Optimization of motion and power performance of a wind-wave hybrid system with built-in wave energy converters

To reduce the cost of wave energy exploitation, various wind-wave hybrid systems have been developed by integrating wave energy converters (WECs) into existing floating offshore wind turbines (FOWTs). Although the power performance of such hybrid systems has been widely optimized, the dynamic stability in waves often shows poor performance. In this research, a novel hybrid system consisting of a semi-submersible FOWT and three built-in oscillating body WECs is proposed. A numerical model simulating its constrained dynamics is developed. A surrogate model-genetic algorithm framework is constructed for rapid optimization. Two optimization criteria of maximizing the wave power generation and minimizing the wind platform motion are set. Single-objective optimizations are first conducted, based on which the effects of three key parameters of the WECs, namely dimensionless stiffness, damping, and mass, on both motion and power performance are investigated. The motion-optimized system reduces the maximum pitch motion by 72.15 % compared with the single platform, whereas the power-optimized system achieves a peak wave power generation of 357.41 kW under unit wave amplitude. A multi-objective optimization is performed to reveal the trade-off relationship between power and motion. The Pareto-optimal solutions are compared with single-objective results and it is shown that as the wave power generation becomes greater, more trade-offs of motion should be made to continually increase the wave power generation. This study provides insights into power-motion interaction and offers guidance for the optimal design of wind-wave hybrid systems.