U.S. Interior Secretary Ken Salazar on Thursday gave Royal Dutch Shell the go-ahead to begin preliminary drilling of a single exploratory oil well off Alaska's northwest Arctic shores.

It's not the final thumbs-up -- Shell's crews must stop drilling nearly a mile short of expected oil and gas -- but the conditional approval will allow Shell to get to work after six years of waiting.

At Shell's headquarters in Anchorage, the mood among employees was optimistic Thursday. One Shell executive said they'd like to frame the Interior Department's letter approving the drilling.

"Ladies and gentlemen, there's no other way to couch this. Today's announcement is extremely exciting," Pete Slaiby, Shell's vice president in Alaska, told a roomful of reporters. "We've been waiting for this for about six years, and our goal of being able to drill in the Chukchi is about to take place."

Shell's drill ship -- the Noble Discoverer -- could be in position as soon as Friday, with preliminary works starting next week, including setting up key safety equipment to stop a runaway well, he said.

Shell can't drill beyond 1,400-feet. To reach oil somewhere beyond 5,000 feet, the Arctic Challenger barge, Shell's final line of defense in the unlikely event of a spill, must be certified in Bellingham, Wash., and be onsite in the Arctic -- a process that could take two weeks or more.