An FPSO mooring line experienced a premature chain link fracture in the top section. The subsequent examination of the failed link identified that the fracture was caused by corrosion fatigue. The fatigue crack initiated at the bottom of a corrosion groove. The groove has a W-shaped cross-section and its bottom is located in the heat affected zones (HAZs) of the flash weld. The crack propagated radially in parallel to the fusion line of the weld and led to the final fracture when the remaining cross-section was insufficient to sustain the applied load.
The groove was approximately 64 mm long, 5 mm wide, and 2.5 mm deep at its deepest location. The metal loss at the groove was within the corrosion allowance per design standards. The corrosion groove formed most likely because of the microstructural differences between the HAZs and nearby material.
The examination also revealed severe metal loss, mainly in the form of pitting, in base metal of the link. Some of the pits were much deeper than the corrosion groove and were at locations with higher stress concentration factors associated with the link geometry itself, i.e., stress concentration factors in the absence of corrosion. The fatigue failure occurred at the groove instead of the deep pits because the groove introduced a high stress concentration factor, which facilitated the initiation of the fatigue crack.
Since the most likely cause of the corrosion groove was the microstructural differences across the weld, it appears to be reasonable to counter the specific failure mechanism by minimizing the microstructural differences though optimizing welding parameters and post weld heat treatment.
The mooring systems of floating production systems (FPSs), once deployed, are expected to keep station for many years without inspection or repair1,2. This is because a failure of mooring components results in not only the significant costs associated with the subsequent replacement or repair but also the risk of environments and lives. In order for improving mooring system integrity, the oil and gas industry has been pursuing better practice in mooring system design, manufacture, transportation, installation, operation, and others3. Among all of the efforts, learning from field service failures, although not desired, is unique and in fact constructive.