PAPER Interdisciplinary Aspects of Offshore Structures AUTHOR ABSTRACT Peter W. Marshall While some institutions treat Ocean Engineering as a single discipline, much of the MHP Systems Engineering progress in this area has been brought about by the interdisciplinary collaboration of Chair, MTS Offshore Structures experts in different areas, such as: Committee Structural engineering 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 oceanography/ 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 structural engineer, 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 construction 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 sea 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 forces, 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 tide). 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, Buoyancy 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 pressure 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 weight of the displaced wa- ends of large submerged elements. For lateral magnitude of pressures 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 force 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).