Numerical Modeling Of Local Scour Beneath Offshore Pipeline(pile)in Steady Currents

At present,with the development of ocean economy and the requirement of national defense,marine structures such as submarine pipeline,oil platform and so on were wildly built in the ocean.But when exposed to the complex ocean environment,local scour always happen where the flow field was changed because of the submarine structures,which may lead to the failure of structure.It's necessary to do the research of the mechanic of scour,and find the way to protect the marine structures.In this paper,based on the CFD software Flow3 D,two-dimensional numerical scour models with both fixed pipeline and vibrating pipeline and three-dimensional numerical scour models of pipeline and pile were built.Flow field is simulated by solving the Reynolds averaged Navier-Stokes equations with RNGk-? turbulence model.Wall function is adopted for the bed shear stress.The bed profile development is solved by conservation of sediment mass.Both bed load and suspended load sediment are considered in this model.The 2D numerical scour model of fixed pipeline was established for simulating scour below offshore pipeline in steady currents.Effects of two parameters--pipeline embedment depth and diameter are assessed.The numerical model setup is based on the laboratory experiment in both clear-water and live-bed scour situations.The bed profile and the max scour depth agree well with the experimental result.It reveals that this numerical model is capable of predicting the scour in complex conditions.And the research to the effect of embedment and diameter showed that the scour depth will grow deeper with smaller embedment and larger diameter.To explore the scour development under the situation of vibrating pipeline induced by vortex shedding,a two dimensional sour model was built.In the scour model of vibrating pipeline,the pipeline was no longer fixed but influenced by the flow field.A virtual spring system was added to realize it.The control equation of the couple motion of pipeline and fluid was solved by implicit finite difference methods.The motion of pipeline was verified with experimental result.The result showed: the scour depth and width were greater than that of fixed pipeline.In the three-dimensional scour model of pipeline,the scour development in the span wise direction was investigated.At last,the effect of angle of incoming flow was assessed.The scour speed in the span wise direction was relatively smaller than the experiment result.It may be due the fact that the numerical model lacked the consideration of seepage at the initial stage.The numerical result showed: the scour developed in both upstream and downstream when the incoming angle was not large and sand ridge was found.But with larger angle of incoming flow,the scour development would be limited.Three-dimensional pile scour model was based on physics experiments and the accuracy of the simulation to flow field especially the vortex around the pile was accessed.The numerical result showed a good agreement with experimental result in scour depth and profile,but the speed of scour development in the upstream was a little small and the accuracy of scour profile in the downstream needed to be improved.