Only luck prevented a well incident earlier this year at a Statoil-operated platform in the North Sea from becoming a major accident, the Norwegian oil safety watchdog said.
The Petroleum Safety Authority Norway (PSA) described as "very serious" an incident in May that caused an evacuation of the platform after changes in well pressure led to a fault on one of two valves designed to prevent a blowout.
The PSA's report comes as Statoil and Norway's oil industry campaign to allow oil and gas exploration off a pristine archipelago in Norway's Arctic, in the face of criticism from green groups and some political parties.
Production at Gullfaks only resumed in mid-July after Statoil installed three cement plugs and one mechanical plug in the faulty well.
"Only chance averted a sub-surface blowout and/or explosion, and prevented the incident from developing into a major accident," the PSA said in a statement.
The watchdog said it had identified "serious deficiencies" in the way Statoil planned the drilling of the well and in the management checks done to ensure the work was properly done.
"These (deficiencies) concerned such key factors as risk management and change control, experience transfer and use of expertise, knowledge of and compliance with governing documents, and documentation of decisions," it said.
The watchdog also asked Statoil to hold an independent assessment of why measures adopted after a 2004 gas blowout at the Snorre platform had not had the desired effects on Gullfaks. It said both incidents held "similar causes".
The watchdog's conclusions come two weeks after Statoil's own inquiry of the incident, which the firm described as "serious", and four days after it suspended drilling operations at all three Gullfaks platforms to review work routines.
Statoil repeated it would learn the lessons from the events at Gullfaks C, which it said showed the need "to intensify our efforts in order to prevent serious incidents".