Civil Engineering Reference
In-Depth Information
Most of the time required for an underwater welding repair is accounted for
by the preparatory work, rather than the welding process, and the preparation
must be planned with considerable care. Around the more complex node geo-
metries, the time required for assembling and sealing the welding chamber can
equal the time needed for the welding operation. A compromise in the execu-
tion is to divide the chamber into as few pieces as possible, thereby minimizing
the amount of underwater assembly work (which increases cost), and minimiz-
ing the safety risk to the divers by reducing the size of the chamber compo-
nents, especially where such work is to be carried out in tidal or splash-zone
conditions. Furthermore, it should not be assumed that the actual geometry
of the structure is precisely as designed, and it is important to perform a survey
of the location before building the welding chamber. It will be necessary to
clean portions of the structure to ensure that an effective seal can be achieved,
allowing the chamber to be dewatered. The welding chamber must be made
sufficiently large to enable the welders to have effective access to the weld site.
Dry Welding Topside
For repair of the topside structure above the water level, dry welding is routine.
The area where dry welding is undertaken is normally designated a temporary
hazardous area and all routine safety procedures must be followed. Hot works
are a consideration when welding topside. Platform shut-in may be required,
depending on the location of the welding relative to the well bay and equip-
ment/piping.
Dry welding topside is the most widely used form of welding. The only lim-
itation is the requirement for hot works.
Dry Welding at or below the Sea Surface
Since a large body of welding technology exists relating to normal atmospheric
pressure, a logical approach to underwater welding repair is to duplicate surface
welding conditions by providing a one-atmosphere environment at the repair
site, but this strategy is limited to shallow water depths. Two methods are avail-
able that can achieve this:
Cofferdam: This essentially is a watertight structure that surrounds the repair
location and is open to the atmosphere. The structure can be open-topped or
it can have a closed top with an access shaft to the surface.
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Pressure-resistant chamber: The work site is surrounded by a chamber con-
structed as a pressure vessel, capable of withstanding the water pressure at
the depth of the repair location. Once the chamber is in place and sealed to
the structure, it is dewatered and the pressure can then be reduced to one
atmosphere. The repair crew can transfer to the welding chamber in a
one-atmosphere environment, within a diving bell, to perform the repair.
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