The fact that electric discharge could run was known for over a 100 old ages. The first of all time underwater welding was carried out by British Admiralty – Dockyard for sealing leaking ship studs below the H2O line. Underwater welding is an of import tool for submerged fiction plants. In 1946. particular waterproof electrodes were developed in Holland by ‘Van der Willingen’ . In recent old ages the figure of seaward constructions including oil boring rigs. grapevines. platforms are being installed significantly. Some of these constructions will see failures of its elements during normal usage and during unannounced happenings like storms. hits. Any fix method will necessitate the usage of submerged welding. Categorization
Underwater welding can be classified as
1 ) Wet Welding
2 ) Dry Welding
In wet welding the welding is performed submerged. straight exposed to the moisture environment. In dry welding. a dry chamber is created near the country to be welded and the welder does the occupation by remaining inside the chamber.
Wet Welding indicates that welding is performed submerged. straight exposed to the moisture environment. A particular electrode is used and welding is carried out manually merely as one does in unfastened air welding. The increased freedom of motion makes wet welding the most effectual. efficient and economical method. Welding power supply is located on the surface with connexion to the diver/welder via overseas telegrams and hosieries.
In wet welding MMA ( manual metal arc welding ) is used.
Power Supply used
: District of columbia
: -ve mutual opposition
When DC is used with +ve mutual opposition. electrolysis will take topographic point and cause rapid impairment of any metallic constituents in the electrode holder. For wet welding AC is non used on history of electrical safety and trouble in keeping an arc underwater.
The power beginning should be a direct current machine rated at 300 or 400 amperes. Motor generator welding machines are most frequently used for submerged welding in the moisture. The welding machine frame must be grounded to the ship. The welding circuit must include a positive type of switch. normally a knife switch operated on the surface and commanded by the welder-diver. The knife switch in the electrode circuit must be capable of interrupting the full welding current and is used for safety grounds. The welding power should be connected to the electrode holder merely during welding.
Direct current with electrode negative ( consecutive mutual opposition ) is used. Particular welding electrode holders with excess insularity against the H2O are used. The submerged welding electrode holder utilizes a turn type caput for gripping the electrode. It accommodates two sizes of electrodes. The electrode types used conform to AWS E6013 categorization. The electrodes must be waterproofed. All connexions must be exhaustively insulated so that the H2O can non come in contact with the metal parts. If the insularity does leak. saltwater will come in contact with the metal music director and portion of the current will leak off and will non be available at the discharge. In add-on. there will be rapid impairment of the Cu overseas telegram at the point of the leak.
Hyperbaric Welding ( dry welding )
Hyperbaric welding is carried out in chamber sealed around the construction O be welded. The chamber is filled with a gas ( commonly He incorporating 0. 5 saloon of O ) at the prevalent force per unit area. The home ground is sealed onto the grapevine and filled with a breathable mixture of He and O. at or somewhat above the ambient force per unit area at which the welding is to take topographic point. This method produces high-quality dyer’s rocket articulations that meet Xray and codification demands. The gas wolfram arc welding procedure is employed for this procedure. The country under the floor of the Habitat is unfastened to H2O. Thus the welding is done in the dry but at the hydrostatic force per unit area of the sea H2O environing the Habitat.
There is a hazard to the welder/diver of electric daze. Precautions include accomplishing equal electrical insularity of the welding equipment. closing off the electricity supply instantly the discharge is extinguished. and restricting the open-circuit electromotive force of MMA ( SMA ) welding sets. Second. H and O are produced by the discharge in wet welding.
Precautions must be taken to avoid the build-up of pockets of gas. which are potentially explosive. The other chief country of hazard is to the life or wellness of the welder/diver from N introduced into the blood steam during exposure to air at increased force per unit area. Precautions include the proviso of an exigency air or gas supply. stand-by frogmans. and decompression Chamberss to avoid nitrogen narcosis following rapid come uping after impregnation diving.
For the constructions being welded by wet submerged welding. review following welding may be more hard than for dyer’s rockets deposited in air. Guaranting the unity of such submerged dyer’s rockets may be more hard. and there is a hazard that defects may stay undetected. Advantages of Dry Welding
1 ) Welder/Diver Safety – Welding is performed in a chamber. immune to ocean currents and marine animate beings. The warm. dry home ground is good illuminated and has its ain environmental control system ( ECS ) .
