A simple prediction model for VIM of multi-column floaters was developed. The model solved the one degree of freedom equation of motion which included inertia, damping, restoration and external force term. Inertia, damping and external force terms were obtained experimentally from measurements of forced oscillation tests with and without towing velocity (current incidence). The numerical model using the force coefficients from experiments was tested for an array with four circular cylinders in a square configuration with column spacing ratio (distance between column centers) equal to 2 and 3. The numerical results were compared with VIM experiments, and the estimated motion amplitude showed a good agreement. The numerical results were able to evaluate the influence of the spacing ratio for the array with four circular columns.
The Floating Offshore Wind Turbine (FOWT) presents complex motion because the wind turbine is subject to wind and the floater is subject to waves and currents. Moreover, in a current, the floating structure is known to present periodic motion excited by the pressure oscillation caused by the vortex shedding. This behavior is called Vortex-Induced Motion (VIM), which has a similar generation mechanism to the so-called Vortex-Induced Vibration (VIV). VIM may cause severe effects on the fatigue of the mooring system. From the viewpoint of cost reduction, it is desirable not to exchange mooring cables. However, the influence of VIM on floating structures has not been quantitatively estimated sufficiently.
Although the VIM and VIV generation mechanisms are the same, there are some different characteristics between them. While VIM occurs for a structure with small aspect ratio with oscillations as a rigid body, VIV occurs for a structure with large aspect ratio and vibrates with elastic deformation. Since a structure with low aspect ratio has a small draft, the effect of three-dimensional flow becomes stronger than that with high aspect ratio. VIM usually exhibits a long oscillation period as compared to VIV.