Dynamic design considerations for offshore wind turbine jackets supported on multiple foundations

摘要

To support large wind turbines in deeper waters (30-60 m) jacket structures are currently being considered. As offshore wind turbines (OWT's) are effectively a slender tower carrying a heavy rotating mass subjected to cyclic/dynamic loads, dynamic performance plays an important role in the overall design of the system. Dynamic performance dictates at least two limit states: the Fatigue Limit State (FLS) and the overall deformation in the Serviceability Limit State (SLS). It has been observed through scaled model tests that the first eigen frequency of vibration for OWTs supported on multiple shallow foundations (such as jackets on 3 or 4 suction caissons) corresponds to low frequency rocking modes of vibration. In the absence of adequate damping, if the forcing frequency of the rotor (so called 1P) is in close proximity to the natural frequency of the system, resonance may occur affecting the fatigue design life. A similar phenomenon commonly known as "ground resonance" is widely observed in helicopters (without dampers) where the rotor frequency can be very close to the overall frequency causing the helicopter to a possible collapse. This paper suggests that designers need to optimise the configuration of the jacket and choose the vertical stiffness of the foundation such that rocking modes of vibration are prevented. It is advisable to steer the jacket solution towards a sway-bending mode as the first mode of vibration. Analytical solutions are developed to predict the eigen frequencies of jacket supported offshore wind turbines and validated using the finite element method. Effectively, two parameters govern the rocking frequency of a jacket: (a) the ratio of the super-structure stiffness (essentially the lateral stiffness of the tower and the jacket) to the vertical stiffness of the foundation; (b) the aspect ratio (the height to width ratio) of the jacket. A practical example considering a jacket supporting a 5 MW turbine is considered to demonstrate the calculation procedure which can allow designers to choose a foundation. It is anticipated that the results will have an impact in choosing foundations for jackets.