A new model for fatigue load sequence effects in offshore wind turbine substructures and its implications for design life

Measurements at Offshore Wind Turbine (OWT) substructures show that typical loading patterns include overloads, underloads, and shifts in mean stress cycles, due to the everchanging wind direction. These events follow each other in time and are further expressed as load sequences. Load sequences may affect the fatigue life. A novel model consisting of a set of explicit equations is developed to estimate the fatigue crack growth including effects of load sequences. The model is based on both experiments and finite element simulations. The model allows for (combinations of) over- and underloads and mean changes. With this model, an extrapolation is made towards realistic loading on OWT substructures to gain insight into the expected effect of load sequences on the fatigue life.