Reliability-based Design Optimization For Offshore Wind Turbine Tower Structure

Tower is an important component of offshore wind turbine. According to the properties of the tower structure, it usually exist three potential failure modes of yielding failure, buckling failure and resonance failure. Structural parameters of the tower directly affect the safety and the economy. Moreover, the structure size, material properties and environmental load of the tower exist inherent uncertainties. In this work, according to the structural properties of the tower, the reliability of the tower structure of NREL offshore 5-MW wind turbine installed on a monopile under three potential failure modes, which is subjected to the extreme loading, is analyzed. Moreover, to get better safty and economy, the optimization design of the tower of NREL 5MW baseline model under reliability constraints, is presented.First, the probability distribution of the structure parameters of tower is described. Structure size and material properties parameters of the tower obey normal distribution, the extreme loading of tower obeys the Gumbel distribution. The first-order reliability method of JC method are used for the reliability calculation from a specified limit state function based on the yielding, buckling and resonance failure mode. The reliability of each segment of the tower under the single failure mode is got. To obtain the whole reliability of the tower structure under multiple failure modes, the correlation coefficient between the limit state function need to be calculated, and combining with the probability network estimate technology(PNET). Compared with the results of reliability analysis, which is calculated by the method of monte carlo simulation(MCS), it is concluded that the reliability value, which is calculated by combining the methods of JC and PNET, has certain accuracy. Secondly, in order to ensure that the tower structure has better security and economy, the optimization design of the tower structure of NREL 5MW baseline wind turbine under reliability constraints, is presented. The mass of the tower is considered as the objective function. The thicknesses of segments and the radius of the bottom and top of the tower are adopted as design variables of the optimization. The yield stress, buckling stress and natural frequency are specified as the reliability constraints. Polak-He optimization algorithm is employed for the optimization and the JC method is used for the reliability calculation. Finally, to get the structure properties of the optimized tower structure under the extreme loading, the static analysis, the buckling analysis and the modal analysis are carried out using the finite element software ANSYS.