The Effect of Mean Tension on Fatigue of Mooring Chain for a 12 MW Semi-Submersible Wind Turbine

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.