Effectiveness of the stress relief procedures and their effects on local buckling behaviour of buried pipes

Buried pipelines are the prime mode used by energy industries in North America for transporting natural gas, crude oil, and other hydrocarbon products. At unstable slopes such pipelines in operation are often subjected to combination of large axial deformation and rotation because of geotechnical and environmental conditions. Large strain may be accumulated in the pipes under these loadings, and it may eventually result in the formation of local buckling or even fractures in the pipe wall. 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 procedures is dependent on the severity of loading and soil conditions.;A full-scale test program consisting of twelve pipe specimens was conducted. Six tests were loaded axially and other six were loaded under combined axial load and bending. Under each type of loading, the specimens were loaded either monotonically or cyclically. The pipes were under different levels of internal pressure. Results from full-scale tests and numerical analyses show that the load cycling has minimal effect on the global response of the pipes. However, there is more accumulated strain after peak response of pipe at buckling location in cyclic bending than in cyclic axial compression. The general behaviour of the pipe walls was very predictable by the numerical model.;Monitoring programs on two pipelines (Pembina River Crossing and Simonette River Crossing) constructed at active landslides in Alberta are used to obtain information necessary for the analysis, assessment, and possible mitigation of geotechnical hazards. Slope movement and pipeline deformation data were collected for the calibration of the numerical model developed in the program. Data before and after the stress relief procedure were recorded for investigation of the effectiveness of the stress relief procedure.;A finite element model was developed to simulate the slope movement and the pipeline response at Pembina River Crossing situated at the active soil movement locations. Shell elements were used for pipe and 3D solid elements for soil. Soil-pipe interaction was simulated by setting a special layer of soil surrounding the pipeline. The model incorporates nonlinear material, soil creep and water table changes. The Modified Drucker-Prager Cap Model was used to model the soils based on parameters determined from the direct shear test results. The finite element model was calibrated by slope indicator data and strain gauge data obtained from the monitoring program with satisfactory agreement. The model was used to simulate the strain accumulation in the pipeline before and after the stress relief procedure. Reasonable agreement was achieved when compared to the field data. The model was also used to determine the critical section of the pipeline and to develop the optimum stress relief procedure and schedule for the pipeline at Pembina River Crossing. The guidelines for the stress relief schedule and procedures for the pipeline were given. The model was used in a parametric study to further understand the behaviour of buried pipelines under repeated soil movement and stress relief procedure.;This research program was designed to investigate the effectiveness of stress relief procedures and evaluate the behaviour of buried pipes subjected to repetitive stress relief procedure, and assess the timely implementation of the procedure. In order to achieve the objectives, the research program was divided into three phases: Phase I: full-scale laboratory tests on pipeline segments under repetitive cyclic loading; Phase II: pipeline field monitoring program; Phase III: development of finite element model for buried pipelines under stress relief procedure.