The effect of internal pressure on the longitudinal behaviour of pipelines

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Pipelines are one of the most important modes of transportation for oil and gas from the location of the resources to the refineries. They constitute an influential part of the Canadian economy. To achieve their design objective, pipelines have to safely transport a highly pressurized flammable substance. In doing so, pipelines need to sustain a combination of circumferential stresses due to the internal pressure and longitudinal stresses induced by external loads. These external loads are primarily the result of soil movement and temperature effects, both induced by the harsh Canadian environment. As the pipe traverses areas of discontinuous permafrost abundant throughout Canada, it is subjected to differential settlement exacerbated by the large seasonal temperature differential which induces large longitudinal stresses. In addition, the pipelines are usually required to cross various water and road ways as well as varying surface topography. The locations of these can potentially have a concentration of longitudinal pipe deformation deeming them very critical for a safe pipeline design. This proposal addresses three areas in the current Canadian design guidelines in which the code equations do not consider the interaction between the internal pressure and the longitudinal response of the pipeline.  The current code equations for the stress based design of pipelines do not consider the complex material properties of the newly developed high strength steel (HSS). In the first area, we are proposing to provide new equations that would allow the proper design of HSS against longitudinal loading and internal pressure. Similarly, the code equations do not consider the effect of the internal pressure for the strain based design of pipelines with circumferential girth weld flaws. However, our previous results suggest the dependency of the critical longitudinal strain on the level of internal pressure. We are therefore proposing to further investigate the interaction between the critical longitudinal strain and the internal pressure using numerical models. Our long term objective is to provide equations to describe that dependency. In the third area, we are proposing to study the post-buckling behaviour of pipelines under the effect of various levels of internal pressure. We are hoping to provide equations that describe the required level of depressurization in the post-buckling phase allowing the safe operation of the pipelines until the wrinkle or buckle is removed. Our project has a large impact on the pipeline industry in Canada. The results of our project will be used by the pipeline industry to make informed decision in the design and operation of the thousands of kilometers of pipelines buried under the Canadian soil.