The incident occurred in connection with a well wash during completion of a drilling operation at Visund.
An inflow and pressure increase were registered in the well's annulus. Production on the field was halted and the operator began demanning the facility.
The BOP was intact, but problems were reported in operating shutoff valves on the top of the drillstring.
The situation was normalised on 18 March.
The incident did not cause any hydrocarbon leaks.
The PSA has decided to instigate an investigation of this incident. The investigation will focus on Statoil's planning and execution of the well operation.
The objects of the investigation include clarifying the chain of events and identifying proximate and underlying causes.
The investigation will result in a report that will be published on ptil.no.
Visund is an oil field located east of the Snorre field in the northern sector of the North Sea. The development comprises a semi-submersible steel production, drilling and quarters platform (Visund A). Water depth in the area is around 335 metres. The northern part of Visund has been developed using a subsea template, around 10 kilometres north of Visund A.
The operator of Visund is Statoil.
Eileen Brundtland, press contact
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