Control optimization and dynamic response analysis of a combined semi-submersible floating wind turbine and point-absorber wave energy converters

Hybridization of a floating offshore wind turbine (FOWT) with wave energy converters (WECs) is a solution providing additional stability and energy reliability to the FOWT. This paper investigates a wind-wave hybrid energy system (WWHES) consisting of a 5-MW semi-submersible FOWT and three point-absorber WECs. The main focus is on the effects of the integrated control optimization of the FOWT and WECs on the stability, loads and power outputs of the WWHES. Numerical simulations are performed using the Coupled Analysis Tool for Integrated Floating Energy Systems. The optimization of the FOWT control is implemented in Wind Energy with an Integrated Servo-control framework to minimize tower-base fore-aft bending moment, and the WEC control parameters are optimized to maximize the WEC power production. Various combinations of FOWT and WEC control strategies are investigated in terms of platform pitch motion, tower-base fore-aft bending moment and nacelle acceleration in the fore-aft direction. The considered FOWT control strategies include: baseline controller, Reference Open-Source Controller (ROSCO) optimized for the FOWT (without accounting for the presence of WECs), and ROSCO optimized for the WWHES where the platform and three WECs are included as a rigid body. The results demonstrate that the integrated control optimization benefits the WWHES by simultaneously suppressing motion and enhancing power production, although sacrificing a small portion of power under conditions very close to the rated wind speed.