Study On Damping Characteristics And Natural Period Of Ship Anti-rolling Tanks

Anti-rolling tank is one of the most widely used anti-rolling devices on ships, which is easy to use and has simple structure and a good damping effect in the both low speed and high speed, especially suitable for ships moored on the sea operating for a long time. The philosophy it works is to use the additional restoring force due to the difference of water levels in the water tank, which will hinder the rolling motions of the ship when the ship roll occurs. To make the anti-rolling tank achieve the best effect, the oscillation of the water in the tank must fall a quarter of the period behind the roll of ship. So while designing an anti-rolling tank, damping and period are very important parameters.At present, at the early stage of the design for anti-rolling tank, when its size and location have been confirmed, we can't forecast the damping characteristics accurately, which must rely on experiment. There are two methods we always use to test the performance of the anti-rolling tank, one is using a model which scaled down the anti-rolling tank, the other one is testing on the actual ship. Experimental studies usually consume a lot of manpower,material resources and financial resources. Moreover, the condition of test is conditioned by the natural environment, and the results will also be affected by many external factors.Simulating the rolling motions of anti-rolling tank by CFD can avoid interferences stated above.In order to conclude a set of effective empirical formula about the damping of anti-rolling tank, which can help the designers confirm the internal structure of anti-rolling tank at the beginning of design, and reduce unnecessary waste. In this paper, the relationship between the amplitude-frequency curves and the damping of water tank was calculated using FORTRAN programs, then the analysis of the results and the best solutions was performed taking a basic design of a RORO ship as an example. Thirdly, the numerical simulations of the rolling motions of anti-rolling tank by CFD were performed, monitoring the change of speed in the water tank, so we could know its natural period. Comparing the results with the experimental data, and summarizing the variety law of anti-rolling tank cycle with damping structure, a new and more accurate empirical formula was proposed. Finally, we got the eliminating curves of water tank from CFD simulations of the free damping of the water tank,then applying it to research of calculating the damping of different water flow chamber combination structure. Through calculation and analysis of data, an estimation method of calculating damping of water tank was given. Although the use of CFD simulation technology in the study is not mature, it will be widely used in the future.