PAPER Interdisciplinary Aspects of Offshore Structures

AUTHOR ABSTRACT Peter W. Marshall While some institutions treat Ocean as a single discipline, much of the MHP progress in this area has been brought about by the interdisciplinary collaboration of Chair, MTS Offshore Structures experts in different areas, such as: Committee Mineral resources Ocean energy Offshore economic potential Structural Engineering Remotely operated vehicles Marine law & policy n a broad sense, like architecture, struc- Dynamic positioning Marine education tural engineering may be defined as the art Moorings Marine materials Seafloor engineering Iand science of conceiving, integrating, analyz- Physical / meteorology ing, and successfully executing the physical base of an offshore project in a cost-effective Large offshore platforms are usually designed by teams of engineers. Although the lead engineer often may be a , many elements of the other technologies are manner. However, the details depend on the particular type of structure, as dictated by site involved. This paper is an update to earlier summaries by Marshall (1980, 1993), but retains and mission. many of the pre-Internet classic references. Papers from the Offshore Technology Confer- ence are listed separately, and cited by OTC number. Fixed Steel Platforms In the 1850s, tubular wrought iron struc- Today there are 4,000 such structures in the or grouting to the annular space between pile tures were built several miles offshore sup- Gulf of Mexico (less the 60 or so that failed or and jacket. Vertical and overturning loads on porting lighthouses to warn of reefs along the were seriously damaged in hurricane Katrina), the structure are resisted by axial capacity of Florida Keys. However, offshore in water depths up to 1340 ft (400 m). Off- the piling. Lateral and torsional loads at the as we know it today began in 1947 with the shore drilling platforms have also been built base of the jacket are carried into the soil via installation of the first steel template-type struc- off Southern California and Alaska. Interna- shear and bending in the piles. Skirt piles are ture in the Gulf of Mexico, for drilling off tionally, they can be found in every ocean and sometimes added at the base of the platform Louisiana in 20 ft (6 m) of water (Lee, 1968). offshore from every continent except Antarc- to increase the foundation capacity. These are tica. After thirty years (i.e. by 1977) fixed plat- driven through and grouted into guides or Figure 1 form technology had matured in the sense sleeves which do not reach all the way back to 12-pile platform for 40 m water depth (1964) that much of the pioneering had been done, the surface. and the pattern was fairly well set for what are A superstructure, or deck section, is set on today’s routine structures. These structures top to complete the structure. It carries the func- consist of three major elements: jacket, foun- tional loads for which the structure was built, dation, and deck. See Figure 1. keeping men and equipment out of reach of A welded tubular steel jacket extends from wave action. Conventional well drilling and the seafloor to slightly above the waterline. production operations are carried out through Hollow legs provide a guide for driving piles; rigid conductor pipes, which are driven into these are braced by a space frame that resists the seafloor and extend up to deck level, whilst lateral loads imposed by nature. The typical being laterally supported by the jacket. jacket is prefabricated onshore in one piece, In recent years, fixed platform technology carried offshore by barge, launched at sea, and has been refined and extended to include set on bottom by ballasting, assisted by a sea- problems of dynamic amplification, fatigue, going crane or derrick barge. and seafloor instability, as well as earthquake The platform foundation is established and oceanographic loadings of unprecedented by driving tubular steel piling through the severity. The present state of practice is codi- jacket legs to a penetration of 100 ft - 500 ft fied in API RP2A (2000). A draft interna- (30 m - 150 m) into the seafloor. Pilings are tional standard for fixed steel structures, ISO attached to the jacket by welding above water CD 19902 (2002), is also in preparation. Brief

Fall 2005 Volume 39, Number 3 99 reviews of the fundamentals of wave kine- The crest can be as much as 96% of the wave ponents normal to the member axis are used matics, hydrodynamic , and tubular height, which is limited by breaking to 78% in Morison’s equation. For template structures structures follow. of the storm water depth (including ). with widely spaced slender members, it is gen- erally assumed that the presence of the struc- Wave Kinematics Hydrodynamic Loads ture does not modify the wave kinematics. In the deep water Airy theory of waves, forces on a body in the water For more massive structures, e.g. concrete grav- motions at the undulating water surface are are a result of the vertical gradient ity-based platforms and compliant towers with described as simple sine and cosine functions. acting on its surface, although equivalent re- buoyancy tanks, it becomes necessary to ac- Water particles travel in circular orbits, rotat- sults can be obtained from consideration of count for axial pressure forces acting on the ing in the direction of wave travel, with the the volume and of the displaced wa- ends of large submerged elements. For lateral magnitude of and motion decaying ter. When a body, e.g. cylindrical jacket leg or forces, diffraction theory may be used to de- with depth as a simple exponential decay func- piling, is subjected to a horizontal pressure rive modified values of Cm, where the ratio of tion. Horizontal velocity peaks at the wave gradient, lateral forces, analogous to buoy- member diameter to wavelength exceeds 1/3. crest. Forward horizontal acceleration peaks at ancy, result; furthermore, since the body par- For very massive structures, diffraction theory 90° (1/4 wavelength) ahead of the wave crest. tially blocks the flow (lateral acceleration) of is also essential in accounting for modifica- Vertical velocity and acceleration have similar the surrounding water, an ‘added mass’ ef- tions to the wave field (Garrison, 1974). For expressions, except that signs, sines, and co- fect creates an additional in phase with structures which undergo significant dynamic sines are switched. Vertical velocity also peaks the pressure gradient and water particle ac- motion, a modified form of Morison’s equa- (upward) 90° ahead of the crest, while verti- celeration. A turbulent wake behind the body tion is used. Relative velocity (water particle cal acceleration peaks (downward) under the creates a drag force which is proportional to velocity minus structure velocity) is used in crest (Weigel, 1964). velocity-squared. the drag force term. The inertial wave force This two-dimensional linear regular wave It has been empirically observed (Morison term is unchanged, but ‘added mass’ is in- theory has some serious limitations. Random et al., 1950; Weigel, Beebe & Moon, 1957) cluded with the structure’s inertial mass (us- waves in the real sea are short-crested and that reasonable results are obtained by super- ing Cm-1 for the added mass coefficient). change form as they propagate, being the re- imposing these effects, using the MOJB sult of many wavelet frequencies traveling at (Morison, O’Brien, Johnson & Schaff) equa- Tubular Structures different speeds and in different directions. tion to compute the lateral force per unit length The art and science of welded steel tu-

In shallow water of depth less than half a of vertical pile, where Cd and Cm are empirical bular space frame structures has grown up wave-length, the celerity is reduced from its drag force and inertial force coefficients, re- with the offshore platform industry, and is deepwater value, hyperbolic sine and cosine spectively. These depend on surface rough- codified in AWS D1.1, as further described expressions appear, and the water particles ness (e.g. the presence or absence of marine by Marshall (1992). travel in elliptical orbits. Since the period of growth), Reynolds number, the ratio of mo- Design guides for tubular members have incoming waves stays the same, wavelength is tion amplitude to pile diameter, and whether been issued by Sherman (1976), CRC (1976), likewise reduced; this causes refraction of the or not the oscillating flow sweeps the wake Chen (1985), and AISC (1997). Large diam- shoaling waves. Near the seafloor, vertical mo- back across the member. They are also used as eter, thin wall tubes are efficient for axial loads tions are suppressed, increasing pressures and calibration coefficients, so that the wave kine- and bending, but must be checked for local the horizontal motions. matics theory being used yields the correct buckling (Ostapenko, OTC 3086). Research For waves whose height exceeds 2% of forces. Full scale wave and force measuring has confirmed the ultimate strength of axially the wavelength, the water surface is no longer programs in the ocean are described in loaded columns (Galambos, OTC 2203; sinusoidal; the wave crests are steeper (up to Bretchsneider (1950), Reid & Bretchsneider Chen, OTC 2683) and of beam-columns 68% of the waveheight in deep water), and (1953), Evans (OTC 1005), and Haring (Sherman et al., 1976; Matlock, OTC 2953). the troughs are flatter. Higher order wave theo- (1979). The data show a lot of scatter. Gener- At deep submergence, tubes are subject to ries, e.g. Stokes 3rd, Stokes 5th, or stream func- ally accepted design values for use with regu- hydrostatic collapse (Kinra, OTC 2689; tion (Dean, 1972) give more accurate solu- lar storm waves acting on template type off- Miller, 1981). Earthquake response benefits tions and a better fit to the boundary shore structures range from 0.5 to 0.8 for Cd from inelastic behavior of tubular members, conditions. The limiting steepness H/L for and 1.3 to 2.0 for Cm. Various kinematics with early studies reviewed in Marshall (1978). deep-water waves is 1/7. models for random waves, and the corre- In the early days offshore, the inadequate For large waves in very shallow water (less sponding force coefficients, are discussed in static strength of welded tubular connections than 1 or 2% of the deepwater wavelength), Forristall (1978) and Marshall & Inglis (1986). was responsible for a number of structural fail- solitary non-periodic wave theory gives a con- For inclined and horizontal members, the ures (Marshall et al., OTC 1043). These oc- venient and accurate description of short steep total wave particle velocities and accelerations curred during severe hurricanes, when the wells crests rising above flat troughs of large extent. are first computed, and then the vector com- were shut in and the personnel evacuated. For

