Numerical Investigation Of Local Scour Below Submarine Pipelines Under Waves And Currents

To study the local scour process of submarine pipelines under waves and currents not only has significant academic value,but helps to grasp or predict practical engineering significance.However,the previous works mostly focused on a single pipeline scour under currents.The formation and development of the scour pit below a single submarine pipeline under waves have not been studied in detail.Moreover,the local scour of the submarine pipelines under waves/currents have not been systematically studied yet.Therefore,in present work,the detailed local scour process,the mechanism and typical characteristics of local scour below a single and two submarine pipelines under waves/currents were investigated in detail,and the influence of dimensionless parameters,i.e.,Froude number(Fr),KeuleganCarpenter(KC)number,still water depth h',wave period T',pipeline diameter D',distance between the pipeline and sand bed ?z',and the spacing between the two pipelines L',on the local scour process below a single and two submarine pipelines under waves/currents was systematically studied.The main research work can be summarized as follows:Firstly,the RANS equation combined with the RNG k-? turbulence model was selected to solve the flow field around the pipelines.The Tru-VOF method was used to track the free surface and the water-sediment interface.A numerical tank model for studying local scour of the submarine pipelines under waves/currents was proposed by combining the empirical sediment transport model.The accuracy and applicability of the numerical tank model were verified through comparative analysis with the experimental results.Secondly,the mechanism and typical characteristics of local scour below a single pipeline under currents was studied.It is found that the shear stress on the sediment particles below the pipeline is larger than the critical shear stress at the beginning of local scour,and the sediment particles start to transport with the water movement,and a scour pit was formed below the pipeline.With the continual effect of sediment transport,the shear stress imposed on the sediment particles decreases,and the depth of the scour pit gradually increases,resulting in a gradual slowdown of the water movement and a decrease in the rate of sediment transport.When the depth of the scour pit no longer changes,namely,after reaching the equilibrium depth,the shear stress imposed on the sediment particles gradually tends to a stable state.Then,the effects of Fr,h',D',?z',and L' on the local scour process below a single and two submarine pipelines under currents were systematically investigated.It is found that,the larger the Fr number,the smaller the h',the smaller the D',the smaller the ?z',the stronger the local scour below the pipeline by the water movement,and the greater the equilibrium depth of the scour pit below the pipeline.When 3?L'?5,the scour pit profile below the upstream pipeline is relatively close to the single pipeline cases,and the sediment accumulation between the two pipelines is relatively increased.When L'=7,the scour pit profile below the upstream pipeline as well as the sediment accumulation are mostly the same as the single pipeline cases,while the depth of the scour pit below the downstream pipeline is larger compared to the smaller L'conditions.Finally,the mechanism and typical characteristics of local scour below a single pipeline under waves was also studied.It is found that,the shear stress curve of the sediment below the pipeline also shows a periodic change,namely,the direction of the local scour below the pipeline under waves shows a periodic change.Moreover,the sediment siltation on both sides of the pipeline is relatively less during the local scour process.When the shear stress on the sediment particles exceeds the critical shear stress,the sediment particles start to transport by the water movement,and a scour pit was formed below the pipeline.With the depth of the scour pit below the pipeline increases,the water movement slows down,the shear stress decreases,and the sediment transport slows down.When the depth of the scour pit reaches the equilibrium depth,the shear stress reaches a stable state,and the local scour process of the pipeline also shows an equilibrium stage.Then,the influence of dimensionless parameters,i.e.,KC number,h',T',D',?z',and L' on the local scour below a single and two submarine pipelines under waves is systematically investigated.It is found that,the larger the KC number,the larger the h',the larger the T',the smaller the D'and the smaller the ?z',the greater the shear stress imposed on the sediment particles,resulting in a larger depth of scour pit below the pipeline.When L'=2,the interaction between the two pipelines is larger,the sediment elevation between the pipes is lower than the initial elevation of the sediment interface.When L'?5,the profiles of the scour pits of the upstream pipeline basically the same as the single pipeline cases,and the sediment accumulation is relatively less between the two pipelines than the smaller L' conditions.