Fatigue life assessment of nylon versus polyester mooring lines

Abstract

Mooring systems are essential for station keeping of floating offshore structures under environmental loads. Synthetic fiber ropes such as polyester and nylon are considered strong candidates for mooring systems in both deep and shallow waters due to their low cost, corrosion resistance, and low stiffness. Accordingly, predicting their fatigue damage has become an important area of research. This study evaluates the fatigue performance of two mooring systems: chain-nylon-chain and chain-polyester-chain, for a semi-submersible floating offshore wind turbine in shallow water. Following industry guidance from the American Bureau of Shipping, two different stiffness values were used for polyester and nylon: a “Static EA” for static simulations and a “Dynamic EA” for dynamic simulations. To achieve comparable performance, adjustments to mooring line lengths, diameters, and anchor positions were made to reflect their differing mechanical properties. Sixty-six sets of loading conditions representing DLC 1.2, DLC 6.1, and SLC I.1 were simulated. Fatigue analysis was conducted by post-processing the tension time histories to extract stress cycles via the Rainflow counting method, followed by fatigue life estimation using Palmgren-Miner's cumulative damage rule. To validate this approach, a parallel fatigue analysis was conducted using the built-in fatigue toolbox in the commercial software OrcaFlex. The comparison between the two methods showed strong agreement, with a maximum percentage difference of approximately 1.38 %. Results indicate that nylon mooring lines offer superior fatigue life compared to polyester, with the platform–chain segment in the nylon configuration also showing significantly enhanced durability.