Analysis And Numerical Simulation On Dynamic Response Of Buried Pipeline Caused By Rockfall Impaction

With the economic development the demand of petroleum resources is larger and larger in our country. Because of disproportion regional distribution of resources, the transportation of petroleum resources is becoming more and more important. As a high effectual and low consume way, transport pipeline has been commonly used. Many pipelines have been laid in the Midwest region in our country, it is a mountainous area with complicated geological conditions and all kinds of geo-hazards, such as landslides, rock-fall. Gas transmission pipeline is shallow buried thin-wall steel tube which has weak capability to resist the influence of geo-hazards, for the depth of burial is about 0.8m～2.5m. Take the gas pipeline from ZhongXian to WuHan as an example, which was laid in 2004, in this period, several dangerous rock near the pipeline failed and caused damage to the pipeline. It shows that, rock-fall is one of the most serious threat to the pipeline.Will the rock-fall damage the pipeline? The question must be answered before the plan of control project has been made. Usually it's answered by subjective experience, because the size and the shape of the rocks are different, also depth of burial is different in different regions, which will lead to blind plans.This paper tries to find a quantitative answer for this question. Theoretical analysis and numerical simulation have been done to research the dynamic response of buried pipeline caused by rockfall impaction. Based on the strength of pipe material, the ultimate bearing capacity of pipeline is researched, in the model the falling rock impacts the soil upright of pipe with a vertical velocity.Some research work has been done. In theoretical analysis, movement route of falling rock is analyzed, the impaction position and the velocity of rock can be used to primarily estimate the influence to pipe; the impact force is been calculated and analyzed based on non-perfect elastic collision method; the impact load on pipe is calculated based on Boussinesq method; the longitudinal stress and transverse stress are calculated respectively, distortion energy yield criteria is used to estimate the state of pipe material. ANSYS/LS-DYNA software is used to simulate the process of impaction. The model is the same as the one used in theoretical analysis. The results are compared and conclusions were made:1) The impact force can be calculated by conservation of energy principle and non-perfect elastic collision method. The impact force is about several decades to several hundred times of gravitation. When the size and shape of rock and the properties of layer are confirmed, the impact force has approximately direct ratio with the velocity of rock. The impact duration is about 10^-3～10^-2s.2) The vertical stress in soil cause by impaction load has a stress concentration region near the surface of pipe. The vertical stress distribution along longitudinal direction and cross direction has the similar pattern with that in static load, but has a faster attenuation from the impaction center to sidepiece.3) The most dangerous cross-section appears in the underside of impaction center. In theoretical analysis, both longitudinal internal force analysis and transverse internal force analysis are needed to obtain the biaxial stress. When the pipeline is shallow buried, the impact load can be regarded as a concentrated force, and the pipe can be regarded as a beam in elastic foundation during the calculation of longitudinal intemal force. It can be predigested as a plane problem in the calculation of transverse internal force, impact load can be regarded as uniformly distributed load, and the pipe cross-section can be regarded as a closed curved beam in elastic foundation,л.М.Емелъяновmethod is used to calculate transverse internal force.4) In the center of impaction, the pipe has a downwarping and the cross-section deformed from circular shape to elliptical shape. The maximum effective stress appears in the sharp peak of the cross-section. When the size and shape of rock and the properties of layer are confirmed, the maximum effective stress has a approximately direct ratio with the velocity of rock.5) The depth of burial has a major influence to the pipeline. According to the numbering simulation, in which only impaction load is considered, when the depth is lm, it satisfies the safety standards of pipe's material strength if the velocity is lower than 11.8m/s, but when the velocity is higher than 13m/s, the pipe will be damaged because the material will have plastic yield. When the depth is 2m, the velocities become 13.8m/s and 17m/s respectively.