EvolutionofOffshoreDrillingunits近海钻井平台.pptVIP

* Steel jacket structure A steel jacket structure is a unit of 6, 8, 10 or more legs of steel tubulars which are adequately braced with horizontal and diagonal cross-members to form a rigid structure (see Figure 7.1.6). The height of the unit is sufficient to ensure that when the unit is placed in position, the bottom of the deck will be clear of the highest expected wave. The jacket is built on land under carefully controlled conditions of quality assurance and testing, with all members being welded together. When built, or partially built if very heavy, the structure is loaded onto a barge and towed out to its location. Flotation aids enable the jacket to be suspended from the barge and slowly sunk into position on the sea bed. Once in position, the jacket is pinned to the sea bed using piles around the legs which are driven in from the surface or more recently with sub-sea hammers. This ensures that the jacket provides a stable base for the addition of a steel deck and the various modules for accommodation and for housing drilling and process equipment, which have to be installed after the jacket is in place offshore. These structures are relatively cheap and have been installed in water depths of over 400 metre * Gravity structure The concrete gravity structure is constructed by a process known as slip forming, in sheltered water and in a floating mode and comprises a base unit which is often subdivided into compartments used as storage tanks. They require relatively firm soil conditions for their foundations, but can usually easily absorb additional loads after installation. Sheltered deep water sites are required to enable them to be constructed. From the base a number of legs, usually three or four, depending on design, take the height of the structure above water level. The deck and then the various modules are placed on top of the legs (see Figure 7.1.7). The concrete gravity structure has sufficient buoyancy to float, with its topside structures instal