A gas leak occurred in December at Statoil's Gullfaks B production facility in the North Sea. The leak lasted about an hour. No injuries were reported.
The PSA found it was "impossible" to operate emergency shutdown valves on the well and called on Statoil to assess the situation.
"The investigation concludes that serious deficiencies have been revealed in Statoil's planning, approval and implementation of activities related to the maintenance of choke valves on Gullfaks B," the PSA said in a statement. "That includes deficiencies in management work on supervising that this activity was pursued in an acceptable manner."
The PSA said it was concerned that Statoil wasn't taking the situation seriously because of responses to previous incidents.
Statoil Executive Vice President Oystein Michelsen in a statement said his company took the December incident "very seriously" and had "drafted actions to ensure increased skills and good risk management."
Statoil said it has until April 29 to respond to the PSA's questions about the Gullfaks leak.
"The frequency of serious incidents offshore has been more than halved since 2004," said Michelsen "However, when an incident like the one at Gullfaks still occurs, it shows that we still have some room for improvement."