UNDER WATER WELDING

INTRODUCTION

             Welding is an important process of modern engineering – in all branches. It is used in fabrications and erections in infrastructures and installations. It joins metals or thermoplastics. Forming a pool of molten mass, the weld puddle, and allowing it to cool to become a strong joint is the basis of the process of welding. For repairing to be carried out underwater, there is a separate process. That is called underwater welding. If damaged ships are to be repaired, underwater welding is the basic technology to be used. It is a highly-specialized profession – more employed in the oil or shipping industry and also in the defense operations.

             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’. In recent years the number of offshore structures including oil drilling rigs, pipelines, and 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.

WELDING

            Welding is a process of joining metallic components with or without application of heat, with or without pressure and with or without filler metal. A range of welding processes have been developed so far using single or a combination above factors namely heat, pressure and filler.

2.1 classification of welding process:

            Welding processes can be classified on the basis of following technological criteria:

• ·         Welding with or without filler material
• ·         Source of energy for welding
• ·         Arc and non-arc welding
• ·         Fusion and pressure welding

UNDERWATER WELDING

                 Underwater welding is a process whereby metals are melted together underwater to either repair a structure or create a new structure. Used on oil wells, ships, and other underwater structures, underwater welding is done by one of two methods. The first is dry welding or hyperbaric welding, in which a structure is created around the weld and a pressurized environment created. The second is wet welding or arc welding, in which the welding electrode contains a flux coating that releases gases to preserve the integrity of the weld. Because of the dangers of shock, explosion and poisoning, underwater welding is only performed by professionals with both diving and welding certification. 

Classifications:

Underwater welding can be classified as:

• 1.      Wet Welding
• 2.      Dry Welding

In wet welding the welding is performed underwater, directly exposed to the wet environment. In dry welding, a dry chamber is created near the area to be welded and the welder does the job by staying inside the chamber.

3.1 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. The increased 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.

3.1.1 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 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 tip 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.

3.1.2 Working:

In wet welding MMA (manual metal arc welding) is used.

Power Supply used: DC

Polarity: -ve polarity

           When DC is used with +ve polarity, electrolysis will take place and cause rapid deterioration of any metallic components in the electrode holder. For wet welding AC is not used on account of electrical safety and difficulty in maintaining an arc underwater.

           The power source should be a direct current machine rated at 300 or 400 amperes. Motor generator welding machines are most often used for underwater welding in the wet. The welding machine frame must be grounded to the ship. The welding circuit must include a positive type of switch, usually a knife switch operated on the surface and commanded by the welder-diver. The knife switch in the electrode circuit must be capable of breaking the full welding current and is used for safety reasons. The welding power should be connected to the electrode holder only during welding. Direct current with electrode negative (straight polarity) is used. Special welding electrode holders with extra insulation against the water are used. The underwater welding electrode holder utilizes a twist type head for gripping the electrode. It accommodates two sizes of electrodes.

            The electrode types used conform to AWS E6013 classification. The electrodes must be waterproofed. All connections must be thoroughly insulated so that the water cannot come in contact with the metal parts. If the insulation does leak, seawater will come in contact with the 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.

                 In underwater welding the arc does not behave as in air. The activity of the gas bubbles being particularly important, as this tends to create a rather unstable arc condition, compared with surface welding, together with a somewhat more confusing weld puddle, which must be mastered by the diver before successful welding can take is no difference between surface MA welding and underwater wet-stick welding. Place. Apart from this, with regard to the actual physical principles of operation, there both processes use basically the same equipment with the exception of necessary waterproofing for the electrodes and certain other safety equipment.

(Figure 1: Shielding of the welding arc and molten pool with a covered stick electrode)

The electrodes themselves may be either carbon manganese (C/Mn) or mild steel if you prefer, and stainless steel (duplex).With the rutile mild steel electrodes being the most widely used, but more about electrodes later.

3.1.3 Advantages of Wet Welding:

·         The versatility and low cost of wet welding makes this method highly desirable.

·         Other benefits include the speed. With which the operation is carried out.

·         It is less costly compared to dry welding.

·         The welder can reach portions of offshore structures that could not be welded using the other methods.

·         No enclosures are needed and no time is lost building. Readily available standard welding machine and equipments are used. The equipment needed for mobilization of a wet welded job is minimal.

3.1.4 Disadvantages of Wet Welding:

·         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.

·         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.

·         Another disadvantage is poor visibility. The welder sometimes is not able to weld properly.

·         The amount of voltage that can be employed is very limited. Care has to be taken so that the welder is not harmed by probable electrical shocks.

