[T]he Exxon Valdez...caused the worst oil spill in U.S. history...Twenty years after the March 24, 1989, disaster...Exxon...has kept using tankers with one hull even as 151 countries have decided two are better than one for preventing oil spills and pledged to ban single-hull vessels by 2015...BP says it won’t hire them because of the risk of leaking...Sunoco, Chevron, ConocoPhillips, Koch Industries, and Total didn’t hire one such vessel last year...Had the [Valdez] been fitted with a double hull, the scale of any spill would have been reduced, said Apostolos Papanikolaou, a ship-design professor at the National Technical University of Athens...Hull design is only one of “hundreds of variables” Exxon uses in monitoring safety, and cost isn’t one of them, said Rob Young, a company spokesman.