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.

Low-cycle fatigue assessment of offshore mooring chains under service loading

“…4(b) shows the damage parameters on the critical plane of the Kt point are lower than those of the crown with a mean absolute difference of 0.0352 and a maximum absolute difference of 0.049, across the range of selected load amplitudes. The result is in line with full-scale experimental fatigue results [8] [9] [40], where the crown is found to be more susceptible to fatigue damage than the Kt point. This is because the Kt point has higher residual compressive stresses than at the crown upon release of the proof load.…”

“…11. The pre-tension affects the mean loads as the higher the pre-tension, the higher the mean loads, and mooring systems in the field normally have a pre-tension between 10% and 20% MBL [40], where the pre-tension in the case study is 17% MBL. As the mooring chain failures occurred in harsh sea environments [1] [3], it is hypothesised that the pre-tension of the greatest loaded line increases with the mean loads, which leads to an increase in the proportion of low-cycle fatigue causing the reported early failures in the first three years of operation.…”

Fatigue reliability of mooring chains, including mean load and corrosion effects

“…Leira). chains have revealed that (i) the fatigue capacity of chains is strongly dependent on the mean load, and (ii) realistic corrosion pits have a detrimental effect not well represented by the simplified approach prescribed by the standards (Gabrielsen et al, 2019;Fernández et al, 2019;Zhang and Smedley, 2019;Ma et al, 2019;Lone et al, 2021). Hence, proper treatment of mean load effect and degradation due to corrosion in the calculations seems imperative to enable improved estimation of mooring line fatigue life.…”

Effect of cyclic softening and mean stress relaxation on fatigue crack initiation in a hemispherical notch

“…Offshore mooring chains are subject to general and microbiologically influenced corrosion attacks. 17,18 During cyclic loading, corrosion pits act as stress raisers, and may become initiation sites for corrosion fatigue cracks. 18 The current work aims to improve the understanding of MSR and its influence on fatigue initiation.…”

“…17,18 During cyclic loading, corrosion pits act as stress raisers, and may become initiation sites for corrosion fatigue cracks. 18 The current work aims to improve the understanding of MSR and its influence on fatigue initiation. Since corrosion pits vary in size and shape, 19 machined, hemispherical notches are considered here in order to develop a generic approach for MSR in R4-grade steel chains.…”