Analysis of a Hybrid Mooring System Concept for a Semi-Submersible Wind Turbine in Intermediate Water Depth Under Operational, Extreme, and Yaw Error Conditions

Abstract A hybrid mooring system consisting of catenary chains and a series of clump weights was designed for the intermediate water depth, for a floating semi-submersible wind turbine. The use of multiple clump weights in the mooring line provides smaller footprint area and shorter bottom section length, as compared to a normal mooring line without clump weights. This suits the use for intermediate or shallow waters. The performance of this mooring concept is analyzed through both experimental and numerical methods. The experimental and numerical mean values of the pitch motion, surge motion and fairlead tension match well under both operational and extreme conditions. The wave-induced low-frequency responses match well after tuning the drag coefficients. The low-frequency components are slightly over-estimated by the numerical model under wind-only condition, because the wind loads and damping acting on the blade model in FAST and in model test are different. However, these components are simulated well under a combined operational wind-wave-current condition. Under extreme condition, the mean and dynamic tensions at clump weights are noteworthy. For extreme condition, the surge motion and fairlead tension of line#4 are more severe when yaw error angle is larger.