Seismic Response Analysis And Strain Design Method For Buried Pipelines Across Faults

Most of the long distance pipelines in our country are situated in poor geological environments, and the pipelines across the active fault zone are in the most severe conditions. Therefore, the earthquake resistance design and technology of buried pipelines is crucial. The failure mechanisms of buried pipelines under the action of cross fault, the seismic response analysis of buried pipes at fault crossing and the seismic design of the buried pipelines at the fault zone are in urgent need of study. This paper focuses on the research on seismic response of buried pipelines at fault areas. The theoretical basis of this research is the theories of nonlinear analysis and pipeline seismic response. The main methods of this research are literature research, earthquake data analysis and numerical analysis. The mechanical properties of pipelines under the action of fault zone are analyzed, while instructions on strain-based anti-seismic design are given. The main contents and results are as follows:(1) Through literature research and theoretical study, the failure modes and mechanisms of buried pipelines under the influence of fault ruptures are analyzed. The influences on seismic response are summarized into four aspects:fault zone property, pipeline stock, burial pattern and site conditions. Different analytical methods are studied. These set the theoretical basis and approach for the paper.(2) Using ABAQUS simulation analysis software, the finite element analysis model of buried pipelines across the fault is established to study strike-slip faults. Pipeline model is set as "crust unit". Soil model is C3D8 entity unit. After several model trials, the physical model size is determined to be 50 m X 10 m X 10 m. Nonlinear contact model of tube and soil is established. Three line model and Mohr-Coulomb model are adopted for the pipe and soil materials respectively. Model subdivision and mesh grid refinement are implemented to improve the model accuracy. According to site soil conditions, fault rupture characteristics of the movement and pipeline performance, reasonable model boundary conditions and mechanical load are set. Through engineering sample calculation and comparison calculation with analytical methods, the reliability of the finite element model is verified.(3) Based on the finite element analysis model of buried pipelines across the fault, considering 8 factors:fault displacement, wall thickness, internal pressure, the angle of the buried depth, thickness/diameter ratio, backfill soil, using the method of numerical simulation, controlling 8 groups of variables, setting different parameter values,47 different working conditions are calculated. The deformation response of buried pipelines under the effect of fault movement is discussed, and the deformation laws of pipelines under different fault displacements are analyzed. Based on the research of stress and strain distribution along the tube axis direction, locations of potential pipeline damages are identified. With the stress and strain contours and the mechanical characteristic curves of pipelines, systematic analysis of earthquake response of buried pipelines across the fault under different factors is conducted. Thus the principles of how various factors influence fault rupture reactions are obtained.(4) In terms of the strain-based pipeline design method, using the finite element model, the method for determining pipeline design strain is studied. According to the principles of how different factors influence the buried pipelines across the fault pattern, instructions on increasing pipeline design strain are put forward. The differences and scopes of strain-based/stress-based design methods are clarified, and criteria and methods of strain design of buried pipelines across the fault are given. Based on the finite element analysis model, certain pipeline working conditions are selected, the maximum strain in each case simulated and calculated. The principles of how different factors influence the pipeline maximum strain are analyzed. Finally, regarding the strain design of buried pipelines across the fault,22 maximum strain values under different working conditions are obtained, which might serve as a numerical reference for the designs of actual engineering pipelines.