Material test requirements for strain-based pipeline design

The pipe metal properties commonly derived from a standard tensile test are the yield strength, ultimate tensile strength and strain at failure. Knowledge of the yield strength and the yield to tensile (Y/T) ratio allows assessing, in stress-based designs, the condition of plastic collapse when defects occur. For strain-based designs, additional information is required. In particular, the strain hardening capacity or the Y/T ratio, the shape of the stress-strain curve, the uniform elongation and the level of weld strength mismatch determine the amount of plastic deformation that can be achieved. It is also to be noted that a strain-based design requires the actual properties and not the specified ones. This paper addresses issues that arise in the determination of acceptable defect sizes of girth welds subjected to longitudinal plastic strains. The issues discussed include the effect of (a) the variability of the measured material properties, (b) the toughness and (c) the strength mismatch. For this purpose, reference is made to an empirical model that allows predicting the strain limit. This model, derived from the results of Curved Wide Plate (CWP) experiments, reflects the observation that, beyond a threshold level of toughness, the tensile properties have a much greater effect on the strain limit than increased toughness. By using data from welded Grade X70 and X80 pipe, it is demonstrated that high-toughness girth welds do not lead to a higher remote strain capacity than girth welds of moderate but adequate toughness. Throughout the paper, guidelines as to how to handle these issues are provided. (AU)