A Tapered Column Deep Draft Semi-submersible (TCDD-Semi) concept has been developed for dry tree drilling and production applications. The new concept features enlarged lower hull geometry with gradually varying column cross-section, in contrast to conventional semi-submersible design with uniform columns. The enlarged lower columns, in combination with properly sized upper columns and pontoons, provide sufficient buoyancy and stability for topside integration at quayside and offshore pre-service operations such as submergence and wet-tow. Most importantly, this new TCDD-Semi concept with tapered columns optimizes the cancelation effect of wave force and is able to reduce the heave motion in harsh environmental condition for dry tree applications with top-tensioned risers.

This paper will present the topside and hull configuration, tank arrangement and ballasting system, stability, as well as mooring system, riser system and global performance design, platform global strength analysis and global fatigue analysis, hull vortex induced motion analysis. In addition, platform fabrication and topside integration methods will also be discussed.

The design analyses show that the proposed TCDD Semi-submersible platform has advantages in heave motion performance, topside integration flexibility, and transition stability during pontoon submergence. Its excellent heave motion performance enables the application of top-tensioned riser system with 35 feet (10.5 m) stroke limit. The new design also significantly reduces platform VIM compared with conventional deep draft semi-submersible.

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