Shell's drilling rig, Kulluk, ran aground off the coast of Alaska in late 2012 due to the company's "inadequate assessment and management of risks" in icy, storm-tossed waters, said the U.S. Coast Guard on Thursday in its final report on the accident. (See related "Pictures: Errant Shell Rig Runs Aground Off Alaska.")

The new findings further complicate the oil industry's efforts to launch a new era of exploration in Arctic, which is estimated to hold 13 percent of the world's undiscovered oil and 30 percent of its undiscovered natural gas. (See related "Quiz: What You Don't Know About Energy in the Changing Arctic.") The Kulluk accident occurred weeks after Shell* made the first foray into drilling in U.S.