100 Marine Technology Society Journal simple joints, punching shear design solved onds. They have been used for oilfield devel- oil fields, and considered for floating LNG the problem (Marshall, 1974 & 1977). De- opment in water depths of 300 m to 500 m, (liquefied natural gas) processing, offloading, sign formulas can also be expressed in limit with special articulations to reduce their sway and receiving facilities. As a result of regula- load format (Yura & Zettlemoyer, OTC mode stiffness, and have been extensively stud- tions against the flaring of gas, FPSOs in U.S. 3690), and extended to more complex joints ied for deeper water. Compliant (i.e. resilient) waters would be limited to use within the (Marshall, 1986). More recent extensions to restoring forces are provided by buoyancy, range of gas pipelines, even though the oil the data base using inelastic finite element guylines, or flex-piles which have reduced axial may be shipped by shuttle tanker. methods, has led to updated criteria for the stiffness by virtue of extended free length above Specialized steel-reinforced flexible flow- static strength of tubular connections. the mudline. Around 600 m water depth, in- lines accommodate the wave-induced motion (Pecknold et al., OTC 17310). Leonardo creasing the hydrodynamic added mass elimi- and are used to transfer fluids to and from the DaVinci’s theory of bending is applicable to nates the need for an articulation. In 900 m floating hull. In mild wave environments, e.g. grouted tubular T connections, which are very water depth, a tall slender jacket with a large West Africa and Java Sea, large hulls can be strong in compression, but weaker (and duc- topside load is sufficiently compliant without moored in fixed orientation, permitting the tile) in tension. either articulation or mass trap. Beyond this flexible flow-lines to be connected directly to Repeated overloads during a severe storm, water depth, construction costs and the whip- the hull. In more severe environments, very or cumulative damage by wave cycling over ping mode become insurmountable problems. large deep-draft hulls can moderate the wave- the years, can cause fatigue failure in the welded Although such designs may be on the induced motions, provided the hull is allowed tubular connections of offshore platforms. De- cutting edge, a principal advantage of compli- to pivot into a favorable orientation. Here, the sign for fatigue of welded details is traditionally ant towers is that they permit conventional flow-lines must be connected to a turret, which done in terms of the nominal stress classifica- drilling and production operations from a maintains fixed orientation globally while ro- tion method (Gurney, OTC 1907; Munse, passive structure that just sits there. If oil field tating within the hull. Swivel connections, OTC 3300). Because branched connections development requires a large number of wells stacked on a common axis with the turret, of large-diameter thin-wall tubes can be sub- at one location, with frequent heavy interven- complete the flow paths. ject to high stress factors, depend- tion during production, the savings in life- ing on geometry, their design is done in terms time well costs could offset a lot of structural Semi-submersibles of the localized shell bending and membrane steel (at the old prices). While a submerged lower hull provides stresses, or hot spot strain range measured adja- most of the buoyancy, substantial columns cent to the joining weld (Toprac, OTC 1062). Floating structures rise through the water surface to provide sta- Experimental databases (Rodabaugh, 1977; Even though they operate at a fixed loca- bility and support the working deck. Reduced UKOSRP, 1978) provided and confirmed the tion, floating offshore structures depend upon volumes near the waterline reduce wave forces empirical basis for the corresponding hot spot the unfailing maintenance of buoyancy to keep and the associated motions. A lateral mooring S-N Curve X in AWS D1.1. A more recent afloat, and upon moorings to keep them on system keeps the hull on location, within 10% database (Mohr et al., 1995) has led to an location. Vessels which operate in transit, or of water depth and in a fixed orientation. update of the fatigue criteria for tubular con- which temporarily keep location by dynamic For a given displacement, structural steel nections (Marshall et al., OTC 17295). The positioning, are beyond the scope of this paper. weight is higher, while topside payload capac- hot spot method is now being extended to Because these facilities are permanently ity and onboard storage volumes are less than more complex and non-tubular welded con- moored at one location, and carry similar con- for conventional hulls. However the lower nections (Marshall & Wardenier, 2005). sequences of failure, their hull structure and motions are more operations-friendly, and the moorings are designed to resist the same sever- fixed orientation is advantageous when the Compliant towers ity of met-ocean conditions as for fixed plat- number of wells and other flow-paths become As the water depth increases, the sway forms at the same site. However, dynamics too great for a turret (i.e. more than one export mode natural periods of fixed platforms in- and motions are more important to their de- riser and roughly a dozen flow lines). crease. For periods greater than 3 to 5 seconds, sign than static applied forces. Wells may be drilled through vertical ma- they may experience problems with dynamic rine risers, steel pipes held up by tensioners, amplification of wave forces, and fatigue from Conventional Surface Hulls which must accommodate vertical stroke up the resulting cyclic stresses. While surface hulls suffer more wave-in- to 15 m and provide rotational degrees of free- Compliant towers avoid the wave reso- duced motions than advanced types of float- dom. Typically, the wells are drilled at several nance problem and extend the feasible water ing structures, their large payload capacity and nearby seafloor sites by a mobile offshore drill- depth range of fixed platform architecture by deck space (relative to structural steel weight) ing unit, and tied back to the semi-submers- arrangements that give them sway periods make them attractive for many applications. ible, which serves as a processing hub. Flow- greater than 22 to 30 seconds, with higher They have been used as FPSOs (floating pro- paths from seafloor wells, or for product export, whipping mode periods less than about 5 sec- duction, storage, and offloading) on offshore are provided through vertical risers, flexible flow-

Fall 2005 Volume 39, Number 3 101 lines, or steel catenary risers. The latter are typi- Spars are compliant in all six degrees of Having a hull displacement in excess of its cally used in water depths greater than 600 m, freedom, and can be used in very deep water, weight makes for lively lateral accelerations, and supported at a fixed point on the lower up to 4000 m. Although resonance with waves more like a spar than a semi-submersible. hull, with an elastomeric ball-joint to accom- is avoided, vortex-induced lateral motions in Proprietary variants of the TLP have a modate small relative rotations due to pitch, high events can and do occur. The single central column (or column cluster), with roll, and riser motions. Flexible umbilicals with single large column provides ample space in- massive cantilevered arms at keel level extend- multiple electrical or hydraulic conductors pro- side for air-cans to support the weight of well ing out to the corner tendon locations vide data, control, and chemical injection func- risers, but also exacerbates wave impact and (Leverette, OTC 10855). tions to the seafloor facilities. green water run-up problems. Semi-submersibles have been used for more A proprietary variant uses a tubular space Concrete Structures than forty years in water depths of 60 m to frame truss between the hard tank and bal- Large-volume marine structures are also 3000 m. They are compliant in all six degrees last, instead of a soft tank. Oilfield adapta- built using steel-reinforced and/or prestrressed of freedom, and do not have a resonant water tion of the spar was patented by Horton concrete shells. Cylindrical and spherical shell depth problem. However, their slender hull (1987), who earlier had invented (OTC forms in compression are particularly efficient form and complex intersections require careful 1263), optimized (OTC 1553), and proto- in resisting hydrostatic pressure, and create large structural design for strength and fatigue. type tested the TLP (below). interior volumes for buoyancy and oil storage. Vertically moored spars, compliant in five Offshore applications are described in ISO CD Spars degrees of freedom and with well risers supple- 19903 (2002). The first use of a large manned spar was menting the vertical stiffness, have been stud- Since 1944, concrete gravity-based fixed the U.S. Navy’s “Flip” vessel, built in the ied and found to be feasible for water depths structures have been installed in 6 m to 300 1960s to provide a nearly stationary floating up to 1750 m. “Barefoot” riser-in-caisson con- m water depths, in the and many platform for acoustical measurements in rough figurations are robust in unstable seafloor con- other parts of the world. Typically, an initial . Shell’s “Brent” spar was built in the 1970s ditions. High tensions due to the mooring raft is built in a graving dock, completed by to provide oil storage and offloading func- function exacerbate riser bending moments slip-forming in deeper water, towed offshore tions for a large North Sea oilfield in 160 m at the keel due to pitching motion of the spar, using its own buoyancy, and sunk at the in- water depth, where the wells had been drilled but these can be relieved by special moment- tended site. Storm overturning forces are re- from three massive fixed platforms. Stability is reducing devices (Marshall, OTC 7528). sisted by the platform’s massive weight (in- achieved by having “hard tank” buoyancy at cluding ballast) resting as a spread footing on the top end and heavy solid ballast at the keel. Tension Leg Platforms the seafloor. Perimeter skirts penetrate the sea- The intervening “soft tank” provided oil-over These platforms generally have a hull- floor to resist sliding, and any spaces under water storage, hopefully to be balanced with form similar to semi-submersibles, but are the raft are filled with cement grout to pro- the external hydrostatic pressure. After twenty moored vertically by pipe tendons at each vide intimate contact with the irregular seaf- years service, this spar achieved considerable corner. Tension created by excess hull buoy- loor. While gravity foundations are most ef- notoriety when Greenpeace opposed its ini- ancy gives these “tension legs” an inverted fective on strong seafloor soils, softer soils can tial decommissioning plan. pendulum effect to keep the platform on be accommodated by making the raft larger, Very large spars, with drafts of 200 m, be- location. Elastomeric ball joints accommo- extending the skirt to greater depths where gan to be used as the sole platform for oil field date angular motions at the ends of the ten- stronger soils are mobilized, and using short- development in the 1990s. They can accom- dons. TLPs are compliant in surge, sway, and term suction instead of ballast to resist dy- modate drilling, production, storage and yaw, but relatively stiff in pitch, roll, and namic uplift forces. offloading functions, with multiple wells typi- heave, reducing the stroke requirement for Concrete hulls have been proposed and/ cally clustered at the platform site. Most motions vertical risers to almost nil. This makes TLPs or used offshore for processing barges, very are less than for semi-submersibles, with vertical attractive for drilling large numbers of wells large semisubmersibles, and spars (Marshall riser stroke requirements of only about 6 m, per- at the platform site. Riser and tendon angles & Chabot, OTC 7258). Their larger weight mitting the use of “dry tree” topside wellheads. (ratio of storm wave motion to water depth) penalizes usage for deep-water TLPs. Near the keel, where moorings and other types become untenable for water depths less than of risers attach, motions are much less. Taut lat- 140 m. The stiff modes create wave reso- Structural Reliability eral moorings can be used to reduce lateral excur- nance problems which make applications in A computed safety factor represents the sions. However, pitch-induced lateral accelera- water depths greater than 1200 m turn into ratio of the nominal strength to the assumed tions at topside deck levels can exceed 0.15g. a high-feedback exercise in futility: stiffer ten- load. However, structural failure occurs when Although one spar lost its drilling derrick over- dons require a more massive hull to support the actual load exceeds the actual strength. board during hurricane Ivan, its measured per- their weight. Lighter carbon-fiber tendons Both are random variables, including bias as formance was otherwise within design limits. may eventually extend the water depth range. well as uncertainty.

