Wave Scattering and Hydrodynamic Interaction Effects Among the Columns of the OC4-DeepCwind Semi-Submersible Floating Offshore Wind Turbine

Abstract

This study investigates the hydrodynamic behavior of the OC4-DeepCwind floating offshore wind turbine, with a specific focus on the influence of wave reflection and hydrodynamic interaction effects between the platform components. The OC4-DeepCwind semi-submersible platform, supporting the NREL 5 MW reference wind turbine, is analyzed using the commercial software ANSYS AQWA 2024 R1 and the in-house codes HAMVAB and SEMISUB. While ANSYS AQWA and HAMVAB account for multiple wave scattering effects within the multi-column configuration, SEMISUB neglects hydrodynamic interactions, enabling a systematic assessment of their influence on the predicted response. Interaction effects are most pronounced in the surge degree of freedom, where neglecting wave reflection distorts the exciting wave force above 0.65 rad/s, and in the surge, heave, and pitch added mass and radiation damping coefficients, with substantial deviations above approximately 0.6 rad/s. The influence of column separation distance on these diffraction loads is also examined. Stochastic-wave simulations of the moored wind turbine under realistic JONSWAP sea states show normalized errors across all examined sea states of 16.6% (heave), 14.6% (surge), and 9.9% (pitch) in platform motions when interactions are neglected, whereas tower-base loads and mooring line tensions are less sensitive, with errors of 11.5% (vertical shear force), 9.4% (horizontal shear force), 9.2% (bending moment), 8.7% (downstream mooring tension), and 5.6% (upstream mooring tension). These results indicate that hydrodynamic interaction effects are critical for predicting platform motions and hydrodynamic coefficients but have a comparatively limited effect on design-governing structural and mooring loads, offering quantitative guidance on when simplified interaction-free models remain adequate for semi-submersible FOWT design.