Influence of Tower Design on Floating Offshore Wind Turbine Dynamics

As research and industrial interests around FOWTs has grown, an aspect that has recently emerged is the fact that, because a floating foundation is inherently more compliant than a fixed one, in many instances tower resonance frequencies are effectively shifted upwards. While onshore wind turbine towers are typically designed with a soft-stiff approach, increasing tower natural frequencies often pushes them into or near the 3P excitation range; this aspect is exacerbated by the fact that blade-pitch controllers are often de-tuned to avoid controller-induced floater-pitch instabilities. A prudent solution to this issue is to stiffen the tower, placing the first side-side and fore-aft natural frequencies above the 3P excitation, with a stiff-stiff approach. This may turn however into heavier towers, negatively influencing system stability but more importantly increasing tower base loads significantly. In this context, the current study aims at exploring the potential influence of careful tower design on FOWT dynamics and loads. The IEA 15MW RWT is used as testcase, since in its floating configuration it features a stiff-stiff tower. Four different lightweight towers are proposed. Base and top outer diameters are kept equal to 10 m and 6.5 m, whereas wall thickness and distributed structural properties vary with the height following similar patterns with respect to the fixed-bottom 15MW reference tower. The impact of these different design solutions is discussed in terms of performance, extreme and fatigue loading on the turbine and the floater. Results prove that improved and lighter tower designs may represent a key aspect to further improve FOWT design and operation.