Background

This work has been developed in the framework of a PhD devoted to the numerical simulation of the real response of offshore floating wind turbines. Nowadays, there are already many works using operational modal analysis for the monitoring of civil engineering structures, but in the last years the need to test and monitor wind turbines brought new challenges. Due to the lack of experimental data available for floating wind turbines the procedures being used in other structures need to be tested with numerically simulated data, so a reliable computational model is needed. Before proceeding to the off-shore model, a first on-shore model regarding a wind turbine monitored in the context of the WindFarmSHM project was created and validated through multiple tests.

Experimental Data

The Supervisory Control and Data Acquisition (SCADA) operating in each turbine provides wind direction, mean wind velocity, wind turbulence, pitch and yaw angles and power generated every 10 minutes. In parallel, the wind turbine is being monitored through a series of extensometers located in strategic points near the tower base and in the blades roots providing a full description of the internal forces in both locations. Furthermore, a set of accelerometers permits the estimation of the modal properties for alternative operating conditions, and 3 bi-axial clinometers distributed in the foundation and tower permit the characterization of the foundation and tower stiffness.

Computational Model

The model was created with FAST, an open source software that integrates numerous modules to compute the response of wind turbines, that is being continuously developed by the National Renewable Energy Laboratory (NREL).

Knowing the geometric properties of the tower and blades, the structural and mechanical properties of the structure are usually of easy computation, but a correct definition of the blades aerodynamic properties needs a more careful treatment. The aerodynamic properties of different sections of the blades were computed with a 2D model created with the ANSYS Fluent module as well as through the XFOIL software.

Having the wind turbine fully characterized, the control mechanisms (torque and pitch control) need to be characterized. Their mathematical models were obtained from the knowledge of the SCADA system.

Results

Having the model fully characterized, numerous time series of wind excitation for different operation conditions were generated, and the structural response computed.

Both measured and simulated response allowed the identification and validation of structural dynamic properties and static and dynamic internal loads. The following quantities were compared:

• Natural frequencies
• Total trust acting on the rotor
• Bending moments at the tower base (originated from static and dynamic loads)

Conclusions

The results obtained from the numerical model were in good agreement with the ones measured in the field. This preliminary validation is very important to extrapolate the model to more demanding scenarios, as the ones experienced by floating wind turbines.