Reliability analysis of mooring lines using copulas to model statistical dependence of environmental variables

• Copulas model dependence between significant wave height and peak spectral period.
• Degree of dependence characterized using Kendall’s τ as measure of association.
• Influence of dependence on reliability of mooring lines studied.
• Study and parametric analysis of mooring lines of an FPSO model reported.
• Dependence of environmental variables has a great effect on reliability of moorings.

The influence of the statistical dependence between significant wave height and peak period on the reliability assessment of mooring lines is examined. Provided that copulas are functions that fully define the joint probability distribution of a set of random variables, they are used to describe the dependence structure of the environmental variables. Results of a case study and parametric analysis of mooring lines of an FPSO are presented and discussed. Copula models, such as Frank, Gumbel and Gaussian, fitted to hindcast data of extreme sea states at a site in the Gulf of Mexico are used. They are estimated in terms of Kendall’s τ measure of association between the environmental variables. Use of the Gaussian copula is equivalent to the widely known and practical approach of assembling joint distributions using Nataf distribution where the dependence structure isparameterized with the linear correlation matrix. Marginal distributions of extreme value data of significant wave height and peak period for the site were employed. The results showed that the dependence structure has considerable effect on the computation of the reliability index. The differences in the estimates of reliability using the Gaussian copula compared to the other copulas can be significantly large. The influence of the uncertainty in significant wave height, and of the mean and uncertainty of the breaking strength is also analyzed.