Only "chance" prevented a major accident at a well run by Norway's Statoil, the country's oil safety watchdog has said.
The Petroleum Safety Authority (PSA) said May's incident, which led to the evacuation of the Gullfaks C platform in the North Sea, was "very serious".
Changes in well pressure led to a fault on one of two valves designed to prevent a blowout, it added.
Statoil said it must "intensify efforts in order to prevent serious incidents".
"The PSA report... points out that the underlying causes should be more closely looked into," said Oeystein Michelsen, Statoil's head of exploration and production in Norway.
Norwegian environmental group, Bellona said it would report Statoil to the police for allegedly violating rules at Gullfaks C.
In its report the watchdog said there had been "serious deficiencies" in the way the company planned its drilling of the well and also in management checks on the work.
"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.
"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."
At its peak, Gullfaks produced 180,000 barrels of oil a day. Production only resumed in mid-July.