2 ) Good Quality Welds – This method has ability to bring forth dyer’s rockets of quality comparable to open air dyer’s rockets because H2O is no longer present to slake the dyer’s rocket and H2 degree is much lower than wet dyer’s rockets.
3 ) Surface Monitoring – Joint readying. pipe alliance. NDT review. etc. are monitored visually.
4 ) Non-Destructive Testing ( NDT ) – NDT is besides facilitated by the dry home ground environment. Disadvantages of Dry Welding
1 ) The home ground welding requires big measures of complex equipment and much support equipment on the surface. The chamber is highly complex.
2 ) Cost of home ground welding is highly high and increases with deepness. Work deepness has an consequence on home ground welding. At greater deepnesss. the arc constricts and matching higher electromotive forces are required. The procedure is dearly-won – a $ 80000 charge for a individual dyer’s rocket occupation. One can non utilize the same chamber for another occupation. if it is a different one.
Advantages of Wet Welding
Wet submerged MMA welding has now been widely used for many old ages in the fix of offshore platforms. The benefits of moisture welding are: 1 ) The versatility and low cost of wet welding makes this method extremely desirable. 2 ) Other benefits include the velocity. With which the operation is carried out. 3 ) It is less dearly-won compared to dry welding.
4 ) The welder can make parts of offshore constructions that could non be welded utilizing other methods. 5 ) No enclosures are needed and no clip is lost edifice. Readily available criterion welding machine and equipments are used. The equipment needed for mobilisation of a moisture welded occupation is minimum. Disadvantages of Wet Welding
Although wet welding is widely used for submerged fiction plants. it suffers from the following drawbacks: 1 ) There is rapid extinction of the dyer’s rocket metal by the encompassing H2O. Although slaking increases the tensile strength of the dyer’s rocket. it decreases the ductileness and impact strength of the weldment and increases porousness and hardness.
2 ) Hydrogen Embrittlement – Large sum of H is present in the weld part. ensuing from the dissociation of the H2O vapor in the arc part. The H2 dissolves in the Heat Affected Zone ( HAZ ) and the dyer’s rocket metal. which causes Embrittlement. clefts and microscopic crevices. Cracks can turn and may ensue in ruinous failure of the construction. 3 ) Another disadvantage is hapless visibleness. The welder some times is non able to weld decently.
Principle of operation of Wet Welding
The procedure of submerged moisture welding takes in the undermentioned mode: The work to be welded is connected to one side of an electric circuit. and a metal electrode to the other side. These two parts of the circuit are brought together. and so separated somewhat. The electric current jumps the spread and causes a sustained flicker ( discharge ) . which melts the bare metal. organizing a weld pool. At the same clip. the tip of electrode thaws. and metal droplets are projected into the weld pool. During this operation. the flux covering the electrode thaws to supply a shielding gas. which is used to stabilise the arc column and shield the transportation metal. The discharge Burnss in a pit formed inside the flux covering. which is designed to fire slower than the metal barrel of the electrode.
Developments in Under Water Welding
Wet welding has been used as an submerged welding technique for a long clip and is still being used. With recent acceleration in the building of offshore constructions submerged welding has assumed increased importance. This has led to the development of alternate welding methods like clash welding. explosive welding. and stud welding. Sufficient literature is non available of these procedures. Scope for farther developments
Wet MMA is still being used for submerged fixs. but the quality of wet dyer’s rockets is hapless and are prone to hydrogen checking. Dry Hyperbaric dyer’s rockets are better in quality than wet dyer’s rockets. Present tendency is towards mechanization. THOR – 1 ( TIG Hyperbaric Orbital Robot ) is developed where frogman performs pipefitting. installs the trac and orbital caput on the pipe and the remainder procedure is automated. Developments of diverless Hyperbaric welding system is an even greater challenge naming for annexe developments like pipe readying and aligning. automatic electrode and wire reel altering maps. utilizing a automaton arm installed. This is in proving phase in deep Waterss. Explosive and clash welding are besides to be tested in deep Waterss.
1 ) D. J Keats. Manual on Wet Welding.
2 ) Annon. Recent progresss in dry submerged grapevine welding. Welding Engineer. 1974. 3 ) Lythall. Gibson. Dry Hyperbaric underwater welding. Welding Institute. 4 ) W. Lucas. International conference on computing machine engineering in welding. 5 ) Stepath M. D. Underwater welding and cutting outputs easy to research. Welding Engineer. April 1973.
6 ) Silva. Hazlett. Underwater welding with Fe – pulverization electrodes. Welding Journal. 1971.