Underwater Welding: Techniques, Equipment & Applications
The fact that electric arc could operate was known for the over a 100 years. The first ever underwater welding was the carried out by British Admiralty Dockyard for the sealing leaking ship rivets below the water line. Underwater welding is the important tool for underwater fabrication works. In the 1946, special waterproof electrodes were developed in the Holland by ‘Van der Willingness’. In the recent year’s number of the offshore structures including oil drilling rigs, pipelines, and the platforms are being installed significantly. Some of these structures will experience failures of it’s the elements during normal usage and the during unpredicted occurrences like the storms, collisions. Any repair method will be the require use of the underwater welding.
Principle operation of Wet Welding
The process of underwater wet welding takes in the following manner: The work to be welded is connected to the 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 the separated slightly. The electric current jumps gap and the causes a sustained spark (arc), which melts bare metal, forming the weld pool. At the same time, tip of electrode melts, and the metal droplets are projected into the weld pool. During this is the operation, flux covering electrode melts to the provide a shielding gas, which is the used to stabilize arc column and the shield transfer metal. The arc burns in a cavity formed inside flux covering, which is the designed to burn slower than metal barrel of the electrode.
Scope for further developments
Wet manual metal arc (MMA) is the still being used for underwater repairs, but quality of the wet welds is poor and are prone to the hydrogen cracking. Dry Hyperbaric welds are better in the quality than wet welds. Present trend is the towards automation. THOR -1 (TIG Hyperbaric Orbital Robot) is the developed where diver performs pipefitting, installs track and the orbital head on pipe and rest process is the automated. Developments of the driverless Hyperbaric welding system is an the even greater challenge calling for the annexe developments like pipe preparation and the aligning, automatic electrode and the wire reel changing functions, using a robot arm installed. This is in the testing stage in deep waters. Explosive and the friction welding are also to be tested in the deep waters.
Developments in Underwater Welding
Wet welding has been used as and the underwater welding technique for a long time and is the still being used. With recent acceleration in the construction of offshore structures underwater welding has been the assumed increased importance. This has led to the development of alternative welding methods like friction welding, explosive welding, and the stud welding. Sufficient literature is the not available of these processes.
Application of Underwater Welding
The important applications of underwater welding are:
- Offshore construction for tapping sea resources.
- Temporary repair work caused by ships collisions or unexpected accidents.
- Salvaging vessels sunk in the sea.
- Repair and maintenance of ships.
- Construction of large ships beyond the capacity of existing docks.
- Repair and maintenance of underwater pipelines.
Underwater welding can be the classified as
1) Wet Welding
2) Dry Welding
In the wet welding welding is the performed underwater, directly exposed to the wet environment. In the dry welding, a dry chamber is the created near area to be welded and the welder does job by the staying inside chamber.
1. Wet Welding
Wet Welding indicates that welding is the performed underwater, directly exposed to the wet environment. A special electrode is the used and welding is carried out manually just as one does in the open air welding. The increased freedom of the movement makes wet welding most effective, efficient and the economical method. Welding power supply is located on the surface with connection to the diver/welder via cables and the hoses. In the wet welding MMA (manual metal arc welding) is the used.
2. Hyperbaric Welding (dry welding)
Hyperbaric welding is carried out in the chamber sealed around structure to be the welded. The chamber is filled with the gas commonly helium containing the 0.5 bar of the oxygen at prevailing pressure. The habitat is sealed onto pipeline and the filled with a breathable mixture of the helium and oxygen, at or slightly above ambient pressure at which welding is to the take place. This is the method produces high-quality weld joints that meet X-ray and the code requirements. The gas tungsten arc welding process is the employed for this is the process. The area under floor of the Habitat is open to the water. Thus welding is the done in dry but at hydrostatic pressure of the sea water surrounding the Habitat.
Future Scope for Underwater Welding
Considerable industrial effort has been made to the improve process performance and control strategies for the various underwater welding processes. For future scope in the underwater welding, major efforts on research and the development should be focused on the following topics:
- Automation of the underwater joining and the inspection of the welded structures.
- Mechanized underwater welding for the actual usage of a very large floating structures.
- Investigation of the potential of using a robot manipulator for the underwater ultrasonic testing of welds in joints of the complex geometry.
- Application of the advanced welding technique, like friction, laser welding and the understand the behaviour of materials after welding and the process optimization.
- Invention of the new welding techniques and explore possibility of its application in the underwater welding.
- Generation of the research data book on weld ability of the materials during underwater welding.
Challenges in Underwater Welding
The main difficulties in underwater welding are presence of the higher pressure due to the water head under which welding takes place, chilling action of the water on the weld metal which might change metallurgical structures and the properties possibility of producing the arc mixtures of hydrogen and the oxygen in pockets, which might set up an explosion, and the common danger sustained by divers, of the having nitrogen diffused in blood in the dangerous proportions. Furthermore, complete insulation of the welding circuit is an essential requirement of the underwater welding. In the practice, use of underwater wet welding for offshore repairs has been.
