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Results from full scale fatigue tests of offshore mooring chains performed in recent years have revealed considerable influence of both mean load and corrosion condition on the fatigue capacity. It has been shown that a reduction of the mean load gives an increase in fatigue life, whereas the corrosion experienced by used chains have a significant negative impact. Neither of these effects are properly addressed by current S-N design curves or design practice. This paper suggests an extended S-N curve formulation, that includes the effects of mean load and corrosion condition. The parameters of the extended formulation are estimated empirically from mooring chain test data that includes new and used chains, with various mean loads and with different degrees of corrosion. The fitted capacity model is then used for fatigue calculation for the mooring system of a semi-submersible, showing the importance of using realistic mean loads and mooring chain corrosion in fatigue assessments.

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No abstract

Studies published in recent years have documented a significant mean load effect on fatigue capacity for offshore mooring chain, and show that a reduction of the mean load gives an increase in fatigue life. However, current S-N design curves are based on fatigue tests performed at a mean load of 20% of minimum breaking load (MBL), which is well above the typical mean loads for most mooring systems. This paper investigates the mean loads experienced during fatigue damage accumulation for the mooring system of a typical production semi-submersible, operating in Norwegian Sea conditions. The study is based on numerical, time-domain simulations, using environmental conditions defined from a series of hindcast data. A parameterized S-N design curve suggested by Fernández et al. (2019), incorporating a Smith-Watson-Topper mean stress correction model, is applied for fatigue damage calculation and compared to results for the S-N design curve prescribed by current standards. For the semi-submersible unit considered there is negligible difference in basing the correction on 3-hour mean load compared to the mean load of individual stress cycles, due to small low frequency tension variations. On this basis, a single correction factor is proposed to allow for mean load correction based on results available from a standard fatigue analysis.