THE EFFECT OF PITTING CORROSION ON FATIGUE PERFORMANCE OF MOORING LINE CHAIN LINKS

Mooring systems play an important role in the safety of floating production units and must comply with ultimate, accidental, and fatigue limit states criteria defined by design standards and classification societies. However, in the last few decades, many mooring lines failures have been reported in the literature. Most of these were related to fatigue and corrosion degradation in the top chain links, becoming a major concern. Since then, a great effort has been made to improve fatigue assessment methodologies of these elements and to model the effects that are not properly considered in the current design practice, such as the mean tension effect and the chain-link surface roughness due to localized corrosion. The design practice of mooring lines usually assumes a uniform corrosion model with a constant corrosion rate. That way, corrosion degradation is considered only by reducing the net area of the chain-link. As a consequence, effects of stress concentration due to localized pitting are not directly addressed, and this may lead to an unconservative design. The present work aims at assessing the stress concentration associated to chain links with pitting corrosion employing finite element analyses and by comparing the localized pitting against the uniform corrosion stresses conditions in the chain link hotspots. Results highlight that, even for small pits, fatigue damages are higher than considering a uniform area reduction, showing one feasible way of improving the current fatigue design methodologies.