Optimum Design Of Bulk Carrier Structures Based On Response Surface Methodology

Bulk carriers play a vital role in the international shipping industry. Over 30% of ocean cargo in the world is transported by bulk carriers. With the trendy that bulk carriers are developed to be faster and larger, research of structural optimization, which does not only aim to reduce the weight of hull but also ensure the security of structures, has become one of the most important projects in developing new and advanced bulk carriers.As for large bulk carriers, the bottom girder and strength deck are the main structural members which not only bear the longitudinal bending moment, but also transfer the local load. Bottom girder bears the relatively high longitudinal bending moment and shear force under alternate hold loading (AHL) condition. And ballast operating condition is one of the most dangerous conditions for strength deck. Therefore, the rational design for main structural members of hull is very significant to the security and economy of ships.In this article, structural optimization for bulk carrier using response surface methodology is proposed and verified to be useful by optimizing main structural members of a bulk carrier. First of all, in accordance with the rules of CCS, two'1/2+1+1/2'finite element models of cargo holds in the middle of a 76000DWT bulk carrier, of which the bottom is double and the side skin is single, are created. By creating corresponding boundary conditions, relative loads and analyzing the results of calculation, the most risky operating conditions of bottom girders and strength deck in different cargo holds are defined. Additionally, design variables of each structure members are defined. Design variables which affect strength of structures relatively obviously are selected as independent variables of response surface functions by carrying out sensitivity analysis. On the basis of calculating von Mises stress and maximum shear via finite element method, the functional expressions of maximum stress and thickness of structural members are established via uniform designed experiment of response surface methodology. Moreover, two different forms of quadratic response surface functions are contrasted by assessing the fitting degree. The pure quadratic form is defined as the functional form that not only meets the accuracy requirements of the projects in structural optimization, but also saves time. Finally, by setting the minimum mass as objective function and subjecting strength and required minimum thickness to be constraint conditions, bottom girders and strength deck are optimized respectively. In this example, the weights of bottom girders and strength deck in NO.6 cargo hold are respectively reduced by 6.89% and 10.78%, while the weights of bottom girders and strength deck in NO.5 cargo hold are respectively reduced by 6.77% and 10.24%. The result shows that the optimization design method is applicable.