Fatigue Capacity of Used Mooring Chain - Results From Full Scale Fatigue Testing at Different Mean Loads

Abstract

Fatigue capacity of mooring chains is one of the important parameters in design of mooring systems for floating offshore structures. Fatigue life is often a limiting factor. With life extension of existing offshore installations, the fatigue capacity and effects of corrosion become even more important, as there will be large costs for mooring line replacements if safe life extension can not be granted, and the effect of fatigue failure can be fatal. Estimation of the fatigue capacity of mooring chains is thus of high importance both for safe and cost-effective design of new mooring systems, and for the safe life extension of older mooring systems. The standards used for design of mooring systems outline a somewhat simplified approach for fatigue analysis, where load cycle range is the only parameter included in the analysis. The fatigue capacity curves used are based on full scale fatigue tests of new chains, where effects of heavily corroded surfaces are not considered. Further it is indirectly assumed that mean load does not have any effect on fatigue capacity. Work presented the last years has indicated a strong effect of both mean load and surface condition, where also formulas for fatigue capacity including these parameters have been developed and presented. The conclusions are based on a large set of full-scale fatigue tests of both new chains and used chains, where the used chains are tested at different mean loads and different levels of corrosion. Equinor has run a large number of used chain fatigue tests. For these tests, each set of tests is typically made from one chain length, with similar condition on all links, and usually run at one mean load only. There are test sets with some variation in either mean load or surface condition, which have added valuable data for the understanding and verification of the effect of these parameters. The effects are well documented, but due to small variation within each set there are uncertainties regarding the quantification of the effects. The latest full-scale fatigue test results, from a chain with significant corrosion pits, include a systematic approach to quantify the effect of mean load. For the chain tested, five tests have been run at low mean load, and five tests at high mean load. This paper presents the results from these fatigue tests. The results are discussed and compared with other fatigue test results on both new and used chain, and with the formulas for fatigue capacity accounting for mean load and surface corrosion.