A procedure is presented for the ultimate strength system reliability analysis of offshore structures. The structural system reliability model is generated through a so-called Critical Path Incremental Loading Procedure in which incrementation is performed along a failure path reflecting the lowest kinematic energy absorbed by the most significant failure path. This procedure reflects the existence of model dependence m structural system reliability, and environmental dependence due to the high loading roughness to which structural systems are generally subjected. No aprion load incrementation scheme is selected, but incrementation is applied on the most contributive load to fail member with the lowest reliability. The procedure can Incorporate various member post-failure behaviour models A semi-probabilistic characterization of structural system redundancy is described which is suitable for early design situation in which different structural layouts leading to different significant failure paths and system reliabilities are contemplated. The porocedure is demonstrated for the system reliability analysis of truss models of jacket structures results show that parallelization strategy to provide alternative and multiple failure paths is not always effective in attempts to increase the reliability of structural systems.
The main decisions affecting the safety of structures are made during design when layout and scanthings, as well as fabrication, operations, and inspection procedures are determined [Moan and Amdahl (1988)]. As the question of optimal layouts inevitably leads to their redundancy characteristic, and provision of multiple failure paths, the structural system reliability becomes an Important factor in the search for the optimum. Moreover, the redundant characteristic of a design is important in connection with catastrophic failure modes. The system reliability of large structures such as an offshore platform is difficult to analyze, however. This analysis is complicated due to the many uncertainties in the choice of the most representative distributions for load and strength, the existence of correlation between possible failure modes, and the behaviour of components in post-failure ranges.
