Underwater welding


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Underwater welding

  1. 1. 1. INTRODUCTION The fact that electric arc could operate was known for over a 100 years. The first ever underwaterwelding was carried out by British Admiralty – Dockyard for sealing leaking ship rivets below the waterline. Underwater welding is an important tool for underwater fabrication works. In 1946, special waterproofelectrodes were developed in Holland by ‘Van der Willingen’. In recent years the number of offshorestructures including oil drilling rigs, pipelines, platforms are being installed significantly. Some of thesestructures will experience failures of its elements during normal usage and during unpredicted occurrenceslike storms, collisions. Any repair method will require the use of underwater welding. The concept of underwater dry environment habitat welding on pipelines essentially began with theOsborn Patent of 1954, which described an underwater enclosure that was to become the predecessor ofwelding habitats used today. The patent was ahead of its time and received little attention until shortly beforeit expired. During the intervening seventeen years, marine pipelines advanced from marshland to open sea inever-increasing numbers following the first offshore well in 1947, 42 miles south of Morgan City, Louisiana. The technology of laying marine pipelines kept pace with the increasing water depths, even though pipediameters increased considerably. However, the technology of joining the large diameter flanged pipe on thesea bottom and the setting of long risers in deep water did not. These remained very time-consuming anddifficult tasks. Around 1966, companies that were involved in diving and related activities built varioustypes of underwater welding habitats and made a few repairs on offshore platforms and pipelines, andperformed a few hot taps. Because Taylors largest customer was, and still is, the Marine Pipeline Divisionof Brown & Root and since diving was also our business, we too began to consider habitat welding. 1
  2. 2. 2. CLASSIFICATION OF UNDERWATER WELDINGUnderwater welding can be classified as:1) Dry welding2) Wet Welding In wet welding the welding is performed underwater, directly exposed to the wet environment. In drywelding, a dry chamber is created near the area to be welded and the welder does the job by staying insidethe chamber.2.1 Hyperbaric Welding (dry welding) Hyperbaric welding is carried out in chamber sealed around the structure to be welded. The chamber isfilled with a gas (commonly mixture of helium and oxygen) at the prevailing pressure. The habitat is sealedonto the pipeline and filled with a breathable mixture of helium and oxygen, at or slightly above the ambientpressure at which the welding is to take place. This method produces high-quality weld joints that meet X-ray and code requirements. The gas tungsten arc welding process is employed for this process. The areaunder the floor of the Habitat is open to water. Thus the welding is done in the dry but at the hydrostaticpressure of the sea water surrounding the Habitat. Fig 1:- In Dry welding sealed chambers are used 2
  3. 3. Dry welding is of two types:1. Mini habitat welding2. Large habitat welding2.1.1 Mini-Habitat welding It makes use of a small, easily portable, gas-filled, often plexiglas enclosure, which is placed over thejoint by a diver. Water is displaced by an inert gas or air supplied from the surface. In this instance weldingis performed at elevated ambient pressures. Depending on the size of the enclosure, the diver is partiallyimmersed in water (only the divers hands and the welding torch are inside the habitat). This method requiresadequate visibility and is limited to areas with clear access. The diver/welder welds with each of severalMMA electrodes. Fig 2:- Mini habitat welding2.1.2 Large Habitat welding In this a specially designed chamber is built and positioned around the intended weld and thewelder/diver enters the chamber in order to undertake the work. The habitat is sealed and water is excludedby introducing an appropriate gas. Hyperbaric welding produces high-quality weld joints that meet X-ray and code requirements. Mostwelding processes can be operated at hyperbaric pressures, but all processes suffer a reduction in capabilityand efficiency results as the pressure increases. Hyperbaric welding, using MMA (SMA), TIG (GTA) are thepreferred processess for high integrity welds, particularly for deep water welds, including tie-ins in pipelinesand risers in the oil and gas industries, however GTA is the method most commonly employed forhyperbaric welding operations. 3
  4. 4. Fig 3:- large habitat welding2.2.3 Advantages of Dry Welding 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, etc. are monitored visually.2.2.4 Disadvantages of Dry Welding 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. At greater depths, the arc constricts and corresponding higher voltages are required. 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. 4
