Evaluation Of Seismic Capacity Of Existing Oil And Gas Pipelines Across Faults

China is one of the most seismically active countries in the world.The experience of earthquake damage at home and abroad shows that seismic fault dislocation is the most important factor causing the damage of oil and gas pipelines.Long-distance oil and gas pipelines often have to pass through active faults because of the constraints of the alignment of mountains and river valleys.Because of steel pipe corrosion,material fatigue and other factors,the anti-seismic capacity of the existing pipeline with the passage of time than the initial construction of continuous decline,faced with a high risk of earthquake damage.Therefore,it is one of the urgent needs for seismic safety of oil and gas pipelines in China to carry out seismic capacity assessment of existing oil and gas pipelines across faults.Based on the characteristics of earthquake damage of buried pipelines,some key problems of seismic capacity evaluation of existing pipelines across faults are studied in this paper,the main contents include:the relationship between fault rupture and seismic factors,the law of pipe-soil interaction and the improvement of soil spring model,the law of pipeline damage caused by corrosion factors,the influence of backfill density,temperature and humdity on the pipeline,the characteristics and shortcomings of pipeline seismic design codes in different periods.The main research contents and achievements of this paper are as follows:(1)The experiences and lessons of earthquake damage to oil and gas pipelines at home and abroad are summarized.Based on the evolution of the analysis methods for pipelines under the action of faults,the characteristics and differences of the ASEISMIC design codes for pipelines in different periods are analyzed from the aspects of fault displacement,allowable strain of pipelines,aseismic checking methods,etc.,the actual case of west-to-east gas transmission in service in China is analyzed.For the need of seismic safety,the current seismic code(Gb/t 50470-2017)in China is more strict than the earlier code in terms of seismic fortification requirements,allowable strain of pipelines and analysis methods.In other words,piping designed in accordance with earlier specifications presents a potential safety hazard.(2)Based on the multi-level requirement(including the maximum displacement and the average displacement)of fault seismic fortification in the current seismic code for pipelines in our country,and on the basis of increasing actual earthquake cases by using Monte Carlo method,in this paper,a new formula of fault coseismic dislocation considering magnitude and focal depth is presented,and the distribution of fault coseismic displacement along fault strike is studied.It is suggested that 2/3 of the maximum displacement of the fault should be used for seismic fortification.(3)Considering the anti-seismic measures of the pipeline crossing the fault by backfilling the loose sand soil,and through the mechanical analysis of the pipeline moving in the loose sand soil for the thrust fault,based on the existing theories and methods of wedge failure,a sand-soil pushing model is proposed.The passive Wedge Failure Model,which can only calculate the horizontal pipesoil Interaction Force,is improved,and a new method for calculating the horizontal and vertical force of soil spring is proposed.The reliability of the three-dimensional nonlinear finite element model is verified by taking the west-to-east gas transmission through fault as an example.(4)The law of pipeline corrosion caused by acid soil and alkaline soil is analyzed and summarized.This paper studies the corrosion rate of different backfill soil to pipeline,compares the influence of different backfill soil on the corrosion of steel pipeline,and develops the law of the influence of backfill soil density,temperature and humidity on the corrosion of pipeline.The greater the density of backfill,the lower the temperature and the humidity,the greater the damage to the pipeline.The results show that the corrosion rate of backfill sand is the smallest.The corrosion model of sand to pipeline is established and the case study is carried out.(5)Based on the analysis of multi-parameter influencing factors,two evaluation methods of pipeline cross-fault aseismic capability are proposed,which are fully quantitative and semiquantitative.The full quantitative evaluation method is suitable for the situation of single environment condition and high accuracy of data.The results show that the improved Kent Index analysis method can be used in the complex pipeline environment and incomplete data,but it is a fast evaluation method.The evaluation method can be used to evaluate the ASEISMIC capability of existing pipelines when they pass through active faults.