Life Extension of Mooring Chains - Including Effects of Mean Load and Corrosion Condition to Fatigue Capacity

Abstract

The mooring systems on floating offshore installations are typically designed for a life of 25 years. Life extension of installations beyond the original design life thus requires either mooring system replacement or assessment to document life extension of the mooring system components. Mooring chains are normally the components with the shortest design life, where the design codes define how break strength and fatigue capacity reduce with time, due to an annual expected corrosion loss. Equinor has replaced many mooring chains, and these have been thoroughly inspected, where some have been tested to document actual break capacity and fatigue capacity. The effect of corrosion on breaking strength is found to be low, while the effect of corrosion on fatigue capacity is found to be significantly larger than specified in design codes. Another key finding is that the fatigue capacity is dependent on mean load. For mooring systems with low pretension or mean load, the mooring lines may have better fatigue capacity than initially calculated in design. By analyzing full scale fatigue test data from both new and used chain, it is now possible to establish formulas for mooring chain fatigue capacity which also account for the effects of mean load and corrosion condition. The total number of chain replacements has, together with installation specific chain replacements, made it possible to estimate corrosion condition and forecast future corrosion development. It has thus been possible to perform fatigue reassessment including fatigue capacity with the effect of chain degeneration. This paper presents the fatigue life reassessment of the mooring chains for an FPSO on the Norwegian Continental Shelf (NCS), utilizing the new knowledge. The reassessment documents a significant increase of the service life for certain mooring line segments, while some are confirmed due for replacement. As a result, the initially planned chain replacement scope has been reduced, with large cost savings. The paper presents and discusses the assumptions and data used in the analysis, including comparison with standard fatigue analysis for the same mooring system.