Norway's PSA Completes Investigation of Visund Incident

Visund Field, North Sea
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The Petroleum Safety Authority Norway (PSA) has investigated the leak that took place on Visund on January 19, 2006 in which a metal plate in the flare drum collapsed, causing a large hole in the flare pipe. The incident could have led to a major accident, and is the largest gas leak to occur in process areas on the Norwegian shelf in recent years. Our investigation has revealed seven nonconformities in relation to the regulations. Design flaws and lack of pressure retention in the flare drum were the direct causes that triggered the incident.

The Visund leak occurred right after flaring started as expected following an interruption in operations. The crew observed a large flame on the flare when the metal plate in the KO drum ("the flare drum") collapsed, causing a hole measuring approx. 0.5 meter in the flare pipe.

The incident developed over a short period of two minutes. For the next approx. 50 minutes, depressurization entailed that the gas leak continued through the hole in the flare pipe.

There were 91 persons on board the facility, which is operated by Statoil. Crew members not assigned emergency response tasks were mustered to the lifeboats and 17 people were evacuated by helicopter to Gullfaks A. No one suffered physical injury.

Gas leak rate: 900 kg/s
The incident is the largest gas leak to occur in a process area on the Norwegian shelf in recent times: Statoil estimated the leak rate at more than 900 kg/s when the incident occurred.

For purposes of comparison, hydrocarbon leaks with a rate greater than 10 kg/s are classified in the most serious category used on the Norwegian shelf. The total volume of gas that blew through the hole has been estimated at approx. 26 tons.

If the course of events had been only slightly altered, the leak could have led to consequences including possible loss of lives and loss of the facility.

The likelihood of the leak igniting is unknown, but the leak did occur in the immediate vicinity of the flare stack, where the flare was burning throughout most of the incident. It was also observed that live cables were destroyed as a result of the incident.

Safety functions worked as intended
The incident did not entail personal injuries or harm to the external environment. The actual consequences relate to significant damage to process equipment, with the largest loss linked to delayed production from the Visund field. In addition, there are also economic losses associated with delayed start-up of drilling and repair costs for the process system.

The investigation shows that all the automatic safety functions involved after the incident occurred did function as intended. This includes process and emergency shutdown systems, as well as the sprinkler systems in affected areas.

The emergency response factors associated with mustering of emergency response personnel, mustering to the lifeboats and evacuation were handled in a satisfactory manner.

Seven nonconformities
The PSA's investigation has revealed seven nonconformities in relation to regulatory requirements. Two of the nonconformities were the direct triggering factors for the incident.

These are related to design flaws in connection with design and dimensioning of the outlet layout from the flare drum, and deficient pressure retention in the drum.

The other five nonconformities are underlying causes which have contributed to design flaws not being discovered until it was too late.

Both Norsk Hydro, which was operator in both the development phase and the first few years of operation, and Statoil, which took over operatorship as of 1 January 2003, have been involved in connection with the nonconformities.

Following this incident, Statoil implemented a number of measures to identify causal relations and implement measures to normalize the situation on Visund.

We regard Statoil's follow-up of the incident as being so comprehensive that we do not currently see a need to issue a notification of order or to apply other more formal measures.

The Petroleum Safety Authority Norway will continue to follow up this incident and has, among other things, asked Statoil to submit an account of how the regulatory nonconformities will be handled.

On behalf of the PSA, DNV has carried out technical examinations. The DNV report can be downloaded together with our investigation report and the MTO diagram (In Norwegian only).

The PSA has also sent several identical letters to the industry, informing it of the preliminary findings of our investigation.

The Visund field is located 22 kilometers northeast of the Gullfaks field in the Tampen area offshore Norway.

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