When the alarm sounded, part of the plant was closed and personnel were evacuated. No personal injuries have been reported in connection with the incident.
Statoil is operator for the Mongstad plant.
The PSA has now decided to investigate this incident.
Its investigation team will begin its inquiries at Mongstad this week.
The goals of this work include clarifying the course of events and identifying the direct and underlying causes in order to contribute to learning and experience transfer.
The investigation will result in a report which will be published on ptil.no.