Underwater Welding Process
Underwater welding process usually refers to the wet welding technique where there is no mechanical barrier that separates the welding are from water. It is also known as submerged water arc welding. For deep water welds and other applications where high strength is necessary, dry water welding is most commonly used.
It is a unique form of welding process and it has got multiple applications which are rare to be worked upon worked upon by other conventional methods of welding.
Large numbers of offshore structures such as oil drilling rigs, pipelines, platforms are installed, during normal usage or during storms and collisions which are unpredictable. The failure of some of the elements may occur and any repair work requires the use of underwater welding.
Water proof welding electrodes are used which are quite unique and work as normal electrodes do in normal working conditions.
In general, the integrity of underwater welds is difficult especially wet underwater welds because defects are difficult to detect. For the structures being welded by wet underwater welding process, inspection following welding may be highly challengeable than welds deposited in air.
The diving tender should always maintain a written record of the following parameters in order to repeat during the next welding.
- The welding amperage as read from the tong meter.
- Both open and closed-circuit voltage as read from the volt-meter.
- Electrode diameter, type, manufacture and waterproofing material.
- Electrical polarity.
- Length of welding cable.
- Depth of work site.
Applications of underwater welding:
- Under water welding is applied in marine applications and installation of offshore oil pipelines.
- It is also used to apply weld repair on massive ships inside the water.
Risks associated with Underwater Welding
- The risks of underwater welding include the risk of electric shock to the welder. The welding equipment ought to be properly insulated to prevent it and the voltage of the welding equipment should be controlled.
- Under water welders must also consider the safety issues that normal divers face about the risk of decompression sickness due to the increased pressure of inhaled breathing gases.
- Heat created by burning or torch can ignite trapped gases. Trapped gases need to be removed by venting or jetting gas. In closed spaces, small amount of gas can get trapped and remained with the diver. Gas must be vented if it can be trapped. Vent holes are drilled to allow the gas to escape to the surface.
- Another risk is associated with wet underwater welding which is buildup of hydrogen and oxygen pockets in the weld because these are potentially explosive. When using grinders or drills sufficient heat can ignite hydrocarbons causing an explosion. Material being burned might contain pockets that can trap burning gas. Gasses need to be properly vented. The solution is to slow down the drill bit to avoid generating the heat levels required to ignite any gas.
Characteristics of Underwater Welding Arc
The welding are does not behave underwater as it does on the surface and the activity of the gas bubble is particularly important to successful completion of the underwater weld.
When the arc is struck, the combustions of the electrode and the detachment of water create a gas bubble or envelope. As the pressure within the bubble increases, it is forced to leave the arc and meet with the surrounding water while another bubble forms to take its place.
When this pressure head becomes greater than the capillary force, the bubble will break down. Therefore, if the electrode is being too far from the work, the weld will be destroyed as the gases explode and blow through.
If the travel speed is too slow, the bubble will collapse around the weld and destroy the possibility of producing an effective weld.
Classification of Underwater Welding
The two main categories of underwater welding techniques are as follows:
- Wet underwater welding
- Dry underwater welding
Wet Underwater Welding
In wet welding, both weldment and welder are exposed to water. Wet welding is performed by the driver welder normally using surface diving or saturation diving or saturation diving techniques using the manual metal arc welding process with electrodes specially coated with insulating varnishes to keep them dry.
Current is supplied by a generator directly to the welding torch head. Interchangeable collets hold electrodes of required diameters and they are tightened by the twist grip control which is also used to eject the stub when electrodes are changed.
Watertight glands and washers prevent seepage of water into the body of the torch which is tough rubber covered thus reducing the danger of electric shock.
In underwater welding, are stability is less due to the large volumes of gas and steam which are evolved making vision difficult and as a result touch welding is generally used.
The presence of water in the immediate vicinity of the weld except at the molten pool under the arc stream results in the very rapid quenching of the weld metal which produces a hard narrow heat affected zone and it can give a rise to severe hydrogen cracking. Weld properties are inferior to air welds.
Standard air welding equipment can be used. There is no fit-up time. This process is more convenient in welding the parts which are located under the water.
