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Underwater Welding

It is the welding process done under the water with the help of two methods : Dry Welding and Wet Welding. The presentation provides basic knowledge on the underwater welding and it's respective techniques.

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Underwater Welding

  1. 1. Knowledge Corridor, Raisan, Gandhinagar-382007,Gujarat Underwater Welding AMIT K. SONCHHATRA (11BME081) ATHAR KOTHAWALA (11BME082) KARAN C. PRAJAPATI (11BME083) YUDHISTHIR RAMNANI(11BME084) RAJ ANUPAM (11BME085)
  3. 3. • The fact that electric arc could operate was known for over a 100 years. The first ever underwater welding was carried out by British Admiralty - Dockyard for sealing leaking ship rivets below the water line. • Underwater welding is an important tool for underwater fabrication works. • In 1946, special waterproof electrodes were developed in Holland by ‘Van der Willingen’.
  4. 4. CONTD. • In recent years the number of offshore structures including oil drilling rigs, pipelines, platforms are being installed significantly. • Some of these structures will experience failures of its elements during normal usage and during unpredicted occurrences like storms, collisions. Any repair method will require the use of underwater welding.
  5. 5. CONTD. • Underwater welding is the process of welding at elevated pressures, normally underwater. Underwater 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 underwater welding are diverse—it is oeen used to repair ships, offshore oil platforms, and pipelines. Steel is the most common material welded
  6. 6. CONTD. • Dry hyperbaric welding is used in preference to wet underwater welding when high quality welds are required because of the increased control over condiVons which can be exerted, such as through applicaVon 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
  7. 7. CONTD. • Research into using dry hyperbaric welding at depths of up to 1,000 metres (3,300 e) is ongoing. In general, assuring the integrity of underwater welds can be difficult (but is possible using various non-destructive testing applications), especially for wet underwater welds, because defects are difficult to detect if the defects are beneath the surface of the weld.
  8. 8. Classification 1.  Dry 2.  wet
  9. 9. DRY WELDING • 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. Gas tungsten arc welding is most commonly used.
  10. 10. CONTD. • The degradation is associated with physical changes of the arc behavior 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.
  11. 11. • 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.
  12. 12. CONTD. • 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.
  13. 13. • 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
  14. 14. ADVANTAGES 1)  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 environmental control system (ECS). 2)  Good Quality Welds - This method has ability to produce welds of quality comparable to open air welds because water is no longer present to quench the weld and H2 level is much lower than wet welds. 3)  Surface Monitoring - Joint preparation, pipe alignment, NDT inspection, etc. are monitored visually. 4)  Non-Destructive Testing (NDT) - NDT is also facilitated by the dry habitat environment.
  15. 15. DISADVANTAGES 1)  The habitat welding requires large quantities of complex equipment and much support equipment on the surface. The chamber is extremely complex. 2)  Cost of habitat welding is extremely high and increases with depth. Work depth has an effect on habitat welding. 3)  At greater depths, the arc constricts and corresponding higher voltages are required. 4)  The process is costly - a $ 80000 charge for a single weld job. One cannot use the same chamber for another job, if it is a different one.
  16. 16. WET WELDING • Simply means that job is performed directly in the water • It involves using special rod and is similar to the process in ordinary air welding
  17. 17. PRINCIPLE OF OPERATION • The process of underwater wet welding takes in the following manner: • The work to be welded is connected to one side of an electric circuit, and a metal electrode to the other side. • These two parts of the circuit are brought together, and then separated slightly. The electric current jumps the gap and causes a sustained spark (arc), which melts the bare metal, forming a weld pool. • At the same time, the top of electrode melts, and metal droplets are projected into the weld pool. During this operation, the flux covering the electrode melts to provide a shielding gas, which is used to stabilize the arc column and shield the transfer metal. •  The arc burns in a cavity formed inside the flux covering, which is designed to burn slower than the metal barrel of the electrode.
  18. 18. WET WELDING
  19. 19. PARAMETERS • Power supply : DC 300 OR 400 AMP • Polarity : straight polarity
