The vortex-induced motions (VIM) phenomenon of semi-submersibles has drawn increasing attention with the development (mainly increase of column size) of new semi-submersibles. Due to the elongated submerged columns and the enlarged projected area to current, deep-draft semi-submersible platforms are susceptible to higher in-line drag forces and transverse vortex-induced lift forces, resulting in considerable horizontal motions in a current environment. In order to check the influence of draft conditions on VIM of the semi-submersible platform with four square columns, experimental investigations with five draft ratios varying from 0.87 to 1.90 were carried out in a towing tank.

The 6-degree-of-freedom (6-DOF) motions of the model were recorded by the motion acquisition system, in synchronisation with restoring forces provided by four load cells, one for each horizontal mooring spring. This paper discusses the dynamic behavior of a semi-submersible platform in five different draft conditions, including coupled motions at the water surface plane, drag and lift forces, and spectral analysis. It is shown that the largest transverse amplitudes are around 75% of the column width in the range of 6.0 ≤ Ur ≤ 8.0 for the deep-draft semi-submersible (H/L = 1.90). With 50% of the immerged column height of the deep-draft model, a 30% decrease in the transverse motion amplitude can be seen. Furthermore, the effects of the draft condition on yaw responses and current loads are also addressed.

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