Influence of Mean Tension on Mooring Line Fatigue Life

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.

Fatigue Assessment of Mooring Chain Considering the Effects Of Mean Load and Corrosion

“…When only the mean load effect is included, the fatigue damage at the end of the service life is 36% of the reference value. This is slightly higher than reported in [13], which is consistent with the previous comparison to the SWT curve (Fig. 7).…”

“…Based on this transformation, they calculated S-N curve parameters at different mean load levels and established a S-N design curve for studless chain with parameterized dependence on mean load. Lone et al [13] used this capacity model to calculate mooring line fatigue damage for a typical production semi-submersible in the Norwegian Sea, and showed that the calculated fatigue damage was reduced by a factor of 2.5-3.0 depending on the orientation of the line. These factors should however be regarded as upper limits for fatigue damage reduction for the unit considered, as the negative effect of corrosion was not accounted for.…”

“…To demonstrate the impact on calculated fatigue life when both mean load and corrosion grade are accounted for, we will consider an example based on the case presented by Lone et al [13]. They calculated mooring line fatigue for a typical semisubmersible operating in the Norwegian Sea, with mean loads in the range from 10% to 15% MBL.…”