Study On Analytical Methods Of Buried Steel Pipelines Under Active Faults

As an important part of the urban infrastructures such as transport, communication, heating supply, water supply and gas supply, and the most efficient, economical and reliable transportation way of oil and natural gas, buried steel pipelines are called as the underground urban lifelines and the lifelines of oil and gas fields. With the development of urbanization and exploitation of oil and gas resources in China, the construction of buried steel pipelines increases day by day, and the structural integrity and reliability are paid more and more attention in practice. China with a vast territory is an earthquake-prone country, and almost all provinces take place strong earthquakes in history. Damages and ruptures of buried pipelines may result in catastrophic disruption of important urban infrastructures and leakages of oil and gas, and even cause fire and explosion. It has serious influence on the relief during earthquakes, the reconstruction after earthquakes and the ecological environment. Fault movements probably represent one of the most severe earthquake effects on a buried pipeline. The aseismic design for buried steel pipelines crossing a fault and the seismic safety evaluation for existing pipelines can avoid the above accidents to some degree. Therefore, it is of great significance to research the earthquake response and failure mechanism of buried steel pipelines subjected to fault movements.To ensure the calculating accuracy and reduce the calculating time, the improved nonlinear finite element (FE) model of buried steel pipeline subjected to active faults is proposed by introducing the pipe-shell element model that allows the cross-section to ovalize and warp. The visualization of the cross-section deformation of the pipe-shell element is achieved by the secondary development based on ADINA. Considering the nonlinearities of pipe steel constitutive model, pipe-soil interaction and geometric, the FE model can be simultaneously applied to various operation loads such as internal pressure and temperature, and accidental loads including permament ground deformation. After the validations of the correctness and effectiveness of the FE model, the various influencing factors are analyzed. The results indicate that the nonlinear models of pipe steel and pipe-soil interaction should be used in the earthquake response analysis of buried steel pipelines under faults. The geometric nonlinear characteristics must be considered at large fault displacements. Furthermore, the effects of the single and combined actions of the internal pressure and the thermal loading on the earthquake responses of buried steel pipelines crossing faults also are studied.Based on the nonlinear pipe-shell FE model of buried steel pipeline subjected to active faults, an axial nonlinear spring located on each end of the pipe is employed to simulate the response of a buried pipeline far away from a fault. Thus, the pipe calculating length is reduced and the analysis efficiency is improved. Using the tri-linear and Ramberg-Osgood stress-strain relationships for pipe steel and the nonlinear pipe-soil interaction model recommended from the standard of American Lifelines Alliance (ALA), which can consider the effects of soil types, the force-displacement relationship of the equivalent boundary nonlinear spring is derived under the condition that the pipe stress with small deformation between pipeline and soil is less than or equals to the ultimate stress of pipe steel. An updated equivalent boundary pipe-shell FE model of buried steel pipeline under active faults is established. The pipe calculating length of the equivalent boundary FE model of buried steel pipeline subjected to a strike-slip fault under uniform site soils can be reasonably obtained by Kennedy method. Compared with the results from conventional FE model, the equivalent boundary FE model is validated.With the bilinear and Ramberg-Osgood models for pipe steel and the nonlinear characteristic of soil-pipeline interaction, a refined analytical methodology of buried steel pipelines subjected to strike-slip faults under uniform site soils is proposed. Based on the elastic-beam and beam-on-elastic-foundation theories, the position of pipe potential destruction, the strain and deformation distributions along the pipeline and the pipe maximum axial total stress and strain are derived. Compared with existing analytical methods and three-dimensional (3D) nonlinear FE analysis, the results show that the pipe maximum axial total strains from the analytical methodology presented are in good agreement with the FE results at small and intermediate fault movements and become gradually more conservative at large fault displacements. The position of pipe potential failure and the lateral pipe-soil relative displacement are fairly consistent with the FE results. Furthermore, for large-diameter buried steel pipelines at smaller fault displacements, the effects of pipe-soil nonlinear interaction have to be taken into account, especially with smaller crossing angles.Based on the Ramberg-Osgood model for pipe steel, an analytical methodology of buried steel pipelines under active faults is proposed. The analytical method is applicable to various faults that lead to pipe elongation and can analyze the effects of heterogenous site soils in both sides of the fault. The position of pipe potential failure, the analytical formulas of deformation and strain distributions along the pipeline, and the pipe maximum axial total stress and strain can be obtained, as well as the rotational angle at the intersection of pipeline and fault subjected to a strike-slip fault under uniform site soils. With increasing the partitions of pipelines, the accuracy of analytical solutions can be improved further. Compared with 3D nonlinear FE analysis, the analytical method presented is validated, and some significant conclusions can be drawn. Meanwhile, a software system of buried steel pipelines under active faults is developed by integrating the proposed analytical methods in the paper and existing analytical methods.