A Jackup-Installed Platform for Marginal Fields

Abstract

An innovative platform for marginal oil and gas field developments is described. Its principal features are that it can be installed by a jack-up drilling rig as part of the drilling program, it does not require foundation piles and has superior boat impact resistance when compared with other designs. This paper describes the design of the structure, with particular emphasis on jack-up installation aspects, boat impact resistance and foundation design. A number of practical applications are described. Introduction A large number of innovative platform structure designs have been developed in recent years that are cheaper to construct and install than conventional three or four legged steel jackets. Ref. 1 gives a good summary of the range of platform designs currently available. The platform designs usually comprise a superstructure supported on a single vertical caisson (monotower) with bracing near its base that attaches to piles driven into the sea floor. Such structures are simpler than conventional jackets, making them cheap to construct. They are also lighter, so they can sometimes be installed by a jack-up drilling rig rather than by a specially chartered crane vessel, making them cheap to install. The guyed caisson platform design introduced here is believed to have advantages over these established designs in that it is still cheaper and easier to construct and install. In addition, it has superior resistance to boat impact forces, which will usually govern the design of small platforms in moderate water depths. Further, some of the conductors and risers can be on the outside of the central caisson, without the requirement for a boat impact frame. This allows the conductors to be spaced well apart, which leads to a good layout of the wellheads and production equipment in the superstructure and aids drilling operations. For multiple conductors it also has the effect of reducing the wave loads and the self-weight of the structure. Platform General Description Figure 1 shows an example of the structure design for a central North Sea site in 60m water depth with a 5,000kN topsides operational weight, 9 well slots and an export riser. The platform is unmanned and requires only limited access during normal operation. The platform comprises a superstructure supported above the wave crest level on a guyed caisson founded on a shallow spud-can on the sea floor. Six of the conductors are supported on the outside surface of the caisson through guides; the export riser and other conductors are inside the caisson. The caisson is 2500mm-diameter fabricated from high strength steel plate with maximum thickness 50mm. Three pairs of wire rope guy lines are attached to the caisson. One end of each line is attached to the caisson near the top, above the water surface with the other end attached to an anchorage at the sea floor. Anchorages may be vertical load plate anchors, suction anchors, piles or anchor blocks, depending on the soil conditions and the loads to be resisted.