Hydrodynamic analysis of a semi-submersible FOWT by hybrid panel-stick models

A diffraction-radiation analysis is usually required when the hydrodynamic interactions between structural members occur in short waves. For bracings or small cylinder members, which play important roles in the vicinity of the natural frequency of a floating platform, special care should be taken into account for the effect of viscous damping. Two hybrid panel-stick models are therefore developed, through the combination of the standard diffraction-radiation method and the Morison's formulae, while considering the effect of small members differently. The fluid velocity is obtained directly by the panel model. The viscous fluid force is calculated for individual members by the stick model. A semi-submersible type platform with a number of fine cylindrical structures, which is designed as a floating foundation for multiple wind turbines, is analysed as a numerical example. The results show that viscous force has large influence on the behaviour of the floating body and can successfully be considered by the proposed hybrid models.