Dynamic response of a semi-submersible floating wind turbine-point absorption wave energy hybrid energy system under rated and extreme conditions

In this paper, the response of a semi-submersible offshore wind turbine, combined with an absorbing wave energy converter (WEC), is analyzed within the FAST to AQWA coupling framework after single mooring line (ML) failure, under rated and extreme (1 in 10 years wind and wave) conditions. Comparisons are made among the six-degree of motions of the platform, power output, and tensions of the unbroken MLs. With upwind mooring line, the hybrid system suffers the most: surge up to 3.3 times the rotor diameter (D) due to the large hub forces and turbine power reduction by 80%, owing to the pitch response of the platform in both rated and extreme conditions; more likelihood of fatigue in unbroken ML under extreme conditions. Regardless of the failure line condition, the mean tension of the unbroken MLs reduces at the expense of larger motions, compared to a non-faulty line condition. The shutdown measures can reduce significantly the tensions in the MLs at rated conditions, owing to turbine larger influence on the platform than WEC, higher motion response and damping processes. The effect of the WEC on the platform is mainly reflected in heave and roll, providing more stability than in a stand-alone floating turbine case. During extreme conditions, the turbine shuts down, and thus, the response is more attributed to the drift of the platform rather than turbine or WEC operation. On a wider level, research is also needed to determine the response in simultaneously ML failures and misaligned operating flows.