Performance study of the QuLAF pre-design model for a 10 MW floating wind turbine

Abstract

This paper presents a comparison study of the simplified model QuLAF (Quick Load Analysis of Floating wind turbines) and a FAST model of the Technical University of Denmark (DTU) 10 MW reference wind turbine mounted on the LIFES50+ OO-Star Wind Floater Semi 10 MW floating substructure. The purpose is to investigate how accurate results can be obtained from this simplified model for different load cases. The two models are briefly presented and the limitations of QuLAF are discussed. These are (a) an under-prediction of the wave excitation loads for large sea states; (b) a simplified representation of the rotor-induced forcing and damping; (c) an over-predicted aerodynamic damping for the tower mode motion and (d) restriction to planar motion. All the limitations are linked to approximations applied for achieving the substantial model speedup relative to the state-of-the-art model. The comparative study is based on the planar version of design load cases (DLCs) 1.2, 1.3, 1.6, 2.1 and 6.1, and the overall analysis shows that the simplified model is generally good at estimating the bending moment at the tower base and the floater motions in heave and pitch. The largest tower-base bending moments are slightly over-predicted, but it is observed that while stronger wind leads to an over-prediction, stronger waves lead to an under-prediction. Thus, in DLC 1.6, where the largest load was obtained at 10.3 m s−1, a good match in tower-base bending moments between the two models is found. The nacelle acceleration, however, is generally under-predicted, which is linked to an over-prediction of the aerodynamic damping on the tower mode. Furthermore, the floater response in large sea states is influenced by the omission of viscous hydrodynamic drag forcing, which leads to an under-prediction of the wave excitation loads. A further investigation of the model limitations confirms these findings with respect to the tower mode damping and viscous drag loads, while the simplified approach to rotor-induced loads is found to provide remarkably accurate forcing results. Although a full design load basis evaluation with a state-of-the-art model must be carried out for the final design, the present results show the potential of applying simplified models in the preliminary design phase.