Protesters board Cairn oil rig offshore Greenland

LONDON (Reuters) - Protesters from environmental group Greenpeace boarded a drilling rig operated by UK oil explorer Cairn Energy on Monday to try and stall development of what the oil industry hopes will become a major new producing center.

Greenpeace said four campaigners 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.

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 climactic conditions and remoteness would make capping a leak and cleaning up spilled oil especially difficult.

The Greenpeace activists have suspended themselves in tents from the rig and have enough provisions for days, said spokesman Ben Stewart, on Greenpeace ship the Esperanza.

The 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 sea between Greenland and Canada is known as "iceberg alley."

Commandoes from a Danish warship which has been trailing the Esperanza failed to stop campaigners from boarding the rig.

"We caught them napping," Stewart said.

Cairn declined comment.

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 drillship, 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.

Cairn shares traded down 0.5 percent at 1242 GMT, outperforming a 1.7 percent drop in the STOXX Europe 600 Oil and Gas index.

Cairn's wells are the first to be drilled in Greenland in over a decade. Six wells were drilled in the 1990s but failed to find oil or gas in commercial quantities. However, companies hope that better technology will now allow them to discover big reservoirs.

Big oil companies including Exxon Mobil and Chevron have bought exploration licenses.