## 3 Experimental Setup
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Item** & **5 MW NREL** & **1/35** & **AAUE-TLP** \\ \hline Nacelle mass (kg) & 24,000 & 5.60 & 10.2 \\ Hub mass (kg) & 56,780 & 1.32 & 2.02 \\ Blade mass (kg) & 17,740 & 0.41 & 0.76 \\ Total Top mass (kg) & 350,000 & 8.16 & 12.0 \\ Hub diameter (m) & 3 & 0.09 & 0.13 \\ Blade length (m) & 61.5 & 1.76 & 0.99 \\ Cut-in/Rated/Cut-out wind speed (m/s) & 3/11.5/25 & 0.5/1.9/4.2 & 3/7/11 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Nacelle and rotor properties. Figure 4: Offshore laboratory at AAU Esbjerg where a wave tank with a 1:35 scaled model of a 5 MW wind turbine on a TLP is installed and wind generator wind a wind filter in front of it. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Item** & **5 MW NREL** & **1/35** & **AAUE-TLP** \\ \hline Hub inertia (kg m\({}^{2}\)) & 115,926 & 2.2 \(\times\) 10\({}^{-3}\) & 4.0 \(\times\) 10\({}^{-3}\) \\ Blade inertia (kg m\({}^{2}\)) & 11,776,046 & 0.22 & 0.22 \\ Generator inertia (kg m\({}^{2}\)) & 5,025,500 & 0.10 & 8.4 \(\times\) 10\({}^{-3}\) \\ Equivalent inertia (kg m\({}^{2}\)) & 40,469,564 & 0.77 & 0.66 \\ Gearbox ratio (-) & 1:97 & 1:97 & 1:28.9 \\ Gearbox effciency (\%) & 100 & 100 & 75 \\ Viscous friction (Nms) & 6,215,000 & 0.70 & 0.43 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: TLP properties. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Item** & **MIT-TLP** & **1/35** & **AAUE-TLP** \\ \hline No. of tethers lines per spoke & 2 & 2 & 1 \\ Young’s modulus (GPa) & 118.4 & 3.38 & 57.30 \\ Diameter (m) & 0.127 & 3.6 \(\times\) 10\({}^{-3}\) & 1.0 \(\times\) 10\({}^{-3}\) \\ Unstretched length (m) & 151.73 & 4.33 & 1.05 \\ Spring stiffness (kN/m) & 19,700 & 16.08 & 34.11 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Tethers properties. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Item** & **5 MW NREL** & **1/35** & **AAUE-TLP** \\ \hline Height (m) & 90.0 & 2.57 & 2.11 \\ Base length (m) & 10 & 0.29 & 0.30 \\ Tower mass (kg) & 347,460 & 8.10 & 4.74 \\ Diameter (t/b) (m) & 3.87/6.0 & 0.11/0.17 & 0.09/0.09 \\ Length (m) & 77.6 & 2.22 & 1.81 \\ Thickness (t/b) (mm) & 19/27 & 0.54/0.77 & 6.70/6.70 \\ Young’s modulus (GPa) & 210 & 6.0 & 3.15 \\ Shear modulus (GPa) & 80.8 & 2.31 & 1 \\ Density (kg/m\({}^{3}\)) & 8500 & 8500 & 1503 \\ Struct.-damp. ratio (\%) & 1.0 & 1.0 & 2.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Tower Properties. On the campbell diagram in Figure 5 the white area represents the operation range of the rotor speed of the AAUE-TLP, while grey areas represent the rotor speed outside the allowed operational range. The periodic excitation from the revolutions of the rotor is indicated by the solid blue lines. These indicate the frequencies associated with the one per revolution (1P) associated with the rotation of the rotor and the three per revolution (3P), six per revolution (6P) and so forth. Recommendations from DNV in [42] is to keep the tower frequencies outside ranges defined as the rotor frequency \(\pm\) 10% and the blade passing frequencies \(\pm\) 10%. This recommendation is to avoid load amplifications arising from vibrations near the natural frequency. This is illustrated in the Campbell diagram by the shaded dark shaded area along the solid blue line. The frequency range of the scaled JONSWAP wave spectrum is visualized by the blue shaded area. From the Campbell diagram it can be seen that the first natural frequency of the tower in fore-aft (F-A) and side-side (S-S) is placed between the 1P and 3P frequencies in the operational range of the FOWT. The tower can therefore be classified as a soft-stiff structure, which is also the case for the 5 MW NREL reference wind turbine [38]. The blades natural frequency is found from experiments conducted at rotor standstill, however, it is well known that the natural frequencies of the blades are rotor speed dependent [43], this is not taken into account in this paper. From the Campbell diagram it can be seen that the 1P loading does not coincide with any natural frequency of the FOWT.