New techniques are introduced into the Bayesian modeling procedure to estimate directional wave spectra based on ship motion data. In the proposed method, the triple-valued function problem in following seas is strictly taken into account and the optimum solution is obtained from the stochastic viewpoint. Using the estimated directional wave spectrum, it is shown that the future states of ship motions and longitudinal bending stresses can be evaluated based on the assumption of linear responses. In order to examine the reliability of the proposed method, on board experiments were carried out; Comparisons between the results of the proposed method and onboard experiments show good agreements.
In rough seas, it is very important for the mariner to monitor the sea conditions and to keep both the ship and its cargo safe by suitable ship maneuvers. In spite of recent developments in nautical instruments, estimating sea conditions still depends on visual observation and many years' experience for deciding these maneuvers. Therefore, it is considered necessary to develop a shipbome guidance system for heavy weather operation. Going back a little in the past, many onboard guidance systems have been developed. Most of these systems can be described as monitoring systems for responses such as ship motions and induced wave loads that are measured by a lot of sensors and strain gauges directly. Though they had some practical value, few of them were adopted on merchant ships because of its cost and difficulty of maintaining the data acquisition hardware. In the case of following seas, three true-wave frequencies contribute to the measured power spectrum at certain encounter frequencies, thus the relationship between encounter and true wave frequencies can be described as a triple-valued fimction.