Directional focused wave group response of a Floating Wind Turbine: Harmonic separation in experiment and CFD

Abstract

The offshore wind sector relies on floating foundations for deeper waters but faces challenges from harsh conditions, nonlinear dynamics, and low-frequency resonant motions caused by second-order hydrodynamic loads. We analyze these dynamics and extract higher harmonic motions for a semisubmersible floating foundation under extreme wave conditions using experimental and numerical approaches. Two focused wave groups, with and without spreading, are considered, and experimental data is obtained from scaled physical model tests using phase-shifted input signals for harmonic decomposition of the wave responses. The responses are reproduced numerically using a novel CFD-based rigid body solver, FloatStepper, achieving good agreement. The study quantifies the effects of wave severity, spreading, and steepness on odd and even harmonics of the surge and pitch responses and mooring line tensions. A stronger sea state notably increased odd harmonics in surge and pitch. Additionally, the pitch subharmonic response, less noticeable in milder states, became apparent. Wave spreading influenced the overall response, with pronounced effects on odd and even superharmonic responses. The results reveal a front-back asymmetry in mooring line tensions, with the back lines experiencing greater tension. Increasing wavegroup amplitude caused shifts in subharmonic and superharmonic responses, transitioning from low-frequency surge-dominated behavior to coupled surge-pitch interaction. The cause of this pitch dominance is identified and discussed via CFD.