102 Marine Technology Society Journal The derivation of statistics for extreme met- The resulting estimates of annual average available. The world’s largest barge has trans- ocean design criteria is described in the section risk rate are as follows: Storm risks of 0.1-1% ported and launched a 1350 ft (400 m) jacket on and meteorology. annually (for structures just meeting API crite- weighing 50000 t (Digre & Marshall, OTC These lateral forces may be normalized on the ria) are added to 0.5% annual risk of loss due to 6050), and there are several which can handle API reference level design criteria. The uncer- fire and collision. This range appears to bracket 700 ft (210 m) jackets weighing 25000 t. tainty here is mostly that due to random storm the historical loss rate of 0.3-0.6% for storms, Launching requires two parallel jacket legs, occur-rence. and falls on the pessimistic side of estimates by although these need not extend the full Estimates of the actual failure load for typi- other investigators who include less engineer- length of the structure. Fabrication by the cal template type offshore platforms may also ing uncertainty. Bayesian calibration on plat- bent roll-up method is facilitated by having be expressed in relation to the design lateral form survival experience also indicates that the flat jacket faces. Thus, we are not likely to see load. Inelastic pushover analysis (Marshall, true risks are near the lower end of the indicated many offshore structures having the configu- 1982) and a ductile-redundant probabilistic range. See Figure 2. Overstating the epistemic ration of the Eiffel tower, however attractive model (Marshall & Bea, 1976) yield median uncertainties overstates the risk, leading to crude that might seem for in-place considerations. failure loads of about twice the design load, suggestions on how to treat the problem. and come closest to being consistent with the Similarly, the lifetime probability of fail- Fabrication and Welding survival experience of older platforms. The ure is obtained by comparing the strength The de facto international standard for uncertainty here comes from random varia- distribution with the extreme value distribu- fabrication and welding of tubular space frame tions in material strength and foundation re- tion for loads experienced during the structure’s structures, as used for offshore platform jack- sistance, and from epistemic uncertainties in- service life. For an N-year life, the lifetime prob- ets, is the AWS Structural Welding Code, D1.1 herent in the calibration of engineering models ability of non-exceedance of a given load level (2004). The tubular work is efficiently per- for structural loading and response. is the annual probability raised to the Nth formed by specialized contractors, and this The annual probability of failure, or risk power. Values of the safety index, b, com- can become a painful learning experience for rate, may be obtained by convolution of these puted on this basis, range from 2 to 3, for the uninitiated. Shop drawings and mold lofts two distributions (Marshall, 1967), or by us- structures designed for lateral loads having a have largely given way to computer modeling ing the lognormal safety format. The prob- l00-year return period (Moses, OTC 3027). and automated cutting systems for plates and ability of failure, PrF, as a function of the safety In the Gulf of Mexico, hurricanes are in- tubes. Large tubes are fabricated as “cans” by index, β, is given by standard normal prob- frequent events, for which there is sufficient cold bending plate and making automated ability tables: warning that personnel are evacuated and the welds in a pipe mill, and their diameter and wells secured against leakage in the event of thickness are easily adapted to the needs of the β 0 1 2 3 4 platform collapse. For periods while the plat- structural design. Cans or standard pipe

PrF 50% 15% 2.3% 0.1% <0.01% form is manned and the wells are flowing, the lengths are assembled into longer structural extreme winter storm produces less than half brace and jacket leg members on brace and the design load. If the structural design of the piling racks. Brace ends are given a saddle- platform provides redundant parallel load shaped beveled cope for their critical connec- paths, it can suffer considerable degradation, tion welds to the jacket leg joint can. Shop- Figure 2 even loss of members, and still be considered prefabricated nodes are sometimes used to Platform in 100 m water depth after hurricane safe. Such structures may be termed “fail-safe achieve pressure-vessel quality levels. Camille in 1969. Damage to deck facilities indi- while manned” (Marshall, 1979). cated 80-ft waves at the site. The jacket and foun- Loadout, Transportation & Launch dation were undamaged. Constructability Typically, jackets undergo a complex jour- Platform configuration must anticipate ney during their construction: loadout, trans- the methods of construction, and resulting port and launch. Skids, backed up by trusses, limitations imposed. With today’s offshore are built into the jacket for this purpose. construction equipment, these limitations For loadout, the completed jacket, lying have been greatly expanded, although the on its side, is analyzed for the succession of largest equipment may not always be avail- support cases as it travels from the fabricat- able in all areas. In normally consolidated clay, ing yard, across a gap, and onto the launch piling up to 84 in (2.1 m) diameter can be barge. Aside from the obvious aspects of grav- driven 400 ft (120 m) into the seafloor, ity space frame analysis, things to be consid- achieving capacities as high as 8500 t. Lifts ered include: crushing of the skid timbers; of 10000 t are possible, although derrick friction and traction forces; flexibility of the barges with 1500-t capacity are more widely ground ways and barge; and rise and fall of

Fall 2005 Volume 39, Number 3 103 the barge with changes in water level, deck Behavior of the structure after it is in the have been developed. Existing installations in load, and ballast condition. water, its upending and setting on bottom, water depths less than 20 m are supported by During transport, the jacket is welded to can be studied by analysis or model test. steel caissons up to 5 m in diameter and 50 mm the barge via temporary tiedown braces. In wall thickness. These are driven into the seaf- the open sea, the barge and jacket undergo loor, often with the aid of internal excavation dynamic motions due to waves. Pitch and Ocean Energy (jetting or drilling) for difficult soils, and are at roll inclinations, accelerations in six degrees The use of offshore platforms for oil & the limit of conventional wisdom for driven of freedom, and hog/sag deflections of the gas development has already been described. piling. There are a number of promising off- barge, can cause significant forces in the Other energy sources for which offshore plat- shore sites in deeper water, ranging from 20 m jacket, skid, and tiedown braces. Several jack- forms may be considered include nuclear, to 80 m in water depth. Soil conditions range ets have been destroyed during storms by OTEC (ocean thermal energy conversion), from soft clay, to hard clay, to dense sand, to these forces, and others have suffered fatigue wind, wave & current, geothermal, and meth- rock, in order of installation difficulty. Finding damage during long ocean tows. Motions ane hydrates. Since the generated power must structural within the constraints of can be analyzed by computer programs us- be exported to market via an undersea electri- cost, dynamics, fatigue, and constructability will ing strip theory hydro-dynamics and ran- cal cable, a stationary site is required. be challenging. dom wave methods widely used by naval . The results are then used to com- Nuclear Waves and Currents pute dynamic forces, which are then trans- In the 1970s, it was proposed to put A number of schemes for extracting en- ferred to a structural analysis program. nuclear power plants offshore, so they would ergy from offshore waves or currents have been When the jacket is launched at sea, it dy- not be in anybody’s back yard. The power proposed, and these all involve structures of namically passes through a succession of posi- plants would be of a standardized design, built some sort to capture the mechanical energy tions in which it is supported by the barge at a central factory, and fully outfitted on a and convert it to useful form. Some utilize the skidways (or by shorter tilt beams at the end of barge for transport to an offshore site pro- rise and fall of the water surface to compress air the ways) and by buoyancy on those parts of tected by a breakwater. The venture folded in an internal chamber, which is used in turn the structure already in the water. The time- after Three Mile Island, and the factory site to drive turbines. Some use the relative mo- step dynamic problem of two bodies can also became Jacksonville’s container port, but the tions in articulations between linked hulls to be solved specialized launch analysis programs, idea of a standardized nuclear design is still drive hydraulic fluid. Some propose to extract including drag and inertial forces on the jack- being pursued. energy directly from the current, e.g. using et, and friction on the skidways. Again, these structures anchored in the Gulf Stream off- forces are transferred to a structural analysis pro- OTEC shore from Miami. All face the daunting prob- gram, at a succession of ‘snapshots’ during the Ocean Thermal Energy Conversion uses lem of first costs that are large compared to launch. The highest load on the launch truss warm surface waters to vaporize the working present value of the revenue stream from pro- typically occurs when the jacket first tips rela- fluid in a thermodynamic power cycle, and duced energy. tive to the barge. The highest local hydro-dy- cool water from below the to con- namic loads occur as the tail end of the jacket dense it. Major structural elements are the cold Geothermal free falls into the water after leaving the barge. water pipe and the floating hull which sup- Geothermal energy (hot water and steam) Hydrostatic pressures, as the jacket dives into ports it. The author’s grandfather (Campbell, has been commercially developed onshore in the water, may also be higher than in service. 1906) described a massive cold water pipe California, Iceland, and elsewhere. Oceanic Jackets may be installed by lifting, instead having near , with articula- crustal spreading zones have such energy in of launching. Deck structure and topside mod- tion at the interface with a conventional float- abundance, but water depth and distance to ules are almost always installed by lifting. This ing hull. Today, spars come to mind as a more market make their economic viability doubt- creates a quite different set of construction load suitable hull form. ful. Hot brine from water zones in played-out cases. Analysis of the transport barge, jacket or offshore oilwells, with a useable delta-T over module, derrick barge, and its lifting gear, as the Wind ten times that of surface waters, might be use- lift is executed in waves, becomes an interesting Wind farms are among the more commer- ful for regasifying LNG at receiving terminals. problem in three-body dynamics (Dekker, OTC cially viable renewable energy sources. Onshore 5819). Fortunately, offshore heavy lift gear is sites raise concerns over spoiled vistas and en- Methane Hydrates quite resilient, having about 3 ft (1 m) of stretch dangered birds. Offshore sites have less of these Seafloor methane hydrates are widespread as it goes from zero to full capacity, and catenary problems, plus the advantage of steadier, stron- in water deeper than 1000 m. They are poten- action of a pyramid sling arrange-ment can ac- ger surface winds in the marine boundary layer. tially a vast resource, exceeding conventional commodate quite a bit of slack without any- Wind turbines up to 104-m diameter, capable hydrocarbon reserves, but they are also relatively thing kinking or coming loose. of generating 3.6 megawatts of electric power, diffuse—roughly one foot of net pay. Commer-

