Wave- and drag-driven subharmonic responses of a floating wind turbine

The nonlinear hydrodynamic responses of a novel spar-type soft-moored floating offshore wind turbine are investigated via analysis motion measurements from wave-basin campaign. A prototype the TetraSpar floater, supporting $1:60$ scale model DTU 10 MW reference turbine, was subjected to irregular wave forcing (with no wind) and shown exhibit subharmonic resonant motions, which greatly exceeded wave-frequency motions. These slow-drift excited nonlinearly, since rigid-body natural frequencies system lie below incident-wave frequency range. Pitch is examined in detail, allowing for identification different mechanisms. response found contain odd-harmonic components, addition even harmonics expected priori by second-order difference-frequency interactions. Data utilising harmonic separation signal conditioning suggests that Morison drag excitation or third-order potential flow could be at play. In extreme survival-conditions sea state, odd identified drag-driven. Their importance tested floater appreciable, as their magnitude comparable effects. Under such severe conditions, would not operating, neglecting aerodynamic damping likely reasonable. Additionally, other possible drivers pitch explored. Both Mathieu-type parametric wavemaker-driven error waves have negligible influence. However, we note slight contamination arising sloshing.