Formulation of ship roll damping models from free-decay data

Abstract

Estimation of roll damping of a ship is essential to the prediction of its response in waves. The formulation of ship roll damping models from experiment or CFD data involves two parts: the determination of the model form and the calculation of the model coefficients, and both have significant influences on damping accuracy. To improve the roll motion prediction of a particular ship, this paper made an investigation of the typical damping models and its influence factors based on benchmark experiment data. The results show that the typical damping models can not reflect the fluid physics of roll motion. A conclusion is given that all roll angle, angular roll velocity, and angular roll acceleration should be considered in damping model determination. For the convenience of solving the roll motion equation, damping models with angle and angular velocity as independent variables are proposed due to the coupling of the angle, angular velocity, and the angular acceleration in roll. A procedure of damping model formulation is developed based on the Prony-SS method which was introduced into roll damping identification recently. Examples that follow the procedure show good performance. Furthermore, forward speed and flow memory effects on ship roll damping are discussed.