104 Marine Technology Society Journal cial development, if it happens at all, could in- ance on dynamic positioning for the full pro- matics and vessel motions as input. However, volve roving deepwater facilities rather than sta- duction life of an oil or gas field is expensive in when the combined mass of deepwater moor- tionary platforms, similar to those developed for terms of energy usage, and would require con- ings and risers is no longer small compared to mining seafloor manganese nodules. The role of tingency planning for emergency disconnects. the floating platform, a coupled analysis of hydrates as a geohazard would further discour- the entire system should be performed. age the use of platforms at a fixed site. Moorings Vertical tendons are essential for the struc- Seafloor Engineering Remotely Operated Vehicles tural stability of tension leg platforms, and This is a broad subject area, covering sea- ROVs are operated by a technician on the guylines are similarly essential for guyed towers. floor characterization, foundation design, surface support vessel, and provide underwa- For other types of floating offshore structures, geohazards, and seafloor construction. ter vision (TV), depth, and positioning data. lateral moorings serve principally as station-keep- Their manipulator arms can perform almost ing, to maintain position and orientation. Seafloor Characterization human tasks, and they can carry payloads or Conventional moorings with drag anchors Characterizing the seafloor soils at an off- apply modest thrust where it is needed. They may utilize chain, steel cable, or a combina- shore platform site is essential for proper de- are not depth-limited like human divers, and tion of these in catenaries which have a por- sign. This can be a major undertaking, requir- have become essential to offshore platforms tion of their length on the seafloor. Replace- ing significant lead time. Overviews are during construction, and subsequently for able chain sections are often used at the provided in the book by McClelland & Reifel inspection and maintenance. platform interface to deal with angular mo- (1986), and by Focht & Kraft (1976). During fixed platform construction, ROVs tions and contact wear. Chain sections may Site exploration occurs in several stages have been used to operate flooding valves, ob- also be used at the mudline touchdown point (Fugro, OTC 2246). A useful starting point serve jacket set-down, guide pile stabbing, and to give compliance to an otherwise semi-taut is to examine the geological setting (Bernard connect grouting hoses. For mooring and hook- combination system. and LeBlanc, 1965; McClelland, 1966). Geo- ing up floating platforms, ROVs retrieve tag Synthetic fiber mooring lines (polyester physical methods may come next (Doyle, lines, make and break mooring line connec- rope) are neutrally buoyant and have a low OTC 5758). Eventually, a soil boring will be tions, and pump out suction caissons. modulus of elasticity, making them attractive needed, with borehole and wireline sampling Inspection and maintenance tasks include for taut moorings in deep water. They can (Perkins, 1967) supplemented by in situ vane close visual inspection, surface cleaning, mag- accommodate wave frequency vessel motions and cone tests (McClelland, OTC 2626 and netic particle crack detection, flooded mem- by elastic stretching, are stiffer than catenaries 2787). Finally come laboratory tests and in- ber detection, anode potential readings, an- for low frequency and static drift, and do not terpretation (Focht, 1967) which form the ode replacement, and making light structural tax the buoyant load-carrying capacity of the basis for design. attachments with studs. AUVs (autonomous floating platform. Taut moorings reduce the underwater vehicles) are increasingly being footprint of the moored system, reducing the Foundation Design used for survey and inspection tasks. problem of interference with other moorings Lateral storm forces imposed on surface- and seafloor activities. piercing offshore structures impose large lat- Advanced anchor types include self-em- eral and overturning loads on their founda- Dynamic Positioning bedment plate anchors and suction caissons. tions, in addition to the usual vertical loads. Surface vessels and semi-submersibles have The vertical holding capacity of such an- Gravity-based structures resist vertical and been fitted with azimuthing pod and fixed chors permits the use of taut moorings. The overturning loads as spread footings, often with tunnel thrusters, satellite or seafloor acoustic soil mechanics of predicting their behavior vertical skirts to enhance sliding resistance. Pile- positioning systems, and computerized con- during embedment and subsequent service supported offshore platforms impose large trols so that they can maintain a fixed surface loading make an interesting branch of seaf- axial and lateral (shear) loads on the tubular position and orientation in various met-ocean loor engineering. elements of their foundation. Deformation- conditions, depending on the amount of Specialized dynamically positioned vessels induced loads may occur due to mudslides, power available. are used to transport and pre-install deepwater earthquakes, and other geohazards. Temporary positioning, for periods of days moorings, and to connect them to the floating or months, has become routine during con- platform when it arrives. Winches, fairleads, Analysis struction and for well drilling. Systems of high connectors, and other mooring hardware are Laterally loaded piles are analyzed numeri- theoretical reliability have been certified for also vital parts of the business. cally using a discrete element beam-column this service, but experience suggests that they Risers and moorings use similar software model (Focht and McClelland, 1955) with remain vulnerable to human factors and un- for their dynamic analysis. Traditionally, they nonlinear soil support algorithms in the form expected events. Long-term, permanent reli- are analyzed independently, using wave kine- of P-Y curves (Matlock and Reese, 1962).

Fall 2005 Volume 39, Number 3 105 For soft plastic clays, Mattock (OTC foolproof approximation is to attach the pile push this predictive limit. When they do, pile- 1204) developed design correlations by back- to linear springs (no rotational or off-diagonal driving fatigue may become a problem (Doyle, fitting the reactions P required to match the terms) at a depth of Mt/Pt below the mud line, OTC 10826). bending moments in an instrumented test pile, where Mt and Pt are pile moment and shear at In stiff soils, where the desired pile capaci- with the bending strains also being used to the pilehead, respectively. ties cannot be achieved by driving, piles may calculate pile deflections Y. The ultimate lateral be grouted into predrilled holes (Kraft, OTC resistance (stated as a bearing pressure) devel- Axial Pile Design 2081). Where piles are driven with underwa- oped as the pile moves through the soil varies The ultimate axial pile capacity is esti- ter hydraulic hammers, rather than extending from 3c near the soil sur-face (where c is soil mated by summing skin friction along the them to the surface, they are connected to shear strength), to l0c at sufficient depth of length of the pile, plus the contribution of skirt sleeves at the base of the structure by confinement for the soil to deform in a plane end bearing. In soft clays, the ultimate skin grouting (Billington, OTC 3083; perpendicular to the pile. Because offshore friction is the end result of the process of soil Halliburton, OTC 3671). structures are cyclically loaded by waves, the disturbance during pile driving and Axial pile capacity reductions due to the cyclically degraded behavior is used in design. reconsolidation under the influence of lateral interactions of piles in groups have been de- P-Y criteria for laterally loaded piles in stiff confining pressure (Seed and Reese, 1956). scribed by McClelland (1966). clay and in sands have been given by Reese & The following method is heuristically consis- Piles in fixed steel jacket structures are pre- Cox (OTC 2312) and O’Neill et al. (1983), tent with more modern effective stress con- dominantly loaded in compression, whereas respectively. In stiff clay, the cyclic degrada- cepts, and served the Gulf of Mexico offshore those in TLPs and taut moorings are loaded in tion is even more pronounced than for soft industry well for over 25 years (McClelland, tension. The latter require special consideration clay. P-Y criteria for sand (Reese and Cox, 1963): In normally -or under-consolidated of their greater uncertainty in pullout behav- OTC 2079-2080) and for pile groups clays, the skin friction is taken as equal to the ior and degradation under repeated loading (Matlock, 1965; Poulos, 1971) have also been undrained shear strength, or cohesion, c. In (O’Neill, OTC 7796). developed. over-consolidated clays, the skin friction is taken Axially loaded piles may be analyzed nu- as the larger of 1000 psf (50 kPa) or that which Other considerations merically using non-linear T-Z curves to rep- would be developed by the clay reconsolidat- Dynamic and cyclic effects have been resent their skin friction (Matlock, OTC ing under the present overburden. studied by Idriss (OTC 2355), Sangrey 2186), although the plastic limit state is com- The foregoing method is comparable to (OTC 2944), and others. Given unlimited monly used where the soil behavior does not today’s methods. A paradox in using the ef- space, one could also discuss such interest- degrade significantly. fective stress method is that the lateral con- ing foundation design subjects as mud mats, Several methods of achieving compatibil- fining pressure available for reconsolidation skirt plates, mudline scour, and debris accu- ity at the pile-to-jacket interface are used. If after pile driving is not always the same. Pre- mulation. one has inexpensive access to a super-com- existing lateral earth pressure can be more puter, detailed models of each entire pile, to- than overburden, as in the case of glacially Geohazards gether with non-linear elements representing consolidated North Sea clays—or less in the Geohazards such as earthquakes and the soil P-Y behavior, can be included with case of desiccated soils exposed by a low mudslides have been extensively studied, al- the space frame analysis (Bea et al, OTC seastand. This is further modified by the dis- though memories of the latter tend to be- 2749). Alternatively, the entire jacket and its placement created by pile insertion, which come obscured by the passage of time be- loads can be reduced to a linear substructure, for pipe piles depends on whether the pile tween triggering events like Camille and with boundary nodes at points of attachment tip has become plugged or cuts into the soil Ivan. Seafloor methane hydrates have been to the piles, and compatibility achieved by ‘cookie-cutter’ style. Today’s design capaci- penetrated by deepwater wells for years, only iteratively accessing the non-linear for ties exceed that which can be confirmed di- recently receiving attention as a to each pile (Matlock, OTC 1699). One of the rectly in pile tests (McClelland, 1967). platform foundations. earliest methods is to analyze the jacket and A challenging aspect of pile design pre- piles separately, using spring intercept matri- dicting the depth to which they can be driven, Earthquakes ces to describe the tangent modulus behavior and thus their calculated capacity. Computa- The general origin and nature of earth- of the pile at the anticipated design point, tional methods for pile driveability have been quakes, and structural response to them, repeating the cycle manually until satisfactory developed by Lowry (OTC 1055 & 1202); has been described by Weigel’s extensive convergence is achieved (Meith & Gooch, McClelland (OTC 1600); and Audibert text (1970). Faults exist under the ocean 1966); great care is required in getting the off- (OTC 3273). With today’s large pile driving as well as on land, but must be detected diagonal terms right, so that the all-important hammers, it is theoretically possible to achieve geophysically, as the usual geological field relationship between pilehead shear (lateral ultimate axial capacities of 13,000 tonnes in methods and reliance on historical records deflection) and moment is preserved. A more soft clay. However, designers are reluctant to do not apply.

