Underwater hull structures may experience failures of its elements during normal usage and during unpredicted occurrences like storms, collisions. Any repair method will require the use of underwater welding.

Classification

Underwater welding can be classified as

• Wet Welding- The welding is performed underwater, directly exposed to the wet environment
• Dry Welding- A dry chamber is created near the area to be welded and the welder does the job by staying inside the chamber.

Dry Welding (Hyperbaric Welding)

Hyperbaric welding is carried out in a 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.

Advantages Of Dry Welding

• Welder/Diver Safety – Welding is performed in a chamber, immune to ocean currents and marine animals. 
• Good Quality Welds – This method can produce welds of quality comparable to open air welds because water is no longer present to quench the weld and H2 level is much lower than wet welds.

Disadvantages Of Dry Welding

• This welding requires large quantities of complex equipment and much support equipment on the surface. The chamber is extremely complex.
• The cost of this welding is extremely high and increases with depth. Work depth has an effect on habitat welding. At greater depths, the arc constricts and corresponding higher voltages are required. The process is costly.

Wet Welding

Wet Welding indicates that welding is performed underwater, directly exposed to the wet environment. A special electrode is used and welding is carried out manually just as one does in open air welding. The increased freedom of movement makes wet welding the most effective, efficient and economical method.

Welding power supply is located on the surface with connection to the diver/welder via cables and hoses

Principle Of Operation Of Wet Welding

The work to be welded is connected to one side of an electric circuit, and a metal electrode to the other side. These two parts of the circuit are brought together, and then separated slightly. The electric current jumps the gap and causes a sustained spark (arc), which melts the bare metal, forming a welding pool. At the same time, the tip of electrode melts, and metal droplets are projected into the welding pool. During this operation, the flux covering the electrode melts to provide a shielding gas, which is used to stabilize the arc column and shield the transfer metal. The arc burns in a cavity formed inside the flux covering, which is designed to burn slower than the metal barrel of the electrode.

Advantages Of Wet Welding

• The versatility and low cost of wet welding makes this method highly desirable.
• Other benefits include speed. With which the operation is carried out.
• It is less costly compared to dry welding.
• The welder can reach portions of offshore structures that could not be welded using other methods.
• No enclosures are needed, and no time is lost building. Readily available standard welding machines and equipment are used. The equipment needed for mobilization of a wet welded job is minimal.

Disadvantages Of Wet Welding

• There is rapid quenching of the weld metal by the surrounding water. Although quenching increases the tensile strength of the weld, it decreases the ductility and impact strength of the weldment and increases porosity and hardness.
• Hydrogen Embrittlement – Large amount of hydrogen is present in the weld region, resulting from the dissociation of the water vapour in the arc region. The H2 dissolves in the Heat Affected Zone (HAZ) and the weld metal, which causes Embrittlement, cracks and microscopic fissures. Cracks can grow and may result in catastrophic failure of the structure.
• Another disadvantage is poor visibility. The welder sometimes is not able to weld properly.