Model Test On Air-Liquid-Solid Coupling Mechanism And Floating Characteristics For Bucket Foundation During Air-Floating Towing Process

The composite bucket foundation is a kind of emerging environment-friendly offshore wind turbine foundation,with the characteristic of simple construction,high construction efficiency and low comprehensive cost.It has gradually received attention and recognition from the offshore wind power engineering community.The bottom is an open structure,which can displace an amount of water by the air pressure generated by the compression of the internal air cushion,and form a buoyancy force by forming a water pressure difference between the inside and the outside of the bucket.This self-floating property is a prerequisite for the bucket foundation to perform air-floating towing.For the single-bucket with subdivision structure,the honeycomb sub-cabin structure is equivalent to the coupling of a plurality of unconnected air cushion structures,and the air cushion in the multi-cabin interior can effectively improve the hydrodynamic characteristics of the foundation.It is embodied in the influence of parameters such as structural frequency,additional mass,air cushion characteristics in the bucket,air-liquid conversion ratio and air-water coupling stiffness.Compared with the common solid floating structure,in addition to considering the interaction between fluid and foundation(fluid-solid coupling),the air-floating foundation needs to consider the air cushion-water cushion interface motion characteristics(air-liquid coupling)and the interaction of the air-water spring on the dynamic response characteristics of the structure(air-liquid-solid coupling).In this paper,a bucket foundation engineering prototype is taken as the research object.Through the theoretical calculation,model test and numerical simulation method,the air-liquid-solid coupling mechanism and floating characteristics for bucket foundation during air-floating towing process are studied.This provides a reference for the towing construction of the bucket foundation.The specific research contents are as follows:Firstly,based on the theory of solid floating body stability and the formula of air cushion pressure,the formula of the floating static stability of the air-cushion floating bucket foundation is deduced.The ratio of air cushion/water cushion in the bucket is introduced and the air cushion reduction coefficient is introduced to calculate the air cushion of different drafts.The ultimate oblique air leakage angle of the bucket foundation is calculated.The air-liquid coupling variation law of the foundation from floating until sinking to the seabed surface is analyzed.Furthermore,the hydrodynamic parameters such as additional mass are solved by combining the equations of motion,and the characteristics of the air-liquid-solid coupling air cushion,the air-liquid interface transition,the air-water coupling stiffness variation and the structural air-floating stability evolution of the bucket foundation are clarified.Secondly,the constant flow column test method is used to compare and analyze the difference of towing resistance,sway and heave motion amplitude and air cushion pressure of single-bucket with subdivision structure and single-bucket without subdivision structure.The turbulent flow numerical model is used to analyze the flow field distribution around the solid floating body,single-bucket with subdivision structure and single-bucket without subdivision structure,and the dynamic water pressure coefficient,towing shielding coefficient and towage energy consumption rate are introduced.The reasons for the difference in the drag force of different structures are analyzed.The fluid-solid mechanism of the bucket foundation and the surrounding flow field under constant air cushion is clarified.Thirdly,the wave towing test method is used to compare and analyze the difference mechanism of the trawling motion response of the single-bucket with subdivision structure and the solid floating foundation under complex wave conditions.By considering the different drafts,towing speed,towing direction,wave height,wave period on the towing resistance,the air pressure,the bottom water pressure and the six degrees of freedom of the structure.The influence of the existence of air cushion and the volume change of the air cushion on the movement frequency and other characteristics of the bucket foundation is explored.Combined with the numerical model of three-dimensional potential flow,the influence of the air-liquid-solid interaction on the wave resistance of the air-floating structure is analyzed.The mechanism clarifies the dynamic response characteristics of the air-floating towing of the bucket foundation.Finally,for the case of the possible damage in the actual towing of the single-bucket with subdivision structure,the experimental design and leveling strategy of the instability of a certain mid-room or side-room is carried out.The coupling attitude change between the air cushion,the water cushion and the structure during the whole process of the bucket foundation from the instability of the damage to the new steady state after the leveling is analyzed.Combined with theoretical derivation and experimental results,the air-water ratio of each room required by different leveling strategies is given.The wave towing feasibility test is carried out again for the flattened bucket foundation,and the wave motion response under the new steady state of the bucket foundation is studied.The difference in the motion response between the bucket foundation with the mid-room failure and the bucket foundation without the mid-room failure is analyzed.The operability of the bucket foundation from the instability of the damage to the leveling and wave towing is verified.