106 Marine Technology Society Journal The onshore characteristics of earthquakes Hydrates system. The riser attaches to this wellhead. are modified in several ways when it comes to Gas hydrates are ice-like lattice structures Temporary wet tree risers may have an elas- offshore applications. For vertical motions, the containing a mixture of water and gas, typi- tomeric flex joint at this point. Permanent free surface effect is at the waterline rather than cally methane. Gas hydrates are stable under dry tree risers each have a steel or titanium the mudline, reducing vertical amplitudes at high pressures and at moderately low tem- tapered stress joint which allows small an- the structure’s foundation (Smith, 1996 & peratures, and are found in the pore space of gular rotation while preserving the long- 1997). On the other hand, offshore struc- soils underneath the seafloor and in perma- term integrity of the flow paths. A back- tures are often sited on deep deposits of ma- frost regions (Maslin, 2004). Gas hydrates are span stress joint may be used to increase the rine soils, which can greatly amplify lateral likely to constitute a serious geohazard due to range of angular motion at either end of the earthquake motions. Also, the water in which the adverse effect of warming on the stability riser (Marshall, OTC 7258). the structure is immersed provides an ‘added of gas hydrate deposits. Local consequences Seafloor facilities serve as tie-backs mass’ effect, which increases inertial forces on could include swelling or fracturing of the to nearby offshore platforms, or more re- the moving structure. soil, mud volcanoes, and gas boils. So far, these mote production hubs. In addition to wet API-recommended design criteria are effects have been brief and fairly mild at indi- tree wellheads, these may include control based on modal dynamic analysis and earth- vidual exploration wells, but could be more and chemical injection manifolds for flow quake response spectra, rather than the quasi- severe due to the continued presence of hot assurance, pipeline termination and junc- static approach taken in many onshore design fluids in a cluster of producing wells. Similar tion structures, and even limited separation codes. For the most earthquake-prone regions, to Mt. St. Helens, phase change in the pore and pumping facilities. the elastic design spectra feature peak ground space is suspected to be the cause of the mas- accelerations of 0.4 g, amplified to 1.0 g for sive Storegga seafloor slide, offshore Norway, structural modes having natural periods of and a resulting tsunami in coastal Scotland. Mineral Resources 0.125 to 0.75 s. For structures founded in In addition to oil and gas, sulfur has been deep strong alluvium, the peak spectral veloc- Seafloor Construction extracted from offshore platforms using the ity is nearly 1.2m/s (4ft/s), and the spectral Seafloor construction activities are increas- Frasch hot water solution process, with the displacement ranges up to 0.9 m (3 ft) for ingly being used as an integral part of offshore heat source also being used to co-generate elec- structures having a 5-s natural period. Fur- platform installation. Pre-driven pilings, driven tric power. Seafloor subsidence becomes a sig- thermore, offshore structures are required to with followers or underwater hammers and nificant issue for platform design. In principle, demonstrate the ability to withstand rare terminating near the mudline, have been used any mineral which could be profitably recov- events producing twice the foregoing motions as temporary or permanent foundations for ered through wells using a solution process with-out collapsing, by either performing an structures which are set thereon (Riles, 1987; could be done this way. inelastic analysis or by following prescriptive Edel et al., OTC 10919). Tension pile clus- Coal mines have been extended under rules which assure system and ters in a seafloor template, or individual high- the sea from onshore mine-heads. In prin- element ductility such that load redistribu- capacity piles are used to anchor the bottom ciple, offshore gravity-based structures tion and inelastic deformation can occur safely. end of pipe tendons for TLPs, with the ten- could also be used at the mine-head, but don bottom termination incorporating an elas- the economics of underground mining have Mudslides tomeric ball joint and a remote latching/un- already become tenuous enough without Weak, unconsolidated seafloor sediments are latching device similar to the clicker on a Parker the added personnel and logistics costs of subject to mass movements under the action of ball-point pen. Suction caissons are pre-in- working offshore. storm waves or earthquakes, even on relatively stalled to anchor vertical TLP tendons and the Other minerals may exploited offshore flat slopes. Several off-shore structures have been lateral mooring systems of semi-submersibles by roving suction dredges, the seaworthi- destroyed by this phenomenon (Sterling OTC and spars. ness and water depth capabilities of which 1898). It is now well enough understood (Bea, The bottom structure of the vertical ris- can be improved by applying offshore tech- OTC 1899) that specialized platforms have ers becomes an indispensable adjunct to the nology. Sand and gravel are the second big- been successfully built in these unstable areas, dry tree wells on TLPs and spars, and on gest resource that we are producing offshore, and this “obscure” technology has its cadre of the wet tree wells drilled from semi- in both state and federal waters. Gold has lifetime devotees. Both flow and deep- submersibles and conventional surface hulls. been commercially recovered offshore Alaska. seated mass movement models have been pro- Typically, the subsea wellhead is supported Howard Hughes’ Glomar Explorer was built posed, with the platforms which fail being the founded on a spud pipe driven or jetted to recover scrap iron from the depths of the ones that were only designed for shallow turbid- into the seafloor, and by the first string of Pacific. Billiton built and tested an equally ity flows (including one in hurricane Ivan). Shut- cemented conductor casing. These supports spectacular deepwater dredging system to in pipelines damaged by mudslides caused great deserve the same attention to structural recover manganese nodules, but it was not economic disruption after hurricane Ivan. loads and fatigue as the rest of the platform commercialized.

Fall 2005 Volume 39, Number 3 107 Offshore Economic Potential construction; deferred operating, inspection, designer has done his job properly, the risk of In addition to oil & gas drilling structures, and maintenance costs, reduced to present structural failure by service of storm loads be- offshore platforms have been built for light value; and the cumulative risk costs related comes small in comparison to the risks of fire, stations, oceanographic research, undersea test- to undesirable events or failures. PrF is the explosion, and collision. For these latter risks, ing, electronic surveillance, power plants, sul- corresponding probability or risk of failure; human factors can be more important than fur mining, pipeline pump and compressor and CF is the cost (inclusive of all disadvan- technical design factors. stations, supertanker terminals, human accom- tages) which follows event F, also reduced to modation, casinos, and even pirate broadcast- present value. Sustainability ing. Infrastructure potential, e.g. as transpor- In responsible calculations, all costs Offshore oil & gas projects often run into tation and processing hubs, is becoming should be internalized, regardless of who environmental opposition because their very recognized as a strong economic incentive to bears them. That is, the expected cost in- purpose is seen as contributing to global warm- build, enlarge, and preserve offshore platforms, cludes oil spill clean up expenses, possible ing, and offends the notion of renewable re- beyond the demands of their initial function human casualties, liability for damages, and sources rather than depletion. Yet, the world is at the site (Marshall, OTC 7867). indirect consequences (e.g. political back- so dependent on hydrocarbons in the short lash), as well as the loss of investment, cash term, that blocking their utilization invites the flow, and tax revenues. Total failure cost can unthinkable: economic collapse, wars, and Marine Law and Policy range from 2 to 5 times the initial capital famine. Fixed offshore platforms are so ben- Safety Considerations investment, and today’s mega-projects carry eficial as artificial reefs, that ways are sought to Risk is a complex subject, having human billion-dollar risks. prolong this function long after hydrocarbon emotional and political elements, as well as From the application of the safety opti- production has ceased. technical and economic ones (Adams, 1995). mization approach, present value dollar risks, The philosophy of making rational trade-offs taken over the platform lifetime, are com- between cost and risk in selecting design crite- bined with initial and maintenance costs, with Marine Education ria permits optimal allocation of resources in the total cost plotted as a function of safety Nurturing the next generation of engi- offshore development projects, provided the factor or the normalized level of design forces. neers to design offshore structures in the fu- technical and economic considerations are for- Epistemic uncertainties may be represented ture has several approaches. Where this is seen mulated so as to consider human and social as a shaded band in such a plot. Increasing as an interdisciplinary field, the hiring prefer- consequences. The rationale of formal reli- the design level relative to API decreases the ence is for the top engineers from the top ability assessment can also be applied usefully risk and increases construction costs. Under schools, who understand broadly applicable to such questions as: the need for redundancy optimistic assumptions, the API-RP2A-1977 engineering principles (structural mechanics, in the architecture of structural systems, cali- level represents the least total cost. Under materials, hydrodynamics) really well, and can bration of the design algorithms, proof testing more risk-adverse conditions, e.g. North Sea work independently to solve novel or difficult of components, quality control during con- post-Cullen, a higher design force might be problems and adapt to changing needs, e.g. struction, and in-service inspections. justified, but we would be making a trade- from fixed platforms to floaters (Marshall, There are many risks attendant upon each off which puts more steel into platforms, leav- 1993a). Degree programs which sacrifice in- venture into offshore oil activity: market price ing fewer resources available for finding new depth technology, in favor of survey courses collapse, political uncertainty, dry holes, energy. Furthermore, the optimum point is or premature how-to specialization, do not blowouts, fires, collisions, storm damage, not sharply defined. accomplish this. Nor does outsourcing. earthquakes, etc. Some of these risks are un- The current API RP 2A (2000) includes avoidable and require difficult trade-offs. consequence-based design criteria, with the However, the risks related to structural de- 1977 level being for moderate consequence Marine Materials sign can be minimized by the expenditure of platforms which can be evacuated for severe Any well-engineered structure requires money (for increased safety factors, strength hurricanes, and higher criteria for platforms that a number of factors be in reasonable bal- and redundancy) and technical effort (for which would be manned during the design ance. Factors to be considered in relation to increased knowledge), and are amenable to event or which have catastrophic consequences economy and safety in the design and steel rational value analysis. of failure, e.g. massive pollution and loss of selection for tubular connections include: (1) Ideally, the objectives of engineering and vital resources. static strength, (2) fatigue resistance, (3) notch any attempts at risk analysis should be di- The catastrophic Piper Alpha explosion toughness, (4) homogeneity and resistance to rected towards optimizing the total cost of and fire in the North Sea, and subsequent lamellar tear-ing, and (5) weldability (Carter, the project or structure, which must be mea- report by Lord Cullen, emphasized the need Marshall, et al., OTC 1043). Concrete struc- sured against its benefit or return. This cost to consider total system risks in assessing the tures have their own set of materials selection includes the initial cost for engineering and safety of offshore platforms. If the structural and quality control considerations.

108 Marine Technology Society Journal Steel At an intermediate level of criticality, stress receives a lot of attention at offshore exhibi- Most fracture control problems in off- concentration, and restraint, steel for brace tions (six-plus papers at the first OTC). Ca- shore structures occur in the tubular connec- stub-ends and non-redundant deck trusses thodic protection, both impressed current tions, or nodes. As these approach the ulti- and girders may warrant Charpy testing at the and sacrificial aluminum or zinc anodes, is mate strength upon which their design loads service . For the least critical appli- used in the submerged zone. Sacrificial steel are based, the ‘hot spot’ regions experience cations, e.g. piling and redundant jacket braces, (e.g. corrosion allowance of 19 mm) or other triaxial stresses, localized yielding, strain hard- ordinary steels which barely meet Barsom’s coatings, e.g., composite (fiberglass) or ening, mobiliz-ation of fully plastic sections, (1977) temperature-shifted criteria may still metallic wraps are used in the splash zone. large deflection shell/ membrane effects, and be used, as wave loading is several orders of Multi-layer paint systems are used in the load redistribution via plastic deformation. magnitude slower than impact testing. atmospheric zone. Recent updates include These phenomena, which must occur in the More advanced fracture control proce- workshops held by MMS (1999) and NIST presence of weld toe notches, place extraor- dures, e.g. da/dN and CTOD, are described (2004). dinary demands on the materials being used, in Marshall (1990). particularly in the main member at tubular In ordinary structural steels, inclusions Fatigue connections. Typical design practice is to use from the ingot are flattened out in the process Fatigue may be defined as damage that high quality heat treated steel plates (e.g. nor- of rolling plate, forming planes of weakness results in fracture after a sufficient number of malized API Spec 2H, Grade 50) for the for through-thickness stresses (normal to the stress fluctuations. Performance may be char- joint can. plate surface), as occur in the joint cans. Even acterized as a plot of stress range versus num- Once the designer has taken care of the where the inclusions are too small to be de- ber of cycles to failure (S-N curve). Specific most fundamental requirement, static tected visually or ultrasonically, the steel may criteria for tubular structures have been dis- strength, then the other modes of failure must fail in lamellar tearing under the influence of cussed earlier in the paper. A fatigue analysis be considered, e.g. fatigue. In the as-welded weld shrinkage strains. Steels exhibiting 25% for off-shore structures includes the following condition, most structural quality steels fall reduction of area in a through thickness ten- elements: in the same S- N scatter band, so that higher sion test are relatively immune to the prob- 1)Long-term wave climate is the starting point strength steels do not allow any reduction of lem (Wold, OTC 1915). Improved steel of fatigue analysis. This is the aggregate of thickness where fatigue governs. Where 50 cleanliness, e.g. very low sulfur levels, can all seastates occurring yearly (or for longer ksi (350 MPa) satisfies the strength require- achieve this. periods of time). Obtaining this data often ments at the same thickness as required by In many ASTM specifications, residual or requires a major effort, with significant lead fatigue, as is often the case, using higher trace elements (e.g. Cr, Mo, V, N) may be times. strength steel has no advantage—only higher present in sufficient quantities to adversely 2)Global scale space frame analysis is per- cost and more potential problems with weld- affect weldability. Too rich a chemistry pro- formed to obtain structural response in ing and fracture. motes the formation of very hard heat affected terms of cyclic member stresses for each Conventional practices for the control zones, subject to cracking upon cooling when sea state of interest. of brittle fracture are based on Charpy tests. hydrogen (moisture) was present in the weld- 3)Geometric stress at all These are admittedly qualitative, but may ing atmosphere. The problem can be allevi- potential hot spot locations within the be correlated empirically to more rational ated somewhat by the application of preheat tubular connections must be considered, engineering approaches, such as the NRL as part of the welding procedure. AWS D1.1 since fatigue failure initiates as a local Fracture Analysis Diagram (Pellini, 1963). Annex XI provides guidelines for dealing with phenomenon. In order to avoid propagation of small the problem in terms of steel chemistry, de- 4)Accumulated stress cycles are then counted, cracks at stresses approaching the ultimate gree of restraint, hydrogen level, preheat, and and applied against suitable fatigue criteria tensile strength, and provide for crack ar- heat input during welding. (e.g. Miner’s rule) to complete the analysis rest at yield stress level, as well as to cover of fatigue damage. scatter in the correlations, steel specifica- Corrosion In view of the scatter and uncertainty in tions for joint cans and other critical loca- The survival of early lighthouse struc- fatigue, the choice of target calculated fatigue tions may require 25 ft-lb (35 J) at -40° tures offshore Florida for over a century is life requires careful evaluation of the economic for service of 14°F air and remarkable, given the inexorable ravages of and risk factors involved. Typically, the target 40°F water (-10 and 5°C). corrosion and fatigue on modern structures. life is a multiple of the required service life. Weld metal and heat affected zones should The complex and mysterious world of cor- have notch toughness requirements compat- rosion has its own technical society, NACE. Corrosion-fatigue ible with the base metal in which they occur, Sea water submergence, the splash zone and Studies of long-term fatigue of welded enforced via consumable selection and proce- the atmospheric zone (sea spray) are all ag- components in sea water reveal a complex set dure qualification. gressive environments. Corrosion protection of synergistic influences (Marshall, 1976).

