Analysis And Experimental Study On Bearing Capacity Of Monopile For Offshore Wind Turbines

Wind energy is the most prospective regenerated energy and the wind energy development plays an important role in improving the energy structure, saving land and realizing the sustainable development of the economy and society of our country. The foundation takes the main content of wind turbine design, as the final taker of the load transmitted from wind turbines. Extra-large diameter monopile foundation has been most common foundation of offshore wind turbines in foreign countries now. The foundation of offshore wind turbines with large bodies, bearing the coupled load resulting from wind, wave, ocean current etc. Thus, the failure mechanism of that is extremely complex, which makes it difficult to design. So the analysis of bearing capacity of monopile and deformation control is one of the key problems in the design of offshore wind turbines. Then the soil-pile interactions and the deformation properties of the foundation of wind turbines under lateral monotonic load becomes the base of research on this problem, and formes the main content of this thesis.Model tests on monopile foundation under lateral monotonic load in three different loading positions were carried out for the in-situ monopole widely used in Europe, with the diameter of5m. In the tests, three different loading heights were1D,3D and6D, where D is the diameter of the pile. And pile-soil interactions and bearing behavior of rigid piles were studied by analyzing the changes of the soil pressure and pore pressure around the pile when loaded, measured by earth pressure producers and pore pressure sensors; load-displacement relationship investigated in tests by LVDTs and axis dynamometer; and law of the rotation center movement and the whole pile deformation calculated from the data measured by the four LVDTs above the mudline. Some conclusions can be drawn in the light of the experimental results:firstly, the load-displacement curve of the test mono-pile has the trend of work hardening, and using the tolerant deflection and angle as a control standard is an effective means to ensure offshore wind turbine work properly; secondly, mh has double logarithmic linear relationship with the deformation; thirdly, the soil pressure along pile shaft under lateral loads increases linearly along the depth, and reaches the maximum value in the depth of2.8D, then decreases to0at0.75multy by the depth, and there is a sharp reverse increasing as the depth continues to increase. Meanwhile, a theory model on radial distribution law of soil pressure was developed based on the test results and previous research. From which, analytic solutions of lateral loaded model of monopile were obtained. An analytical method for the lateral bearing capacity of monopile based on deformation was suggested. The results were compared with those computed by p-y curve method in API, and fit well.At last, the capacity of in-situ piles with diameters between3-6m was calculated and analyzed by the deformation-based capacity method, and then compared with the analytical results from other calculating method. It is shown that the capacity calculation method based on ultimate ground resistance method cannot consider the deformation character of monopile under lateral load, and that the API p-y curve method is quite conservative for monopile, though it considers the nonlinearity of soil. The method in this thesis based on deformation predicts the relation between load and deformation properly, providing the scientific analysis and technical support for the designing of super large diameter monopile. And several parameters affecting the bearing capacity of pile foundation were calculated and analyzed in detail, and it is found that the m value is the most important one among the soil parameters, so getting the m value against the deformation through experiment is the key to predicting the deformation of monopile.