In recent years, the number of offshore structures, pipelines, and platforms being installed in deeper waters has increased. Some of these pipelines and structures will experience failures. Any repair for these on location will require the use of underwater welding.
When confronted with the issue of underwater welding, we often question: “Why should we consider underwater welding in the first place?” The immediate answer is “Why not?”
Risks and precautions
Welding underwater can be a dangerous profession if precautions aren't taken. The main risks are electric shock and the possibility of producing in the arc mixtures of hydrogen and oxygen in pockets, which might set off an explosion. The other common danger is breathing nitrogen in the air mix, which is absorbed into the blood but not metabolised by the body at depths under pressure. This could turn into bubbles on ascent and paralyse the diver. Curiously, the risk of drowning is not considered in commercial diving because that is the first hurdle to overcome in this profession.
The quantity of dives, dive repetitiveness, depth of the operations, time spent underwater and the exhausting nature of a specific task increase these risks significantly. Appropriate safety measures are provided to the diver via emergency air or gas supply, stand-by divers and decompression chambers. The diving-related health and safety procedures are managed by strict governing guidelines and work procedures.
When subsea welding is completed, both the welder and the structures being welded are at risk. The welder has to be very careful to avoid receiving an electric shock. For this, adequate precaution is taken by insulating the welder and limiting the voltage of welding sets. Continuous control of hydrogen and oxygen build-up is managed by removal and kept away from the arc to minimise any potential explosion.
Lastly, the welder’s time under water is controlled by using saturation diving chambers and regular rest periods in between. Inspection of an underwater weld is very difficult and complicated when compared to surface welding, but as it is the only controlling process of the quality of the weld, it is always done. The weld is inspected very carefully to confirm that no defects remain.
There are many underwater welding schools located in different parts of the world, including Australia, to train commercial divers. Historically, underwater welding was restricted to salvage operations and emergency repair work with limited depths of less than 9 m.
In underwater welding, the environment around the welder
is wet. He or she wears a dive suit, and uses welding equipment which
has been customized for wet environments. This equipment is designed to
be as safe as possible for the welder, reducing the risk of electric
shock and the development of dangerous situations. Someone who practices
underwater welding must be both a skilled welder and a skilled diver,
with the ability to safely and effectively prepare a scene for welding
and to confirm that the welds are of high quality.
For
some welding situations, a diver may create a dry chamber around the
objects being welded. This type of welding is known as hyperbaric
welding. Welders performing hyperbaric welding must still have diving
skills and the specialized skills to weld at high pressure, but they are
not working in an actively wet environment. Constructing a dry chamber
can be time consuming, but there are a number of advantages to working
in a dry environment which can make hyperbaric welding preferable for
certain applications.
http://www.wisegeek.com/what-is-underwater-welding.htm
http://pipeliner.com.au/news/application_of_underwater_welding_processes_for_subsea_pipelines/063794/
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