Non Linear System Identification of Offshore Floating Structures

Floating structures such as spar platforms are typically designed to be compliant or move with environment loadings, rather than resisting them. Hence they are designed so that there is no dynamic amplification in wave frequency response. However, higher order nonlinear effects are produced in low frequency wind excited regions, especially in Spar platforms. It is difficult to separate the nonlinear behavior of the model response from the loading using conventional methods. In this paper, Empirical Mode Decomposition and Hilbert Transform (EMD/HT) is used to identify the nonlinear response of a spar from the model test results. From the measured response the dynamic parameters are estimated as follows: 1) The multi-component response of the floating structure is decomposed into IMF components. 2) Hilbert transform of the input and the IMF signal in the time domain is done to extract the instantaneous dynamic characteristics. 3) Amplitude and frequency dependent frequency response function is used to represent the result of HT identification. The EMD method can identify any changes in system properties in real time and can be effectively used for repair and retrofit.