This paper examines the influence of internal pressure on the strain capacity of pipelines in a pressurized full-scale pipe tension test and curved wide plate (CWP) tests. Finite element analysis (FEA) was also used to investigate the strain behavior of X80 linepipe under large axial loading with high internal pressure. Full-pipe and CWP tensile tests of girth welded joints were performed using high-strain X80 linepipes. In particular, these tests focused on the effects of defect location and internal pressure on global strain behavior in X80 linepipes. A SENT test with a shallow notch specimen was performed to investigate the material resistance curve for ductile cracking, and FEA was conducted to predict ductile crack propagation behavior from a surface defect on the HAZ of a girth weld. The influence of internal pressure was clearly observed in the strain capacity. The critical tensile strain is drastically reduced by increased crack driving force under a high internal pressure condition.
Use of linepipes with high strength grades such as X80 and X100 is under study, with the aims of improving transportation efficiency and reducing welding costs (Glover, 2003). In these regions, it can be expected that pipelines will be subjected to large deformation due to large ground movement associated with liquefaction or faults in seismic regions, and frost heave and thaw settlement in permafrost regions. Because the conventional stress-based design method cannot be applied in cases where strain greatly exceeds the yield stress of the material, application of strain-based design (SBD) is under active study. Assuming bending deformation of the pipeline, the key parameters for SBD design of pipelines are local buckling of the pipe itself on the compression side of bending, and the critical strain for fracture from defects, particularly in girth welds, on the tension side of bending.