  5. 5. 2.2 Wet welding Wet Welding indicates that welding is performed underwater, directly exposed to the wet environment.A special electrode is used and welding is carried out manually just as one does in open air welding. Theincreased freedom of movement makes wet welding the most effective, efficient and economical method.Welding power supply is located on the surface with connection to the diver/welder via cables and hoses. Fig.4:- Showing a welder doing wet welding.In wet welding MMA (manual metal arc welding) is used.Power Supply used : DCPolarity : -ve polarity When DC is used with +ve polarity, electrolysis will take place and cause rapid deterioration of anymetallic components in the electrode holder. For wet welding AC is not used on account of electrical safetyand difficulty in maintaining an arc underwater.The power source should be a direct current machine rated at 300 or 400 amperes. Motor generator weldingmachines are most often used for underwater welding in the wet. The welding machine frame must begrounded to the ship. The welding circuit must include a positive type of switch, usually a knife switchoperated on the surface and commanded by the welder-diver. The knife switch in the electrode circuit mustbe capable of breaking the full welding current and is used for safety reasons. The welding power should beconnected to the electrode holder only during welding. Direct current with electrode negative (straight polarity) is used. Special welding electrode holders withextra insulation against the water are used. The underwater welding electrode holder utilizes a twist typehead for gripping the electrode. It accommodates two sizes of electrodes. 5
  6. 6. The electrodes must be waterproofed. All connections must be thoroughly insulated so that the watercannot come in contact with the metal parts. If the insulation does leak, seawater will come in contact withthe metal conductor and part of the current will leak away and will not be available at the arc. In addition,there will be rapid deterioration of the copper cable at the point of the leak.2.2.1 Principle of operation of Wet Welding The work to be welded is connected to one side of an electric circuit, and a metal electrode to the otherside. These two parts of the circuit are brought together, and then separated slightly. The electric currentjumps the gap and causes a sustained spark (arc), which melts the bare metal, forming a weld pool. At thesame time, the tip of electrode melts, and metal droplets are projected into the weld pool. During thisoperation, the flux covering the electrode melts to provide a shielding gas, which is used to stabilize the arccolumn and shield the transfer metal. The arc burns in a cavity formed inside the flux covering, which isdesigned to burn slower than the metal barrel of the electrode.3. EFFECT OF UNDERWATER ENVIRONMENT Although the quality of wet weld has improved, mechanical properties are still not equal to thoseexhibited by dry welds. There is an increased occurrence of defects, such as porosity and inclusions.Ductility and toughness are not equal to those of dry welds due to porosity as well as changes in thechemical composition and microstructure caused by the wet environment. When water is exposed to high temperatures in the welding arc, it decomposes into oxygen andhydrogen which dissolve into the weld pool. As molten iron cools, the solubility of oxygen and hydrogendecreases. Oxygen can come out of solution in the form of solid or liquid oxide inclusions, or gases, whichcan cause porosity. Hydrogen can form pores of molecular hydrogen, react with oxygen to form watervapour, or cause embrittlement and cracking after the metal cools. As underwater depth increases, thehydrostatic pressure increases at a rate of 1 atm for every 33 ft (10m). For a given mole fraction, the partialpressure of a gas increases as the total pressure increases. Therefore for a constant composition of gases inthe arc atmosphere, the activities of oxygen and hydrogen above the weld pool increase with depth. Theresult is a change in chemical composition, microstructure and porosity content of the weld metal withdepth.3.1 Effect of depth on chemical composition. Elements with an affinity for oxygen are increasingly partitioned from the weld metal to the slag andoxide inclusion with depth. Volume present of oxide inclusion in weld metal increases with weld metaloxygen content as seen in figure. Therefore it can be concluded that oxide inclusion content of underwaterweld increases with depth. The net effect of increasing depth is a decreases in hardenability due todecreasing alloy element content and increasing oxygen content. 6
  7. 7. Fig 5 :-Effects of underwater depth on weld metal composition( Ibrra et al 1987 ).Fig 6:- Effect of underwater depth on weld metal oxygen content (Ibarra et al 1987) 7
  8. 8. 3.2 Effect of depth on Porosity Porosity in underwater wet weld is caused by hydrogen, techniques to reduce the hydrogen content ofwet should also be effective at reducing porosity. Increasing the calcium carbonate content of a rutile-baseelectrode coating has been shown to reduce porosity in wet welds. Sanchez-Osio et al [1995] increasedcalcium carbonate from 9 to 12.5 pct., which decreased porosity from 2.2 to 1.0 pct at a depth of 30 ft(9m).Carbonates decompose to form carbon dioxide and carbon monoxide in the arc, reducing the partial pressureof hydrogen, and thus reducing the amount of hydrogen absorbed into the weld pool.Fig 7:- Effect of underwater depth on porosity. (Suga and Hasui 1986) 8
