The Petroleum Safety Authority Norway (PSA) has completed its investigation of the fire which occurred at Equinor’s refinery at Mongstad on 3 July 2022.
he fire at the Mongstad plant occurred because of a leak in a thermowell flange on a pipe carrying a mix of naphtha and hydrogen. With a temperature of 520°C, this blend auto-ignited on contact with the air.
A jet fire probably occurred at the auto-ignition site, while fires with less flame broke out at several points along the pipe. The leak rate is estimated to have been 0.55 kilograms per second. Film from the fire, as well as observations by personnel in the area, show that the flames may initially have been 10-15 metres high.
Emergency response personnel mustered quickly and opted for a controlled burning-off of the gas from the leak in the reactor column, while cooling down surrounding areas with large quantities of fire water to maintain structural integrity and thereby prevent escalation of the incident. The fire broke out at 05.43 and was reported to have been extinguished at 11.25.
Nobody was injured as a result of the fire, but the incident led to the shutdown of several process plants for just under three weeks and a substantial loss of production.
The fire broke out in an area where people are not normally present. Had the incident occurred while somebody was in the area, they could have been exposed in the worst case to fatally high heat radiation.
In the worst case, ignition of the medium in overlying pipe tracks could have caused an escalation of the fire, with large-scale local destruction of material assets.
Had the fire been extinguished earlier in its progression, before the column had been pressure-relieved, an uncontrolled gas cloud might have formed and possibly suffered delayed ignition. Not extinguishing the fire was a conscious action to avoid that scenario.
Direct and underlying causes
The direct cause of the fire was a leak of auto-ignitable hydrogen-rich naphtha, probably because of a loss of tensioning in the bolts on the thermowell flange. The investigation has indicated several underlying causes:
- pretensioning of bolts
- the quality of planning for the job package
- insulation implementation in the job package
- quality control of the job package
- lack of follow-up of thermowells in general
- high pressure of work.
The investigation has identified several nonconformities from the regulations, covering:
- deficiencies in design and maintenance of the plant
- deficiencies in the work process for surface treatment
- deficiencies in risk analysis.
The following conditions have also been observed, which the PSA team has opted to characterise as improvement points:
- depressurisation during an incident could be improved
- notification and reporting of hazards and accidents could be improved
- learning from incidents could be improved
- securing technical findings and evidence could be improved.
The PSA’s investigation has been completed. Equinor has been asked to explain by 31 January 2023 how the nonconformities will be handled and to provide the PSA with its assessment of the improvement points which were observed.