Simulation of Ship Drift Motion with a Simplified Mathematical Model under the Wind

Abstract

Dead ships in rough sea make strong drifting motions and sometimes cause the significant accidents such as grounding or destroying offshore structures. For the prediction of such drift motion, it is necessary that the suitable mathematical model should be provided. Although many mathematical models for the conventional maneuvering ship motion are proposed and widely used, most of them are limited within the conventional maneuvering motion. They cannot be used for the large drift angle such as 90° including the zero-ship speed turning. It is very difficult for the conventional mathematical model to express the hull forces in such drift motion. One of the authors tried to make them using crossflow drag model1), 2), 4), 5), 6). However, the above models include the longitudinal integral terms, which makes the difficulties when using for real-time simulators or system identifications. In this paper, the authors have developed a simple mathematical model that has the almost equivalent hydrodynamic force characteristics for conventional crossflow model instead of using the integral terms. The new model can also express the hydrodynamic forces with large drift and turning motion including zero ship speed condition. In order to validate the mathematical model, the drift tests in the uniform wind were carried out and the simulated results were compared with the measured data. From the comparison between experimental results and simulated them, it is found that the proposed mathematical model as well as the original crossflow drag model make it possible to predict the wide range of drift motion. Furthermore, the parameters in the proposed mathematical model can be easily obtained from the principal particulars of ship based on the regression analysis. Then the drifting simulations become very easy by using the proposed simple mathematical model and the empirical formulas of the parameters.