Dynamic Response and Fatigue of Offshore Wind Turbines - Effect of Foundation Type and Modelling Method Using Software FAST
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This thesis presents dynamic response and fatigue analyses of several bottom-mounted offshore wind turbine models, simulated in the aero-hydro-servo-elastic simulation tool FAST v8. A 5 MW offshore wind turbine supported on a monopile was used as a reference model, and the effect of different foundation modelling methods, concepts and dimensions was studied. Default modelling of the wind turbine foundation in FAST is by means of a rigid connection to the seabed, implying that foundation flexibility and soil-structure interaction are not considered. The first objectiveof this thesis was to study the effect of including foundation flexibility in the FAST model. The soil-foundation stiffness was represented by adding a fictive beam below the mudline, a procedure called the apparent fixity method. A simplified approach was implemented first, followed by the development of a more exact approach. The second objective was to perform sensitivity analyses of both monopile foundations and suction caisson foundations with different dimensions, with respect to dynamic response and fatigue damage. It was concluded that foundation modelling is an important topic in offshore wind turbine research and development. Neglecting the flexibility of the foundation lead to underpredictions of structural dynamic response and fatigue damage, while modelling the foundation with the simplifications made in the first method lead to overpredictions. Furthermore, reducing the monopile subsoil length had no significant effect on the FAST output and the fatigue life, while reducing the monopile wall thickness lead to large increases in fatigue damage, and hence large reductions in fatigue life. Regarding both the suction caisson foundations and the monopile foundations, it was concluded that further analyses with respect to soil capacity and buckling would have to be performed to make valid conclusions on required dimensions.