Exxon Mobil Corp's push to persuade workers at its Beaumont, Texas refinery to sign a five-year contract, nearly twice as long as the last one, is part of an effort to avert labor stoppages during a possible expansion that could make it the largest such plant in the United States, sources familiar with refinery operations said. In a bid to win support, the company has offered a $4,500 bonus to hourly workers represented by the United Steelworkers union, which normally only signs three-year deals in the refining industry, the sources said.