This paper discusses the evolution of line-pipe steel against the background of the failure incidence and the design basis for transmission pipelines, with a focus on those transporting natural gas. Working-stress design (WSD) is introduced as background for analysis of incident experience. It is shown that failure incidence does not correlate with the WSD factor of safety on pressure-induced stress, leading to the underlying causes of failure and discussion of alternative design philosophies, and consideration of safety factors other than those based on stress, or the effect of pressure. Full-scale test data are discussed to rationalize why failure frequency does not correlate with factor of safety. These results point to a very large factor of safety on pressure, with failure pressure found much in excess of the specified minimum yield stress (SMYS), the reference stress for WSD-based pipeline design. Full-scale failure at pressures much in excess of that for in-service incidents motivates discussion of causes of such failures and brings into question the utility of alternative design philosophies. The role of toughness is introduced as key to the success of WSD and alternative design philosophies. The historical evolution of both strength and toughness is then introduced along with apparent differences in toughness depending on how it is characterized. Historical trends are contrasted to those for modern steels, with diametrically opposing trends evident. The implications for design are discussed with reference to fracture control plans and methods to characterize required arrest toughness.

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