The accident on Deep Sea Trym on 4 December has attracted sharp criticism from the Norwegian Petroleum Directorate (NPD).
These critical remarks are contained in separate letters to rig owner Odfjell Drilling and to Statoil as operator of the Gullfaks field in the North Sea, where the incident occurred.
Odfjell is ordered to ensure that the direct and underlying causes of the accident - which allowed oil to pour under high pressure onto the rig deck - are clarified and corrected. The company must implement the necessary measures and correct any damage caused to Deep Sea Trym by the accident.
In its letter to Statoil, the NPD instructs the group to ensure that Odfjell actually implements the instructions it has been given. Although Norway's new Petroleum Act assigns much greater responsibility to rig owners than before, Statoil has an overall duty of care as operator.
Statoil takes a very serious view of the incident, and its own internal inquiry team will be submitting a preliminary report in the middle of next week.
The likely cause is that a heave compensator - used to keep the drill string stable during wave motion - locked after production testing in a well on Rimfaks, one of the Gullfaks satellites. Test equipment weighing six-seven tonnes came loose from the string and was thrown up inside the rig. Oil leaked out briefly before the well shut in automatically.
Nobody was on the drill floor when the accident happened, but the NPD takes the view that a disaster was only narrowly averted. The state agency has banned further use of the rig. Statoil immediately halted all operations on board.
No decision has yet been taken on whether to send Deep Sea Trym to land for repairs.