The methods of structural reliability, taking uncertainties related to both loading and ultimate strength into account, have been applied to mooring system design. This paper outlines the philosophy and a general methodology for a reliability-based design, followed by an extensive case example of a semisubmersible, moored by an eight-point chain leg system in 350 m water depth. The failure mode considered is failure due to tensile overload in a single line. The calculated annual probability of failure and results from a number of sensitivity analyses are presented. It is demonstrated that the mean and the variability of the chain link strength and the viscous drift forces have large impact on the calculated failure probability. Both low-frequency and wave-frequency loads are found to be important for the reliability of the system. Minor sensitivity is found to the modelling of tension variation along the length of the line, and to the type of wind gust spectrum applied.
The purpose of this paper is to outline the philosophy and a general methodology for reliability-based design of mooring systems against tensile overload, and apply the theory to a case example of a drilling semisubmersible, operating in 350 m water depth. The outlined methodology is primarily applicable to permanent moorings, and the use of a typical drilling semi in the case examples should therefore be considered as an example of a small permanently moored semi. Mooring system failures occur from time to time. For an eight-line chain mooring system, an annual line failure rate of 0.33 for chain manufactured before 1985, and 0.08 for chain manufactured after 1985, are reported by Andreassen (1993) based on empirical data.
