ANCHORAGE, Alaska — Hours after a repaired Shell icebreaker eased past protesters in Oregon to join an Arctic drilling operation, the oil giant used other vessels in its flotilla to begin excavating off the coast of Alaska in hopes of confirming the presence of billions of barrels of crude below the ocean floor.

Royal Dutch Shell PLC now only needs the 380-foot Fennica, repaired at a Portland shipyard, to be on hand at the drill site to ask federal regulators for permission to dig into oil-bearing rock already included on its Arctic offshore leases.

The drilling would come over the protests of environmental groups in Portland and elsewhere who say Shell has not proved it can clean up a major spill in a region covered by ice much of the year.

Critics have decried Arctic offshore drilling since before the federal government auctioned off billions of dollars in leases to Shell and other companies in 2008. But after failed lawsuits and protests, opponents have few options to stop drilling.

Protesters seized one of their last chances this week after the Fennica was gashed by an underwater obstruction in the Aleutian Islands and sent to Portland for repairs.

Thirteen members of Greenpeace USA dangled from a bridge over the Willamette River and kayakers converged on the Fennica to block the repaired icebreaker from heading toward the Pacific Ocean and on to Alaska.

The Greenpeace protesters persisted even after a federal court judge in Anchorage ordered $2,500 hourly fines. They were finally removed by authorities. Two people were arrested.

The U.S. Geological Survey estimates U.S. Arctic offshore reserves at 26 billion barrels of recoverable oil and 130 trillion cubic feet of natural gas.

Shell officials have said offshore Alaska offers some of the most prolific, undeveloped hydrocarbon basins in the world, a potential game-changer that could increase domestic supply by more than 1 million new barrels of oil a day.