Computational Simulation of Line-Pipe Fabrication Processes

Line pipe fabrication involves plate forming and seam welding, for which the forming process is continuous for some processes. Residual stresses that are important in line pipe performance, particularly for sour service, reflect the interaction between the plate forming, radial expansion, and welding-induced thermal and local plastic deformation. These mechanisms interact to produce a complex three-dimensional residual stress field. This paper uses an advanced computational simulation procedure to simulate the detailed forming and welding interactions and their combined effects on thermal and residual stress development. Such technology can be used to guide fabrication procedures and optimize radial expansion as well as welding parameters, which more typically reflect trial and error type decisions. The simulation process is first outlined and then applied to a specific pipe diameter and wall thickness ratio. Detailed three-dimensional finite-element-based simulations of the spiral seam welding process are then presented considering both plate forming/radial expansion effects and their interactions with seam welding. Validation experiments are presented whereby simulation outputs are directly compared to measured data. Finally, use of pipe split-ring testing is simulated to evaluate its ability to quantify the residual stresses. Not surprisingly, such results show steep through-wall gradients in the hoop and axial components, with levels for some components well above the yield stress. Cold expansion is shown to be beneficial in terms of relief of the hoop component, but potentially detrimental for the longitudinal weld component. Pipe parameters like diameter to thickness and grade are considered. The utility of split-ring testing is evaluated analytically, and compared to typical results for expanded line pipe.