Mooring systems for floating production systems are currently designed on the basis of individual components (lines and anchors). A spar design originally developed by an industry consortium was chosen as the study spar. The mooring system was designed for three different water depths: 3,000, 6,000 and 10,000 feet. A numerical model was employed to compute global motions and tensions in the mooring lines given met-ocean conditions. Component and system reliability analyses were conducted using representative probabilistic descriptions of the extreme met-ocean conditions (hurricanes and loop currents) in the Gulf of Mexico.
Based on a detailed analysis of component and system reliability for spar mooring systems in water depths from 3,000 to 10,000 feet, the following major conclusions have been drawn:
- Existing design guidelines provide for levels of system and component reliability against extreme loading that are above typical target levels that have been proposed by industry.
- Levels of reliability between mooring lines and anchors are not necessarily consistent; anchors have failure probabilities that are more than an order of magnitude smaller than those for lines under extreme loading.
- Mooring systems exhibit redundancy in that failure of the most heavily-loaded component during an extreme event does not necessarily lead to failure of the system. The redundancy is greater for the taut versus semi-taut systems and is greater during loop current events versus hurricane events.
- The reliability for the taut systems is higher than that for the semi-taut system due to the relatively small contribution of environmental loading versus pre-tension for the taut systems.
- The reliability for a design that is governed by loop current events is greater than one that is governed by hurricane events due to smaller uncertainty in the environmental loading conditions during loop currents compared to hurricanes.
Although this project is based on the analysis of the mooring system for a specific spar design and design environment, the procedures for reliability analyses produced in this project and the major conclusions are intended to provide general tools and guidance that will be useful for the design of mooring systems for floating production systems in deep water.
