Ultimate Limit State Analysis and Snap Load Assessment of Mooring Lines for Floating Offshore Wind Turbines Across Varying Water Depths

Abstract Mooring line snap loads occurs more frequently for floating offshore wind turbines (FOWTs) deployed in intermediate water depths (50–100 m) due to the use of shorter mooring lines compared to deep water applications (>100 m). Current design standards estimate maximum mooring line tension values based on the assumption that snap load occurrences are rare. However, this approach risks underestimating the maximum design tensions in systems where snap loads occur frequently. In this study, different mooring systems for a 15MW VolturnUS-S reference FOWT targeted for deep (200 m) and intermediate (60 m) water depths are investigated. The analysis focuses on the effect of water depth on the occurrence of snap loads and their influence on maximum mooring line tensions. The chain-catenary mooring (Cat-200) system, designed for 200 m water depths, serves as the baseline for comparison against two alternative mooring systems for intermediate waters, i.e. the chain-catenary (Cat-60) and taut (Taut-60) systems. Dynamic analysis results show that the mooring systems in the intermediate water depth result in higher tension standard deviations compared to that of the deep water mooring system, due to their stronger wave-frequency responses. Moreover, the maximum mooring line tensions under the 50-year storm environmental condition are analysed using the mean up-crossing rate (MUR) method with a threshold level determined experimentally. Among the systems analyzed, the Cat-60 mooring system experiences the highest maximum mooring line tension—348% and 95% greater than the Cat-200 and Taut-60 systems, respectively. In terms of the snap load occurrence, the Taut-60 mooring system is effective in reducing the occurrence of snap events, offering a more stable performance. Lastly, results in this study also indicate that the MUR method accurately predicts maximum mooring line tensions for systems with both high and low snap event occurrences.