Analysis of wave-induced vertical ship responses by Hilbert-Huang transform method

Time series of wave-induced ship responses are analysed by the Hilbert-Huang Transform (HHT) method. The results of the analysis of numerically simulated motions and loads on a chemical tanker in head waves are compared with the analysis of experimental data from laboratory model tests. The Empirical Mode Decompo-sition, which is a first part of HHT method is applied to the sea surface elevations of two sea states (high and low seas) and the wave-induced vertical motions and bending moments and the results are discussed. It is found that the decomposition of the experimental data into intrinsic components compares well with the decomposition of the numerically simulated ones for both high and low seas. The first two or three intrinsic mode functions carry the major part of the energy of both experimental and numerical simulated data. The time-frequency distribution of energy, called Hilbert spectrum, is calculated by the second part of HHT method. The Hilbert spectrum of the sea surface elevation is compared with Hilbert spectrum of wave-induced ship motions and moments in an attempt to study the time-frequency characteristics of ship response. The results of the present work provide confidence in the usage of numerical simulation data for the decomposition into intrinsic components of ship responses, avoiding the need for experimental data from laboratory model tests.