A rupture of buckled steel pipes on the tensile side of a cross-section is studied in this paper as the most plausible case of ultimate failure for the pressurized buried pipelines under monotonically increasing curvature. Finite element simulation of full-scale bending tests on two pressurized X80 pipes with different yield-to-tensile strength (Y/T) ratios were conducted. The Y/T ratio and internal pressure were identified as the crucial factors that have a coupled effect on the ultimate failure mode of buckled pipes. That is, the high values of Y/T ratio and internal pressure mutually trigger the rupture of buckled pipes on the opposite side of the wrinkling.
Steel pipelines are so ductile and can accommodate a large amount of post-buckling deformations while preserving their operational safety and structural integrity. To benefit from this outstanding quality and prevent the buckled (wrinkled) pipelines from premature rupture, the postbuckling behavior of the steel pipes should be well understood.
Rupture is one of the major failure limits to the integrity of pipelines that endangers the environment as well as the public safety and property. Comprehensive experimental and numerical studies on the fracture of buckled steel pipes (Das, 2003; Sen, 2006; Mohajer Rahbari, 2017) show that under increased monotonic curvature, successive buckles (wrinkling) are formed on the compressive side of the wall, and the occurrence of rupture at the wrinkling location is unlikely because of the ductile nature of steel material. Rupture of wrinkling can occur once buried pipelines are subject to a very rare and changing boundary conditions accompanied by extremely large plastic deformations toward tearing the wrinkled wall (Ahmed, 2011). However, experiments have shown that the increasing curvature can easily trigger the postbuckling rupture of the tensile wall on the opposite side of the wrinkling (Sen, 2006; Mitsuya et al., 2008; Tajika and Suzuki, 2009; Igi et al., 2011; Tajika et al., 2011; Mitsuya and Motohashi, 2013; Mitsuya and Sakanoue, 2015). This mode of failure seems very likely to be the rupture limit of the wrinkled pipes, as it occurs following the same regime of monotonic bending deformations that have previously made the pipe buckle.
Das, Sreekanta (Department of Civil and Environmental Engineering, University of Windsor, Windsor, Ontario, Canada) | Cheng, J.J. Roger (Department of Civil and Environmental Engineering, University of Alberta, Alberta, Canada) | Murray, David W. (Department of Civil and Environmental Engineering, University of Alberta, Alberta, Canada)
An experimental study based on limited full-scale tests on a 305-mm-diameter wrinkled gas/oil pipeline indicates that the integrity of this pipeline is severely threatened and fracture may occur quickly if it is subjected to cyclic load-deformations. However, this kind of test is extremely time-consuming and expensive, so that an experimental study is not a realistic solution for understanding the behavior and assessing the risk associated with every scenario of wrinkled pipelines in the field. This work was then undertaken to develop an accurate numerical model for better understanding the post-wrinkling behavior and predicting the remaining fracture life of field wrinkled steel pipelines subjected to cyclic deformations.
The oil and gas industry in North America uses steel pipelines as the primary mode for transporting natural gas, crude oil and various petroleum products. In Canada alone, about 700,000 km of energy pipelines are in operation (Yukon Government, 2006). Many additional pipeline projects are underway, especially in West Canada and Alaska, such as the Mackenzie Gas Project and the Alaska Highway Pipeline. Field observations of buried energy pipelines indicate that the subsurface geotechnical movements with or without thermal loads can introduce large forces and displacements on these pipelines, resulting in localized curvature, strains and associated deformations in the pipe wall (Yoosef-Ghodsi et al., 1995; Jayadevan et al., 2004). Often the local deformations of the pipe wall result in local buckling of the pipe wall (called wrinkling) and, in its post-buckling range of response, local buckles (wrinkles) in the pipe wall grow under sustained deformations. The wrinkling usually occurs under the combinations of internal pressure, axial load, and with or without bending moment (Yoosef-Ghodsi et al., 1995; Doreyet al., 1999; Bai et al., 2000).