Three-mode pneumatic management of marine U-tank systems

This paper deals with a new pneumatic control strategy for the roll damping enhancement of marine U-tank stabilizers. The proposed technique consists in a three-mode operation, where the control is active only within a limited resonant range around the ship natural frequency, whereas the control valves are kept closed in the remaining frequency range. Moreover the connection valve between the two air chambers is either closed or partially opened for the low or high frequencies, respectively. The pressurized air for the active control is fed by a turbo-blower set aboard and operates accelerating the motion of the water mass in the U-duct. The theoretical analysis is conducted in the hypothesis of harmonic excitation and includes an equivalent linearization procedure for the turbulent air flow through the upper valve regulating the efflux from the one to the other tank. The application of the laws of thermodynamics to the open air system overhanging the water in each tank chamber reveals the negligibility of the heat transfer through the tank wall in comparison with the mass transfer effects. As a practical result, the mathematical model permits developing a suitable optimization process with the aim at the best damping operation, which however must be subjected to two fundamental constraints: the system stability and the pre-imposed limit power absorbable from the control equipment. Comparing with natural damped passive systems, a reduction of the order of 20–40% can be roughly attained for the resonant roll amplitude, depending on the supplied control power.