown that the actual shear F = 2.5·c ·c ·d·t·f / γ force is lower than the shear resistance b,R 1 2 u m

of the bolt material incl. the relevant c and c can be taken from Table A-9 partial safety factors 1 2 • d diameter of the bolt it shall be shown that the actual shear t thickness of the plate force is lower than the bearing fu characteristic ultimate strength resistance of the hole inclusive the for the plate material. relevant partial safety factors. γm partial safety factor for material

If movements due to clearance in the holes 1.2d ≤e <3.0d c = e /3d Low- are not allowed in the connection (e.g. 0 1 0 1 1 0 2.2d ≤p <3.75d c = p /3d -1/4 est caused by change in load direction) tight- 0 1 0 1 1 0 1.2d ≤e <1.5d c = e /0.9d -2/3 value fitted bolts should be used. In that case the 0 2 0 2 2 0 2.4d ≤p <3.0d c = p /1.8d -2/3 tolerances for bolt/hole shall be h13/H11. 0 2 0 1 2 0 Table A-9. Magnitude of reduction factors c1 and c2.

Pre-stressing of the bolt is not required, but the parts shall be drawn together in “snug- tight” condition. “Snug-tight” condition can generally be identified as that achievable by the effort of a man using a normal size spanner without extension.

A – Bolt Connections 247 Guidelines for Design of Wind Turbines − DNV/Risø

Bolt Normal Oversize Slotted diameter holes holes holes d d0 - d d0 - d l - d 12 1 3 4 14 1 4 4 16 2 4 6 20 2 4 6 22 2 4 6 24 2 6 8 ≥ 27 3 8 10 Figure A-8. Definition of hole distances. Table A-11. Standard and oversize clearance for bolt holes of diameter d0. Min. distance Optimal distance e1 1.2d0 3.0d0 Characteristic coefficients of friction for some commonly used surfaces may be taken p1 2.2d0 3.75d0 from Table A-12 and A-13. e2 1.2d0 1.5d0

p2 2.4d0 3.0d0 Table A-10. Min. bolt distances. Class Treatment A • Sand blasted Category B: • Sand blasted and sprayed For this category the same as for Category A with aluminium /zinc should normally be shown in the ultimate B Sand blasted and painted with limit state, but also that the friction between zinc-silicate painting 50-80µ the mating surfaces alone can adopt the C • Cleaning with steel brush or shear force at serviceability limit state flame cleaning including the relevant partial safety factors. • Hot dip galvanized and sand stringed Category C: D Untreated surfaces For this category it shall be shown that it is Table A-12. Categories of surfaces. slip-resistant at the ultimate limit state including the relevant partial safety factors. Class Characteristic friction coefficient Eurocode 3 defines the capacity as follows: A 0.50 B 0.40 FS,R = c4·n·µ / γm·Fp C 0.30, however for hot dip galvanized without sand FS,R design carrying capacity in friction stringing 0.10 D 0.20 c4 = 1 for normal holes Table A-13. Characteristic friction coefficients of c4 = 0.85 for oversize normal holes different surfaces. c4 = 0.7 for slotted holes n number of friction surfaces µ characteristic friction coefficient A.10 Bolts subjected to tensile load γ partial safety factor for material m The load carrying capacity of bolts without F mean pre-stressing force, which p pre-stressing can according to Eurocode 3 be is maximum 0.7·f u calculated as

248 A – Bolt Connections Guidelines for Design of Wind Turbines − DNV/Risø

Ft,R = C·fu,b /γm·AS Pretension of bolts For pre-tensioned bolts the mean pre-tension C = 0.9 for finally thread form rolled shall not exceed 0.7 times the characteristic bolts ultimate bolt strength using the stress area of C = 0.85 for finally heat treated bolts the bolt. fu,b ultimate strength of bolt material AS stress area of the bolt In mechanical engineering higher utilizing of the bolt-material are used commonly 80- 100% of the characteristic σ02 value A.11 Bolts subjected to tensile load depending of the pretension method. and shear

The following requirement is given in A.13 Codes and Standards Eurocode 3 for bolts subjected to tensile load and shear: National codes for steel structures might be mandatory to use in some countries. This is 2 2 (Fv,S / Fv,R) + (Ft,S / Ft,R) ≤ 1 often the case for bolt connections in those parts of the wind-turbine, which are defined Fv,S and Ft,S actual shear load and tensile by the authorities as building structures. The load respectively. requirements in these national codes may Fv,R and Ft,R load carrying capacity in deviate from the methods described above shear and tension respec- with respect to both design and installation. tively.

REFERENCES A.12 Execution of bolt connections VDI 2230, Systematische Berechnung The following apply, according to ENV hochbearspruchter Scraubenverbindungen 1090-1 and Eurocode 3. October 2001

Straightness of mating surfaces in Zeit- und Daurfestigkeit von schwarzen und compression feurverzinkten hochfesten scrauben, Must be < 0.5mm between an arbitrary Bauingenieur 61 (1986) located straight edge and the surface.

ENV 1090-1, General rules and rules for Washers buildings. For connections in category A washers are not required. ENV, Eurocode 3. Design of steel

structures, 1993-1-1. For connections in category B and C with class 8.8 bolts tempered washers are DS 412 Steel Constructions, 1998-07-02 required beneath the turning component.

RCSC, Specification for Structural Joints For connections in category B and C with using ASTM A325 or A490 Bolts, 1985. class 10.9 bolts tempered washers are required beneath both head and nut. Bossard Handbuch der Verscraubungs-

technik, Expert Verlag

A – Bolt Connections 249 Guidelines for Design of Wind Turbines − DNV/Risø

B. Rules of Thumb B.2 Rotor The following rules apply to stall regulated Rules of thumb and laws of similarity, or turbines under the assumption of a constant scaling laws, are of great importance to the Reynolds number. designer during the initial phase of the de- velopment of a wind turbine. In particular, At constant rotational speed the rotor power when adjustments or modifications are to be P is proportional to the power coefficient CP made on an existing turbine to adapt it to times the third power of the reciprocal of the 3 another environment, or eventually to redes- tip speed ratio λ as well as to the third ign a turbine by means of scaling. It should power of the tip speed Vtip. be emphasized, however, that caution should 3 3 be exercised when using highly simplified P = ½⋅ρ⋅A⋅Vtip ⋅(λ ⋅CP) rules as the ones presented in this section. Different turbine designs will to a varying λ = V/Vtip degree be in conformity with these rules. ρ Air density A Rotor area B.1 Loads Vtip Rotor tip speed V Wind speed B.1.1 Rotor loads

As an initial estimate of the blade loads it Now, assuming that the variation of the has been common to use static pressure of power coefficient Cp versus the tip speed 300 N/m2 over the entire rotor area. ratio is independent of the rotational speed for a given set of λ and CP, the following For fatigue loads a pressure of 150 N/m2 rule applies: distributed on the three blades in 107 cycles has commonly been used. 3 3 P ,(λ,C )  ω   n  1 p 1 =  1  =  1  λ  ω    B.1.2 Fatigue loads P2 ,( ,C p )2  2   n2 

For the purpose of comparison it is normal P Rotor power to calculate an equivalent load for an ω Angular speed equivalent number of cycles representing the same accumulated damage as the actual load n Rotational speed spectre, see section 4.4.1. The equivalent load varies with the equivalent number of Further, assuming that the CP curve is valid cycles also for the variation of the rotor radius, the following rule is applicable:

N1 5 3 S = S ⋅ m 2 1 P1  R1   n1  N 2 =     P  R   n  2  2   2  in which Si is the equivalent load corre- sponding to the equivalent number of cycles P Rotor power Ni. R Rotor radius n Rotational speed

250 B – Rules of Thumb Guidelines for Design of Wind Turbines − DNV/Risø

B.3 Nacelle B.3.1 Main shaft A rough estimate of the main shaft diameter signifies that it amounts to 1 % of the rotor diameter.

B.4 Noise Other things being equal, sound pressure will increase with the fifth power of the speed of the blade relative to the surround- ing air.

REFERENCES

Petersen, H., Simplified Laws of Similarity for Wind Turbine Rotors, Risø-M-2432, Risø National Laboratory, 1984.

Krohn, S., www.windpower.org, Danish Wind Turbine Manufacturers Association, 2001

B – Rules of Thumb 251 Guidelines for Design of Wind Turbines − DNV/Risø

C. Fatigue Calculations This time series is extracted from an aeroelastic computer code as described in Section 4.3. Fatigue failure takes place by the initiation and propagation of a crack until the crack Through a cycle counting method, as becomes unstable and propagates fast, if not described in Section 4.4.1, a stress range suddenly, to failure. distribution, or design spectrum, illustrated in Figure C-2, is established. From this To ensure that a structure will fulfil its stress range distribution fatigue analysis is intended function, fatigue assessment, performed. supported where appropriate by a detailed fatigue analysis, should be carried out for each type of structural detail, which is subjected to extensive dynamic loading. It should be noted that every welded joint and N attachment, or other form of stress concentration, is potentially a source of fatigue cracking and should be individually considered. Fatigue design can be carried Stress range out by methods based on S-N curves from fatigue tests in the laboratory and/or Figure C-2. Stress range distribution. methods based on fracture mechanics. Great caution shall be exercised in that

machine element design most often refers to True fatigue life is a function of workman- stress amplitudes, whereas construction ship related to fabrication and corrosion design most often refers to stress ranges. protection. Therefore, it is important that the Mixing these terms will result in great fabrication is performed according to good errors. practice with acceptance criteria fulfilled as assumed for the prediction.

C.2 Fracture mechanics

C.1 Stress ranges The fatigue life consists of three stages: • Most components in wind turbines are crack initiation • subjected to stress ranges of great variation crack propagation • as illustrated in Figure C-1. final fracture

Since the final fracture happens rapidly, the fatigue life N can be expressed as

N = Ni + Np

Ni initiation time Np propagation time

Figure C-1. Simulated time series of bending stress in The initiation and the propagation time for the main shaft during production at 12 m/s. steel components is often expressed by

252 C – Fatigue Calculations Guidelines for Design of Wind Turbines − DNV/Risø formulas on the following form, where the defects, which may often be located in areas latter is known as Paris' power law with stress concentrations, see Section C.5. Fracture mechanics will mostly apply to n calculation of the remaining life of detected  ∆K  N = B ⋅   cracks. i  ρ   

C.3 S-N curves da = C ⋅ ∆K m Fatigue strength data are usually reported as dN p S-N curves, also known as Wöhler curves. The S-N curve is established through n empirical material constant numerous sample tests with different load B empirical material constant ranges, resulting in pairs of stress range S ρ dependant on geometry of stress and a number of stress cycles N to failure at concentration this stress range. The S-N curve can usually a crack length be expressed as C empirical material constant m empirical material constant N = K⋅S -m ∆ stress intensity range K

On logarithmic form this results in a linear ∆ The stress intensity range K for cracks relationship between logS and logN through a thickness is in its general form written logN = logK − m logS

∆K = ∆S πaF K empirical material constant determining the level of the S-N curve ∆S far-field range of applied stress m Wöhler exponent, slope of S-N curve a instantaneous crack length F form factor for crack and surrounding logS

geometry SB

m1 As indicated, a fracture mechanics approach 1 to fatigue requires explicit data of material behaviour and stress conditions that are SD m2 often difficult to obtain. 1 SE

