Abstract: An underwater pipe laying vessel for laying a continuous pipeline, applying what is known as the "J" shape pipe laying method, uses shorter lengths of pipeline welded together on board end to end to form pipe assemblies; the pipe assemblies are stored and subsequently transported to a pipe laying boom unit which guides pipe assemblies to be welded end to end to a pipeline which is already in the "process of laying" under the vessel. The pipe laying boom is located substantially in the center of the vessel and is swivelled about a horizontal axis which is perpendicular to the length of the vessel; the boom includes a flash butt welder and an underlying jacketing-unit for suitably treating a welded surface of the pipeline to prevent subsequent corrosion; a pipeline section guide and pipeline section support located under the jacketing-unit assist in supporting and guiding the treated pipeline for proper positioning.