  9. 9. 3.3 Effect of depth on microstructure. The loss of alloying elements with depth alters the microstructure and causes decreased strength andtoughness. The microstructure of wet welds consists mainly of coarse primary ferrite(PF) and ferrite withaligned carbides(FS)(Ibarra 1987). In contrast, surface welds frequently contain large fraction of acicularferrite, which is preferred due to the resistance of acicular ferrite to cleavage fracture. Addition of titaniumand boron to wet welding electrodes has been shown to produce up to sixty pct acicular ferrite in themicrostructure at a depth of 30 ft(9m).3.4 Effect of depth on mechanical properties Few investigations have focused on the increasing depth on mechanical properties. The effect of depthon tensile strength and charpy toughness are presented in figure 8. Each set of data points corresponds towelds made with the same type of SMA welding electrode. Tensile strength and toughness decreases in anapproximately linear with depth to 330ft(110m). Changes in microstructure, chemical composition, andporosity undoubtedly contribute to the decline in mechanical properties.3.5 Effect of alloying elements on weld metal microstructure and properties.3.5.1 Manganese Increasing the Mg content of low carbon manganese steel weld metal results in micro structuralrefinement through an increase in hardinability. As shown in the figure manganese contents of underwaterwet welds are typically less than 0.5 wt. pct. The manganese content will lead to formation of acicular ferritein the as deposited microstructure and refinement in the reheated microstructure by increasing thehardinability.3.5.2 Titanium and boron Several investigations have shown that it is possible to produce weld metal microstructure high inacicular ferrite by alloying with titanium and boron. By adding titanium alone we can produce 70 pct acicular ferrite. Boron and titanium when addedtogether enhances the formation of acicular ferrite. Titanium protects boron from reacting with oxygen andnitrogen and forms fine intragranular inclusions of titanium oxide and nitride. Boron is then free to diffuse toaustenite grain boundaries, where it reduces the grain boundary energy, and retards the formation of grainboundaries ferrite. Finally the titanium rich intragranular inclusions provide nucleation sites for acicularferrite.3.5.3Rare earth metals To reduce oxygen content as REMs are some of the strongest deoxidents. They form more stable oxidesthan titanium and manganese. Efimeko(1980) found that there was a maximum in room toughness with anaddition of .2 to .4 pct metallic REM(yttrium) to the coatings of welding electrodes. Addition of REM tounderwater wet welds should improve recovery of alloying elements. A reduction in oxygen content ofwelds should be beneficial to upper shelf toughness. 9
  10. 10. Fig 8:- Chemical composition and mechanical properties of steel wet welds as a function of depth 10
  11. 11. 3.4 Advantages of Wet Welding Wet underwater MMA welding has now been widely used for many years in the repair of offshoreplatforms. The benefits of wet welding are: - 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. Readily available standard welding machine and equipments are used. The equipment needed for mobilization of a wet welded job is minimal. Fig 9:- showing the wet welding process and freedom for welder. 11
  12. 12. 3.5 Disadvantages of Wet Welding Although wet welding is widely used for underwater fabrication works, it suffers from the followingdrawbacks: - 1. There is rapid quenching of the weld metal by the surrounding water. Although quenching increases the tensile strength of the weld, it decreases the ductility and impact strength of the weldment and increases porosity and hardness. 2. Hydrogen Embrittlement – Large amount of hydrogen is present in the weld region, resulting from the dissociation of the water vapour in the arc region. The H2 dissolves in the Heat Affected Zone (HAZ) and the weld metal, which causes Embrittlement, cracks and microscopic fissures. Cracks can grow and may result in catastrophic failure of the structure. 3. Another disadvantage is poor visibility. The welder sometimes is not able to weld properly. 12
  13. 13. 4. RECOMMENDED EQUIPMENT FOR UNDERWATER WELDING AND CUTTING Category Item Diving equipment MK 12, MK 1 or Superlite 17B Surface Supported Diving System with Welding shield and assorted welding lens Heavy-duty rubber gloves and Playtex-type or surgical gloves Communications System Cutting and welding equipment DC welding power source 400 amperes Safety switch rated at 400 ampers, 250 volts Welding cables, sizes 1/0 and 2/0 with bolt-together cable connectors and necessary bolts and nuts “C”-type ground clamp Amperage tong meter Electrician’s DC volt meter Cutting equipment Underwater cutting torches Oxygen hose Manifold to connect the cylinders; if more than one is to be used 2-stage, high flow, high volume regulator Leak detector - LEAK-TEC or soap suds Welding equipment Electrode holder Weighted wire brush Chipping hammer Scraper Ultrasonic thickness and flaw detection device Dye Penetrant test kit 13
