US operator Marathon Oil has been fined £1.16 million ($1.48 million), one of the largest penalties ever for UK offshore safety failures, over a massive high-pressure gas blast and leak on the Brae Alpha platform in the UK central North Sea that occurred three and half years ago.

Aberdeen Sheriff Court heard this week that on 26 December 2015 an eight-inch diameter high-pressure pipework in module 14 of the platform suffered a catastrophic rupture as a result of corrosion under insulation, allowing about 2150 kilogrammes of methane gas to be released almost instantaneously.

It happened while most of the 100 workers on the platform were in the accommodation block in readiness for their Boxing Day meal and away from the source of the blast.

Staff heard a loud bang at about 4.30pm before lights went out and the detection of the gas in the module triggered an initial emergency shutdown and blowdown on the platform.

The force of the high-pressure blast caused significant and widespread damage within the module, which was discovered at about 8pm when workers entered it after the gas dissipated.

Nobody was hurt and the gas did not ignite, but safety inspectors said that if any workers had been in the module at the time of the incident they would almost certainly have been killed or seriously injured.

Analysis showed the pipe had corroded over a period of 20 to 30 years. The corrosion was caused by sea water escaping from fire sprinkler nozzles close to the pipe that had found its way into and accumulated in the insulation.

An investigation by the Health & Safety Executive (HSE) found that Marathon had failed to undertake any suitable inspection of the pipework that would have allowed the company to identify the risk and prevent the hazard from materialising.

HSE scientists calculated that the pipe wall — originally 22 millimetres thick — had corroded at a rate of about one millimetre per year and was less than two millimetres thick when the blast happened.

The investigation showed that an external consultancy had advised Marathon in 2010 on pipework inspection, but Marathon did not implement the recommendations, which might have prevented the blast.

The failures resulted in personnel on board the facility being exposed to an unacceptable risk of serious personal injury or death from fire and explosion and also led to HSE serving an improvement notice on Marathon in January 2016, requiring the company to implement an effective hydrocarbon pipework inspection and maintenance regime.

Marathon admitted breaching prevention of fire and explosion regulations and health and safety legislation.

HSE inspector Ahmedur Rezwan said: “This incident is a further reminder of the ever-present hazards in oil and gas production, that if not rigorously managed can easily result in a potentially life-threatening event.

“Corrosion under insulation is a well-known risk and this incident should not have occurred. During any normal period of operations personnel could easily have been working in, or transiting through Module 14, and they would almost certainly have been killed or suffered serious injury. The timing of the incident and fact that the gas did not ignite was fortuitous.”

Corrosion under insulation is caused when moisture finds its way into an insulation system and is trapped against the pipework.

Alistair Duncan, head of the health and safety investigation unit at Scotland's Crown Office & Procurator Fiscal Service, said: “This incident could have been prevented had suitable and sufficient measures been put in place.

“Hopefully this prosecution and the sentence will remind other employers that failure to fulfil their obligations can have potentially very serious consequences and that they will be held to account for their failings."

Marathon said in a statement issued after the hearing that it fully co-operated with the HSE and complied with the measures suggested in the improvement notice.

"The safety of our personnel is a top priority and we have taken action to resolve issues relating to this incident," it said.

In February, London-based RockRose Energy announced it was to become a UK North Sea operator for the first time after striking a deal to acquire Marathon’s UK upstream business for $140 million, signalling the US company’s exit from the region after three decades.

The deal is expected to close in the second half of the year and has an effective date of 1 January 2019.

On 22 December 2015, four days before the Brae incident, Total was fined a record £1.125 million for failings that led to a 51-day gas leak in 2012 after a blowout in a high-pressure high-temperature well at its Elgin process, utilities and quarters facility in the central North Sea.

The Elgin facility was hurriedly evacuated of all 238 personnel and, by the time it was brought under control, 6000 tonnes of gas and condensate had been released.

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Marathon fined for 2015 blast