Mitigation of Ship Motion Using Passive and Active Anti-Roll Tanks

Because excessive roll motions of ships in rough seas badly affect their performance, there is a continuous interest in efficient ways to mitigate these undesirable motions. There are different devices to mitigate the roll of ships with different levels of performance and operating limits. Anti-roll tanks are more effective than other roll stabilization devices when the ship is not underway or moves slowly. Here, we investigate the application of passive and active anti-roll tank systems. The tank system consists of three tanks: each one consists of two columns connected at the bottom via a horizontal pipe equipped with a pump. The tanks are arranged along the length of the ship, symmetrically located about its center of gravity. The motion of the liquid in the tank is 1-D, but it exerts loads on all degrees of freedom of the ship. The equation governing the tank-liquid motion is coupled with the equations governing the 6-DOF motion of the ship in waves; the coupled system is solved simultaneously in time. First, we derive expressions for the forces and moments exerted on the ship by the tanks. Then, we study the roll response at different sea heading angles in rough sea conditions in the absence of the tanks to identify the critical heading angles where the roll is large. We demonstrate the nonlinear behavior of roll through frequency-response curves for different beam wave amplitudes. These curves exhibit typical nonlinear phenomena (jumps and hysteresis) for high wave amplitudes. Spectral analysis shows a two-to-one frequency relationship between the roll and pitch in rough head and follower seas, which are the most critical sea headings. For passive and active tanks, we study the effect of the frequency of the tank system on its effectiveness. We consider active anti-roll tanks in which the pump power is controlled via a proportional-derivative (PD) control law using the roll angle and its rate. We compare the performances of passive and active tanks in rough sea for the critical heading angles. We found that active anti-roll tanks outperform passive ones in terms of roll reduction and size and weight, but they require power consumption. To achieve a specified roll reduction, the weight of a passive tank might be as large as five times that of an active tank. We also found that the performance of passive tanks depends strongly on their frequencies, in contrast with active tanks, which are insensitive to their frequencies.