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Hyperbaric welding

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Diver wearing a diving helmet is welding a repair patch on a submarine
A US Navy diver at work.
Underwater welding

Hyperbaric welding is the process of welding at elevated pressures, normally underwater.[1][2] Hyperbaric welding can either take place wet in the water itself or dry inside a specially constructed positive pressure enclosure and hence a dry environment. It is predominantly referred to as "hyperbaric welding" when used in a dry environment, and "underwater welding" when in a wet environment. The applications of hyperbaric welding are diverse—it is often used to repair ships, offshore oil platforms, and pipelines. Steel is the most common material welded.

Dry hyperbaric welding is used in preference to wet underwater welding when high quality welds are required because of the increased control over conditions which can be exerted, such as through application of prior and post weld heat treatments. This improved environmental control leads directly to improved process performance and a generally much higher quality weld than a comparative wet weld. Thus, when a very high quality weld is required, dry hyperbaric welding is normally utilized. Research into using dry hyperbaric welding at depths of up to 1,000 metres (3,300 ft) is ongoing.[3] In general, assuring the integrity of underwater welds can be difficult (but is possible using various nondestructive testing applications), especially for wet underwater welds, because defects are difficult to detect if the defects are beneath the surface of the weld.

Underwater hyperbaric welding was invented by the Russian metallurgist Konstantin Khrenov in 1932.

Dry

Dry hyperbaric welding involves the weld being performed at the prevailing pressure in a chamber filled with a gas mixture sealed around the structure being welded.

Most welding processes SMAW, FCAW, GTAW, GMAW, PAW could be operated at hyperbaric pressures, but all suffer as the pressure increases.[4] Gas tungsten arc welding is most commonly used. The degradation is associated with physical changes of the arc behaviour as the gas flow regime around the arc changes and the arc roots contract and become more mobile. Of note is a dramatic increase in arc voltage which is associated with the increase in pressure. Overall a degradation in capability and efficiency results as the pressure increases.Welding processes have become increasingly important in almost all manufacturing industries and for structural application [Khanna, 2004]. Although, a large number of techniques are available for welding in atmosphere, many of these techniques can not be applied in offshore and marine application where presence of water is of major concern. In this regard, it is relevant to note that, a great majority of offshore repairing and surfacing work is carried out at a relatively shallow depth, in the region intermittently covered by the water known as the splash zone. Though, numerically most ship repair and welding jobs are carried out at a shallow depth, most technologically challenging task lies in the repairing at a deeper water level, especially, in pipelines and occurrence/creation of sudden defects leading to a catastrophic accidental failure. The advantages of underwater welding are of economical nature, because underwater-welding for marine maintenance and repair jobs bypasses the need to pull the structure out of the sea and saves valuable time and dry docking costs.

Special control techniques have been applied which have allowed welding down to 2500m simulated water depth in the laboratory, but dry hyperbaric welding has thus far been limited operationally to less than 400m water depth by the physiological capability of divers to operate the welding equipment at high pressures and practical considerations concerning construction of an automated pressure / welding chamber at depth.[5]

Wet

Underwater welding commonly uses a variation of shielded metal arc welding, employing a waterproof electrode.[2] Other processes that are used include flux-cored arc welding and friction welding.[2] In each of these cases, the welding power supply is connected to the welding equipment through cables and hoses. The process is generally limited to low carbon equivalent steels, especially at greater depths, because of hydrogen-caused cracking.[2]

Risks

The risks of underwater welding include the risk of electric shock to the welder. To prevent this, the welding equipment must be adaptable to a marine environment, properly insulated and the welding current must be controlled. Commercial divers must also consider the safety issues that normal divers face; most notably, the risk of decompression sickness following saturation diving due to the increased pressure of inhaled breathing gases.[6] Many divers have reported a metallic taste that is related to the breakdown of dental amalgam.[7][8][9] There may also be long term cognitive and possibly musculoskeletal effects associated with underwater welding.[10]

See also

References

  1. ^ Keats, DJ (2005). Underwater Wet Welding - A Welder's Mate. Speciality Welds Ltd. p. 300. ISBN 1-899293-99-X.. {{cite book}}: Check |isbn= value: invalid character (help)
  2. ^ a b c d Cary, HB and Helzer, SC (2005). Modern Welding Technology. Upper Saddle River, New Jersey: Pearson Education. pp. 677–681. ISBN 0-13-113029-3.{{cite book}}: CS1 maint: multiple names: authors list (link)
  3. ^ Bennett PB, Schafstall H (1992). "Scope and design of the GUSI international research program". Undersea Biomedical Research. 19 (4): 231–41. PMID 1353925. Retrieved 2008-07-05.
  4. ^ Properties of the constricted gas Tungsten (Plasma) Arc at Elevated Pressures. Vol. Ph.D. Thesis. Cranfield University, UK. 1991.
  5. ^ Hart, PR (1999). A Study of non-consumable welding processes for diverless deepwater hyperbaric welding to 2500m water depth. Vol. Ph.D. Thesis. Cranfield University, UK.
  6. ^ US Navy Diving Manual, 6th revision. United States: US Naval Sea Systems Command. 2006. Retrieved 2008-07-05.
  7. ^ Ortendahl TW, Dahlén G, Röckert HO (1985). "Evaluation of oral problems in divers performing electrical welding and cutting under water". Undersea Biomed Res. 12 (1): 69–76. PMID 4035819. Retrieved 2008-07-05. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  8. ^ Ortendahl TW, Högstedt P (1988). "Magnetic field effects on dental amalgam in divers welding and cutting electrically underwater". Undersea Biomed Res. 15 (6): 429–41. PMID 3227576. Retrieved 2008-07-05. {{cite journal}}: Unknown parameter |month= ignored (help)
  9. ^ Ortendahl TW, Högstedt P, Odelius H, Norén JG (1988). "Effects of magnetic fields from underwater electrical cutting on in vitro corrosion of dental amalgam". Undersea Biomed Res. 15 (6): 443–55. PMID 3227577. Retrieved 2008-07-05. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  10. ^ Macdiarmid JI, Ross JAS, Semple S, Osman LM, Watt SJ, Crawford JR (2005). "Further investigation of possible musculoskeletal and cognitive deficit due to welding in divers identified in the ELTHI diving study" (PDF). Health and Safety Executive. Technical Report rr390. Retrieved 2008-07-05.{{cite journal}}: CS1 maint: multiple names: authors list (link)


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