Eni confirmed it had successfully started production on its landmark Goliat oilfield, two years after its initial first oil target day.
The project is the first oilfield to start producing from the Norwegian sector of the Barents Sea.
Goliat, which will peak at 100,000 barrels a day, has been hit by a string of delays, including problems building its FPSO, which has the capacity to store one million barrels of oil.
As a result, project costs soared 52% to NOK48.6billion from the original estimate of NOK30million.
Goliat has a subsea system comprised of 22 wells and is estimated to hold 180million barrels of oil.
The Italian operator owns a 65% stake in the field with Statoil owning the remaining 35%.