RMS-3: A Systems Reliability Case Study of an Eight-leg Jacket Platform

This report describes a systems reliability analysis of an idealized eight-leg, steel-frame offshore jacket platform located in 140-ft of water, and designed for a 100-year return period wave. The reliability of the steel-frame structural system (excluding piling) under gravity and extreme wave loads was estimated using a structural systems reliability analysis procedure devel­oped at Stanford University /1/. Wave loads and member resistances were treated as random variables, and reli­ability calculations were performed using first-order reliability methods. Reliability analyses were conducted to study the effect of using X-bracing versus K-bracing in the principal load-carrying bents, the effect of wave load variabili­ty, the decrease in reliability under damaged conditions (simulated by removing selected frame members), and the adequacy of horizontal bracing to transfer loads among the frame bents. The results reported herein are to some degree sensitive to simplifications made in modelling the post-failure behaviour of structural members. In particular, the compressive braces are treated in a conservative manner. Also, the jacket structure used in this study is a highly idealized design. Consequently, the analytical results of this study should be viewed as more qualita­tive than quantitative, and the conclusions cannot be directly generalized to real structures. On the other hand, the report illustrates the kinds of studies and comparisons that are possible using systems reliability. The measures of structural redundancy and robustness introduced in this report can be used when comparing the structural reliability characteristics of alternative steel-frame platform designs.