ABSTRACT A new semi-automatic welding gun has been developed for underwater welding. The principle consists of producing a water-free zone within a rapidly rotating bell-shaped transparent cavity. A protective gas environment can thus be maintained within the vortex zone, providing suitable conditions for performing the welding operation in all positions. Butt and fillet welds on plates up to 1" thickness have been made. The encouraging results obtained indicate the possibility of a large field of application on offshore structures and in shipyards, where manual or automated welding of good quality is required. The paper describes the conceptual design of the system, the test procedures which have been adopted and summarizes the most pertinent results obtained in the laboratory and under hyperbaric testing conditions. INTRODUCTION The technique of underwater wet welding has made some progress within the last few years, mainly by the development of new coated electrodes; suitable for operation in the presence of water. Nevertheless, the water in the welding zone limits the quality of welds for the following reasons:–the dissociation of the water in the arc zone yields atomic and molecular hydrogen, which increases the hydrogen level and hence the risk of hydrogen cracks;–the rapid cooling of the weld in the presence of water results in additional local quenching effects, which may increase the hardness, in the weld and the heat affected zone;–the presence and the rapid evaporation of water affects the stability of the arc which may cause the occurence of porosity. For these reasons, large underwater dry welding operations, as for instance on pipelines in the offshore oil and gas industries, are generally carried out in dry habitat welding. The installation of such chambers around pipeline sections to be welded is a time-consuming and expensive operation. In recent years, a number of devices have been developed and used with different degrees of success. These consist of carrying out the weld in a small welding chamber supplied with a protective gas environment and sealed either by means of flexible or spongy seals or else by discharging a water jet through an annular nozzle surrounding the gun. As will be explained below, none of these systems has proved to be truly successful in every day underwater welding applications, a fact which has led us to consider new approaches for underwater welding in suitably dry conditions, with a robust, reliable, light-weight hand-held tool. The principle consists of producing a water-free zone within a rapidly rotating bell-shaped transparent cavity. A protective gas environment can thus be maintained within the vortex zone, providing suitable conditions for performing the welding operation in all positions (see Fig. 1). Patents for this process have been applied for in several industrialized countries.