Assessment of Roll Stabilization Systems for Heavy Lift Ships

Abstract

Heavy-lift ships are vessels designed to move very large loads that cannot be handled by normal vessels, and lately heavy lift ships have been widely used in the offshore industry for offshore installation projects which requires large operational time window. As majority of the lifting operations are done at zero speed condition the dependency on roll motion is very high, which make it necessary to install roll stabilization system on such vessels. There are many roll stabilization systems which are being commonly used in the maritime industry, but, most of the stabilization systems become obsolete at zero speed condition and hence the only effective systems left are passive fins and anti roll tanks. But, both these systems have constraints ,like, anti-roll tanks utilizes large stowage area of the vessel which effect the profit of the company, as well as, retrofitting of these systems are really complicated. Similarly, passive fins have the limitation on its size, as the size of the fins increases the resistance. So a new stabilization system has to be designed which is effective, economical and has the possibility to be fitted to a vessel whenever required without affecting its stowage area or the hull form resistance. As a solution it was found that if there is a possibility to fit a large plate on the vessel, then, it will act as a damping system which can reduce the roll motion. Even though, the system seems simple, there can be ‘n’ number of possibilities which make it necessary to perform an optimization analysis. Before running the optimization, it is necessary to accurately estimate the roll motion by considering the non-linearity due to viscosity. Majority of the motion analysis softwares used in the industry are potential solvers, which do not analyze the viscous component. So to consider the non linear viscous damping, a roll decay test is performed but, since experimental tests are expensive, numerical simulation using RANSE solver (Star CCM+) was used. Finally, by using roll decay results and ITTC recommendations, the non-linear damping coefficient is estimated and the non linear roll RAO of the vessel is computed accurately using a potential solver. Now the optimization using genetic algorithm can be performed for finding the best plate which gives minimum roll RAO and minimum structure weight. So by considering the oscillatory drag coefficient of the plate it’s possible to estimate the roll RAO of the vessel with plate and using simple beam theory the best plate structure for a pressure distribution was calculated which is used to compute the weight of the structure. Finally, the best plate configuration obtained is compared with all the possibilities of fixing a passive anti roll U-tank in the vessel and for designing the U-tank the theory put forth by Lloyd in 1989 is used. Ultimately the most feasible solution out of the above is proposed.