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

Abstract

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.

Fatigue reliability of mooring chains, including mean load and corrosion effects

“…In this section, we review the mean load and corrosion dependent fatigue capacity model presented in Lone et al (2021) and the probabilistic fatigue damage model proposed in Lone et al (2022) in the context of the present paper. We apply the usual convention of describing random variables by capital letters (e.g., 𝑋), small letters to describe a realization of a random variable (e.g., 𝑥), and bold symbols to denote vectors or matrices (e.g., 𝑿, 𝒙).…”

“…To account for the effect of mean load and corrosion on the fatigue capacity, Lone et al (2021) expressed the intercept parameter as function of these parameters:…”

“…where log(⋅) is the common logarithm, (𝐵 𝑗 ) 𝑗∈{0,1,2} are coefficients and 𝑔 1 (𝜎 𝑚 ) and 𝑔 2 (𝑐) are monotonically increasing functions of the mean stress 𝜎 𝑚 and a corrosion grade 𝑐, respectively. The corrosion grade applied here is based on a customized scale ranging from 1 (new chain or mild corrosion) to 7 (severe corrosion), see Lone et al (2021) for details.…”

“…Leira). chains have revealed that (i) the fatigue capacity of chains is strongly dependent on the mean load, and (ii) realistic corrosion pits have a detrimental effect not well represented by the simplified approach prescribed by the standards (Gabrielsen et al, 2019;Fernández et al, 2019;Zhang and Smedley, 2019;Ma et al, 2019;Lone et al, 2021). Hence, proper treatment of mean load effect and degradation due to corrosion in the calculations seems imperative to enable improved estimation of mooring line fatigue life.…”

“…Based on test results for new and used chain, tested at a range of mean load levels and with various degrees of corrosion, Lone et al (2021)…”