Dynamic Responses of a Semi-type Offshore Floating Wind Turbine

This paper deals with wave-wind induced global motion and mooring system responses of a semi-type offshore floating wind turbine. The analysis in this paper is conducted in time domain, using an aero-hydro-servo-elastic simulation code-FAST. Due to the limitation of the initial drag force model in FAST, a quadratic damping model is developed to gain a better consideration of viscous performance. Simulations involve free-decay in still water, dynamic responses in moderate sea states with and without wind. Aerodynamic effects on platform dynamic responses are investigated. Analysis indicates that wind force would accelerate platform's decay motion and increase natural periods of surge and pitch, while heave motion is little influenced. Pitch motion is decreased at wave frequency range under joint wave-wind loads condition. Little wind-induced pitch resonant response has been observed. Nevertheless, instabilities are induced for surge motion due to negative aerodynamic damping. A typical mooring line force is found to be greatly excited by wind force.