The National Transportation Safety Board blames the 2012 grounding and wreck of Shell’s Arctic drilling rig Kulluk on the oil giant’s “inadequate assessment of the risk” of towing it across the Gulf of Alaska in a winter storm. “No single error or mechanical failure led to this accident,” the NTSB said in findings released this month. “Rather, shortcomings in the design of a plan with an insufficient margin of safety allowed the accident to take place.