Photo: Troll C platform; Source: Equinor

After finding cracks on gas coolers, Equinor ordered to ensure maintenance of North Sea platform

Norwegian offshore safety regulator has finished investigating an incident with gas cooler cracks on one of the Equinor-operated North Sea platforms and followed it up with an order to the operator.

The Petroleum Safety Authority Norway (PSA) informed on Monday that it had completed its investigation of an incident involving the discovery of cracks in the outer shell of second-stage gas coolers on Troll C platform, which was launched in November 2021. According to the PSA’s report, breaches of the regulations have been identified, and operator Equinor has now been given an order.

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The regulator explained that cracking in a gas compressor cooler was identified on 24 October 2021, elaborating that a black substance was identified under a second-stage gas cooler during an inspection of the Troll C facility in the Norwegian North Sea to check the status of an ongoing job.

Based on the Norwegian regulator’s statement, 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. In addition, this was found to be the case for a similar cooler in the parallel process train.

While the incident caused no harm to people or the natural environment, it led to a production shutdown and substantial repair work. The PSA further stated that this 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, which held hydrocarbon gas under a pressure of 60 bar.

The offshore safety watchdog further advised that these 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 and the coolers are protected by fire insulation on tank and flanges.

The regulator said that based on technical material investigations, the cracks were through-wall and caused by chloride stress corrosion cracking (CSCC), which had begun externally. In the PSA’s view, the condition had developed over time, and it is difficult to date when cracks began to develop, while underlying causes of the incident include the design/construction of the coolers and their follow-up during the operational phase.

While conducting its investigation, the PSA identified several breaches of the regulations. The six nonconformities that the regulator found include a lack of risk reduction related to material degradation; maintenance deficiencies; deficiencies in consequence classification; failure to use information; deficiencies in governing documents; and late notification.

Furthermore, the Norwegian safety regulator observed two conditions, which have been categorised as improvement points. These are maintenance programme and documentation of passive fire protection.

On that basis, Equinor has been 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.

In addition, the Norwegian state-owned giant has been ordered to 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 Petroleum Safety Authority confirmed that the deadline for compliance with this order is 1 December 2022, adding that it must be notified when the order has been complied with.

Located in the northern part of the North Sea, the Troll field lies about 65 kilometres west of Kollsnes while the Troll C – a semi-submersible living quarters and production platform in steel – is part of Troll Phase II, which Equinor developed from Troll Vest with Troll B.

The oil in Troll Vest is produced from multiple subsea templates tied into Troll B and Troll C via pipelines. The production from the Troll C installation started in 1999 and the water depth in the area is about 340 metres. The Troll C platform is also used for production from Fram, Fram H-Nord, and Byrding.

When it comes to Equinor’s electrification plans for the Troll field, it is worth noting that the Norwegian Ministry of Petroleum and Industry approved the energy giant’s plan for the development and operation of this field in December 2021, enabling it to turn its Troll West electrification plans into reality.

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The project includes major and complex modifications to the Troll B and C platforms along with an 85-kilometre long cable from Kollsnes to Troll B, and a 20-kilometre long cable from Troll B to Troll C.