What are the principles of underwater wet welding?

Frequently Asked Questions

Arc welding is achieved by arranging an electrical circuit in which current passes from the welding power source through the welding electrode, across an arc to the workpiece, and through a return lead to the power source. For the safety of the welder/diver during wet underwater welding, it is vital that he or she is completely insulated from the welding circuit. An insulated electrode holder or welding gun is used, and the welder/diver wears rubber gloves. For safety, on the verbal directions of the welder/diver, the power supply (which is controlled from the surface, via a double-pole 'knife' switch) is switched off immediately welding stops. For a discourse on underwater electrical safety, see ^Ref.1.

Once the welding circuit is completed by striking an arc, the heat of the arc is sufficient to vaporise the surrounding water. Hence, the arc is surrounded by a vapour shield, similar to the gas shield which surrounds an arc during welding in air. However, reaction with the molten metal influences the composition of this vapour, and it comprises typically ~70% hydrogen, ~25% carbon dioxide and ~5% carbon monoxide. The resulting weld metal diffusible hydrogen level can be extremely high (up to 100 ml/100g of deposited metal for rutile electrodes), but is much lower for oxidising electrodes (~25 ml/100g of deposited metal). These levels increase as the welding depth increases, thereby increasing the risk of hydrogen-assisted cracking. See Refs ^[2,3] for information on the metallurgy of wet underwater welding.

Most wet underwater welding is carried out using manual metal arc electrodes, which have a waterproof coating over the flux coating. Semi-automatic welding equipment has been developed which uses a solid wire surrounded by a gas shield, retained within a 'water curtain'. Following its early use in Germany ^[4], the process was later revived in Japan ^[5-8], where it was used on Megafloat. A variant of the process uses a wire brush ^[9,10] instead of a water curtain to contain the gas shield. Weld metal diffusible hydrogen levels are comparable with those for welds deposited in the dry. An alternative semi-automatic process, developed in the former Soviet Union, uses a flux-cored wire without any shielding gas ^[11-13]. Wet underwater welding has been used for permanent repairs to harbour works, offshore structures, pipelines and ships. See Refs ^[2,11,12,14] for information on underwater repair.

Further information

• Assessment of the mechanised flux cored arc welding process for underwater wet welding.

References

1. Underwater Electrical Safety For Divers; www.underwater.com/archives/arch/044.01.shtml
2. Proc. International workshop on Underwater welding of marine structures, New Orleans, U.S.A. 7-9 December 1994, American Bureau of Shipping.
3. Liu S, Olson D L and Ibarra S: 'Underwater welding' ASM Metals Handbook, Vol 6, 1993 1010-1015.
4. Hoffmeister H, Küster K and Schafstall H-G: 'Weld joint properties of medium strength steels after underwater wet MIG-welding by the water curtain process', Proc. Second Int. Conf. on Offshore welded structures, 16-18 November 1982, London, The Welding Institute, 1983, p17-1 - p17-8.
5. Irie T, Ono, Y, Matsushita H, Kitamura N, Tohno K, Morita, T and Ogawa, Y: 'Formation of local cavity by water curtain nozzle'. Proc. 16th International Offshore Mechanics and Arctic Engineering (OMAE) Conference, Yokohama, Japan, 13-17 Apr.1997. Ed: M. M. Salama, M Toyoda, S Liu, J F Dos Santos, H Endo, O B Andersen, Publ: New York, NY 10017, USA; American Society of Mechanical Engineers, 1997, Vol.3. pp.43-50.
6. Irie T, Ono Y; Sato M, Kitamura N and Ogawa Y: 'Underwater butt welding down to 10m'. Proc. 17th International Offshore Mechanics and Arctic Engineering (OMAE) Conference, Lisbon, Portugal, 5-9 July 1998. Ed: C Guedes-Soares. Publ: New York, NY 10017, USA, ASME International; 1998, Materials, Paper 2232.
7. Ogawa Y, Kitamura N, Tohno K, Iire T and Matsushita H: 'Automatic underwater welding for construction of offshore structures'. Proc. 8th International Conf. On Behaviour of Offshore structures (BOSS '97), 1997, Ed. J H Vugts, Vol.3 279-291.
8. Anon: 'New material takes welding subsea', Marine Engineers Review, December/January 2003 p.53.
9. Hamasaki M, Sakakibara J and Arata Y: 'Underwater MIG welding using a wire brush nozzle'. Metal Construction, 1979 11(6) 288-289.
10. Hamasaki M, Sakakibara J and Arata Y: '"Wet" underwater MIG welding in deep water'. Proc. Int. Conf. on Welding Research in the l980's. Osaka, 27-29 Oct.1980. Publ. Osaka University Welding Research Institute, Osaka, Japan, Sept. 1980. Session B. Paper B-11. pp.61-66.
11. Kononenko V Ya and Rybchenkov: 'Experience with wet mechanised welding with self-shielding flux-cored wires in underwater repair of gas and oil pipelines' Paton Welding Journal 1994 6(9-10) 401-404.
12. Kononenko V Ya and Savich I M: 'Wet mechanised welding using self-shielded flux-cored wires in underwater pipelines repair' Proc. 2^nd Int. Pipeline Technology Conf., Ostend, Belgium, September 1995, R. Denys, Ed., Elsevier, 1995, Vol. II 73-78.
13. Lucas W: 'The flux-cored arc process for wet welding and cutting - an assessment', The Welding Institute Research Bulletin May/June 1997 38(3) 44-48.
14. Nixon J: 'Underwater repair technology', Woodhead Publishing Limited, 2000.