This paper describes a practical and accurate procedure for safety assessment of offshore jackets in undamaged or damaged conditions. Deterministic safety measures based on maximum load-carrying capacity or absorbed energy capacity for the whole structure which is predicted by using the idealized structural unit method (ISUM) are defined Verification examples of the method are given. Application example to safety evaluation for a realistic offshore jacket under accidental collision loads is shown. It is observed that reduction of the structural safety level due to accidental damage is significant.
In traditional offshore industry, structural design has been carried out by "component-based method", in which strength of only local member is checked against the design loads. Calculation of the maximum load-carrying capacity for the whole structure is not conducted in this process. As a result, it is not ensured how much safety margin beyond the design event exists. For more rational design of new structures, it is necessary to evaluate the safety for the whole structure as well as the local member. Furthermore, the existing offshore structures may suffer accidental damage or experience over-loading by changing operational requirements. The structure must remain intact without catastrophic collapse even after such accidents. It may be needed to develop a proper repair-procedure for the situation of relatively minor damage. First of all, in order to ensure whether the structure is still safe enough, safety assessment for the whole structure in damaged as well as in undamaged condition is essential. Along this line, a lot of efforts for the development of safety assessment method for offshore structures are increasing (Aggarwal: 1990, Lalani: 1990, Billington: 1991). This means that in order to predict the maximum load -carrying capacity or the absorbed energy capacity for the whole structure, the progressive collapse analysis should be performed.