The present paper focuses on the different issues encountered when assessing the failure probability of a moored system. The example of a moored FPSO is considered. Uncertainties associated with the description of the mechanical system, the load calculations and environmental conditions are discussed. Methods to perform reliability analysis and to assess the failure probabilities are presented.
The present paper focuses on the different issues encountered when assessing the failure probability of a moored system. The framework of reliability analysis allows computing the failure probability of a system and to calibrate the safety factors which are required to reach a given level of reliability. However, assessing accurately the failure probability of a system requires all the involved uncertainties to be considered and possibly included in the limit state function. The first goal here is therefore to identify and characterise the main uncertainties present in the modelling of a moored structure. Uncertainties associated with the mechanical systems may evolve with time. For example, the maximum breaking load (MBL) for a mooring chain depends on the corrosion status of the chain link which initial mechanical properties will weaken with time. Examples of failure probability computations for chain link and steel wire rope will be presented. Hydrodynamic and aerodynamic polar coefficients or any other coefficient such as added mass and inertia, damping parameter and restoring force and moment carry their own level of uncertainty. But it should be kept in mind that exposed areas to wind and current loadings are also involved in the hydrodynamic force calculation. These last quantities depend on draft and trim of the FPU, which may in turn depend on other parameters such as oil and water volumes stored in each of the compartments as in the case of a FPSO hull.