3.2 Dry Welding

          In case of dry welding, otherwise known as Hyperbaric welding, Hyperbaric welding is carried out in chamber sealed around the structure to be welded. The chamber is filled with a gas (commonly helium containing 0.5 bar of oxygen) at the prevailing pressure. The habitat is sealed onto the pipeline and filled with a breathable mixture of helium and oxygen, at or slightly above the ambient pressure at which the welding is to take place. This method produces high-quality weld joints that meet X-ray and code requirements. The area under the floor of the Habitat is open to water. Thus the welding is done in the dry but at the hydrostatic pressure of the sea water surrounding the Habitat. The operation is carried out in higher pressure. The gas tungsten arc welding process is used. 

3.2.1 Classification of Dry Welding:

There are two basic types of dry welding:

• 1.      Hyperbaric welding
• 2.      Cavity welding

3.2.2 Hyper baric welding:

• ·         It is carried out in chamber sealed around the structure to be welded
• ·         The chamber is filled with a gas at the prevailing pressure, to push water back
• ·         The welder fitted with breathing mask and other protective devices on the pipe line
• ·         Mask filled with a breathable mixture of helium and oxygen in the habitat 
•     The area under the floor of the habitat is open to water, so hyper baric welding is termed as “habitat welding”.
• ·         As depth increase pressure also increases, it affects both for driver and welding process

  3.2.3 Cavity welding:

1. ·         Cavity welding is another approach to weld in water free environment
2. ·         Conventional arrangements for feeding wire and shielding gas
3. ·         Introducing cavity gas and the whole is surrounded by a trumpet shaped nozzle through which high velocity conical jet of water passes.
4. ·         It avoids the need for a habitat chamber and it lends itself to automatic and remote control.
5.       The process is very suitable for flat structures.

3.2.4 Advantages of Dry Welding:

·         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).

·         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.

·         Surface Monitoring – Joint preparation, pipe alignment, NDT inspection, etc. are monitored visually.

·         Non-Destructive Testing (NDT) – NDT is also facilitated by the dry habitat environment.

·          Welding can be carried out without getting affected by ocean currents and marine animals.

·         Better quality welds can be used.

3.2.5 Disadvantages of Dry Welding:

·         The chamber is very complex. Large support equipment is needed at the surface to support the chamber.

·         The expense will be very high. The cost increases proportionately to the depth under water.

·         The chamber has limited reusability.

4. UNDERWATER WELDING USING ROBOTIC TECHNOLOGY

            Welding in offshore and marine application is an area of research and understanding, where many problems are still unsolved. The important application of the off shore welding is the ship building and pipeline construction. Since underwater welding is done at the elevated pressure, the care must be ensured to improve the welder’s safety. Hence the robotic technology is recommended to overcome the problem relating to the life threat of the welder’s.

            The robot that is designed for the underwater welding is based on the submarine design. The main parts of the robot is

• ·         Propeller
• ·         Electromagnetic Wheels
• ·         Welding Rod holder and Rod
• ·         Stepper Motors
• ·         AT mega 16 Microcontroller
• ·         Camera
• ·         Lights

             The operation of the robot is controlled by the micro controller. The microcontroller gets its input from the computer system. The wired interface is used because it’s unable to construct a wireless design efficiently in the place having the elevated pressure. The electromagnetic wheels and propellers are actuated by the stepper motors. This stepper motors are interfaced to the microcontroller using the H bridges. The movement of the robot is done using the propellers and electromagnetic wheels are used to stick to the surface. The camera helps to find the area of the weld and converts it into the coordinate system. This co-ordinate system helps to monitor the weld and make a weld accurately on the surface. [5]

            Diver-assisted remotely controlled TIG butt welding of pipelines has for many years proved to be a reliable and cost effective method both for tie-ins and repair. To cover the depth-range beyond the diving limit, a fully remote controlled system for repair welding of pipelines, is under development. This new concept is based on MIG fillet welding of an outer sleeve, and is well underway to be qualified for pipeline operations offshore down to 1000 msw. The Last year’s research and development of the MIG welding process shows a great potential to apply this welding process down to water depths of 2500 meters. [6]

5. RISKS FACTORS INVOLVED

            The offshore industries carry out welding activities in the wet environment. It is evident that the wet environments possess difficulties in carrying out underwater welding. This type of welding is very dangerous process, especially when a person goes underneath the surface of the water, and works with electric equipment. Following are some of the risk factors involved in the process of underwater welding: 

