The optimization of the ship structure with design variables of the plate thickness and the shape of stiffener is a popular study filed. But, the optimization of the structure with the design valuables of the number and the position of stiffners are necessary for obtaining a better optimal solution. Therefore, the optimization with theses design valuables is performed in this paper. The optimization with theses design variables needs a lengthy calculation time because recreating FEM models and re-analyzing must be performed. In order to overcome this problem, the calculation method for evaluation of the strength of structures without recreating FEM models and the method of reducing calculation amount of FEM are proposed. There is another problem in optimization part. Since the number of stiffeners is changed the number of design variables may vary during an optimization process. In order to solve this problem, an effective optimization method for this problem is proposed.
The structure which imitate a double hull part of oil tanker is selected for target of optimization. These proposed methods are applied to this optimization model and their effectiveness an verified.