Operated by Esso Norge A/S, the Slagentangen oil refinery is located in Tønsberg local authority on the Oslo Fjord.

The naphtha leak occurred as part of normal operations in the section of the refinery known as the pipestill, where crude oil is distilled in an atmospheric distillation tower.

A decision to investigate the incident was taken by the PSA on 25 August 2020.


The investigation has found that the direct cause of the leak was corrosion under the insulation in a four-inch pipe for heavy naphtha from the distillation tower.

Lack of inspection and repair meant that the corrosion had continued long enough for a hole to occur in the pipe.

Piping and equipment in a refinery can be exposed to both external and internal corrosion. They are inspected at specified intervals to monitor and repair such damage.

The pipe where the leak occurred had been removed from the maintenance programme by mistake in 2008. This error was discovered in 2013 but, instead of being restored to the programme, the pipe continued to be let out of the equipment strategy.

This pipe was accordingly last inspected for corrosion under its insulation in 2004.


The leak rate has not been calculated, but is estimated to have been about 10 litres per minute. Total leakage is put at 4 400 litres. Since the leaked naphtha was collected by the refinery’s open drain system, no harm was caused to the environment.

Potentially, the leak could have ignited and caused a fire in the pipestill. The absence of fire and gas detectors means that the detection of any ignition would have depended on its discovery by refinery personnel.

A fire could potentially have spread and escalated as a consequence of fracturing in other equipment or structural failure of process facilities in the area. Had the leak been ignited after the establishment of fire-extinguishing resources, the investigation team has concluded that this would probably have hindered an escalation.

The probability of a leak igniting in a process plant where explosion-proof equipment remains intact is low. Had it ignited while being investigated by operations personnel, however, they could have been exposed to a fire. They were particularly vulnerable when close to the leak point on a platform which could only be accessed via a ladder.


The investigation team has identified four nonconformities and four conditions it has chosen to categorise as improvement points.


  • risk assessment of the need for fire and gas detection in the pipestill
  • maintenance and inspection
  • ensuring the safety of the operations and maintenance personnel involved
  • performance requirements related to emergency response.

Improvement points

  • overview of impairments in the plant
  • marking in the plant
  • shutdown procedures for the plant
  • investigation mandate.

What happens now?

The PSA’s investigation has been completed. Esso has been asked to explain how the nonconformities will be dealt with, and to give the PSA its assessment of the improvement points observed. The deadline for these responses has been set to 31 January 2021.