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OSLO, March 22 (Reuters) - Greenpeace activists have boardeda rig that was due to sail to the Arctic to drill prospects onbehalf of oil firm Statoil, the environmental groupsaid on Thursday.

"We have two people on board and 13 on the water to ensurethe rig does not leave port. Our activity started at 8am (0700GMT) this morning," Truls Gulowsen, the head of Greenpeace inNorway, told Reuters.

"We are prepared to stay as long as necessary," he added.The action took place at the Skipavik yard on Norway's westcoast, where the drilling unit was being prepared for theupcoming work.

The West Hercules rig belongs to Seadrill and iscontracted to Statoil via its North Atlantic Drilling affiliate.

Statoil said last month the rig would drill two explorationwells during the upcoming summer season, with options for afurther five wells, adding the West Hercules would provide for asafe and efficient operation.

On Thursday, the oil company said the Greenpeace actionwould not affect operations.

"There's a contract for upcoming work, but we haven't begunany operations yet in that regard. It's from this summer andonwards," Statoil spokesman Morten Eek said.

Greenpeace and other groups in January lost a lawsuitagainst the Norwegian government over the continued drilling inthe Arctic, arguing that the award of exploration licencesviolated citizens' right to a clean environment.

The environmental groups have since appealed theverdict.(Reporting by Gwladys Fouche and Camilla Knudsen, editing byTerje Solsvik and David Evans)