| Underwater crossing pipelines in rivers are prone to be exposed during the scouring process all year round and protection measures such as heavy block are required to ensure their stability in place.However,the scour characteristics of pipeline under action of water flow are still not clear.The uncertainty of terrain changes around pipeline poses a threat to the safe operation and subsequent maintenance of pipeline.Therefore,study the flow field distribution and scour characteristics around pipeline under heavy block protection measures can provide important theoretical support for design,construction and maintenance of underwater pipeline heavy block protection measures.This paper studied the flow velocity around the pipeline under different working conditions through fixed-bed model test.On basis of the fixed-bed model test,a three-dimensional fluid-solid coupling numerical model was used to further analyze the flow field distribution law around pipeline.Through movable bed model test,the scour characteristics of riverbed around the pipeline are studied,and following main conclusions are drawn:(1)The fixed-bed model test is used to study the velocity distribution under different spacing between heavy block,length of heavy block,water flow velocity,and water depth of the river channel near pipeline.The results show that: The local scour velocity around pipeline increases with water flow velocity and length of heavy block,and decreases with water depth of river and spacing between heavy block.The maximum value of local scour flow rate is located at 0.5 times of pipe diameter D from the top of pipeline,where velocity fluctuates greatly,presenting an "S" shape.(2)The flow velocity data of fixed bed model was used to verify the reliability of three-dimensional numerical model.The numerical model further analyzed the flow pattern of water around pipeline,the pressure distribution of river bed and the bed shear stress.The results showed that: Under action of water flow,clockwise vortices will appear at the front and back of pipeline.The vortex on the upstream side will cause erosion pits on the upstream side of pipeline,and the vortex on backstream side will cause accumulation of sediment.When spacing between heavy block and length of heavy block are changed,the pressure difference in the river bed before and after pipeline will also change.A larger pressure difference will cause seepage damage to the river bed sediment at the bottom of pipeline.The riverbed shear stress is the largest at 1.5 times of pipe diameter D on the upstream side of pipeline,and it can be seen that the upstream side of pipeline is most prone to scour.(3)The scouring process,scouring mechanism,scouring topography and maximum scouring depth of riverbed around pipeline under different working conditions are studied through movable bed test.The results show that: The scour process of bare river-crossing pipeline under protection of heavy block can be divided into four stages,the formation of scour pits,the suspension of heavy block on upstream side,the suspension of pipeline on upstream side and the balance of erosion.In the scour balance stage,the maximum scour depth of pipeline is located in pipeline section where heavy blocks are arranged.In the process of changes in water flow velocity and river depth,the maximum scour depth is between 0.9 and 1.6 times of pipeline diameter D.The maximum scour depth of pipeline is positively related to the length of heavy block and flow velocity,and negatively related to the spacing between heavy block and depth of river.The length of heavy block,the spacing between heavy block,the hydrodynamic conditions and the characteristics of sand grain all affect the maximum scour depth of pipeline.An empirical formula for predicting the maximum scour depth of pipeline exposed through river under the protection of heavy block is proposed by dimensional analysis method,and its reliability are verified by experimental data. |
PDF Full Text Request