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

An IEA 15MW semi-submersible FOWT in focused waves is investigated by using a consistent second-order hydrodynamic model in the time domain, uniquely formulated in the body-fixed coordinate system (BFCS). The quadratic drag loads on the cross sections along the horizontal pontoons are estimated based on the local Keulegan-Carpenter (KC) numbers instantaneously estimated in the time domain, and the required KC-dependent drag coefficients are pre-calculated by dedicated two-dimensional (2D) CFD analysis for the relevant cross sections. Soft linear springs in the horizontal plane are used to approximate the restoring mechanism due to mooring system. The proposed empirical viscous correction works very well in comparison to the model tests of free decays in calm water. For the FOWT exposed to focus waves, significant low-frequency (LF) surge and pitch motions are observed in the referred model tests, which are reproduced by the present second-order time-domain model with reasonably good accuracy. The linear model is shown to significantly underestimate the motion responses close to the wave-focusing time and the large-amplitude transient LF oscillations when the focused waves have propagated away.