  14. 14. 5. REQUIREMENT REASONS FOR UNDERWATER WELDING5.1 Power Supply Requirements. The preferred power supply for underwater cutting and welding is a 400 amp or larger, engine drivenDC welding generator with a minimum of 60 percent duty cycle. The generator shall have independentvoltage and amperage controls. A welding power source with a minimum capacity of 300 amperes isacceptable, however, cutting time is considerably longer whenever power is reduced. 400 amp and over arerequired for some operations. DC generators, motor generators and rectifiers are acceptable power supplies.In an emergency, a 200-ampere machine set for peak or near-peak load may be sufficient for short periods oftime; rectified or motor driven type machines may also be used.5.2 Power converters. Power converters are just what the name implies; the input alternating current (AC) primary power ispassed through a circuit breaker to a rectifier, where the input is transformed to DC power. Portable powerconverters, though somewhat limited, are particularly useful for salvage work when ship or shore power isavailable. Power converters should not be used when the work is deeper than 100 FSW or with more than200 feet of welding lead. To work beyond these boundaries would overwork the machine. This type machinewill support cutting operations using the exothermic technique for short periods of time, however, for longburning jobs where power is to be used, a welding generator is recommended. This type of unit will notaccommodate cutting operations using the steel-tubular electrode. When using electric-driven machines,ensure that the primary power supply cables are laid separately and away from welding supply cables. Motorgenerator power sources consume large amounts of energy during their entire running period. A weldingrectified power supply will require significant energy only during the welding cycle. This may be animportant consideration when away from normal logistic support.5.3 Welding Generator, Pre-Setup Inspection. Successful underwater welding and cutting is highly dependent on the efficient running of the powersupply unit. Before any underwater work takes place, the welding power source should be fully inspected byqualified personnel. The commutators on motor-generators should be clean. Brushes must not be excessivelyshort or worn. Slack brush rigging springs must be replaced. Operation of a welding generator beyond itscapacity and duty cycle will melt the soldered rotor winding connections to the commutator. This will reducethe generator’s capacity during future use. If the generator is incapable of achieving rated capacity for itsduty cycle, then the interior of the housing should be checked for traces of soldered deposits. All powersources should be checked for confirmation of rated capacity before use. Motor generators designed solelyfor use with semi-automatic welding equipment are not suitable for underwater applications. 14
  15. 15. 5.4 Polarity. Underwater cutting and welding operations are usually performed with DC ELECTRODE NEGATIVE,i.e., STRAIGHT POLARITY. A typical equipment arrangement is illustrated in Figure 10. The cable fromthe electrode holder is connected to the negative (-) terminal of the DC power supply and the positive (+) orground cable is conncted to the work. The majority of electrode manufacturers call for a DC, ELECTRODENEGATIVE setup. Occasionally, there may be a requirement for reverse polarity, sometimes referred to asDC, ELECTRODE POSITIVE for a particular electrode or for improved welding. This will normally be atthe recommendation of the electrode manufacturer and will be printed on the electrode box or included inaccompanying literature. Fig 10:- A schematic representation of a welding circuit with straight polarity.5.5 Diesel Driven Welding Generator Amperage and Voltage settings. In addition to setting up for the desired polarity, the machine must also be set for the correct amperageand voltage. Setup procedures for welding machines vary according to the type of machine and according tothe manufacturer. Therefore, the manufacturer’s operating manuals and instructions for the particularmachine should be consulted and closely followed. Most diesel driven welding generators have two controldials, one for adjusting desired amperage and one for voltage adjustment. There are several types of weldinggenerators in use throughout the Navy and the commercial industry. The overall functions and operatingsetup are essentially the same. 15
