The crippled oil tanker Exxon Valdez leaves Alaskan waters today for California on a 3,000-mile journey that will be the first major movement of the tanker since it ran aground three months ago, causing the biggest oil spill in U.S. history. "We're not expecting anything to go wrong," said Coast Guard Lt. Alan Carr. "The belief is that it's structurally sound enough to make the transit." No crude oil is on the Exxon Valdez; 11 million gallons spewed out of the ruptured hull when the ship ran aground March 24 and another 42 million gallons were pumped out. Exxon has promised to keep the damaged tanker 100 miles offshore until it reaches San Diego, where it will undergo a $25-million repair job at the National Steel & Shipbuilding Co.