Influence Of Signal-noise Ratio On Global Damage Detection To Offshore Platform

In damage detection to offshore platform，Nondestructive Damage Detection（NDD）methods based on vibration measurements in complex structures were shownto be promising by researchers. For large structures as offshore platforms, the ambientvibration measurement is mainly used for modal identification and damage detection.However, in the real ocean test, modal identification and damage detection areinterfered by many factors. Measurement noise is one of them. A jacket platformmodel is researched in this paper, signal-noise ratio is used to present the noise levelin test signal, and a new concept as Damage Element Modal Influence Index (DEMI)is also proposed, the relationship between damage detection result and Signal-NoiseRatio (SNR) of impulse response and DEMI are obtained. The main work in this paperis described as follows:Signal-Noise Ratio is used to described the noise level of measurement signal oranalog signal. Signals of different SNR can be extracted by designing model test.Meanwhile, it is found that signal’s SNR exerts different influences to conditions ofdifferent damage location or damage degree, and DEMI is proposed to research thisdifference. This concept can be used to evaluate the incidence of different member tothe dynamic characteristics of offshore platform. A series of damage condition isdesigned to simulate diversified DEMI, and damage detection in every condition isperformed to study the influence of SNR to damage detection with different DEMI.The conclusions of model experiment indicate that both SNR and DEMI havecertain influence to damage detection. When DEMI is larger than5, the damagelocation and damage severity can be identified correctly even with small SNR signals;when DEMI is smaller than0.5, the damage identification cannot be realized evenwith largest SNR signals; when DEMI is in the critical value of0.9-1.5, SNR becomes the key factor of damage identification, that is when SNR is larger than20, thedamage location can be identified correctly, as SNR decreases to a certain value10,both of damage location and damage severity evaluation cannot be realized.The simulation result indicates that as DEMI decreases in different conditions,the SNR needed to identify damage correctly increases correspondingly, which showsthe same law as physical model experiment. But the relationship of DEMI and criticalSNR in simulation differs seriously from the experiment, a quantitative referencecannot be confirmed.Based on the concept and analysis method proposed in this paper, in actualoffshore platform testing SNR can be estimated by collecting acceleration signal ofkey nodes, and DEMI will be calculated by modal frequency identified as well, thenthese two indexes can be used to offer references for the detectable possibility ofdamage identification.