Dynamic responses of a semi-submersible wind turbine platform subjected to focused waves with viscous effects

In most previous studies on the dynamic responses of floating offshore wind turbines, regular wave conditions are assumed in the analysis with the inviscid flow theory. The focused waves, however, have not been considered even though they may have larger wave heights and more concentrated energies, in general, to cause more significant responses in a floating platform. In this study, the characteristics of the dynamic responses of a semi-submersible wind turbine platform subjected to focused waves are studied using a sliding mesh technique with the three-dimensional shear stress transport k - ω turbulence model. Effects of wave steepness, fluid viscosity, and wave nonlinearity on the dynamic responses are investigated. The high-order wave loading in the transverse direction is found significant under high wave steepness conditions. The viscous effects of fluid notably aggravate the pitch and surge dynamics of the floating platform compared to those from under the inviscid flow conditions. Due to the nonlinear characteristics of the focused wave, the floating platform is found to experience a long vibration period and slow drift dynamics in the surge direction after the focused time with significant fluctuation.