Fall 2005 Volume 39, Number 3 109 Early studies of S-N behavior were made by Physical Oceanography/ Friction also gives rise to a turbulent at- Bouwkamp (1966), Havens (OTC 1046), Meteorology mospheric boundary layer near the sea sur- Kochera (OTC 2604), Hartt (OTC 2380 & The author joined MTS in 1967, count- face, which has a rather complex structure 5663,) and others. Crack Propagation was ing membership in the American Society for in space and time. In defining and using studied by Bristoll, Jaske (1977), and others, Oceanography, which was subsequently ab- wind speeds, considerable confusion results with exhaustive European work summarized sorbed. Understanding wind, waves, and cur- when the reference height and duration are at the SIMS conference (1987). Seawater is rents was fundamental to the early definition not clearly specified. The usual standard ref- always detrimental to crack propagation, but of design criteria for offshore platforms. erence elevation is 10 m (33 ft). The veloc- cathodic protection can restore air-like behav- Hurricanes are cyclonic circulations with ity at other elevations is empirically related ior at low stress levels, near the initiation or sustained winds over 74 mph (category 1). to the velocity at the reference elevation by non-propagation threshold. Design guidelines and regulations are typi- a power law. Hourly mean velocity describes cally based on conditions which occur on av- the sustained wind, and this is what is used Concrete erage once per 100 years at a particular plat- in the calculation of wind- driven waves Concrete structures are a composite of form site. These generally correspond to and currents. In the turbulent boundary the concrete matrix and reinforcing bars, category 3 storms with waves up to 72 ft. The layer, the wind undergoes random fluctua- mesh, or fibers. The concrete matrix is an resulting designs have reserve strength which tions about this mean, with the RMS (root- admixture of portland cement, coarse and can generally survive storms up to category 4. mean-square) fluctuation being 10-15% of fine mineral aggregates, water, and additives, After withstanding 80-ft waves in Camille and the mean. The structure (i.e. frequency con- which is placed as a fluid within the casting Ivan, the 38-year-old structure in Figure 2 tent) of these fluctuations can be described formwork and around the reinforcing, sub- was reported lost in Katrina. by wind spectra (Vickery, 1985; Forristall, sequently curing into a solid. Concrete is 1988). For structures with short natural weak and brittle in tension, but can have Wind periods, it suffices to design for the maxi- compressive strength upwards of 70 MPa. Wind is responsible for generating waves, mum velocity of a gust large enough (of Tension loads are carried by the reinforcing, and wind forces are directly applicable to the sufficient duration) to envelope the struc- which is typically steel, but composite mate- design of above-water portions of offshore ture in question, relating this velocity to the rials may also be used. Materials issues in- platforms, and the equipment placed mean with a gust factor. Such maximum clude compatibility between the concrete thereon. Winds are generated in response to gust factors range from 1.25 to 1.45 for constituents, and corrosion prevention for gradients (defined on extra tropical storms, and from 1.3 to 1.7 the reinforcing. Concrete construction is la- weather maps by isobars), Coriolis forces (due for tropical storms. bor intensive, and quality control requires to the rotation of the Earth), centrifugal constant attention. forces, and friction. The geostrophic wind, Waves which is in equilibrium between pressure Empirical relationships for estimating Composites gradients and the Coriolis force, blows paral- wave heights, given the windfield, were de- Fiberglass and carbon fiber composites lel to the isobars, with the higher pressure on veloped during WWII, to assist in planning have a cured polymer as the matrix. The fi- the right in the northern hemisphere. Thus, amphibious landings. These were published ber reinforcing can be made stronger, stiffer, the wind blows in a counterclockwise direc- in 1947 by Sverdrup & Munk (1947). With lighter, and more corrosion resistant than steel, tion around the low pressure center of a storm the advent of , research con- making composites attractive for offshore (clockwise in the southern hemisphere). At tinued at Texas A&M, with the classic rela- structure applications. The matrix is weaker, an altitude of many hundreds of meters, the tion-ships being published by Bretschneider although its strength and ductility can be geostrophic wind speed is proportional to et al. (1958). Starting with theoretical con- varied by design. Analysis and design follow the pressure gradient, increasing as the isobar sideration of the energy budget of waves, some of the same principles as reinforced con- spacing gets smaller, but also depending on with appropriate nondimensionaliz-ation, crete, albeit on a smaller scale, with fiber ori- latitude. Curved isobars indicate the pres- and extensive field calibration, they estab- entation customized to the strength and stiff- ence of centrifugal forces, which can support lished relationships between significant ness needs of the structure. Composite risers greater pressure gradients at a given velocity. waveheight and period, as a function of wind and tendons have been “just around the cor- In storms at sea, the surface wind speed is fetch, duration, and velocity, as well as ef- ner” for almost 20 years, but prototypes are 50% to 70% of the geostrophic wind speed fects of shoaling and decay distance. In a natu- now being field tested. Designing their ter- (depending on air-sea temperature differ- ral seastate, waveheights vary, following the minations and connectors has been particu- ence); and the wind blows at a 10-15° angle Rayleigh probability distribution. The sig- larly difficult, and the quantities required for to the isobars, spiraling in towards the low nificant is the average height of a large offshore project would tax world pro- pressure center; with both effects being due the highest third of the waves, 1.41 times duction capacity. to friction (Weigel, 1966). the RMS wave height, 4 times the standard

110 Marine Technology Society Journal deviation of the sea surface, and close to what and currents operations with deep-seated currents in excess visual observers typically report. Of the waves, Tides may be classified as astronomical of 1 m/s (2 knots) over considerable depth. 10% exceed this height. The most probable tides and storm surge. The latter consists of Tidal currents, wind-driven storm cur- extreme value of the highest wave is a mul- wind tide (the pile-up of wind-driven cur- rents, and ocean circulation currents at the tiple of significant waveheight (Hs), depend- rents against the shore) and pressure-differ- surface affect the placement and operability ing on the length of observation (or storm ential tide (water rising in response to the of boat landings and barge bumpers. Extreme duration), as follows: low pressure center of a storm). The total of currents, and their profile with depth, affect all these defines the datum upon which de- the design loads for the platform and its ap-

1 hour - 1.7 Hs sign waves are superimposed. The daily tides purtenances, especially those vector compo-

3 hours - 1.86 Hs (1/1000 waves) are important in operational design of boat nents which are in line with the design wind

8 hours - 2.0 Hs landings and barge fenders, and in defining and waves, and occur simultaneously. the limits of splash zone corrosion and ma- Wave periods also vary. Wave energy as a rine growth. Ice function of frequency may be described by When the wind blows over water, In cold regions, ice accumulation is related an energy spectrum. General seastates have a microscale boundary conditions at the inter- to the cumulative number of degree-days be- broad-banded spectrum, as described by face indicate that the surface current should low freezing. For example, in Cook Inlet, Pearson-Moskowitz and others (Michel, be downwind, at 3-6% of the frictional veloc- Alaska, a cold month with 500 degree-days 1967). In these seas, the peak of the energy ity (windspeed measured right at the surface), below -7°C can produce ice floes about 1 m spectrum, corresponding to the period of the or 2-3% of the mean windspeed at the usual thick. More-or-less conventional steel tower- significant waves, is 1.4 times the zero-cross- elevations of measurement. Due to viscous fric- type platforms (with very large legs and no ing period. Spectra for intense storm seas are tion and turbulent mixing, this velocity gradu- bracing at the waterline) have been designed more sharply peaked (Brannon, 1967). Fetch ally spreads downward. Mass transport mani- and built to crush their way through first-year is the length of ocean over which the wind fests itself in higher order wave theories, as ice floes of this thickness. See Figure 3. Load blows to generate waves. A certain minimum well as in superimposed steady current. At measurements on a cylindrical test structure duration is required to achieve fully devel- 30º N, the Coriolis effect tends to turn the indicated applied forces to be of the order of oped waveheights. As the waves move out of mass transport to the right, at 7.5° /h; in the 55% of the ice compressive strength (which the area in which they were generated, they absence of wind changes or other boundary varies with temperature and loading rate), gradually decay in height, and their periods conditions, the steady state current would be yielding a design crushing strength of 300 psi lengthen, eventually becoming . Storm 90° to the right at the surface, spiraling fur- (2 MPa), as described by Peyton (1968). waves which are generated in, or move into, ther to the right and decaying in velocity with Multi-year ice floes and pressure ridges, broad areas of shallow water have reduced depth (the Ekman spiral). A strong ther- icebergs and ice islands, would require much heights. Further modifications of the waves, mocline tends to confine wind-driven mass more massive structures for permanent pro- e.g. refraction due to shoaling, apply to coastal transport to the upper layers. A barrier coast- locations. Swell moving onto a beach may line requires that the steady state net mass trans- achieve several times its deep-water port be parallel to the coastline and the Figure 3 waveheight. bathymetry (although there may be onshore Ice-resisting platform in Cook Inlet, Alaska (1965) With a wind field varying in time and current at the surface and return current at space, defining fetch, duration, and decay depth, or vice versa); however, a low coast-line distance could be a very subjective process. will admit significant shoreward transport to A ‘standard project hurricane’ was often de- account for the volume of flooding fined for use in the design of coastal defense (Bretschneider, 1967). structures (CERC, 1966). Wilson (1966) Several numerical grid-type describes a numerical process for following models have been developed to deal with these a traveling windfield and integrating its ef- phenomena. Calibration of these models on fect on significant waves moving along vari- currents measured during storms remains an ous predefined rays. Cardone (OTC 2332) ongoing effort. Ocean circulation boundary generalized the numerical procedure, to in- currents, e.g. the Gulf Stream, transport huge tegrate growth and decay for all wave direc- volumes of water (tens of cubic kilometers per tions and frequencies, with the synoptic hour). Anti-cyclonic eddies which break off wind and directional energy spectrum be- from these currents (e.g. the loop in the east- ing described at points on a grid covering ern Gulf of Mexico) persist for months, and large areas of ocean. have impacted deep-water offshore drilling

