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Underwater Welding by Diver
The concept of conducting underwater welding involves welding performed below the water surface at a certain depth, in a dry or wet environment. Wet underwater welding implies that the welding process is carried out directly in the water without any kind of insulation barrier to prevent the contact of the ambient water with the place of work, the weld pool, the electric arc, the filler material and the welder. It is clear that in such conditions there are a number of parameters that considerably hamper the actual welding process and also affect the quality of welded joints. In case of dry underwater welding there is no direct contact of surrounding water on electric arc and weld pool as it is divided by mechanical barrier which ensure dry environment under atmospheric or hyperbaric pressure, depending on water depth and object shape or type involved. The search for offshore hydrocarbons has taken the oil industry into increasingly deep water. Over the past decade activities have gone beyond the continental shelf in the Gulf of Mexico, Brazil, West Africa, northwest Europe and the Mediterranean Sea. Oil is now being produced from fields far above 1000 m water depth, with field developments in progress to double these depths. Importance of the underwater welding and inspection technology is well proven in numerous cases of installation, repair and maintenance of naval objects. Moreover, as exploitation of oil and gas seems to move in deep waters, technical level of those techniques demands further investments and development. Although it is clear that automation is unavoidable, conventional diving is irreplaceable in certain activities and following that, great importance is on the field of education and training of human resources. Also, it is obvious that development level reached the point where application of these technologies is possible and applicable in different situations. It is well known that in last 30 years, number of projects was profiled in order to develop underwater technology in «fit-forservice» level. Some of that projects gave good results, but number of projects collapsed due to large cost and poor flexibility. Underwater wet welding technique was misunderstood for a long time, and it was a synonym for low quality weld full of porosity and cracks with poor mechanical properties like low ductility and due to micro structural issues prone to cracking. This lack of experience and knowledge was present in companies which did not understood all underwater welding issues which caused development of inadequate welding procedures, poor welder technique and inappropriate filler materials. Through time, that status has been changed, and today underwater welding projects, both dry and wet, are used in most complex and difficult objects with a high level of quality assurance
Classification of underwater welding
1. Underwater wet welding
Underwater wet welding is flexible and applicable on various types and shapes of underwater structures. Diver-welder and electric arc are in direct water environment which causes number of negative impact factors not only for weld quality but also for welder safety. These problems appear proportionally with depth and therefore the depth is a limiting factor when taking in account wet welding procedures operational accessibility. On the other hand, equipment and other technical facilities are far more complex and cheaper comparing to underwater dry welding procedures so very often underwater wet welding is proper technology to use for maintenance of underwater structures and repair of ships.
2. Underwater dry welding
Comparing to underwater wet welding which is primarily used at shallow depths when need for larger depths emerge underwater dry welding procedures is first choice. Moreover, usage of underwater dry welding methods offers completion of full penetration welds with mechanical properties adequate to welding in normal conditions. Additionally, it is possible to perform preheating or post weld heat treatment in order to decrease hydrogen content and improve weld properties.
Application of underwater dry welding can arrange normal open air visibility and communication between welders and other workers greatly enhance work progress and safety. Further, there are better conditions for surface cleaning and weld preparation as well for maneuverability and manipulability of equipment and working parts. Due to good working conditions preparation and welding time is shortened in a great deal comparing to underwater wet welding. Regarding choice of welding processes, in this case beside MMAW which is most often used, FCAW-flux cored arc welding and GMAW- gas metal arc welding as well as TIG- tungsten inert gas welding also come into perspective depending on welding but also on depth requirements. As for problems normally expected using underwater wet welding like brittle microstructure prone to cracking in this case this problems are completely avoided. On the other hand, usage of underwater dry welding demand for significant time and financial resources just for preparation of underwater habitats, manipulating equipment like cranes and also other technical resources. Comparing to underwater wet welding procedures larger working crew is needed. Very often habitats can be used only for one type or shape of underwater structure because of positioning and sealing demands. However, underwater dry welding is widely used in maintenance and erection of underwater structures.
Application of underwater dry welding can arrange normal open air visibility and communication between welders and other workers greatly enhance work progress and safety. Further, there are better conditions for surface cleaning and weld preparation as well for maneuverability and manipulability of equipment and working parts. Due to good working conditions preparation and welding time is shortened in a great deal comparing to underwater wet welding. Regarding choice of welding processes, in this case beside MMAW which is most often used, FCAW-flux cored arc welding and GMAW- gas metal arc welding as well as TIG- tungsten inert gas welding also come into perspective depending on welding but also on depth requirements. As for problems normally expected using underwater wet welding like brittle microstructure prone to cracking in this case this problems are completely avoided. On the other hand, usage of underwater dry welding demand for significant time and financial resources just for preparation of underwater habitats, manipulating equipment like cranes and also other technical resources. Comparing to underwater wet welding procedures larger working crew is needed. Very often habitats can be used only for one type or shape of underwater structure because of positioning and sealing demands. However, underwater dry welding is widely used in maintenance and erection of underwater structures.
