The FHSB Semi (Free-Hanging Solid Ballast Semisubmersible) is an innovative floater design that maintains the advantages of the conventional semisubmersibles while offering the favorable motions of the Spars. Therefore, it is a suitable solution for supporting dry tree applications in ultra-deepwater harsh environmental conditions. This new design resembles the conventional semisubmersible hull design with an added feature of a retractable Free-Hanging Solid Ballast tank located deep below the hull keel level and connected through four groups of chains (on its four corners) to the semisubmersible hull; hence the name FHSB Semi. In this paper, a parametric study is performed to optimize the key design parameters that govern the performance of the FHSB Semi response in ultra-deepwater harsh environment. For the purpose of this study, the payload, number of Top Tensioned Risers (TTRs), export Steel Catenary Risers (SCRs), environmental condition and water-depth similar to those used in designing an existing Spar platform in the Gulf of Mexico (GoM) have been used to design an equivalent FHSB Semi platform. Results are obtained using fully coupled nonlinear analysis and are presented to illustrate the sensitivity of the platform performance to each of these parameters. Finally the added safety in design, and cost benefits offered by the optimized FHSB Semi design are quantified and presented.
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