Underwater Welding
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Hyperbaric welding is the process of welding
at elevated pressures, normally underwater. Hyperbaric welding can either take
place wet in the water itself or dry inside a specially constructed positive
pressure enclosure and hence a dry environment. It is predominantly referred to
as "hyperbaric welding" when used in a dry environment, and
"underwater welding" when in a wet environment. The applications of
hyperbaric welding are diverse—it is often used to repair ships, offshore oil
platforms, and pipelines. Steel is the most common material welded.
Dry hyperbaric welding is used in preference
to wet underwater welding when high quality welds are required because of the
increased control over conditions which can be exerted, such as through application
of prior and post weld heat treatments. This improved environmental control
leads directly to improved process performance and a generally much higher
quality weld than a comparative wet weld. Thus, when a very high quality weld
is required, dry hyperbaric welding is normally utilized. Research into using
dry hyperbaric welding at depths of up to 1,000 metres (3,300 ft) is ongoing.
In general, assuring the integrity of underwater welds can be difficult (but is
possible using various nondestructive testing applications), especially for wet
underwater welds, because defects are difficult to detect if the defects are beneath
the surface of the weld. Underwater hyperbaric welding was invented by the
Russian metallurgist Konstantin Khrenov in 1932.
Dry welding
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.
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.
Special control techniques have been applied
which have allowed welding down to 2,500 m (8,200 ft) simulated water depth in
the laboratory, but dry hyperbaric welding has thus far been limited
operationally to less than 400 m (1,300 ft) water depth by the physiological
capability of divers to operate the welding equipment at high pressures and
practical considerations concerning construction of an automated pressure /
welding chamber at depth.
Wet Welding
Wet underwater welding directly exposes the
diver and electrode to the water and surrounding elements. Divers usually use
around 300–400 amps of direct current to power their electrode, and they weld
using varied forms of arc welding. This practice commonly uses a variation of
shielded metal arc welding, employing a waterproof electrode. Other processes
that are used include flux-cored arc welding and friction welding. In each of
these cases, the welding power supply is connected to the welding equipment
through cables and hoses. The process is generally limited to low carbon
equivalent steels, especially at greater depths, because of hydrogen-caused
cracking.
Wet welding with a stick electrode is done
with similar equipment to that used for dry welding, but the electrode holders
are designed for water cooling and are more heavily insulated. They will
overheat if used out of the water. A constant current welding machine is used
for manual metal arc welding. Direct current is used, and a heavy duty
isolation switch is installed in the welding cable at the surface control
position, so that the welding current can be disconnected when not in use. The
welder instructs the surface operator to make and break the contact as required
during the procedure. The contacts should only be closed during actual welding,
and opened at other times, particularly when changing electrodes.
The electric arc heats the workpiece and the
welding rod, and the molten metal is transferred through the gas bubble around
the arc. The gas bubble is partly formed from decomposition of the flux coating
on the electrode but it is usually contaminated to some extent by steam.
Current flow induces transfer of metal droplets from the electrode to the
workpiece and enables positional welding by a skilled operator. Slag deposition
on the weld surface helps to slow the rate of cooling, but rapid cooling is one
of the biggest problems in producing a quality weld.
Sumber: https://en.wikipedia.org/wiki/Hyperbaric_welding
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