NAME 345Welding Technology

Lecture – 11

Underwater Welding

Md. Habibur Rahman

Lecturer Department of Naval Architecture & Marine Engineering

Bangladesh University of Engineering & TechnologyDhaka-1000, Bangladesh

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Underwater Welding

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Underwater Welding

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Underwater Welding

• Underwater welding method enables us to weld properly underthese conditions.

• Underwater welding is an important tool for underwater fabricationworks.

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Underwater Welding

• The welding circuit must include a positive type of switch, usually aknife switch, operated on the surface and commanded by the welderand is used for safety reason.

• When DC is used with +ve polarity, electrolysis will take place andcause rapid deterioration of any metallic components in the electrodeholder.

Principle of Operation

• For wet welding AC is not used on account of electrical safety anddifficulty in maintaining an arc underwater.

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Underwater Welding

• Power supply requirements – 400 amps or larger. DC generators,motor generators, and rectifiers are acceptable for power supplies.

• Power converters.

• Welding Generator, Pre-Setup.

• Polarity.

• Diesel driven welding generator amperage and voltage settings.

• Gas manifolds.

Requirements for Underwater Welding

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Underwater Welding

Schematic diagram for underwater welding or cutting

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Underwater Welding

The important applications of underwater welding are:

a) Offshore construction for tapping sea resources,

b) Temporary repair work caused by ships collisions orunexpected accidents,

c) Salvaging vessels sunk in the sea,

d) Repair and maintenance of ships,

e) Construction of large ships beyond the capacity of the existingdocks,

f) Repair and maintenance of underwater pipelines.

Applications

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Underwater Welding

i. Wet Underwater Welding &

ii. Dry Underwater Welding

Types of Underwater Welding

• In wet welding the welding is performed underwater, directlyexposed to the wet environment.

• In dry welding, a dry chamber is created near the area to bewelded and the welder does the job by staying inside the chamber

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Underwater WeldingWet Welding

• As the name implies, underwater wet welding is done in anenvironment where the base metal and the arc are surroundedentirely by water. In wet welding MMA (manual metal arc welding)is used.

• Wet welding increases freedom of movement which makes thewelding process the most effective, efficient and economical.

• Welding power supply is located on the surface with connection tothe diver/ welder via cables and hoses.

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Underwater WeldingWet Welding (Contd.)

Power Supply Used: DC• Polarity: -ve polarity‒ When DC is used with +ve polarity,

electrolysis will take place and causerapid deterioration of any metalliccomponents in the electrode holder.

• For wet welding AC is not used onaccount of electrical safety anddifficulty in maintaining an arcunderwater.

• The power source should be a directcurrent machine at 300 or 400amperes.

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Underwater WeldingWet Welding (Contd.)

Principle of operation of Wet Welding• Work to be welded is connected to one side of an electric circuit, and a metal

electrode to the other side.• 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 transfermetal.

• The arc burns in a cavity formed inside the flux covering, which is designed toburn slower than the metal barrel of the electrode.

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Underwater WeldingWet Welding (Contd.)

Advantages Disadvantages

i. The versatility and low cost of wetwelding makes this method highlydesirable.

ii. Other benefits include the speed withwhich the operation is carried out.

iii. It is less costly compared to dry welding.

iv. The welder can reach portions of offshorestructures that could not be welded usingother methods.

v. No enclosures are needed and no time islost building. Readily available standardwelding machine and equipments areused. The equipment needed formobilization of a wet welded job isminimal.

i. There is rapid quenching of the weldmetal by the surrounding water. Althoughquenching increases the tensile strength ofthe weld, it decreases the ductility andimpact strength of the weldment andincreases porosity and hardness.

ii. H2 Embrittlement – Large amount of H2 ispresent in the weld region, resulting fromthe dissociation of water vapour in the arcregion. The H2 dissolves in the HeatAffected Zone (HAZ) and the weld metal,which causes embrittlement, cracks andmicroscopic fissures. Cracks can grow andmay result in catastrophic failure of thestructure.

iii. Another disadvantages is poor visibility.The welder some times is not able to weldproperly.