  16. 16. 5.6 Gas Manifolds. Manifolds, sometimes referred to as coupler blocks, are used for the purpose of connecting two or morecylinders of the same kind of gas to discharge all of the cylinders through one regulator. The use ofmanifolds is desirable when it is necessary to furnish an uninterrupted supply of gas or when it is needed tosupply gas at greater rates than can be supplied from a single cylinder.5.7 Underwater Oxygen-Arc Welding Torches. Underwater oxygen arc welding/cutting torches are designed for gripping the cutting electrodes and fordelivering power to the electrode and oxygen to the electrode bore. Safety is a primary consideration in theirdesign and construction. All current-carrying parts of the torches must be fully insulated to afford fullprotection to the diver from electric shock and to protect the torch from rapid corrosion due to electrolysis.The torches also should be lightweight, simple in construction, easy to maintain and should incorporate anelectrode clamping device which will make the changing electrodes underwater a relatively simple task.Only torches which have been designed specifically for underwater cutting shall be used. The torches mustalso have a sufficient capacity for the maximum current required by the electrodes with which the torch is tobe used. The elements of an oxygen arc cutting torch are:1. Collet or grip which holds tubular cutting electrodes and provides for entry of oxygen into the cutting electrode bore.2. Oxygen valve for controlling oxygen flow rate.3. Electrical connector for attaching a power cable.4. Flash arrester to prevent hot metal particles from entering the torch.5. Insulated coupling between the electrode and the oxygen valve to safeguard the operator from electric shock and to prevent damage of the valve due to electrolysis.6. Complete insulation for all current-carrying metal parts of the torch to safeguard the operator against electric shock and to protect the metal parts from deterioration by electrolysis.5.8 Compressed gas supplies Oxygen is used in the oxygen arc welding/cutting process. The gas is stored in high pressure cylindersand delivered at a reduced rate via the regulator to the cutting torch. Of all the high pressure gases handledduring diving operations, oxygen is the most hazardous. This is true because it lowers the ignitiontemperature of flammables and greatly accelerates combustion. Hydrocarbons ignite almost spontaneously inthe presence of oxygen and oxygen fires create intense heat. The following paragraphs describe precautionsto be followed when handling compressed gases. MAPP gas is a mixture of stabilized methylacetylenepropidene.Acetylene is very unstable at pressures above 15 psi and is NOT used for underwater cutting. 16
  17. 17. MAPP Gas cutting is the only oxygen-fuel underwater cutting process approved for use by the Navy. Itis used only when oxy-arc cutting equipment is not available or when there is an absence of sufficientelectric power to conduct oxy-arc cutting. MAPP gas is a mixture of stabilized methylacetylene propidene.MAPP gas cutting is performed with a gas torch rather than with a cutting electrode. The technique forunderwater cutting is exactly the same as oxygen-acetylene cutting topside. However, a great deal moreskill is required using the MAPP gas process as opposed to Oxy-Arc underwater cutting. Like Kerie Cable,this process is useful in ship salvage operations especially in areas where stringing long electrical powerleads is difficult. Also, the necessary equipment can be carried in a small boat. The standard acetyleneoxygen cutting torch is approved for Navy use. A spacer sleeve is attached over the cutting tip which aidesthe diver in maintaining the proper stand-off from the metal to be cut.5.9 Waterproofing Surface Electrodes. Unlike commercially manufactured wet welding electrodes, it is necessary to waterproof the fluxcoating of surface welding rods. This is accomplished by dipping the electrode in one of the suitablewaterproofing solutions listed in fig 7. One or two dips may be required, depending on viscosity ofwaterproofing material. Be sure to completely cover all of the flux coating. One or two dips are requiredbecause when the waterproof coating is inadequate, water forced into the interstices will turn to steam whenthe arc is struck and will blow off the coating. The waterproofing coatings must be thoroughly dry beforeapplying additional coatings and also before they are used. The grip ends of the electrodes should be cleanedto prevent the waterproofing material from interfering with electrical contact between the electrodes andholder. It is desirable to send the diver only a few electrodes at a time since waterproofing protects theelectrode covering for a limited amount of time. Fig 11 :- Electrodes suitable for wet welding and water proofing materials. 17
  18. 18. 6. PRINCIPAL OF OPERATION OF CUTTING The principle of cutting metal underwater with a gas torch is virtually the same as that employed topside, except that the acetylene has been replaced with MAPP gas. The torch mixes the MAPP gas and oxygen which burn and generate sufficient heat to melt the metal to be cut. A small area on the metal is heated to a molten puddle and then oxygen is directed at that point. The oxygen instantly converts the molten puddle into a gaseous and chemical state, while simultaneously blowing it away. The flame is angled slightly in order to preheat the metal ahead of the cut, thus allowing the cut to continue.7. DANGERS AND DIFFICULTIES FACED IN UNDERWATER WELDING AND CUTTING. Underwater cutting and welding processes generate explosive gases.1. When cutting with power on or welding, hydrogen and oxygen are dissociated from the water and will travel separately as bubbles. These bubbles can collect in a trapped or confined space overhead. As the hydrogen and oxygen gases combine they will ignite, causing