IIW White Paper

9 Needs and challenges of major industry sectors for future applications

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).

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Through Optimum Use and Innovation of Welding and Joining Technologies

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