The application of Hilbert-Huang Transform (HHT) method in conjunction with the masking signal for modal parameter identification to structures with closely spaced modes of vibration is investigated in this paper, and the masking signal technique is improved. A 2DOF system with closely modes of vibration is first analyzed to demonstrate the strongpoint and basic procedure for identifying modal parameters of the system using the HHT method. Then, a physical model of an offshore platform is analyzed using the FFT method, the scalar ARMA method and the HHT method in conjunction with the improved masking signal technique respectively. The results show that the HHT is able to identify the modal parameters of the structure with closely spaced modes of vibration. The HHT method performed in the timedomain seems to be a very promising tool for system identification of offshore platform structures.
INTRODUCTION Hilbert-Huang Transform (referred to as HHT), introduced by Huang in 1998, put forward a new time-domain data-processing method which is composed of two parts, Empirical Mode Decomposition (referred to as EMD) and Hilbert transform. Compare with the traditional modal parameter identification methods, EMD can effectively deal with nonstationary data. And the Fourier spectrum, from Hilbert spectrum can be seen not only in amplitude and frequency can be seen with time, which the Fourier spectrum cannot be reflected. In addition, for the non-stationary Time-domain signal, Fourier spectral resolution may be lower, while Hilbert spectrum, because it combines both the frequency and time to coordinate analysis, easy to remove interference and will be beneficial to retrieve signal resolution. First of all, introduce the HHT method, using masking signal technology to solve the problem of mode mixing in EMD, improve the parameter selection methods of masking-signal and prove the validity of the improved masking signal technology through the numerical examples.