A fracture mechanics analysis may be used logN to predict the number of load cycles in the NB ND NE crack propagation stage of an actual Figure C-3. S-N curve. structure. The extent to which a fracture S Endurance limit or ctt-off limit mechanics calculation can provide E S constant amplitude fatigue limit comparable information on fatigue life with D S often set equal to the ultimate strength that derived from S-N curves will depend on B the number of load cycles in the initiation Given a specified stress range S, the S-N stage. Frequently, the initiation stage for curve gives the number of cycles N to welded joints is almost negligible, because a failure. The S-N curve shown in Figure C-3 fatigue crack will develop from existing

C – Fatigue Calculations 253 Guidelines for Design of Wind Turbines − DNV/Risø

is bilinear which is commonly used for steel. When σlogN is not known, it can be set equal For such a curve two different sets of logK to 0.46 for plain steels free from welds, see and m are given depending on whether N < DNV (1984). ND or N > ND. The endurance limit SE under which failure will not occur, is a property The S-N curve valid for analysis of a specific for ferrous and titanium alloys. structural detail is to be applicable for the material, the structural detail, the state of The material will exhibit a natural variability stress considered and the surrounding of logN about the mean as given by this environment. The S-N curve should take into curve - often expressed in terms of the account possible material thickness effects. standard deviation σ. Usually, the mean curve is specified in such a way that in 50% Recommended characteristic S-N curves for of the cases the true value of logN will be steel are described in Section C.6 and C.7, larger than given by the curve, and in 50% and recommended characteristic S-N curves of the cases it will be smaller than given by for steel bolts are given in Appendix A. For the curve. For design of wind turbine determination of S-N curves for cast iron, structures, it is required to use a reference is made to Hück et al. characteristic S-N curve, which is to be chosen in a conservative manner such that For design purposes, fatigue analysis based logN in a large fraction, 1−p, of the cases on S-N curves from fatigue tests is normally exceeds the value given by the characteristic straightforward and the most suitable curve. Usually, 1−p is taken as 97-98%. method. It is then used in conjunction with When the mean S-N curve is given, the Palmgren-Miner’s rule for prediction of characteristic S-N curve can be found by cumulative damage. keeping the values of logN fixed and changing the corresponding S values according to the following formula C.4 The Palmgren-Miner rule The fatigue life – or in other terms the σ log N cumulative damage – under varying loads S − = S exp(−k ) 1 p 50% m can be predicted based on the S-N curve approach under the assumption of linear in which S50% is the stress range cumulative damage by Palmgren-Miner’s corresponding to the mean S-N curve for rule. The total damage that a structure will given N, S1−p is the corresponding stress experience during its design life may be range corresponding to the characteristic S- expressed as the cumulative damage from N curve with 1−p probability of exceedance, each load cycle at different stress levels, independent of the sequence in which the σlogN is the standard deviation of logN representing the natural variability in logN stress cycles occur, i.e. no sequence- for given S, and k depends on p and can be dependency or so-called ”load cycle effect” taken from Table C-1. is present.

Table C-1. Quantile coefficient k vs. fraction p According to Palmgren-Miner’s rule, the Fraction 20 10 5 2.3 2 1 0.1 accumulated damage D can be predicted as p % follows K 0.84 1.28 1.65 1.96 2.05 2.33 3.09

254 C – Fatigue Calculations Guidelines for Design of Wind Turbines − DNV/Risø

k ∆n(S ) arrive is important. In such cases, the = i D ∑ expression for D by the Palmgren-Miner’s i =1 N(S ) i sum may underpredict the true accumulated damage. ∆ in which n denotes the number of stress cycles of stress range S in the lifetime of the Note that the Miner’s sum formulation for structure, and N is the number of cycles to the cumulative damage used with the quoted ∆ failure at this stress range. n is determined S-N curve model disregards a possible from the long-term distribution of the stress dependency on the stress ratio R = σmin/σmax. ranges as the one shown in Figure C-2, and This is sufficient for most components in N is determined from the expression for the wind turbines since they are subjected to − S-N curve; logN = logK m logS. The sum fatigue loading with an approximately zero is over all stress ranges Si in a sufficiently mean stress σm, as is the case for the main fine discretisation of the stress range space shaft in Figure C-1. However, in some cases into k blocks of constant range stress cycles. the influence of the mean stress needs to be taken into account. Different S-N curves for It is noted that the expression for the different values of σm and R can be accumulated damage D can be interpreted as evaluated or a fatigue life can be assessed a sum of partial damage owing to load through methods that take the mean stress cycles at various stress ranges, regardless of directly into account. See Section C.9. the sequence in which the load cycles occur.

When the long-term stress range distribution C.5 Fatigue in welded structures is a Weibull distribution with scale parameter s0 and shape parameter h, Welds in structures usually possess defects which may form the basis for crack growth and eventually lead to fatigue failure. For = − − s h FS (s) 1 exp( ( ) ) , welded joints involving potential fatigue s 0 cracking from the weld toe, an improvement in strength by a factor of 2 on the fatigue life and the total number of stress cycles in the can be obtained by controlled local design life is ntot, then the cumulative damage D can be calculated as

n m m D = tot Γ(1+ )s K h 0 in which Γ() denotes the gamma function.

The criterion for design is usually formulated as

D ≤ 1 Figure C-4. Grinding a weld toe tangentially to the plate surface (A) will produce little improvement in strength. Grinding must extend below the plate surface (B) in Caution should be exercised in cases for order to remove toe defects, from DNV Class. Notes which load cycle effects are present, and for 30.2. which the sequence in which the load cycles

C – Fatigue Calculations 255 Guidelines for Design of Wind Turbines − DNV/Risø machining or grinding of the weld toe. This C.7 Characteristic S-N curves for can be carried out by means of a rotary burr. forged or rolled steel The treatment should produce a small S-N curves for alloyed steel are not concave profile at the weld toe with the standardised in the same manner as in the depth of the depression penetrating into the case of structural steel. Based on empiric plate surface at least 0.5 mm below the relationships between the static material bottom of any visible undercut. The strength and the fatigue strength, so-called undercut should be small enough to allow synthetic S-N curves are established. the maximum required grinding depth to remain within 2 mm or 5% of the plate For forged and rolled steel subject to fully thickness, whichever is the smaller. reversed stress cycles about a zero-mean Reference is made to Figure C-4. σ σ stress, i.e. the stress ratio R = min/ max. = − By grinding the weld profile such that a 1, the following applies: circular transition between base material and The fatigue strength limit for rotating weld surface is achieved, the stress 6 concentration becomes reduced as the bending with 50% survival probability at 10 grinding radius is increased. This may lead cycles can be calculated as to additional improvement of the welded σ ⋅ ⋅σ ⋅ σ ⋅ joint. D = 1.25 (0.436 y + 77) f(Ry, B) f(d,r)

σ The benefit of grinding may be claimed only y yield strength or 0.2% proof stress for for welded joints, which are adequately the shaft material, related to the actual protected from seawater corrosion. The dimension. (MPa) σ benefit of grinding should usually not be B ultimate strength for the shaft material, taken into account during design, as related to the actual dimension. (MPa) grinding is considered as the only improvement, which can be carried out f(d,r) is a size factor, which depends on the during fabrication, to increase the fatigue shaft diameter d and the fillet radius r. For strength. unnotched parts, the size factor is

2 f (d,r) = 0.8 + C.6 Characteristic S-N curves for d structural steel and for notched shafts it is S-N curves for welds and base material of structural steel are classified in DS412 and 0.9 +1/ r for r ≥ 10 mm Eurocode 3. In these standards, guidance is f (d,r) =  given to selection of S-N curves, on the form  1.0 for r < 10 mm logN = logK − m logS for numerous construction and weld details, to be used in These expressions for the size factor require conjunction with the Palmgren-Miner rule. d and r to be given in units of mm.

f(Ry,σB) is a surface roughness factor, which is calculated as

256 C – Fatigue Calculations Guidelines for Design of Wind Turbines − DNV/Risø

σ R f (R ,σ ) = 1− B log ( y ) y B 4000 10 10

Ry peak-to-peak roughness, Ry ≈ 6Ra Ra mean roughness

The S-N curve slope m is expressed as

12 m = + 3 Figure C-5. Characteristic S-N curve for non-welded, β 2 forged or rolled machine steel.

β = 1+η(α −1) Note that the standard S-N curve gives number of cycles to failure N at stress

amplitude σ. This is in contrast to structural η = + + σ −4 400 0.62 0.2log10 r y 10 log10 ( ) engineering, where S-N curves give number r of cycles to failure N at stress range S, and caution should thus be exercised to avoid β notch factor confusion and errors. Note also that in α stress concentration factor structural engineering, the characteristic S-N η notch sensitivity curve for structural steel is usually defined r fillet radius differently as the S-N curve, which represents 97.7% survival probability (i.e., The static notch factor is defined as the 2.3% quantile in the realisation of fatigue tests). Thus, it is usually combined σ with a smaller requirement to the materials β = + α − y 2 m 1 ( 1)( ) γ 1000 factor m than the 1.8 quoted above for non- welded machine steel. A standard S-N curve, in principle applicable to all non-welded machine steels, can now be constructed, expressed in terms C.8 S-N curves for composites of the above quantities, as shown in Figure S-N curves for composite materials such as C-5. This standard S-N curve, which fibre reinforced plastics may vary from case represents 50% survival probability (i.e. the to case, depending on the composition of the 50% quantile in the realisation of fatigue material. However, many different glass tests), is to be considered as a characteristic fibre reinforced composites follow the same S-N curve for the material. A materials strain-life curve, denoted ε-N curve. γ factor m should be applied to all stress Standard ε-N curves for design, established values of this characteristic S-N curve to get from fatigue tests in the laboratory on a a design S-N curve for use in design. For number of different materials, can be found critical components such as the main shaft, in Mayer (1992). γ m = 1.8 should be used. Note that the standard S-N curve in Figure C-5 cannot be applied to case-hardened steels.

C – Fatigue Calculations 257 Guidelines for Design of Wind Turbines − DNV/Risø

C.9 Other types of fatigue   σ  σ = S 1 −  m  assessment a em      Sut  In order to take the mean stress σ ≠ 0 into m account, several different approaches have The Goodman line is combined with the been developed, some of which are yield line in “the modified Goodman described below. These empirical fatigue diagram” shown in Figure C-7. This curves are constructed from static material diagram includes compressive stresses. properties. Each line in Figure C-6 From the diagram can be read the maximum represents different approximations allowed stress amplitude σa in order to avoid describing the relationship between the σ mean stress and the stress amplitude above fatigue failure for a given mean stress m. which failure will occur. σ a ° 45 line

σa Su

Syt Yield line Sy a

R = 0 Se a

45° Sem Goodman line σm

Gerber line

Se

σm 45° S Syt Sut y Figure C-6. Different fatigue lines for non-zero mean Figure C-7. Modified Goodman diagram. stress.

Sy yield strength σ stress amplitude a Su ultimate strength σ mean stress m Se endurance limit Syt yield strength in tension Sut ultimate strength in tension Sem modified endurance limit, i.e. reduced REFERENCES for effects of surface, size, reliability etc. DNV, Fatigue Strength Analysis for Mobile σ σ R = min/ max Offshore Units, Classification Notes No. 30.2, Det Norske Veritas, Høvik, Norway, The Gerber diagram is a parabola given by 1984.

2   σ   DNV, Fatigue Assessment of Ship σ = S 1 −  m   Structures, Classification Notes No. 30.7, a em   S     ut   Det Norske Veritas, Høvik, Norway, 1998.

