LOAD MEASUREMENTS ON A 2MW OFFSHORE WIND TURBINE IN THE NORTH SEA

T. R. Camp, Garrad Hassan & Partners Ltd, St Vincent’s Works, Silverthorne Lane, Bristol, BS2 0QD, UK. W. Grainger, Amec Wind Ltd, Haugh Lane Industrial Estate, Hexham, , NE46 3PU, UK. A. Henderson, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands. K. Argyriadis, Germanischer Lloyd WindEnergie GmbH, Johannisbollwerk 6-8, D-20459 Hamburg, Germany. P. H. Lauritsen, Wind Systems A/S, Smed Hansens Vej 27, DK-6940 Lem, Denmark. V. Weighill, Powergen Renewables Development Ltd, Westwood Way Business Park, Coventry, CV4 8LG, UK.

ABSTRACT: One of the two offshore wind turbines installed at Blyth during 2000 has been comprehensively instrumented in order to measure the sources of wind and wave loading and the resulting structural response. This paper describes the structural and environmental measurements that have been made over the winter of 2001/2002. These include measurements of extreme loads resulting from breaking waves impacting on the support structure and fatigue loads due to the combined action of wind, waves and currents. The use of this data to validate numerical models of wave loading in shallow waters is discussed and some preliminary comparisons between measurements and theory are presented.

1 INTRODUCTION turbines to experience the full force of North Sea wave conditions. They therefore provide an ideal opportunity to Blyth offshore is situated off the study wave loading at full scale in an aggressive Northumberland coast, on the north-east of . It environment. comprises two Vestas V66 2MW wind turbines, situated approximately 1km offshore. The turbines were installed The measurement programme at Blyth and supporting between August and October 2000 in rock-socket theoretical studies are being performed in a collaborative foundations on a submerged rock known as the ‘North project sponsored by the European Commission (JOR3- Spit’. Both turbines are sited in a mean water depth of CT98-0284), the UK Department of Trade and Industry approximately 9m. and Novem, The Netherlands agency for energy and the environment. The project, named ‘Offshore Wind Of the two turbines installed at Blyth it was decided to Turbines at Exposed Sites’ (OWTES), is being instrument the southern-most turbine. This turbine is undertaken collaboratively by Delft University of positioned at the top of a steeply shelving region of the Technology, Germanischer Lloyd WindEnergie, Vestas sea bed which was considered would increase the Wind Systems, AMEC Wind and Powergen Renewables likelihood of breaking waves at this turbine. Breaking Developments under the leadership of Garrad Hassan and waves were indeed experienced at the site during Partners. installation of the turbine and subsequently, as described in Section 3.1. 2 MEASUREMENT SYSTEM The offshore wind turbines installed at Blyth are the first The measurement system installed at Blyth comprises three main elements: (i) measurement of the turbine Blade flapwise and edge- structural loading, (ii) measurement of the sea-state, and wise bending moments at (iii) measurement of wind conditions at an onshore the root of each blade. meteorological mast close to the turbine. The instrumentation comprising each of these sub-systems is described briefly below.

2.1 Structural loading measurements X&Y bending moments & The turbine loading is measured using a large array of torsion of low-speed shaft strain-gauges which have been applied to every major structural element of the turbine. On the tower and pile foundation, strain-gauges are used to measure bending X&Y bending moments at moments in two dimensions at eight vertical stations. tower top & base, plus Blade loads are measured as flapwise and edgewise torsion at tower top bending moments at the blade roots. The low-speed shaft of the turbine is also instrumented to measure torque and bending moments in two orthogonal directions. In X&Y bending moments at: addition, signals related to the control and operational Mean sea level (MSL), status of the turbine are recorded, including blade pitch two stations between MSL angles, the speed and position of the rotor, nacelle and mudline, orientation, brake status and generated power. The two stations between mud- locations of the strain gauges on the turbine and support line and pile base. structure are shown schematically in Figure 1. Torsion at MSL and lowest pile station A large number of measurement stations were instrumented on the pile to provide redundancy, because it was believed that the strain-gauges might not have a Figure 1: Structural measurements long life in the salt water environment. In fact, after seven Tower top

