The trans-Alaska Pipeline is the largest conduit of domestic oil, a funnel for crude from the North Slope’s Prudhoe Bay to the Port of Valdez. But the Prudhoe wells are drying up—and the prospect of replacing them appears ever more grim. Despite the capacity to carry 2 million barrels a day, the pipeline’s current flow is less than 700,000 gallons and falling at least 6 percent a year. Now its operators have commissioned a study to see how low the supply can get before crude freezes in transit. The most common estimate is about 500,000 barrels, a figure that ConocoPhillips recently predicted would be reached by 2015.