Mooring Line Fatigue Calculator

Single-stress-range fatigue screen for mooring chain using the DNV-OS-E301 T-N curve

DNV-OS-E301 T-N Curve Mooring Chain

Input Parameters

DNV-OS-E301 T-N curve class. Studlink has ~2x the fatigue capacity of studless.

Nominal chain bar diameter. Nominal area = 2 x (pi/4 x d^2) (two bars carry the load).

Representative peak-to-trough tension range for the screen.

Number of stress cycles applied per year at this tension range.

Target service life to check against.

Default 1.0. DNV-OS-E301 applies a Design Fatigue Factor (typically 3-10 depending on consequence/inspectability); enter a higher value to reflect it.

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Fatigue Life (years)
Standard Reference: DNV-OS-E301 "Position Mooring". T-N fatigue curve N = a_D · S-m. Studless chain: m = 3.0, a_D = 6.0e10. Studlink chain: m = 3.0, a_D = 1.2e11. (S in MPa.)
Single-stress-range screen: This tool applies ONE representative tension range to the whole cycle count. A real fatigue analysis sums Miner's-rule damage over a full rainflow-counted tension-range histogram from the mooring time-domain response. Treat the result as a first-pass screen, not a design value.
Disclaimer: This calculator is for educational and illustrative purposes only. Final engineering calculations should be performed by qualified engineers using validated software and verified against applicable codes and standards.

About This Calculator

Mooring lines on floating offshore units (FPSOs, semi-submersibles, spars, and floating wind platforms) accumulate fatigue damage from the cyclic tension induced by waves, low-frequency vessel motions, and the platform's mean offset. Chain links are a common fatigue-critical component, and DNV-OS-E301 "Position Mooring" provides a tension-range vs. cycles (T-N) curve to estimate their fatigue capacity.

Method

The tension range is first converted to a nominal stress range by dividing by the chain's load-bearing cross-section. A chain link is built from two parallel bars that together carry the load, so the nominal area is:

A = 2 × (π/4 × d²)

where d is the chain bar diameter. The nominal stress range is then S = ΔT / A (with ΔT in newtons and A in mm², S comes out in MPa).

The cycles to failure at that stress range come from the DNV-OS-E301 T-N curve:

N = aD · S−m

with curve parameters (per DNV-OS-E301): studless chain m = 3.0, aD = 6.0×1010; studlink chain m = 3.0, aD = 1.2×1011 (S expressed in MPa). The annual damage is the cycles applied per year divided by N, and the fatigue life is the reciprocal of the annual damage (divided by any safety factor on damage). A fatigue life at least equal to the design life passes the screen.

Limitations

  • Single representative tension range only — real analysis uses a rainflow histogram + Miner summation.
  • No mean-load (mean tension) correction or thickness/size effect.
  • The default safety factor is 1.0; DNV applies higher Design Fatigue Factors based on consequence and inspectability.
  • Corrosion, out-of-plane bending (OPB), and proof-load effects are not modelled.

Need Professional Mooring Fatigue Analysis?

For full time-domain mooring analysis, rainflow-counted fatigue with histograms, OPB/IPB assessment, and Design Fatigue Factor application, we offer professional consulting services with validated methodologies.

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