Numerical Simulation of Stress Relief of Buried Pipeline at Pembina River Crossing

Pipelines in operation often experience various loadings due to operational and environmental conditions. Large strain may be accumulated in the pipes under these loadings, and it may eventually induce local buckling or even fractures on the pipes. It is a common practice that a stress relief procedure is applied to a pipe by removing the soil around the pipe, allowing the pipe to spring back to a zero load state. The frequency of stress relief procedure is dependent on the severity of loading and soil conditions. This project is intended to study the behavior of buried pipes subjected to repeated stress relief procedures. The buried pipeline at Pembina River Crossing in Lodgepole, Alberta was simulated using the finite element method and the results were compared with field measured data. The pipeline at Pembina River Cross is situated at the active soil movement locations. A finite element model was developed to simulate the slope movement and the pipeline response. The correlation between soil movement and precipitation was investigated. With shell elements for pipe, 3D-solid elements for soil, this model captures the global and local behavior of pipeline. Soil-pipe interaction was simulated by setting a weak layer of soil surrounding the pipeline. The model incorporates nonlinear material, slope soil creep and water table change. Modified Drucker-Prager Cap Model was applied to soils based on direct shear test results. The finite element model was calibrated by slope indicator data and strain gauge data with satisfactory agreement. The model was used to simulate the strain accumulation and the stress relief in the pipeline, before and after the stress relief operation. Reasonable agreement was achieved when compared to the field data. The model can be used to further understand the behavior of pipe under repeated soil movement and stress relief procedure. It can also be used to develop the optimum stress relief procedure and operating schedule.