Norway's Petroleum Safety Authority (PSA) is investigating a hydrocarbon leak which happened on the Eldfisk field in Block 2/7 in the North Sea early on Thursday 7 August.
The leak led to the discharge of stabilized oil to the sea from the ConocoPhillips operated Eldfisk field terminal platform (FTP).
The facility was in the process of starting up after an emergency shutdown (ESD) and loss of power on 6 August, said the PSA.
It said: "According to ConocoPhillips, a blowdown valve in the separator was left open during the start-up, sothat stabilized oil flowed to the sea via a sea sump and knockout drum."
The leak was discovered and halted at 5am on 7 August. A helicopter was dispatched to monitor and assess the size of the spill, and mechanical recovery of the oil initiated.
The PSA added: "The PSA’s decision to launch its own investigation reflects the seriousness of the incident and the information received about it. Among other goals, the inquiry will seek to establish the course of events, identify the direct and underlying causes, and follow up ConocoPhillips’ own investigations of the ESD and leak."