The activists claim that Cairn Energy, which runs the Leiv Eiriksson rig, is not taking enough precautions to avoid accidents like the Gulf of Mexico blowout in 2010, and demand the group release a plan for how to manage a potential oil spill.

The activists launched five inflatable speed boats from the Greenpeace ship Esperanza early Saturday morning, bypassing the Danish navy and scaling the giant rig to make their way to the drill manager’s office.

The group said an oil response plan has not been published.

“Cairn Energy is hiding its oil spill response plan so we’re going to the one place where there must be a copy of it,” Greenpeace campaigner Ben Ayliffe said. “It’s obvious why Cairn won’t tell the world how it would clean up a BP-style oil spill here in the Arctic, and that’s because it can’t be done,” he said. The group claims the remoteness and freezing temperatures in the area would make a cleanup extremely difficult.

Cairn, which last month won permission to drill up to seven oil exploration wells off the Arctic island’s west coast, confirmed protesters had climbed onto the semi-submersible drilling vessel, breaching the safety exclusion zone and causing it to suspend drilling.

It insisted that it “seeks to operate in a safe and prudent manner” and that Greenland authorities have established stringent operating regulations, similar to those applied in the Norwegian North Sea.

Cairn added that authorities on the island have been alerted.

Greenland’s government has called the Greenpeace action a publicity stunt at the expense of Greenland’s “legitimate right” to develop its economy.

Earlier this week, two Greenpeace activists were arrested by Danish authorities after they had attached themselves under the rig in a pod, hanging just a few meters from the drill-bit that would be used to strike oil, and preventing Cairn from starting its drilling for four days.

Greenpeace said it has received a legal summons from Cairn’s lawyers after the action for having cost it up to $4 million for every day it could not drill. It said the case will be heard in a court in the Netherlands on Monday.