Uns N07725 Nickel Alloy Connection Failure

ABSTRACT A deep water well in the Gulf of Mexico experienced the failure of a completion component manufactured from nickel alloy UNS N07725 (alloy 725). This precipitation hardenable nickel-based super alloy is recognized as one of the oil industry’s most reliable high-strength corrosion resistant alloy. The incident occurred after 22 months of well suspension that was required for commissioning of subsea infrastructure. The tubing string separated below the tubing hanger in a 5-1/2 in (0.139 m) premium box connection. Examination of the failed alloy 725 box revealed a fracture morphology characteristic of hydrogen assisted cracking (HAC). The failure investigation systematically eliminated potential overload and metallurgical failure mechanisms through testing protocols and identified a single most likely root cause. Results concluded that various chemicals sealed within the failed 5-1/2 in (0.139 m). box connection during make-up reacted in a deleterious manner such that the internal box environment became a source of hydrogen. This hydrogen enriched environment was in intimate contact with the box threads that were highly stressed due to the required tensile pre-load of the proprietary connection make-up. The combination of nascent hydrogen (due to chemical reactions) at the metal surface and high localized stress established the basic conditions necessary for hydrogen embrittlement. Laboratory testing was conducted with ten alloy 725 specimens, 3 copper plated and 7 un-plated, that were ‘cleaned and doped’ using chemical solutions and compounds required by the approved doping protocol. Test results demonstrated that 5 alloy 725 specimens that were cleaned and doped incorrectly failed within 35 days. Analysis of the failed specimens found fracture morphology characteristic of HAC that very closely replicated the failed completion component. Comparative testing of alloy UNS N07718 (alloy 718) did not produce any failures.