On the Horizontal Slow Oscillation and the Dynamic Stability of Ships Moored to Single Point Moorings (2nd Report)

Abstract

In the last report, the relation of the shapes of single point moored ships and their slow oscillation in current was investigated. In practice, not only current but wind and wave have to be considered, and it has been said that the slow oscillation is excited by current and stabilized by wind. However the interaction of different external forces, especially the effect of the relative direction of them on the slow oscillation, has not been examined sufficiently, and almost all of the studies on this problem have been carried out only numnerically, and moreover the results of them are not always same. Aiming at the interaction of current and wind, the effect of the relative direction of them were investigated in this paper by the two theoretical approaches, and model tests were conducted to verify the theoretical results. And it is ascertained that the slow oscillation becomes small when the directions of current and wind are different and becomes large when the directions are identical. Dynamic stability analysis cannot indicate the amplitude of the oscillation, it shows only whether the ship is stable or not, but can make the effects of various factors on the oscillation clear, and it can make it easy to find out the direction for the system modification. Examples of the application of the dynamic stability analysis to investigate the system to suppress the oscillation are also denoted in this paper.