Union workers at Exxon Mobil Corp's Beaumont, Texas, refinery agreed to an unusually long six-year contract on Thursday night, which sources say the company pushed to assure no work stoppages during a contemplated multibillion-dollar expansion. Members of United Steelworkers union Local 13-243 voted by secret ballot on the contract offer, which is two years longer than the national agreement reached by the USW and U.S. refinery owners in March.