Recent Advances in Underwater Dry Pipeline Welding
Authors: Lynch Robert P. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.

Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Until recently all underwater welding was performed in the wet by divers using the conventional stick electrode technique. The quality of weld produced could not favorably compare with the same weld made in air. Ocean Systems, Inc. developed a new underwater welding technique which enables an operator to consistently and reliably produce a maximum quality weld. The weld is performed in a dry ambient pressure performed in a dry ambient pressure enclosure that contains a fire proof atmosphere. With this technique, weld quality exceeds the requirements set forth in the American Petroleum Institute, Standard 1104.

This paper discusses the technical aspects of the welding process and the specialized equipment developed to bring the process to the sea floor. Also discussed are several operations in which the process was applied as well as a description of anticipated future applications.

Introduction

It has long been recognized by pipeliners that conventional underwater wet stick electrode welding would not give them the joint quality that they require. Consequently, until recently all offshore pipeline welding of any type was performed pipeline welding of any type was performed on the surface. There are many types of welds that are much more suitable to being performed in situ for economic, mechanical performed in situ for economic, mechanical and safety reasons. An example of such a weld would be a hot tap intersection. To raise a pipeline to the surface for a hot tap requires the removal of the overburden for a considerable distance to either side of the weld point. The distance is of course dependent on water depth and pipe size. Large diameter pipe in deep water poses a tremendous excavation problem. poses a tremendous excavation problem. Once the line is uncovered as many as three derrick barges may be required to raise the line to the surface while maintaining a critical catenary. Unless the water depth is shallow it is most probable that the pipeline will have to probable that the pipeline will have to be cut in order to get it to the surface. This will then likely involve tensioning problems to maintain the catenary. An problems to maintain the catenary. An additional length of pipe will have to be welded in with the hot tap to make up the distance between the cut ends. When the line is relayed on the bottom it now has a slack loop which will lay over to one side and thus induce a significant compound torsional and compressive stress. When the pipeline is at the surface the entire operation is in a vulnerable position. if heavy weather should arise necessitating a run for port, the open ends will have to be capped and the line reset on the bottom. This occurrence could double the cost of the operation and greatly increase the period of line downtime. Another problem with a surface hot tap is the difficult fit-up for the tie-in weld on a rolling, pitching and heaving barge.