Underwater dry welding alternatives
Depending on water depth, shape and type of underwater structure as well as ease of arranging other
necessary facilities underwater dry welding could be performed in several options.
necessary facilities underwater dry welding could be performed in several options.
Dry welding at one atmosphere
Dry underwater welding in cofferdam is characterized by the fact that the welder and the welding site are in dry atmosphere, without the presence of water. To isolate the working site from water steel, or more recently aluminum box-like construction is used, which has an exit towards the sea surface, so that the activities are performed at the atmosphere pressure, thus eliminating the diving problem and influence of increased pressure on the welding parameters.
Cofferdam is set along and fixed to the structure where the works are being carried out. Before underwater assembly, a seal of polymeric foam is adhesion bonded to the contact edge of the cofferdam, and it adheres to the contact surface and prevents water breakthrough. After setting and fixing the cofferdam, the water is pumped out through a special system of valves, necessary safety measures are carried out, and the welders who work in dry atmosphere are lowered through an entry shaft into the cofferdam. Special problem in working in such a narrow confined space is the occurrence of vapors and gases which may have an explosive character, so that it is necessary to solve an adequate system of ventilation and fresh air supply.
The main advantage of such welding is better quality of weld related to wet underwater welding, flexibility in the selection of the procedure and the welding technology and higher working safety. The drawbacks are additional costs of realization and positioning of cofferdam, as well as problems that occur in structure sealing. This concept is often used in practice and there are a number of examples where this technology has been successfully implemented. After installing the cofferdam it is necessary to insure a surveillance system or to install adequate sensors to detect water leakage, and it is necessary to install a system for pumping out water. Very often, the problems of good sealing are present in practice. This indicates that the very design and construction of the cofferdam represent an additional cost and investor’s engagement.
The main advantage of such welding is better quality of weld related to wet underwater welding, flexibility in the selection of the procedure and the welding technology and higher working safety. The drawbacks are additional costs of realization and positioning of cofferdam, as well as problems that occur in structure sealing. This concept is often used in practice and there are a number of examples where this technology has been successfully implemented. After installing the cofferdam it is necessary to insure a surveillance system or to install adequate sensors to detect water leakage, and it is necessary to install a system for pumping out water. Very often, the problems of good sealing are present in practice. This indicates that the very design and construction of the cofferdam represent an additional cost and investor’s engagement.
On the other hand, the application of underwater welding in dry atmosphere substantially increases the productivity of welding, providing a selection of different welding and cutting procedures, and the weldquality is improved. From the aspect of welding, it is very positive that more productive cutting and welding procedures may be applied compared to wet underwater welding. Apart from MMAW procedure, MAG procedure and flux cored wire welding can be considered. However, MMAW procedure is recommended because of the problems that occur due to the separate wire feeder and contamination of the working ambient by shielding gas. Regarding the composition and the thickness of material, gas cutting and air plasma cutting are suitable, the latter being especially favorable since supply of dangerous gases (flammable gas and oxygen) into the cofferdam is thus avoided.
In case when cofferdam is used, the diving activities are implemented in the preparation of works, and during installation and dismantling of the cofferdam. However, it is necessary, during the works, to maintain the continuity of control activities from the outside, i.e. by visual control which is done by the diver, so as to be able to identify on time the damaged seals or elements of the positioning and fixing system of the cofferdam.
In case when cofferdam is used, the diving activities are implemented in the preparation of works, and during installation and dismantling of the cofferdam. However, it is necessary, during the works, to maintain the continuity of control activities from the outside, i.e. by visual control which is done by the diver, so as to be able to identify on time the damaged seals or elements of the positioning and fixing system of the cofferdam.
Figure 1: Preview of usual cofferdam configuration for ship hull repair with entrance shaft from upper side
Dry welding in a habitat
Welding at ambient pressure in a large chamber from which water was
displaced and where such atmosphere is achieved that welder has no need to use diving equipment. As it is shown on the figure 11 welders are completely in dry environment and weld properties are equivalent to one welded in normal conditions. However, much more fit-up time is necessary to fix the habitat and prepare it for welding.
Figure 2: Specially designed habitat for repair of K-nodeon offshore platform
Dry chamber welding
Welding at ambient pressure in a simple open bottomed dry chamber that at
least accommodates the head and shoulders of a diver-welder in full diving equipment. Welder-diver is partly immersed in water but welding is performed in a dry atmosphere. Habitat is smaller and less
complex than in case of dry habitat welding. Due to smaller size of habitat other operating facilities are also less expensive.
least accommodates the head and shoulders of a diver-welder in full diving equipment. Welder-diver is partly immersed in water but welding is performed in a dry atmosphere. Habitat is smaller and less
complex than in case of dry habitat welding. Due to smaller size of habitat other operating facilities are also less expensive.
Dry spot welding
Welding at ambient pressure in a small, transparent, gas filled enclosure with the diver-welder outside in the water. Welding process itself is performed in dry atmosphere and welder is situated outside of welding area and physically divided by transparent barrier. Equipment is much smaller than in case of dry habitat or chamber welding but problems are possible due to welding fumes and visibility.
Figure 4: The Hydrobox system for riser repair
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