Welding is a fabrication process that joins metals or thermoplastics by using coalescence. Melting the work pieces and adding a filler material to form a pool of molten material known as the weld puddle that cools to become a strong joint describes the process of welding. This is in contrast to soldering and brazing which involves melting of a material with lower melting point material between the work pieces.
Underwater welding provides a means to assemble or repair underwater. This is a highly useful technology available that allows repairs of ships damaged during hurricanes or wars. There are a couple of alternatives available, which include clamped and grouted repairs and bolted flanges. However, these alternatives do not always provide satisfactory results and also introduces high loading at offshore structures. Underwater welding is a highly specialized trade. Most are employed in the oil or shipping industry and the military.
Underwater welding process can be classified into the following two categories:
- Wet Welding
- Dry Welding
In the case of Wet Welding, the welding is performed underwater, directly exposed to the wet environment. A special electrode is used and welding is carried out manually as in open air welding. As freedom of movement is permissible, wet welding is most effective, efficient and economical. The welding power and supply is located on the surface and is connected to the welder using cables or hoses.
The advantages of wet welding are:
- The cost is very low especially when compared to dry welding.
- The speed with which the task can be carried out is very high.
- As the equipments required are very less, the welding can be carried out immediately with minimal planning.
The disadvantages of wet welding are:
- The weld is quenched rapidly in the water. Although, this is good in a way as quenching increases the tensile strength of the weld, it decreases the ductility and impact strength and also makes the weld very porous.
- The visibility of the welder is poor.
- The amount of voltage that can be used is very limited and a lot of precaution has to be taken to ensure that the welder does not receive electric shocks.
In the case of Dry Welding, also known as Hyperbaric welding, it is carried out in a chamber sealed around the structure to be welded. The chamber is filled with a gas, normally helium containing 0.5 bar oxygen. The chamber is fitted on to the pipeline and filled with the breathable mixture of air described above and the pressure is maintained slightly above the pressure at which welding is to take place. The gas tungsten arc welding process is used.
The advantages of dry welding are:
- Welding can be performed immune to ocean currents and marine animals.
- The welds are of a better quality when compared to the welds of wet welding as water does not quench the welds.
- Joint preparation and pipe alignment can be monitored visually from the surface.
The disadvantages of dry welding are:
- Large support equipment is required at the surface to support the chamber which by itself is very complex.
- The cost involved in dry welding is high and the cost increases with increase in depth where the welding is to be carried out.
- The reusability of the chamber is very limited.
Risks and Precautions
Both the welder and the structures being welded are at risk when underwater welding is used. The welder has to be very careful so as to avoid receiving an electric shock. For this, adequate precaution must be taken to insulate the welder and limiting the voltage of welding sets. Secondly, pockets of hydrogen and oxygen are built up by the arc and are potentially explosive. Lastly, the welder has to keep in mind that nitrogen is built up in the bloodstream when exposed to air at high pressure under the water and hence must take adequate precaution.
For the structures that are being welded underwater, although inspection is very difficult when compared to surface welding, it is a must. The weld must be inspected very carefully to confirm that no defects remain.
All welding precautions form a subset of underwater welding precautions. In addition, all safe arc-welding precautions must be observed with particular attention being paid to using electricity underwater safely.