Wave-induced loads on semisubmersible platforms in operational and survival conditions are usually evaluated from the Morison equation. If the pontoons are unusually large, closely spaced, or near the sea surface, then interaction effects that are not included in the Morison equation approach become significant. In the transit condition, when the pontoons are floating in the sea surface, the wave loads must be calculated by other methods. An improved strip theory approach is presented utilizing “opposed motion potentials.” Interaction effects between the pontoons are included. The present improvement provides additional information about the spatial distribution of the diffraction forces, enabling the determination of the internal forces between the pontoons. Comparisons are shown between results from the Morison equation approach, the improved strip theory, and model tests for a recent semisubmersible design at operational draught. The improved strip theory is also compared to 3-D sink-source calculations for the same rig at transit draught.

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