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Sunday, 15 February 2015

Underwater Welding

Trained Engineers - 10:42

Underwater Welding
Underwater welding can be divided into main types that have been in use for many years: “wet” welding, which is done in the open water; and “hyperbaric” welding which is either performed “dry” inside a specially constructed positive pressure enclosure, or in the open water.
Underwater welding is a risky but necessary form of welding. There are many ships and structures that require underwater assembly or repair, including offshore oil rigs and oil tankers.
Welding is simply the best way to assemble or repair most metals, and the alternatives like grouted or clamped repairs are unsatisfactory in many instances.
Wet underwater welding
In "wet" underwater welding, MMA, or manual arc welding is the most common method. This is a variation of shielded metal arc welding, which employs a waterproof electrode. The other common processes in use are friction welding and flux-cored arc welding. In both of these methods, the power supply that operates the welding equipment is connected via a set cables and hoses.
The wet welding method is typically limited to fairly low carbon steels, or equivalents, especially at greater depths because of the risk of hydrogen-caused cracking.
In the future, there will be a need for improved power sources and revised electrode coatings in order to weld higher carbon steels at greater depths.

Hyperbaric or dry underwater welding
The use of hyperbaric or dry welding is a process of welding in high pressure environments, usually underwater. While hyperbaric welding can be done wet in the open water, it′s more commonly performed inside a specially constructed hyperbaric chamber that is a dry environment. This type of welding is used to perform the same kind of assembly and repair work as wet welding, namely underwater pipelines, offshore oil rigs, barges and ships of all sizes.
In hyperbaric welding, the diver/welder works from outside the chamber. The special MMA electrodes are positioned in advance through a flexible port, and the welder manipulates them as he looks through windows in the enclosure. Some of the newer chambers are completely transparent enclosures that surround the weld area, and water is evacuated by pumping gas inside.
When high quality welds are needed, dry hyperbaric welding is usually the method of choice. More control over the conditions are possible inside a dry enclosure, including the application of pre and post weld heating. This controlled environment results in a much improved welding process and higher quality welds when compared to wet welds of the same type.
Dry hyperbaric welding is typically used in depths of less than 400 meters, but there are special techniques being developed that may one day allow welding down to 2000 meter or more.
Risks of underwater welding
There are many risks of underwater welding, including the rest of electrical shock to the diver/welder. Special welding equipment has to be used, equipment that have been insulated and adapted to the rigors of marine environments.
Then there are the safety issues that all divers face, including the risk of the "bends," or decompression sickness due to the increased pressure inherent in saturation diving. An additional risk when wet welding is the buildup of pockets of gas and oxygen, which are potentially explosive under the right conditions.
Common precautions for underwater diving include a system for administering emergency air, a team of stand-by divers to perform a rescue, and decompression chambers to help prevent decompression sickness following saturation diving.

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