Reliability of offshore mooring systems has been an issue in the past years. For both mobile and permanent moorings, it is believed that the reliability can be improved. This paper assessed the factors of safety (FOS) prescribed by industry standards to see how they perform among different mooring patterns. The common FOS in most codes/standards and Class Rules are 1.67 and 1.25 for intact and one-line damaged condition, respectively. The basis of these two prescribed numbers is arguably empirical or semi-empirical, and they correspond to 60% and 80% of the minimum break strength (MBS) of a mooring line. This paper assesses how these two numbers may be adjusted through a reliability analysis so that a more consistent reliability level can be achieved among different mooring design patterns. Four mooring patterns of increasing number of lines and reducing size are analyzed. The probability of system failure is calculated for each to determine if there is a trend. It is found that the FOS pair of 1.67 and 1.25 exhibits an inconsistency in the level of reliability achieved among the four different patterns. The system with the highest number of small lines achieves a higher reliability level. The result suggests that a more consistent level of reliability among mooring patterns may be achieved by increasing the intact FOS and/or reducing the redundancy FOS.

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