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Underwater WeldingHyperbaric (Dry) Welding

• Inside a specially constructed positive pressure enclosure and hence a dryenvironment.

• Use for high quality welds as more control over conditions.

• Involves the weld being performed at the prevailing pressure in a chamberfilled with a gas mixture sealed around the structure being welded.

• Most arc welding processes such as shielded metal arc welding (SMAW), flux-cored arc welding (FCAW), gas tungsten arc welding (GTAW), gas metal arcwelding (GMAW), plasma arc welding (PAW) could be operated at hyperbaricpressures.

• The area under the floor of the habitat is open to water. Thus the welding isdone in the dry but at the hydrostatic pressure of the sea water surrounding thehabitat.

• The habitat is filled with a gas (He and O2) at the prevailing pressure.

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Underwater WeldingHyperbaric (Dry) Welding (Contd.)

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Underwater WeldingHyperbaric (Dry) Welding (Contd.)

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Underwater WeldingHyperbaric (Dry) Welding (Contd.)

Methods Usedi. Pressure Welding: Working in a pressure vessel measuring one

atmosphere unit of pressure (same as pressure at sea level).

ii. Habitat Welding: Using a chamber in ambient pressure (same assurrounding pressure at working depth) about the size of a smallroom to weld. Before entering, the chamber displaced its water intothe surrounding ocean or lake.

iii. Dry Chamber Welding: It is habitat welding, but with a smallerchamber. The chamber holds the head and shoulders of a diver(dressed in a diving gear) and is open at the bottom for the fit in.

iv. Dry Spot Welding: Here the habitat is even smaller. The habitatshrinks to the size of about the welder-diver’s head and itscompletely clear. It is placed on the weld site and the welder insertshis electrode inside the habitat, which seals around.

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Underwater WeldingHyperbaric (Dry) Welding (Contd.)

Advantages Disadvantages

i. Better diver safety.

ii. Better quality welds.

iii. No build up of hydrogen and oxygenpockets.

iv. Allows for heat treatment before andafter welding.

v. Nos destructive testing.

vi. Surface monitoring is possible.

i. Require large, complex equipment.

ii. Chamber has to be fabricated differentlyfor different applications.

iii. Cost is very high and increases withdepth.

iv. More energy requirement.

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Underwater WeldingDangers of Underwater Welding

i. Chances of an Electric Shock• This happens when the welding equipment that is used is not adopted

to work under water.

• The equipment should be tested properly, well-insulated and awaterproof electrode should be connected to it.

ii. Possibility of an Explosion• The chances of such explosions are more in processes, wherein both,

H2 and O2 are involved, and may lead to the formation of numerousgas pockets.

• During hyperbaric welding, the formation and combination of H2 andO2 pockets are dangerous because they are explosive, when ignited.

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Underwater WeldingDangers of Underwater Welding (Contd.)

iii. Decompression Sickness• Decompression sickness, also known as diver’s disease.

• Diver inhales harmful gases such as N2, when he dives quickly from a highpressure zone to a low pressure zone.

• If the welder dives too fast to the surface of the water, N2 bubbles enters hisbloodstream. These bubbles then spread inside the diver’s body, and startshowing numerous adverse symptoms.

• Decompression sickness may lead variably to rashes, joint pain, paralysis oreven death of a person

iv. Breakdown of Dental Amalgam• As a result of metallic taste in the mouth.

• Recent studies have shown that in the process of electric welding and cuttingunder the water, a magnetic field with alternating current gets created. Thismagnetic field, in turn, induces a secondary current in the oral tissues of thewelders, due to which their dental amalgam breaks down.

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Underwater WeldingFurther Developments

• Dry Hyperbaric welds are better in quality than wet welds. Presenttrend is towards automation. THOR – 1 (TIG Hyperbaric OrbitalRobot) is developed where diver performs pipefitting.

• Developments of diver less Hyperbaric welding system is an evengreater challenge in developments like pipe preparation andaligning, automatic electrode.

• Explosive and friction welding are also to be tested in deep waters.

• Application of advanced welding technique, like friction, laserwelding.

• Invention of new welding techniques and explore the possibility.

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Underwater Welding

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