Mooring system reliability in tropical cyclone and North Sea winter storm environments

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3 Citations (Scopus)


The characteristics of waves, winds and currents in a tropical cyclone environment differ significantly from those in a winter storm environment, like the North Sea. This can have a significant effect on the reliability of a mooring system that is designed to satisfy 100 yr conditions with specified Factors of Safety in accordance with ISO19901-7 or API RP 2SK. This paper presents reliability analysis of the mooring system of a permanently connected Floating LNG vessel, placed at two locations: (a)a tropical cyclone environment of the North West Shelf of Australia and (b)a winter storm environment of the North Sea. It is demonstrated that as a result of differences in the long term distribution of environmental parameters (waves, winds)between a North Sea environment and a tropical cyclone environment, the long term distribution of the mooring line response differs significantly in these two locations. This paper shows that a mooring system which is designed in accordance with ISO (or API), in these two environments, will achieve very different reliability levels because of the significant differences in environmental characteristics. In order to achieve the same reliability for the mooring system at these two geographical locations, Factors of Safety for use with 100 yr environmental conditions (Ultimate Limit State)were derived to achieve the same target probability of failure of 10 −4 /annum. It was found that for the North Sea environment, a factor of 1.5 is required for both the mooring chain and the pile, while for the tropical cyclone environment the required Factor of Safety has to be increased to 2.1. These differences are very significant and design standards need to be revised to reflect these findings.

Original languageEnglish
Pages (from-to)306-316
Number of pages11
JournalApplied Ocean Research
Publication statusPublished - 1 Jul 2019


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