The incident caused a gas leak that pushed seawater more than 30 m (98 ft) up the derrick, although the well was closed by the annular preventer inside the BOP around one minute later.
Statoil found that loss of life could have occurred if the safety equipment had failed to function as intended, or if the gas had been ignited. The BOP was quickly activated and stopped the gas leak, and five gas detectors automatically turned off equipment that might have generated sparks.
The report concluded that two circumstances led to weakening of the barriers, helping gas reach the drill floor.
The first was that downhole valves were used as barriers against the reservoir and were unintentionally opened. The second related to the annular preventer inside the BOP, which should have been closed before the operation was started, because it was not possible to measure the pressure below the wellhead sealing.
After the incident actions were taken to ensure the downhole valves are not used as barriers, with a deeply set plug reintroduced as a barrier during use of vertical Christmas trees. Various other measures are currently being implemented.
Margareth Øvrum, executive vice president Technology, Projects and Drilling at Statoil, said: “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.”