Mooring line fatigue

What you can ask it to do

• Estimate fatigue life of a chain or wire mooring line from its tension-cycle history.
• Apply T-N / S-N data for the component and report the damage and the safety factor.
• Flag the governing line and where life falls short of the design service life.

Why it matters

A station-keeping engineer on a floating system — FPSO, FPS or semi — has to show that every mooring line survives its design service life with the code safety factor. The tension histories come out of time-domain analysis across many sea states, and the fatigue bookkeeping then runs per component: chain, wire and connector each have their own T-N data, and the line that governs is rarely obvious by inspection. A standards-traceable screening check turns a pile of tension cycles into a damage number, an implied life and a clearly identified governing segment.

The method — governing relationships

API RP 2SK and DNV-OS-E301 accumulate fatigue from the tension-range spectrum of each line against component T-N (tension–cycles-to-failure) data using Miner's rule:

where a_D and m are the component-specific T-N intercept and slope (chain, wire rope or connector), RBS the reference breaking strength, T_service the design service life and FDF the fatigue design factor set by the code according to component type and inspectability. The governing line or segment is the one with the highest accumulated damage.

Symbols and notation follow API RP 2SK / DNV-OS-E301. No numeric example is shown here on purpose — the calculator and Deckhand compute the damage, life and safety factor deterministically from your tension input and T-N data, so every figure is reproducible and traceable.

What a screening check covers — and what it does not

The screening assumes the tension input represents the design environment; lines flagged near a limit warrant a full code fatigue analysis with the upstream model.

Questions