The Stabilizing Effects of U-Tanks as Passive and Controlled Anti-Rolling Devices

In this study a U-type tank, consisting of two lateral reservoirs connected by means of an horizontal channel, is considered either as a passive or controlled anti-rolling device for ships. Initially, forced oscillation tests have been conducted at CEHIPAR for the U-type tank on a moving platform for different frequencies and also for different amplitudes of the pure roll forced motions adopting two distinct configuration: passive and controlled by means of two air valves installed on top of two lateral reservoirs. The most important aspect was to obtain a large set of roll restoring moments, for both passive and controlled configurations, which could give a discrete distribution of the roll restoring moments along the entire range of wave frequencies of interest. On a first study, a detailed account of model tests was presented, the method of analyzing the experimental results was discussed, and the experimental data of the U-type tank was compared with results from Lloyd's linear frequency domain model, where non-linear effects with the amplitude of the forced motions was thoroughly addressed. In this paper, the stabilizing effects of the same U-tank installed onboard a typical containership has been first analyzed with two distinct tools: WaMIT wave interaction 3D analysis software, and the two degrees-of-freedom numerical model referred above. In second place, the experimental data of the controlled U-type tank has been compared and contrasted with results from a nonlinear time domain model, where the controller of the air valves has been also included. Therefore, some preliminary conclusions could finally be drawn about the stabilizing effects of U-type tanks as passive and controlled anti-rolling devices.