Underwater Joining and Cutting - Present and Future

Abstract

ABSTRACT Most welding in underwater salvage and emergency repair is done in the water environment (wet) with the conventional manual shielded metal-arc process. This is a very slow process and the weld quality generally is very poor particularly in the high strength steels required for platforms and pipelines. Welding in a dry environment in a pressurized chamber that encloses the welding site permits high-quality welding to be done in high-strength steels. Arc welding processes that have a gaseous shield around the arc are used in dry welding. Dry welding however is slow and costly because the chambers are cumbersome and difficult to install. The hyperbaric pressures also may have deleterious effects on the welding arc. Other welding processes show promise for future utilization in underwater operations. These include plasma-arc welding, brazing, adhesive bonding, and explosive welding. Underwater cutting conventionally is done by flame or arc processes. Flame cutting is being used less and less because the equipment is bulky and it requires added skills. Arc cutting is faster and easier to use. Plasma-arc cutting holds promise for the future because it can cut thicker metal and its cutting speeds are high. Also, it is not limited to the cutting of steel as are arc and gas cutting. Aluminum, copper alloys, and stainless steel can be plasma cut with equal ease. Future efforts to bring underwater welding and cutting to the same degree of development as surface welding and cutting will require indepth, broad-range, long term research efforts. A multitude of interrelated, difficult problems must be solved. Success is expected but it will not be overnight. INTRODUCTION Welding and cutting underwater is now a hot item, technically speaking. This intense interest has been stimulated by the huge growth- in offshore oil and gas exploitation. Unfortunately, underwater welding and cutting technology just hasn't been able to keep up, and for a simple reason until recently, there just hasn't been a strong economic motivation. Now, the money motive is here and the technology is not. Underwater welding and cutting has been done for years, but limited to salvage and emergency repairs. Present underwater construction requirements demand high-quality fabrication techniques that these old procedures and equipment can't meet. The shortcomings of existing welding and cutting methods are due primarily to:The surrounding water environmentPoor visibilityEquipment not specifically designed for underwater useSevere limitations on diver performanceEffects of pressure on the behavior of the process. Very little effort has been directed toward developing processes and techniques for underwater welding and cutting. The extent of underwater fabrication operations in the past has been too limited to justify large outlays of research funds in the face of little, if any, economic return. However, the motivation now has appeared in the form of very high costs of underwater construction and repair and the rapid increase in offshore activity. As a result, some efforts now are being made to develop new techniques for underwater welding and cutting and to improve the existing processes.