Mean and Low Frequency Roll for Semi-submersibles in Waves

The paper demonstrates how the findings of a series of model tests and diffraction calculations, performed to determine the motion behavior of a new semi-submersible design were used to influence the final design. From the model tests at different drafts, wave headings and current speeds it was observed that a semi could take on a steady list angle in regular head waves. It was observed that this steady list angle increases for smaller vessel drafts and higher current speeds. This mean list angle appeared to be induced by shallow water wave effects on top of the pontoons and resulting set-down. Increasing the stability of the semi by changing the vertical centre of gravity reduced the steady list angle, and in some cases removed the list completely. As a result the design of the stability columns was changed to provide a substantial increase in initial stability at both operating and survival drafts. The model test results were confirmed by a diffraction analysis that also provided a good assessment of the forces causing the steady list angle.

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