Study On Output Based Modal Parameter Identification Of Offshore Platform

Vibration response contains the inherent characteristics of the structure. The identified modal parameters can just reflect the structural properties, which could be applied to structural damage detection and health monitoring. In the field of aerospace, civil engineering and mechanical engineering, modal parameter identification technology has made rapid development. But in the offshore engineering, offshore platform is served in harsh environment. As a result of the disturbance and un-sufficient ambient excitation, the modal identification is not satisfactory. Noise removal and model order determination from measured data are important for platform structural modal parameter identification.A large amount of literatures related to output based modal parameter identification are reviewed. More focus on the theory and algorithm, application background, significance and current research status, solved and open problems. Based on this summarization, noise removal for measured data and false mode eliminate come to the most important, and in which model order is the key problem.The theory of modal analysis and methods of modal parameter identification are introduced, such as Ibrahim time domain method, Eigensystem realization algorithm and Least squares complex exponential method. Several commonly used model order method are briefly discussed and compared, including Curve fitting method, Stability diagram, Error graph and rank estimation of Hankel matrix. Rank estimation of Hankel matrix is more intuitive and theoretical based. Focused on a recently proposed method——Cadzow iteration based noising method. A five degree of freedom system is employed to validate that the methods of model order, signal de-noising and parameter identification.Under the case of low background noise, rank estimation of Hankel matrix and Cadzow iteration based noising method work well. Sometimes, false mode remains after mode order and signal de-noising when measured signal contains strong noise interference. For the above problem, a method for false mode excluding is proposed, and then used for parameter identification of a three-dimensional frame structure and an offshore platform structure simulated. Results show the effectiveness of the method.Impulse excitation applied on the platform is simulated, including two cases of direction: Case1, along the diagonal direction on one leg of platform; Case2, along the vertical diagonal direction. According to the simulation results, the third-order mode damping ratio of case1 is inaccurate, that of case2 is much better. Comparative the parameter identification error of different measured signal, the results of which can be used as reference for optimal arrangement of the sensor.