Prediction Of Environment Loads And Response Of Offshore Wind Turbine Support Structure

With the development of world economy, the demand for energy is growing and the energy problem becomes more and more serious. On one hand, the fossil fuel is nonrenewable resource that is facing exhaustion. On the other hand, the excessive use of fossil fuel energy will lead to a series of environmental problems that are endangering the survival of mankind. Therefore, the whole world is making adjustment to energy structure for the third time, transferring the energy pattern of oil, coal and gas into the sustainable development energy pattern based on the renewable clean energy. Wind energy is one of the first kinds of energy used by mankind, and is currently the most competitive renewable energy which has biggest prospect of large-scale exploitation. Developing and utilizing wind energy is of profound significance to improve energy structure and protect ecological environment.So far, developing and utilizing wind energy is mainly through wind power generation. With the development of wind power technology, the capacity of wind turbine is increasing continuously. The size and weight of wind turbine are also increasing greatly. That brings forward a higher requirement for supporting structure. For the kilowatt wind power turbine, the weight of upper structure (including rotor, engine, etc) and the loads on the tower are limited, so the requirement in the strength and stability of the support structure is easy to meet. However, for the megawatt wind power turbine, the size and weight of the rotor are increasing greatly. This makes the support structure bear larger loads. In addition, compared to the wind turbine on land, the working environment of the offshore wind turbine is more complicated. Besides of the wind load, the current and wave loads should be taken into account. Therefore, the security of the wind turbine support structure has received more and more attention of the designers and scholars. At present, the research of the support structure is mainly for the wind turbine on land, and the research for the offshore wind turbine is relatively few. In this paper, the support structure of offshore wind turbine is modeled with simplification, and then its dynamic characteristics are analyzed. On that basis, the environment loads on the offshore wind turbine can be calculated, and the dynamic response of support structure to the environment loads can be predicted.Firstly, the classical Euler-Bernoulli beam theory is used to model the support structure of offshore wind turbine. The lumped mass is adopted to simplify the structure on the top of the tower such as the rotor and the engine and the connection platform between the tower and the foundation. Based on the widely applied m method, the linear spring is used to simulate the interaction between the foundation and the soil.Secondly, the dynamic characteristics of the support structure of offshore wind turbine (natural frequency and modal) are analyzed. The influence of the top lumped mass, the gravity of the wind turbine system and the sea water on the natural frequency are studied.Thirdly, the environment loads on the offshore wind turbine, including rotor axial load, wind load on the tower, the current load and the wave load on the foundation are determined. Base on the blade element momentum (BEM) theory, by choosing the appropriate airfoil, the Wilson method is adopted to calculate the rotor axial load. The universal wind pressure formula is used to determine the wind load on the tower. The drag force formula is applied to calculate the current load. The semi-empirical and semi-theoretical Morison formula is used to compute the wave load.Finally, the response of the support structure of offshore wind turbine to the environment loads is analyzed and predicted. The contributions of various environment loads are compared, and the dangerous position of the support structure is predicted.In this paper, the particularity of the support structure of offshore wind turbine is comprehensively considered. The research results can provide reference for the offshore wind turbine design.