U-tank active roll dampening system for and method for active roll dampening of a vessel

A U-tank active roll dampening system for a vessel (100) defining a port side and a starboard side, comprising:

• a U-tank formed by one side tank (11a) arranged at port side of the vessel (100) and one side tank (11b) arranged at starboard side of the vessel (100), each of the side tanks (11a, 11b) being sealed at its respective upper end and the side tanks (11a, 11b) being connected to one another by a crossover duct (12) at their bottom ends, the U-tank (10) being partially filled with a fluid,
• a motion reference unit (50) measuring accelerations of the vessel (100) roll motion, and
• a control unit (60), wherein
• at least one vacuum and air pressure manipulation unit (20) is arranged to an upper part of one of the respective side tanks (11a, 11b) and at least one aeration device (30) against open air is connected to an upper part of the other of the side tanks (11a, 11b),
• the at least one vacuum and air pressure manipulation unit (20) is configured to supply air pressure or vacuum on the fluid surface in the respective side tank (11a) to which it is arranged, and
• the control unit (60) includes one or both of software and means for controlling the at least one vacuum and air pressure manipulation unit (20) to thereby control a level of fluid in both of the side tanks (11a, 11b) in anti-phase in advance of vessel roll motion period by predicting and calculating a fluid level in the respective side tanks (11a, 11b) in anti-phase in advance of vessel roll motion period based on measurements of the motion reference unit (50).