Modal Identification of Fixed Jacket Platforms Using Frequency Domain Decomposition

Frequency Domain Decomposition method (FDD), is the reliable and powerful approach for system identification ofoutput-only systems without knowing the exciting forces. In this approach, Power Spectral Density matrix (PSD) iscalculated from matrix of responses, and then it is decomposed into a set of single degree of freedom systems, eachcorresponding to an individual mode. Natural frequencies are estimated by peak picking and the part of the SDOF PSDaround the peaks, bells, are then transformed back to the time domain by an Inverse Fast Fourier Transform (IFFT) anddamping ratios are estimated by logarithmic decrements of the corresponding free time decays. This approach wasapplied to a 5 DOF fixed jacket platform model. Displacement time histories of different levels of the jacket under theunknown loading were processed and modal parameters of the system including natural frequencies, mode shapes and damping ratios were estimated using FDD approach. Results were compared with analytical, Exact values and errors of estimated values were calculated. It was shown that, modal parameters estimated using FDD method, are in quite good agreement with exact values

Keywords:

frequency domain decomposition , FDD , modal identification , fixed jacket platforms