| Offshore wind is a safe, clean and stable source of renewable energy, which is a strategic option to overcome the problems of energy shortage and environment contamination. China is rich of offshore wind energy along the east and southeast coastline, so, it is promising to invest into the offshore wind farm. With the wind turbines shifting from onshore to offshore, and moving into deeper and deeper water area, high requirements for the construction as well as the service of the foundation and the overall wind turbine structure are necessary to meet the adverse offshore environment. Therefore, the difficulty of the technology and the cost to construct the offshore wind turbines are remarkably increasing. And it will become an obstacle to develop the offshore wind energy. Suction caisson, because of its advantages such as simplicity of installation, high bearing capacity against overturning moment and ease of retrieval for reutilization, will be becoming one of the most popular foundation options for the offshore wind turbines in future. Because of its high and slender nature, accompanied with the mass and stiffness distribution, offshore wind turbines are dynamically sensitive structures. Its dynamic properties are also subjected to change under the effects of environmtal loading and foundation stiffness.The suction installation behavior of suction caisson, the dynamic behavior of the overall wind turbine structure supported on different types of foundations, involving the crossing field of ocean engineering and geotechnical engineering, there still exist many unresolved problems, which urgently needs to be studied under current circumstances of rapid development of offshore wind energy. Aiming to investigate these critical issues, studies about suction caisson installation in silt and dynamic behavior of integrated support structure had been conducted in this thesis, which could be listed as follows:1. Based on the model tests platform, a series of suction caisson installation tests were carried out in silt. According to the development trend of external side friction, internal friction, tip resistance, as well as the average hydraulic gradient around the tip and internal soil plug, a systematic analysis as to the effects from the suction induced seepage on the soil resistance and stability of soil plug was given; an innovative concept of filter layer consisting of geotextile and gravel was introduced to improve the quality of caisson installation, the feasibility and advantage of this kind of filter layer were proved by the tests results, some advice to its real applicability in field were also given.2. The cross coupling stiffness was considered in the improved analytical method, a systematical investigations were carried out to study the influence from the lateral stiffness, rotational stiffness and cross coupling stiffness on the first four orders of natural frequency of the whole wind turbine structure; an approximate method was introduced to calculate the first order natural frequency, as to the real turbine tower, the’taper effects’were taken into account in the calculation; the improved analytical and approximate method were verified by some real field examples, and relevant sensitivity analysis on several parameters were also carried out.3. Scaled model tests were carried out to investigate the dynamic behavior of monopile supported wind turbine model in sand bed; based on the simplified model along with the sand behavior under cyclic loading, an sound explanation to its observed phenomenon of dynamic development trend was given; tests were also performed to investigate the dynamic behaviour of turbine model founded on different types of foundations; according to the similitudes and the tests results, suggestions as to the foundation and structure selection for the prototype wind turbine were given. |
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