The present study attempts to formulate a simple mechanical model for the damaged tubular member shown in offshore structures such that the progressive collapse strength analysis for damaged offshore structures after accidental collision becomes to be possible with a reasonable computational efforts. The theoretical background of the present study is laid on the Idealized Structural Unit Method(ISUM) firstly suggested by Ueda and Rashed and also a simplicity for the numerical procedure is emphasized on the present formulation, in which the personal computer-based application is aimed. In order to verify the applicability of the present method to the actual structures which may be modelled by frame structures, experiments using large scale frame models composing of tubular member are conducted, in which three plane frame models for intact, damaged and removed condition and one spatial frame model for damaged condition of the local member are treated. Since the results by the present method are in good agreement with those by experiments, it is concluded that the present method can be practically employed for the progressive collapse strength analysis of tubular offshore structures.
For instance, if the local element in the offshore structures has the damages resulting from the action of the accidental collision, the internal forces in the structure are abruptly redistributed and also adjacent members becomes to be progressively failed as the external load increases further and finally the structure may be collapsed as a whole. Thus to evaluate the progressive collapse strength of the offshore structures must be one of the most important tasks in the design process for safer and more reliable structure. In general, finite element method is considered to be a powerful tool to carry out detail analysis of the nonlinear behaviour until the structure reaches the ultimate limit state.