UNDERWATER WELDING
Underwater welding is
performed while the welder is submerged, often at elevated barometric
pressures. Because of the adverse conditions and inherent dangers associated
with underwater welding (also known as wet welding) divers must be trained to
an exceptionally rigorous standard with highly specialized instruction.
Underwater
welding is an important tool for
underwater
fabrication works. In 1946, special waterproof electrodes were developed in
Holland by ‘Vander Willingen'’. In recent years the number of offshore
structures including oil drilling rigs, pipelines, platforms are being
installed significantly.
CLASSIFICATION
Under water welding can
be classified as
I. Wet welding
II. Dry welding
WET WELDING
It is the Key technology for repairing
marine structure. In this Welding is performed under water directly exposed to
the wet environment. Increased freedom movement makes more effective, efficient
and economical. Supply is connected to the welder/driver via cables or hoses.
Complete insulation of the cables and hoses are essential in case to prevent
the chance for electric shock. MMA (Manual Metal Arc) welding is commonly used process
in the repair of offshore platforms.
Wet welding
entails the diver to perform the weld directly in the water. It involves using
a specially designed welding rod, and employs a similar process used in
ordinary welding. Here are advantages to wet welding:
·
Cheap and fast
·
high tensile strength
·
ease of access to weld spot
·
no habitat
·
no construction
Disadvantages
·
Rapid quenching of the weld metal
by the surrounding
water.
·
Welders working under water are
restricted in
·
manipulating arc.
·
Hydrogen embrittlement causes
cracks.
·
Poor visibility due to water
contaminance.
PRINCIPLE OF OPERATION
The work
is connected to the positive side of dc source and electrode to the negative. The
two parts of the circuit are brought together and then slightly separated. The
two parts of the circuit are brought together and then slightly separated. An electric
current occurs in
the gap
and causes a sustained spark which melts the bare metal forming a weld pool. The
flux covering the electrode melts to provide a shielding gas. Arc burns in the
cavity formed inside the flux covering, which is designed to burn slower than
the metal barrel to the electrode
Advantages
The
versatility and low cost.
Less
costly than dry welding.
Speed
with which it is carried out
No
enclosures so no time is lost for
Building
Disadvantages
Rapid
quenching of the weld metal by the surrounding
water.
Welders
working under water are restricted in
manipulating arc.
Hydrogen
embrittlement causes cracks.
Poor
visibility due to water contaminance.
DRY WELDING
A
chamber is created near the area to be welded and the
welder does the job by staying inside the
chamber.
It produces
high quality weld joints.
The
gas-tungsten arc welding process is used mostly for
pipe works
Gas
metal arc welding is the best process for this welding.
CLASSIFICATION OF DRY
WELDING
There are
two basic types of dry welding:
I. Hyperbaric
welding
II. Cavity
welding
Hyper baric welding: -
It is
carried out in chamber sealed around the structure
to be
welded
The
chamber is filled with a gas at the prevailing
pressure,
to push water back
The
welder fitted with breathing mask and other
protective
devices on the pipe line
Mask
filled with a breathable mixture of helium and
oxygen in
the habitat
The
area under the floor of the habitat is open to water,
so hyper
baric welding is termed as “HABITATWELDING”
Limitation: -
As
depth increase pressure also increases, it affects both
for
driver and welding process
Cavity welding: -
Cavity
welding is another approach to weld in water free
environment
Conventional
arrangements for feeding wire and shielding
gas
Introducing
cavity gas and the whole is surrounded by a
trumpet
shaped nozzle through which high velocity
conical
jet of water passes.
It
avoids the need for a habitat chamber and it lends itself
to
automatic and remote control.
The
process is very suitable for flat structures
Advantages: -
Welder/diver
safety
Good
quality weld
Surface
monitoring
Non-destructive
testing
Disadvantages: -
The habitat
welding requires large quantities of complex
equipment
and much support equipment on the surface
Cost is
extremely high
