A novel small footprint shared anchor (SFSA) mooring system concept is proposed for deepwater floating wind turbines based on a new "length-varying tension-fixed" (LVTF) theory. Theoretical discussions are provided, and an example is given using a simplified 2-line system to illustrate the LVTF mooring theory and SFSA concept. Preliminary design and numerical analyses of a 3-line SFSA LVTF mooring system are performed for a 15 MW reference semi-submersible floating wind platform in 200 m and 1000 m water depth. The results show that the novel SFSA LVTF mooring concept is feasible, thus providing a cost-effective and environmental-friendly mooring solution for large scale deepwater floating offshore wind energy development.
As the demand for offshore wind power increases, there are growing needs to deploy large wind turbines supported by floating foundations in water depth greater than 60 meters where conventional bottom-fixed foundations become less attractive economically. During the past decade, remarkable advancements in floating offshore wind turbine (FOWT) platform technologies have been achieved. There are currently more than 30 different design concepts of FOWT platforms as summarized in a recent report to BOEM by ABS (2021). In contrast, mooring system technologies for station-keeping of floating offshore structures have not advanced significantly for several decades. Existing mooring system concepts (mainly catenary, taut leg and tension leg), rules and design practices developed for the offshore oil and gas industry over the past 60 years are being adopted for floating offshore wind energy development. The existing design tools, modeling practices, and industry standards are well established, mature and project tested (ABS, 2022; Chen and Kim, 2021; Ma, Wu, Stolen, Bello, Horst, and Luo, 2021), thus are extremely valuable to the nascent floating offshore wind industry. However, there are significant differences between these two industries. For deepwater offshore oil and gas development, the mooring system is typically custom-designed (not sensitive to cost) for one standalone high-value (often more than $1 billion) floating production and/or drilling platform installed in water depth greater than 500 meters located at a remote oil and/or gas field far away from the shoreline. On the other hand, for floating offshore wind energy development, the mooring systems must be of standardized design (sensitive to cost) for large quantity (over 50 units) of relatively low-value (less than $50 million per unit) FOWT platforms installed in relatively shallow waters (60 - 200 m water depth) not far away from the shoreline. These differences pose unprecedented technical, environmental and supply chain challenges. In particular, for a commercial floating wind farm, the mooring lines of conventional catenary mooring systems will have footprints over a large area of the ocean causing potentially negative impact to the marine environment and co-use issues. It is recognized that simple adoption of the existing mooring concepts cannot meet the dual challenges of high cost and high environmental impact the floating wind industry is facing today.
