Underwater welding

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Underwater welding

مُساهمة من طرف Admin في الأربعاء يونيو 25, 2008 12:38 am

Underwater welding






- a key technology for repairing marine structures


TWI has been involved in underwater welding research for over 25 years for the marine, offshore and oil and gas industries. Consultancy and development work has covered consumables, automated underwater welding, and equipment.
Description


Underwater welding can be divided into three main types:




  • Wet underwater welding, where manual metal arc welding (MMA) is the most common process. Flux-cored arc welding (FCAW) has been widely used in the former Soviet Union. Friction welding, which has the advantage of being relatively insensitive to depth, and which lends itself to robotic operation, has the potential for use in deep water repair.
  • Coffer dam welding, which is carried out in the dry, in air, where a rigid steel structure to house the welders is sealed against the side of the structure to be welded, and is open to the atmosphere.
  • Hyperbaric welding, 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.


Current status


The materials which are most commonly welded are microalloyed C-Mn steels of pipelines and offshore structures, and C-Mn steels on ships and harbour works. Extensive repairs have been carried out to offshore platforms following hurricane or explosion damage. Coffer dam welding is most likely to be employed in harbour works or ship repair, although wet underwater welding is also widely used. Semi-automatic FCAW has been used in the former Soviet Union for repairs to (shallow) pipelines and to the hulls of sunken ships, prior to refloating.
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.
Important current issues


The technology for hyperbaric welding is well established, and generally well researched. There is, however, a need to be able to use the process at ever greater depths, with research being carried out in the range 500 to 1,000m.
Wet underwater MMA (SMA) welding has now been widely used for many years in the repair of offshore platforms, including those which have suffered hurricane or war damage. Future needs will include the requirement to repair offshore structures in deeper waters, fabricated from higher carbon equivalent steels. In view of the high weld metal diffusible hydrogen levels generated in wet underwater welding, there is a risk of hydrogen-assisted cracking which increases with increasing carbon equivalent. Revised electrode coating formulations and improved power source technology will be required.
Wet underwater FCA welding may find wider use in the west, if it proves capable of giving acceptable weld quality.
Benefits


Underwater welding provides a means of assembly or repair underwater. Alternatives, which include clamped and grouted repairs (which may introduce unacceptably high loading on offshore structures), and the use of bolted flanges for tie-ins, are not necessarily always satisfactory.
Risks


For the humans involved, the risks are of three main types. Firstly there is a potential 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 and cutting. 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 bloodstream 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 decompression sickness following saturation diving or too rapid return to the surface from a deep dive.
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.
Expertise


TWI's arc welding engineers and technicians have a wealth of know-how and experience gained through many years of industrial problem solving and R&D. Consultancy and development work has covered:




  • MMA welding consumables
  • flux-cored wire welding consumables & equipment
  • remotely operated, mechanised welding equipment
  • wet cutting techniques


Resource






  • equipment & consumables for wet welding
  • wet welding tank for preliminary trials
  • indoor, heated deepwater tank with secure, 24 hour facilities at TWI Technology Centre (North East), Middlesbrough
  • advice on process and consumable selection, weld procedures & best practice
  • troubleshooting & feasibility studies
  • process & equipment development


Underwater facilities


TWI Technology Centre (North East), Middlesbrough provides a total support package for the specialist and operational requirements of the sub-sea/offshore industry. This state-of-the-art facility meets the needs of sub-sea/offshore operators, from underwater training and qualification of personnel, to R&D, equipment testing and development, inspection and calibration.
The underwater tank is the most modern underwater operations centre in the UK






  • The tank is 8m in diameter and 6.5m deep, with a volume of 318 cubic metres


Underwater welding work expands at TWI North ( Connect, July 1996)
At its Great Abington headquarters, TWI has a smaller wet welding tank, which will accommodate a welder/diver.
Welder/diver training is also carried out in Thailand.
Case studies






Underwater butt welding trials, designed to demonstrate to the Ministry of Defence the feasibility of sub-sea ship repairs, have been successfully completed in TWI North's diving tank


Navy underwater trials completed in TWI tank ( Connect, July/August 1999)
Team helps in Magnus repair
When BP completed a difficult underwater repair in a closure weld at a water depth of 182m on its Magnus platform in the North Sea, TWI staff from several disciplines provided support on various technical aspects of the repair.
Demonstration of in-situ weld repair to a FPSO hull
A satisfactory approach for the repair of a fatigue crack in the bottom of the hull of an FPSO has been demonstrated. The wet underwater weld, deposited in the flat position, simulated the repair of a fatigue crack. The exercise proceeded satisfactorily, giving a weld of good integrity.
Further information


How is it that arc welding can be carried out in water?
Can sound welds be made by wet underwater welding?
The flux-cored arc process for wet welding and cutting - an assessment ( Bulletin, May 1997). This article covers an in-depth appraisal of the use of the flux-cored arc (FCA) process in comparison to conventional manual metal arc (MMA) for wet welding and cutting. Equipment, comparisons of welding and cutting performance and cost analyses are included.
Assessment of the mechanised flux-cored arc welding process for underwater wet welding. Summary of a report on the equipment, operating characteristics and potential applications of the self-shielded flux cored arc welding process. A mechanised system for producing underwater wet welds using the FCAW process was commissioned and the performance of a range of power sources and alternative consumables was assessed.
All TWI's confidential consultancy and project work is conducted to ISO 9001 quality standard.

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رد: Underwater welding

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