NEW WELDING TRENDS

Under sea Welding process

Santhosh Kumar .S,

Venkatesh .S,

KARPAGAM INSTITUTE OF TECHNOLOGY

COIMBATORE

DEPARTMENT OF MECHANICAL ENGINEERING

Abstract

The process of joining together two pieces of metal-Welding is carried out by the use of heat or pressure or both and with or without added metal-types of welding methods-under water or under sea welding methods- 1.Dry welding 2.Wet welding. this welting process arise during the last world war an urgent need was felt for salvaging vessels sunk into the deep sea and this

need raised the status of underwater welding from almost a pipe dream to a practical process. Moreover international interests to develop and utilize ocean which covers 70% of the earth and its resources such as development of offshore gas and oil field, fisheries multiplication, large offshore construction and mineral resources, mining in sea bottom etc. have led to the

development of underwater welding. The interesting matter is an Underwater welder earns

$100,000 to $200,000.

Introduction

Welding is a fabrication process that joins metals or thermoplastics by using coalescence. Melting the work pieces and adding a filler material to form a pool of molten material known as the weld puddle that cools to become a strong joint describes the process of welding. This is in contrast to soldering and brazing which involves melting of a material with lower melting point material between the work pieces. Welding process is many types such as Solid state Welding-Resistance Welding-Soldering-Oxyfuel gas welding-Brazing-other process such as Induction welding; electron beam welding, thermit welding, laser beam welding and electro slag welding. Other than this now there is an interesting topic that is underwater welding or undersea welding. Underwater welding provides a means to assemble or repair underwater. This is a highly useful technology available that allows repairs of ships damaged during hurricanes or wars. There are a couple of alternatives available, which include clamped and grouted repairs and bolted flanges. However, these alternatives do not always provide satisfactory results and also introduces high loading at offshore structures. This is a highly specialized trade and most people are employed in the oil or shipping industry and the military.

Underwater welding process can be classified into the following two categories:

Wet Welding Dry Welding

How under water welding arise

Russian metallurgist Konstantin Khrenov made the first underwater weld in 1930’s he made simple experiments in his lab to bring out the new method in welding process. The first under water welding was carried out by British Admiralty – Dockyard for sealing leaking in ship rivets below the water line. Underwater welding advanced through World War II, particularly during Pearl Harbor salvage ops. Under water welding is an important tool for underwater fabrication works. In 1946, special waterproof electrodes were developed in Holland by Van der Willingen. In 1970’s Whitey Grubbs and Dale Anderson of Chicago Bridge & Iron (CB&I) qualified an underwater wet welding procedure to American Welding Society (AWS) standards. 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.

Under water Welding

Hyperbaric welding  is the process of welding at elevated pressures, normally underwater Hyperbaric 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 hyperbaric welding are divers. It is often used to repair ships, offshore oil platforms, and pipelines. Steel is the most common material welded.

Types of Underwater welding

There are two main categories in the underwater welding process which are; Wet Welding and Dry Welding. Wet Welding is performed underwater where the welding is being done while being exposed to water. Wet Welding is most effective as it provides the freedom of movement for the welder, because of this wet welding is efficient, economical and the most utilized choice for repair work underwater.

Wet welding

Hydroweld and Underwater Technical Services pooled their technical and administrative skills to successfully complete a contract for DML (Devonport Management Ltd), which carry out work on behalf of the MoD (Ministry of Defence). The project, the first ever of its kind, required the wet welding of over 40 blanking plates to the ballast tanks of a submarine, the largest being 2.6 x 1.2 meters. The contract was completed on time, in budget with no lost time accidents and at a fraction of the cost of dry docking.

All of the wet welding was completed using Hydro weld FS wet welding electrodes, which are the only underwater welding electrodes have been approved, by the MoD, for permanent and temporary repairs to war ships. This project further proved the electrodes capabilities and welding techniques used by Hydroweld, through the successfully completion of wet

welding procedures and production welding on materials with a relatively high carbon equivalent, up to 0.47%. The accepted normal upper limit is 0.40% using ferritic electrodes. The welding was carried out by a selected team of 20 coded wet welder divers, who’s variety of skills and commitment to the project was invaluable and who must be commended. All of the wet welds were subjected to 100% visual inspection and MPI by the client with a 100% pass rate.

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. Power Supply used: DC Polarity: -ve polarity

Principle of Wet Welting

The process of underwater wet welding takes in the following manner: The work piece 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.

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.

We prefer Wet Welting method for

Wet underwater MMA welding has now been widely used for many years in the repair of offshore platforms. 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 building. Readily available standard welding machine and equipments are used. The equipment needed for mobilization of a wet welded job is minimal.

We oppose Wet Welting method for

Although wet welding is widely used for underwater fabrication works, it suffers from the following drawbacks: -

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.

Dry welding methodHyperbaric Welding is the process

in which a chamber is sealed around the structure to be welded, and is filled with a gas (commonly helium containing 0.5 bar of oxygen) at the prevailing pressure.

The hyperbaric welding may be one of two types:

A) Mini-Habitat welding - makes use of a small, easily portable, gas-filled, often Plexiglas enclosure, which is placed over the joint by a diver. Water is displaced by an inert gas or air supplied from the surface. In this instance welding is performed at elevated ambient pressures. Depending on the size of the enclosure, the diver is partially immersed in water (only the diver's hands and the welding torch are inside the habitat). This method requires adequate visibility and is limited to areas with clear access. The diver/welder welds with each of several MMA electrodes, positioned in advance through the flexible port.

B) Large Habitat welding - a specially designed chamber is built and positioned around the intended weld and the welder/diver enters the chamber in order to undertake the work. The habitat is sealed and water is excluded by introducing an appropriate gas. Hyperbaric welding

produces high-quality weld joints that meet X-ray and code requirements. Most welding processes can be operated at hyperbaric pressures, but all processes suffer a reduction in capability and efficiency results as the pressure increases. Hyperbaric welding, using MMA (SMA), TIG (GTA) or FCAW, is the preferred process for high integrity welds, particularly for deep water welds, including tie-ins in pipelines and risers in the oil and gas industries; however GTA is the method most commonly employed for hyperbaric welding operations.

Risk of Dry weldingThere 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 stream 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. 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.

We prefer Dry welding for

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.

We oppose Dry welting for

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.

Concluded as Even through under water welding is more advantageous, there are more risk factor faced by the workers. 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.under water welding has it scope; Developments of diver less Hyperbaric welding system is an even greater challenge calling for annexes 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.