Idealise the structure by assuming that the substructure and tower are a single tube, of constant diameter and thickness, completely fixed at the mudline. Consider two different sections, one of diameter 4.0 m and wall thickness 31 mm and the other of diameter 5.5 m and wall thickness 25 mm. Are either, or both, of these sections appropriate for the wind turbine tower, i.e. is the natural frequency in a safe range? (You can assume Young's modulus is 210 GPa, density of steel is 7850 kg/m3)