OFFSHORE STRUCTURAL SYSTEM RELIABILITY UNDER CHANGING LOAD PATTERN

This paper considers structural system reliability analysis when the spatial force pattern changes as the load increases, specifically when the nodal forces on a steel offshore jacket structure change as the base shear increases under increasing wave height. This paper demonstrates the so-called "fragility approach" to systems analysis, wherein the reliability of the system is obtained for each of a set of increasing wave heights. These probabilities are conditional on the wave height. These results are multiplied by the (discrete) probability levels of these wave heights, and the products are summed to obtain the (total) system reliability. The method is applied to analyze a fixed offshore jacket in 140 feet of water and designed for a 100-year (63 foot) wave. The change of the relative importance of different member-failure sequences with wave loads corresponding to different wave heights is studied. In this case study, the inclusion of wave slamming forces on the deck at higher wave heights led to an increase of the failure probability by an order of magnitude. The use of an improved fixed wave-load pattern treatment is explored. A comparison of the member-level and system-level failure probabilities is presented in terms of factors measuring different system effects.