Numerical Simulation Of Local Scour Of Cylindrical Pile In Self - Elevating Platform

With the development of our country,the demand for petroleum is gradually increasing.So petroleum exploitation is turning to the ocean.Marine exploitation mainly concentrated in the offshore, and jack-up platform is the mainly device for offshore petroleum exploitation. In the marine environment,column feet will affected by local scour.It will affect the stability of the platform and lead to accidents.Scholars at home and abroad for such problems mainly focuses on the study of local scour of bridge piers or model test research.Few study on the local scour for actual size of the offshore platform column feet leg.Therefore,this paper established a simulation model of current under the local scour,the wave model and the waves-current model with local scour model.Use the scholars at home and abroad’s experiment results to verify the rationality of the numerical model. Simulate the influence law of column feet leg under the different influence factors,local scour’s forming process, the local scour’s mechanism and the hole of local scour.the results can provide important reference for local scour protection measures, structural optimization and safety assessment of offshore platform column feet leg.In this thesis, research work’s mainly endeavor and contributions are as follows:(1) Literature research and analysis. Widely collected and analyzed local scour mechanism under current condition and summarized the research achievements of other researchers.Discussed local scour characteristics and former achievements under the wave-current condition and the classification of sediment transport and force.(2) Overview on the mathematical model and numerical method used in the simulation. Simply described the software,governing equation and turbulence model used in the simulation research.(3) Created the mathematical model of local scour under the current condition.Established the cylinder’s local scour model under the current condition,and compared the flow field characteristics and scouring results with experiment results of Melville.(4) Created the wave simulation model and study on reflect wave absorbed in porous structure.Established a series of three-dimensional wave models to study on reflect wave absorbed in porous structure.Study on porosity,pore structure material particle size and structure of the slope gradient,to obtain the optimal wave factors of pore structure which influence wave absorbing.(5) Created and verify the local scour simulation model under the wave-current condition.Created the local scour model and set up pressure boundary to instead of sammerfeld boundary under wave-current condition to solve the the water level rise when using Sammerfeld boundary.Use Van Rijn and Qi’s experiment results to verify the rationality of the numerical model’s vertical flow velocity and local scour results under the wave-current condition.(6) Simulation the column feet leg’s local scour under different influence factors condition.Investigated analyzed the cause of "Shengli NO.3" platform’s accident,and points out the relationship between local scour and accident.Simulation column feet leg’s local scour under different flow velocity, water depth, the diameter of pile legs and different wave width.Studies those influence and mechanism of different factors’s on column feet leg’s local scour.In conclusion, this thesis utilizes the computational fluid dynamics, mechanics of sediment transport,and discipline knowledge such as wave theory to study on local scour around cylinder with a series of numerical models.Obtain the change rule of local scour under different environmental conditions.This thesis’s research results can provide safety assessment of local scour, protection measures and structural optimization for platform’s column feet legs,which has high theoretical significance and practical value in engineering.