Exploration of new energy resources located in areas of complex ground and ambient climate imposes strict requirements on pipeline material and design. One of the major research issues in such areas is differential ground movement, which, possibly, is associated with large longitudinal straining in addition to plastic circumferential elongation. Hence, common design principles need thorough re-consideration, notably with respect to strain hardening properties of both base metal and girth welds. This paper summarizes results of a research, which was conducted within Europipe GmbH and supported by Salzgitter Mannesmann Forschung GmbH. Central to the discussion is the numerical representation of strain hardening behavior of pipeline steels, which mainly was focused on base metal. In particular, the discussion comprises stress-strain behavior in transverse as well as in longitudinal direction, the effect of material ageing and the significance of uniform elongation.
With the sustained rise in both oil and gas price to record levels, hydrocarbon reserves that were considered too expensive to justify production only a few years ago are now being considered as attractive. Fortunately, advances in technologies for exploration and production do allow uneconomic reserves to be accessed (Martin, 2006). These "new" reserves tend to be in regions where difficult ground prevails. That is to say, more and more pipelines will be prone to large differential ground movements. Differential ground movements may have many reasons such as soil subsidence, frost heave, thaw settlement and landslides, to name a few. A phenomenon, which is common to such load scenarios, is that they may evoke large longitudinal strains in addition to plastic circumferential elongation. This is very much different to the case of bare pressure containment where, mainly, circumferential and radial components of strain tensor undergo plastic deformation. Since longitudinal strains may be tensile or compressive complex multi-axial stress states accrue with plastic deformation developing in more than two co-ordinate directions, which, within a cross section, are not uniform any more.
