Cracking in a gas compressor cooler was identified on 24 October 2021. During an inspection of the Troll C facility in the Norwegian North Sea to check the status of an ongoing job, a black substance was identified under a second-stage gas cooler.

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. This was also found to be the case for a similar cooler in the parallel process train.

Actual and potential consequences

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.

Reasons for the incident

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 millimetres. The coolers are protected by fire insulation on tank and flanges.

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.

Underlying causes of the incident include the design/construction of the coolers and their follow-up during the operational phase.

Observations

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 categorised as improvement points. These are:

• maintenance programme
• documentation of passive fire protection.

Order

The investigation has identified serious breaches of the regulations, including maintenance deficiencies (section 8.2 in the investigation report).

On that basis, Equinor has been given the following order.

Pursuant to section 69 of the framework regulations on administrative decisions, see sections 45 and 47 of the activities regulations on maintenance and on maintenance programme, and sections 58 and 59a of the technical and operational regulations on maintenance and on maintenance programme, Equinor is ordered 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
• ensure that failure modes related to corrosion under insulation on stainless steel materials are systematically prevented through maintenance programmes on all Equinor’s facilities and at its onshore plants.

The deadline for compliance with the order is 1 December 2022. We must be notified when the order has been complied with.