It's been a while, but when I was up there it did not matter in Prince William Sound either way, speed restriction & no line up from tug. In Puget Sound if twin screw, still escort but the single screw needed a line up from escort tug. The twin screw could make better speed w/o tug, so the cost of missing a tide at Bellingham / Cherry Point could add up over the life of the tanker. Not sure of added cost of having a line up for tug rates vs. no line up escort. Also, ATC is pretty much owned by BP, one of their competitors Arco then ConnocoPhillips had twin screw/twin engine rooms on this run before the Alaska Class where built. Could be bad press if anything happens. ExxonMobil did not care about the press aspect as they built single screw plants.