Dynamic Behavior Of Offshore Wind Turbine Monopile Structure

With the rapid development of China’s economy, the energy shortage has become increasingly prominent. Offshore wind power is a new form of energy, which has potential around the world and is an effective method to solve the problem. Large diameter monopile is widely used in offshore wind farms because of its construction speed and economy. Offshore wind turbine is a slender structure with long tower, large top-mass and mainly affected by lateral load. The natural frequency of offshore wind turbine is limited in a narrow range between 1P and 3P in most project, which makes the dynamic characteristic the key point. Soil conditions of offshore wind farms are complex and difficult to survey. In addition, there is high randomness of wind and wave load, so that the introduction of probabilistic analysis is necessary. The following work is carried out based on these problems.(1) A test platform is developed to carry out a 1:50 scaled 1g test to study the dynamics of offshore wind turbine. The results show that the stiffness of soil increase rapidly when nondimensionalized parameter H/GD2 is less than a value between 7.71×10-4 and 9.79×10-4. Otherwise, the stiffness of soil will not increase, even a decrease of stiffness can happen before it recovered. Damping ratio can reach 20%increase or 70% decrease under cyclic loading, which appears opposite to natural frequency. A function based on maximum horizontal load, dynamic amplification and amplitude of cyclic load to predict the cumulative displacement was developed. The test shows that the natural frequency of offshore wind turbine would reduce by 3%under side cyclic loading, which should be noticed by engineers.(2) A numerical model is developed based on the Euler-Bernoulli beam using API p-y nonlinear curve. The dynamic responses of the offshore wind turbine structure under external load are calculated. The results come out that the initial stiffness of API p-y curve in sand can be equivalent to the secant stiffness with y=0.001m. Changes of monopile length will affect the relative stiffness between soil and pile, which cause the transition between rigid pile and slender pile. The design parameters of offshore wind turbine have great impacts on the natural frequency of OWT.(3) Probabilistic analysis method is introduced to quantify the effect of random variables on the failure of offshore wind turbine. Firstly, the Monte Carlo simulation is used to generate 100,000 sets of uncertain parameters based on their distributions.Secondly, the influence of each parameter is determined qualitatively by hypothesis test.Then Bayesian analysis is used to further quantify each parameter’s influence. The results show that the monopile diameter is the most important factor for natural frequency based failure,while the relative density of sand is vital for the mud line rotation based failure.