MS10.5- Dynamics of Wind Energy Systems

The interest in improving our understanding on the seismic response of the support structures of large Offshore Wind Turbines (OWTs) is gradually increasing due to the present commitment to the replacement of fossil fuels with renewable energies, and the consequent expansion of offshore wind farms in the world. In this line, this paper explores and compares the seismic responses of the support structures of three different reference OWT of increasing power and size: the NREL 5 MW RWT, the IEA Wind 10 MW RWT and the IEA Wind 15 MW RWT, all of them on monopiles. The seismic response of these OWTs is studied considering three different working conditions (parked, operating and producing electrical power, or during an emergency shutdown due to the arrival of an earthquake), and the structural response under the arrival of different earthquakes is compared to that of the structures submitted only to the different environmental loads. From the point of view of the structural model, the responses computed using a simplified fixed-base model are compared with those computed using a model in which the flexibility of the foundation is included using a simplified lumped parameter model (LPM) that allows to introduce the dynamic properties of the soil-foundation system and, especially, its capacity to dissipate energy back into the soil. In this regard, the needed impedance and kinematic interaction functions are computed using an advanced Boundary Element – Finite Element (BEM-FEM) model in which the pipe pile is modelled using FEM shell structural elements, and the unbounded layered soil is modelled using the BEM. Then, the structural response of the system is evaluated using OpenFAST, adapted to include the LPM to model the soil-foundation system at the base of the structure, and to include the possibility of considering the seismic signal as filtered Foundation Input Motions. The influence of aspects such as Soil-Structure Interaction, rated power, operating mode, direction of shaking or time of arrival of the seismic signals are summarized and discussed. This study was supported by the Consejería de Economía, Conocimiento y Empleo (Agencia Canaria de la Investigación, Innovación y Sociedad de la Información) of the Gobierno de Canarias and FEDER through research project ProID2020010025, and by the Ministerio de Ciencia e Innovación and the Agencia Estatal de Investigación of Spain, and FEDER, through research project PID2020-120102RBI00 (MCIN/AEI/10.13039/ 501100011033).

Wind energy has increased its importance among renewable energy sources in recent years due to its high energy production capacity and the return of investment cost in a short time and has become a better alternative to conventional energy sources. The share of relatively large wind turbines in the renewable energy market, which can produce more energy in a short time, is increasing rapidly. Being able to accurately and consistently define the dynamic characteris-tics of turbines is crucial to ensure that these structures remain operational throughout their economic life and even serve more than their planned lifespan. In this study a novel autono-mous data acquisition and system identification framework is developed. The subject of the work is an in-service 2.5 MW horizontal axis wind turbine with three rotor blades in Iz-mir/Turkey. Vibration, temperature and relative humidity data is recorded through data acqui-sition system designed distributed along the turbine tower height (80 m.). Data acquisition sys-tem contain three types of sensors at different levels: One tri-axial, three uni-axial accelerome-ters, temperature and relative humidity sensors at foundation level; two uni-axial accelerome-ters at 20-meter level; two uni-axial accelerometers and temperature sensor at 40-meter level; two uni-axial accelerometers at 60-meter level; two uni-axial accelerometers, temperature and relative humidity sensors at 80-meter level. Preprocessed vibration data is transmitted from wind farm to the university campus with operational and environmental data (e.g., wind speed, wind direction, temperature, humidity, rotor speed, nacelle direction, and pitch angle) collect-ing from turbine SCADA system synchronously. Modal parameters are estimated using p-LSCF method providing more clear stabilization chart than other operational modal analysis methods. A clear stabilization chart makes more easier to choose stable system poles that is significant to automate system identification. Whole data acquisition and system identification process is done without any user interference. A preliminary correlation work is done between the dynamic characteristics of the turbine and operational/environmental factors.

Floating wind turbines (FWT), which enable access to substantial wind resources in deep waters, have been anticipated to contribute significantly to achieving the carbon-neutral target. Popular simulation tools for this relatively new offshore technology adopt the linear potential flow theory borrowed from the offshore oil and gas industry to evaluate the hydrodynamic forces, which are calculated about the equilibrium position of the platform. However, the compliance of the floating platform can potentially lead to large displacements and rotations under combined wind and wave actions. In this case, the validity of the system’s description through its original linearization should be reinvestigated. The present work proposes a new piecewise linearization approach that can capture the nonlinearity by re-linearizing the wave-platform interaction system at instantaneous platform positions (operating points). An open-source boundary element method code, Nemoh, is utilized to calculate the hydrodynamic force for the linearized wave-platform system at each operating point. In addition, the blades-controller-platform coupling effect is investigated in this work by interfacing the wave-platform re-linearization scheme with a wind turbine model developed in Simulink with robust aerodynamics and controller simulation modules. The integrated model can be used to conduct a fast evaluation of the full-system nonlinear dynamics for operating FWTs. The results obtained by this method are compared versus the common practice of linearizing around the equilibrium and comparisons are drawn.

The exploration of the wind energy available offshore is of vital importance to overcome the energy dependence on fossil fuels and, depending on the deepness of different locations, this task may only be feasible if based on floating solutions. For onshore wind turbines, the fatigue damage experienced by the structure towers is significantly impacted by the wind conditions and the implemented control algorithms. In the case of floating offshore wind turbines (FOWT), the platform pitch and roll motions, induced by both wind and waves action, introduce additional sources of time varying stresses through two distinct mechanisms: by inducing low frequency wind action as perceived by the wind turbine and through the time varying rotor mass eccentricity. In order to properly extrapolate the fatigue damage, these effects must be taken into consideration. In this work, we have characterized and correlated the fatigue damage with different environmental conditions and measurable variables (e.g. accelerations) and discuss different methodologies to extrapolate the results to non-monitoring periods based on these findings.