Burst Strength of Gouges in Low Toughness High Pressure Transmission Pipes

Abstract

The “State-of-the-art review of the existing fitness-for-purpose assessment methods for damaged pipelines” [1] performed recently by Advantica identified, among others, the need to set a limit for the use of failure criteria for gouges and corrosion defects in the toughness-independent regime. This issue has very practical implications, as the length of older pipelines currently operated and having toughness values potentially leading to toughness-dependent failure behavior is not negligible. EPRG launched a project aimed at establishing a limit on the toughness value that separates toughness-dependent from toughness-independent burst failure behavior of gouges. The mainly experimental work checks the toughness related behavior of gouges on varying diameters of gas transmission pipes; small diameter (150 mm), medium diameter (350 mm), and large diameter (900 mm). Pipe material is carefully characterized in terms of tensile properties, Charpy energy, and shear area. Then, based on the toughness independent criterion, a set of gouges is defined, of different depths / lengths, so as to span the different regions of the criterion, covering both short and long defects. These defects are manufactured by spark erosion, resulting in thin slits. Each such slit is incorporated into a vessel that is submitted to a burst test, with a number of additional measurements, like strain gauges on the pipe surface, a clip gauge at the center of the defect, etc. The results are interpreted both in terms of comparison with the available assessment criteria, and also in terms of analysis of the failure surface, to identify failure mechanisms. A new interpretation in terms of constraint is proposed in order to explain some a priori unexpected experimental results.