At present, this method is used for non-critical welds and critical welding is being carried out under dry conditions.
Specially designed electrodes are used to perform wet welding and power supply is cated on the surface connecting to the welder with the help of cables and hoses as.
However, similar to open air welding, wet welding is also performed manually. Wet welding is the most effective, efficient and economical method used as it provides an increased freedom of movement.
Advantages of wet underwater welding:
- Welders can reach positions inaccessible by other methods.
- Process is fast.
- Cost of welding is very low.
- It has more freedom of repair design and fit-up.
- Standard welding equipment can be used.
- The welding process is speedy due to readily available machine, equipment and no enclosures.
Limitations of wet underwater welding:
- Quenching effect is produced at the weld surface due to direct contact of the arc and molten weld-pool with water.
- Both porosity and hardness also increase.
- It produces low visibility to the operator.
- Inspection of welds is very difficult as compared to open air welding. So, producing a good quality of weld is very difficult task.
Dry Underwater Welding
Dry underwater welding, also known as hyperbaric welding, is carried out by creating a chamber filled with a mixture of helium and oxygen gases for breathing purposes near the welding area.
Where the structure is to be welded and the welder is supposed to work from inside the chamber. TIG welding process is mostly employed in dry underwater welding and the weld joints created are of high quality which meets code requirements.
However, all types of arc welding process such as shield metal arc welding, MIG welding and flux cored arc welding can be applied in this technique.
Dry chambers are used to provide safety and isolated environment to weld underwater. These specially designed chambers provide not only a pleasant environment to breathe in but also a clean environment by regular discharge of hazardous gases which are produced as a result of welding procedure.
At the same time, the elevated pressure may cause problems. Thus, hyperbaric welding process is limited to particular depth of water where a welder can sustain himself to operate easily without any operational problem.
Advantages of dry underwater weld:
- Higher welder safety is ensured in a chamber, less immune to ocean currents and marine animals.
- High quality weld joints are produced as compared to open air weld joints.
- All inspections and preparations are done visually.
- Non Destructive Testing (NDT) is also facilitated by dry welding which is pretty much important for ensuring weld quality.
- Good surface monitoring is ensured.
Limitations of dry underwater welding:
- The process requires huge number of complex equipment.
- The operator needs to be more and specifically trained.
- The process is costly which requires an element of precision welding.
- Cost of process is higher and the welder needs a proper training.
- Equipment is complex.
- More deep and energy are required.
- It cannot be weld if the weld spot is at unreachable places.
Comparison of Underwater Weld Over Normal Air Welding
- Electrodes are painted for waterproofing.
- Electrode core wire is usually similar to air welding but for high strength steel, a core wire of stainless steel or special steel is required.
- Waterproofed flux coated or iron-oxide covered electrodes are used in underwater welding.
- The gap between electrode and work is not maintained in underwater welding because the electrode is lifted due to the pressure of water during arc but the gap is maintained between electrode and work in air welding.
- Underwater welding arc is surrounded by steam and gases but it is not in the case of air welding.
- Cooling rate of the part can be controlled in air welding by changing the energy input but it is difficult in underwater welding.
- Hydrogen and oxygen levels are high in underwater welding but it is normal in air welding.
- Electrode holder also insulated in underwater welding to avoid electric shock.
Welding Automation in Aerospace Industries
Aerospace manufacturing is unique and specialized. The parts are complex and often gigantic. While it may come as a surprise, this industry has begun to rely more and more on the standard 6-axis industrial robot, not specialized machinery.
Welding robots are very precise, move smoothly at considerable speed through a programmed path. Being computer-based, they can be programmed and have sensors to follow the seam and apply corrections to the welding parameters.
Reasons for implementation of welding robots in aerospace industry:
The following are the reasons for implementation of welding robots in aerospace industry:
- Robotics automation is economical:
Aerospace manufacturers are attracted to robot’s cost-effectiveness. The typical 6-axis articulated robot is far more affordable than a custom-built machine and it performs just as effectively.
Companies that appropriate for a reconditioned robot can expect to save even more i.e., up to 50%-60% less than the cost of a new robot.
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