The leak happened on 31 December.
"The most serious problem is the failure of the system after the power loss," PSA spokeswoman Inger Anda said, adding that the gas release stopped after "several hours".
She said that excess gas in the processing plant was "released uncontrollably" after a low-pressure flaring system failed.
"We will look into what happened and meet Shell officials next week," Anda said.
Ormen
