Analysis Of Strain Response Of Buried Pipeline At Fault Crossings

The problem of imbalance of oil and gas resources in various regions is effectively solved by the use of buried pipeline transportation.It plays an indispensable role in energy allocation all over the world.Long-distance buried pipelines inescapably go through seismic fracture zones which makes the buried pipelines be easily influenced by the diastrophism.A strong and resilient pipe can withstand a certain degree of seismic oscillation.However,it is really difficult for buried pipes to resist the permanent ground displacement caused by faulting and surface damage.Finite element method(FEM)is used in this paper to study the mechanical responses and influence rules of sensitive parameters of the cross-fault buried pipe conveying fluid.It provides advice and references for the finite element modeling of cross-fault buried pipeline as well as the conduct of anti-seismic design and measures of pipes.The main conclusions are as follows:(1)The analytical methods of cross-fault buried pipe responses domestic and overseas have been investigated and analyzed and be divided into theoretical-analytical method,numerical analysis method as well as experimental study.Limitation of each method is summarized and existing seismic measures of buried pipeline are studied.(2)The faulting mechanism of different types of fault is studied and the empirical formula of magnitude and fault displacement is summarized.The failure modes of buried pipes due to fault displacement are divided into tensile failure and buckling failure and the influence factors of pipeline failure are discussed.The stress and strain based pipeline failure criterions are compared and then the strain-based plastic failure criterion is established.(3)Applicabilities of nonlinear theory,elastoplastic theory,shell theory and fluid-solid coupling theory in this paper are analyzed.ADINA and ANSYS software are chosen after comparison to build the finite element model.The pipe model is established by MITC4 shell element,the fault is modeled by 3D-solid element as well as fluid conveys by the pipe is modeled by 3D-fluid element.Pipe-soil interaction is represented as 3D nonlinear contact and coupling between pipe and fluid is calculated iteratively by FSI.Henceforth,the three-phase coupling model of soil-pipe-fluid is established and its effectiveness has been verified by the full scale experiment conducted by Cornell University.(4)In order to prove the influence between internal fluid and the response of cross-fault pipeline which turns to be an innovation point in this field,the stress and strain responses of pipe under three different conditions including blank pipe,pipe under uniform internal pressure and pipe conveying fluid are analyzed.It proves that the internal fluid forces the pipe to expand and restrains the pipe's deformation due to the fault at some extent.This state enhances the pipeline's ability to adapt to the fault and is more in line with the actual conditions.(5)The influence of fault type,fault displacement,fault and pipe angle,buried depth,pipe diameter and thickness,pipeline steel grade,fluid pressure,fluid velocity,fluid temperature and soil properties on cross-fault pipe responses have been studied.Based on the FEA results,the predictor formula of axial compression strain of pipe under the movement of reverse fault is proposed through the multivariate nonlinear regression theory combined with MATLAB software programming.(6)Based on FEA results combined with the following concept 'The priority is to reduce the computational strain' raised by 'Seismic technical specification for pipelines of oil and gas pipelines'(GB/T50470-2017),general seismic measures have been obtained.Zigzag piping arrangement which means adding elbows to the pipe has been proposed and studied.The influences of bend angle as well as thickness on reducing the pipe's strain have been analyzed in this paper.