Experimental investigation into the influences of pontoon and column configuration on vortex-induced motions of deep-draft semi-submersibles

Deep-draft design of semi-submersibles improves vertical motions but leads to critical vortex-induced motions (VIM) attributable to fluctuating loads on columns. As characteristic components, both columns and pontoons produce complex wake interference and thus have significant influences on the VIM of semi-submersibles. This paper presents an experimental study on VIM of deep-draft semi-submersibles (DDS) to examine the effects of pontoons and column configuration. There were four test models including four-pontoon DDS, two-pontoon DDS, four-square-column structure and four-rhombic-column structure with no pontoons. A number of current headings and reduced velocities were considered. The main results reveal that the two-pontoon DDS can generate larger lift forces in comparison with the four-pontoon DDS; consequently, the transverse motion amplitudes at high reduced velocities are larger. However, the four-square-column structure with no pontoons shows the most significant transverse and yaw responses owing to the largest excitation forces induced by the well-established wake. On the other hand, similar trend and values in the transverse response are observed for the four-square-column structure at the 45° current heading and for the four-rhombic-column structure at the 0° current heading. The incidence angle related to columns has more significant effect on the VIM of DDS than that related to the platform.