Under water welding
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Transcript of Under water welding
UNDER-WATER WELDING
Soumyabrata Basak
INTRODUCTION
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 WELDINGKey technology for repairing marine structure
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.
PRINCIPLE OF OPERATIONThe 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
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
AdvantagesThe versatility and low cost.
Less costlier than dry welding.
Speed with which it is carried out
No enclosures so no time is lost for building.
DisadvantagesRapid 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 WELDINGA 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 worksGas metal arc welding is the best process for this
welding.
CLASSIFICATION OF DRY WELDINGThere 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 weldedThe chamber is filled with a gas at the prevailing
pressure, to push water backThe welder fitted with breathing mask and other
protective devices on the pipe lineMask filled with a breathable mixture of helium and
oxygen in the habitatThe 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
environmentConventional arrangements for feeding wire and shielding
gasIntroducing 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
RISKS V/S PRECAUTIONSRisk of electric shock so achieving electrical
insulation of electrical welding equipmentsHydrogen and oxygen are produced by the arc in wet
welding are potentially explosion so precaution must be taken to avoid the build up of pockets of gas
The life or health of the welder will be in risk from nitrogen introduce into the blood stream, precautions include the provision of an emergency air or gas applied
Scope of further developmentsHyper baric welding is well established and generally
well researched.Research being carried out for welding at a range of
500 to 1000m deep.THOR-1 (Tig Hyperbaric Orbital Robot) is developed
where diver performs pipe fitting, installs the tracks and orbital head on the pipe and rest process is automated.
APPLICATIONSOffshore construction for tapping sea resourcesTemporary repair work caused by ship’s collisions,
unexpected accidentsSalvaging vessels sunk in the seaRepair and maintenance of shipsConstruction of large ships beyond the capacity of
existing docks
CONCLUSION Alternatives which include clamped and grouted
repairs (which may introduce unacceptably high loading on offshore structures) and the use of bolted flanges for the tie-ins are not necessarily and are not always satisfactory
REFERENCESwww.twi.com
www.aws.com
Production Technology- O.P.Khanna
www.howstuffworks.com
THANK YOU