Abstract: First and second underwater tubular members are assembled together so that the outer surface of the first tubular member and the inner surface of the second tubular member are spaced apart to form an annular cavity. The cavity is sealed, cleaned and dried and filled with a gas at a pressure greater than the surrounding water pressure. The pressure in the cavity is reduced prior to detonating an explosive charge within the first tubular member to weld the members together. The cavity may be subdivided into a plurality of zones by further seals, the pressure in the zones being separably adjustable so as to be able to control the pressure difference across each seal.