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.
|Country:||Trinidad & Tobago|
|Published (Last):||16 October 2009|
|PDF File Size:||12.49 Mb|
|ePub File Size:||9.31 Mb|
|Price:||Free* [*Free Regsitration Required]|
The first ever underwater weldingwas carried out by British Admiralty Dockyard for sealing leaking ship rivets below the water line. Underwater welding is an important tool for underwater fabrication works. In recent years the number of offshore structures including oil drilling rigs, pipelines, platforms are being installed significantly.
Some of these structures will experience failures of its elements during normal usage and during unpredicted occurrences like storms, collisions. Any repair method will require the use of underwater welding. The power source should be a direct current machine rated at or amperes.
Motor generator welding machines are most often used for underwater welding in the wet. The welding machine frame must be grounded to the ship. The welding circuit must include a positive type of switch, usually a knife switchoperated on the surface and commanded by the welder-diver. The welding power should be connected to the electrode holder only during welding. Direct current with electrode negative straight polarity is used. Special welding electrode holders with extra insulation against the water are used.
The underwater welding electrode holder utilizes a twist type head for gripping the electrode. It accommodates two sizes of electrodes. The electrode types used conform to AWS E classification. The electrodes must be waterproofed. All connections must be thoroughly insulated so that the water cannot come in contact with the metal parts.
If the insulation does leak, seawater will come in contact with the metal conductor and part of the current will leak away and will not be available at the arc. In addition, there will be rapid deterioration of the copper cable at the point of the leak. Hyperbaric Welding dry welding Hyperbaric welding is carried out in chamber sealed around the structure o be welded. The chamber is filled with a gas commonly helium containing 0.
The habitat is sealed onto the pipeline and filled with a breathable mixture of helium and oxygen, at or slightly above the ambient pressure at which the welding is to take place. This method produces high-quality weld joints that meet Xray and code requirements. The gas tungsten arc welding process is employed for this process. The area under the floor of the Habitat is open to water. Thus the welding is done in the dry but at the hydrostatic pressure of the sea water surrounding the Habitat.
The first ever underwater welding was carried out by British Admiralty Dockyard for sealing leaking ship rivets below the water line. In , special waterproof electrodes were developed in Holland by Van der Willingen. In dry welding, a dry chamber is created near the area to be welded and the welder does the job by staying inside the chamber. A special electrode is used and welding is carried out manually just as one does in open air welding. The increased freedom of movement makes wet welding the most effective, efficient and economical method.
In wet welding MMA manual metal arc welding is used. For wet welding AC is not used on account of electrical safety and difficulty in maintaining an arc underwater. The welding circuit must include a positive type of switch, usually a knife switch operated on the surface and commanded by the welder-diver.
The knife switch in the electrode circuit must be capable of breaking the full welding current and is used for safety reasons. Power Supply work Hyperbaric Welding dry welding Hyperbaric welding is carried out in chamber sealed around the structure o be welded. This method produces high-quality weld joints that meet X-ray and code requirements.
Precautions include achieving adequate electrical insulation of the welding equipment, shutting off the electricity supply immediately the arc is extinguished, and limiting the opencircuit voltage of MMA SMA welding sets. Secondly, hydrogen and oxygen are produced by the arc in wet welding. Precautions must be taken to avoid the build-up of pockets of gas, which are potentially explosive.
Precautions include the provision of an emergency air or gas supply, stand-by divers, and decompression chambers to avoid nitrogen narcosis following rapid surfacing after saturation diving.
For the structures being welded by wet underwater welding, inspection following welding may be more difficult than for welds deposited in air. Assuring the integrity of such underwater welds may be more difficult, and there is a risk that defects may remain undetected. The warm, dry habitat is well illuminated and has its own environmental control system ECS. Disadvantages of Dry Welding 1 The habitat welding requires large quantities of complex equipment and much support equipment on the surface.
The chamber is extremely complex. Work depth has an effect on habitat welding. At greater depths, the arc constricts and corresponding higher voltages are required. One cannot use the same chamber for another job, if it is a different one. Advantages of Wet Welding Wet underwater MMA welding has now been widely used for many years in the repair of offshore platforms.
The benefits of wet welding are: 1 The versatility and low cost of wet welding makes this method highly desirable. With which the operation is carried out. Readily available standard welding machine and equipments are used.
The equipment needed for mobilization of a wet welded job is minimal. Disadvantages of Wet Welding Although wet welding is widely used for underwater fabrication works, it suffers from the following drawbacks: 1 There is rapid quenching of the weld metal by the surrounding water. Although quenching increases the tensile strength of the weld, it decreases the ductility and impact strength of the weldment and increases porosity and hardness.
Cracks can grow and may result in catastrophic failure of the structure. The welder some times is not able to weld properly. Principle of operation of Wet Welding The process of underwater wet welding takes in the following manner: 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 then separated slightly. The electric current jumps the gap and causes a sustained spark arc , which melts the bare metal, forming a weld pool.
At the same time, the tip of electrode melts, and metal droplets are projected into the weld pool. During this operation, the flux covering the electrode melts to provide a shielding gas, which is used to stabilize the arc column and shield the transfer metal.
The arc burns in a cavity formed inside the flux covering, which is designed to burn slower than the metal barrel of the electrode.
Developments in Under Water Welding Wet welding has been used as an underwater welding technique for a long time and is still being used. With recent acceleration in the construction of offshore structures underwater welding has assumed increased importance. This has led to the development of alternative welding methods like friction welding, explosive welding, and stud welding. Sufficient literature is not available of these processes. Scope for further developments Wet MMA is still being used for underwater repairs, but the quality of wet welds is poor and are prone to hydrogen cracking.
Dry Hyperbaric welds are better in quality than wet welds. Present trend is towards automation. Developments of diverless Hyperbaric welding system is an even greater challenge calling for annexe developments like pipe preparation and aligning, automatic electrode and wire reel changing functions, using a robot arm installed. This is in testing stage in deep waters. Explosive and friction welding are also to be tested in deep waters.
J Keats, Manual on Wet Welding. Lucas, International conference on computer technology in welding. D, Underwater welding and cutting yields slowly to research, Welding Engineer, April Related Interests.
Seminar On Underwater Welding -Report and ppt Download
The two types of welding to be covered in this manual are : Wet welding and Dry welding at the splash zone welding process which is done under water. It is used in oil company, ship company. Wet Welding : Wet Welding indicates that welding is performed underwater,directly exposed to the wet environment. A special electrode isused and welding is carried out manually just as one does in openair welding. The increased freedom of movement makes wetwelding the most effective, efficient and economical method. Dry Welding : Hyperbaric welding is carried out in chamber sealed around the structure o be welded.
The power source should be a direct current machine rated at or amperes. Motor generator welding machines are most often used for underwater welding in the wet. The welding machine frame must be grounded to the ship. The welding circuit must include a positive type of switch, usually a knife switch operated on the surface and commanded by the welder-diver. The knife switch in the electrode circuit must be capable of breaking the full welding current and is used for safety reasons. The welding power should be connected to the electrode holder only during welding.