Shell plans to use the drillship M/V Noble Discover and the drilling unit Transocean Polar Pioneer for its planned Arctic drilling this summer. The two vessels will provide relief-well capabilities for each other, BOEM said in a statement. The conditional approval was lambasted by environmentalists Monday, claiming the administration has unnecessarily exposed the Arctic to an oil spill. "Any major Arctic Ocean spill would be impossible to clean up," Franz Matzner, director of the Natural Resources Defense Council's Beyond Oil Initiative, said in a statement. Such a spill would be "catastrophic" since Shell's proposed drilling takes place more than 1,000 miles from a Coast Guard base and other cleanup infrastructure, Matzner said. "Accidents can and do happen; and there is no proven way to respond to an oil spill in icy Arctic waters," Susan Murray, a deputy vice president with Oceana, said in a statement. In March, Interior upheld its 2008 Arctic lease sale, which had been suspended by a federal court ruling. Under that ruling, BOEM updated an estimate of recoverable oil in the Arctic area it had leased the rights to, boosting the estimate from 1 billion barrels to 4.3 billion barrels. Shell has requested a five-year extension on its Arctic leases, which the company said it needs before it invests hundreds of millions of dollars more in the Arctic. Federal officials have not ruled on this request. Shell has already spent about $6 billion on its Arctic drilling efforts.