Reliability assessment of mooring lines for floating structures considering statistical parameter uncertainties

• Uncertain parameters in statistical model of dynamic loading considered.
• Uncertain parameter in statistical model of line resistance considered.
• First-order formulation for assessing uncertain reliability index of mooring lines.
• Uncertainty of reliability index of catenary and taut-leg mooring lines evaluated.
• Predictive reliability and predictive failure probability assessed.

This work focuses on the effect of parameter uncertainty on the reliability index and on the predictive reliability of mooring lines for floating structures under loading from extreme sea-states. A first-order analytic formulation is developed which takes into consideration uncertain parameters in the statistical models of the maximum dynamic tension and the breaking resistance of the mooring lines. Expressions are derived for the mean and the variance of the reliability index in terms of the mean values and the covariance matrix of the uncertain parameters, and of the sensitivity of the reliability index to the uncertain parameters. The predictive reliability index is expressed in terms of the mean and the variance of the reliability index. The formulation is applied to case studies of catenary and taut-leg mooring lines of an offshore structure and the relative effects of the sources of statistical uncertainty are assessed. The case studies demonstrated the applicability and capability of the formulation to capture and represent the relative contributions of the different sources of uncertainty on the predictive reliability and predictive failure probability, as well as on the statistics of the uncertain reliability index.