STABILITY OF SHIPS WITH LARGE BREADTH-DRAFT RATIO IN FOLLOWING AND QUARTERING SEAS

摘要

The International Maritime Organization (IMO) decided to revise its intact stability criteria from prescriptive based criteria to be the performance based one. The new criteria should be capable to investigate stability failure in seaways such as dead ship condition, pure loss of stability, parametric rolling and broaching. In case of pure loss of stability in following and quartering seas, delegation of Japan and USA proposed vulnerability criteria level one based on the hull form characteristics of ships. These proposed criteria have been tested by using several different hull forms. However similar characteristics with the Indonesian ro-ro ferries have never been used to validate the criteria. The main characteristics of Indonesian ro-ro ferries are small draught with large breadth or large ratio of breadth and draught. Therefore it is important to investigate the pure loss of stability in following and quartering seas in order to investigate effect of some variables. This paper discusses about effects of wave direction, significant wave height and ship speed on roll motion characteristics of ships with large ratio of breadth and draught in irregular following and quartering seas. Two degree of freedom model is used to investigate the roll motion characteristics. The irregular wave is developed by using the ITTC wave spectrum and the linear and nonlinear damping coefficients of roll are estimated by using the Ikeda semi empirical method. The results show that the significant wave height has significant effect on the roll motion in following and quartering seas. Eventhought the ship may safely operate in the significant wave height of 2.25 meters with maximum roll angle of 0.545 radian or 31 degrees. In higher significant wave height, capsizing dangerous may occur due to pure loss of stability. The maximum roll angle does not significantly change due to alteration of the heading angle from wave direction. It means that the heading angle does not significant effect on roll motion in following and quartering seas. When the intial forward speed increases, the maximum roll angle decreases. However in a certain initial forward speed, the maximum roll angle increases because occurrence of resonance. This phenomena show that dangerous condition due to pure loss of stability may be avoided by change the ship speed.