Shell has been given final clearance to resume drilling for oil and gas in the Arctic Ocean for the first time since 2012.
The firm interrupted its drilling program in the oil-rich Chukchi Sea off the northwest coast of Alaska in 2012 after a series of problems that included losing control of an enormous rig, from which the Coast Guard had to rescue 18 workers.
Now the Department of the Interior has granted the Anglo-Dutch company a permit to start drilling again after repairs were completed to the Fennica, an icebreaker the company leases that carries emergency well-plugging equipment.
The ship had suffered a gash in its hull after hitting uncharted shoals off southern Alaska and late last month, 13 Greenpeace activists hanging from a bridge in Oregon temporarily blocked the freshly repaired vessel from reaching the Pacific Ocean to return to Alaska.
Green groups have vowed to fight Shell's plans as they want to protect whales, walruses and polar bears in a vulnerable region that scientists say is changing rapidly due to global warming.
Shell was granted the Chukchi leases by the administration of former President George W. Bush and since then it has spent about $7bn (£4.5bn) on exploration in the Arctic, even though oil production is at least a decade away.
However, the Sierra Club, the country's oldest environmental group, has urged the current administration to cancel sales of oil-zone leases scheduled for 2016 and 2017 and to remove the possibility of drilling in the Arctic Ocean.
Head of the organisation Michael Brune said President Barack Obama "must change the course on Arctic drilling set eight years ago by former President George W. Bush and not perpetuate it".
Harsh conditions in the Chukchi have discouraged other oil companies from drilling there so far, but with the US government estimating as much as 20 per cent of the world's undiscovered oil and gas is in the Arctic, expanding exploration looks inevitable.
Shell spokesman Curtis Smith said the company "looks forward to evaluating what could potentially become a national energy resource base".
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