Fall 2005 Volume 39, Number 3 111 duction operations, of which are several have Operational Criteria References been proposed, but none built. These are Once the platform site has been selected, Adams, J. 1995. Risk. London: University College. covered, albeit hypothetically, by API Bulle- experienced specialists should be consulted in tin 2N (1988), and were once the focus of a defining the oceanographic and meteorologi- AISC. 1997. Specification for the Design of technical division of ASME (Offshore Me- cal conditions from which operating as well as Steel Hollow Structural Sections. In: Hollow chanics and Arctic Engineering). In the high extreme design criteria will be drawn. In new Structural Sections Connections Manual. Sec arctic, much of the exploratory drilling to date areas, this data gathering is a major effort, re- 10 pp. i -52. American Institute of Steel has been conducted by floating rigs on a sea- quiring significant lead time. Data which are Construction. sonal basis, or from caisson-retained and earth- used should be carefully documented, includ- filled artificial islands. ing their source or basis, and the degree of API Bulletin 2N. 1988. Planning, Designing, confidence which can be placed in them. Both and Constructing Fixed Offshore Structures in Design Criteria measured data, and data generated from Ice Environments. American Petroleum Institute. The derivation of API reference level hindcasts using well-calibrated mathematical API RP 2A. 2000. Recommended Practice for (1977) design criteria for the NW Gulf of models, should be used cooperatively. Nor- Planning, Designing, and Constructing Fixed Mexico will now be described. The upper mal environmental conditions are important Offshore Platforms. 21st edition. American tail of the probability distribution for ex- in planning construction operations, and in Petroleum Institute. treme hurricane loads on a typical platform defining downtime (e.g. due to resupply prob- has been derived from historical storm data. lems) during the service life of the platform. AWS D1.1-2004. Structural Welding Code – Central pressure, area extent, and track, for The frequently occurring sea states (and the Steel. American Welding Society. 43 storms covering 70 years were used to long-term distribution of wave heights) are Bernard, M.A., and LeBlanc, R.J. 1965. reconstruct wind fields, which were used in also important for fatigue calculations. Sev- Resume of Quarternary Geology of the turn to hindcast the waves and currents, eral years of continuous data are desirable for Northwestern Gulf of Mexico Province. Shell initially for nine sites spread across the Texas- this purpose. EPR Publication 400. Louisiana continental shelf (Evans et al, OTC 1692). Both literal reconstruction of Bouwkamp, J.G. 1966-1970. Tubular Joints history, and random generation of storms Conclusions Under Static and Alternating Loads. Structures and Materials Research Reports from the observed population, produced As listed in the Abstract, many of the tech- 66-15, 67-29, and 70-4. Berkeley: Univ. of similar site-to-site variability, with the most nical committees in MTS are related to the California. likely extreme value of the 100-year wave design, construction, and structural integrity height being 75 ft (22.5 m). These of offshore platforms. This paper has reviewed Brannon, H.R., et al. 1967. Storm wave hindcasts, and others using a grid covering those relationships. characteristics. SPE Journal, March. the entire Gulf of Mexico, produce similar results, have been calibrated against an ocean Bretschneider, C.L. 1957. Evaluation of drag data gathering program (Ward, 1978). and inertial coefficients from maximum total The full set of environmental conditions range of wave forces. Texas A&M Rept. 55-5. requires 8-11 parameters to describe. The Bretschneider, C.L. 1958. Revisions in wave short set includes: significant wave height, forecasting: deep and shallow water. Proc. 6th period and direction; current surface veloc- Coastal Engrg Conf. Council on Wave ity, total transport and direction; mean wind Figure 4 Research. speed and direction. The longer set includes Bullwinkle jacket for 400 m water depth (1988). parameters for spectral peakedness, direc- Last of the dinosaurs. Bretschneider, C.L. 1967. Estimating wind- tional spreading and current profile. A rather driven currents over the continental shelf. intractable problem of joint statistics may Ocean Industry, June. be avoided by using a literal reconstruction Campbell, B.J. 1906. In: Engineering News. of history to see how these parameters com- bine, computing total peak lateral force (or Carter, R.M., Marshall, P.W. et aI. 1969. other structural response parameter of in- Material problems in offshore platforms. Proc. terest, e.g. base shear or overturning mo- OTC 1043. ment, on a prototypical structure) storm by CERC. 1966. Shore Protection, Planning, storm or hour by hour, and then examining and Design. US Army the extreme value statistics of the parameter Research Center, Tech. Report No.4, 3rd edition. of interest.

112 Marine Technology Society Journal Chen, W.F. and Han, D.J. 1985. Tubular Marshall, P.W., ed. 1978. Inelastic Behavior of Matlock, H. 1965. Lateral Load Behavior of a Members in Offshore Structures, Bath: Tubular Members and Structures. ASCE Pile Group in Clay Soil. Report to Shell, 1965. Pitman. 271 pp. Combined Preprint 3302. Chicago: American McClelland, B. 1966. The Progress of Society of Civil Engineers. CRC TG 18. 1976. Circular Tubes and Shells. Consolidation in Delta Front and Pro-delta In: Guide to Stability Design Criteria for Marshall, P.W. 1979. Strategy for monitoring, Clays of the Mississippi River. Presented at. Metal Structures. Bethlehem: Structural inspection, and repair for fixed offshore Research Conference on Marine Stability Research Council platforms. Proc. ASME Conf on Structural Geotechnique, Univ. of Illinois, Urbana. Integrity Technology, Washington, D.C. Focht, J.A. and McClelland, B. 1955. Analysis McClelland, B., Focht, J.A. and Emrich, W.J. of Laterally Loaded Piles by Difference Marshall, P.W. 1980. Fixed Pile-Supported 1967. Problems in Design and Installation of Equation Solution. Texas Engineer, Fall. Steel Offshore Platforms. Journal of the Heavily Loaded Pipe Piles. In: - Structural Division, ASCE, vol.107, No.ST6, ing in the Oceans, ASCE, San Francisco. Focht, J.A. 1967. Interpretation of Boring and p 1083-1094. Test Results. In: Ocean Engineering, Vol. II. McClelland, B. and Reifel, M.D. 1987. Shell Development Co. Technical Training Series. Marshall, P.W. 1982. An overview of recent Planning and Design of Fixed Offshore work on cyclic inelastic behavior and system Platforms. New York: Van Nostrand Reinhold. Focht, J.A. and Kraft, L.M. 1976. Progress reliability. Proc. SSRC. with Marine McClelland Engrs. 1963. Appendix ‘C’, Problems. Presented at the ASCE Bicentennial Marshall, P.W. 1986. Designed of Internally Criteria for predetermining pile capacity. Convention, Philadelphia. Stiffened Tubular Joints. In: Proc IIW-AIJ Found in their soil and foundation reports to International Meeting on Safety Criteria in clients (revised 1967). Haring, R.E. et al. 1979. Total wave forces Design of Tubular Structures, Tokyo. and moments. OTS session, Proc. 4th ASCE McClelland Engrs. 1966. Pile Loading Tests, Conf on in the Oceans, 1979. Marshall, P.W. 1992. Design of Welded Single Piles and Circular Groups, Venice and Tubular Connections: Basis and Use of AWS Harvey, Louisiana. Report to Shell. Horton, E.E. 1987. U.S. Patent 4,702,321. Code Provisions. Amsterdam: Elsevier. 412 pp. Meith, R.M. and Gooch, A.B. 1966. Jaske, C.E. et al. 1977. Interpretive Report on Marshall, P.W. 1993. Offshore Structures. Computer analysis of off-shore drilling Corrosion Fatigue of Welded Carbon Steel for Chapter 6.7, In: Constructional Steel Design. platforms. Paper No. SPE 1414, AIME Application to Offshore Structures. Battelle London: Elsevier. pp.761-789. Symposium on Offshore Technology and Mem Inst report to API Operations, New Orleans. Marshall, P.W. 1993a. Offshore Industry Lee, G.C. 1968. Twenty Years of Platform Perspective on Ocean Engineering Education. Michel, W.H. 1967. Sea spectra simplified. Development. Offshore, June. In: ASEE Annual Conference Proceedings, Presented to SNAME Gulf Section. Marshall, P.W. 1967. Risk factors for offshore Champaign-Urbana. American Society of Miller, C.D. 1981. Summary of Buckling Tests structures. Proc. 1st ASCE Conf on Civil Engineering Education. on Fabricated Steel Cylindrical Shells in the Engineering in the Oceans, San Francisco. Marshall, P.W. and Inglis, R.B. 1986. Wave USA. Presented at: Buckling of Shells in Marshall, P.W. and Toprac, A.A. 1974. Basis Kinematics and Force Coefficients. ASCE Offshore Structures, Imperial College, London. for Tubular Joint Design. Welding Journal, May. Structures Congress, New Orleans. Mineral Management Service. 1999. Marshall, P.W. 1974. Basic Considerations for Marshall, P.W. 1990. Fracture control Proceedings of the MMS/Industry Interna- Tubular Joint Design in Offshore Construction. procedures for deep water offshore towers. tional Workshop on Corrosion Control for WRC Bulletin 193. Welding Research Council. Welding Journal, January. Marine Structures and Pipelines. Galveston, Texas, February 9-11. Marshall, P.W. and Bea, R.G. 1976. Failure Marshall, P.W. and Wardenier, J. 2005. modes for offshore structures. Proc. 1st Intl Tubular vs. Non-Tubular Hot Spot Stress Morison, J.R. et al. 1950. The force exerted by Conf on the Behaviour of Off-Shore Struc- Methods. Proc ISOPE Conf, Seoul. surface waves on piles. Petroleum Transactions, tures, BOSS-76, Trondheim. AIME. Maslin, M. 2004. Gas Hydrates: A Hazard for Marshall, P.W. 1977. A Review of American the 21st Century. Benfield Hazard Research O’Neill, M.W. et al. 1983. An evaluation of Criteria for Tubular Structures-and Proposed Centre, University College, London. P-Y relationships in sand. Report to API. Revisions. Copenhagen: IIW Doc. XV-405-77. Matlock, H. and Reese, L.C. 1962. General- ized Solutions for Laterally Loaded Piles. Transactions, ASCE, Paper 3370.

