Research On The Optimal Design Methodologies Of Ships' Sequential Ballast Water Exchange

Ballast water management and the method chosen to achieve it is a key issue and concerns key technologies in ship design. If the sequential exchange method is the chosen method, the sequence chosen to perform the exchange is very important and affects many aspects including ships'stability, structural strength, maneuverability, operational expenses and building cost, etc. This dissertation focuses on developing optimization methodologies for the large-scale sequential ballast water exchange as well as for the small-scale sequential ballast water exchange. In this dissertation, based on the multiple risk assessment criteria of the sequential method, the problem of finding a feasible and optimum exchange sequence is boiled down to be a multi-objective combinatorial optimization problem with multiple nonlinear constraints.For the large-scale sequential exchange, the method of transversely symmetrical ballast water tanks being exchanged simultaneously is adopted, and the potentials of multiple tanks being exchanged at a time are explored. The mathematical model for the symmetrical ballast water exchanging problem is built with minimizing the occurred maximum trims, maximum hull girder bending moments and shearing forces as objectives and the related safety assessment criteria as constraints. In order to find a set of Pareto solutions, the multi-objective Elitist Nondominated Sorting Genetic Algorithm (NSGA-â…¡) is utilized. In the algorithm, a constraint-domination principle is adopted to handle the multiple constraints and a nondominated sorting method is used to perform the selection of the Pareto solutions. Using the proposed mathematic model and the NSGA-â…¡, a Pareto solution set that meet all the design criteria can be efficiently and accurately obtained for the engineers to choose from within short running times. And a real case study on the symmetrical ballast water exchange of a 176,0OODWT bulk carrier is conducted to illustrate the effectiveness of the proposed approach.For the small-scale sequential exchange, the diagonal exchange strategy is explored to see if better solutions can be found than that of the traditional symmetrical exchanging method. The diagonal exchange mathematical model is built, taking into considerations of the ship's intact stability, structural strength, trims, draughts, and bridge vision. An NSGA-â…¡with some operators specially designed for the problem is presented to obtain a set of approximate Pareto-optimal solutions for engineers to choose from. Comparing with the traditional symmetrical exchanging method, the simulation results show that the proposed method can produce more and better solutions with smaller trims, smaller bridge blind vision ranges and better structural strength performances.There maybe some ships that do not have a feasible sequential exchange solution. The key to this problem is to consider the sequential exchange at the preliminary design stage. Among all the factors that influence ships'sequential exchange performances, the subdivision arrangement is a factor that are comparatively more adjustable and more importantly, will add no building cost to the ship. So this dissertation also focuses on finding a methodology for ships'subdivision arrangement that can guarantee ships'offshore sequential ballast water exchanging (SBWE) performances in the preliminary design stage. Subdivision arrangement of a ship is also a multi-objective combinatorial optimization problem with multiple nonlinear constraints. In the dissertation, a mathematical model is built using minimizing trims and hull girder longitudinal bending moments and shearing forces occurred in the SBWE as the objectives, and the multiple safety criteria of the SBWE as the constraints. The longitudinal lengths of the ballast water tanks (BWTs) are taken as design variables that will alter within a reasonable length range. Like the above mentioned two problems, the operators of the nondominated sorting mechanism, the constraint-domination principle of the NSGA-II are utilized in the optimization algorithm to handle the multiple objectives and constraints, separately. A special crossover operator called the Dispersion Apportioned Allelic (DAA) Crossover is introduced to perform the reproduction of the special problem. A real case study of the subdivision arrangement based on the SBWE of a 50,000DWT double hull product tanker is conducted to demonstrate the feasibility and effectiveness of the proposed approach.Based on the above works, the key technologies that are needed for building the sequential ballast water exchange optimization system (SBWEOS) are studied. The overall framework and the detailed sub-frameworks of the SBWEOS that are based on the AutoCAD software are built. On the basis of fully analyzing the System's necessary functions and the users'characteristics, the working procedure of the System is studied, and the problems of data transfers are worked out.Hopefully, this work can make some contributions to the research and popularization progress of the sequential method.