Impact of high order wave loads on a 10 MW tension-leg platform floating wind turbine at different tendon inclination angles

Floating wind technology is being developed rapidly with the aim of harvesting high-energy wind resources in medium and deep water areas, unreachable using fixed bottom solutions. Given the complexity of these systems, the interactions between the structure and incident hydro-aerodynamic forces need to be well understood. While numerous solutions are being explored, an optimal design is yet to be established within the industry.

This study explores the effects of tendon inclination on the dynamic behaviour of a 10MW tension-leg platform (TLP) floating offshore wind turbine (FOWT), and the interaction of different design solutions with higher-order hydrodynamic loading. The model was subject to an extreme sea state in order to capture second and third-order wave effects, and the nonlinear waves were generated via the high-order spectral (HOS) method. The analysis was performed using the hydrodynamic engineering tool CALHYPSO, in-house developed by EDF Lab.

Second and third order inertial hydrodynamic loads were included in the time-domain simulations in order to capture low frequency loads and ringing effects respectively. Results show that difference-frequency second order effects have a negligible impact on motions and tendon tensions of the analysed floating wind turbine model, while third order terms can significantly enhance the dynamic response of the system to extreme incident waves. While inclined-leg floater configurations presented improved motion and tendon tension responses under linear loading, the inclusion of quadratic and triple-frequency contributions showed that tendon inclination can in fact increase tension variations in the mooring lines when subject to extreme wave climates. This can lead to slacking in the mooring lines being observed more frequently in inclined-leg configurations. The results therefore suggest that neglecting third order effects, as commonly done in industry, can lead to significant underestimations of motion and tendon tension responses of tension-leg platform wind turbines.