An investigation carried out by the U.S. Bureau of Safety and Environmental Enforcement (BSEE) has found that hydrogen embrittlement was the cause of a flowline jumper failure in the Gulf of Mexico in 2020 that resulted in a subsea leak.
The incident that occurred in July 2020 saw approximately 479 barrels of oil released into the Gulf of Mexico from a subsea jumper. The BSEE investigation identified this was a result of a failed flange connection on the jumper, between the multiphase flowmeter (MPFM) and Blind Tee.
According to the BSEE, the failure was due to the fracture of four of the eight fasteners (studs) at the flange connection, which was attributable to hydrogen embrittlement. The fractured studs were composed of the corrosion-resistant nickel-based alloy (Inconel 718) which is not alloy steel.
Inconel 718 fasteners installed in subsea environments with cathodic protection are said to be susceptible to hydrogen embrittlement and should comply with certain American Petroleum Institute (API) standards to mitigate the embrittlement risks.
The U.S. Bureau found the causes of the incident to be failure to install API 6ACRA compliant studs in the jumper flange connection, engineering documentation used for procurement not specifying API 6ACRA compliance for subsea flange fasteners, a gap in the QA/QC process allowed for nonAPI 6ACRA-compliant subsea flange fastener installation, and management of change (MOC) process not being followed.
Therefore, BSEE is recommending that operators and contractors use an industry-knowledgeable metallurgist to evaluate all current and future subsea fasteners to verify they are fit for service and not prone to hydrogen embrittlement or any other environmental cracking failures, share subsea leak detection system learnings with industry, and emphasize that company, contract and sub-contract personnel enact MOC when there are modifications associated with equipment, operating procedures, personnel changes, materials and operating conditions.
The recommendations also include validating and documenting flange fastener make-up torque values, ensuring SSLD notification system alarms are appropriately set with effective alerts and properly monitored during shut-in, transient and steady-state production operating conditions, as well as considering the subsea leak potential while investigating significant subsea flowline pressure drops even when the pressure trends are above the ambient pressure of the sea.
The Bureau’s panel also encourages the industry to develop a standard means for determining shear and torsional capacities of API flange connections.