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Through Optimum Use and Innovation of Welding and Joining Technologies
Improving Global Quality of Life
Normalisation of the narrow welding will be required as for FBW and ERW pipe fabrication but this is not a
major drawback. In the McDermott trials, the Vickers hardness was less than 200 DPN after normalisation
in a PWHT that raised the temperature to 900˚C for 3 minutes, followed by air cooling; hardness values
are acceptable for sour service.
The University of Texas investigated the procedure and economics of homopolar welding for installation
of offshore pipelines and concluded that the procedure could reduce both cost and installation time. The
second phase of studies to develop homopolar welding for J-lay application began in February 1993 funded
by a consortium which now includes oil companies, welding contractors and government.
In friction stir welding the pipe ends are butted together and a ceramic tool is rotated at high speed in the
butt, heating and shredding the steel as it progresses. The shredded steel then reforms behind the tool. The
technique appears suitable for duplex stainless steels that are complex and slow to weld by conventional
techniques.
Many new offshore fields must handle high temperature and corrosive fluids and the need to use corrosive
resistant materials has increased. Of the solid Corrosion Resistant Alloys (CRAs), the duplex stainless steels
and weldable 13% chrome alloys present the major materials for topside welding. Welding of these materials
is difficult and slow, and alternative welding techniques are attractive. Of these, friction stir welding shows
the most promise; the method largely overcomes the shift in the ratio of ferrite to austenite, and the lower
temperature reduces the risk of formation of sigma phase.
The on-shore gas industry is developing fields in remote areas and economical transport of gas to market
requires long distance large diameter, high pressure pipelines. The use of API 5LX100 and X120 steels is
favoured. To date, some 500 km of the high grade steel pipelines have been installed. Welding procedures
have been developed but require very careful control. The alternative one shot welding technique for the
relatively modest wall thickness high grade pipeline materials is friction stir welding.
Mechanical failures in the offshore sector do occur. The BP failure in the Gulf of Mexico and the Montara
Oil Spill in the Timor Sea are two dramatic examples. If they do occur, their ecological and economic
consequences can be most severe. With increasing understanding of the mechanical behaviour of structures
and improving material properties, this industry sector is therefore striving for the continued reduction in
failure rates through the application of better predictive material and process models. At the same time,
increased competitiveness and cost awareness and the ageing of infrastructures have led the industry to
invest in R&D developments to justify reduced safety margins, more cost-effective design methods and
extensions to design life.
IIW has contributed for many decades to achieve these goals and many of its member organisations have
extensive experience in the development of structural integrity-related issues faced by this industry.
Relevant activities include applied research into the material property, performance and inspection
of offshore structures and components. The work is based on capabilities and experience relating to
metallurgical behaviour; fracture; fatigue; design; residual stresses; failure investigations; advanced NDT;
plant assessment; and materials testing. The application of this whole spectrum of technologies is often
referred to as fitness-for-service (FFS) or engineering critical assessment (ECA).
9
Needs and challenges of major industry sectors for future applications
