Hydrodynamic analysis and optimisation of a novel wind-wave hybrid system combined with the semi-submersible platform and various wave energy converters

The wind-wave hybrid system, comprising floating offshore wind turbines (FOWTs) and wave energy converters (WECs), is emerging as a promising solution for efficient ocean renewable energy extraction due to its multiple reciprocal benefits. However, challenges related to coupling mechanisms and structural optimisation continue to limit its potential for significant motion reduction. Numerical models based on empirical theories are often insufficient to produce fully coupled, high-fidelity aero-hydro-mooring simulations. This study proposes a novel wind-wave hybrid system and investigates its hydrodynamic performance through the optimisation of coupling layouts and key parameters using the computational fluid dynamics (CFD) software OpenFOAM. A new DeepCwind-WEC hybrid system is introduced, combining oscillating-body WECs (OBWECs) and Wavestars on the DeepCwind platform. A multiphase solver, overWaveIsoFoam, is developed and validated, integrating the isoAdvector algorithm, overset mesh technique, and the waves2Foam toolbox. Using overWaveIsoFoam, various hybrid layouts and key parameters are optimised. The hydrodynamic motion-reduction effect, mooring dynamics, and wave-shedding characteristics are analysed to evaluate the performance of the optimised configuration. The results demonstrate that: (i) the hybrid system, incorporating 12 inner OBWECs and 15 outer Wavestars, achieves an optimal layout with notable reductions in surge, pitch, and heave motions compared to the standalone DeepCwind platform; (ii) parameter optimisation of the outer Wavestars — arm projection Lh = 25 m and featured angle φ = 20° — further enhances motion stability; (iii) the integration of diverse WEC types increases mean mooring loads, though these remain within acceptable and predictable limits; (iv) the hybrid system intensifies the wave-shedding effect, enhancing wave reflection and mitigating diffraction; and (v) the extended WEC configuration exhibits increased sensitivity to wave directionality, while showing limited sensitivity to wave–current interaction.