Singal Noise Removal And Model Parameter Indentification Experiment Study Based On Offshore Structure

At present, A lot offshore platforms which designed before recent years are suffering from aging and damaging in the process of long-term operation, some of them have reached their service lives. In the practical engineering, these aging platforms are still useful in the exploitation of nearby coastline, And it is also out of considerations for financial saving. Moreover, the damage of platform caused by improper construction process or misuse could also lead to the security problems of platform. So it is necessary for engineers to assess the health conditions of platforms regularly or irregularly. When damage occured, the intrinsic property of offshore platform will changed simultaneously. embodied in the change of the modal parameters, so to identify the structural damage, it’s significant to identify the modal parameters of offshore structures accurately. In this paper, the study for the modal parameters are as the following aspects:1. This paper studied Fourier analysis method and some time domain methods like least squares complex exponent method (or Prony method), Eigensystem Realization Algorithm (ERA), Stochastic Subspace Identification (SSI) by the numbers. Fourier analysis is the basis of frequency domain method, and it’s vital in the modal identification field. Prony method and ERA are based on impact excitation response and SSI is a identification method based on random excitation response.2. The paper uses the theory of Cazdow and signal decomposition and reconstruction method to research noise elimination. The theory of Cazdow is a noise removal method that bases on truncated singular value decomposition. Signal decomposition and reconstruction method is used to extract weak mode, but the method is applied to eliminate noise in this paper. The numerical studies show that the two methods could eliminate noise in response signal very well, but the signal decomposition and reconstruction method need to have a proper bandwidth, the results will produce great error when the bandwidth is too large or little.3. The impulse excitation and random excitation experiments of jacket physical model have been implemented, using response data to study modal parameter identification. Form the numerical study, the ERA and SSI could accurately identify the modal frequencies and damping ratios, and SSI has better anti-noise ability. Applying ERA and SSI to jacket physical model response data, the results validate ulteriorly that ERA is better in identifying response data with impulse excitation and SSI is much better with random excitation.The paper applies signal decomposition and reconstruction method to eliminate noise in signal, and the method achieves good result. In order to indentify modal parameter of measured signal, the software of Offshore Structure Modal Identification and Vibration Detective System (MIVDS) is used, which is developed by our team. The analytical results indicate that ERA applies to the signal of impulse excitation response and SSI applies to the stochastic excitation response signal. This paper can provide a certain reference for identifying the modal paraments of measured signal of offshore structure. Especially when these signals are filled with noise, it’s also helpful in the modal paraments identifying under the different excitation condition.