Hydrodynamic Modelling Effects on Fatigue Calculations for Monopile Offshore Wind Turbines

Abstract

For shallow and intermediate water depths, monopile foundations are the most commonly used concept, and is considered the most promising with respect to the cost of energy. The typical highest structural eigenperiods of these structures are usually between 3-5 seconds, and often coincide with the wave frequencies. Estimates of the fatigue life is therefore very dependent on accurate hydrodynamic modeling of the wave forces. In these cases, the waves are often relatively short compared to the structure, and the often used Morison s Equation is no longer accurate, as it does not account for diffraction. In this thesis, a comparison between the standard Morison s equation and MacCamy-Fuchs diffraction theory has been performed. The responses and fatigue life of a 5 MW wind turbine was investigated, using both hydrodynamic theories. The analyses showed that the wind turbine experienced small differences as a result of the hydrodynamic modeling. Since wave diffraction affects small waves, the results were different at different depths. At the mean sea level, MacCamy-Fuchs resulted in approximately 3% lower fatigue damage. However, at the bottom, where only the large waves affect the structure, MacCamy-Fuchs resulted in approximately 3 % higher fatige. This suggests that the wind turbine in question is not very sensitive to diffraction. It was however seen that the fatigue damage was dominated by load cases with large aerodynamic thrust, suggesting that the structure is more sensitive to aerodynamic loads than hydrodynamic loads, as should be expected from a monopile structure at shallow water depth.