As oil and gas production has moved into deeper water, and with the common requirement for platform drilling and/or frequent well intervention, new solutions have been developed to support rigid risers and surface trees. The four types of dry completion unit employed to date - jackets, compliant towers, TLPs and spars - are described and compared, along with their advantages and current limits of applicability. A database of deepwater production platforms is included.
Platform optimisation cannot be performed in isolation from riser considerations, as the requirements of the risers are intimately related to the operating characteristics of the production facility. This interrelationship, and resulting trends in both platform concepts and production riser configuration, is explored. A key issue is the tendency for the risers to displace vertically relative to the facility, which becomes very significant for softly-moored hulls in ultra-deepwater. This displacement may be accommodated by modifying the riser shape, make-up and method of support.
The advantages of dry completion units in allowing direct well accessibility are well known. This paper describes the various types of dry completion unit currently found in deepwater, defined here as greater than 300m (nearly 1,000ft). Deepwater production began in 1979 from a jacket-supported platform. In 1984 a tension leg platform and compliant tower were both commissioned, and the first production spar commenced operation in 1997.
These facilities have been developed specifically to enable deeper water production with surface trees. There are significant differences between the platforms, including substructure/hull configuration, foundation/mooring system, motions response, construction sequence, drilling and workover operations, and water depth range. These variations are outlined in the following sections.
Each evolution has forced changes to the production risers to enable them to accommodate increasing lateral displacement and, in particular, vertical displacement relative to the facility. Methods of dealing with this vertical incompatibility will be seen to be strongly dependent on the mooring stiffness, and whether the hull includes a deep moonpool, as well as the riser configuration.
The vast majority of offshore platforms have fixed-base steel substructures, called jackets. A jacket has a number of legs to support the topside weight. The legs are linked by bracing members in the horizontal and vertical planes, which enable the overall structure to transmit the environmental loading (wave, current, wind, etc) down to the seabed. The corner legs are commonly battered (eg. Bullwinkle, 1989, in Fig. 1) so that the system has sufficient strength and stiffness to match the increasing global overturning moment down the structure.
Details of jackets around the world in greater than 300m water depth are given in Appendix A. For completeness, the Appendix also includes the only concrete gravity structure (CGS) situated in more than 300m of water, although this concept is not discussed further.