The effect of internal pressure on the longitudinal behaviour of pipelines

管道是石油和天然气从资源所在地到炼油厂的最重要的运输方式之一。他们是加拿大经济的重要组成部分。为了实现其设计目标，管道必须安全地输送高压易燃物质。在这样做时，管道需要承受由于内部压力引起的周向应力和由外部载荷引起的纵向应力的组合。这些外部荷载主要是土壤移动和温度影响的结果，两者都是由加拿大恶劣的环境引起的。由于管道穿越加拿大各地丰富的不连续永久冻土区，它会受到因季节温差大而加剧的不均匀沉降，从而引起较大的纵向应力。此外，管道通常需要穿过各种水路和道路以及变化的表面地形。这些位置可能会有纵向管道变形的集中，认为它们对于安全的管道设计非常关键。本提案涉及加拿大现行设计指南中的三个方面，其中规范方程未考虑管道内部压力和纵向响应之间的相互作用。 目前的管道应力设计规范公式没有考虑新开发的高强度钢（HSS）的复杂材料特性。在第一个方面，我们建议提供新的方程，使HSS的适当设计，以对抗纵向载荷和内部压力。类似地，对于具有环向环焊缝缺陷的管道的基于应变的设计，规范方程没有考虑内压的影响。然而，我们以前的结果表明，依赖于临界纵向应变的水平上的内部压力。因此，我们建议使用数值模型进一步研究临界纵向应变和内部压力之间的相互作用。我们的长期目标是提供方程来描述这种依赖关系。在第三个领域，我们建议研究管道在各种水平的内压作用下的后屈曲行为。我们希望提供描述后屈曲阶段所需降压水平的方程，以允许管道安全运行，直到褶皱或屈曲被移除。 我们的项目对加拿大的管道行业有很大的影响。我们项目的结果将被管道行业用于在埋在加拿大土壤下的数千公里管道的设计和运营中做出明智的决定。

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.