The primary objective of a mooring system is to maintain vessel position, protect the risers, and prevent collision with nearby infrastructure. So at what point is a mooring system considered to have failed: single line failure, two line failure, or at the point of collision or riser failure?
Most mooring systems are designed to allow failure of one line. When one mooring line fails, the overall stationkeeping ability of the system, as a whole, should still exceed maximum environmental loads; hence conventional understanding that the failure of two lines is considered a mooring failure.
However, the fact that mooring systems are designed with one line redundancy creates false confidence in the remaining system capacity in the event of a single line failure. The root cause of single line failures are rarely isolated events – corrosion, fatigue and other type of degradation affect multiple mooring lines. When one line fails prematurely can an operator be confident that the remaining, similarly degraded, mooring lines are still able to sustain normal operating loads along with the added load of the failed line?
See what technical authorities at Chevron, SOFEC, BP, Total, and Shell have to say about mooring failures: