Research On Wave Estimating Based On Data Of Ship Motion Response

The Dynamic Positioning technology is playing a more and more important role in theexploration of sea. According to the actual working situation of DP vessel, we could derivethat most of the operation is used to counteract the influence of external disturbance. So it isof great importance if the sea state of working region is obtained. The sea state could betreated as a kind of forward feed, thus the DP controller could operate more effectively. Onthe one hand it could make the vessel consume less power during operation; on the other handit could help the vessel achieve the desired target faster.Ship motion time series could be obtained from sensors installed on the ship. Afteranalyzing cross spectrum of these time series, the paper proposed a method to estimate waveswith the help of wave directional spectrum and ship response amplitude operator.Three motions-roll, pitch and heave-are used in the paper. As these motions are mainlycaused by waves, so the paper first studied waves. As traditional long crest wave spectrumscontain no information about directions, the paper studied short crest waves which is alsocalled wave directional spectrum. Two methods to simulate waves are studied, and thevalidity of one of the methods is tested.The model of kinematics and dynamics of a ship with six freedoms is build. With thehelp of mss toolbox in MATLAB, roll、pitch and heave is simulated, thus time series of shipmotions are obtained. RAO is introduced as it would be used in the latter part of the paper.Cross spectrum of time series should be calculated after they are obtained. Aiming at theproblems existed in the traditional methods of spectrum calculation, spectrum leakage andmanual configuration for example, the paper proposed a method to calculate cross spectrumbased on MAR model. With the simulated time series, behavior of MFPE and BIC is studiedand cross spectrum of these time series is calculated.Theoretical cross spectrum of ship motions is calculated combining wave directionalspectrum and ship motion RAO. Set the cross spectrum of ship motion time series obtainedthrough MAR-based spectrum analysis as the target spectrum, and then apply GA inparameter optimization. Through altering parameters of wave directional spectrum, minimumvalue between theoretical cross spectrum and target spectrum could be found theoretically. Atlast, simulations of proposed method were carried out under low and moderate sea state.