The Goodman line is given by DS 412, Code of Practice for the structural use of steel, 3rd edition, 1998-07-02.

258 C – Fatigue Calculations Guidelines for Design of Wind Turbines − DNV/Risø

Eurocode 3, Design of steel structures – part 1-1: General rules and rules for buildings, 2nd edition, September 1993.

Hück, M., Trainer, L., Schütz, W., ABF 11, Berechnung von Wöhlerlinien für Bauteile aus Stahl, Stahlguss und Grauguss, Synthetishe Wöhlerlinien, Industrieanlagen- Betriebsgesellschaft mbH, 1983.

Mayer, R.M., Design of Composite Structures against Fatigue, Applications to Wind Turbine Blades, Mechanical Engineering Publications Ltd., Bury St. Edmunds, Suffolk, U.K., 1996.

Osgood, C. C., Fatigue Design, 2nd edition, Pergamon Press, 1982.

C – Fatigue Calculations 259 Guidelines for Design of Wind Turbines − DNV/Risø

D. FEM Calculations Frequency analysis Frequency analysis is used to determine the eigenfrequencies and normal modes of a If simple calculations cannot be performed structural part. to document the strength and stiffness of a structural component, a Finite Element The FEM program will normally perform an analysis should be carried out. analysis of the lowest frequencies. However, by specifying a shift value, it is possible to The model to be included in the analysis and obtain results also for a set of higher fre- the type of analysis should be chosen with quencies around a user-defined frequency. due consideration to their interaction with the rest of the structure. Note that the normal modes resulting from such analysis only represent the shape of the Since a FEM analysis is normally used when deflection profiles, not the actual deflec- simple calculations are insufficient or tions. impossible, care must be taken to ensure that the model and analysis reflect the physical Dynamic analysis reality. This must be done by means of Dynamic FEM analysis can be used to carrying out an evaluation of the input to as determine the time-dependent response of a well as the results from the analysis. structural part, e.g. as a transfer function. Guidelines for such an evaluation are given The analysis is normally based on modal below. superposition, as this type of analysis is much less time consuming than a ‘real’ time dependent analysis. D.1 Types of analysis Though different types of analyses can be Stability/buckling analysis performed by means of FEM analysis, most Stability/buckling analysis is relevant for analyses take the form of static analyses for slender structural parts or sub-parts. This is determination of the strength and stiffness of due to the fact that the loads causing local or structures or structural components. FEM global buckling may be lower than the loads analyses are usually computer-based causing strength problems. analyses which make use of FEM computer programs. The analysis is normally performed by applying a set of static loads. Hereafter, the Static analysis factor by which this set of loads has to be In a static analysis structural parts are multiplied for stability problems to occur, is commonly examined with respect to found by the program. determining which extreme loads govern the Thermal analysis extreme stress, strain and deflection re- By thermal analysis, the temperature sponses. As the analysis is linear, unit loads distribution in structural parts is determined, can be applied, and the response caused by based on the initial temperature, heat input/ single loads can be calculated. The actual output, convection, etc. This is normally a extreme load cases can subsequently be time-dependent analysis, however, it is examined by means of linear combinations - usually not very time- consuming as only superposition. one degree of freedom is present at each

modelled node. Note that a thermal analysis set-up as mentioned here can be used to

260 D – FEM Calculations Guidelines for Design of Wind Turbines − DNV/Risø analyse analogous types of problems define the model and the boundary con- involving other time-dependent quantities ditions. Hence the coordinate system valid than temperature. This applies to problems for the elements and boundary conditions governed by the same differential equation should be checked, e.g. by plots. This is as the one which governs heat transfer. An particularly important for beam elements example of such an application can be found given that it is not always logical which axes in foundation engineering for analysis of the are used to define the sectional properties. temporal evolution of settlements in foundation soils. Similarly, as a wrong coordinate system for symmetry conditions may seriously corrupt Types of analyses the results, the boundary conditions should The analyses mentioned above only be checked. encompass some of the types of analyses that can be performed by FEM analysis. Insofar as regards laminate elements, the Other types of analyses are: plastic analyses default coordinate system often constitutes and analyses including geometric non- an element coordinate system, which may linearities. have as a consequence that the fibre directions are distributed randomly across a Furthermore, combinations of several model. analyses can be performed. As examples hereof, the results of an initial frequency Material properties analysis can be used as a basis for Several different material properties may be subsequent dynamic analysis. Finally, the used across a model, and plots should be result of a thermal analysis may be used to checked to verify that the material is form a load case in a subsequent static distributed correctly. analysis. Drawings are often made by means of using units of mm to obtain appropriate values. D.2 Modelling When the model is transferred to the FEM program, the dimensions are maintained. In The results of a FEM analysis is normally this case care should be taken in setting the documented by plots and printouts of material properties (and loads) correctly, as selected extreme response values. However, kg-mm-N-s is not a consistent set of units. It as the structural FEM model used can be is advisable to use SI-units (kg-m-N-s). very complex, it is important also to document the model itself. Even minor Material models deviations from the intention may give The material model used is usually a model results that do not reflect reality properly. for isotropic material, i.e. the same

properties in all directions. Note, however, D.2.1 Model that for composite materials an orthotropic The input for a FEM model must be docu- material model has to be used to reflect the mented thoroughly by relevant printouts and different material properties in the different plots. The printed data should preferably be directions. For this model, material stored or supplied as files on a CD-ROM properties are defined for three orthogonal directions. By definition of this material, the Coordinate systems choice of coordinate system for the elements Different coordinate systems may be used to has to be made carefully.

D – FEM Calculations 261 Guidelines for Design of Wind Turbines − DNV/Risø

D.2.2 Elements codes, and for some programs the thickness can be shown by 3D views. For a specific structural part, several different element types and element The stresses at connections such as welds distributions may be relevant depending on cannot be found directly by these elements the type of analysis to be carried out. either. Usually, one particular element type is used for the creation of a FEM model. 3D

By the use of solid elements the correct However, different element types may be geometry can be modelled to the degree of combined within the same FEM model. For detail wanted. However, this may imply that such a combination special considerations the model will include a very large number may be necessary. of nodes and elements, and hence the

solution time will be very long. Further- Element types more, as most solid element types only have 1D three degrees of freedom at each node, the Models with beam elements are quite simple mesh for a solid model may need to be to create and provide good results for frame denser than for a beam or shell element like structures. model.

One difficulty may be that the sectional Combinations properties are not visible. Hence, the input Combination of the three types of elements should be checked carefully for the direction is possible, however, as the elements may of the section and the numerical values of not have the same number of degrees of the sectional properties. Some FEM freedom (DOF) at each node, care should be programs can generate 3D views showing taken not to create unintended hinges in the the dimensions of the sections. This facility model. should be used, if present.

Beam elements have six degrees of freedom Naturally, the stresses in the connections in each node – three translations and three cannot be calculated accurately by the use of rotations, while solid elements normally beam elements only. only have three – the three translations.

Shell elements normally have five degrees 2D of freedom – the rotation around the surface Shell and plate elements should be used for normal is missing. However, these elements parts consisting of plates or constant may have six degrees of freedom, while the thickness sub-parts. As shell elements stiffness for the last rotation is fictive. suitable for thick plates exist, the wall thickness does not need to be very thin to The connection of beam or shell elements to obtain a good representation by such solid elements in a point, respectively a line, elements. These elements include the introduces a hinge. This problem may be desired behaviour through the thickness of solved by adding additional ‘dummy’ the plate. The same problems as for beam elements to get the correct connection. elements are present for shell elements as Alternatively, constraints may be set up the thickness of the plates is not shown. The between the surrounding nodal thickness can, however, for most FEM displacements and rotations. Some FEM programs be shown by means of colour

262 D – FEM Calculations Guidelines for Design of Wind Turbines − DNV/Risø programs can set up such constraints Furthermore, the shape and order of the automatically. elements influence the required number of elements. Triangular elements are more stiff Element size/distribution than quadrilateral elements, and first-order The size, number and distribution of elements are more stiff than second-order elements required in an actual FEM model elements. depend on the type of analysis to be performed and on the type of elements used. This can be seen from the example below, in which a cantilever is modelled by beam, Generally, as beam and shell elements have membrane, shell and solid elements. five or six degrees of freedom in each node, good results can be obtained with a small E = 2.1⋅105 N/mm2 100 N number of elements. As solid elements only have three degrees of freedom in each node, 10 mm they tend to be more stiff. Hence, more 100 mm elements are needed. Figure D-1. Cantilever.

Element Number of u σ σ Description y x,node x,element type elements [mm] [N/mm2] [N/mm2] Analytical result - 1.9048 600 600 Beam element, 2 nodes per 10 1.9048 600 600 BEAM2D element, 3 DOF per node, u , x 1 1.9048 600 600 uy and θz Membrane element, 4 nodes PLANE2D per element, 2 DOF per 10 x 1 1.9124 570 0 node, ux and uy Membrane element, 3 nodes 10 x 1 x 2 0.4402 141 141 TRIANG per element, 2 DOF per 20 x 2 x 2 1.0316 333 333 node, ux and uy 40 x 4 x 2 1.5750 510 510 Shell element, 3 nodes per SHELL3 20 x 2 x 2 1.7658 578 405 element, 6 DOF per node Solid element, 8 nodes per SOLID element, 3 DOF per node ux, 10 x 1 1.8980 570 570 uy and uz Solid element, 4 nodes per 10 x 1 x 1 0.0792 26.7 26.7 TETRA4 element, 3 DOF per node ux, 20 x 2 x 1 0.6326 239 239 uy and uz 40 x 4 x 1 1.6011 558 558 Solid element, 4 nodes per TETRA4R 20 x 2 x 1 1.7903 653 487 element, 6 DOF per node Table D-1. Analysis of cantilever with different types of elements.

Element quality Several measures for the quality of elements The results achieved by a certain type and can be used, however, the most commonly number of elements depend on the quality of used are aspect ratio and element warping. the elements.

