A SIMPLIFIED APPROACH TO WAVE LOADING FOR FATIGUE DAMAGE ANALYSIS OF MONOPILES

摘要

The monopile is the dominating substructure concept used for offshore wind turbines. Offshore wind farms have so far been built at a depth of up to 24 m. As the importance of wave loads increase when deeper waters are considered for wind farms, it becomes increasingly important to correctly and efficiently include wave loading in structural analysis. Also, for deeper waters monopiles are expected to gradually become less economical compared to alternative substructure concepts, and wave loading is important to rate different alternatives. Although monopiles is the focus of this paper, the methodology presented is largely relevant to alternative substructure concepts.A rational and efficient methodology is suggested to construct a scatter diagram based on growth curves, wind speed, fetch, and the duration of winds. First, a scatter diagram for wind speed is generated based on the 1-year and 50-year wind speeds. The wave scatter diagram is then constructed using growth curves and considering the average duration of winds compared to the length of the wave growth phase.Wave loading is discussed in the context of the wave climate found most relevant for fatigue loading. Drag effects are ignored due to low Keuligan-Carpenter (KC) number. For short wave periods, diffraction effects should be considered, but they are nevertheless ignored as very little of the fatigue damage occur for these periods. An analytic expression for the mudline moment amplitude is used to find the stress range for a regular wave.Fatigue damage and fatigue life is calculated using the SN-approach. The results indicate that wave loading can be very important for fatigue damage in an analysis also including wind loading and dynamic effects.