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The Mongstad refinery. Image courtesy of Helge Hansen/Statoil.

An October well control incident at an offshore Statoil platform and a separate incident at a refinery could have been fatal, according to a company report.

An investigation conducted by Statoil found that the incident had a “high degree of seriousness” and “at worst it could have led to loss of life” if safety equipment had failed or leaked gas had been ignited.

The incident occurred on October 15 during well plugging by the mobile rig Songa Endurance at the Troll field.

The well control incident caused a gas leak that pushed seawater more than 98 feet up the derrick before the well was closed by the annular preventer inside the blowout preventer (BOP) about one minute later.

Statoil found that the BOP was quickly activated and halted the gas leak.

Five gas detectors also automatically turned off equipment that could have produced sparks.

No injuries occurred in connection to the well control incident.

Statoil said the investigation found that existing downhole valves were “used as barriers against the reservoir and were unintentionally opened.”

The company said a finding was related to the annular preventer inside the BOP that “should have been closed before the operation was started, because it was not possible to measure the pressure below the wellhead sealing.”

The report found that the two main findings “weakened the barriers and helped gas reach the drill floor.”

Statoil said that some immediate actions were taken after the incident to ensure that the downhole valves are not used as barriers.

The company added that a deeply set plug was reintroduced as a barrier during use of vertical Christmas trees.

The company said the report also outlined several other actions that are currently being taken.

“This is a very serious well control incident. The actions taken will improve our ability to assess risk, both before and during operations. We will share our experience from this incident with the rest of the industry,” Statoil executive vice president for Technology, Projects and Drilling Margareth Øvrum said.

Statoil also investigated a hydrogen leak that occurred at the Mongstad refinery at the October 25.

The Mongstad incident occurred when a hydrogen leak occurred due to external corrosion of a pipe socket at the Mongstad processing complex during surface maintenance.

The investigation found that, during pipe inspection in connection with surface maintenance in the isomerization plant at Mongstad, a portable gas detector was triggered close to a valve.

“When an attempt was made to close the valve, the pipe socket broke and high-pressure hydrogen-rich gas was released,” Statoil said.

Two people were in the vicinity of the area when the leak occurred.

Statoil said the evacuation alarm was immediately activated and followed by evacuation of employees, a shutdown of the facility involved and pressure relief to flare.

The incident was clarified after about an hour.

After about one hour the situation was clarified.

Statoil said its investigation points to external corrosion as the triggering cause.

The company found that “wrong prioritization” of maintenance as a result of insufficient risk understanding was identified as root causes.

The report concluded that the incident was “serious and at worst it could have led to loss of life,” Statoil said.