Research On Strength Behavior Of Long-distance Buried Pipe For Oil And Gas Transportation In The Landslide Area

With the development of global economy and industrialization process, demand for resources such as natural gas and oil grows rapidly around the world. At present, buried pipes have become one of the significant ways to transport these resources from remote area for the advantages of high transport and economic efficiency. Because the long-distance buried pipes have to pass through vast territory with complex geology, it will inevitably suffer from the adverse effects from surrounding environment. With increasing frequency of geological disaster and extreme weather, accidents of buried pipes caused by landslide, which is one of the typical permanent ground deformation actions, has roused wide attention in recent decades. So far, there are two significant problems that are urgent to be solved in the academics and engineering:the first one is how to predict the limit load-bearing ability of buried pipe caused by landslide actions accurately and effectively; the second one is to evaluate whether the buried pipe that suffers from deflection load during landslide process can still service in safety without substitution.In order to solve the two problems mentioned above, research on the strength behavior of buried pipes under landslide action is given in this paper. This paper is supported by two special funds for quality supervision research in the public interest (Grant No.200910096-2and201210242). The main contents are given as follows:(1) A3D numerical model is established to predict the mechanical and deformation response of buried pipe due to deflection during landslide process. In the model, the nonlinear effects of material, geometry and contact problems are comprehensively considered. First, the external loads arising from the deformed soil during landslide process are controlled by deflection displacement, and the loading location is applied on the surface of surrounding soil that is close to the buried pipe; Second, the contact pairs with finite sliding property is used to simulate the interaction between the pipe and soil. Third, the true stress-strain curve of pipe material is corrected properly according to the Bridgman law, which ensures the accuracy of finite element analysis.(2) Based on the theory of elastic-plastic mechanics, the limit load-bearing ability of buried pipes under landslide action is predicted using the arc-length algorithm and non-linear stabilization algorithm, and the stress-strain responses of buried pipe at plastic collapse and strain softening stages are obtained. Besides, effects of internal pressure, D/t ratio, width of landslide, mechanical properties of soil and pipes on the flexible deflection ability are investigated. It is revealed that the maximum value of1st principle strain only depends on the mechanical properties of soil and buried pipe when the structure comes into the plastic collapse stage. Thus, this strain can be used as a reference value to determine the allowable strain in engineering later.(3) Based on the theory of elastic-plastic fracture mechanics, the whole fracture process of buried pipe such as crack initiation and propagation under complex loads are demonstrated using the extended finite element method. The simulation contents include:(a) material performance tests of tensile and three-point bending;(b) Burst tests of straight pipe without deflection;(c) Fracture process of buried pipe due to deflection arising from landslide actions. According to the finite element analysis, some conclusions can be obtained:First, the limit value of mesh size near the crack-tip field is obtained which can ensure the computational efficiency and accuracy; Second, σmaxps and Gc are two significant parameters to affect the crack behavior and limited load-bearing ability of buried pipes. The former one represents great influence on the damage initiation property, while the latter one mainly controls the fracture toughness to resist crack propagation; Third, effects of internal pressure, D/t ratio, width of landslide, performances of soil and pipes on the crack behaviors are also investigated. Finally, the values of1st principle strain as crack propagates unstably and pipeline fractures are obtained. Since the two strains only depend on the mechanical properties of soil and buried pipe, they can be used as another reference values to determine the allowable strain later. (4) A common strength failure criterion based on the1st principal strain is proposed to determine the safe properties of buried pipeline under this special failure issue. In the criterion, the value of allowable strain is determined by the results of finite element analysis and limited values in the corresponding standards. For the buried pipe without defects, the allowable strain is given by the strain as buried pipe comes into plastic collapse stage, the strain as buried pipe ultimate fractures, and the strain constrained in the ASME B31.8standard. For the buried pipe with crack-like defects, the results evaluated by XFEM and FAD in API579standard are compared.(4) A typical accident of buried pipeline arising from landslide actions is used to verify the finite element model and strength failure criterion provided in this paper. The contents of failure analysis consist of the base metal/welding mechanical testing, fractographic testing and site inspections, which can provide technical support for the design and risk-based inspection of buried pipes.