Vibration Behavior Analysis And Dynamic Parameter Identification Of Offshore Wind Turbine Structure

Exposed to the complex operational environment, the vibration of offshore wind turbine structure, on one hand, is induced by the ambient loads such as wind, wave and flow and form a random vibration model about the coupling dynamic system composed of unit, tower, foundation and foundation soil under the environment excitation. On the other hand, the operational turbine may also bring many problems including machinery, electromagnetism and working equipment so as to the relevant harmonic excitations will lead to the structural forced vibration phenomenon. The different dynamic characteristic indicators which can reflect structural vibration, for example structural Operational Modes, structural Vibration Sources and structural Excitaitons, can be generated under the various external excitaitons acting on the offshore wind turbine structure. These indicators are beneficial to grasp accurately the dynamic vibration characteristics of wind turbine structure in the working status, and simultaneously have an important significance on the online realtime monitoring and security evaluation for offshore wind turbine. Hence, in view of the above key issues, some corresponding researches and innovative research results have been achieved and obtained in this paper:( 1)Based on the measured data, the structural vibration response characteristics under different operating conditions and the change regularity of structural vibration with the variation of operation factors have been analyzed overally.Firstly, On the basis of measured displacement data obtained from an offshore wind turbine, the fundamental structural vibration characteristics under different operating conditions were described particularly. Secondly, the study gives a definite influence relation between structural vibration and wind speed, rotate speed during the typhoon. Finally, the synchronism among the operational factors and the influence mechanism on structural vibration induced by the change of operation factors have been verified and discussed. Whereafter, a better conlusion on the security evaluation of structural vibration in the diverse operational status was easily acquired.(2)Considering the harmonic excitations in different degree, a set of operational modal identification method for the offshore wind turbine structure has been proposed to reduce the influence on the identification accuracy induced by the strong harmonic interference.In allusion to dissimilar operational vibration characteristics of offshore wind turbine structure caused by the harmonic excitations in different degree, a set of method and system appropriate for the structural operational modal identification under the harmonic excitations was proposed combined with a compound de-noising theory in this paper. The presented method, which adds the known periodic element corresponding to strong harmonic frequency into the Hankel matrix and achieves an accurate and complete separation between the structural modal information and added harmonic component, provides the online realtime monitoring of offshore wind turbine structure with an effective method as the theoretical basis of eigensystem realization algorithm(ERA) and probability density function(PDF). Besudes, a modified stochastic subspace identification(SSI) method considering harmonic interference called harmonic modification SSI(HM-SSI) method was also suggested in this study. Then the effectiveness and accuracy of the new method were verified through a simple numerical model of cantilever beam and the superior robustness and better accuracy of identification results affected by different harmonic frequency, the level of noise and the proportion of harmonic energy were reflected. Further, the modal parameter information under different operational conditions was obtained and the resonance safety assessment of the operational wind turbine was accomplished.(3)Based on the principle of spectral kurtosis, a certain kind of vibration source identification method has been established to identify and define vibration source type, major vibration sources and corresponding change rule of offshore wind turbine structure under the various actual operation conditions.In view of undefined type and mechanism of vibration source which give rise to the structural vibration of offshore wind turbine, the research on the vibration source identification and the corresponding change law with the variation of the operational factors must be carried out. Firstly, the study innovatively introduced the theory of spectral kurtosis to differentiate the random or harmonic character refected from structural vibration in specific frequency and discuss the main frequency information from the responses and formation mechanism of the possible structural vibration sources. Then the major vibration scoure type and relevant frequency have been identified and confirmed. Secondly, the rules about vibration source distribution and energy change with the variation of structural location as well as the dominated vibration source and corresponding energy with the change of operational factors were studied deeply and determined. The relationship between the whole vibration and component vibration has also been explained.Finally, it was demonstrated that the rotating frequency(1P) had an important significance on resonance check for the offshore wind turbine by extracting respectively the homologous energy of rotating frequency(1P) and frequency multiplication(3P) from harmonic excitation source.(4)Based on the measurement vibration responses, a type of quantitative identification method of dynamic load parameters for offshore wind turbine structure has been proposed. Further, the influence factors and relevant change rule of the static and dynamic load for the wind turbine structure were analyzed comprehensively.In view of the problem that environmental and operational excitations acting on wind turbine structure cannot be measured definitely in the practical engineering, on one hand, the theoretical solution method on equivalent static load acting on top of tower structure and the change rule of overall equivalent static load have been given and researched qualltatively based on the dynamic strain data. On the other hand, perceiving the offshore wind turbine strucutre at standstill as research object, a quantitative identification model for load parameters was proposed by mastering accurately the general structural load mechanism. Then, the feedback analysis on the critical load parameters of the presented model can be achieved by a cyclic calling to compute the equivalent numerical model based on the optimization identification program combined the particle swarm optimization algorithm(PSO) with the various measured vibration data. The dynamic rule about change of steady drag coefficient and unsteady lift coefficient with the variation of operational factors which is suitable for the practical offshore wind power projects has also been studied and declared. Finally, this paper suggests the selection and standard on the critical load parameters in the excitation simulation in the practical engineering.