UNDER WATER WELDING:-
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.
Synopsis:-
The Welding is an unavoidable process of modern engineering – civil, electrical,
mechanical, automobiles, marine aeronautical – in all branches. It is used in
fabrications and erections in infrastructures and installations. It joins metals
or thermoplastics.
The Hyperbaric welding OR UNDER WATER WELDING is the process of welding at elevated pressures, normally in underwater. It is of 2 types DRY & WET Welding. The Hyperbaric welding can either take place wet in the water itself OR dry inside. Here a specially constructed positive pressure enclosure is used and hence it is called dry welding. Similarly Hyperbaric welding term used in dry environment & underwater welding term used as in wet environment. The hyperbaric welding is used to repair damaged ships, under water pipe line & etc.
INTRODUCTION:-
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, 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.
UNDERWATER WELDING PRINCIPLES:-
Underwater welding can be performed under wet or dry conditions. Dry underwater welding refers to underwater welding which is performed under hyperbaric conditions (an underwater habitat is displacing the water with a gas at the prevailing pressure), and wet underwater welding refers to welding which is performed in fresh- or salt-water without a protecting habitat.
Wet underwater welding will here after be referred to as ―wet welding‖, and wet/dry underwater welding will simply be referred to as ―underwater welding‖. The most commonly used wet welding technique is shielded metal arc welding (SMAW), also known as manual metal arc (MMA) welding or informally as stick welding. The main differences for the welding equipment (wet welding equipment versus ―workshop‖ welding equipment) are shown in Figure 1. Note that wet underwater welding is performed by using a (usually motor generated) power source generating DC current only (for wet welding AC is not used on account of electrical safety and difficulty in maintaining a welding arc underwater), the inclusion of a single or dual pole knife switch (circuit breaker), which protects a welding diver from electrocution, and the use of double insulated cables. Both the power source and knife switch (capable of breaking the full wet welding current) are secured above water (grounded) and controlled (on the diver welders command) by an operator. The cross section of the welding cable is adjusted to the length of the cable and should be highly wear resistant (marine growth) and sufficiently flexible; special welding electrode holders are used with extra insulation against the ingress of water.
CLASSIFICATION OF UNDERWATER WELDING:-
The Underwater welding can be classified as:-
1) Wet Welding:- In wet welding the welding is performed underwater, directly exposed to the wet environment.
2) Dry Welding :- 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
COMPONENTS OF WATER WELDING:-
Electrode Holder:-
This new generation compression type welding stinger is engineered with features that let you know it’s from Broco. Lightweight, durable and designed to hold the electrode at the optimum angle to reduce diver fatigue. The new BR-21... Setting the wet welding standard.
Water Resistant Electrodes:-
The fundamental process of welding underwater does not differ from welding on land. An electrode carrying a direct current creates an arc of electricity. That arc is then applied to melt the target metal. The difference is that the electrodes used underwater are waterproof. Extra insulation around the connections ensure that the current will be able to generate the arc and not degrade the copper wiring. Doing this underwater presents a new set of risks. Both the risk of electrocution and the technical difficulty of maintaining the electrical arc underwater, is the reason why direct currents are used instead of alternating currents.
Line Diagram :
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.
Risks Associated with Underwater Welding:-
There is a risk to the welder/diver of electric shock. 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.
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, standby
divers, and decompression chambers to avoid nitrogen narcosis following rapid surfacing after saturation
diving.
SAFETY TOOLS AND EQUIPMENTS
The MK 12 welding shield is secured to the helmet by two spring clips which hook to the forward edge of the side viewport retainers. A rubber cushion on the rear of the shield body helps to hold it in place. The lens is clip-mounted in a hinged section. The hinged section can be flipped up or down and is held in position by a flat detent spring. The replaceable lenses, made of gold foil laminated with polycarbonate, are supplied in #4, #6 and #8 commercial shades of light density. Darker shades are available but are seldom required.
Personal Protective safety
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, NDT inspection, etc. are monitored visually.
4) Non-Destructive Testing (NDT) – NDT is also facilitated by the dry habitat environment
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 One cannot use the same chamber for another job, if it is a different one.
Advantages of Wet Weldin
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 building. Readily available standard welding machine and equipments are used. The equipment needed for mobilization of a wet welded job is minimal.
Disadvantages of Wet Welding
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.
Application of Underwater Welding
The important applications of underwater welding are:
(a) Offshore construction for tapping sea resources,
(b) Temporary repair work caused by ship’s collisions or unexpected accidents.
(c) Salvaging vessels sunk in the sea
(d) Repair and maintenance of ships
(e) Construction of large ships beyond the capacity of existing docks.
Underwater Welding – Future Scope of Research
Considerable research effort has been made to improve process performance and control strategies for the
various underwater welding processes over the last half century. However, there are still many problems to
overcome. The major efforts on research and development should be focused on the following topics:
a. Automation of the underwater joining and inspection of the welded structures.
b. Mechanized underwater welding for actual usage of a very large floating structures.
c. Investigation of the potential of using a robot manipulator for underwater ultrasonic testing of welds in
joints of complex geometry.
d. Application of advanced welding technique, like friction, laser welding and understand the behavior of
materials after the welding and process optimization.
e. Invention of new welding techniques and explore the possibility of its application in underwater welding.
f. Generation of research data book on weld ability of materials during underwater welding.
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