a popping sound.2. Oxygen cutting is about 60 percent efficient, resulting in approximately 40 percent pure oxygen being released into the environment. This gas can become entrapped above the work area, and when combined with a fuel such as hydrocarbons, can easily be ignited by a hydrogen bubble or a spark trapped in the bubble. Any pop is a sign of explosive gases collecting above the underwater work area and is the point when cutting or welding must stop and the cause investigated. Prior to the start of any underwater cutting or welding, as built drawings and physical configuration of the work area must be studied to determine all these areas and voids that could contain or trap explosive gases. These areas and voids must be vented or made inert. Care should also be taken when cutting or welding on enclosures that are on or above river beds, especially in mud, because trapped methane gas in the proper concentrations can explode. Explosive gases may be produced by any one or a combination of the following: 1. Petroleum products such as gasoline, fuel oil or greases 2. Paint mixing mediums, such as linseed oil or thinners 3. Epoxies, adhesives and solvents 4. Ammunition or bulk explosives 5. Decaying vegetable or animal matter 6. Unburned gases from cutting torches3. There is a risk to the welder/diver of electric shock.4. Precautions must be taken to avoid the build-up of pockets of gas, which are potentially explosive. 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. Precautions include the provision of an emergency air or gas supply, stand-by divers, and decompression chambers to avoid nitrogen narcosis following rapid surfacing after saturation diving. 18
  19. 19. 5. For the structures being welded by wet underwater welding, inspection following welding may be more difficult than for welds deposited in air. Assuring the integrity of such underwater welds may be more difficult, and there is a risk that defects may remain undetected.8. Safety measures Every precaution must be taken to prevent an underwater explosion. To minimize the possibility of explosions from trapped gasses, the following procedures are recommended: 1. Start cutting at the highest point and work downward. 2. When cutting thick material, i.e., propeller shafting, cut from the outside and work around the circumference. By withdrawing the electrode every few seconds to allow water to enter the cut, exceedingly high temperatures can not build up inside the metal. 3. A brushing or stroking action in the direction of the intended cut should be used. 4. 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. 5. When working under a sea chest, gas can be vented by briefly opening an internal valve or by the above method. 6. Precautions include achieving adequate electrical insulation of the welding equipment, shutting off the electricity supply immediately the arc is extinguished, and limiting the open-circuit voltage of MMA (SMA) welding sets. Secondly, hydrogen and oxygen are produced by the arc in wet welding. 19
  20. 20. 9. DEVELOPMENTS IN UNDERWATER WELDING 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 assumedincreased 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.9.1 Scope for further developments Wet MMA is still being used for underwater repairs, but the quality of wet welds is poor and is prone tohydrogen cracking. Dry Hyperbaric welds are better in quality than wet welds. Present trend is towardsautomation. THOR – 1 (TIG Hyperbaric Orbital Robot) is developed where diver performs pipefitting,installs the trac and orbital head on the pipe and the rest process is automated.Developments of diverless Hyperbaric welding system is an even greater challenge calling for annexedevelopments like pipe preparation and aligning, automatic electrode and wire reel changing functions, usinga robot arm installed. This is in testing stage in deep waters. Explosive and friction welding are also to betested in deep waters. 20
  21. 21. 10. CONCLUSION 1. The chemical composition of steel wet welds can be controlled by addition a strong deoxidant, such as titanium, to the system. 2. The proportion of acicular ferrite in the as deposited microstructure was significantly increased through titanium-boron additions( 60-80%AF) over welds with only ferro additions(10-20%AF) over the range of depths tested, 70 to 300 ft(21 to 91m). 3. Addition of titanium and boron, which led to refinement of the as deposited weld metal, also refined the ferrite grain size of the reheated weld metal through a memory effect. 4. Combined additions of titanium, boron, and manganese increased the tensile strength of the steel wet welds due to microstructural refinement and increased hardenability. 5. Upper shelf toughness of wet welds is most likely controlled of the concentrations of defects, such as pores, micro cracks, and oxide inclusions. 21
  22. 22. 11. REFERENCES1) 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 Institute.4) W.Lucas, International conference on computer technology in welding.5) Stepath M. D, Underwater welding and cutting yields slowly to research, Welding Engineer, April 1973.6) Silva, Hazlett, Underwater welding with iron – powder electrodes, Welding Journal, 1971. 22