document

Abstract

Statoil has carried out a comprehensive analysis of the underlying causes after a gas seabed blow-out on the Snorre field in the North Sea. The Norwegian authorities have described the incident as one of the most serious ever to occur in the North Sea. The causal analysis is based on the "Pentagon model". The results show the importance of viewing the technological, organisational and human factors in conjunction in order to best understand the underlying factors and to draw up relevant, focused follow-up measures.

Introduction

The Snorre field has been producing oil and gas since 1992. Up until 1999, when Norsk Hydro took over, the field was operated by Saga. Statoil became operator at the turn of the year 2002/2003.

Snorre A is an integrated production, drilling and accommodation platform anchored at a depth of approximately 300-350 m in the North Sea. On 28 November 2004, an uncontrolled gas blow-out took place on the seabed under the platform. The incident occurred in connection with the preparation of well P-31A for the drilling of a sidetrack. During pulling of a 2,578 m scab liner, gas was drawn into the well and it leaked out through a known hole in the 9 5/8" casing then through an unknown damage or weakness in the 13 3/8" casing.

There were 216 persons on board at the time of the incident, 181 of whom were evacuated to other installations while the other 35 persons remained on Snorre A to carry out emergency response and well control tasks. The gas blow-out was stopped and the well brought under control the day after. No one was physically injured in connection with the incident.

The Norwegian authorities under the auspices of the Petroleum Safety Authority Norway (PSA) have investigated the incident and described it as one of the most serious incidents ever in the North Sea based on the potential of the accident. The PSA's investigation concluded that Statoil had not followed its governing documentation, that risk analyses were inadequate and that there were deficiencies in management involvement and breaches of well barriers. The PSA ordered Statoil to find the causes of these non-conformities, to explain why the incident occurred and to draw up and implement measures to prevent similar incidents. On this basis, Statoil carried out a causal analysis which was submitted to the PSA on 1 November 2005.

The causal analysis is based on a review of documents, a questionnaire survey and interviews.

The PSA's investigation report documents a number of weaknesses and non-conformities in the chain of decision that resulted in the gas blow-out on 28 November 2004. They include failures of judgement, failures in work processes and in relation to organisational control mechanisms.