Strain based design considerations for spiral welded pipelines

Abstract

Pipelines are constructed in hostile environments where the occurrence of imposed plasticdeformations can necessitate a strain based design approach. Under such conditions not only the strengthand toughness properties have to be considered; also the strain capacity of pipe and weld metal becomecrucial. Considering the use of spirally welded linepipe sections, the helical seam weld and anisotropicmaterial properties pose real challenges to pipeline designers. In our work, the tensile strain capacity anddefect tolerance of high strength, high toughness spiral pipes will be investigated. This paper brieflydiscusses the different steps in the spiral pipe manufacturing process and their influence on the mechanicalproperties of the pipe. The forming angle is a key parameter as it determines (a) the anisotropy in strengthand toughness of the pipe steel, and (b) the orientation of possible seam weld defects. Each mechanicaloperation (forming, expansion) and each thermal operation (welding, coating) will affect local or globalstrength, toughness and ductility properties of the pipe metal. A thorough material characterization at eachprocess step is needed for a qualitative and quantitative understanding of these effects.