Conclusions
The aim of this study was to find the fatigue damage accumulated in the offshore wind turbine (OWT) monopile structure under service loading conditions. A mixed (isotropic and kinematic) hardening approach was used to model the non-linear cyclic response of S355 G10+M material. The findings can be summarised as follows:
(1) Strain controlled cyclic load testing of S355 showed the cyclic softening behaviour with a 10% drop in stress level in the stabilised hysteresis loop for all the tested strain amplitudes.
(2) Realistic service loads were calculated by using the wind speed and wave height data recorded from an offshore wind farm in the North-sea using SCADA and wave buoy.
(3) A non-linear constitutive equation was proposed to model the cyclic deformation behaviour of S355 for large strain applications (upto 3% strain). However, the present study showed that the peak stress and strain values in the structure were within the elastic regime.
(4) The cumulative fatigue damage calculated for the OWT monopile was 7.13% after 20 years of operation under the realistic service loads. This indicates that the structural design can be optimised for reducing the material cost, thereby making wind power generation more economic.