WASHINGTON — One of Shell Oil’s two Arctic drilling rigs is beached on an island in the Gulf of Alaska, threatening environmental damage from a fuel spill and calling into question Shell’s plans to resume drilling in treacherous waters north of Alaska in the summer.
The condition of the rig, the Kulluk, which broke free from a tow ship in stormy seas and ran aground Monday night, was unknown late Tuesday. The Coast Guard was leading an effort to stabilize the vessel, 266 feet in diameter, to prevent further damage and keep more than 150,000 gallons of diesel fuel and lubricants from spilling onto the rocky shoreline.
At a Tuesday news conference in Anchorage, Captain Paul Mehler III, the federal on-scene coordinator, said a reconnaissance flight showed the Kulluk was upright and stable.
‘‘No sign of breach of hull, no sign of release of any product,’’ he said, adding that the response team hoped to get salvage experts aboard the ship to get a better picture of damage.
Steven Russell of the Alaska Department of Environmental Conservation said that, so far, there was no sign of harm to the environment or wildlife.
The Kulluk’s 18 crew members were evacuated by Coast Guard helicopters on Saturday.
It was the latest in a series of mishaps to befall Shell’s ambitious plans to prospect for oil in the Beaufort and Chukchi seas off Alaska’s North Slope.
Shell halted drilling in September after equipment failures, unexpected ice floes, operational missteps, and regulatory delays forced the company to scale back its plans. Its drilling rigs completed two shallow pilot holes and left the Arctic in late fall to return to Seattle for maintenance work but have encountered problems in transit.
If the Kulluk, which Shell upgraded in recent years at a cost of nearly $300 million, is wrecked or substantially damaged, it will be hard for the company to find a replacement and receive the numerous government permits needed to resume drilling in July, as it had planned. Under Interior Department rules governing Arctic drilling, the company must have two rigs on site to provide a backup vessel to drill a relief well in case of an uncontrolled escape of oil or gas.
A separate containment system designed to collect oil in the case of a well accident failed during testing, preventing Shell from drilling into oil-bearing formations during its abbreviated exploration season.
Shell’s Alaska vice president, Pete Slaiby, said company officials were working with a Coast Guard-directed unified command and could not comment separately.
An official involved in the operation who spoke on condition of anonymity said: ‘‘We don’t know about the damage. It’s too dark. The weather is horrendous.’’ The official said the fuel tanks on the vessel were well protected inside the hull, making a spill unlikely.
The Kulluk is in the Sitkalidak Strait, home to a threatened species of sea lion.