Pre-FEED decision making for mooring system configuration using Progressive Reliability Method

Abstract

Abstract The safety of an offshore structure is significantly affected by the decisions made early in the Pre-FEED and FEED stages; however, a quantitative measure to evaluate the effect of each decision on system safety is usually not available. Mooring systems are particularly vulnerable offshore structures to such early decisions because they are exposed to uncertain extreme environments and already have a relatively high rate of failure in their components. This paper presents a Pre-FEED quantitative reliability analysis for the configuration selection of a mooring system, accounting for the uncertainties associated with the environmental loads and component strengths. By estimating the annual probability of failure of alternative mooring configurations, using Progressive Reliability Method (PRM) and some simplifying assumptions, the additional safety versus additional cost of different mooring configurations compared to the based configuration are compared and discussed. In the analysis of this study, the redundancy of the mooring system is considered by defining the system failure as if two or more mooring lines fail. The environmental conditions and the mooring component capacity variations are selected based on Gulf of Mexico projects. By estimating the response of the mooring system based on limited dynamic analysis for various return periods and loading directions, a regression model is used to develop the probability distribution of the conditional probabilities of failure under each return period and estimate the annual probability of failure of the mooring system. This simplified method decouples the environmental demand probabilistic analysis from the component demand and capacity probability distributions and therefore uses a practical probabilistic model to compute the safety of alternative mooring configurations. The results are useful for identifying the safest alternative configuration as well as quantifying the effect of adding more lines to the system or risk of each configuration. This study discusses the strength failure of the mooring components under extreme loads and other failure modes are beyond the scope of this work.