ANCHORAGE, Alaska (AP) — There's no indication of a fuel leak from a petroleum drilling vessel that ran aground on a remote Alaska island, the Coast Guard said of a maritime accident that has refueled debate over oil exploration in the U.S. Arctic Ocean.

The Royal Dutch Shell PLC ship Kulluk was being towed to a Pacific Northwest shipyard for maintenance and upgrades when it went aground in the Gulf of Alaska during a vicious New Year's Eve storm.

"There are still no signs of any sheen or environmental impact, and the Kulluk appears to be stable," Coast Guard Capt. Paul Mehler said Wednesday night, after flying over the rig with a Shell representative and an Alaska Environmental Conservation Department official.

The Kulluk is a non-propelled, 266-foot diameter barge with a funnel-shaped hull designed to operate in ice. It is carrying more than 140,000 gallons of diesel and about 12,000 gallons of lube oil and hydraulic fluid. Centered on the vessel is a 160-foot derrick.

Mehler said he saw four lifeboats on the shoreline but there was no indication that other debris had been ripped from the ship.

The flyover in rain and 35 mph winds showed a few birds but no marine mammals near the rig, said Steve Russell of the Environmental Conservation Department.

Also Wednesday, calmer weather allowed five salvage experts to be lowered by helicopter to the barge. They conducted a three-hour structural assessment. Mehler said the assessment team was working with salvage planners but it was too early to speculate on a timeline for moving the vessel.

After the grounding, critics quickly asserted it has foreshadowed what will happen north of the Bering Strait if drilling is allowed.

Environmentalists for years have said conditions are too harsh and the stakes too high to allow industrial development in the Arctic, where drilling sites are 1,000 miles or more from the closest Coast Guard base.

Two national organizations kept up the drumbeat Thursday by calling for a halt to all permitting for Arctic offshore drilling in the wake of the grounding.

"This string of mishaps by Shell makes it crystal clear that we are not ready to drill in the Arctic," said Chuck Clusen of the Natural Resources Defense Council. "Shell is not Arctic-ready. We have lost all faith in Shell, and they certainly don't have any credibility left."

Lois Epstein, a civil engineer who works for The Wilderness Society in Anchorage, said Shell has made troubling, non-precautionary decisions that put workers and the Coast Guard at risk.

"These ongoing technical and decision-making problems and their enormous associated costs and risks taken by our military personnel once there were problems should lead the federal government to reassess its previous permitting decisions regarding Shell," Epstein said.

In the short term, she said, damage to the Kulluk may prevent it from being ready for the 2013 open water season. Besides drilling in the Beaufort, the barge was supposed to be on hand for drilling a relief well if Shell's drill vessel in the Chukchi Sea, the Noble Discoverer, experienced a wellhead blowout and was damaged, Epstein said.

Shell has maintained it has taken a heads-up approach to anticipating and reacting to problems.

Shell Alaska spokesman Curtis Smith said Wednesday the Kulluk had been towed more than 4,000 miles and had previously experienced similar storm conditions. Shell staged additional towing vessels along the route in case there were problems, he said.

"We know how to work in regions like this," Smith said. "Having said that, when flawless execution does not happen, you learn from it, and we will."