PDF | The paper describes principles of underwater welding and recent trends in research works undertaken for enhance welding technology. PDF | Welding in offshore and marine application is an area of research and understanding where, many problems are still unsolved. In the present paper. UNDERWATER WELDING Amit Mukund Joshi (Mechanical Engineer) Junior Research Fellow Mechanical Engineering Department Indian Institute of.
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In the present paper, a brief description of the different commercial underwater techniques has been made. The problems in underwater welding have also been . When I was young, I was always fascinated by welding carried out in underwater environment. Basically, flame and water are mutual exclusive as the. Underwater Welding techniques. Esam F. Alajmi. The Public Authority For Applied Education And Training. ABSTRACT. Welding demand in offshore and marine.
Power Supply Electrode Holder Electrode work Knife Switch 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. 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 X- ray 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.
Precautions include achieving adequate electrical insulation of the welding equipment, shutting off the electricity supply immediately the arc is extinguished, and limiting the open-circuit 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: 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: Although quenching increases the tensile strength of the weld, it decreases the ductility and impact strength of the weldment and increases porosity and hardness.
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Show More. No Downloads. Views Total views. Actions Shares. Embeds 0 No embeds. No notes for slide. Seminar report on underwater welding 1 1. The first ever underwater welding was 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, and 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.
In recent years the number of offshore structures including oil drilling rigs, pipelines, platforms are being installed significantly. Underwater wet welding technique was misunderstood for a long time, and it was a synonym for low quality weld full of porosity and cracks with poor mechanical properties like low ductility and due to micro structural issues prone to cracking. This lack of experience and knowledge was present in companies which did not understood all underwater welding issues which caused development of inadequate welding procedures, poor welder technique and inappropriate filler materials.
Through time, that status has been changed, and today underwater welding projects, both dry and wet, are used in most complex and difficult objects with a high level of quality assurance. Underwater Welding It is the process of welding at elevated pressures, normally underwater. Underwater welding can either take place wet in the water itself or dry inside a specially constructed positive pressure enclosure and hence a dry environment. 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.
Equipment and other technical facilities are far more complex and cheaper comparing to underwater dry welding procedures so very often underwater wet welding is proper technology to use for maintenance of underwater structures and repair of ships.
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. In wet welding MMA manual metal arc welding is used. Power Supply used: DC Polarity: For wet welding AC is not used on account of electrical safety and difficulty in maintaining an arc underwater. Fig 1; working process of a wet welding 6. Motor generator welding machines are most often used for underwater welding in the wet.
The welding machine frame must be grounded to the ship.
Types of Underwater Welding
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. 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.
Electric arc instability, which causes irregular geometry in the welded joint, slag inclusions, porosity and insufficient penetration. Ambient pressure has a significant influence on the behaviour of a welding arc, the performance of the welding process and the resultant weld properties. Increasing pressure leads to destabilisation of the arc plasma resulting from escalating turbulence in the arc column.
The rapid cooling leads to great hardness in the heat-affected zone, low toughness in the welded joint and the appearance of porosity due to the capture of gas bubbles. The high content of hydrogen in the column of the electric arc, molten metal in the transfer and in weld pool, which results in hydrogen capture in the metal of the weld and in the heat-affected zone. This increases the susceptibility to the appearance of cold 7. The high oxygen content in the electric arc column, molten metal in the transfer and weld pool, which leads to oxidation, reduction of the proportion of alloy elements and the degradation of mechanical properties.
The disintegration and solving of the coating of the electrodes, which results in electric arc instability and the appearance of porosity. Fig 2; figure shows person doing underwater welding in ocean 2. Their hardness reading sometimes exceeds HV10 in the heat affected zone. Specification for underwater welding according to the following expression: In underwater dry welding, it is possible to implement pre-heating and maintenance of inter-layer It is applicable for the repair of both ship hulls and underwater structures but is considered a second-rate technology because of the poor mechanical characteristics of the welded joint.
Today, wet underwater welding has an important Industrial, commercial and economic potential in the development and maintenance of underwater structures and because of the much lower costs and better flexibility has an advantage over dry underwater welding Techniques, high quality welds also being achieved.
However, further development of the steels used in underwater structures, the large number of installed and planned pipelines and increasing depth require further Development of the procedure of underwater wet welding.
Underwater Welding - A Review
Clearly in this the MMAW technology is facing the major barrier of automation solutions, which restricts it to use at lower depths, down to 60 m, while other Welding techniques like FCAW are becoming of primary importance for further research. Professionally skilled and educated diver-welders are cornerstone of underwater welding activities. It is noticed that incorrect working technique increase hydrogen level in weld metal, as well as quantity of porosity.
Also, bad electrode angle and higher welding speed are influencing significantly on slag inclusion, what is well known problem in underwater wet welding. Measuring of weld hydrogen content showed that apart from Welding position, type of electrode coating etc. Further, good quality stick electrodes are needed to establish and maintain electric arc and to deposit weld metal.
So far, developed coated electrodes are very good in their operational characteristics. Especially, it is of crucial impact that coating provides easy slag removal and low level of hydrogen as possible.J Keats, Manual on Wet Welding. Developments in Under Water Welding Wet welding has been used as an underwater welding technique for a long time and is still being used. Embed Size px. Special welding electrode holders with extra insulation against the water are used.
Dry 2. DC Polarity: