Design and analysis of taut mooring systems for a combined floating offshore wind and wave energy system at intermediate water depth

The present study proposes an engineering approach to mooring system design for the Semi-Submersible Torus Flap Combination (STFC), a floating offshore combined wind and wave energy system deployed at a water depth of 50 m. During the initial design phase, an analytical method is proposed to provide quick estimations of key design criteria for mooring configurations considering different of anchor radii and mooring line stiffnesses. These analytical results are then validated against the results calculated using fully coupled numerical models in SIMA. To assess the impact of wave energy converters (WECs) on STFC's dynamic responses, its behaviour under aligned wind and wave loadings are compared with that of a semi-submersible offshore wind turbine utilizing the same supporting platform and mooring system. Subsequently, the global responses of STFC with 5 different mooring configurations are compared in terms of platform motion responses, mooring line tensions and wind and wave power productions. The findings indicate that the analytical method efficiently estimates mooring system characteristics with reasonable accuracy, and it can serve as a valuable tool in the preliminary stages of taut mooring system design. In addition to wave power absorption, the WECs introduce wave-induced damping to the platform motion which reduces the mooring line tension responses, enhances mooring line fatigue life and minimizes the fluctuations in wind turbine power production. • Polyester-rope-based taut mooring systems are proposed for the Semi-Submersible Torus Flap Combination (STFC) deployed at 50 m waters. • An efficient analytical method is proposed for evaluating preliminary taut mooring design criteria using key design variables such as mooring line stiffness and anchor radius. • Based on the analytical results, fully coupled numerical analyses of STFC are conducted with 5 different mooring configurations. • The higher wave-induced responses for STFC as compared to CSC result in an effective damping of the low-frequency platform surge motion. • The global responses of the STFC with various mooring configurations are compared and the results provide recommendations for mooring system design in intermediate waters.