D – FEM Calculations 263 Guidelines for Design of Wind Turbines − DNV/Risø

The aspect ratio is the ratio between the side include element models of structural parts lengths of the element. This should ideally other than the particular one to be be equal to 1, but aspect ratios of up to 3-5 investigated. One situation where this comes do usually not influence the results and are about is when the true supports of a thus acceptable. considered structure have stiffness pro- perties which cannot be well-defined unless Element warping is the term used for non- they are modelled by means of elements that flatness or twist of the elements. Even a are included in the FEM model. slight warping of the elements may influence the results significantly. When such an extended FEM model is adopted, deviations from the true stiffness at Most available FEM programs can perform the boundary of the structural part in checks of the element quality, and they may question may then become minor only. As a even try to improve the element quality by consequence hereof, the non-realistic effects redistribution of the nodes. due to inadequately modelled boundary conditions are transferred further away to The quality of the elements should always the neighbouring structural parts or sub- be checked for an automatically generated parts, which are now represented by mesh, in particular, for the internal nodes elements in the extended FEM model. and elements. It is usually possible to generate good quality elements for a Types of restraints manually generated mesh. The types of restraints normally used are constrained or free displacements/rotations With regard to automatically generated or supporting springs. Other types of high-order elements, care should be taken to restraints may be a fixed non-zero displace- check that the nodes on the element sides are ment or rotation or a so-called contact, i.e. placed on the surface of the model and not the displacement is restrained in one just on the linear connection between the direction but not in the opposite direction. corner nodes. This problem often arises when linear elements are used in the initial The way that a FEM program handles the calculations, and the elements are then fixed boundary condition may vary from one changed into higher-order elements for a program to another. One approach is to final calculation. remove the actual degree of freedom from the model, another is to apply a spring with Benchmark tests to check the element a large stiffness at the actual degree of quality for different element distributions freedom. The latter approach may lead to and load cases are given by NAFEMS. singularities if the stiffness of the spring is These tests include beam, shell and solid much larger than the stiffness of the element elements, as well as static and dynamic model. Evidently, the stiffness can also be loads. too small, which may again result in singularities. D.2.3 Boundary conditions An appropriate value for the stiffness of Definition of boundary conditions such a stiff spring may be approximately 106 The boundary conditions applied to the times the largest stiffness of the model. model should as a matter of course be as realistic as possible. This may require that the FEM model becomes extended to

264 D – FEM Calculations Guidelines for Design of Wind Turbines − DNV/Risø

As the program must first identify whether Structural loads consist of nodal forces and the displacement has to be constrained or moments and of surface pressure. Nodal free, the contact boundary condition requires forces and moments are easily applied, but a non-linear calculation. may result in unrealistic results locally. This is due to the fact that no true loads act in a Symmetry/antimetry single point. Thus, application of loads as Other types of boundary conditions are pressure loads will in most cases form the symmetric and antimetric conditions, which most realistic way of load application. may be applied if the model and the loads possess some kind of symmetry. Taking Load application such symmetry into account may reduce the The loading normally consists of several size of the FEM model significantly. load components, and all of these com- ponents may be applied at the same time. As The two types of symmetry that are most a slightly different load combination in a frequently used are planar and rotational new analysis will require an entirely new symmetries. The boundary conditions for calculation, this is, however, not very these types of symmetry can normally be rational. defined in an easy manner in most FEM Instead, each of the load components should programs by using appropriate coordinate be applied separately as a single load case, systems. and the results found from each of the corresponding analyses should then be The loads for a symmetric model may be a combined. In this way, a large range of load combination of a symmetric and an combinations can be considered. To antimetric load. This can be considered by facilitate this procedure, unit loads should be calculating the response from the symmetric used in the single load cases, and the actual loads for a model with symmetric boundary loads should then be used in the linear conditions, and adding the response from the combinations. antimetric loads for a model with antimetric boundary conditions. As only one or more parts of the total structure is modelled, care should be taken If both model and loads have rotational to apply the loads as they are experienced by symmetry, a sectional model is sufficient for the actual part. To facilitate such load calculating the response. application, ‘dummy’ elements may be added, i.e. elements with a stiffness Some FEM programs offer the possibility to representative of the parts which are not calculate the response of a model with modelled – these are often beam elements. rotational symmetry by a sectional model, The loads can then be applied at the even if the load is not rotational-symmetric, geometrically correct points and be as the program can model the load in terms transferred via the beam elements to the of Fourier series. structural part being considered.

D.2.4 Loads D.3 Documentation The loads applied for the FEM calculation are usually structural loads, however, D.3.1 Model centrifugal and temperature loads are also The result of a FEM analysis can be relevant. documented by a large number of plots and

D – FEM Calculations 265 Guidelines for Design of Wind Turbines − DNV/Risø printouts, which can make it an Loads – boundary conditions overwhelming task to find out what has The loads and boundary conditions should actually been calculated and how the be plotted to check the directions of these, calculations have been carried out. and the actual numbers should be checked from listings. To be able to check the The documentation for the analysis should correspondence between plots and listings, clearly document which model is documentation of node/element numbers considered, and the relevant results should and coordinates may be required. be documented by plots and printouts. The above listed aspects can and should be Geometry control checked prior to performing the analyses. Control of the geometric model by means of Possibilities of checking the results – checking the dimensions is an important and besides comparison with hand calculations – often rather simple task. This simple check also exist in the various available programs. may reveal if numbers have unintentionally been entered in an incorrect manner. Reactions The reaction forces and moments are Mass – volume – centre of gravity normally calculated by the FEM programs The mass/volume of the model should and should be properly checked. As a always be checked. Similarly, the centre of minimum, it should be checked that the total gravity should correspond with the expected reaction corresponds with the applied loads. value. This is especially relevant when loads are applied to areas and volumes, and not Material merely as discrete point loads. For some Several different materials can be used in programs it is possible to plot the nodal the same FEM model of which some may be reactions, which can be very illustrative. fictitious. This should be checked on the basis of plots showing which material is A major reason for choosing a FEM analysis assigned to each element, and by listing the as the analysis tool for a structure or material properties. Here, care should be structural part is that no simple calculation taken to check that the material properties can be applied for the purpose. This implies are given according to a consistent set of that there is no simple way to check the units. results. Instead checks can be carried out to make probable that the results from the FEM Element type analysis are correct. Several different element types can be used, and here plots and listing of the element Mesh refinement types should also be presented. The simplest way of establishing whether the present model or mesh is dense enough Local coordinate system is to remesh the model with a more dense With regard to beam and composite mesh, and then calculate the differences elements, the local coordinate systems between analysis results from use of the two should be checked, preferably, by plotting meshes. As several meshes may have to be the element coordinate systems. created and tried out, this procedure can, however, be very time-consuming. Moreover, as modelling simplification can induce unrealistic behaviour locally, this

266 D – FEM Calculations Guidelines for Design of Wind Turbines − DNV/Risø procedure may in some cases also result in For models with rotational symmetry a plot too dense meshes. Instead, an indication of of the deflection relative to a polar whether the model or mesh is sufficient coordinate system may be more relevant for would be preferable. evaluation of the results.

D.3.2 Results Stress All components of the stresses are calculated Unrealistic results and it is should be possible to plot each Initially, the results should be checked to see component separately to evaluate the if they appear to be realistic. A simple check calculated stress distribution. is made on the basis of an evaluation of the deflection of the component, which should, The principle stresses could also be plotted naturally, reflect the load and boundary with an indication of the direction of the conditions applied as well as the stiffness of stress component, and these directions the component. Also, the stresses on a free should be evaluated in relation to the surface should be zero. expected distribution.

Error estimates Strain Most commercial FEM programs have some Similar to the stresses, the components of means for calculation of error estimates. the strains and the principle strain could be Such estimates can be defined in several plotted for evaluation of the calculated ways. One of the most commonly used results. estimates is an estimate of the error in the stress. The estimated ‘correct’ stress is found by interpolating the stresses by the same interpolation functions as are used for displacements in defining the element stiffness properties.

Another way of getting an indication of stress errors is given by means of comparison of the nodal stresses calculated at a node for each of the elements that are connected to that node. Large variations indicate that the mesh should be more dense.

Load combinations If the results are found as linear combinations of the result from single load cases, the load combination factors should of course be clearly stated.

Displacement The global deflection of the structure should be plotted with appropriately scaled de- flections. For further evaluation, deflection components could be plotted as contour plots to see the absolute deflections.

D – FEM Calculations 267 Guidelines for Design of Wind Turbines – DNV/Risø

E. Material Properties E.1 Steel E.1.1 Structural steel In this appendix, some mechanical Structural steel is commonly used in the properties for the most commonly used tower structure, the base frame and different materials in wind turbines are given. parts of the transmission system. In Table E-2 and Table E-3 values for yield- and The values given for mechanical properties ultimate stress is given for different grades are characteristic values that are to be of structural steel. In the table t is the nomi- divided by appropriate material partial nal thickness. The values given apply in coefficients to yield the design values. The combination with Eurocode 3 and DS 412. values given apply in combination the Danish design codes. Table E-1. Mechanical properties of steel. Modulus of elasticity E 210 000 MPa Poisson’s ratio ν 0.3 Shear modulus G E/2(1-ν) Unit mass ρ 7850 kg/m3 Coefficient of linear ther- α 12⋅10-6 °C-1 mal expansion

Table E-2. Yield strength for structural steel.

Reference Grade Minimum yield strength fy [MPa] Standard Thickness t [mm] ≤ 16 > 16 > 40 > 63 > 80 > 100 > 150 > 200 ≤ 40 ≤ 63 ≤ 80 ≤ 100 ≤ 150 ≤ 200 ≤ 250 EN 10 025 S235 235 225 215 215 215 195 185 175 EN 10 025 S275 275 265 255 245 235 225 215 205 EN 10 025 S355 355 345 335 325 315 295 285 275

Table E-3. Tensile strength for structural steel.

Reference Grade Tensile strength fu [MPa] Standard Thickness t [mm] < 3 ≥ 3 ≥ 100 ≥ 150 < 100 < 150 < 250 EN 10 025 S235 360 – 510 340 – 470 340 – 470 320 – 470 EN 10 025 S275 430 – 560 410 – 560 400 – 540 380 – 540 EN 10 025 S355 510 – 680 490 – 630 470 – 630 450 – 630

268 E – Materials Properties Guidelines for Design of Wind Turbines – DNV/Risø

E.1.2 Alloy steel The values given in Table E-4 apply to smooth specimens with diameter d [mm] or thickness t [mm].

Table E-4. Properties of selected alloyed steel. Grade d ≤ 16 16< d ≤ 40 40 < d ≤ 100 100 < d ≤ 160 160 < d ≤ 250 t ≤ 8 8 < t ≤ 20 20 < t ≤ 60 60 < t ≤ 100 100 < t ≤ 160

fy fu fy fu fy fu fy fu fy fu 1000 - 900 - 800 - 750 - 700 - 34CrMo4 800 650 550 500 450 1200 1100 950 900 850 1100 - 1000 - 900 - 800 - 750 - 42CrMo4 900 750 650 550 500 1300 1200 1100 950 900 1100 - 1000- 900 - 850 - 800 - 50CrMo4 900 780 700 650 550 1300 1200 1100 1000 950 1100 - 1000 - 900 - 800 - 750 - 36CrNiMo4 900 800 700 600 550 1300 1200 1100 950 900 1200 - 1100 - 1000 - 900 - 800 - 34CrNiMo6 1000 900 800 700 600 1400 1300 1200 1100 950 1250 - 1250 - 1100 - 1000 - 900 - 30CrNiMo8 1050 1050 900 800 700 1450 1450 1300 1200 1100 fy yield strength [MPa] fu tensile strength [MPa]

E.2 Cast iron Table E-6. Mechanical Mat lamina, Weaved The values given in Table E-5 are valid for a properties for GRP. 30 weight roving wall thickness below 50 mm. For other % glass lamina, 50 weight % dimensions the values must be reduced in 1 accordance with DIN 1693 and proper glass Values to be used in reduction factors for surface roughness and strength calculation notches must be included. Tensile strength 80 MPa 170 MPa Compressive strength 100 MPa 100 MPa Table E-5. Properties of selected cast iron (minimum Bending strength 150 MPa 170 MPa values). Shear strength Grade Yield Tensile Elongation interlaminar 10 MPa 10 MPa strength strength at rupture in plane 20 MPa 20 MPa fy [MPa] fu [MPa] δ [%] Flexural modulus 4 000 MPa 8 000 MPa GGG-35.3 350 220 22 Values to used in de- GGG-40 400 250 15 formation calculation GGG-40.3 400 250 18 Modulus of elasticity 6 000 MPa 12 000 MPa GGG-50 500 320 7 Shear modulus 2 200 MPa - GGG-60 600 380 3 Poissons ratio 0.35 - 1. Applies to balanced web, loaded in the main direction.

