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A discrete optimization method using genetic algorithms is developed for the optimization of ship structures. In this method, the constrained minimization problem is first transformed into an unconstrained one by a penalty term depending on the degree of constraint violation. Since the search procedure of the genetic algorithm is done based on the evaluation of fitness function, the unconstrained minimization problem is further converted into a maximization of the fitness function. The discrete design variables are coded into a binary string of finite length. The search procedure from generation to generation is carried out by a simple genetic algorithm with the genetic operators of reproduction , crossover and mutation. A cargo ship with large hatch opening is taken as the numerical example for the illustration purpose. The influences of penalty coefficient, population size, crossover probability and mutation probability on the optimum design are investigated. The comparison between the genetic algorithm and the multiplier method is also made. It demonstrates that the present method can handle the optimization of ship structures with discrete design variables well .

SummaryIn this paper a procedure for the optimization of ship structures is developed based on plastic design of ship structures consisting of the transverse and longitudinal members using the Sequential Unconstrained Minimization Technique (SUMT) . Using the developed method, the optimization of a segregated ballast tanker is carried out considering the weight of one hold length as the objective function. Midship section scantlings are chosen as the design variables. Constraints are considered with respect to plastic collapse of hull girder under longitudinal bending, of structural members subjected to axial force, shear force and bending moment, and of platings subjected to lateral pressure. Constraints are also set in buckling strength of platings under inplane load and in design variables such as minimum plate thickness. The variations of maximum bending moments and shear forces due to changes in design variables are calculated based on Taylor series expansion method so as to reduce the computation time of iterative calculation for function minimization in the SUMT procedure. The tanker structure got optimized and the results show that the proposed method based on plastic design is an effective tool in the optimization of ship structures. A comparison of the proposed method with the optimization based on elastic design is also made and the results are presented.