Numerical modelling and dynamic response analysis of a two-rotor semi-submersible wind turbine with integrated wave energy converters

Mounting multiple rotors on a shared supporting structure offers a scalable pathway to increase offshore wind energy production without complete rotor redesign. Additionally, integrating wave energy converters (WECs) into floating offshore wind turbines (FOWTs) enhances overall system efficiency through shared use of infrastructure and marine space. In this study, a novel combined wind and wave energy (CWWE) system is proposed, featuring a semi-submersible FOWT with two 5 -MW rotors and a torus WEC. The system is developed using an in-house, fully coupled simulation framework built on the OpenModelica platform. A detailed description of the tool, along with a code-to-code comparison with commercial software, validates the newly developed features not previously published. Dynamic response analyses under various wind and wave conditions confirm the feasibility of the two-rotor configuration installed on a semi-submersible platform and assess the influence of the WEC. The system’s yaw motion is effectively controlled using a linear quadratic regulator (LQR), which limits the platform yaw standard deviation (SD) to a maximum of 1.27° under all investigated environmental conditions. The addition of the torus WEC increases heave restoring stiffness and shifts the natural frequency into the wave excitation range. However, damping from the power take-off (PTO) system successfully mitigates the resulting resonant motion. To evaluate site adaptability, the CWWE system is assessed at two representative water depths: 200 m (deep) and 50 m (intermediate). In intermediate waters, increased mooring stiffness reduces platform mean offset and raises surge natural frequency, making mooring tension more sensitive to higher-frequency excitations compared to that of the deep-water mooring system. Finally, a configuration with multiple torus WECs is explored. Despite the use of a simplified PTO model with constant stiffness and damping, the system achieves an increase in wave energy capture from 1.35% to 8.85% of the total mean power output, while simultaneously reducing platform pitch resonance. These results highlight the potential of multi-WEC hybrid systems for offshore renewable energy.