Impact of mean stress on mooring line fatigue assessment: An analytical study

Abstract

Mean stress significantly impacts material fatigue behaviour, extending lifetimes under compressive loads and reducing them under tensile loads. Mooring systems experience complex, dynamic loads, with lines continuously under tension. This tension introduces a tractive mean load influenced by various factors, including environmental conditions and design pretension. Despite the critical role of mean stresses in the structural integrity of mooring lines and its contribution to fatigue damage, standard offshore mooring design procedures do not explicitly account for their effects. Numerous studies have investigated mean stress effects in fatigue, resulting in various correction methods. This study presents a systematic approach to incorporate mean stress into mooring system design, evaluating six common correction methods (i.e., Goodman, Gerber, Soderberg, Smith-Watson-Topper (SWT), Morrow, and Kwofie) against the DNV offshore specification. The findings underscore the importance of accounting for mean stress, which can reduce estimated lifetimes by up to 50%. Furthermore, selecting a correction method tailored to material-specific responses is essential as estimations vary significantly. Experimental data indicate that the SWT method provides the most accurate results among those tested.