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The investigation of a hydrocarbon leak on Oseberg A in the North Sea on 17 June 2013 has identified serious breaches of the regulations. The Petroleum Safety Authority Norway (PSA) has now notified operator Statoil of an order.

Direct and underlying causes Wells on the Oseberg field centre began to produce sand around 2000. They are tested with the test separator to identify the acceptable rate of flow for sand production. Sand is accordingly produced into the system during well testing. The test manifold’s blowdown line is unfavourably configured with regard to sand production: - its connection to the manifold means that sand can accumulate in the line - a 90° bend positioned directly downstream from the line’s orifice plate means that sand carried in the gas stream hits the outer wall of the bend at very high speed. This caused sand erosion, which led to a hole being eroded in connection with the pressure blowdown on 17 June 2013. The main reason why this was able to develop over time and eventually cause a gas leak was that no adequate review of the plant has been conducted to verify that it could handle sand production.

Direct and underlying causes

Wells on the Oseberg field centre began to produce sand around 2000. They are tested with the test separator to identify the acceptable rate of flow for sand production. Sand is accordingly produced into the system during well testing.

The test manifold’s blowdown line is unfavourably configured with regard to sand production:

- its connection to the manifold means that sand can accumulate in the line

- a 90° bend positioned directly downstream from the line’s orifice plate means that sand carried in the gas stream hits the outer wall of the bend at very high speed.

This caused sand erosion, which led to a hole being eroded in connection with the pressure blowdown on 17 June 2013.

The main reason why this was able to develop over time and eventually cause a gas leak was that no adequate review of the plant has been conducted to verify that it could handle sand production.

Nonconformities

The investigation identified five nonconformities related to:

- overpressure protection of the test manifold

- risk assessment in using the equalising and blowdown system for gas injection

- work processes for operating wells and process plant

- the inspection programme

- the design dealing with sand production.

Notification of order

The investigation has revealed that key work processes for operating the Oseberg field centre are inadequate. Several of these activities are pursued in normal operations and have continued for a number of years without the work processes being risk assessed, updated or prepared.

On the basis of the findings made by the investigation, the PSA has given Statoil notice of the following order:

“Pursuant to section 69 of the framework regulations on administrative decisions, see sections 8, 11 and 13 of the management regulations on internal requirements, the basis for making decisions and decision criteria, and work processes respectively, and sections 24, 27 and 30 of the activities regulations on procedures, critical activities and safety-clearance of activities respectively, Statoil is ordered to review, risk-assess and update work processes for operation of the wells and process plant on the Oseberg field centre, see chapters 6.1.2 and 6.1.3 of the report.

The deadline for compliance with the order is 31 March 2014. We must be informed when the order has been complied with.”

January 13, 2014; Image: Statoil