Design optimization with genetic algorithms: How does steel mass increase if offshore wind monopiles are designed for a longer service life?

Knowledge about the scaling of steel mass of monopiles is needed to decide for which service life an offshore wind farm should be planned. A computer-aided method to optimize monopiles for different fatigue lifetimes was developed. The optimization was performed with a genetic algorithm. Fatigue constraints were evaluated with aero-hydro-elastic load simulations in the time-domain. Importance sampling was applied to reduce the required number of load cases to 120 (only 7% of total amount of load cases). The optimization was tested for an 8 MW offshore wind turbine. Results prove that the developed method using importance sampling is suitable to gain fast and accurate optimization results. Only 5% more steel is needed to raise the fatigue lifetime from 25 to 35 years for a design without inspections. The increase of steel mass flattens out towards longer fatigue lifetimes since the structure becomes stiffer and less prone to wave excitation. This is valuable input to decide on the ideal service lifetime and maintenance strategies.