Tower base Saab Video MSL WaveRadar camera

Pile depth 1

Pile depth 2

Mudline Coastal Leasing Microspec & Foundation depth 1 Nortek ADCP

Foundation depth 2

40m 0 0306090Time (s) Figure 2: Example pile strain gauge output (Y BM) Figure 3: Sea-state instrumentation

Northern turbine

Southern turbine

Met. mast

BLYTH thousand) per (parts Probability

0 Hs 6 Wind speed (m/s)

500m Figure 5: Example wind / wave scatter diagram

statistics describing the wave climate and the current Figure 4: Meteorological mast location profile. These instruments include a wave and tide recorder (Coastal Leasing Microspec) and an acoustic months, only one gauge has failed, resulting in excellent doppler current profiler (Nortek ADCP). The sea-state strain gauge coverage on the pile. Figure 2 illustrates the instrumentation is shown schematically in Figure 3 correlation of the strain gauge output at different points on the pile for a typical sea-state. In this figure, each 2.3 Wind measurements gauge output has been normalised to give the same range. Wind conditions are measured using anemometers and The tower and pile strain gauges will be calibrated during wind vanes mounted on an onshore meteorological mast summer 2002. and on the turbine nacelle. Although the meteorological mast for the project would be ideally located offshore, 2.2 Sea-state measurements close to the monitored turbine, the large cost of such an The wave and current climate is recorded using installation was beyond the budget of the OWTES instruments mounted both above and below water level. project. The mast has therefore been positioned on the A Saab WaveRadar unit is mounted on the turbine coast, approximately 1km from the southern turbine. The walkway to measure the instantaneous water level at the mast features anemometers at heights of 10m, 20m, 30m turbine base, including time-history profiles of passing and 40m above ground level and instruments to measure waves. Simultaneously, instruments mounted on the sea atmospheric pressure, temperature and precipitation. The bed approximately 40m from the foundation record 0.25 12 0.20 10 0.15 0.10 8

0.05 6 Current speed (m/s)

0.00 29 -Aug-01 400 4 350 2

300 (m) bed sea above Height 250 0 200 0.00.10.20.30.4 150 Speed (m/s)

Current direction (deg) 100 29-Aug-0129-Aug-01 30-Aug-01 30-Aug-01 31-Aug-01 31-Aug-01 Figure 7: Mean current profile at high water

Figure 6: Depth-averaged current speed and direction doppler current profiler (ADCP) is shown in Figures 6 and 7. Figure 6 shows the variation in depth-averaged position of the meteorological mast relative to the current speed and direction with the tidal cycle, while offshore turbines is shown in Figure 4. Figure 7 shows the mean velocity profile at high water. The very sheared nature of this profile (which has 2.4 Measurement programme curvature of opposite sign to that of a conventional The turbine structural loads, the sea-state at the turbine boundary layer profile) may reflect the position of the base and the wind characteristics are recorded instrumentation on the top of the North Spit rock. simultaneously using a Garrad Hassan T-MON measurement system. This well-proven, robust system is well suited to offshore applications. Analogue-to-digital 3 NUMERICAL MODELLING converters are distributed around the turbine structure and send data via a fibre-optic link to an onshore control While data is being recorded at Blyth, the OWTES room. Here, the collection and recording of data is partners have modelled the V66 turbine using several controlled by a central computer which has a modem codes. Garrad Hassan have used the Bladed for Windows connection to allow remote interrogation. program [1], while Delft University have used the DUWECS code [2] and Germanischer Lloyd The measurement period began in November 2001 and WindEnergie have used the DHAT program [3]. will continue to November 2002. During this period, every sensor on the turbine is being sampled at 20Hz and To check the basic structural modelling of the V66 using data collected in two formats: ‘summary’ datasets and the Bladed code, measured and predicted natural ‘campaign’ datasets. Summary datasets consist of the frequencies of vibration of the blades and tower were basic statistics of each channel, measured every ten compared. Figures 8 and 9 show measured power spectra minutes, comprising the minimum, maximum, mean and for the blade root flapwise and edgewise bending standard deviation of each signal. Campaign datasets moments respectively, for each of the 3 blades. The peaks store time-history data from each measured channel and in these spectra corresponding to the first modes of will be recorded in order to establish the dynamic vibration are identified and the measured and predicted behaviour, fatigue load spectra and extreme loads frequencies are given. In both cases, the predicted and experienced by the turbine. The recording of campaign measured frequencies are very similar. datasets is triggered automatically by the central computer when pre-defined trigger conditions are met. Similar results are shown for the tower in Figure 10. In this case, both the first and second modes of vibration A database of campaign datasets is currently being have been identified and the frequencies of both are seen compiled for a range of ‘normal’ conditions, covering to be predicted accurately. variations in mean water level, mean wind speed and significant wave height. Campaign datasets are also being 3.1 Breaking wave modelling used to record extreme wave loading events. It is intended to use the database of measurements in combination with the numerical models of the V66 to Summary datasets are being used for a number of verify and enhance the computer models. As an example purposes, one of which is to compile a database of wind of this, some measurements and predictions of breaking and wave characteristics at the site. Figure 5 shows an wave loads are presented. example of this data, in the form of a scatter diagram with significant wave height (Hs) and wind speed as Breaking waves were observed at the southern Blyth independent variables. turbine during installation and subsequently during storms which occurred in November 2001. Figure 11 The sea-state instrumentation mounted on the sea bed is shows a measured time-history of the water surface providing good data. Typical output from the acoustic elevation, recorded using the wave radar, during one of First flapwise mode First edgewise mode Measured: 1.000 Measured: 1.680 Predicted: 0.985 Predicted: 1.698 Power spectral density (log) density spectral Power Power spectral density (log) density spectral Power

