Seven months ago the giant tanks in Cushing, Okla., the largest crude oil storage hub in North America, were three-quarters empty. After spending the last few years brimming with light, sweet crude unlocked by the shale drilling revolution, the tanks held just less than 18 million barrels by late July, down from a high of 52 million in early 2013. New pipelines to refineries along the Gulf Coast had drained Cushing of more than 30 million barrels in less than a year. As quickly as it emptied out, Cushing has filled back up again. Since October, the amount of oil stored there has almost tripled, to more than 51 million barrels. As oil prices have crashed, from more than $100 a barrel last summer to below $50 now, big trading companies are storing their crude […]