There are a few people in the powerplant construction business who would love to see James M. Bernhard Jr., the smooth-talking multimillionaire chairman of what is probably the world's most sophisticated pipe fabricator, The Shaw Group, fall flat on his face. Bernhard, through a series of contracting acquisitions, has gone into competition against some of his customers, including powerful Bechtel Group Inc. He began easing his way into the turnkey powerplant construction business from 1997 to 1999, adding value, he would say, to the pipe fabrication work at which Shaw Group excels. Eighteen months ago, he upset the power pecking