Two Dry Tree Semisubmersible Designs for Ultra Deep Water Post-Katrina Gulf of Mexico

The increased interest in floater designs for ultra-deep water has produced a number of dry tree semisubmersible designs that allow the use of top tensioned risers (TTRs). The primary advantage of the TTRs is that they facilitate direct vertical access to production wells and thereby offer access for well completions and interventions. The principle behind several of the dry tree semisubmersible designs is to reduce the motions of a traditional shaped semisubmersible to a level that can accommodate TTRs. This is accomplished by using heave plates that are positioned beneath the semisubmersible hull and are supported by a lower structure, such as a truss. To a certain extent, the motion responses above and below the response amplitude operator (RAO) cancellation period can be manipulated by designing the relative sizes of the pontoons and columns as well as the size and depth of the heave plates such that the sum of the forces interacts to minimize the heave motion. The paper presents and discusses two semisubmersible designs that assume a common topsides and riser payload. The two designs are sized and analyzed for the new Gulf of Mexico metocean criteria. The comparison is based on hull dimensions, including heave plate and structural support construction. In both cases, results of the hull performance predicted by numerical simulations from fully coupled models are compared and discussed.