(CNN) -- Coast Guard aircraft have found no sign of a spill from a Royal Dutch Shell oil drilling barge that ran aground off a southern Alaska island during a fierce winter storm, authorities reported Tuesday.

The 266-foot Kulluk "is sound. There is no sign of a breach of the hull. There is no sign of a release of any product," Coast Guard Capt. Paul Mehler III told reporters Tuesday afternoon.

Mehler said the Kulluk had about 143,000 gallons (4,500 barrels) of diesel fuel and 12,000 gallons of combined lube oil and hydraulic fluid on board. The rig -- a key part in Shell's controversial Arctic oil exploration project -- ran aground off Sitkalidak Island, about 200 miles south of Anchorage, on Monday night.

The Kulluk had been working in the Beaufort Sea, off Alaska's North Slope, until October. It was being towed back to its winter home in Seattle when it ran into a severe storm off the Alaskan coast. The Coast Guard evacuated its 18-man crew Saturday night, and it drifted for 10 hours on Sunday after the tug that was towing it lost power.

Monday night, tug crews had to cut the rig loose during a storm that whipped up 24-foot waves, leading to its grounding.

Sean Churchfield, Shell's Alaska operations manager, said Tuesday that the rig "is upright, rocking with a slow motion, and is stable" -- but he added, "There is still a lot of work to be done to bring this to a safe conclusion."

A joint command was set up to head off any possible environmental damage, but ongoing bad weather has kept crews from getting aboard the vessel, Mehler said. Weather conditions were expected to improve through the rest of the week, with seas subsiding from 24 feet Tuesday to 11 feet by Friday, according to the National Weather Service.

The rig grounded in an area of Ocean Bay, where water depth is 32 feet to 48 feet, according to a release from the response team.

Shell's Arctic exploration plans caused widespread concern among environmentalists and were held up after BP's Deepwater Horizon disaster in the Gulf of Mexico. Shell says it's working at far less depth and lower pressures than the BP well that erupted off Louisiana, killing 11 men aboard and unleashing an undersea gusher that took three months to cap.

The U.S. Geological Survey estimates more than 90 billion barrels of oil and nearly 1,700 trillion cubic feet of natural gas may be recoverable by drilling. And the shrinking of the region's sea ice -- which hit record lows in 2012 -- has created new opportunities for energy exploration in the region.

Climate researchers say that a decrease in sea ice is a symptom of a warming climate, caused largely by the combustion of carbon-rich fossil fuels. The science is politically controversial but generally accepted as fact by most scientists.