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

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