Hyperbaric welding
Hyperbaric welding is carried out in a chamber sealed around the structure to be welded (Figure 3 and Figure 4). The chamber is filled with a gas (commonly Helium containing 0.5 bar of oxygen) at the prevailing pressure. 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 other test requirements. The gas tungsten arc welding process, or a combination of gas tungsten arc welding for the starting layers and manual arc welding for the remaining layers (see Figure 5), 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.
Subsea hyperbaric welding is widely used for joining offshore oil and gas pipelines and for underwater repairs to production platforms. The welding is performed by divers or equipment installed by divers in the habitat.
The welding processes generally used are Tungsten Inert Gas (TIG), also known as "Gas Tungsten Arc (GTA)" and Manual Metal Arc (MMA) also known as "Shielded Metal Arc (SMA)", welding. The steels welded together in the offshore oil and gas industry are typically between 12 mm and 35 mm thick. The weld is done in a series of "passes". Manual hyperbaric welding procedures normally involve making at least the first two "passes" with TIG and the subsequent "passes" with MMA.
Hyperbaric TIG welding is widely used as a technique for making the root pass and some subsequent passes in manual hyperbaric welding procedures. TIG (Figure 6) is the process used on fully mechanised orbital hyperbaric welding robots such as the THOR system, THOR standing for TIG Hyperbaric Orbital Robot. In this process electric arc is maintained between a non-consumable tungsten electrode and the molten weld pool. A separate filler wire is added. The electrode, arc and weld pool are shielded by a stream of inert gas normally argon or an argon helium mixture. Manual Metal Arc or "Shielded Metal Arc" welding is widely used for hyperbaric welding at moderate water depths. The weld metal is deposited from a covered steel electrode with a "Basic" coating. This coating contains approximately 30% calcium carbonate. An electric arc is maintained between the electrode and the workpiece. During welding the coating decomposes to from CO and CO2 gases and a calcium oxide slag that covers the molten metal.
Health and safety considerations in hyperbaric welding
At the present time approximately 50% of the hyperbaric welds involve use of manual welding procedures. The mechanised robot welding systems involved also currently require the presence of divers in the welding habitat to perform intervention tasks such as changing tungsten electrodes and grinding. The health and safety aspects of hyperbaric welding are therefore of considerable importance.
Hyperbaric welding fumes and gases
Arc welding processes produce various contaminants and the exposure of divers to these must be less than that permitted in the health and safety legislation. Hyperbaric welding is performed in a generally closed environment and ventilating the welding area with a large flow of air is not a practical solution as it is in many welding operations in a normal workshop. Control of exposure is normally achieved by the combination of a gas regeneration system, which removes the pollutants, and the use of respirators. The concentration of pollutants must also be monitored and recorded.
Welding fumes and gases from the hyperbaric Manual Metal Arc (MMA) welding
Carbon monoxide and carbon dioxide are produced during Manual Metal Arc welding from the decomposition of the calcium carbonate in the electrode coating. Particulate consisting mainly of oxides of the filler metal and the metals being welded are also generated. These pollutants are controlled by the use of filters, absorbents and catalyst in the gas regeneration systems in the welding habitat or the chamber.
Welding fumes and gases from hyperbaric Tungsten Inert Gas (TIG) welding
The amount of particulate produced by TIG welding is generally much less than produced by MMA welding. TIG welding generates significant amounts of ozone. Ozone is removed by contact with filters and chemicals in gas regeneration system.
Ozone concentration
The changes in ozone readings during an experiment where the welding gas regeneration system was not operated.
Argon gas concentration
If argon is used as a shielding gas for hyperbaric welding this represents a source of potential contamination in the habitat. Argon is not toxic but is approximately twice as narcotic as nitrogen. It is also a slower transferring gas with respect to decompression than helium. As far as it is known, the concentrations of argon present during normal hyperbaric welding do not present a hazard, but it is important to beware of the consequences of accidental over exposure.
Fire risks in hyperbaric welding habitat
The risk of fire in welding habitat is an important consideration at moderate depths. Where the oxygen content of a breathable atmosphere is within the range that can support combustion.
Advantages of dry welding
- Welder/diver safety: Welding is performed in a chamber immune to ocean currents and marine animals. The warm, dry habitat is well illuminated and has its own environment control system (ECS).
- Good quality welds: This method has the ability to produce welds of quality comparable to open air welds because water is no longer present to quench the weld and hydrogen level is much lower than wet welds.
- Surface monitoring: Joint preparation, pipe alignment, NDT inspection, etc, are monitored usually.
- Non-destructive Testing (NDT): NDT is also facilitated by the dry habitat environment.
Disadvantages of dry welding
- The habitat welding requires large quantity of complex equipment and much support equipment on the surface. The chamber is extremely complex.
- Cost of the habitat welding is extremely high and increases with depth. Work depth has an effect on habitat welding. At greater depths, the arc constricts and corresponding higher voltages are required.
- The process is costly. One weld may cost $ 80000 (about Rs 40 lac). One cannot usually use the same chamber for another job.
Scope for further developments
Wet MMA welding is still being used for underwater welding repairs but the quality of wet welds is poor and the welds are prone to hydrogen embrittlement. Dry hyperbaric welds are better in quality than wet welds. Present trend is towards automation as mentioned earlier about THOR - (TIG Hyperbaric Orbital Robot) which has been developed where the diver performs the pipe fitting, installs the trac and orbital head on the pipe and the rest of the process is automated. 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.
Coffer dam welding, is another variation of underwater 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.
Other processes include underwater laser welding (with a protective gaseous atmosphere contained in a rotating skirt) and water curtain welding. The latter is a mechanised process where a conical jet of water produces an enclosure to confine the welding gas. The process was used in Japan in the late 1990s and in the early years of this century to deposit the sealing runs for welds to join together the modules of Megafloat, the prototype structure intended to be used for such purposes as floating airships.
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