Verification of RAOs in sea trials

A method to estimate the directional wave spectrum from measured ship motions is currently under development. The algorithm relies on response amplitude operators (RAOs) to link ship motions to the sea state. In order to verify numerically obtained RAOs in realistic sea states, two campaigns of extensive sea trials were performed on the RV Simon Stevin in the North Sea close to Ostend. Ship motions were recorded in six degrees of freedom, in this paper analysis was restricted to roll and pitch. Data from several directional wave buoys in the vicinity of the trial runs were available. RAOs were calculated using OCTOPUS and HYDROSTAR. Spatial and temporal fluctuations were found in the wave buoy data, so there is some uncertainty in the wave spectrum seen by the vessel. Considering these uncertainties, HYDROSTAR gave reasonable agreement for pitch, larger discrepancies were found for roll especially close to the resonance frequency.