رفتار خط لوله X60 در معرض تغییر شکل های محوری و جانبی

ABSTRACT Behavior of X60 Line Pipe Subjected to Axial and Lateral Deformations

The energy related industries in North America use 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. Many additional pipelines projects especially in West Canada and Alaska of various scales such as the Mackenzie Gas Project and Alaska Highway Pipeline are underway. The Alaska Highway Pipeline Project, which will run between Prudhoe Bay in Alaska to various parts of USA through Yukon, British Columbia, and Alberta, will alone cost about U.S. $20 billion [1]. The majority of these pipelines run below ground and in the arctic and subarctic regions.

Field observations of these buried pipelines indicate that it is not uncommon for geotechnical movements to impose large displacements on buried pipelines, resulting in large localized deformations, strain, and curvature in the pipe wall. Such displacements may be associated with river crossings, unstable slopes, or regions of discontinuous permafrost. Often the deformation of the pipe wall results in local buckling (called “wrinkling”) and, in its post-buckling [post-wrinkling] range of response, wrinkle develops rapidly to a significant magnitude.

This can occur under the loading conditions that may be idealized as combinations of variable internal pressure, compressive axial load, lateral load, and moment.

Several studies [2–9] were completed to understand how wrinkle in the field line pipe initiates and grows under monotonically increasing axisymmetric axial deformation and under monotonically increasing bending deformation with and without internal pressure. Typically, for this type of loading, the specimen will begin to buckle locally when the longitudinal stress reaches a magnitude in the vicinity of the yield stress [10]. The buckle then amplifies in an elastic-plastic manner and develops into a wrinkle of large amplitude as the specimen shortens and the load falls off.

That is, the specimen “softens.” The study by Das et al. [5] shows that a X52 grade wrinkled pipe subjected to monotonically increasing axisymmetric axial deformation and internal pressure may not rupture, and multiple wrinkles can form that look like an “accordion” shape [Fig. 1]. Other studies by Das et al. [11,12] show that the wrinkled pipe may lose its structural integrity because of rupture formation in the wrinkle region, if it is subjected to elastic-plastic strain reversals due to axial and bending cyclic deformations that develop due to temperature variations, pressure fluctuations, and freeze-thaw cycles of the ground [Fig. 2].

The objective for the current study is to investigate the wrinkling behavior and failure mode in the wrinkle under combined lateral and axial loadings. The motivation of this study was due to the diagnosis and exposure of a wrinkle shape that looks like a cowboy hat, and the rupture that occurred in the wrinkle location of a field X60 grade NPS10 (line pipe with nominal diameter of 10 in. or 254 mm) energy pipeline with a wall thickness of 7.9 mm, as shown in Fig. 3. This field specimen was obtained from a segment on of the pipeline on an unstable slope. The operating pressure in that segment of the line pipe ranges from 0.3py to 0.35py. The physical inspection of the NPS10 field line pipe showed physical marks of soil thrust and rubbing in the portion of the pipe just above the wrinkle, and therefore, it was felt that the pipe wall in that region was subjected to a lateral load (load not aligned to the longitudinal axis of the pipeline). However, the real load combination was not known. Das et al. [13] conducted one laboratory test on a X52 grade pipe to understand the possible load combination that may have caused the rupture in the field NPS10 line pipe. The test load and specimen end conditions could not be controlled systematically in that test though the final deformed shape showed some similarities with the field NPS10 pipeline. Thus, a detailed and carefully designed experimental research program was undertaken at the University of Windsor for better understanding of the load and boundary conditions that possibly produced a wrinkle and the rupture in the wrinkle region that look similar to the field NPS10 line pipe. Four full-scale laboratory tests on NPS6 (pipe with a nominal diameter of 6 in. or 152 mm) X60 grade pipe [14] specimens were successfully completed, and the results are discussed in this paper.