"The gas leak occurred in connection with a routine operation (on the Heimdal platform) where a valve was overloaded and gas leaked into the surrounding area," the company stated. "The investigation concludes that this was a serious gas leak."
Statoil said, however, there were no injuries as a result of the incident nor were there threats to the integrity of the platform. Emergency response systems worked as intended, the company added.
There were 98 workers on the platform in the North Sea at the time of the incident.
The company said gas leaked from the platform for about 4 minutes. It said the incident was caused by a "complex chain of events" related to design, planning and a "failure to communicate."
Jannicke Nilsson, head of regional operations for Statoil, said the company was working to avoid similar incidents, adding there were operational improvements made since the incident.
"The Heimdal incident is still a strong reminder of the need for continuous learning and improvement in our operations," Nilsson said in a statement.