The development of a rigorous mathematical model for simulating the processes leading to the transition of an initially stable through-wall defect into an elongated running fracture in pressurized pipelines is described. The application of the model to the Puncture of a gas pipeline followed by its isolation indicates significant and rapid localized cooling of the pipe wall to temperatures well below its ductile to brittle transition temperature. The resulting order of magnitude drop in the fracture toughness coupled with the pressure stresses at the defect plane are shown to lead to catastrophic pipeline failure in the form of a running fracture some 2000 s after its puncture. Significantly, delay in pipeline isolation after its puncture is shown to have a profound effect in circumventing such failures. The above study for the first time quantitatively highlights the importance of taking into account the expansion-induced cooling effects as a credible failure scenario when undertaking safety assessment of pressurized pipelines. (c) 2005 American Institute of Chemical Engineers.