Mooring chain is a standard component in mooring systems for floating offshore installations. Corroded chains, with both significant general corrosion and preferential weld/weld-line corrosion (PWC) have been taken out service, where strength and fatigue capacity have been investigated by full-scale break testing and full-scale fatigue testing. General corrosion loss has shown to have a significant effect to both break strength and fatigue, whereas surface corrosion condition and PWC has no or limited effect to break strength, but a significant effect on fatigue strength. This paper presents the chain conditions and test results from fatigue testing of chains with significant corrosion loss and PWC.
Many of Equinor's floating offshore assets have reached or will soon reach their initial design life. To assess integrity and possible life extension, mooring chains have been taken out of service. The actual chain conditions have been mapped, and remaining strength and fatigue capacity has been assessed by full-scale fatigue testing. The results from the assessments are used to conclude on replacement or life extension of remaining mooring chains.
As mooring chains are not protected from corrosion, degrading is expected. Degrading is however different between assets and pending positions in the mooring line. A large variation in condition is seen, both on surface conditions, such as general roughness and pits from typically bacterial corrosion, and general corrosion loss. The effects of surface degradation have previously been studied, where a mean load and surface condition dependent fatigue capacity has been defined (Hørte et al 2022, Lone et al 2021; 2023). Fatigue capacity as a function of general corrosion in combination with surface corrosion has however so far not been studied in detail or quantified.
Lately, attention has been drawn to a special corrosion phenomenon close to the weld-line, defined as preferential weld/weld-line corrosion (PWC). PWC can be defined as more aggressive corrosion in or close to the weld fusion line and is observed at different degrees on most used chains. On some chains PWC appears only as one or two thin shallow lines, whereas on other chains deeper grooves are seen. The deeper PWC corrosion is the condition that is now addressed by the industry, as no issues has been observed with the thin shallow appearance.
