PIPELINE LIFE EXTENSION through condition assessment and integrity management relies on the availability of material specification and construction records as well as data from inspection surveys. For corrosion, minimum wall thickness criteria are based on operating pressure, nominal pipe wall thickness and pipe grade. For stress corrosion cracking and fatigue, assessing the risk of a large rupture relies in part on asset-specific material fracture toughness data. Significant progress has been made regarding the measurement and assessment of pipe geometry and material anomalies for these fitness for service evaluation. However, the determination of the maximum allowable operating pressure and maximum allowable flaw size for existing assets can be improved through in-the-ditch testing methods for mechanical properties. This paper describes two mechanical testing methods recently developed and implemented for nondestructive testing of vintage pipeline steel. In the first method, hard blunt styluses of different geometries slide over the pipe surface at constant loads to measure material hardness. The hardness values for dissimilar styluses are input into predictive equations to determine the yield strength and ultimate tensile strength of the steel. When performed over longitudinal seams or girth welds, the tests identify the heat treatment including normalization which enhances the toughness of the steel. In the second method, the instrument is equipped with a wedged-shaped stylus that includes an opening, or stretch passage, where material is locally subjected to tension that results in microvoid growth and coalescence. This material response matches observations from laboratory ductile fracture tests, and is correlated with the fracture toughness through measurements of the crack tip opening displacement on features of the fracture surface that remains on the sample after testing. Ongoing field testing and third-party validation provide examples of application and performance of the methods.
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