Equipment failure on a Seadrill-connected rig working for Wintershall Dea offshore Norway that resulted in production equipment slipping to the seabed is set to keep the unit offline for weeks.

Seadrill offshoot Northern Offshore said in a statement to the Oslo Stock Exchange on Friday that the semi-submersible West Mira had suffered an "operational incident" on 19 March while working on the Nova field for the German operator.

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"During the process of lowering production equipment to the well location in the Nova field, the rig experienced equipment failure resulting in the production equipment descending to the seabed," Northern Offshore said.

Wintershall Dea had earlier identified the subsea equipment as a christmas tree.

A subsea christmas tree is a set of valves, spools, pressure gauges and chokes connected to the top of an exploration or development well to direct and control the flow of formation fluids from the well.

"No one was injured in the incident and the well was secured with three barriers in place."

The rig player added that manager Seadrill Europe Management and Wintersall Dea "are conducting investigations into the incident and have agreed a plan to secure the production equipment".

"Based on current plans to inspect the impacted equipment and perform between well maintenance, the company is forecasting that the West Mira will return to work by mid-April, subject to weather conditions and other factors which may arise.

"Both the manager and client remain in close communication with the [Norwegian] Petroleum Safety Authority."

In an earlier statement, Wintershall Dea said: "An investigation into the incident, which is considered serious, has been initiated."

"While lowering a christmas tree from West Mira, the winch wire snapped when the tree was five metres below the sea surface. The christmas tree sunk to the seafloor 368 metres below water level.

"Eight people were working in the area of the rig where the incident occurred in safe distance from moving equipment."

The German player said a remotely operated vehicle was sent to the seafloor to assess the situation.

"The ROV survey showed no risk of discharge of well fluids or hydrocarbons and the christmas tree has been localised on the template.

"The well is under control and secured with three barriers."

Last May, Wintershall Dea gained consent from the Petroleum Safety Authority to use the West Mira for production drilling and completion work at the Nova field, with an estimated duration of 388 days.

The 2019-built seventh-generation harsh-environment rig was chartered to drill six wells under a firm contract worth $106 million, with options for 10 additional wells.

This work was originally due to start in March 2020, although the operator had an option to bring this forward to the third quarter of last year that apparently did not pan out.

Wintershall Dea holds a 45% operating stake in production licence 418 that hosts Nova, with partners Spirit Energy on 20%, Edison on 15%, and Solveig Gas unit Capricorn and One-Dyas on 10% apiece.

Lundin in minor oil discovery

Elsewhere offshore Norway, the NPD revealed on Friday that Swedish independent Lundin Energy has made what the NPD termed a "minor oil discovery" at the 16/4-13 S well on production licence 359 in the North Sea.

The well was drilled about 15 kilometres south of the Edvard Grieg field in the central part of the North Sea and 190 kilometres west of Stavanger.

"The objective of the well was to prove petroleum in reservoir rocks from the Triassic to Paleozoic Age," the NPD said.

"The well encountered an oil column of 10 metres in conglomerate sandstone in these rocks, about seven metres of which with moderate to poor reservoir quality.

"The oil/water contact was encountered 1950 metres below the sea surface.

"The entire reservoir, including the water zone, consists of conglomeratic sandstones and in a thickness of about 380 metres.

"Preliminary estimates place the size of the discovery between 0.5 and 1.4 million standard cubic metres of recoverable oil equivalent [between 3.14 million and 8.81 million barrels of oil equivalent].

"The licensees will assess the discovery with regard to a possible tie-in to the Solveig field."