1.E-14 1.E-14

012345 012345 Normalised frequency Normalised frequency Figure 8: Measured and predicted blade flap frequency Figure 9: Measured and predicted blade edge frequency

8

1st tower mode Measured: 1.000 Predicted: 1.002 6

2nd tower mode Measured: 6.461 Predicted: 6.294 4

2 Power spectral density (log) density spectral Power Surface elevation (m above LAT)

1.E-14 0 0246810 0 50 100 150 Normalised frequency Time (s) Figure 10: Measured and predicted tower frequencies Figure 11: Breaking wave surface elevation history these storms. Waves of up to 8m height can be seen. A CONCLUSIONS video recording of the south turbine, taken using a camera mounted on the north turbine, shows that waves This paper has described work on the ongoing OWTES of this height are indeed breaking waves. project. The main items of progress are summarised below: A sequence of extreme waves of the same period and height were modelled in Bladed using Dean’s stream • A comprehensive monitoring system has been function theory. The predicted overturning moment at the installed on one of the V66 turbines at Blyth. This sea bed is compared qualitatively with the measured includes instruments to measure structural loading bending moment in Figure 12. Both the predicted and and properties of the sea-state and wind. measured results show high loads at the instant of wave • A twelve-month monitoring programme is currently impact, followed by oscillation of the turbine support underway. Databases of ‘summary’ and ‘campaign’ structure at its natural frequency until the impact of the datasets, which characterise both normal and next wave. Following calibration of the strain gauges, a extreme loading environments, are being compiled. quantitative comparison of the results will allow the • Numerical models of the V66 turbine have been values of hydrodynamic drag coefficient (Cd) and inertia created using the Bladed, DUWECS and DHAT coefficient (Cm) to be verified. It will also be possible to codes. Comparisons of measured and predicted investigate the component of the breaking wave loading structural frequencies have shown the structural due to slam effects. components of the models to be accurate. • The database of measurements will be used to verify 2.9 and enhance computer models and design standards 2.8 for offshore wind turbines. Preliminary modelling of breaking wave loading has resulted in good 2.7 qualitative agreement with measurements. 2.6

2.5

ACKNOWLEDGEMENTS Measurement (mV) 2.4 The partners of the OWTES project wish to acknowledge the financial support of the European Commission (Joule 0 102030405060 project JOR3-CT98-0284), the UK Department of Trade Time (s) and Industry and Novem, The Netherlands Agency for 6 Energy and the Environment. 4 2 REFERENCES 0

[1] Bossanyi, E.A., “Bladed for Windows: User -2

Manual”, GH report 282/BR/101, January 2002. (MNm) Prediction -4 [2] Zaaijer, M.B., “DUWECS Reference Guide”, -6 Institute for Wind Energy, Delft University of Technology, 1999. 0 102030405060 [3] Argyriadis, K., “DHAT User Manual”, internal Time (s) GL WindEnergie report, 2002. Figure 12: Measured and predicted breaking wave loads