Quantifying the effect of low-frequency fatigue dynamics on offshore wind turbine foundations: a comparative study

Offshore wind turbine support structures are fatigue-driven designs subjected to a wide variety of cyclic loads from wind, waves, and turbine controls. While most wind turbine loads and metocean data are collected at short-term 10-minute intervals, some of the largest fatigue cycles have periods over one day. Therefore, these low-frequency fatigue dynamics (LFFD) are not fully considered when working with the industry-standard short-term window. To recover these LFFDs in the state-of-the-industry practices, the authors implemented a short-to-long-term factor applied to the accumulated short-term damages, while maintaining the ability to work with the 10-minute data. In the current work, we study the LFFD impact on the damage from the Fore-Aft and Side-Side bending moments and the sensors' strain measurements and their variability within and across wind farms. For an S-N curve slope of m=5, up to 65 % of damage is directly related to LFFD.