Power Performance of a Floating Wind Turbine Model under Individual and Coupled Pitch and Surge Motions

Floating offshore wind turbines (FOWTs) can experience complex coupled motions that substantially influence their power performance. This study combines theoretical analysis and wind-tunnel experiments to assess the effects of individual and coupled surge and pitch motions across a moderate range of frequencies, amplitudes, and phase differences. Kinematic analysis reveals that phase difference governs the coupling effects by modulating the motion-induced rotor velocity. Results show that power variations are periodic, with pitch-induced changes in rotor orientation exerting the dominant influence. Within the tested range of dynamics, surge motion reduces mean power, whereas pitch motion can mitigate this reduction and even lead to a net power increase in certain in-phase coupled cases. Theoretical predictions show reasonable agreement with experimental results, though discrepancies at higher frequencies (i.e., Strouhal numbers) highlight the role of unsteady aerodynamics due to the modification of local blade-element kinematics and induction effects. Additionally, results indicate that platform motions of the upstream FOWT, particularly pitch, can benefit downstream turbines through enhanced wake deflection and mixing.