Study On Local Scour Of Seabed Around Marine Pile Foundation And Pipeline

The exploitation of marine wind energy needs a large number of marine structures such as the offshore wind turbines.The number of offshore wind turbines increase dramatically all over the world in recent years.The marine pile foundation and submarine pipeline,as important components of wind turbine,are subjected to long-term oceanic loadings such as waves and currents.The seabed around the pile foundation and pipeline may undergo local scour,which may induce pile foundation and pipeline become unstable or even collapse.Therefore,it is of great practical significance to investigate the stability of pile foundations and pipelines in marine environment.In this paper,the local scour around marine pile foundation and pipeline were investigated through numerical simulation and theoretical analysis.The main contents and conclusions of this dissertation are as follows:A three-dimensional fluid-pile foundation-seabed coupled numerical model is established to investigate the local scour around marine pile foundations.The governing equations of the fluid motion are the N-S equation and the RNG k-? turbulence model.The local scour around the pile foundation is calculated by adopting the sediment scour model.The local scour around the monopile and pile group are investigated,respectively,and the effects of pile group arrangement and pile spacing on the local scour are investigated.The results show that the local scour around pile mainly caused by the horseshoe in front of the pile,the acceleration of the flow at the sides of the pile,and the existence of the wake vortex at the back of the pile.The maximum local scour depth appears at an angle of approximately 45° with respect to the flow direction in front of the pile.The interaction between each pile in the pile group is significant.The effects of pile spacing and flow angle are significant on the local scour of pile foundations.The formulas for calculating the local scour depth of monopile and pile groups was established,respectively.Wave-induced seabed pile foundation responses are investigated numerically.A set of three-dimensional coupled numerical model are established to investigate the dynamic responses of pile foundation and seabed under wave loads.The governing equations of the fluid sub-model are the N-S equation and the RNG k-? turbulence model,and the governing equations of the soil sub-model is the Biot's partly dynamic model.The two sub-model is coupled calculated through data exchange.The numerical results show that the maximum bending moment of the pile foundation appears near the height z=-1.5 m.The liquefaction depth around the pile reaches 4.13 times of the pile diameter under the current calculation condition.The interaction between the solitary wave and the pile foundation on the slopping seabed is strong.The pile spacing and platform have significant effects on the dynamic response of pile foundation under wave loads.Local scour around pile has significant effects on the response of pile and seabed under combined wave and current.Both the displacement and bending moment of pile increases as the maximum scour depth increases.A two-dimensional coupled numerical model of the fluid-pipeline-seabed is established to investigate the local scour under pipeline under the action of ocean current.The governing equations of the fluid motion are the N-S equation and the RNG k-? turbulence model.The local scour around the pipeline is calculated by adopting the sediment scour model.The effects of fluid reduced velocity,the initial gap-to-diameter ratio of pipeline,and median particle size of sediment particles on the local scour under pipeline are studied.Based on dimensional analysis and multi-regression method,the formula for calculating the local scour under fixed pipeline is established.The accuracy of the formula is verified with present numerical results and previous experimental results.The formula can provide a reference for predicting the local scour depth under pipeline in practical engineering.The interaction between vortex-induced vibration of pipeline and local scour is investigated.The local scour under fixed pipeline,1 DOF vibrating pipeline,and 2 DOF vibrating pipeline are investigated in detail.The effects of reduced velocity,the initial gap-to-diameter ratio of pipeline,and pipeline diameter on the local scour and pipeline vibration are studied.Also,the local scour under two vibrating pipelines is investigated.The results show that pipeline vibration will greatly increase the scour depth and scour hole range.The shape,strength and direction of motion of the vortex that shed behind the pipeline and the morphological changes of the scour hole interact with each other throughout the scouring process.The additional scouring depth is primarily dependent on the pipeline transverse vibration amplitude no matter for a 1 DOF or 2 DOF vibration pipeline.However,the in-line vibration in 2 DOF vibration pipeline weakens the sediment accumulation at the downstream of the pipeline.The interaction between upstream and downstream pipelines is significant.The maximum upstream and downstream pipelines vibration magnitudes occurs when G/D=2,but the maximum scour depth under the upstream and downstream pipelines occurs when G/D=1.When G/D=1 and 2,the vortices that shed behind the upstream and downstream pipelines have strong interaction.When G/D is greater than 2,the local scour depth under the upstream pipeline is not affected by downstream pipeline,but the scour depth of downstream pipeline decreases as G/D increases.