Comparison of Fatigue Characteristics of Polyester and Nylon Mooring Lines for Shallow Water Floating Offshore Wind Turbines

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Abstract The offshore wind industry is growing, and turbines with power production of 10 MW or greater are likely to become the norm in the future for both shallow and deep waters. For up to 50 m water depth, bottom fixed structures like monopiles and jackets are considered technically and economically feasible. Beyond this water depth, floating sub-structures offer the promise of cost reduction. Synthetic fiber ropes like nylon and polyester are good candidates for mooring system designs not only for their low cost, corrosion resistance, and fatigue life compared to steel wires and chains but also for their low stiffness which can reduce the wave frequency loads on the floater, especially in shallow water. In an IOWTC 2022 paper, the present authors discussed the strength characteristics of a shallow water mooring system comprising of chain-heavy chain-polyester-chain combination. This combination was specially chosen to work in shallow water conditions (60–100 m). Using simulations, the most probable maximum values of the mooring line were presented and discussed. In this paper, fatigue analysis has been performed on a similar mooring configuration wherein we consider two synthetic material options: polyester and nylon. The IEA 10 MW reference wind turbine and a semi-submersible floating platform were used in our simulation in Nantucket, MA (Water Depth (WD) = 60m). Three different design load cases DLC 1.2, DLC 6.1, and SLC I.1 were conducted in our simulations and the mooring line fatigue damages converted to represent 25-year service life. The analysis of mooring line tension-tension fatigue damage is calculated using the T-N curves. The fatigue damage is estimated using the built-in fatigue analysis toolbox in OrcaFlex after performing a set of OrcaFlex simulation files based on different mooring line configurations and design load cases. In this work, the behavior of nylon vs. polyester in shallow water hybrid mooring applications is investigated and discussed.