Recently published studies have documented a significant mean load effect on fatigue capacity for offshore mooring chains, showing that a reduction of mean loads gives an increase in fatigue life. Standard design practice has been to base S-N fatigue design curves on tests carried out at 20% of the chain minimum breaking load (MBL). These curves are then used to compute damage for all tension cycles, regardless of their actual mean value.

This paper investigates the mean load dependency in mooring chains for the INO WINDMOOR 12 MW floating offshore wind turbine by describing the mean load tension associated with each stress cycle in two different ways: The average of the maximum and minimum load in each cycle (cycle mean) and half-hour mean load. A parametrized S-N curve design approach developed by Fernández et al. was used to account for the mean tension.

With both methods for accounting for mean stress, the mean tension was below 20% MBL, for the least extreme load conditions. More extreme load cases caused half-hour means above 20% of MBL, and single cycle mean tensions were far beyond the 20% limit, and contributed with large fatigue damage. This caused some difference between the methods.

Compared to the DNVGL-OS-E301 S-N curve, a significant reduction in fatigue damage was seen for the mooring lines least affected by the extreme conditions (leeward). However, mean tensions beyond 20% of MBL in the windward line contributed to larger fatigue damage in extreme cases, and ended up with a total accumulated damage similar to the design curve base case. The validity range of the method was 7%–20%, and many observations were outside of this range.

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