Experimental study on vortex-induced motions of a semi-submersible at different spacing ratios

Vortex-induced motion (VIM) is a primary cause of fatigue damage to the mooring systems and risers of semi-submersible platforms. For multi-column platform structures, flow refraction and diffraction around individual columns generate complex ambient flow fields, and the underlying VIM mechanisms have not been fully clarified. In this work, towing tank model tests are performed to investigate the effects of column spacing ratio (S/D) on the VIM responses of a semi-submersible platform under a 45°current incidence angle, focusing on the quantitative correlations of motion amplitudes and frequency characteristics with spacing ratio and reduced velocity. The experimental results indicate that distinct lock-in phenomena appear in platform sway motion under all tested S/D conditions. According to frequency characteristics, the VIM evolution can be classified into three stages: pre-lock-in, lock-in and post-lock-in. In particular, the frequency coupling behavior in the post-lock-in stage is highly sensitive to column spacing ratio. Although all S/D cases exhibit consistent amplitude variation trends with reduced velocity, significant differences are observed in amplitude magnitudes. Under identical test conditions, larger column spacing ratios generally induce more intensive VIM responses, with a maximum sway amplitude difference of 30.8%. Further power spectral density (PSD) analysis verifies that surge galloping occurs in the post-lock-in region, while yaw galloping is synergistically dominated by reduced velocity and column spacing ratio. Although large column spacing can mitigate inter-column flow interference, such layout schemes substantially increase the risks of multi-degree-of-freedom VIM and galloping excitation. Therefore, simply increasing column spacing is not an effective measure for VIM suppression in practical semi-submersible platform design. The optimal S/D should be comprehensively determined by balancing the sway lock-in range and the critical onset velocities of surge and yaw galloping, as well as fully considering the current spectrum of the operating sea area. This multi-objective design strategy can achieve effective overall control of platform VIM risks.