07-10-2016, 02:48 PM
1458210762-UNDERWATERWELDING.pptx (Size: 164.21 KB / Downloads: 5)
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 :
Wet welding
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
APPLICATIONS
Offshore construction for tapping sea resources
Temporary repair work caused by ship’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