Optimal Mooring Pattern for a Semi-Submersible FOWT in a Typhoon Environment

Abstract The scarcity of suitable shallow waters for fixed-bottom offshore wind turbines has prompted developers to explore deeper waters, albeit with caution due to the significant capital expenditure (CAPEX) associated with Floating Offshore Wind Turbines (FOWTs). A major cost component in FOWTs is the mooring system, a concern exacerbated in regions with the presence of typhoons, necessitating a more robust and therefore expensive 3 × 3 mooring solution compared to 3 × 1 in areas such as the North Sea. This study tried to propose a 3 × 2 mooring arrangement tailored for FOWTs in the typhoon region, offering a potential cost reduction of up to one-third compared to the original 3 × 3 configuration. To achieve cost savings, an in-depth analysis of spreading angles from 10° to the widest angle of 126° is performed using the pre-tension-diameter vs offset-tension 3D response surfaces technique. The research reveals that, theoretically, a 60° mooring angle minimizes floater offset with the least tension. However, the discrepancy in safety factors between intact and damaged conditions makes grouped mooring a more cost-effective choice. Utilizing the American Bureau of Shipping (ABS) safety factor, a 10° spread angle mooring system’s chain cost emerges as roughly one-tenth cheaper than alternative configurations. Additionally, the study explores an innovative V-Share mooring 1, wherein a single pile anchor connects to two different columns via two mooring lines. Depending on the anchoring conditions, it can be the most cost-effective. In conclusion, a 3 × 2 mooring pattern for a 15MW semi-submersible FOWT in typhoon regions might be theoretically achieved on paper. However, it may not be practically achieved, if the size limit of the manufactured chain is considered.