Abstract
Abstract Nylon ropes are showing promising properties for the use in mooring lines for offshore wind energy systems. Determining the long-term reliability of nylon mooring ropes is an important consideration for the safe operation of offshore wind energy installations. These ropes are mainly composed of multiple sub-ropes; each formed from high-tenacity polyamide 6 (PA6) multifilament yarns. A reliability-centered design approach requires not only knowledge of deterministic mechanical properties but also a probabilistic estimation of performance under uncertainty. Detailed yarn-level creep and recovery data facilitate the development of stochastic models that capture the variability in material properties, loading conditions, and environmental effects. Such models support the estimation of service life distributions, identification of critical failure modes, and quantification of the probability of rope failure over the operational lifetime. This investigation conducts a comparative analysis of structural reliability methodologies to estimate the failure probability of nylon mooring lines employed in floating wind turbines. The research leverages both simulated and empirical data, with extreme tension loads estimated by means of a numerical wind turbine model subjected to a Gulf of Maine environment, and with tensile strength obtained based on measurements from nylon yarn experiments. The evaluated methodologies encompass the FirstOrder Reliability Method (FORM), standard Monte Carlo simulation (in both original and transformed spaces), and Importance Sampling. The analysis incorporates two distinct levels of parameter uncertainty to test the robustness of the analysis. The results show strong consensus among the different methods, thereby validating their application for assessing mooring line reliability. This study, conducted under the NYMOOR project, concludes that sophisticated approaches, such as FORM and Importance Sampling, provide more computationally efficient and practical alternatives for conducting the reliability-based design at the scale of an individual line. While this work mainly focuses on the ultimate strength and extreme loading scenarios, the long-term performance of nylon mooring lines is also affected by fatigue under repeated cyclic loading, which models the dynamic stresses generated by wind and waves in the offshore environments.