  20. 20. EFFECT OF WET ENVIROMENT • WATER = HYDROGEN + OXYGEN • Dissolve in weld pool • Solubility decreases and then comes out = porosity • Oxygen as solid , liquid inclusions or gases • Hydrogen combines with oxygen forming vapor. V-groove wet weld deposited at 100 m depth (a) and its radiographic image
  21. 21. Formation of Hydrogen • 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 vapor shield. However, reaction with the molten metal influences the composition of this vapor and it comprises of about 70% hydrogen, 25% carbon dioxide and 5% carbon monoxide. • The risk of hydrogen induced cracking increases as welding depth increases in water (sea/ocean).
  22. 22. • D4301, D4313 and D4327 are electrode classification in JIS (Japanese Welding Society). • Actually these electrodes are used in arc welding • For underwater welding, special type of ilmenite coating is applied around the electrodes • Picture Courtesy: Kobelco Welding and Hyundai Welding Co. Ltd.
  23. 23. ADVANTAGES 1)  The versatility and low cost of wet welding makes this method highly desirable. 2)  Other benefits include the speed. With which the operation is carried out. 3)  It is less costly compared to dry welding. 4)  The welder can reach portions of offshore structures that could not be welded using other methods. 5)  No enclosures are needed and no time is lost in building. Readily available standard welding machine and equipments are used. The equipment needed for mobilization of a wet welded job is minimal.
  24. 24. DISADVANTAGES • Rapid quenching decreases impact strength and ductility. • Hydrogen embrittlement. • Poor visibility in water. • Higher energy density of hydrogen, higher efficiency.
  25. 25. REQUIREMENTS • Power supply requirements - 400 amp or larger. DC generators, motor generators and rectifiers are acceptable power supplies • Power converters. • Welding Generator, Pre-Setup • Polarity. • Diesel Driven Welding Generator Amperage and Voltage settings. • Gas Manifolds
  26. 26. Figure showing schematic diagram for underwater welding or cutting
  27. 27. DANGERS AND DIFFICULTIES • Hydrogen and oxygen are dissociated from the water and will travel separately as bubbles. • Oxygen cuung is about 60 percent efficient • Above river beds, especially in mud, because trapped methane gas in the proper concentrations can explode.
  28. 28. CONTD. • There is a risk to the welder/diver of electric shock. • There is a risk that defects may remain undetected • The other main area of risk is to the life or health of the welder/diver from nitrogen introduced into the blood steam during exposure to air at increased pressure
  29. 29. SAFETY MEASURES • Start cuung at the highest point and work downward • By withdrawing the electrode every few seconds to allow water to enter the cut • Gases may be vented to the surface with a vent tube (flexible hose) secured in place from the high point where gases would collect to a position above the waterline. •  Precautions include achieving adequate electrical insulation of the welding equipment •  Areas and voids must be vented or made inert.
  30. 30. DEVELOPMENTS • Wet welding has been used as an underwater welding technique for a long time and is still being used. • With recent acceleration in the construction of offshore structures underwater welding has assumed increased importance. • This has led to the development of alternative welding methods like friction welding, explosive welding, and stud welding. Sufficient literature is not available of these processes.
  31. 31. SCOPE • Wet welding is still being used for underwater repairs, but the quality of wet welds is poor and are prone to hydrogen cracking. •  Dry Hyperbaric welds are better in quality than wet welds. Present trend is towards automation. THOR - 1 (TIG Hyperbaric Orbital Robot) is developed where diver performs pipe fixing, installs the trace and orbital head on the pipe and the rest process is automated. • Developments of driverless Hyperbaric welding system is an even greater challenge calling for annexed developments like pipe preparation and aligning, automatic electrode and wire reel changing functions, using a robot arm installed. This is in testing stage in deep waters. • Explosive and friction welding are also to be tested in deep waters.
  32. 32. REFERENCES 1)  D. J Keats, Manual on Wet Welding. 2)  Annon, Recent advances in dry underwater pipeline welding, Welding Engineer, 1974. 3)  Lythall, Gibson, Dry Hyperbaric underwater welding, Welding InsVtute. 4)  W.Lucas, InternaVonal conference on computer technology in welding. 5)  Stepath M. D, Underwater welding and cuung yields slowly to research, Welding Engineer, April 1973. 6)  Silva, Hazlel, Underwater welding with iron - powder electrodes, Welding Journal, 1971. 7) 8) Amit Mukund Joshi, Underwater Welding, JRF, IIT-Bombay,