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.
The majority of today's deep-water developments are located in the Gulf of Mexico, Brazil, and West Africa. These are all areas with moderate environments characterized by much lower wave heights than are found in areas subject to harsh environments such as the North Sea and Eastern Canada. A semi-submersible designed specifically to operate only in moderate environments will have acceptable motion characteristics even if the height of its stability columns is less than the column height of a rig designed to maintain sufficient air gap in storm waves and to have acceptable motion characteristics in larger operating waves. The capital cost of such a moderate environment semi-submersible will be substantially less than the cost of its harsh environment cousin.