Degradation Mechanisms of Chain Links in Offshore Mooring Lines

Abstract

Degradation mechanisms contributing to mooring line failures are identified and discussed with respect to the service conditions for the defined mooring line segments. Corrosion, environmental assisted cracking (EAC) and wear are the degradation mechanisms believed to reduce the fatigue life of mooring lines. It is difficult to predict where corrosion may occur and it is considered to be more purposeful to determine the critical areas where corrosion may cause the most damage on the remaining service life. The critical areas and the possible degradation mechanisms have therefore been the deciding factors when suggesting the outer bend section on chain links in the splash zone as the area where scanning technology could be used. Chain links located at the seabed for 14 years showed extensive corrosion damage. Localized corrosion in the form of pitting and large areas of material removal, were the most prominent corrosion damage on the links. Microbiologically influenced corrosion (MIC) are suspected as the cause of the localized corrosion attacks. The corrosion pit features are fairly consistent for MIC, although extensive testing are necessary to determine MIC as the cause for the corrosion attack. Cup-type hemispherical pits, striation pits, often gathering in clusters, and pits following the longitude direction of the link straight section, are characteristic for pits associated with MIC. Cross section measurements on the bottom chain links showed little reduction in the straight section and greater reduction in the crown section. The maximum reduction rates in both areas were above than the maximum ISO recommended value. This is considered to be of less concern, as the average cross section reduction rate for the straight section is close to negligible and the original cross sections at the crown are probably less than the assumed 114 mm after the manufacturing processes. A fatigue testing program is presented for specimens with rough surface geometry in order to determining the effect of surface condition on fatigue life and compare the results with analytical models of the chain links. Material testing with artificial corrosion pits are presented as well.