Short-term Variability of Cross-Spectral Analysis for Ship Responses in Waves

Short-term variability of ship responses is investigated by cross-spectrum analysis. In a steady state condition, it is well-known that a certain length of sampled data is required for stable results of the spectral analysis. However, the phase lag between responses, in terms of the phase angle obtained from the (coupled) cross-spectra, has not been discussed in detail. In a previous but similar study, the authors pointed out that the short-term variability of the relative phase angle of the cross-spectra is harmful to sea state estimation using the wave buoy analogy, i.e. ship as a wave buoy. In the present report, an investigation based on the multi-variate auto-regressive model method has been applied to the same data. In the results, it has been observed that the short-term variability of the relative phase angle can be reduced.