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6226 |
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Tuesday, September 17, 2019, Saal 3 12:00 PM Offshore |
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Unterwasserschweißen mit umhüllten Stabelektroden für Schiffbau- und Offshore-Anwendungen |
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Jan Hilkes* / voestalpine Böhler Welding UTP Maintenance GmbH, Deutschland Jürgen Tuchtfeld/ voestalpine Böhler Welding UTP Maintenance GmbH, Deutschland |
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Since a vast part of many Marine and Offshore constructions, such as pipelines, manifolds, jackets, corrosion preventing anodes, steel waterway constructions and so on are to a great extend underwater. Hence, it is obvious that welding as well as cutting activities for maintenance and repair is supposed to take place underwater as well. Shielded Metal Arc welding (SMAW) with covered stick electrodes is a very versatile, simple and practical welding process, for which reason often used for underwater maintenance and repairs. A covered stick electrode as the consumable and a power source is all that is required to be operational. Mind you, the power source has to be adapted and prepared for underwater welding to assure the safety aspects of such an underwater welding operation. Under water, welding can take place under two circumstances or conditions. The first one is directly in the water with wet diver-welders that dive with compressed air or an enriched air mixture, with which the depth and residence time is restricted to about 50m of water depth. The second option is dry hyperbaric welding with a chamber around the structure that needs welded upon, where the water has been pressed out to create a relatively dry environment. Hyperbaric welding can go to very deep water-depths of up to hundreds of meters. The hyperbaric welders are so-called saturation divers that remain up to weeks at the specific depth or under the same water pressure. Saturation here means that the blood of the welders saturates with the breathing gas mixture of Helium and a bit of Oxygen. This set up allows that GTAW can be applied to produce high X-ray quality welds in, for example, subsea pipelines. However, being underwater, the stick electrodes are exposed to one: a wet or very high humidity environment, the seawater and two: to an increased pressure caused by the water pressure, whereas every ten meter of water depth equals one Bar of pressure. In order for the stick electrodes not to take up moisture before or during operation in such a wet environment, a protective layer of paraffin, clear lacquer of PE is applied, that will melt or burn away gradually during the welding operation itself. To guarantee dry stick electrodes on site and to just before the welding operation, vacuum-sealed packaging is available to have the best assurance. To deal with the water pressure, the electrodes have to be designed to generate enough arc force to keep the surrounding water away from the electrical arc and to ensure sufficient positive penetration of the welds produced. This means sufficient gas forming elements as, for example, cellulose, as it is also being used for that purpose in pipe-welding electrodes. The increased pressure also influences the metallurgical processes in the electrical arc as for example the Manganese content reduces with increasing pressure (water depth). This implies the development of electrode coatings should account for these influences. It also means that the electrodes can or have be specifically designed for certain water-depths in order to meet the mechanical properties. In addition, one has to realize that due to water surrounding the work piece and welding, the cooling rates are faster and the t8/5 shortens, which has an influence on the mechanical properties including the Hardness. Since Hardness and Hydrogen do not go together, the design of the covered electrodes has to be suited to the circumstances to enable solid welds be produced under water. The presentation will cover the diving, welding and metallurgical aspects of wet underwater welding covered electrodes with industrial examples and applications. |