In early 2011, a single point mooring system for a deepwater Gulf of Mexico (GOM) project failed at the tether chain for a free-standing hybrid riser, allowing the buoyancy air can and the free-standing flowline riser to separate. The 440-ton buoyancy air can rose suddenly to the surface while the free standing riser collapsed. Based on the investigation of this event and a review of historical events, BOEMRE is revising and re-issuing Safety Alert #259.
The investigation determined that a 6 3/4-inch diameter, 862-pound chain link in the tether chain had fractured and separated near its butt weld. Analysis of the fracture indicated that the chain link had a weld repair and the fracture initiated in the middle of the weld. Three links of the 24-link tether chain were found to have weld repairs. After the chain had been heat treated, the non-US based manufacturer had made weld repairs to the chain by grinding defects and filling the void with weld material. The chain was being built in accordance with Det Norske Veritas (DNV) Offshore Mooring Chain standard. Post heat treat weld repairs are disallowed per DNV’s Offshore Mooring Chain standard. The post heat treat weld repairs made the chain susceptible to hydrogen induced stress cracking due to the extreme hardness of the weld material and the residual stress within the weld.