Royal Dutch Shell PLC's foray into Arctic offshore drilling has suffered a serious setback after one of its two Alaskan drilling rigs ran aground off a small island while trying to escape a fierce storm.

The Kulluk drilling ship grounded Monday night on rocks off the southeast side of Sitkalidak, an uninhabited island in the Gulf of Alaska, according to officials at a command center run by the U.S. Coast Guard, Shell and Alaskan state emergency workers.

The Kulluk was being towed by a 360-foot anchor handler, the Aiviq, and a tugboat, the Alert. The vessels were moving north along Kodiak Island, trying to escape the worst of a North Pacific storm that included winds near 70 mph (113 kph) and ocean swells to 35 feet (11 meters).

About 4:15 p.m., the drill ship separated from the Aiviq about 10 to 15 miles (16 to 24 kilometers) offshore and grounding was inevitable, Coast Guard Cmdr. Shane Montoya told reporters.

"Once the Aiviq lost its tow, we knew the Alert could not manage the Kulluk on its own as far as towing, and that's when we started planning for the grounding," he said.

The command center instructed the nine tug crew members to guide the drill ship to a place where it would cause the least environmental damage. The tug cut the unmanned ship loose at 8:15 p.m. and it grounded at 9 p.m. near the north tip of Ocean Bay on Sitkalidak Island.

The Coast Guard planned to fly out early Tuesday to plan a salvage operation and possible spill response. The drill ship drafts up to 40 feet (12 meters) and is carrying 150,000 gallons of diesel and about 12,000 gallons of lube oil and hydraulic fluid, Montoya said.

Susan Childs, Shell's on-scene coordinator, said it was too early to know how the vessel would react to the pounding of the storm. Still, she was optimistic about its salvage prospects and chances for staying intact.

"The unique design of the Kulluk means the diesel fuel tanks are isolated in the center of the vessel and encased in very heavy steel," she said. "When the weather subsides and it is safe to do so, we will dispatch crews to the location and begin a complete assessment."

The Kulluk is designed for extended drilling in Arctic waters and underwent $292 million in technical upgrades since 2006 to prepare for Alaskan offshore exploration. The drill ship worked during the short 2012 open water season in the Beaufort Sea off Alaska's north coast. Its ice-reinforced, funnel-shape hull can deflect moving ice downward and break it into pieces.

Attached to a drilling prospect, the Kulluk is designed to handle waves 18 feet (5.5 meters) high. When disconnected from a well, it's designed to handle seas to 40 feet (12 meters) high.

Garth Pulkkinen of Noble Corp., the operator of the drill ship, said it was never in danger of capsizing.

The vessel's tow line first broke Thursday night south of Kodiak Island as it was moving from Dutch Harbor in the Aleutian Islands to Seattle for maintenance.

Before a line could be reattached, the Aiviq's engines failed, possibly from contaminated fuel. The Coast Guard cutter Alex Haley attempted to secure the drifting drill ship but that line also failed and wrapped itself around one of the cutter's propellers.

With bad weather predicted, the Kulluk's crew was evacuated Saturday.

The Aiviq, with its engines restored, and a tug re-established lines to the drill ship, but lines broke Sunday. During a lull in the storm early Monday, the crew of Alert grabbed the original 400-foot (122-meter) line trailing the drill ship and later the Aiviq grappled aboard one of the emergency lines.