Validation of a strip theory approach to estimating ship motion from sea state

Abstract

A method to estimate the ship motion on board of a vessel underway is presented in this work. Oscillatory ship motions in four degrees of freedom (heave, surge, roll, and pitch) are measured using a low-cost combined gyro and accelerometer-based instrument placed near the center of mass of the vessel. Measurements were recorded from the SA Agulhas II, a South African Polar Supply and Research Vessel on a regular trip from Cape Town to the polar region in the Southern Ocean. The motion power spectra for different degrees of freedom are determined from time series. The electronic filter analogy is used in this study. Response Amplitude Operators are used to predict the ship motion response to the wave through various frequencies and directions. Response Amplitude Operators were computed numerically for roll and pitch as well heave and surge using a hydrodynamic strip code for seakeeping for different ship speeds. The ship motion obtained from the sensors are compared to motion spectra computed from visual observation entries recorded from the vessel’s logbook using the Bretschneider Ansatz model and wave spectra computed from a re-analysis of remote sensing data from the European Centre for Medium Weather Forecast. In the conclusion, considering there were a certain margin of error in the visual observations, a good agreement was obtained for all degrees of freedom. A much better agreement was achieved when compared with the reconstructed sea state from a re-analysis of remote sensing data.