Equinor – Statfjord A – investigation of oil leak after overpressurising storage cell

Equinor notified the PSA on 26 November 2019 that oil had been observed in the sea around the North Sea platform. This was reported to come from a sludge cell, and the leak stopped when oily water was diverted to another cell.

The PSA decided to launch an investigation of the incident, and that work has now been completed.

The incident

Sludge cell 11 on Statfjord A was subject to an overpressure of 2.6 bar on 26 November. The cell is designed to be operated with an internal pressure five bar below the ambient seawater pressure.

The overpressure occurred because oily produced water was added to the cell while the ballast-water valve to the outlet had been left closed following maintenance of its actuator.

This overfilling caused in the lines down to the cell to fill with water, and the consequent liquid column resulted in an overpressure of 2.6 bar.

That in turn caused cracking in the cell dome and subsequent leakage of oil and sludge to the sea.

Actual and potential consequences

The overpressure in the sludge cell resulted in local areas of cracking in the cell dome, where the concrete’s tensile strength had been exceeded. That allowed oil and sludge to leak to the sea from the cell for about four hours.

Estimated to total about 150 m³, the discharge consisted primarily of oil.

Under different circumstances, the leak could have continued for longer or caused a larger discharge.

The potential consequence if the prestressing steel in the concrete structure had been damaged would probably have been a permanent manning-reduction criterion for the rest of Statfjord A’s producing life. In the event of extensive damage, the weather criterion might have resulted in substantial downtime for the facility – particularly during the winter.

Equinor’s experience in the period since the incident was normalised indicates that the crack in the cell dome has largely closed by itself and is sealed. Had the incident resulted in a crack which lost material and created a hole in concrete, it might have been impossible to re-establish the underpressure required for cell 11’s structural integrity.

A hole in the concrete shell could have caused a much large discharge (the cell has a volume of 18 000 m³). If the underpressure could not be restored, cell 11 would have had to be isolated, structural integrity would have had to be reanalysed and possible weather restrictions could have been imposed for manning and operation.

Direct and underlying causes

According to the PSA investigation, modifications and changes to operating methods between storage and sludge cells have introduced system vulnerabilities and allowed a single action to shut off barriers.

The direct cause of the overpressure was filling cell 11 with the ballast-water valve closed.

Underlying causes related to the absence of barriers owing to modifications from storage to sludge cells, lack of risk assessments of the modifications, inadequate risk and system understanding when planning maintenance work on the ballast-water valve, and the failure of applicable system and operating documentation to reflect the technical or operational changes.

Regulatory breaches

The PSA’s investigation has identified nonconformities related to inadequacies in the process safety system against overpressure in the sludge cells, inadequate risk and system understanding when planning maintenance work on the ballast-water valve, lack of safeguards on safety-critical valves, and inadequate governing documentation. It also found deficiencies in Equinor’s own follow-up, which have been categorised as an improvement point.

Notice of order

The investigation has identified serious breaches of the regulations, and the PSA has now given Equinor notice of the following order.

Pursuant to section 69 of the framework regulations on administrative decisions, see section 5 of the management regulations on barriers, section 34 of the facilities regulations on the process safety system, section 20 of the activities regulations on start-up and operation of facilities, and section 21 of the management regulations on follow-up, Equinor Energy AS is ordered to implement the following measures.

1.     Review the use of the storage cells on the Statfjord facilities to ensure that sufficient technical and operational barriers are in place, and review the system description and procedures to ensure safe operation of the cells. See sections 9.1.1 and 9.1.3 of the report.

2.     Present a plan to follow up the effects of your own measures for correcting the identified nonconformities in the PSA’s investigation report and the action areas in your own investigation report. The action areas in your own report cover the conduct of risk assessments, documentation and procedures, technical upgrading of Statfjord A, and facility-specific expertise and compliance with requirements. The plan must also cover other facilities with a similar design.

The deadline for compliance with the order is 1 July 2020.

We must be notified when the order has been complied with. Equinor is requested to take the initiative on a meeting to account for the basis on which the barriers have been improved and made effective.

Equinor has also been asked to provide the PSA with its assessment of the improvement point observed. The deadline for this is set to 5 June 2020.