A time-domain method to evaluate the use of moving weights to reduce the roll motion of a ship

A method to evaluate the use of actively controlled moving weights on board ships to reduce roll motion is developed. The weights can simulate in prin ciple anti-roll-tank systems, or they can be considered a possible anti-rol l device in their own right. The ship, the moving weight, and the control d evice an considered components of a single dynamic system. The full eight-d egree-of-freedom set of coupled governing equations for the complete dynami c system is derived. And a three-degree-of-freedom non-linear approximation for the roll motion only (MOTSIM) is derived from these eight equations. T he reduced set of equations is used to determine the influence of various p arameters and to evaluate control strategies. A PID controller is developed to command the position of the weight and a servomechanism model is used t o predict its actual position. Then, the moving-weight system is incorporat ed into LAMP (Large-Amplitude-Motion Program), a computer code that integra tes the governing equations of the sea and the motion of the ship interacti vely and simultaneously and predicts the motion of the ship in the time dom ain. A comparison of the results from the two simulations shows that there is fairly good correlation between the simple and complex models, but the s imple model is a little optimistic in predicting the effectiveness of the m oving-weight system. The results predict that the moving-weight system can be an excellent roll-suppressing device when the moving weight is as small as 1% of the displacement of the ship and the maximum distance the weight m oves is as small as 15% of the half-beam. (C) 2000 Elsevier Science Ltd. Al l rights reserved.