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  Title Identification of a unique geometry that contributed to pipeline ratcheting: numerical and metallurgical findings
  Author(s) David B. Futch, Dr Melanie Sarzynski, and Brent A. Vyvial  
  Abstract STRESS ENGINEERING SERVICES, INC. (SES) assisted a pipeline operator with the analysis of an in-service natural gas pipeline leak identified in a location where an adjacent pipeline road-crossing had been replaced. The leak coincided with a feature called by ILI data as a top-of- the-pipe dent. After receiving the pipe sample, SES performed a combination of metallurgical and numerical analyses to determine the likely causes of the leak. SES visually examined the pipe and found multiple wrinkles spanning approximately two-thirds of the pipe circumference. Additionally, four crack regions were identified (two through-wall and two with both internal and external surface breaking cracks that did not extend through-wall) in the peaks and valleys of the wrinkles. All of the observed cracks extended via shear fracture at a 45° orientation to the pipe surface and appeared to be ductile overload based on scanning electron microscopy.

Numerical analysis, using finite element methods, indicated that the areas of cracking correlated with the areas of high stress. Furthermore, the analysis demonstrated that an accumulation of plastic strain (ratcheting) occurred in the subject pipeline due to repeated internal pressure cycles. The interaction between adjacent wrinkles led to a scenario whereby the anomaly region alternated between tensile and compressive yielding as the pipeline was pressure cycled. Based on the results of metallurgical and numerical analyses, SES concluded that the complex geometry of the wrinkled region resulted in a ratcheting behaviour leading to crack formation and propagation, and ultimately the failure due to leakage.

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