Global Fatigue Analysis of the Mooring Systems for Offshore Floating Structures

The mooring system is essential for keeping the floating structures in place while performing marine applications under adverse weather conditions. However, mooring systems are subjected to fluctuating stresses due to structural details, material characteristics, and environmental factors resulting in fatigue damage. In addition, most studies only perform a local fatigue analysis focusing on the upper chain. However, a global fatigue assessment is necessary to determine the most critical fatigue location along the mooring line. A turret moored FPSO with nine mooring lines in a 400-meter water depth is utilised in this study. The metocean data is taken from the Gulf of Mexico. Three different mooring system designs have been established to analyse further the global fatigue analysis of the mooring lines induced by low frequency, wave frequency, and combined responses. Results reveal that the fatigue damage of the moorings is greatly influenced by wave frequency motion, and the chain at fairleads is the most critical fatigue location for all three cases. Conversely, the least critical fatigue location is the chain at the top of the bottom chain in case 1, while it is the chain at the seabed interaction point in cases 2 and 3. Results also show that the chain-polyester-chain system has more extended fatigue life than chain-chain-chain and chain-wire-chain systems. This paper also provides further recommendations for future work. Keywords: mooring system, offshore floating structures, global fatigue analysis, fatigue life, FPSO, wave frequency, low frequenc