Experimental and numerical studies on the low-frequency responses of a spar-type floating offshore wind turbine

Accurate modeling of low-frequency (LF) wave loads and responses is important but challenging in the design of spar-type floating offshore wind turbines (FOWTs), especially for shallow and intermediate water depths. A 6 MW spar-type FOWT system designed for a water depth of 100 m is investigated both experimentally and numerically to study the nonlinear hydrodynamics responses in waves. The second-order LF wave loads are modeled in the time-domain tool FAST using two different methods based on full Quadratic Transfer Functions (QTF) and Newman's approximation, respectively. To account for viscous effects in calculating the LF responses, linear and quadratic damping models calibrated from model tests are comparatively studied and the latter is found to be more suitable and easier to implement in the time-domain simulations. It is demonstrated that it is crucial to include the 2nd order difference-frequency wave excitation in the analysis of motions in both horizontal and vertical planes. With the considered water depth, a full QTF method is seen to greatly improve the prediction of LF responses compared with Newman's approximation. In general, it is observed that the wave-frequency (WF) responses are higher in amplitude than the LF responses for surge and heave, while the LF pitch motions are higher than the corresponding WF contribution.