Under water welding is the installation of large patches, as well as the attachment of suitable pad-eyes presents a more complicated problem to the diver than does underwater cutting. Considerable practice is necessary to achieve a consistently good standard of underwater welding for salvage work. As a result, the diver's underwater welding techniques must conform to acceptable standards. Also, the ocean acts as a large heat sink and draws off the heat of the electrode. This may cause blow holes and possible loss of strength between the patch and hull. This is true because the gas cannot escape from the molten pool of metal due to sudden cooling by the surrounding water. Despite the above shortfalls, underwater welds of good strength that are acceptable for salvage work are possible. Unless otherwise specified, the term "underwater welding" as used in this manual refers to the wet welding technique where no mechanical barrier separates the welding arc from the surrounding water.

The two types of welding to be covered in this manual are :

• Wet welding and

• Dry welding at the splash zone welding process which is done under water. It is used in oil company, ship company.

Wet Welding :

Wet Welding indicates that welding is performed underwater,directly exposed to the wet environment. A special electrode isused and welding is carried out manually just as one does in openair welding. The increased freedom of movement makes wetwelding the most effective, efficient and economical method.Welding power supply is located on the surface with connection tothe diver/welder via cables and hoses.

Dry Welding :

Hyperbaric welding is carried out in chamber sealed around the structure o be welded. The chamber is filled with a gas (commonly helium containing 0.5 bar of oxygen) at the prevailing pressure. The habitat is sealed onto the pipeline and filled with a breathable mixture of helium and oxygen, at or slightly above the ambient pressure at which the welding is to take place. This method produces high-quality weld joints that meet X- ray and code requirements. The gas tungsten arc welding process is employed for this process. The area under the floor of the Habitat is open to water. Thus the welding is done in the dry but at the hydrostatic pressure of the sea water surrounding the Habitat.