Dry Tree Semisubmersible Options for Deepwater Production

Recent deep water field developments have made increasing use of dry tree completions from floating production units, principally spars and TLPs. There are a number of development efforts underway to include dry tree semi-submersibles in deep water. The use of dry tree semis requires some form of heave suppression as well as a means for limiting pitch motions. Two concepts using this principle include the DPS 2001 and the TPG 3300. Both of these concepts consist of a deck supported on columns and, in the case of the DPS-2001, pontoons. In each case there is a submerged heave plate to trap mass and to dampen the heave motions. Risers must pass through this heave plate. This paper will present the results of study of options supporting top tensioned risers from these platforms, with a focus on the DPS-2001. Air Cans and tensioners have both been investigated. Air cans have been used on all spar platforms to date, but in this case the cans are shielded by the spar hull and are not exposed to wave kinematics or current loads. Tensioners have been used on all the TLPs, but their stroke is less than would be required for a semi-submersible, even one that is heave compensated. The tensioners used in this study had a relatively high stiffness in order to reduce stroke. This results in a coupling of the motions of the hull with the risers and results in some increase in the peak tensions. It also results in a lower heave period and higher heave motions. The paper will discuss the technical and economic tradeoffs for these options.