• 1.      Electric shock dangers- All welder-divers face risk of electric shock, especially during wet welding since their entire atmosphere is composed of water. Welder-divers that work in “splash zones” (areas intermittently covered by water) face even more risk given the exact position they must hold while they work – water waves can throw them off-balance and cause variables loosen their grounding cable.
• 2.      Underwater explosions- Underwater welding produces gases (oxygen, hydrogen) that have explosive potential if combined in high levels. During wet welding, welder-divers may hear a small popping sound caused from hydrogen and oxygen bubbles traveling upward and collecting. This sound should serve as a warning to stop welding immediately to locate the area where gas is collecting.
• 3.      Health risks- Because many welder-divers work hundreds of feet underwater, they undergo pressure changes that can cause harmful effects on their body on their way up.

                                i.            Diver bends (Decompression sickness) - Decompression sickness or “the bends” happens when welder-divers make their journey to the surface too quickly and pushes dissolved gases into other parts of the body too quickly and pushes dissolved gases into other parts of the body through the bloodstream. Similar to putting your foot on a half-full balloon and creating bulges in odd places. Symptoms of the bends include dull pain; itching and fatigue in parts of the bodies are skin, lungs, ears, brain, joints and spinal cord.

                              ii.            Musculoskeletal Issues and Hearing Impairment-The underwater welders are prone to certain problems pertaining to their musculoskeletal system, in the long run. Added to this, it has been reported that many of the underwater welders suffer from various kinds of hearing impairments ranging from temporary hearing loss to a permanent one.

• 4.      Drowning- Malfunction in breathing equipment such as your mask, hoses or oxygen tank(s) may create major problems for underwater welders, especially in situations where they cannot come to the surface quickly. Because underwater welders use surface supplied oxygen, their umbilical’s can twist and rip. These dangers are amplified during projects with high water current, demolition and salvaging.
• 5.      Freezing- As a rule of thumb, the deeper the descent, the colder it gets. If water penetrates your skin, your body temperature will quickly drop in a cold environment. This can lead to respiratory problems, hypothermia and death.
• 6.      Marine life- Dangers that are posed by the marine animals cannot be ignored. [8]

6. SAFTY RULES

• 1.      Wear rubber suit and gloves.
• 2.      Cables are Watertight and completely insulated. To insulate exposed parts, apply rubber tape, scotch cote then electrical tape.
• 3.      Use water proof electrodes that are fully insulated.
• 4.      Handle loose metallic items carefully so they don’t come in contact with electrode.
• 5.      Safety (knife) switches are open only right before welder is ready to power electrode.
• 6.      Keep power supply on rubber or wooden platform.
• 7.      Welding machine frame must be earthed.
• 8.      Only change or tighten the electrode holder when no current is in the circuit.
• 9.      Always use appropriate grade of welding filter to protect eyes of welder.
• 10.  Regularly inspect diving helmet to prevent deterioration resulting from electrolysis.

7. APPLICATIONS

• ·         Offshore construction for tapping sea resources
• ·         Temporary repair work caused by ship’s collisions, unexpected accidents
• ·         Salvaging vessels sunk in the sea
• ·         Repair and maintenance of ships
• ·         Construction of large ships beyond the capacity of existing docks

8.  CONCLUSIONS

            The joining technique “underwater welding” is discussed here, this type of welding method is too risky and dangerous. 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 stream during exposure to air at increased pressure. Precautions include the provision of an emergency air or gas supply, stand-by divers, and properly insulated electrodes and other equipments. Recent trend in underwater welding is automation, now remote controlled robots are ready to do the welding works, that helps to avoid injuries and other risks involved in the underwater welding.

REFERENCES

[1]   Underwater Welding-Recent Trends and Future Scope-Ketan Verma and Harish K. Garg

[2]   Underwater welding –a review-Jerzy Łabanowski, Dariusz Fydrych, Grzegorz Rogalski

[3]   A Review On Under Water Welding Process Underwater welding-Kunthles kumar and Rickramjeet singh

[4]   Present status and future scope-Jyotsna Dutta Majumdar

[5]   Under-Water Welding Using Robotic Technology-V.Prasanth, S.Sukesh Kumar, Dr.R.Gnanaguru

[6]   Remotely Controlled Hyperbaric Welding of Subsea Pipelines-Hans Fostervoll, Ragnhild Aune, Jan Olav Berge

[7]   Fundamental Difficulties Associated With Underwater Wet Welding-Joshua E.Omajene, Jukka Martikainen, Paul Kah, Markku Pirinen

[8]   Welding process in marine application: a review-A Anand, A Khajuria