Underwater welding is a key technology for repairing marine structures. Underwater welding can be divided into three main types:
- Wet underwater welding, where manual metal arc welding (MMA) is the most common process. Flux-cored arc welding (FCAW) has been widely used in the former Soviet Union. Friction welding, which has the advantage of being relatively insensitive to depth, and which lends itself to robotic operation, has the potential for use in deep water repair.
- Coffer dam welding, which is carried out in the dry, in air, where a rigid steel structure to house the welders is sealed against the side of the structure to be welded, and is open to the atmosphere.
- Hyperbaric welding, in which a chamber is sealed around the structure to be welded, and is filled with a gas (commonly helium containing 0.5 bar of oxygen) at the prevailing pressure.
The materials, which are most commonly welded, are microalloyed C-Mn steels of pipelines and offshore structures, and C-Mn steels on ships and harbour works. Extensive repairs have been carried out to offshore platforms following hurricane or explosion damage. Coffer dam welding is most likely to be employed in harbour works or ship repair, although wet underwater welding is also widely used. Semi-automatic FCAW has been used in the former Soviet Union for repairs to (shallow) pipelines and to the hulls of sunken ships, prior to refloating. Hyperbaric welding, using MMA (SMA), TIG (GTA) or FCAW, is the preferred process for high integrity welds, particularly for deep water welds, including tie-ins in pipelines and risers in the oil and gas industries.
The first ever underwater welding was carried out for sealing leaking ship rivets below the water line. Underwater welding is an important tool for underwater fabrication work. Since 1946, special waterproof electrodes are being built. In recent years, the number of offshore structures including oil-drilling rigs, pipelines, platforms are being installed increasingly. Some of these structures can experience failures of its elements during normal usage and during unpredicted occurrences like storms, collisions, etc. Any repair or reclamation work will require the use of underwater welding.
Categorization Underwater welding can broadly be classified as
1. Wet welding
2. Dry welding
In wet welding, the welding is performed underwater, directly exposed to the wet 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 called a habitat and therefore this type of welding is habitat welding.
Wet welding indicates that welding is performed underwater, directly exposed to the wet environment without a dry habitat. Special electrodes are used and welding is carried out manually just as one does in the open air. The increased freedom of movement makes wet welding an effective, efficient and economical method. Welding power supply is located on the surface with connection to the diver/welder via cables and hoses. In wet welding usually MMA (manual metal arc welding) is used. Flux-cored arc welding (FCAW) has been widely used in the former Soviet Union. Friction welding, which has the advantage of being relatively insensitive to depth, and which lends itself to robotic operation, has the potential for use in deep water repair.
Underwater Manual Metal Arc (MMA) welding
Power supply used: DC
When DC is used with +ve polarity, electrolysis of water taking place causes rapid deterioration of any metallic component in the electrode holder. For wet welding AC is not used on account of electrical safety and for difficulty in maintaining arc underwater. The power source should be a direct current machine rated at 300 or 400 amperes. Motor generator welding machines are most often used for underwater welding in the wet. The welding machine must be grounded to the ship. The welding circuit must include a positive type of switch usually a knife switch (see Figure 1) operated on the surface and commanded by the welder-diver. The knife switch in the welder 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.
Special welding electrode holders with extra insulation against the water are used. The underwater welding electrode holder utilises a twist type head for gripping the electrode. The electrode types used conform to AWS E 6013 (BIS equivalent IS 307264) 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 leaks, sea water coming in contact with the metal conductor results in part of the current leaking away and not being available at the arc. In addition there will be rapid deterioration of the copper cable at the point of leakage. Figure 2 shows wet underwater welding being carried out. The quality of wet welds is lower than that achievable with dry habitat welding because the weld is rapidly cooled by the surrounding water and contains high concentrations of hydrogen and oxygen. Wet welding is nevertheless widely applied on non-critical structures offshore. The latest research and advancements in wet underwater welding has led to production of A class welds (AWS classification) in wet underwater welding.
Risks involved :
There is a risk to the welder-diver of electric shock. Precautions include achieving adequate electrical insulation of the welding equipment, quick shutting off the electric supply when arc is no longer required and limiting the open circuit voltage of MMA (SMA) welding sets. Secondly, hydrogen and oxygen are produced by arc in wet welding. Therefore precautions must be taken to prevent build up of the pockets of gases which are potentially explosive. The other main area of risk is to the life or health of the welder-diver from nitrogen introduced into the blood stream during exposure to air at increased pressure. Precautions include the provision of an emergency air or gas supply, standby divers and decomposition chambers to avoid nitrogen narcosis following rapid surfacing after saturation diving. Nitrogen narcosis is a condition that occurs in divers breathing compressed air. When divers go below depths of approximately 100 ft, increase in the partial pressure of nitrogen produces an altered mental state similar to alcohol intoxication. Nitrogen narcosis, also commonly referred to as "rapture of the deep," typically becomes noticeable at 100 ft underwater and is incapacitating at 300 ft, causing stupor, blindness, unconsciousness, and even death. For the structures being welded by wet underwater welding, inspection following welding may be more difficult than for welds deposited in air. Assuming the integrity of such underwater welds may be more difficult, and there is a risk that defects may remain undetected.
Advantages of wet welding :
Wet underwater MMA welding is now being widely used and has been so for many years in the repair of offshore platforms. The benefits of wet welding are :
1. The versatility of wet welding makes the process highly desirable
2. Other benefits include the speed with which the operation is carried out.
3. It is less costly compared to dry welding.
4. The welder can reach portions of offshore structure that could not be welded using other methods.
5. No enclosures are needed and no time is lost building one. Readily available standard welding machine and equipment are used.
Disadvantages of wet welding :
Although wet welding is 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
2. Hydrogen embrittlement - large amount of hydrogen is present in the weld region from the dissociation of water vapour in the arc region. The hydrogen dissolves in the heat affected zone (HAZ) and the weld metal resulting in embrittlement, cracks and microscopic fissures. Cracks can grow resulting in catastrophic failure of the structure.
3. Poor visibility
Development of underwater welding
Wet welding has been used widely 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.