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

Abstract

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 100yr 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 100yr 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.

Mooring Lines Reliability Analysis Case Study: Fso Abherka,

“…There are a number of previous studies related to reliability analysis on various types of mooring lines, such as FPO [6] , fish cages [7][8][9] , vessels [10,11] , deep water structures [12,13] , and semi-submersible system [14] . Montes-Iturrizaga et al [6] analyzed the reliability of moorings under uncertainty condition of sea states using a nested model.…”

“…Furthermore, it also suggest that the correlation of corrosion model for mooring chains has a remarkable impact on the failure probability of mooring system. Stanisic et al [11] models a reliability analysis of mooring lines on floating liquefied natural gas (FLNG) vessels. It evaluates the failure probability of mooring chain and pile.…”

Effect of mooring line layout on the loads of ship-shaped offshore installations

“…Here, we use Winterstein's model (Ding and Xinzhong, 2016;Hao and Yang, 2020) (Hao and Yang, 2020;Peng et al, 2020). Other approaches for estimating the short-term extreme of a stochastic process, and in particular for mooring line extreme tension, comprise the Peak Over Threshold Method (POT) (Zhao and Dong, 2020), the already mentioned PDM (Stanisic et al, 2019(Stanisic et al, , 2018, and the global maxima approach based on a Gumbel distribution (DNV GL, 2020) [also discussed by Zhao and Dong (2020)].…”