Research On Dynamic Response And Local Scour Of Composite Bucket Foundation

Composite bucket foundation has a lot of advantages such as its applicable of different soils, its good stability, its high speed in construction and its recycle use, good transfer of the large moment and horizontal loads from the tower to the foundation and so on. It has started promotion in offshore wind power development in China. O ffshore wind turbine system is in the complex marine environment and it is affected by wind, wave and current loads which make it technically demanding and costly development. It is the key point of the foundation design in the wind power engineering, and the stability of wind power foundation under wind, wave, current and other complex environments has a great impact on the safety and stability of the wind structure. It is an important guidance for design and safe operation of wind power structures to research dynamic loads, wind and wave- induced vibration responses and local scour around foundation under unidirectional currents and bidirectional currents. Main contents and innovative conclusions in this thesis based on composite bucket foundation are summarized as follows:1. An analytical study on wave force on composite bucket foundation is deduced and proposed, and e xperiments for verification are designed and conducted. Dynamic loads such as wind and wave are summarized. Regarding the problem of low accuracy of wave load calculation, an analytical study on wave force on composite bucket foundation is deduced and proposed. Firstly, based on the theory of diffraction, the analytical solution of wave force on composite bucket foundation is accurately derived by selecting the appropriate boundary conditions. Then, in order to test the validity of the theoretical solutions, a model of arc transition on the scale of 1:60 is made by plexiglass and experiment is designed and conducted in wave tank, in which wave pressure under regular waves of a given frequency on the surface of the model is measured. At last, the experimental results are compared with the theoretical solutions. The influence of the wave height, water depth and wave period on the wave force on the composite bucket foundation is studied and the wave pressure distribution on the composite bucket foundation is presented. From the theoretical analysis and experimental study, it can be concluded that:(1) the analytical solutions of wave force on composite bucket foundation and wave pressure on seabed around the foundation are proposed;(2) compared with previous piecewise integration method of wave force calculation, the analytical method shows good consistent with the experimental results and has high calculation precision, and the defect of previous methods is pointed out;(3) a visual intelligent program for calculating wave force on composite bucket foundation is coded and developed successfully, and the developed program avoid the trouble and possibility educed by traditional method;(4) wave force increases with the growth of wave height and water depth, and the maximum of wave pressure of a certain height on the surface grows with the increase of distance from seabed. Additionally, nonlinear wave force is analysed. The characteristic of wave field around composite bucket foundation is revealed, and the research makes it possible to the dynamic responses of composite buekte foundation.2. Research on the dynamic responses of composite bucket foundation. Regarding the problem of uncertain wind and wave- induced vibration characteristic of the soil, the research on the dynamic responses of composite bucket foundation is carried out. Firstly, the coupling model of wind turbine tower- foundation-soil with three-dimensional visco-elastic artificial boundary is established. Then, Davenport spectrum which is verified is composited to simulate the fluctuating wind load. At last, the analysis of the natural vibration characteristics of wind turbine structure and foundation with different boundaries is done. And wind and wave- induced vibration response in time domain and power spectrum in frequency domain is analyzed. Further the different effects between visco-elastic artificial boundary and fixed boundary to the response of the entire system are studied. Analysis results show that :(1) the physical meaning of the spectral peaks is clarified and explained;(2) the vibration of soil inside of bucket is almost the same as the vibration outside; the vibration of soil inside of bucket is much smaller than the vibration outside because of the above ballast and cyclo-hoop effect;(3) the characteristic of wind- induced vibration of the soil is pointed out;(4) the response at tower tip has no difference between the model using different boundaries, and the vibration frequency of the foundation is significantly reduced in the model of visco-elastic artificial boundary. In addition, the recommendations for engineering application are proposed;(5) the difference of dynamic response of composite bucket foundation whether local scour of seabed is considered is compared. It is emphasized that local scour of seabed can not be ignored in the dynamic analysis. The dynamic analysis of composite bucket foundation lays the foundation of the research of the weakening effect of soil.3. Experime ntal study on local scour around composite bucket foundation of offshore wind turbines. It is rarely published in the literature about the local scour around composite bucket foundation, and this paper carried out the construction of the test site and the experiments of local scour around composite bucket foundation models.(1) A bidirectional current system is developed, and it has been successfully applied to study local scour experiment around composite bucket foundations.(2) Based on the series models tests, the local scour of silt seabed around composite bucket foundation under unidirectional currents and bidirectional currents is studied. The morphology of the scour hole around foundations, equilibrium scour time and maximum scour depth are investigated based on a series of scale model of 1:20, 1:40 and 1:70. The maximum scour depth of the composite bucket foundations on-site is obtained according to the physical tests and the characters the scour hole around foundations under unidirectional currents and bidirectional currents are compared.(3) The impact of water depth and velocity to the equilibrium duration and the equilibrium scour depth is studied, and the formula for calculating the local sour depth around composite bucket foundation is further proposed. According to the experimental study, engineering problems are solved, and the laws which can be widely applied are summed up.