Study on Short-term Variability of Ship Responses in Waves – Part II: Examination of the correlation method

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 of the cross-spectra, has not been discussed in detail. Analyzing long stationary time series by direct Fourier transform, the authors pointed out that the short-term variability of the relative phase angle of the cross-spectra was very harmful to sea state estimation of the wave buoy analogy. In this report, the same investigation based on the correlation (Blackman-Tukey) method was applied to the same data. In the results, it was observed that the short-term variability of the relative phase angle was reduced to a certain extent.