Study On Hydrodynamic Characteristics And Loading Capacity Around Underwater Crossing Pipeline

With the increase in energy demand,the scale of oil and gas pipeline network is gradually expanding,the transportation range of long-distance oil and gas pipelines has become wider and wider,and the transportation environment has become increasingly complex.Underwater pipeline is widely used in practical engineering as the first choice when crossing complex water areas such as mountain rivers.However,due to the influence of natural and man-made factors,it is prone to be exposed and suspended during its year-round service.The suspended pipeline is prone to fracture under the impact of water flow,which not only affects the daily life of residents,but also brings damage to the ecological environment that is difficult to recover due to the ensuing oil spill.Therefore,it is of great significance to study the hydrodynamic characteristics and loading capacity of the underwater crossing pipeline and explore the variation law of the nearby flow field distribution and stress state with the topography in the process of scouring and expansion of the crossing pipeline,which will provide some guidance for pipeline maintenance and protection and safe operation.In this paper,the velocity distribution and pressure distribution around the pipeline under different parameters are measured by physical model tests of fixed and moving beds.With numerical simulation method,the flow field structure behind the pipeline was supplemented by the software ANSYS FLUENT and the LES model based on the verification of the accuracy of numerical simulation results.The hydrodynamic characteristics and loading capacity around underwater pipeline with different velocity,pipe diameter,flow angle and gap ratio under fixed and moving bed conditions are obtained.The main research results are as follows:(1)The velocity distribution around the pipeline and the pressure distribution on the surface of the pipeline were obtained by the physical model test of fixed bed.The results show that the amplitude of velocity change near the pipeline,the maximum velocity loss value and the velocity reversal value at the bottom of the pipeline all increase with the increase of pipe diameter and flow angle,and decrease with the increase of gap ratio.Under the oblique flow,the velocity distribution at different sections of the pipeline has obvious three-dimensional effect.The pressure distribution at all points on the surface of the pipeline tends to be symmetric with respect to the x-axis.The surface pressure and the resultant force on the pipe are positively correlated with the flow velocity,flow angle and pipe diameter,and negatively correlated with the clearance ratio.The empirical formula of the resultant force on the unit length pipeline is fitted by dimensional analysis method,which is in good fit with the test results and has good application in practical engineering.(2)By establishing a three-dimensional numerical model of the stationary bed physical test,a supplementary study was made on the rear wake structure of the pipeline under the stationary bed condition on the basis of verifying the correctness of the numerical model.The results show that there are periodic shedding Karman vortices behind the pipeline,and the magnitude of vorticity is about 10 times of the flow velocity.The size,number,shedding frequency and spacing between adjacent vortices increase with the increase of the flow velocity,flow angle and pipe diameter,and decrease with the increase of the clearance ratio.The turbulence characteristics,the length of the backflow area and the area of the wake area of the water behind the pipeline also have the same change law,and the three-dimensional characteristics of the wake structure behind the pipeline under the oblique flow are obvious.(3)Through the moving bed physical model test,the scour propagation law of the pipeline with initial scour was investigated,and the variation law of the velocity near the pipeline and the point pressure on the surface of the pipeline with the change of topography in the scour propagation process was obtained.The results show that the velocity distribution around the pipeline is greatly affected by the bottom bed surface,and the increase of the nearbottom velocity near the bottom of the pipeline accelerates the spread of the scour pit during the initial scour.With the increase of the scour depth,the near-bottom velocity decreases gradually,and the scour rate slows down until the scour equilibrium.The pressure at each point on the surface of the pipeline,the horizontal drag force and the rising force on the pipeline all decrease with the increase of scour depth.The fitting empirical formula for calculating pipe resultant force is in good agreement with the experimental results and can be used to predict the force of pipeline under the condition of moving bed scour.(4)By establishing a two-dimensional numerical model of moving bed test,the flow field distribution behind the pipeline under different terrain conditions was simulated,and the law of wake flow with terrain was investigated.The study shows that the wake behind the pipeline cannot form the Karman vortex street which falls off regularly due to the influence of the bottom bed surface.In the primary expansion stage,the size of vortex behind the pipeline increases with the increase of scour depth,and then the scour rate slows down.The size of the vortex street behind the pipeline decreases,and the gradually increasing vortex street near the bed surface hinders the development of scour,and the scour rate decreases until the scour equilibrium.