A Practical Procedure for Fatigue Assessment of Mooring Line Chains with Pitting Corrosion

Abstract

Chain link fatigue failures in mooring lines have recently aroused the interest of the technical community in developing new design and monitoring methodologies focused on considering the effects of local corrosion. Experimental breakage test results evidenced that the effects of mean tension, pitting corrosion and surface roughness may influence the fatigue strength of chain links. A recently-developed methodology here denoted as Corrosion Grade considers the effects of mean tension and local corrosion through a visual assessment of the chain. The present work proposes a parametric correction factor to be used in the current S-N fatigue design methodology for considering local corrosion in studless chains. The parametric model is developed by means of finite element-based modeling of various pit shapes in the chain. The correction factor is fitted as a function of the pit aspect ratio. The proposed correction factor is combined with the Corrosion Grade methodology to obtain a final formula that considers the mean tension and the local corrosion. Results from both Corrosion Grade and the proposed correction factor are compared in three corroded experimental test samples. The proposed correction factor is capable of estimating the fatigue life of the corroded components analyzed with conservatism.