The Environmental Protection Agency has approved an air quality permit for a Shell Oil drilling vessel and accompanying vessels that the company hopes to use in Arctic waters next year.
The EPA on Monday approved the air permit for the drilling vessel Noble Discover.
The permit was a key hurdle for Shell to overcome before it can begin exploratory drilling in the Chukchi Sea off Alaska's northwest coast.
Environmental groups and some Alaska Native groups oppose offshore drilling. Drilling was held up this year in part by a successful appeal of two air permits EPA issued to Shell in 2010.
Outer continental shelf reserves in the Chukchi and Beaufort seas are estimated at 26.6 billion barrels of recoverable oil and 130 trillion cubic feet of natural gas.