An investigation by the Norwegian offshore oil and gas industry safety regulator into an incident involving the discovery of cracks in the outer shell of second-stage gas coolers on Equinor's Troll C offshore platform in Norway in October 2021, has found that there had been potential for a major gas leak, and that Equinor had breached regulations.
Cracking in a gas compressor cooler on the Norwegian North Sea platform was identified on October 24, 2021. During an inspection of the Troll C platform to check the status of an ongoing job, a black substance was identified under a second-stage gas cooler, the Norwegian Petroleum Safety Authority has found.
Further investigations established that the substance was asphaltene (a heavy crude oil component), which had leaked out through cracks in the cooler’s outer shell. The PSA said this was also the case for a similar cooler in the parallel process train.
"The incident caused no harm to people or the natural environment but resulted in a production shutdown and substantial repair work. In the PSA’s view, the incident could have developed under slightly altered circumstances into a major gas leak through brittle fracturing in the outer shell of one of the gas coolers. The coolers held hydrocarbon gas under a pressure of 60 bar," the PSA said.
Developed over time
The gas coolers are a shell and tube type, comprising an outer shell in 22%Cr duplex steel where gas circulates and an inner tubing bundle in titanium filled with coolant (seawater). The outer shell has a specified thickness of 36 millimeters. The coolers are protected by fire insulation on tank and flanges, the PSA said.
Technical material investigations have concluded that the cracks were through-wall and caused by chloride stress corrosion cracking (CSCC), which had begun externally.
The condition had developed over time, and it is difficult to date when cracks began to develop.
The underlying causes of the incident include the design/construction of the coolers and their follow-up during the operational phase.
Several breaches of regulations
The PSA investigation has identified several breaches of the regulations. Nonconformities were: lack of risk reduction related to material degradation; maintenance deficiencies; deficiencies in consequence classification; failure to use information; deficiencies in governing documents; late notification.
In addition, two conditions were observed which have been categorized as improvement points. These are related to the maintenance program and documentation of passive fire protection.
Following the findings of the investigation, the PSA has ordered Equinor to ensure a level of maintenance for the HTA and HTB coolers on Troll C "which means they are capable of performing their required functions in all phases of their operating lives."
PSA has also order Equinor to ensure that failure modes related to corrosion under insulation on stainless steel materials are systematically prevented through maintenance programs on all Equinor’s facilities and at its onshore plants.
The deadline for compliance with the order is December 1, 2022.