E.3 Fibre Reinforced Plastics These values are to be reduced in E.3.1 Glass fibre reinforced plastics accordance with DS 456 depending on temperature, load duration, cyclic load and Table E-6 shows the guiding values given in fabrication method. the Danish design code for the mechanical properties of glass fibre reinforced plastics.

E – Materials Properties 269 Guidelines for Design of Wind Turbines – DNV/Risø

E.4 Concrete Danish Energy Agency, Recommendation to Comply with the Requirements in the Reinforced concrete is most often used in Technical Criteria for Danish Approval the foundation structure but also towers Scheme for Wind Turbines, Foundations, made of prestressed concrete are seen. Here August 1998. attention will be paid to reinforced concrete

(RFC) only. DIN 1693 Blatt 1, Gusseisen mit

Kuglegraphit, Werkstoffsorten unlegiert und E.4.1 Mechanical properties niedriglegiert, Oktober 1973. For dimensioning purposes the characteristic strength of concrete and reinforcement DS 411, Code of practice for the structural respectively are given in Table E-7. In the use of concrete, 4. edition, 1999-03-03 same table are for overall calculations (in particular FEM-analyses) informative de- DS 412, Code of Practice for the structural formation values and density given, acc. to use of steel, 3. edition, 1998-07-02. Herholdt et all. (1986). These values are in accordance with Eurocode 2 and DS 411. It DS 456, Code of Practice for the structural shall be noted that concrete itself is not able use of glass fibre reinforced unsaturated to absorb tension stress. For this purpose the polyester, 1. edition, April 1985 reinforcement is introduced. DS 481, Concrete – materials, 1. edition Table E-7. Mechanical properties of RFC 1999

Compression (M) fck >25 MPa strength of (A) >35 MPa 1 EN 10025, Hot rolled products of non-alloy concrete (E) >40 MPa structural steels – Technical delivery Yield strength Ks410S fyk 410 MPa of rein- Ks550S 550 MPa conditions, March 1990 + amendment A1, 2 forcement Tentor 550 MPa August 1993. Modulus of elasticity E 51000 1+13/ f EN 10083, Quenched and tempered steels – ck Part 1: technical delivery conditions for Poisson’s ratio ν 0.1 – 0.25 Shear modulus G E/2(1-ν) special steels, 1997. Unit mass ρ 2500 kg/m3 1. Environmental classes acc. to DS 411, moderate Eurocode 2: Design of concrete structures – (M), aggressive (A) and extra aggressive (E). Part 1-1: General basis for buildings and 2. Selected steel qualities most often used. civil engineering works, ENV 1992-1- 1:1991 Compression strength of plain concrete should not be chosen less than 5 MPa. Eurocode 3: Design of steel structures – Part 1-1: General rules and rules for buildings, 2. edition, September 1993. REFERENCES Herholdt A. D. et all., Beton-Bogen CEB-FIP Model Code 1990 (MC 90) 2.udgave 1986, CtO, Cementfabrikkernes tekniske Oplysningskontor

270 E – Materials Properties Guidelines for Design of Wind Turbines − DNV/Risø

F. Terms and Definitions

This appendix contains terms and definitions that are not explicitly described elsewhere in the book. The list is based on the definitions in the references listed in the end of this appendix.

Actuator: Device that can be controlled to the total kinetic energy contained by the apply a constant or varying force and wind at a given wind speed within a given displacement. capture area. (Established by the German Anemometer distance constant: Quantity scientist A. Betz). related to the response time of an Bin: Interval for test data grouping. anemometer. Blade Angle: Angle between the plane of Angle of attack: Angle between the the rotor and the local chord line of the direction of the resulting wind velocity on blade profile the blade and the chord line of the blade Blade cone angle: Angle between the rotor airfoil section. plane and the blade quarter chord line. Annual average: Mean value of a set of Blade load distribution: Description of the measured data of sufficient size and duration manner in which blade loads are distributed to serve as an estimate of the expected value along the quarter chord line. of the quantity. The averaging time interval Blade root: Part of the rotor blade that is shall comprise a whole number of years to closest to the hub. average out non-stationary effects such as Buckling: Failure mode characterised by a seasonality. non-linear increase in deflection with a Augmenter: Device or structure which change in compressive load. increases an air flow's speed by reducing the Calibration load: Forces and moments area through which it passes. applied during calibration. Autonomous system: Wind turbine in Capacity factor: Ratio of the average power combination with at least one other source produced by a wind turbine to its rated of energy (e.g. diesel generator, solar power. (Usually taken over a period of time collector, biogas system, etc.) commonly - daily, weekly, monthly, annually - used as used to improve the continuity of supply. a prefix). Auto-reclosing cycle: Event with a time Capacity: Total rated power of a wind period, varying from approximately 0.01 turbine or group of wind turbines. In the second to a few seconds, during which a case of electricity producing turbines, it breaker, released after a grid fault, is denotes the rated generator power. automatically reclosed and the line is Capture matrix: Organisation of the reconnected to the network. measured time series according to mean Availability: Percentage of time that a wind wind speeds and turbulence intensities. turbine generator is available for production. Catastrophic failure: Disintegration or Axial: Direction or motion perpendicular to collapse of a component or structure. the rotor plane. Characteristic value (of a material Betz limit: Maximum energy conversion property): Value having a prescribed efficiency theoretically obtainable from a probability of not being attained in a wind turbine rotor, equal to 16/27 ≈ 0.593 of hypothetical unlimited test series. Chord line: Straight line connecting an

F – Terms and Definitions 271 Guidelines for Design of Wind Turbines − DNV/Risø airfoil's leading and trailing edges. amplitude and mean value during a fatigue Complex terrain: Surrounding terrain that test. features significant variations in topography Co-ordinate system: Co-ordinate systems as and terrain obstacles that may cause flow defined in DS 472 and IEC 61400 are shown distortion. in Figure F-1 and Figure F-2, respectively. Cone angle: See Blade Cone Angle. Constant amplitude loading: Denotes the application of load cycles with a constant

zb zb yb xb xb

yb

zh zh

yh xh xh

yh

zt zt

yt xt

Figure F-1. DS 472 Co-ordinate. Figure F-2. IEC 61400 Co-ordinate.

Creep: Time-dependant increase in strain Diurnal: Daily. Used to describe meteoro- under a sustained load. logical conditions (temperature, atmospheric Cup anemometer: Rotating device for pressure, wind speed, turbulence, etc.) that measuring wind speed, characterised by normally vary regularly with the time of cups mounted on radial arms which rotate day. about a vertical axis. Dormant failure (also known as latent Cut-in wind speed (Vin): Lowest mean wind fault): Failure of a component or system speed at hub height at which the wind which remains undetected during normal turbine starts to produce power. operation. Cut-out wind speed (Vout): Highest mean Downwind: Denotes the main wind wind speed at hub height at which the wind direction. turbine is designed to produce power. Dump load: A load connected to a wind Design limits: Maximum or minimum turbine for the purpose of dissipating or values used in a design. storing excess power. It may be electrical Design loads: Loads that the turbine is (e.g. resistive heating coils or a battery) or designed to withstand. They are obtained by mechanical (e.g. a flywheel). applying the appropriate partial load factors Edgewise: Direction that is parallel to the to the characteristic values. local chord of the blade.

272 F – Terms and Definitions Guidelines for Design of Wind Turbines − DNV/Risø

Electric power network: Particular Guy (wire): Cable or wire used as a tension installations, substations, lines or cables for support between the ground and a tower. the transmission and distribution of electri- Horizontal axis wind turbine (HAWT): city. Wind turbine whose rotor axis is Electromagnetic interference: Impairment substantially parallel to the wind flow. of an electromagnetic signal, either by Hub height: Height of the centre of the electrical field interference or by deflection rotor above the terrain surface. of a radio transmitted signal. Idling: Condition of a wind turbine Environmental conditions: Characteristics generator that is rotating slowly and not of the environment (altitude, temperature, producing power. humidity, etc.) which may affect the WTGS Inboard: Towards the blade root. behaviour. Inertial sub-range: Frequency interval of External conditions: Factors affecting the wind turbulence spectrum, where eddies operation of a wind turbine, including wind - after attaining isotropy - undergo regime, electrical grid, temperature, snow, successive break-up with negligible energy ice, etc. dissipation. Fatigue strength: Measure of the load- NOTE - at a typical 10 m/s wind speed, the bearing capacity of a material or structural inertial sub-range is roughly from 0.02 Hz element subjected to repetitive loading. to 2 kHz. Fatigue test: Test in which a cyclic load of Infinite life design: Design where the constant or varying amplitude is applied to operating stresses does not result in damage. the test specimen. In-plane: Direction or motion parallel with Feathering: To change the blade pitch the rotor plane. angle of all or part of a blade to reduce the Kite anemometer: Kite which has been aerodynamic lift. calibrated to give quantitative wind speed Flap: Direction which is perpendicular to data. the swept surface of the non-deformed rotor Leading edge: Part of a blade at the blade axis. incidence of the air flow. Flapping: Blade motion in the out-of-plane Lead-lag: Blade motion in the plane of of rotation direction. rotation (HAWT). Flapwise: Direction that is perpendicular to Loads: Typical term for the different load the surface swept by the non-deformed rotor components with reference to Figure F-3: blade axis. Blade loads Flatwise: Direction that is perpendicular to Term refer to directions in and the local chord, and spanwise blade axis. perpendicular to the rotor plane (while the Freewheeling: Free rotation of the wind terms in parenthesis refer to directions along turbine with the generator disconnected. and perpendicular to the local blade chord). Furling: To adjust the wind turbine Mbe: Leadwise (edgewise) bending structure in order to reduce the driving force moment of the wind, for instance feathering - yawing Mbf: Flapwise (flatwise) bending moment out of the wind. Mbt: Torsional moment Gamma function: The Euler gamma Fbe: Leadwise (edgewise) shear force ∞ − − Fbf: Flapwise (flatwise) shear force function is given by Γ(x) = e u u x 1 du ∫0 Fbs: Spanwise (axial) force Hub loads Grid: Network for transmission and Mhr: Rotor torque/roll distribution of electricity. Mht: Tilt moment

F – Terms and Definitions 273 Guidelines for Design of Wind Turbines − DNV/Risø

Mhy: Yaw moment the natural frequencies, damping and mode Fhg: Gravitational force shapes of a structure. Fhs: Side force Natural frequency: Frequency at which a Fht: Rotor thrust structure will choose to vibrate when Tower loads perturbed and allowed to vibrate freely. Mtl: Tower lateral bending moment Net power: The power available from a Mtt: Tower tilt/normal bending moment wind turbine less any power needed for Mty: Tower yaw/torsion moment control, monitoring, display or maintaining Ftg: Tower gravitational force operation, i.e. power available to the user. Ftl: Tower lateral shear force Unless otherwise specified, P will be 10- Ftn: Tower normal shear force minute average values. Net reactive power: The reactive power supplied by the power system or absorbed Mbf Mbe by the power system. Fbf Fbe Non-destructive testing (NDT): Inspection Mbt methods that do not alter the properties of the structure. Fbs One-seventh power law: Wind speed profile which uses the power law with an exponent M α = 1/7. ht Mhr Operating limits: Set of conditions, defined Fhs Fht by the WTGS designer, that govern the Mhy activation of the control and protection system. Fhg Outboard: Towards the blade tip. Overspeed control: System that limits rotor Mtt speed to a maximum value. Mtl Parking brake: A brake capable of Ftl Ftn

