Fatigue Tests on Corroded Mooring Chains Retrieved From Various Fields in Offshore West Africa and the North Sea

Abstract

When an aged mooring system seeks a life extension, it is necessary to assess the remaining fatigue life of the corroded mooring chain. This paper summarizes the results of fatigue tests performed on mooring chain samples retrieved from six different fields in West Africa and North Sea. The impacts of corrosion on fatigue life on the samples were researched. The tests were managed under a Joint Development Project, “Fatigue of Corroded Chains (FoCCs JDP)”. The objectives of the JDP are (1) to derive a methodology for assessing the remaining fatigue life of corroded chain, (2) to develop guidance for performing reliable FEA of chain links to assess remaining fatigue life, and (3) to provide more rational basis to improve industry guidance on mooring line replacement criteria for life extension. Fatigue test procedure was defined by the fifteen (15) participating members. The procedure specified the testing parameters, including mean tension, tension range, and test frequency. Six sets of fatigue tests have been completed in seawater with the number of cycles to failure recorded. These chain samples were retrieved from floating production and storage units, e.g. FPSOs and FSUs, that were still in service. Fatigue data obtained from the tests were plotted against the design SN curves and results from fatigue testing of new chain. It was found that most of these samples have limited amount of fatigue capacity remained. Most interesting finding is that the sharpness of the surface feature on the corroded chain link has a significant impact on the remaining fatigue life. Another interesting finding is that the surface feature created by corrosion can be quite distinct and unique depending on the geophysical locations where the sample came from. These findings and test results may serve as references for life extension assessment of an aged mooring system.