<P> Underwater hyperbaric welding was invented by the Russian metallurgist Konstantin Khrenov in 1932 . </P> <P> Welding processes have become increasingly important in almost all manufacturing industries and for structural application . Although a large number of techniques are available for welding in atmosphere, many of these techniques cannot be applied in offshore and marine application where presence of water is of major concern . In this regard, it is relevant to note that a great majority of offshore repairing and surfacing work is carried out at a relatively shallow depth, in the region intermittently covered by the water known as the splash zone . Though numerically, most ship repair and welding jobs are carried out at a shallow depth, the most technologically challenging task is repair at greater depths, especially in pipelines and repair of accidental failure . The advantages of underwater welding are largely of an economic nature, because underwater - welding for marine maintenance and repair jobs bypasses the need to pull the structure out of the sea and saves valuable time and dry docking costs . It is also an important technique for emergency repairs which allow the damaged structure to be safely transported to dry facilities for permanent repair or scrapping . Underwater welding is applied in both inland and offshore environments, though seasonal weather inhibits offshore underwater welding during winter . In either location, surface supplied air is the most common diving method for underwater welders . </P> <P> Dry hyperbaric welding involves the weld being performed at raised pressure in a chamber filled with a gas mixture sealed around the structure being welded . </P> <P> Most arc welding processes such as Shielded Metal Arc Welding (SMAW), Flux - cored arc welding (FCAW), Gas tungsten arc welding (GTAW), Gas metal arc welding (GMAW), Plasma Arc Welding (PAW) could be operated at hyperbaric pressures, but all suffer as the pressure increases . Gas tungsten arc welding is most commonly used . The degradation is associated with physical changes of the arc behaviour as the gas flow regime around the arc changes and the arc roots contract and become more mobile . Of note is a dramatic increase in arc voltage which is associated with the increase in pressure . Overall a degradation in capability and efficiency results as the pressure increases . </P>

What are the two types of underwater welding