With appropriate consumable design, however, it is the possible to reduce porosity and to the enhance weld-metal toughness through microstructural refinement. Test welds were performed in the lab for conditions of welding at low depths with application of the various welding conditions like – welding current, painting of the electrode, electrodes polarity, thickness of the flux covering electrodes core, salinity of the water, contamination of electrode (carbohydrates) and time of the wetting of electrode in water; it was found that most relevant variables for the quality of weld were as shown in the following Pareto chart.
In the early 20th century, even though professional diving was already an established industry (and had been for hundreds of years), underwater welding was not. The Soviet engineer, Konstantin Cherenkov, was the first to invent a method to join and cut metals in submerged conditions. Cherenkov learnt that one of the largest hurdles to successful wet welds was the sporadic outflow of gas bubbles from the point of contact with the metal and arc. This reaction triggered major porosity in the welds. With the assistance of his peers, Cherenkov developed a waterproof coating for the electrodes. He initiated experimenting and carried out underwater i.e. immersed welding in labs. In 1932, Cherenkov voyaged with engineers to the Black Sea for the first successful on-field testing.
Defects frequently form on the subsea metal structures and pipes because of the service conditions. Although, a large number of the techniques are available for welding in the atmosphere, many of them cannot be applied in offshore and the marine applications where presence of water is the major concern. When it is not possible to the place metal structure into a dry dock for the repair or maintenance, underwater welding techniques are the applied. Another advantage of the underwater welding is of economical nature, because underwater welding for the marine maintenance and repair jobs bypasses the need to the pull structure out of sea and the saves much valuable time. Sometimes, in the case of sudden defects leading to a catastrophic accidental failure, the becomes important to carry out maintenance on the site.
Underwater Wet Welding Methods
In wet welding technique, even a complex structure may be welded. The most commonly used wet welding technique is the Shielded Metal Arc Welding process (SMAW) and the Flux Cored Arc Welding (FCAW), using the self-shielded flux cored arc welding. However, from an economic point of view, the wet welding technique with coated electrodes (SMAW) comes as the first consideration for general underwater welding. This is carried out by means of special waterproof stick electrodes, with no physical barrier between water and welding arc. Wet underwater welding directly exposes the welder diver and electrode to the water and surrounding elements.
Welders usually use around 300-400 amps of direct current to power an electrode, using varied forms of arc welding, employing a waterproof electrode. Other processes that are used also include the Flux-cored Arc Welding (FCAW). In each of these cases, the welding power supply is connected to the welding equipment by specially isolated cables, installed from topside or platforms above the water. The welder instructs the surface operator to make and break the contact, as required, during the procedure. The contacts should only be closed during actual welding, and opened at other times, particularly when changing electrodes.
Equipment for underwater wet welding
The underwater wet welding equipment has to be the meet all the safety aspects. Due to the limited time spent underwater, diver-welder has to be the provided with satisfactory operating conditions. Equipment has to be regularly maintained according to the stipulated regulations in order to ensure its proper functioning. Main equipment needed for the wet underwater welding can be summarized in the following:
1. Diving equipment
The diving equipment includes a dry diving suit, dry suit full-face mask, surface supply umbilical cord, air tank on the surface with regulation manometer, and compressor for filling the air tank.
2. Welding power sources
Intensive dynamic behavior of the power source is necessary to obtain a stable electric arc in greater depths. The welding power source must be adjusted for underwater wet welding, not only in the matter of good arc behavior but also in the aspect of diver welder safety.
3. Safety switch
For safety reasons, the electrical circuit is fitted with a safety switch, which interrupts or establishes the flow of current necessary for underwater welding or cutting as a diver requests or in case of an accident.
4. Communication system
For constant connection and coordination of works between the surface and the diver, a two-way telephone communication system has to be applied in order to facilitate the realization and organization of underwater activities.
5. Welding cables, and welding and cutting holders
Special cables with special insulation class have to be the used for underwater welding and cutting in order to the prevent breakthrough of electric current into the water or to the platform structure, which may cause safety problems and difficulties in the welding.
6. Mechanical tools system
Mechanical tools for the underwater works may be driven electrically, hydraulically and the
pneumatically. Electric drive is the avoided because of safety problems, and mostly hydraulic and the pneumatic tools are used.
Applications of the Underwater Welding will see increasing in demand in Kuwait due to the country ambitious plans toward starting offshore oil and the gas exploration and production, increasing in the marine projects such as Jabber Bridge and establishing new marine ports. Therefore, underwater welding will play vital roles in the development and shaping of the Kuwait’s future industry. There is a big need to the well understand underwater techniques, secure needed resources and experts and develop national capabilities to be ready meet the country’s needs. Therefore, preparation of the needed resources and development plans to improve skills in underwater welding techniques need to be considered in the country developments plans in order to meet the requirements of Kuwait’s future industry trend.