INTRODUCTION TO UNDERWEAR WELDING

Underwater welding is an important tool for underwater fabrication works.
In 1946, special waterproof electrodes were developed in Holland by ËœVander Willingen’â„¢.
In recent years the number of offshore structures including oil
drilling rigs, pipelines, platforms are being installed significantly.
Underwater welding is a type of welding which takes place underwater.In
underwater welding, the environment around the welder is wet. and uses
welding equipment which has been customized for wet environment,commonly
use under water techniques are hyperbaric enclosure welding, wet
Underwater-welding, high pressure water jet welding, other welding
processes: friction welding, resistance welding, arc welding, tig
welding, mig welding, oxyacetylene welding, electron beam welding, laser
beam welding,and main risks for the welder performing
Underwater-welding are the potential for electric shock,and the
possibility of producing in the arc mixtures of hydrogen and oxygen in
pockets, which might set up an explosion,and There are three main ways
to perform Underwater-welding. One is to build an enclosure, a pit,
around the place of repair and to pump away all the water: that amounts
to prepare the conditions for normal welding in air, although the place
may be deep under sea level. Another method of Underwater-welding
consists in preparing an enclosure to be filled with gas (helium) under
high pressure (hyperbaric) to push water back, and have the welder,
fitted with breathing mask and other protective equipment, weld quite
normally out of water but under pressure. The third is the wet
Underwater-welding method, where no attempts are made to dry up the
location of welding. Instead the power of the arc generates a bubble of a
mixture of gases which lets metal melting and joining occur more or
less normally, using specially covered electrodes to avoid that too much
hydrogen be absorbed in the weld. The skilled welder must also be a
diver, equipped for Underwater-welding, with all the extra equipment and
protection a welder must use. There is also a less used method of
Underwater-welding which features a special torch which sprays a cone of
high pressure water, within which protective gas under pressure
insulates the weld location from the water during welding.

Under water welding
Fig. Under water welding

CLASSIFICATION 

 Under water welding can be classified as :
i. Wet welding
ii. Dry welding


1) WET WELDING
  • Key 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 OPERATION
  1. The work is connected to the positive side of dc source and electrode to the negative
  2. The two parts of the circuit are brought together and then slightly separated
  3. 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.
  4. 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 costlier 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.

2) 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.

Scope of further developments
  • Hyper 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.

APPLICATIONS
  • Offshore construction for tapping sea resources
  • Temporary repair work caused by collisions, unexpected accidents
  • Salvaging vessels sunk in the sea
  • Repair and maintenance of ships
  • Construction 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.

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