Mty preventing rotor movement at wind speeds up to and including the survival wind speed. Peak power ratio operation: Variable speed F tg operation when maintaining the tip speed ratio for maximum power output. Figure F-3. Loads. Peak value: Maximum value occurring during the time under consideration. Load envelope: Collection of maximum Point loading: Load or series of loads that design loads in all directions and spanwise are applied at discrete spanwise positions. positions. Power collection system: Electric connec- Method of bins: Data reduction procedure tion system that collects the power from one by which test data are grouped into wind or more wind turbines. It includes all speed intervals (bins). For each bin, the electrical equipment connected between the number of samples and sum of parameter WTGS terminals and the network connec- samples are recorded. The average tion point. parameter value within each wind speed bin Power conditioning: To change or modify can then be evaluated. (This is a general the characteristics of electric power. technique applicable to a variety of Power density: Amount of power in the parameters). wind per unit of cross-sectional area of the Modal tests: Test carried out to determine

274 F – Terms and Definitions Guidelines for Design of Wind Turbines − DNV/Risø wind stream. Usually given in W/m2. to the rotor axis. Power output: Electrical power delivered Rotor speed: Rotational speed of a wind by a WTGS. turbine rotor about its axis. Prevailing wind: Wind direction occurring R-ratio: Ratio between minimum and most frequently at a site. maximum value during a load cycle. Primary (or main) Generator: Generator Safe life: A prescribed service life with a connected to the mains power grid when the declared probability of catastrophic failure. wind turbine is supplying nominal power. SCF: Stress Concentration Factor. Factor Projected area: Area covered at any instant for increased stress due to geometrical by the rotor blades, as seen from the irregularities such as holes, notches, etc. direction of wind velocity. (Area solidly Scheduled maintenance: Preventive covered by the blades as opposed to the maintenance carried out in accordance with swept area). an established time schedule. Radial position: Distance from the rotor Service loads: Load spectrum, including centre to a point in the rotor plane. sequence, which is representative of the Rated wind speed: Specified wind speed at actual operating conditions. which a wind turbine's rated power is Site electrical facilities: All electrical, achieved. electronic, safety and control sub-systems Reference wind speed (Vref): Basic required to connect the WTG to the main parameter for wind speed used for defining power system. WTGS classes in the IEC standard. Solidity: Rotor projected area divided by the Resonance: Dynamic condition in which swept area of the rotor. the frequency of an applied force is close to Spanwise: Direction parallel to the a system's natural frequency resulting in longitudinal axis of a rotor blade. amplified vibration or oscillation. Spar: Primary lengthwise structural member Rotationally sampled wind velocity: Wind of a wind turbine rotor blade. velocity experienced at a fixed point of the SRF: Stress Reserve Factor. Safety factor rotating wind turbine rotor. The turbulence on top of partial safety factors. spectrum of a rotationally sampled wind SRF = LD / RD in which LD is the design velocity is distinct from the normal load, and RD is the design resistance. turbulence spectrum. While rotating, the Stand-alone: WTG not part of a large blade cuts through a wind flow, which energy network. varies in space. As a consequence hereof, Standstill: Condition of a WTG that is the resulting turbulence spectrum will stopped. contain sizeable amounts of variance at the Start-up wind speed: The lowest wind frequency of rotation and harmonics of the speed at which a wind turbine will begin same. rotation but will not necessarily have a net Rotor axis: Axis through the rotor centre energy output. and the main shaft. Static test: Test in which a specified load of Rotor centre: Point on the centre of the hub constant magnitude and direction is applied in the rotor plane. to a test specimen. Rotor diameter: For a HAWT, diameter of Steady-state operation: State of operation the circular swept area of the rotor and blade during which the turbine and the external assembly. conditions remain in a steady state. Rotor plane: For a HAWT, the plane Stiffness: Ratio of change of force (or perpendicular to the rotor axis where the torque) to the corresponding change in quarter chord lines intersect, or pass closest displacement of an elastic body.

F – Terms and Definitions 275 Guidelines for Design of Wind Turbines − DNV/Risø

Strain: Ratio of the elongation (or shear System which converts kinetic energy in the displacement) of a material subjected to wind into electric power. stress, to the original length of the material. Wind turbine heating system (WTHS): Support structure: Part of a wind turbine Wind turbine where the output energy is in comprising the tower and foundation. the form of heat. Survival wind speed: Maximum wind speed Wind turbine pump (WTP): Wind turbine (normally a 3-second gust) that a WTG has directly coupled to a pump. been designed to sustain without damage to Wind vane: Device for indicating or structural components or loss of ability to recording wind direction. function normally. Wind velocity: Vector describing the speed Swept area: Projected area perpendicular to and the direction of the wind. the wind direction that a rotor will describe Wind-diesel system: See Autonomous during one complete rotation. System. Tare loads: Forces and moments created by Yaw rate: Rate of change nacelle yaw gravity. position (usually measured in °/sec.).” Tip-speed: Linear speed of a blade tip. Vtip = r⋅ω (rotor radius⋅angular speed) Tower shadow: Disturbance of the flow REFERENCES field around, and created by, the tower. Trailing edge: Aft portion of a blade, DS472, Last og sikkerhed for normally pointed. vindmøllekonstruktioner, Dansk Ingeniør- Transient event: Event during which the forening, 1st edition, Copenhagen, Den- state of operation of the wind turbine mark, 1992. changes, such as during shut-down or gust. Twist: Spanwise variation in angle of the Elliot, George, Recommended practices for chord lines of blade cross-sections. wind turbine testing, 8. Glossary of terms, Ultimate strength: Measure of the turbine Centre, Dept. of maximum (static) load-bearing capacity of a Trade & Industry, National Engineering material or structural element. Laboratory, Glasgow, UK, 2nd edition 1993. Unscheduled maintenance: Maintenance carried out, not in accordance with an IEC, Wind turbine generator systems – Part established time schedule, but after 1: Safety requirements, International reception of an indication regarding the state Standard, IEC61400-1, 2nd edition, 1999. of an item. Upwind: Denotes the direction opposite to IEC, Measurement of mechanical loads, TS the main wind direction. 61400-13, 1st edition, 2001-06. Utility grid: Electrical distribution system for public supply. IEC, Full-scale structural testing of rotor Utility interconnection: Electrical blades, TS 61400-23, 1st edition 2001-04. connection between a wind turbine generator system and a utility grid in which energy can be transferred from the WTGS to the utility grid and vice versa. Variable amplitude loading: Application of load cycles of non-constant mean, and/or cyclic range. Wind turbine generator system (WTGS):

276 F – Terms and Definitions Guidelines for Design of Wind Turbines − DNV/Risø

G. Tables and Conversions Fahrenheit temperature = Celsius temperature times 1.8 plus 32.

G.1 English/metric conversion Conversion between US units and metric G.2 Air density vs. temperature units The density ρ of air in units of kg/m3 can be expressed as Metric units US units LENGTH 353.12 0.3048 m 1 foot ρ = 273.15 + T 1 m 3.281 feet 1 m 39.37 inches in which T is the temperature in Celsius. 2.540 cm 1 inch 1 km 0.621 mile 1.609 km 1 mile G.3 Air density vs. height AREA ∆ρ 1 km2 0.3861 square mile The change in air density for an increase 2 ∆ 2.590 km 1 square mile h in the elevation above sea level can be 1 m2 10.76 square feet expressed as 0.093 m2 1 square foot ∆ρ VOLUME = −1.194⋅10 −4 kg/m3. 1 m3 353 cubic feet ∆h 0.0283 m3 1 cubic foot CAPACITY 1 litre 0.0353 cubic feet G.4 Rayleigh wind distribution 28.33 litre 1 cubic foot The long-term distribution of the 10-minute 1 litre 61.02 cubic inches mean wind speed U10 at a given site is 0.01639 litre 1 cubic inch usually well represented by a Weibull 1 litre 0.2642 US gallon distribution 3.7854 litre 1 US gallon MASS and WEIGHT u F (u) = 1− exp(−( ) k ) 1 metric ton 1.1 tons U10 u 1 kg 2.2046 lbs 0 0.4536 kg 1 lb in which u and k are distribution 1 kN 0.2247 kips 0 parameters. When k = 2, which is often a 4.45 kN 1 kip good approximation for wind speed data, the 1 kPa 0.000147 ksi distribution becomes a Rayleigh 6804 kPa 1 ksi distribution, 1 kPa 0.147 psi 6.804 kPa 1 psi u F (u) = 1− exp(−( ) 2 ) U10 u Temperature conversion: 0

Celsius temperature = and the following relationship exists Fahrenheit temperature times 5/9 minus 32. between u0 and the mean value E[U10] in the long-term distribution:

G – Tables and Conversions 277 Guidelines for Design of Wind Turbines − DNV/Risø

2 u = E[]U 0 π 10

Let the cumulative probability be denoted p. The corresponding value of the wind speed, u, can then be found by

= ⋅ u u 0 x , where x is tabulated in Table G-1.

Table G-1. Quantiles of standard Rayleigh distribution Probability p Quantile x 0.0001 0.0100 0.001 0.0316 0.002 0.0447 0.005 0.0708 0.01 0.1003 0.1 0.3246 0.2 0.4724 0.3 0.5972 0.4 0.7147 0.5 0.8326 0.6 0.9572 0.7 1.0973 0.8 1.2686 0.9 1.5174 0.99 2.1460 0.995 2.3018 0.998 2.4929 0.999 2.6283 0.9999 3.0349

278 G – Tables and Conversions Guidelines for Design of Wind Turbines − DNV/Risø

Index

Accessibility ...... 2, 29, 149, 153 Blades Acetal (POM) ...... 165 aspect ratio ...... 65, 264 Active brakes ...... 146 buckling ...... 110, 112, 115 Aerodynamic core ...... 106, 107 coefficients ...... 60, 65, 70, 71 damping ...... 58, 72, 73, 75, 88, 115 damping ...... 58, 74, 88, 115, 175 element momentum method 63, 66, 70 data extrapolation ...... 71 elements ...... 105 modelling ...... 61, 63 gelcoat ...... 106 Aeroelastic calculation 60, 70, 75, 173, geometry ...... 4, 104 180 leading edge ...... 4, 51, 63, 64, 75 Air loads ...... 89, 117, 250 density ...... 49, 250, 277 material ...... 58, 79, 104, 111 kinematic viscosity ...... 174 mould ...... 108 Alloyed steel ...... 126, 138, 143, 269 natural frequency ...... 73, 113 Amplitude decay ratio ...... 73 number ...... 1, 3, 59 Anemometer ...... 63, 236 pitch angle ...... 4, 69, 104 Annual energy production ...... 5, 7, 236 pressure side ...... 104, 150 Aspect ratio ...... See Blades principal axes ...... 68, 69, 105 Asynchronous generator 3, 152, 154, profile ...... 58, 67, 68, 104, 105 155, 224 resin ...... 106, 107 Atmospheric conditions ...... 32, 40, 236 sizing ...... 106 Atmospheric pressure ...... 49 suction side ...... 104 Autocorrelation function ...... 39 tests ...... 26, 110, 113, 114, 116 Availability ...... 6 tip angle ...... 117, 118 trailing edge ...... 4, 63, 65, 110 Beam theory 68, 104, 129, 173, 178, 198, twist ...... 58, 74, 104 262 web ...... 104, 105, 106, 110 Bearings wet hand lay-up ...... 108 alignment ...... 128 Blocking mechanisms ...... 29, 162, 233 basic dynamic capacity ...... 136 Bolts clearance ...... 133 clamping length ...... 244, 245, 246 friction torque moment ...... 164 connections ...... 118, 178, 239 gear ...... 135 fatigue ...... 175, 240 lubrication ..... 130, 136, 156, 163, 165 impact strength ...... 239 main bearing ...... 116, 123, 127 pretension ...... 27, 126, 233, 242, 249 rating life ...... 132, 135 quality classes ...... 239 rolling bearings 132, 136, 163, 166, Boundary layer theory ...... 35, 64 167 Bowen ratio ...... 41 seals ...... 130, 131, 165 Brakes slide bearings ...... 163, 165, 166 aerodynamic 14, 15, 60, 106, 109, 110 spherical roller bearing . 127, 130, 135 callipers ...... 146, 162 tight fits ...... 122, 123 disc ...... 146, 147, 148, 162 Betz' law ...... 6 electrical ...... 156, 163 Bevel gears ...... 134, 135 mechanical .14, 87, 146, 148, 154, 237 pads ...... 146, 148

