Practical evaluation system for post-buckling strength of ship double-hull structures

Since the recent obligation for tankers to adopt a double-hull structure, most types of ships have adopted a double-hull structure. In double-hull structure, the neutral axis lies near the center of the cross section, so that the high tensile and compressive stress of about the same level occur alternately on the skin plate. Considering the transverse strength of hull under the heavy load condition, a compressive stress nearly equal to the allowable stress acts on the skin panels which, in many cases, exceeds the elastic buckling stress of such panels. Once skin panels as attached plate of girder are buckled, the stiffness of the girders decreases consequently. If this stiffness reduction leads the expansion of the region of panel buckling, global strength of hull decreases seriously. On the other hand, such panel elastic buckling dose not have serious effect on the global strength of hull if the buckled region is limited in the small area due to the re-distribution of the applied load. Therefore the assessment of the safety and reliability of double hull structure under the heavy load condition requires the confirmation of the buckling strength of the skin plate panels and the behavior of the global structure after buckling Recent progress of the analysis tool and expanded capacity of the computer system enables to solve such complicated nonlinear problems gradually. However these analyses have not been effectively applied in the actual design stage yet because the global hull structure model is too large for this kind of nonlinear analysis. In order to accurately evaluate the post buckling behavior of whole double hull structure in a practical time, the authors have developed the new assessment system, named SAIAS-PB, by the iteration of linear FEM analysis introducing simplified plate element with equivalent stiffness of buckled panel. This system enabled the evaluation of the post buckling strength of whole hull structure in very short time, and thus, the design for optimizing the arrangement and reinforcement of structure. In this paper, we report the investigation of the stiffness of panel after elastic buckling, development of new plate element for equivalent stiffness of buckled panel and the application to the transverse strength calculation of actual double hull structure.