Position Specific Monopile Fatigue Load in the Frequency Domain

• Published in: Volume 2: Structures, Safety, and Reliability
• Main Authors: Dahl , Niels Christian, Thyssen Nørby Larsen, Richard, Høgsberg, Jan, Nielsen, Martin Bjerre
• Format: Article in Journal/Newspaper
• Language: English
• Published: The American Society of Mechanical Engineers (ASME) 2022
• Subjects: Arctic
• Online Access: https://orbit.dtu.dk/en/publications/babd590e-963e-4b77-950c-4800ef6c7c56; https://doi.org/10.1115/OMAE2022-79907

This article presents a method for frequency domain calculations of fatigue loads for offshore wind turbines from wave loading on the monopile foundation. The method is used to estimate position-specific loads to lower the inherent conservatism to optimise the monopile design compared to the conventional approach where a conservative load envelope is applied for the whole or selected parts of the wind farm. The loads are calculated by transforming a wave spectrum into a stress spectrum, used for frequency domain rainflow counting. The remaining fatigue load is then determined by comparing the wave-only fatigue loads to the loads from an integrated loads analysis including all aero-dynamics associated with the turbine. The remaining contributions to fatigue loading are demonstrated to be nearly constant between positions and can thus be used to calibrate the wave-only load, estimated by frequency domain analysis, which subsequently enables the calculation of position-specific loads. When applying the method on site specific data, load reductions up to 12.5% are obtained on individual positions, while a 1–2% saving is achieved on the overall monopile mass.