Index 279 Guidelines for Design of Wind Turbines − DNV/Risø

Braking loads ...... 60 Cost ...... 5 Braking systems .... 12, 14, 146, 150, 156 Couplings ..120, 122, 123, 126, 141, 145 Brittle fracture . 18, 22, 48, 165, 179, 239 Coupon test ...... 144, 145 Bulk temperature ...... 140 Crack growth ...... 27, 252, 255 Cable twist ...... 3, 12, 13, 159, 166 inspection ...... 145, 214 Capacitor bank ...... 225, 226 monitoring ...... 220 Capacity factor ...... 7 Cumulative damage 24, 25, 76, 77, 117, Cast iron ...... 118, 269 207, 254, 255 Caughey series ...... 72 Cut-in wind speed ...... 7, 81 Centrifugal forces ...... 58, 117, 152 Cyclones ...... 32, 46 Ceramic brake pads ...... 148 Certification ...... 28, 30, 236 Damping Characteristic chord length .....64, 75, 89 aerodynamic ...... 58, 74, 88, 115, 175 Characteristic values ...... 11, 20 negative ...... 60, 74 Charnock’s formula ...... 33 structural ...... 58, 72, 73, 74, 88 Clay 188, 193, 195, 197, 199, 202, 203, Danish approval scheme ...... 7, 154, 188 209, 213, 218 Darrieus ...... 2 Coating 51, 156, 212, 213, 214, 220, 221 Davenport ...... 39, 62, 84, 182 Coefficient of variance ...... 81, 183 Delamination ...... 113, 114, 115 Coherence model ...... 39, 62 Design codes ...... See Standards Common-cause failures .... 14, 16, 17, 18 Design lifetime ...... 11, 112, 157, 176 Computational fluid dynamics ...... 65 Design parameters ...... 21 Concepts Design situations ...... 10, 11, 55 downwind ...... 2 Detail categories ...... 175, 176, 177 horizontal axis ...... 2, 30, 159, 163 Deterministic method ...... 21, 100 upwind ...... 1, 2 Diffraction theory ...... 98 vertical axis ...... 2 Dismantling ...... 2, 126, 214, 215, 221 Concrete ...... 206, 270 Displacement transducers ...... 115 Conductor cable ...... 114 Drag 51, 58, 60, 64, 65, 71, 74, 75, 76, Cone penetrometer test (CPT) ...... 188 87, 95, 96, 97, 98, 182 Configurations Drivetrain ...... See Transmission clusters ...... 7, 194 Ductility ...... 22, 119, 120 parks ...... 8 Duhamel’s integral ...... 52 stand-alone ...... 7 Dynamic amplification factor ... 211, 219 wind farms ...... 8, 36, 227, 229, 231 Dynamic load rating ...... 28, 132, 136 Control system ...... 9, 12, 27, 29 Dynamic response ...... 71, 148, 171 modelling ...... 69 Dynamic stall ...... 74, 76 parameters ...... 12, 69 Conversions ...... 277 Earthquake ...... 51, 52, 99, 201, 202, 203 Cooling system ...... 149, 155, 223, 233 Eccentric loading ...... 193 Coordinate system ...... 129, 160, 261 Economy ...... 5, 8, 10, 21 Coriolis forces ...... 58 Edgewise vibrations ...... 60 Coriolis parameter ...... 44 Effective foundation area ...... 190 Corrosion protection 51, 130, 131, 166, 179, 208, 212, 213, 214, 220, 239, 252

280 Index Guidelines for Design of Wind Turbines − DNV/Risø

Efficiency ...... 2, 4, 6, 7 Flange coupling ...... 145 aerodynamic ...... 1, 6 Flapwise vibrations ...... 74 electrical ...... 6 Flash temperature criterion ...... 140 mechanical ...... 6 Flex4 ...... 70 Efficiency factor ...See Power coefficient Flicker ...... 13, 227, 230 Eigenfrequency ... See Natural frequency Flutter ...... 69, 75 Electrical safety ...... 230 Foundation Embedded foundations ...... 192 bearing capacity factor ...... 192 Emergency stop ...... 29 friction angle ...... 188, 197, 199, 200 Endurance limit ...... 139 gravity base ...... 187, 188, 208 Environmental classes ...... 206 inclination factors ...... 192, 193 Environmental conditions 30, 46, 110, monopile ...... 98, 187, 208, 209 130 shape factor ...... 193 EP additives ...... 131, 132, 137 shear strength parameters ..... 188, 192 Equivalent loads ...... 81, 163, 164, 250 skin friction 194, 196, 197, 215, 217, Euler force ...... 178 218 External conditions ...... 29, 32, 48, 55 spring stiffness ...... 201, 203, 204 Extreme value analysis ..... 43, 77, 82, 85 stability ...... 189 Extreme value distribution ...... 44, 82, 84 stiffness .172, 188, 189, 201, 203, 204 Extreme winds ...... 43 tripod ...... 187, 209, 215 Four-point ball bearing ...... 163, 165 Fabrication .. 26, 209, 214, 252, 256, 269 Fracture mechanics ...... 252, 253 Fail-grace ...... 10 Frechet distribution ...... 34, 35 Fail-safe ...... 14, 15, 150, 154, 157 Frequency analysis ..... 16, 219, 260, 261 Failure mode analysis ...... 15, 16, 18, 55 Frequency converter 154, 155, 156, 223, Failure probability .... 10, 18, 19, 26, 136 226, 227, 228, 229, 230, 232 Fatigue Fretting corrosion ...... 27 assessment 24, 79, 112, 117, 121, 125, Friction coefficient ...... 248 132, 185, 206, 211, 219, 252 Frictional velocity ...... 32, 33, 41 design life ...... 11, 27, 80, 136, 252 Full-scale test ...... 26, 114 equivalent loads ...... 81, 136, 164, 183 Functional load ...... 60 load spectrum ...... 81, 89 loads ...... 76, 80, 125, 170 Galvanization ...... 239, 240 S-N curves 24, 79, 81, 124, 211, 240, Gamma function ...... 7, 24, 35, 111, 255 252, 253, 256 Gear Fault tree analysis ...... 16 design contact stress ...... 138 Feasibility assessment ...... 10, 21 gearless ...... 8, 227 FEM ...... See Finite element method gray staining ...... See Micro pitting Fibre reinforcements 104, 106, 118, 257, heat treatment ...... 137, 143 269 helical ...... 134 Filtering ...... 132, 150, 156 hypoid ...... 134 Finite element method ...... 61, 70, 260 installation ...... 142 Fire ...... 16, 149, 233 materials ...... 137, 142 Firing angle ...... 225 micro pitting ....27, 137, 140, 141, 143 First-order reliability method ...... 23, 24 scuffing ...... 140, 144 Fixed speed ...... 227 surface durability ...... 137, 138, 146 Flange connections ...... 118, 175, 179 tooth form ...... 134, 139

Index 281 Guidelines for Design of Wind Turbines − DNV/Risø

Generator Inertia asynchronous .... 3, 152, 154, 155, 224 coefficient ...... 95, 96 induction ...... 223 loads ...... 58, 96, 98 multiple-poled ...... 157 Inflow angle ...... 64, 65, 66, 71, 76, 166 multipole ...... 228 In-rush current ...... 225 overload ...... 13, 115, 155, 157 Inspection ...... 11, 13, 176 reactive power ...... 224, 225, 226, 227 intervals ...... 26, 176 size ...... 5 Installation 28, 29, 55, 142, 175, 194, slip ...... 152, 224 199, 209, 212, 213, 220, 233 synchronous 152, 155, 157, 223, 224, Insulation ...... 28, 153, 156, 157 229 Integral temperature criterion ...... 140 variable speed 6, 69, 154, 181, 227, Interference factor .. See Induction factor 228, 229, 230 Geological study ...... 187 JONSWAP spectrum ...... 93 Geometrical size effect ...... 121, 122 Geophysical survey ...... 187 Kaimal spectrum ...... 38, 39, 62, 88 Geostrophic wind speed ...... 40, 44 Geotechnical analysis ...... 187, 188, 218 Labour safety ...... 28, 29, 180, 233 Gerber diagram ...... 258 Labyrinth seals ...... 130, 131 Germanischer Lloyd ...... 30 Lift Glauert’s correction ...... 67 coefficients ...... 64, 65, 66, 71, 76, 158 Goodman diagram ...... 258 curve ...... 65, 70, 75 Gravity loads ...... 58, 87, 90, 112, 117 forces ...... 60, 63, 64, 104 Grid connection .... 13, 80, 223, 229, 231 Light conditions ...... 29 Grid loss ...... 148 Lightning ...... 28, 53, 106, 113, 114, 230 Grinding ...... 255 Limit states 11, 18, 19, 23, 24, 27, 56, Gumbel distribution 23, 44, 83, 85, 86, 117, 145, 210, 217, 218, 248 89 Load Gust ...... 43, 55, 57, 87, 152, 174 distribution ...... 24, 25, 89 Gyroscopic effects ...... 3, 58, 59, 161 verification ...... 93, 96, 170, 171 Load combination Hail ...... 51 extreme ...... 11, 20, 99, 174, 260, 267 Harris spectrum ...... 37, 38 fatigue ...... 81, 90, 91, 120, 174 HawC ...... 61, 70 Logarithmic decrement ...... 72, 73 High-cycle fatigue ...... 77 Longuet-Higgins distribution ...... 95 Homogeneous terrain .. 33, 35, 37, 39, 62 Low-cycle fatigue ...... 77, 126 Humidity ...... 41, 50, 110, 153 Lubrication 27, 130, 136, 137, 141, 165, Hurricanes ...... 45 233 grease ...... 130, 131, 166 Ice ...... 33, 50, 214 oil ...... 132, 137, 141 cone ...... 210, 216, 219 relubrication interval .... 131, 132, 165 loads .... 50, 60, 99, 210, 214, 216, 218 viscosity ratio ...... 136, 137 Ideal site ...... 234 Idling . 13, 15, 55, 56, 57, 79, 80, 81, 155 Main shaft ....87, 120, 130, 133, 251, 257 Induction factor ...... 66 Mann model ...... 62, 63 Induction generator ...... 223 Manual operation ...... 56, 159 Manuals ...... 29, 233

