Reliability-based fatigue inspection planning for mooring chains of floating systems

Abstract

The safety of offshore floating units, supporting renewable energy generation or oil and gas production facilities, highly depends on the platform's station keeping capability, which is provided by mooring systems. Lately, there has been a high rate of mooring line failures, leading to increased interest in improving their design and maintenance methodologies. Specifically, upper chain segments at the splash zone are more prone to suffer from the effects of corrosion degradation and fatigue. The fatigue limit state and the corrosion allowance of mooring chains are safety requirements that need to be verified continuously through mandatory regular inspections. Current trends related to mooring chains point to the development of new methodologies for fatigue damage calculation, corrosion degradation models and inspection planning methods. The present work proposes a practical reliability-based method for planning future inspections of such mooring line components, which combines recently-developed fatigue and corrosion methodologies and takes into account the results of the previous inspections. The risk-based assessment of the mooring system is continuously updated with new inspection data, and is a suitable tool for monitoring the current reliability level of the structure. This work presents a case study of a FPSO on the Brazilian coast as a practical example of the application of the proposed inspection planning method. In this way, it aims to help future designers and operators to avoid failures in mooring chains and to optimizing inspection and maintenance costs.