Two production semi-submersibles that were based on Exmar Offshore Company's proprietary OPTI& Hull had been successfully constructed and installed in the Gulf of Mexico in the past five years. The OPTI& hull features pontoon lip extending outboard of five-sided columns. The pontoon lip improves heave motion and the five-sided column design has proven effective in Vortex-Induced-Motion (VIM) reduction.

This paper presents the methodology and main considerations in the OPTI& hull sizing process. With a set of limiting criteria, an in-house hull sizing program is used to generate thousands of hull configurations and to perform diffraction and radiation analysis. Subsequently, the hull steel weight and heave motion is examined to determine a design front on which a better compromise between hull steel and motion is achieved. A few candidate hulls may be selected with further considerations of constructability, mechanical and electrical equipment arrangement and structural design restrictions. Finally, Client's preference, riser design requirements, installation and operation preferences, and economics are fully considered.

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