Reliability assessment of a mooring system

A simplified system reliability assessment of a 12-line mooring system of a semi-submersible is carried out. The main purpose of the paper is to indicate the inherent safety level of the Norwegian mooring line codes. The safety factors of the codes have remained more or less the same for 2-3 decades, while both chain quality and load calculation schemes have been significantly improved during this period. A simplified system reliability analysis is carried out by defining system failure as loss of all lines in one corner. The critical failure path of the system is identified. Failure probabilities are estimated for lines degraded after 10 years in operation. The gain in safety level of replacing the lines with new lines is demonstrated as well as the safety level achieved if lines are replaced with lines fulfilling the rule requirements. The paper includes a discussion of the consequences of utilising weather dependent line optimisation when designing mooring line systems. Furthermore, a brief discussion of a reasonable target probability regarding mooring line safety is included.