Influence of Ship Navigation on the Long Term Prediction of Wave Loads

Abstract

When estimating an expectation value of ship response, a long term prediction is commonly made under the assumption that a ship speed is constant with equally distributed encounter angle to waves. But in severe sea states, a ship speed is naturally reduced and shipmasters often change the speed and the course of ships to avert hull damage and to ease a ship motion. Since ship responses are sensitive to the encounter angle and to the ship speed, it is important to account for the effect of ship handling, speed reduction and course change, on the long term prediction. The present paper describes at first the statistical characteristics of ship's navigation in real seas, based on weather reports by ships in operation. Distribution of mean value and variation of ship speed in some routes are discussed. Then an empirical model to describe navigational behavior is presented by fitting simple formula. Several ship types are chosen as examples. Finally, a new method for the long term prediction is proposed to take in the effect of the ships handling in real sea. Example calculation is made for wave-induced midship bending moment and relative wave motion at the bow. Comparisons with results by conventional prediction method are made.