Study On The Uplift Resistance And Ratcheting Behavior Of Pipelines Buried In Sand

With the vigorous development of the offshore oil industry, as the most efficient, economical and reliable mode of transport, pipeline has been widely used. However, the buried pipeline which may be subject to upheaval buckling under the interaction between soil and pipeline has attracted more and more concerns.Based on plane strain assumption, this paper investigates the uplift resistance and ratcheting behavior of pipelines buried in sand by PFC discrete element numerical simulations and physical model tests. Pipeline may occurs cyclically when buried pipelines are subject to temporary suspensions in supply followed by continuation which is called thermal cycle, due to the flow of sand and the interaction between soil and pipeline, pipeline will move some distance in vertical direction and will not return to the initial position after each cycle, this phenomenon is known as pipeline ratcheting. The main contents of this study are as follows:(1) The monotonic uplift tests of pipelines buried in dense sand and coarse gravel are carried in different cases. The soil flow mechanisms are discussed by PIV. Model tests and numerical simulations reveal that the uplift resistance-displacement curve can be divided into 3 stages, i.e. the uplift resistance increases rapidly in the initial uplift process until reaching a peak value, then reduces gradually due to the strain softening of sand, and finally reaches to a steady phase. In the development stage of pullout force, the shear zone extends from the ’shoulder of pipeline’and development to the soil surface, the angle between the shear zone with the vertical direction is about 2-fold dilation angle; the angle decreases in the softening stage; the angle is zero which means the shear zone is straight up. Based on test results, the three-stage prediction model is proposed.(2) DEM (discrete element method) is adopted to analysis the pullout process of pipeline. The results agree well with the experimental data on coefficient of peak resistance. Comprehensive numerical analysis, model test of standard sands and coarse gravel, the effect of particle size on the change characteristics of resistance is discussed.(3) Model test and numerical model are used to investigate the ratcheting effects of pipeline. The results show that the stress amplitude of cyclic loading is the important factors to cumulative displacement. In the cyclic uplift tests, the distribution of shear zone in unloading and loading stage is similar, but the mobilized direction of soil is opposite. Less cyclic number but larger displacement amplitude of cyclic uplift test reveals that the constraining force on the pipeline can return to the force-displacement curve of monotonic uplift tests.