Mooring systems are usually designed against ultimate and fatigue failure of individual lines as well as to survive a certain environmental condition with one line failed. Depending on the system configuration, failure in one mooring line could obviously increase the loads in the remaining lines, especially in the adjacent lines. Increasing extreme line tension and induced fatigue damage will also depend on the environmental conditions that the mooring system could experience after damage. The annual variation of sea wave conditions also leads to a significant variation of the predicted line tension. In this paper, annual extreme values of mooring line tension have been estimated both by full long-term analyses and by the contour line method for an intact mooring system and for a damaged one, respectively. On the average, an increase of 20% and 30% in extreme tension are obtained in the adjacent line due to a single line failure in a 16-line system and in a 12- line system, respectively. Long-term fatigue damage induced by line tension has also been summed up from the short-term contributions which are estimated by a bi-model fatigue formulation and an average increase of the annual damage by 50% and 90% are obtained due to failure for the 16-line and 12-line systems.
Mooring system of floating structures is usually designed against ULS (Ultimate Limit State), FLS (Fatigue Limit State) and ALS (Accidental Limit State) by using the LRFD (Load and Resistance Factor Design) method. Partial safety factors are calibrated by detailed structural reliability analyses and specified by many design codes, e.g. API RP 2SK (1997), DNV-OS-E301 (2004), ISO 19901-7 (2005), etc. Design checks both for ULS and for ALS are made by comparing line capacity and extreme line tension under a design environmental condition for an intact system and for a damaged system with one line failure, respectively.
