Over recent years, ever more globalised trade connections and the developments in terms of world trade, energy markets, climate change concerns or security threats have stressed the importance of seaborne transport for the prosperity of Europe and its citizens. With over 80% of world trade being carried by sea, maritime transport remains the backbone of international trade. For the EU, the world’s most important exporter and the second biggest importer, shipping and related services are essential in helping European companies to compete globally.
Whereas all angular degrees of freedom (roll, pitch and yaw) are important for sea-keeping characteristics of any ship, roll motion is known to be the critical one. This is because ship roll is typically lightly damped and the restoring moment of the ship is small in the cross-plane in comparison with the other planes. As a result, excessive roll can occur under unfavourable or extreme sea conditions. This can lead to reduced effectiveness of the crew, damaged or lost cargo, limited operability of the on-board equipment, or even to catastrophic sea accidents which include capsizing of the ship and the loss of human lives.
Roll reduction devices include keels, fin stabilizers, rudders, gyro stabilizer, azimuthing propellers, and anti-roll tanks (ARTs). Among these devices anti-roll tanks have raised a considerable attention since they do not cause highly concentrated loads as, for example, gyro stabilisers, they are effective at low forward speeds.
In the traditional design of passive anti-roll tanks (ART), the energy associated with the tank fluid motion is dissipated. The research carried out within this project explores the possibility that this energy is instead harvested.
On the other hand, U-tube ARTs can be made active. With the active approach, the water can be transported from one side reservoir to the other by a pump located in the connecting conduit between the two reservoirs, according to a controller command. This could potentially enable generating a more effective anti-roll moment than that produced by passive ARTs.
Results of research carried out under the STARMAS project suggest that the maximisation of the ART harvested power corresponds to the minimisation of the mean kinetic energy of the ship roll. This is of particular technological importance since the ART tuning that minimises the roll motion also maximises the recuperated energy. Therefore prospective efficient ART energy harvesters would not be conflicting with efficient roll control.
Regarding the active ART technology, it was found that a reduction of the ART natural frequency results in a reduction of power injected into the system by the pump. The same roll control performance can be achieved with a lightly damped, low natural frequency ART as with the heavily damped ART having its natural frequency similar to that of the natural frequency of the ship. The former design however requires much less power to drive the pump.