282 Index Guidelines for Design of Wind Turbines − DNV/Risø

Material certificates ...... 11, 143 Palmgren-Miner rule 24, 77, 125, 176, Material properties ...... 268 177, 207, 211, 254, 255, 256 Mean stress 78, 79, 81, 110, 112, 117, Parametrised load spectra ...... 89 123, 124, 241, 255, 256, 258 Parks See Wind farms Measurements ...... 49, 234 Partial safety factors 11, 20, 21, 22, 23, damping ...... 72, 73 26, 101 full-scale test ...... 26 calibration ...... 20, 21, 22 load ...... 170, 236 Passive brakes ...... 148 model verification ...... 62, 70, 171 Peak counting method ...... 77, 78 noise ...... 237 Peak-over-threshold method ...... 89 power performance ...... 7, 234 Periodical maximum method ...... 89 power quality ...... 237 Personal safety ...... See Labour safety uncertainty ...... 27, 46, 234 Pile foundation 187, 189, 193, 203, 205, wind speed ...... 37, 46, 234 209 Meteorology mast ...... 234 Pile refusal ...... 213 Micro pitting ...... See Gear Pile-driving 194, 210, 211, 213, 215, 220 Middelgrunden ...... 98 Pile-soil-pile interaction ...... 194, 198 Miner ...... See Palmgren-Miner Pitch angle ...... 4, 69, 104 Minimal cut set analysis ...... 17 Pitch control ...... See Power regulation Modal analysis ...... 61, 70, 181 Planetary gears ...... 134 Monin-Obukhov length ...... 41 Polyamide (PA) ...... 165 Monitoring 4, 12, 13, 15, 132, 141, 150, Polyethylene Terephtalate (PET) ..... 165 220 Polyurethane (PUR) ...... 165 Morison’s equation ...... 95 Potential annual energy ...... 7 Power NACA profiles ...... 71 coefficient ...... 6, 250 Natural frequency 73, 88, 91, 113, 115, electrical ...... 6 171, 173, 204, 218, 219, 260 iso-power curve ...... 4 Near-coastal locations ...... 33 mechanical ...... 69 Network connection .. 12, 13, 55, 57, 226 nominal ...... 12, 155 Nitrided gears ...... 145 performance ...... 5, 7, 234 Noise ...... 1, 3, 130, 158, 227, 237, 251 curve ...... 6 Nominal power ...... See Power quality ...... 229, 232, 237 Non destructive testing ..... 118, 127, 214 rated ...... 4, 152 Notch sensitivity factor ...... 122, 123 Power regulation ...... 3, 6, 69 active stall ...... 4 Offshore turbines 8, 33, 50, 51, 179, 187, pitch ...... 4, 6, 155 208 stall ...... 4, 6, 63, 152 Operation range ...... 13 Power spectral density 38, 39, 40, 75, 93 Operation temperature ...... 48, 120 Prandtl’s tip loss factor ...... 67 Operational conditions ...... 55, 60, 79 Probabilistic method 22, 24, 26, 28, 100, Operational load .....See Functional load 181 Operational procedures ...... 29, 233 Protection system 10, 11, 15, 149, 150, Outgassing ...... 108 153 Overconsolidation ratio ...... 197, 202 P-y curves ...... 197, 198, 199, 200 Overloading ...... 155 Overspeed ...... 12, 13, 155, 157, 237

Index 283 Guidelines for Design of Wind Turbines − DNV/Risø

Quality Sensors ...... 9, 14, 152, 159, 166 assurance . 30, 109, 120, 127, 144, 145 cable twist ...... 166 control ...... 26, 142 direction ...... 159 Quarter chord point ...... 64, 75 Service schedule ...... 233 Quasi-static method ...... 87 Serviceability limit state 11, 18, 219, 248 Quasi-steady analysis ...... 74 Shear modulus ...107, 197, 201, 202, 203 Short-circuit ...... 154, 155, 157 Radiation ...... 50 Shrink fits ...... 120, 122, 123, 126, 146 Rain ...... 50, 116, 158 Shutdown ...... 14, 15, 29, 56, 57, 80, 141 Rain-flow counting ...... 77, 112, 174 Site assessment ...... 46 Range counting method ...... 77, 78 Slab foundation ...... 187 Rayleigh damping model ...... 72, 73 Slewing bearing ...... 163 Rayleigh distribution 80, 94, 95, 277, service life curve ...... 164 278 Slide bearing ...... 165 Recurrence period ..... 11, 43, 48, 88, 170 Slip rings ...... 3, 224 Redundant design 14, 148, 150, 151, 166 S-N curve ...... See Fatigue Reinforced concrete ...... 206, 270 Softstarter ...... 225 crack width ...... 207 Soil conditions ...... 51, 93, 187, 189 Relative humidity ...... 50, 153 Poisson’s ratio ...... 203 Reliability skin friction ...... 195, 196 analysis ...... 10, 17 sliding resistance ...... 193 index ...... 19, 21, 23, 24 stratigraphy ...... 188 Reynolds number ...... 64 Solar radiation ...... 46, 50 Richardson number ...... 41 Specific power performance ...... 5, 7 RIX number ...... 47, 48 Specific rotor power ...... 5 Rødsand ...... 98 Spoilers ...... 14 Rotating bending ...... 256 Spur gears ...... 134 Rotor coning ...... 58, 158 Stall angle ...... 64 Rotor loads ...... 59, 87, 91 Stall regulation .....See Power regulation Rotor speed Stall strips ...... 75, 76 fixed ...... 1, 86, 170 Stall-induced vibrations ...... 73 variable ...... 1, 69, 171 Standard components ...... 159 Roughness parameter ...... 33, 34, 38, 44 Standards 19, 20, 21, 30, 33, 111, 127, 141, 151, 192, 239, 249, 268, 269 Safety DIBt Richtlinien ...... 30 classes ...... 10, 14, 22, 218 DS 412 .....48, 175, 177, 179, 247, 256 electrical ...... 230 DS 472 10, 15, 30, 35, 37, 48, 56, 89, philosophy ...... 10 90, 112, 160, 182, 183, 184 requirements ...... 10, 11, 20, 28, 126 IEC 61400-1 15, 30, 35, 37, 38, 55, Safety factors ..See Partial safety factors 57, 130, 172, 231 Safety system ...... See Protection system NVN 11400-0 ...... 30 Saffir-Simpson ...... 45 Start-up 56, 79, 80, 112, 132, 154, 172, Sand . 149, 196, 199, 202, 213, 218, 248 225 Scaling laws ...... 250 Stop ring ...... 133 Scour protection ...... 214 Storms ...... 45, 99 Scuffing ...... 140 Strain gauges ...... 115 Seals ...... 130, 131, 165

284 Index Guidelines for Design of Wind Turbines − DNV/Risø

Stress concentration factors 117, 118, Tower 121, 122, 123, 176, 177, 211, 212, erection ...... 170, 174, 187 253, 256 guided ...... 170 Stress range distribution 24, 25, 79, 80, height ...... 8, 169, 175, 178 112, 211, 252, 255 imperfection ...... 178, 179 Stress ratio ...... 121, 123, 255, 256 lattice ...... 169, 170, 201 Strouhal number ...... 175 loads ...... 22, 30, 60, 88, 170, 173 Structural natural frequency ...... 171, 172, 204 damping ...... 58, 72, 73, 74, 88 relative slenderness ratio ...... 177, 178 modelling ...... 68 shadow ...... 2, 56, 60, 170, 230 redundancy ...... 10 tubular ...... 169, 173 reliability analysis ... 18, 19, 21, 22, 24 vortex shedding ...... 60, 174 safety ...... 11, 18, 19, 20, 21, 27, 28 Transient wind conditions ...... 43, 56 standards ... 20, 48, 127, 175, 177, 179 Transmission system 27, 70, 116, 129, Structural steel ...... 127, 268 146, 151 Student’s t distribution ...... 82, 84 Transportation ...... 28, 55, 169, 170, 233 Surface conditions ...... 117, 121 Trends ...... 1, 93, 231 Surface roughness ...... 121 Turbulence ...... 25, 34, 62, 70, 88 intensity ...... 36, 47, 63, 79 Tail vane ...... 158 length scale ...... 62, 63 Technological size effect ...... 121 model ...... 37, 62 Teetering hub ...... 3 scale parameter ...... 38 Temperatures ...... 11, 48, 49 simulation ...... 43 Terrain wake ...... 36 complex ...... 37, 39, 47, 93, 234, 236 Turkstra’s rule ...... 100 homogeneous ...... 33, 35, 37, 39, 62 Two-dimensional flow ...... 64 roughness parameter ...... 32, 34, 35 T-z curves ...... 196, 217 types ...... 33 Tests ...... See Measurements US units ...... 277 Thermal analysis ...... 260 Thyristor ...... 225 Vacuum bagging ...... 109 Tilt angle ...... 113 Variable speed ...... 227 Tip angle ...... 117, 118 Veers model ...... 62, 63 Tip brake ...... See Brakes Velocity pressure ...... 44, 45 Tip speed ratio ...... 69, 250 Viscosity classes ...... 141, 142 Tolerances 27, 113, 118, 123, 126, 133, Von Karman’s constant .... 32, 33, 35, 44 158, 162, 163, 172, 179, 180, 220, Vortex generators ...... 75, 76 247 Vortex-induced vibrations ...... 175 Topography ...... 41, 47, 53, 188, 234 Tower Wake effect ...... 36, 56, 236 blade clearance ...... 3, 113, 172 Washers ...... 249 buckling ...... 52, 177 Wasp Engineering ...... 47 connections ...... 178, 187, 212 Wave loads ...... 93, 99, 210 cost ...... 169, 170 Waves ...... 33, 51, 99, 203, 212 damping ...... 72, 172 Weak grids ...... 13 door ...... 176 Wear ...... 15, 27, 130, 132, 157

Index 285 Guidelines for Design of Wind Turbines − DNV/Risø

Weibull distribution 7, 32, 46, 80, 255, 277 scale parameter ..... 24, 25, 32, 90, 183 shape parameter ...... 32, 90 Welds 175, 177, 214, 254, 255, 256, 262 Wind climate ...... 7, 32, 37, 99 data ...... 43, 47 direction ...... 42 profile ...... See Wind shear resource ...... 13, 169 rose ...... 42 shear ...... 40, 43, 55, 57, 173 synthetic field ...... 62 Wind farms 36, 42, 56, 93, 229, 231, 232 Wind speed 10-minute mean 32, 37, 43, 45, 46, 277 50-year ...... 33, 43, 45, 82, 100 distribution ...... 7, 182 extreme ...... 43, 47, 57 gust ...... 43, 57, 87, 152, 174 nominal stall ...... 89 profile 32, 33, 40, 42, 47, 55, 57, 61, 173 standard deviation ...... 34, 46, 63 Wind tunnel ...... 64, 71, 76 Wings ...... See Blades Wöhler curve ...... See S-N curve Working areas ...... 29 Worm gears ...... 134

Yaw active ...... 3, 86, 159, 161 brake ...... 29, 146, 162 error 12, 56, 71, 81, 112, 158, 159, 160, 161, 162, 166 loads ...... 60, 160 passive ...... 3, 158, 161 system ...... 2, 59, 146, 157, 158, 233 Yield strength 23, 122, 124, 125, 146, 239, 256, 258, 268, 269 Young’s modulus ...... 138, 165, 203, 204

286 Index