Fall 2005 Volume 39, Number 3 113 National Institute of Standards & Technology. Smith, C.E. 1997. Dynamic Response of a Dean, R.O. 1972. Application of stream 2004. International Workshop of Coatings for Steel Jacket Platform Subject to Measured function theory to offshore design problems. Corrosion Protection: Marine Pipelines and Seafloor Earthquake Ground Accelerations. OTC 1613. Ship Structures. NIST Special Publication Proceedings, 8th International Conference on Dekker, J.N. et al. 1988. Computer analysis of 1035. Biloxi, MS, April 14-16. the Behaviour of Off-Shore Structures. The heavy lift operations. OTC 5819. Hague, Netherlands. Pellini, W.S. and Puzak, P.O. 1963. Fracture Digre, K.A., Marshall, P.W. et aI. 1989. analysis diagram procedures for the fracture- Sverdrup, H. and Munk, W.H. 1947. Wind, Design of the Bullwinkle platform. OTC 6050. safe engineering design of steel structures. Sea, and Swell: Theory of Relations for WRC Bulletin 88. Forecasting. U.S. Hydrographic Office. Doyle, E.H. et al. 1988. Technical Advances in High-Resolution Hazard Surveying, Perkins, R.L. 1967. Soil Boring Equipment and UKOSRP 1978. European Offshore Steels Deepwater Gulf of Mexico. OTC 5758. Techniques. In: Ocean Engineering, Vol .II. Research. Preprints of the Select Seminar, Shell Development Co. Technical Training Series. Cambridge, England. Doyle, E.H. 1999. Pile Installation Perfor- mance for Four TLP’s in the Gulf of Mexico. Peyton, H.R. 1968. Ice and marine structures. Weigel, R.L., Beebe, K. and Moon. 1957. OTC 10826. Ocean Industry. March, Sept. & Dec. issues. Ocean wave forces on circular cylindrical piles. Journal of the Hydraulics Division, Edel, C.J.C. et al. 1999. Installation of the Poulos, H.G. 1971. Behavior of Laterally ASCE, April. Baldpate Compliant Tower. OTC 10919. Loaded Piles: Pile Groups. Journal of the Surveying and Mapping Division, ASCE, Vol. Weigel, R.L. 1964. Oceanographic Engineering. Evans, D.G. 1969. Analysis of wave force data. 97, No. SM5. Paper 8093, p.733f. Englewood Cliffs: Prentice-Hall. OTC 1005.

Reid, R.C. and Bretschneider, C.L. 1953. Weigel, R.L. 1970. . Evans, D.G. 1972. Theoretical hindcasts of Surface waves and off-shore structures: the Englewood Cliffs: Prentice-Hall. currents and water elevations using numerical design wave in deep or shallow water, storm models. OTC1692. Wilson, B.W. 1966. Design sea and wind tide, and forces on vertical piles and large conditions for offshore structures. Presented at Forristall, G.Z. 1978. Storm wave kinematics. submerged objects. Report of the Texas A&M the Offshore Exploration Conference, OTC 3227. Research Foundation. OECON-66. Forristall, G.Z. 1988. Wind spectra and gust Rodabaugh, E.C. 1978. Review of Data factors over water. OTC 5735. Relevant to the Design of Tubular Joints for OTC 1969-2005 Use in Fixed Offshore Structures. Battelle Fugro, B.V. 1975. Site Investigations for References from: Houston: Proceedings of Mem Inst report to WRC. North Sea Forties Field. OTC 2246. the Offshore Technology Conference. Rolfe, S.T. and Barsom, J.M. 1977. Fatigue www.otcnet.com Garrison, C.J. et al. 1974. Wave forces on and Fracture Control in Structures. large volume structures: a comparison between Audibert, J.M.E. 1978. Recent Advances in Englewood, Cliffs: Prentice-Hall. theory and model tests. OTC 2137. Predicting Pile Driveability. OTC 3273. Seed, H.B. and Reese, L.C. 1956. The action Gurney, T.M. 1973. On Fatigue Design Rules Bea, R.G. et aI. 1973. Movements and forces of soft clay along friction piles. Trans. ASCE, for Welded Structures. OTC 1907. developed by wave-induced slides in soft clays. Paper 2882. OTC 1899. Havens, F.E. 1969. Fatigue Strength of Sherman, D.R. 1976. Tentative Criteria for Quenched and Tempered Carbon Steel Plates Bea, R.G. et aI. 1977. A study of soil-pile-structure Structural Applications of Steel Tubing and and Welded Joints in Sea Water. OTC 1046. systems in severe earthquakes. OTC 2749. Pipe. AISI Committee of Steel Pipe Producers. Halliburton Svcs. 1979. An Assessment of Billington, C.J. et al. 1978. The Strength of Sherman, D.R. and Erzurumlu, H. 1976. Grouting Materials, Placement Methods, And Large Diameter Grouted Connections. OTC Ultimate Capacity of Tubular Beam Columns. Monitoring Equipment For Offshore 3083. Presented at: ASCE National Structural Structures. OTC 3671. Engineering Conference. Cardone, V.J. 1975. Hindcasting the Hartt, W.H. 1975. Cathodic Protection directional spectra of hurricane generated SIMS. 1987. Steel in Marine Structures. Criteria for Notched Mild Steel Undergoing waves. OTC 2332. Amsterdam: Elsevier. Corrosion Fatigue in Sea Water. OTC 2380. Chen, W.F. 1976. The Axial Strength and Hartt, W.H. et al. 1988. Fatigue Properties of Behavior of Cylindrical Columns. OTC 2683. Exemplary High-Strength Steels in Seawater. OTC 5663.

114 Marine Technology Society Journal Horton, E.E. and Pauling, J.R. 1970. Analysis McClelland Engrs. 1976. Application of Yura, J. and Zettlemoyer, N. 1980. Ultimate of the Tension Leg Stable Platform. OTC 1263. Remote Vane Results to Offshore Capacity Equations for Tubular Joints. OTC Geotechnical Problems. OTC 2626. 3690. Horton, E.E. et al. 1972. Optimization of Stable Platform Characteristics. OTC 1553. McClelland Engrs. 1977. Seafloor Cone Penetrometer for Deep Penetration Measurements Idriss, I.M. 1975. Soil Response Consider- of Ocean Sediment Strength. OTC 2787. ations in Seismic Design of Offshore Platforms. OTC 2355. Moses, F. 1970. Reliability format for offshore structures. OTC 3027. Kinra, R.K. 1976. Hydrostatic and Axial Collapse Tests of Stiffened Cylinders. OTC 2685. Munse, W.H. 1978. Predicting the Fatigue Behavior of Weldments for Random Loads. Kochera, J.W. et al. 1976. Fatigue of Structural OTC 3300. Steel for Offshore Platforms. OTC 2604. O’Neill, M.W. 1995. The Response of Kraft, L.M. et al. 1974. State-of-the-Art: Suction Caissons in Normally Consolidated Ultimate Axial Capacity of Grouted Piles. Clays to Cyclic TLP Loading Conditions. OTC 2081. OTC 7796. Leverette, S.J. et al. 1999. Morpeth SeaStar Ostapenko, A. 1978. Tests on Two High- Mini-TLP. OTC 10855. Strength Short Tubular Columns. OTC 3086. Lowrey, D.P. et al. 1969. Applications of Pecknold, et al. 2005. New API RP2A Tubular Wave-Equation Analysis to Offshore Pile Joint Strength Design Provisions. OTC 17310. Foundations. OTC 1055. Reese, L. and Cox, W. 1974. Field Testing of Marshall, P.W. and Chabot, L. 1993. Laterally Loaded Piles in Sand. OTC 2079. Concrete Floating Central Production Facility. OTC 7159. Reese, L. and Cox, W. 1974. Analysis of Laterally Loaded Piles in Sand. OTC 2080. Marshall P.W. et al. 1993. Back-span Stress- joint. OTC 7258. Sangrey, D.W. 1977. Response of Offshore Piles to Cyclic Loading. OTC 2944. Marshall, P.W. 1995. Infrastructure for Regional Development in Deep Water. OTC 7867. Smith, C.E. 1996. Response of a Steel Jacket Platform Subject to Measured Seafloor Marshall, P.W. et al. 2005. Background to Seismic Ground Morions. OTC 8110. New API Fatigue Provisions. OTC 17295. Sterling, G.H. et al. 1973. The failure of Matlock, H. 1970. Correlations for Design of South Pass 70 ‘B’ Platform in Hurricane Laterally Loaded Piles in Soft Clay. OTC 1204. Camille. OTC 1898. Matlock, H. et al. 1972. Nonlinear analysis of Toprac, A.A., Erzurumlu, H., and Kurobane, soil-supported frame. OTC 1699. Y. 1969. Research in Tubular Joints: Static and Matlock, H. 1975. Computer Predictions for Fatigue Loads. OTC 1062. Axially-Loaded Piles with Nonlinear Supports. Vickery, B.J. et al. 1985. An investigation of OTC 2186. dynamic wind loads on offshore platforms. Matlock, H. 1977. A Computer Program for OTC 4955. the Analysis of Beam-Columns Under Static Ward, E.G. et aI. 1978. Statistics of hurricane Axial and Lateral Loads. OTC 2953. waves in the Gulf of Mexico. OTC 3229. McClelland Engrs. 1972. Practical Planning Wold, A. et al. 1973. Development of method for Driving Offshore Pipe Piles. OTC 1600. for measuring susceptibility of steel plate to lamellar tearing. OTC 1915.

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