The Research On Anti-Scouring Mechanism And Performance Of Sleeve Around Pile Foundation Under Waves And Current

With the rapid development of economy,a large number of Marine engineering structures are built in the offshore area.As a simple foundation structure,pile foundation has a strong bearing capacity and is widely used in ocean engineering.The action of waves and water flow can cause the local erosion of offshore structural pile foundation.The impact of local erosion that can affect the stability of the structure must be considered.It is difficult to accurately predict the maximum erosion depth around the foundation while the hydrodynamic environment is complex and changeable in the marine environment.Protective measures should be considered to reduce the local erosion around pile foundation to ensure the safety and stability.This paper studies the mechanism and performance of sleeve protection by the combination of physical model experiment and numerical simulation.The three-dimensional numerical tank was established by the computational fluid dynamics software Flow3d.The flow field around the structure was simulated by using a three-dimensional steady RANS equations with RNG k-ε turbulence closure.The VOF method was used to track the free surface.The flow field around the pile foundation with sleeve protect or not under the current and waves conditions were analyzed,and the anti-scouring mechanism of the sleeve is studied.The experiments of local scour around pile foundation with sleeve protect were conducted in a wave-current tank.The scour processes and the developments of scour depth inside and outside the sleeve were recorded under various waves,current velocities,sleeve relative heights and diameters.The topography of bed after scour was measured by a laser scanner.The difference of the form of scour hole under various conditions was analyzed,and the main factors affecting the protecti ve effects of sleeve is discussed.The results show that setting the protective sleeve near the pile foundation can reduce the maximum near-bottom flow rate around the pile,reduce the sediment retention capacity in the cylinder,prevent the sediment from escaping from the sleeve,and limit the action range of the horseshoe vortex,thus reducing the final scour depth in the cylinder.Under waves and current conditions,the effect of sleeve protection is positive.The maximum scour depth inside the sleeve can be reduced by 70%,and the maximum scour depth inside and outside the sleeve can be reduced by about 40%under the test conditions.The impact of the sleeve height on the scour depth inside the sleeve was more significant than the diameter.The scour depth inside the sleeve decreases with the increase of the sleeve height.The diameter of the sleeve mainly affects the scouring outside the sleeve.When the sleeve is buried protruding above the initial bed surface,the maximum scour depth outside the sleeve decreases with the increase of diameter.As the sleeve height decreases,the diameter has less impact on the scouring outside the sleeve.Within this text condition,the sleeve height he=0.25D and sleeve diameter Ds=1.75D is the best arrangement of the sleeve.Under this condition,the maximum scour depth inside and outside the sleeve can reach 38.03%under pure flow condition and 28.53%under wave flow condition,which can basically meet the protection requirements.