The compressive and tensile strain limits of a X80 high-strain pipe and a conventional pipe are compared based on FEA results, whose outer diameter are assumed to be 20, 30 and 40 inches. Geometric imperfections of wall thickness, pipe diameter and longitudinal blister are taken into FEA model in order to predict the compressive strain limits precisely. In girth welded pipeline model, a surface defect with 70mm length x 3mm depth is introduced at the heat affected zone (HAZ) on inner surface of the pipeline. The pipes were pressurized with the hoop stress up to 72 % of SMYS. Validation of FEA was made based on a full-scale bending test. Additionally, SENT tests and its FEA are conducted to obtain ductile crack initiation limit of HAZ based on equivalent plastic strain criterion. The compressive strain limits are compared with the tensile strain limits defined by ductile crack initiation from the defect at HAZ of girth weld, and the discussion is made in which strain limits can significantly affect pipeline integrity in seismic and permafrost regions.
Large-scale pipeline projects demand high-strength and -toughness linepipes providing sufficient strain capacity in order to survive large ground deformations in seismic and cold regions (Glover, 2002, 2003). Strain-based design should focus on tensile properties of high-strength materials which would be beneficial to earn compressive and tensile strain capacity as one of key issues (Zimmerman, 1995, Suzuki, 2001- 2007, Mohr, 2004, 2006). And the pipeline design should point out the tensile strain limits of girth welds based on ductile cracking evaluation (Otsuka, 1980, Toyoda, 2001, Sadasue, 2004, Igi, 2007). This paper discusses the compressive strain limits and the tensile strain limits of 20, 30 and 40" inch diameter X80 linepipes. Two stress-strain curves of the X80 pipes were used for FEA to compare the compressive strain limits, whose Y/T ratios of the high-strain pipe and the X80 standard pipe were 0.77 and 0.89, respectively.
