Under water welding
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UNDER-WATER WELDINGSoumyabrata Basak INTRODUCTIONUnderwater 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. CLASSIFICATIONUnder water welding can be classified as :
Dry weldingWET 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 negativeThe two parts of the circuit are brought together and then slightly separatedAn 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 electrodeAdvantagesThe 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 :
Cavity weldingHyper 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 HABITATWELDINGLimitation:- 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
Good quality weld
Non destructive testingDisadvantages:- 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 gasThe 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 appliedScope 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 ships 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.comwww.aws.comProduction Technology- O.P.Khannawww.howstuffworks.comTHANK YOU