The Kulluk, one of two rigs that Shell used to drill test wells off the North Slope of Alaska as part of the company’s ambitious and expensive effort to open Arctic waters to oil production, was being towed by the Aiviq to a Seattle shipyard for off-season maintenance when the towline initially separated during a storm on Thursday.

The Aiviq then lost power, and other support ships and a Coast Guard cutter were brought in to help with engine repairs and to reconnect towlines to the Kulluk, which does not have its own propulsion system. The 18 workers aboard the rig were evacuated by Coast Guard helicopters on Saturday.

Image Credit... The New York Times

Over the weekend, support crews struggled in 25-foot swells to reconnect towlines, succeeding several times. But each time the lines separated again, leaving the rig in danger of drifting toward land.

The Kulluk, which was built in Japan in 1983 and upgraded over the past six years at a cost of $292 million, is designed for icy conditions in the Arctic. It can drill in up to 400 feet of water and up to 20,000 feet deep. During drilling season it carries a crew of about 140 people, Mr. Smith said.

Shell has spent six years and more than $4 billion in its effort to drill in Arctic waters, one of the last untapped oil-producing regions in the United States. But the effort has faced regulatory hurdles and opposition from American Indian and environmental groups.

Last summer, the Kulluk drilled a shallow test well in the Beaufort Sea while another rig drilled a similar hole in the Chukchi Sea to the west.