There have been several incidents on the Norwegian Continental Shelf with loss of mooring lines caused by failure of high-strength components.
These failures have occurred both during ordinary operating conditions and during installation of mooring lines, and for different levels of tension in the failed mooring lines.
Several explanations for the contributing causes for these failures have been nominated at seminars, conferences and in investigation reports. The proposed causes are related to material technology, installation, design and operation. At present there is little systematic research on the topic and no best practice has yet been established for avoiding loss of mooring lines caused by failure of high-strength components.
An activity to collect and perform an independent critical review of available material on the usability and suitability of high-strength mooring chains has therefore been initiated. This work was carried out in 2022. The evaluations are based on 18 reported incidents, mostly on the Norwegian Continental Shelf, where high-strength mooring components failed on mobile offshore drilling units.
The current data analysis indicates that the most probable leading failure mechanism affecting high strength chains, is hydrogen embrittlement. The main contributing factors for crack initiation and growth are related to susceptibility to hydrogen embrittlement in the material, handling of mooring chain, high tensile loading and local production of hydrogen due to application of cathodic systems for corrosion protection.
It is believed that the report provides a good foundation for establishing mitigation actions for reducing the number of failures in high-strength mooring components. Also, some perspectives of possible future investigations to allow better understanding of the contributing factors are proposed.