Directional focused wave group response of a floating wind turbine: Harmonic separation in experiments and CFD

The offshore wind sector relies on floating foundations for deeper waters. However, these face challenges from harsh conditions, nonlinear dynamics, and low-frequency resonant motions caused by second-order difference-frequency hydrodynamic loads. We analyze these dynamics and extract such higher harmonic motions for a semisubmersible floating foundation under extreme wave conditions using experimental and numerical approaches. Two distinct, focused wave groups, with and without wave spreading, are considered, and experimental data is obtained from scaled physical model tests using phase-shifted input signals to provide the harmonic decomposition of the floating foundation wave responses. The measured responses are reproduced numerically using a novel Computational Fluid Dynamics (CFD) based rigid body solver called FloatStepper, achieving generally good agreement. The study quantifies the effects of wave severity, spreading, and steepness on odd and even harmonics of the surge and pitch responses of the floating foundation and mooring line tensions. The focused wave groups of a stronger sea state showed a notable increase in the amplitudes of odd harmonics for surge and pitch. In addition, the pitch subharmonic response, less noticeable in the milder sea states, became more apparent. Wave spreading primarily influenced the overall response of the spreading case, with a more pronounced effect observed on odd and even superharmonic responses. The results also reveal a front–back asymmetry in the tensions of the mooring lines, with the back lines experiencing greater tension than the front. Similarly to the effect of wave severity, a strict increase in wave group amplitude led to pronounced shifts in both subharmonic and superharmonic responses, transitioning from predominantly low-frequency surge-dominated behavior to a coupled surge-pitch interaction. The underlying cause of this pitch dominance is identified and discussed through CFD.