The isolation tool works at depth in the bore of a pipe whose upper end is to be severed and replaced. The tool isolates the lower pipe bore from the upper bore so that flammable downhole fluid is prevented from reaching a welding site along the pipe wall. The tool incorporates: means for establishing a pair of axially spaced apart annular seals engaging the pipe inner surface; means for venting downhole fluid to ground surface; and means for monitoring for seal leakage. The tool is primarily mechanical in nature. It finds use in connection with off-shore platform riser renewal operations.