16-02-2010, 07:55 PM
UNDER WATER WELDING.pptx (Size: 164.21 KB / Downloads: 1,147)
INTRODUCTION
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.
CLASSIFICATION
¢ Under water welding can be classified as :
i. Wet welding
ii. Dry welding
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
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
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 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.
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 shipâ„¢s 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