Modal Parameter Identification Of Offshore Platform Structures Using HHT Method

The modal parameter identification is the groundwork and key of the structural health monitoring techniques. The modal parameter identification methods based on structural dynamics are involving new methods research on signal testing and data processing techniques, not only including the deep research on theory of structural dynamics and experiment skills as well. The Hilbert-Huang Transform (HHT) is suited to be used in the nonlinear and unstable signal processing for it offers an approach to describe the signal's instantaneous characters. The application of HHT in the oceanography, seismology, bioengineering and structure health monitoring has already proved the advantage of this method. But the HHT still needs further study for it is only proposed recently. The main work of this thesis is to study the mode mixing problem in Empirical Mode Decomposition (EMD) when it is used in modal parameter identification of offshore platform as follows.The first issue is to study the methods which are used to solve the mode mixing problem, point out the problems in these methods. The masking signal method is studied as a main method of these methods, and the parameters of the masking signal are improved. The numerical results show that the improved masking signal method allows Empirical Mode Decomposition (EMD) to be used to separate components that are closely in frequency that would be inseparable with standard EMD techniques better than original masking signal.Two 2DOF systems with closely spaced modes of vibration and different damping ratios are analyzed to demonstrate the precision for identifying modal parameters of the system using the HHT method with different methods for mode mixing. The numerical results show that the HHT in conjunction with the improved masking signal or Fourier filter is able to identify the modal parameters of systems with damping ratios around 0.02~0.05. An offshore platform was modeled by Finite Element (FE) method, of which the analytical modal parameters are obtained by modal analysis. The HHT in conjunction with the improved masking signal and Fourier filter are applied to identify the modal parameters of the FE model and field testing under impulsion excitation. According to the comparison results, the HHT in conjunction with the improved masking signal and Fourier filter were shown to be effective for the structural modal parameter identification, the results of former are more accurate and reasonable when it is used in field testing.