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Numerical Investigations Of Local Scour Around Pipelines Under Unidirectional Flow

Posted on:2023-05-01Degree:MasterType:Thesis
Country:ChinaCandidate:H C ZhangFull Text:PDF
GTID:2530306827972319Subject:Port, Coastal and Offshore Engineering
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As the the lifeline of marine oil and gas,Pipelines are channels for offshore oil and gas transportation with the characteristics of continuous,fast and large transportation and play an important role in the transportation of marine oil and gas.Because pipelines traverse deep and shallow waters,under the long-term action of ocean currents,the sand bed around the pipelines are prone to migration and erosion,resulting in the instability of the pipeline.At the same time,because the monitoring and maintenance of submarine pipelines are difficult,once rupture or leakage occurs,it will bring serious environmental pollution and economic losses.Therefore,it is of great significance for the normal operation of pipelines to fully understand the local scour characteristics around pipelines and guide the safety design of the project.In this paper,the numerical analysis model of computational fluid dynamics is used to analyze the local amplification characteristics of shear stress on the surface of the sand bed under the pipeline under the action of unidirectional flow,the local scour characteristics of pipelines on different sand beds,and the local scour problem of two pipelines,where the shear stress amplification factor below the pipeline refers to the ratio of the fluid shear stress at this point to the shear stress under the condition of no pipeline,which reflects the local shear stress change caused by pipeline laying.In the first chapter of the thesis,the research background is mainly introduced,the current research status in this field are summarized,and the key points of the research work of the thesis is put forward.In the second chapter,the numerical analysis model of local scour of the pipeline and its verification are mainly introduced.In the third chapter,the characteristics of shear stress amplification factor around pipelines are mainly studied.The research results show that under extremely shallow water conditions,the smaller the relative water depth,the greater the change of the water level on the free surface above the pipeline,and even the occurrence of hydraulic jumps,causing a dramatic change in the flow field behind the pipeline,resulting in the scour depth and shear stress amplification factor.Increase.When the water depth is greater than 3D(D is the diameter of the pipe),there is no water level difference on the free surface above the pipe.When the water depth is greater than 6D,the influence of the water depth change on local scour can be ignored.At the same time,the larger the gap ratio between the pipeline and the bed surface,the shallower the equilibrium scour depth,and the smaller the shear stress amplification factor.As the diameter of the pipeline increases,the equilibrium scour depth and shear stress amplification factor below the pipeline decrease.It should be noted that the scour depth used in this paper has been dimensionless,S*=S/D.But after D>0.4m(Reynolds number Re>2.95×105,Re=UD/ν,where U is the incoming flow velocity,D is the diameter of the pipe,andνis the fluid kinematic viscosity),it no longer affects the equilibrium scour depth.In chapter 4,the local scour characteristics of submarine pipelines under different sediment parameters are mainly studies.With the increase of the bedrock coefficientβMPM,the time required for the scour to reach equilibrium decreases,and the equilibrium scour depth increases,but whenβMPM>12,the effect of bedrock sediment transport rate on the equilibrium scour depth is no longer obvious.At the same time,with the increase of sediment density,the time required for scour to reach equilibrium increases,and the equilibrium scour depth decreases.With the increase of the median particle size of the sediment,the equilibrium scour depth under the pipeline increases.In chapter 5,the local scour characteristics of two pipes under different pipe spacing are mainly studies.The numerical analysis results show that when the pipe spacing G≤0.5D,the double pipes form an integral shear layer in the flow field,and at the same time generate an integrated scour topography.With the increase of pipeline spacing,the shear layer and local scour topography of the dual pipelines begin to separate,and the downstream pipeline flow field and scour topography are strongly affected by the upstream pipeline.When G>4.0D,the local scour shapes of the two pipelines are completely separated,and the local scour topography no longer overlaps,but the wake vortex of the upstream pipeline can still affect the local scour process of the downstream pipeline.In chapter 6,the main research results are summarized and an outlook on the follow-up research work are given.
Keywords/Search Tags:Pipelines, Local Scour, Shear Stress Amplification Factor, Cohesive sediment
PDF Full Text Request
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