Gravity Platforms

A typical North Sea gravity platform design. (From Scfqetne, K. et al.. Marine Geotech., 3,369-421,1979. Reprinted with permission. Copyright Taylor Francis Group.) 

The relatively flat seafloor topography and firm near-surface soil conditions are well suited to gravity-type platforms. The principal features of offshore concrete gravity structures are their large size, their construction afloat and deployment reasonably complete, and their installation on a usually unprepared site. 

---

This chapter is organized into sections on loadings, pile structures, gravity platforms, anchor uplift capacity, jack-up platforms, and hydraulic filled islands. 

At a number of locations in the North Sea, large concrete gravity platforms have been used as alternatives to the more conventional pile-supported steel structures (Figure 10.31). Among the many considerations that enter into the decision process between the pile-supported... 

Schematic of base of gravity platform showing dowel, concrete skirt, and steel skirt.

For the North Sea gravity platforms, with a diameter of about 100 m and foundations on overconsolidated clay or dense sand, the estimate has been an average long-term settlement in addition to the consolidation settlement of about 10 mm/yr. This long-term settlement is intended to include both the ordinary secondary settlements and wave-induced settlements. As exemplified by the Ekofisk tank, the rate may be much higher in... 

Eide, O. T., Andersen, K. H., and Lunne, T. (1979b), Observed foundation behaviour of concrete gravity platforms installed in the North Sea 1973-1978, Proceedings of the 2nd International Conference on the Behaviour of Offshore Structures, 79, pp. 435 56. 

Lunne, T., and St. John, H. 1979. The use of cone penetration tests to compute penetration resistance of steel skirts underneath North Sea gravity platforms. Proceedings of the European Conference on Soil Mechanics and Foundation Engineering. 

Each of the main facility types, e.g. steel jacket, gravity structure, tension leg and floating platform, have different options for decommissioning. The main factors which need to be considered and which will impact on costs are type of construction, size, distance from shore, weather conditions and the complexity of the removal, including all safety aspects. The following options are available ... 

Tension leg and floating platforms can easily be released and towed away for service elsewhere, which is cheap and attractive. In the case of the fixed platforms, the topside modules are removed by lift barge and taken to shore for disposal. Gravity based structures can in theory be deballasted and floated away to be re-employed or sunk in the deep ocean, and steel jackets cut and removed at an agreed depth below sea level. In some areas jackets are cleaned and placed as artificial reefs on the seabed. The... 

Ancillary equipment attached to a tall vessel will subject the vessel to a bending moment if the centre of gravity of the equipment does not coincide with the centre line of the vessel (Figure 13.21). The moment produced by small fittings, such as ladders, pipes and manways, will be small and can be neglected. That produced by heavy equipment, such as reflux condensers and side platforms, can be significant and should be considered. The moment is given by ... 

Ahydrate blockage in the export line from Shell s Bullwinkle platform in the Green Canyon Block 65 to the Boxe platform was reported in DeepStar Report A208-1 (Mentor Subsea, 1996, p. 52). The 12 in., 39,000 ft line was not insulated. The seawater temperature was 50°F at the base of the platform in 1,400 ft of water. Gas gravity was 0.7 and the flow rate was 140 MMscf/d at an inlet pressure of 800 psi. 

Nevertheless, in view of the gravity of the illegal drug problem, it may be worth re-appraising the idea from the platform of modem science. Take the poppy plant, Papaver somniferum, as an example. The plant produces opium, which is the air-dried milky exudate collected from incised unripe poppy-seed capsules. Opium contains a mixture of about 20 alkaloids which amount to ca 25% by weight of the opium. Of these 20 alkaloids the main ones are ... 

A large issue of anti-tank mine fuses being entrained from the south face of the underground loading platform at Tunnel Quarry in 1943. The portable gravity runways are linked to the belt conveyor on the platform. 

---