Welding still preferred method of attachment
if new methods can be developed
If welding in water depths beyond 500 meters is viable and economic, the petroleum industry is likely to continue to use that method of attachment. This consensus was developed by Cranfield University (London) before embarking on a welding research program for unmanned deepwater development. Arc welding provides high joint efficiency, low weight, and has a good operational record.
At present, there are alternatives to arc welding for deepwater operations:
- Several types of mechanical connectors have been developed for transmission pipelines and some joints have been completed at remote offshore sites. Although bulky and expensive, the research team states, they could be used for deepwater work, probably using metal-to-metal sealing techniques for maximum reliability.
- A solid phase welding technique, such as explosive welding, could be used, although the danger inherent in the use of explosives could inhibit further development of this idea, which was shown to have a limited application in the late 1970s.
- High performance adhesive techniques have been used widely in the fabrication of automotive and aerospace components. Such techniques could possibly be used to enhance the performance of mechanical joining methods.
The development of deepwater arc welding remains a likely option when the industry faces the task of extracting oceanic hydrocarbon reserves, says Dr. Richardson. Research in various countries has shown that two arc welding processes are effective at depths of 500-1,000 meters - plasma welding and gas metal arc (GMA) welding.
Cranfield's contribution, after the completion of recent research, was to show that plasma welding can be made to remain stable at pressures equivalent to 1000 meters water depth, in spite of the large number of variables which come into play at depths of this order.
Plasma welding is a derivative of gas tungsten arc welding (GTAW), which, with shielded metal arc welding (SMAW), is commonly used in continental shelf waters, usually at a depth of less than 200 meters.
The superiority of plasma welding below 200 meters is attributed to constriction of the arc within a water cooled orifice, which reduces the cross sectional area of the arc. This improves arc stability because the constriction increases the plasma velocity.
Dr Ian Richardson of Cranfield University is shown with a hyperbaric research vessel that will allow welding experiments up to a maximum of 250 bar, equivalent to a water depth of 2.5 km.
Gas metal arc welding has been shown to have acceptable stability and fusion levels at pressures equivalent to more than 1000 meters. Special power supply control systems would be necessary for GMA welding at such a depth, but these would not add significantly to the cost of the whole system. Standard high performance power sources could be used to supply the actual welding current.
Difficult welds
On the other hand, work remains to be done on the development of effective torch manipulation, strategies to deal with difficult welds and the reduction of welding fume. "There are other variables to be looked at," says Dr Richardson, "but industry in general has considerable experience in using GMA techniques in robotic welding and it may be possible to use similar robots inside hyperbaric workchambers."
Dr Richardson says the development of welding techniques for deep water is a new step rather than a stage in an evolutionary process. "It is in fact more than a new step. It is a multi-stage process by which, for any given depth, trials are necessary to establish the viability of welding techniques. These must be combined with process studies to determine the effect of variables at such depths. Our objective is to maximize the stability and consistency of the process across as wide a range as possible of operating conditions.
"The welding operation is, however, only one of a range of factors which must be addressed in the development of a viable remotely operated system. There is also a need for supporting research and development on remote handling systems for weld preparation, the placement of equipment, installation of equipment and weld alignment."
The growing emphasis on the importance of deep sea techniques has been given added impetus by doubts about the wisdom of sending divers deeper than 180 meters, based on emerging data on the long term effects of deep diving, according to Dr, Richardson.
The aim of Dr Richardson and his Cranfield team is to push ahead with arc welding research at pressures greater than 100 bar. To do this they are commissioning, with funds provided by Britain's Engineering and Physical Sciences Research Council, a hyperbaric welding research vessel and associated system which will operate at a maximum pressure of 250 bar, equivalent to water pressure at a depth of 2.5 km. Named Hyperweld 250, the new vessel is based on the experience of more than 20 years of hyperbaric welding research at Cranfield. Initial work, scheduled to begin early next year, will be in the depth range of 1000-1500 meters.
The design is similar to other hyperbaric welding research vessels, having a tube closed at one end and a plug held in place by hydraulic rams at the open end. The plug end is rigidly fixed to the support structure and provides mountings for the penetrators which supply services to the vessel's internal systems. The long end is mounted on a wheeled carriage and can be moved, under hydraulic actuation, to give access to the vessel's interior.
Gas systems
The welding process will be monitored by a closed circuit television link. Three gas systems will be used to store pure helium, pure argon and mixed gas at 20, 60 and 200 bar, with a buffer store at 350 bar for final supply to the vessel. A dedicated programmable logic controller will automatically operate and control the gas supply.
A design objective of Hyperweld 250 is the ability to weld both flat plate and tubular components. It was decided therefore to construct the welding system in the form of a carriage which could be mounted either on a linear or a circular track. The carriage incorporates its own drive motor, the welding torch and manipulator, a consumable feed unit and the weld pool viewing system.
The complete unit can be moved, relative to the weld, vertically and laterally. A development program to provide the very high arc voltages required for plasma welding at high pressure has been undertaken by Fronius, the Austrian power supply manufacturer. Stansted Fluid Power of England joined the Cranfield team in designing the pressure vessel and gas storage/flow control system.
Offshore systems
"The control system responsible for sequencing all welding operations has been designed by Isotek Electronics and is based on the controls used successfully on current offshore systems," says Dr Richardson. "This will ensure that future developments will be compatible with offshore methods, making industrial exploitation of research results relatively easy."
To enable engineers to see what is happening in the welding chamber a sensitive video camera will be used, capable of recording images in almost complete darkness and, at the other end of the scale, when the welding torch is producing light which is expected to be about 1,000 times brighter than a domestic 100 watt bulb. The camera will cope with this brightness range by means of an automatic gain control linked to the lens iris, combined with a series of manually controlled filters.
One of the problems will be the electromagnetic noise generated by the welding power supply. The Cranfield team intends to counteract this by using optical fibres to carry the image to the surface. "Being light and flexible, optical fibres will also be compatible with the need to maneuver the torch during welding," says Dr Richardson.
"High quality dichroic lenses, protected by sapphire windows, will direct the light from the welding torch into the optical fibers. Mirrors will reflect the heat in the other direction. The Cranfield simulator should not only produce beneficial spin-offs for shallow water operations but is also expected to have uses in related research activities. It is envisioned, for example, that in future. many deepwater structures will be made from high strength steels. The Hyperweld 250 could provide valuable information on the effect of welding, especially hyperbaric repair, on the fatigue performance of new steels.
Other aspects of material performance, such as corrosion resistance and fracture toughness, are also likely to be undertaken once appropriately designed welded components have been produced.
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