Motion Analysis of a Semi-submersible Floating Offshore Wind Turbine in Focused Waves by a Consistent Second-Order Model

An IEA 15 MW semi-submersible floating offshore wind turbine (FOWT) in focused waves is investigated by a consistent second-order time-domain hydrodynamic model, formulated in the body-fixed coordinate system. The quadratic drag loads on the horizontal pontoons are estimated by locally calculated Keulegan-Carpenter (KC) numbers, with the required KC- dependent drag coefficients predetermined by two-dimensional computational fluid dynamics analysis for the relevant cross sections. Soft linear springs are used to approximate the restoring mechanism of mooring system. The proposed viscous correction performs optimally compared with model tests for the free decays of a FOWT in calm water. For the FOWT in focused waves, significant low-frequency (LF) surge and pitch motions, as observed in the model tests, are reproduced by the present second-order time-domain model with acceptable accuracy. The linear model notably underestimates the motion responses near wave-focusing time and the large-amplitude transient LF oscillations as the focused waves propagate away.