Greenpeace said four expert climbers climbed aboard the Stena Don, a semi-submersible rig, which has been drilling the Alpha prospect in the Sigguk block, 175 km offshore Disko Island, West Greenland (right).
The Greenpeace activists suspended themselves in tents from the Stena Don rig 15m above the icy Arctic ocean said spokesman Ben Stewart, on Greenpeace ship the Esperanza.
The campaigners, who are protesting against the ''huge risks'' energy companies are taking with the environment by drilling for oil in deep Arctic water, say they have enough supplies to occupy the tents for several days.
Sim McKenna, from the US, who is one of the climbers, said: ''We've got to keep the energy companies out of the Arctic and kick our addiction to oil, that's why we're going to stop this rig from drilling for as long as we can.
''The BP Gulf oil disaster showed us it's time to go beyond oil."
Cairn has said Greenland could have billions of barrels of reserves but the BP oil spill in the Gulf of Mexico has raised fears about the risks of offshore drilling. Greenland's harsh climate and remoteness would make capping a leak and cleaning up spilled oil especially difficult should one occur.
Greenpeace's vessel Esperanza has been in the area for over a week planning activities to prevent Cairn from completing its objectives before the end of the narrow summer drilling season. The Esperanza set sail from London last month with a pledge to target what the environmental group described as one of the 10 most dangerous deep water drilling sites in the world.
Greenpeace said it wanted to highlight problems with oil that went "far beyond" the disaster at BP's Deepwater Horizon rig in the Gulf of Mexico, and pledged to confront the industry head-on over its "reckless" pursuit of oil.
The campaigners are concerned that a discovery of oil could prompt an oil rush by major companies in the Arctic, threatening the fragile environment of the region and providing a new source of a fossil fuel which contributes to climate change. In fact, Chervon and Exxon Mobil have already obtained exploration licenses.
The environmental group had targeted the Cairn exploration rig in the Arctic because the region "is really symbolic of the state of crisis the world is in", a spokeswoman for Greenpeace said.
"We're facing catastrophic climate change, yet the oil companies drill for the last drop of oil."
"It brings it all home to see tugs pulling icebergs away from the rig - it shows how far we've gone to keep our addiction to fossil fuels going." (Right)
The Greenpeace representative went on to say that the exploration of the virtually untouched region, where seals, humpback whales and sea lions could be seen, showed what was wrong with the oil industry and the world's addiction to oil.
She also said it highlighted the impact of fossil fuels on climate change, with Greenland glaciers melting and icebergs falling into the sea as a result of warming temperatures.
At the height of the BP oil spill crisis in the Gulf of Mexico, marine pollution experts warned the "nightmare" oil spill would be one that stemmed from drilling in the Arctic.
Marine scientists warned that, in an Arctic spill, oil would only evaporate and break down slowly and would affect the "albedo", or reflective (right), effect of the region which helps maintain its climate. This would speed up the melting of the polar ice.
Greenpeace activists board rig despite warship protectionCommandos from a Danish warship which has been trailing the Esperanza failed to stop campaigners from boarding the Stena Don rig, Greenpeace said.
"We caught them napping," Ben Stewart of Greenpeace said.
Greenland deputy police chief Nielsen said that police could have stopped the activists before they reached the rig, but chose not to intercept them because of the risks involved, both to the activists and police officers, in such cold waters.
He said the police were supported by the Danish navy but that maintaining law and order in the area was a police matter. Cairn declined comment.
Greenland police said that the activists remained suspended by their climbing gear below the rig in the late afternoon.
"Like any criminal, they can be expected to arrested and prosecuted," Morten Nielsen, deputy chief of police in Greenland, stated.
The Stena Don has been drilling the Alpha-1 well since late June. It was expected to hit target depth in 55 days and Cairn investors are expecting to hear whether it has been successful in the coming weeks.
Edinburgh-based Cairn last week completed drilling of its nearby T8 prospect. Although the T8 well failed to hit oil, the company said the results proved the existence of a hydrocarbon formation.
The Stena Forth drill ship, which drilled the T8 prospect, is scheduled to commence drilling another prospect soon.
Greenpeace declined to say if it might try and disrupt that vessel's operations next.
Reuters, "Protesters board Cairn oil rig off Greenland", accessed September 2, 2010
The Telegraph, "Greenpeace claims to have shut down Greenland oil well", accessed September 2, 2010