Abstract: A pipe-welding assembly is provided in which single-pass welding of one pipe length to another is accomplished by gas lasers mounted to rotate around the axis of the pipes to weld the pipe ends together in the gap between two axially aligned spaced pipe clamps. Weld-testing units are similarly rotated around the pipe axis. The pipe-welding assembly is mounted on a gyroscopically-stabilized platform, which can be located on a pipe-laying land vehicle, or on a pipe-laying ship for laying pipe lines on the ocean bed. If the ship is equipped with a pipe feed tower, with the pipe-welding assembly positioned at the base of the tower and a drawbridge provided for elevating successive pipe lengths angularly into the tower, pipe laying can be performed in the "J" configuration. If the tower is mounted so that it can be inclined angularly into a position approaching the horizontal, and the pipe-welding assembly can be similarly rotated on its stabilized platform, pipe-laying can be performed in the "S" configuration.