MS10.2 - Dynamics of Wind Energy Systems

The rapid growth of the wind industry sector has motivated increasing sizes of wind turbines, with corresponding natural frequencies becoming smaller, and eventually shifted into the dominant portions of the wind and wave loading spectra. As a result of this effect and the increasing regularity of extreme weather events, the fatigue assessment of offshore wind turbines has grown in importance. A key parameter in determining the fatigue life of offshore wind turbines is the stiffness of the foundation. However, high uncertainty is associated with the soil properties, which results in inaccurate estimation of the fatigue life. In order to update these estimates from information from the structure as is, measured data from tower sensors (accelerometers, strain gauges, etc.) can be exploited to update the foundation parameters of offshore wind turbine models. Some challenges however arise when attempting to do so, including the implied identifiability of different parametrizations of the foundation parameters, e.g. whether the stiffness matrix of the foundation should be represented using the stiffness or compliance parameters. The practical unidentifiability of parameters may lead to highly diverging optimal parameter estimates under use of different tools. Combined with the associated uncertainty of the soil properties, this can severely throw off accumulated damage estimates related to remaining useful life. In this work, the NREL 15 MW reference wind turbine has been modelled via use of the Simscape environment. The developed Simscape model has the advantage of being able to separately model the different components of a wind turbine, resulting in a coupled servo-hydro-aero-elastic model of a fixed-bottom offshore wind turbine. The foundation is modelled using linear coupled springs, which comprises translational, rotational, and cross-coupling springs in a series configuration. An additional advantage of the Simscape model is that the representation of the tower uses a greater number of degrees of freedom in comparison to industry standards such as OpenFast. The model is used to generate synthetic data to match those corresponding to an instrumented offshore wind turbine, assuming deployment of accelerometers at four different levels near the nacelle. An operational modal analysis (OMA) method, SSI-cov, is used on noise corrupted data to obtain estimates for the modal parameters (natural frequencies and mode shapes) of the wind turbine. These identified modal parameters are then contrasted to model estimated modal parameters, within a Bayesian model updating approach, to determine not only the optimal estimates of the stiffnesses in the coupled spring foundation but also their associated uncertainties. The results show that the maximum a posteriori (MAP) estimates for all three parameters are close to their true values and that the translational spring stiffness is the most identifiable, whilst the rotational stiffness is characterized by the highest uncertainty. Discussions are produced regarding the parametrizations of the foundation stiffness matrix and the consequences of this choice to the identifiability of the related parameters.

Abstract: The floating offshore wind turbines (FOWTs) have been extensively studied and are being and will be installed more in the world to generate more clean energy to reduce the trend of global warming. The construction and maintenance of wind farms require a large number of ships, and some wind farms are built near waterways. Hence, the FOWTs are often exposed to the threats from collisions by visiting and passing ships. Although the previous literatures have investigated dynamic response and damage of FOWT under the ship collision, they focused on global motions and local deformations with the simplified models or loads. In the present work, a new co-simulation technology using MATLAB and LS-DYNA was developed to consider the aero-hydro-servo-elastic-ship collision coupling analysis of FOWTs. The wind load, wave load and controller are established in MATLAB and then transferred to LS-DYNA by the functional mock-up interface (FMI), and the displacement, velocity and acceleration of high-fidelity finite element Model in LS-DYNA are transferred to MATLAB through the same interface. The proposed model in the present work can capture the responses of FOWTs well comparing with FAST’s results in terms of different cases. Finally, the dynamic responses and damages of a NREL 5 MW spar-type floating offshore wind turbine impacted by a 3500-ton vessel were investigated, considering different impact velocities, wind and wave loads.

Modern wind turbines towers have a circular cross-section (commonly tubular and made of steel), which is needed to have the same behaviour in all wind directions. This entails that it has similar natural frequencies in the wind direction and in the perpendicular direction, the difference being due to the offset of the rotor blades with respect to the vertical axis of the tower, since they are at the end of the nacelle. Thus, there is the possibility to have an internal resonance between along-wind and across-wind oscillations, that can interact when nonlinearities are taken into account at the structural level, a fact that is a consequence of the slenderness of the towers. Thus, it is possible that the wind force acting on the tower produces an oscillation in the perpendicular direction, similarly to what happens for vortex induced vibration, but due to a completely different reason. This investigation is aimed at investigating this phenomenon by means of a simplified nonlinear model, by obtaining an analytical solution allowing to have a full understanding and a rigorous treatment of the problem. As a by-product, this gives the possibility to control the phenomenon by properly modifying the parameters of the structure. This work represents a first attempt to describe the internal resonance problem in wind turbine towers, and will be developed in the future by considering more sophisticated mechanical models, that however will require numerical simulations and thus will not be very performant for parametric analyses.