Sensitivity Analysis of Modal Parameters of a Jacket Offshore Wind Turbine to Operational Conditions

Abstract

Accurate estimation of offshore wind turbine (OWT) modal parameters has a prominent effect on the design loads, lifetime prediction, and dynamic response of the system. Modal parameters can vary during the operation of OWTs. This paper studies the variation and sensitivity analysis of an OWT’s modal parameters with respect to operational and environmental conditions. Three finite element models of a jacket-supported OWT at the Block Island Wind Farm are created within the OpenSees, SAP2000, and OpenFAST platforms and validated using experimental measurements. The OpenFAST model is used to simulate the modal parameters of the turbine under various wind speed, rotor speed, power, yaw angle, mean sea level, blade pitch angle, and soil spring values. The model-predicted modal parameters of the first fore–aft (FA) and side–side (SS) modes are compared to those identified from experimental measurements. Results from the simulations show that the first FA natural frequency and damping ratio mostly depend on the rotor speed and wind speed, respectively, while yaw angle and mean sea level do not have a visible effect. It is observed that there is about 8% stiffening in the first FA frequency and an aerodynamic damping of 7.5% during the operation of the OWT.

Fatigue Analysis of a Jacket-Supported Offshore Wind Turbine at Block Island Wind Farm

“…Jacket fatigue is evaluated based on the stresses obtained from a SAP2000 FE model of the support structure for Turbine B2. This model is verified with the experimental data in the authors' previous studies [9,12]. As shown in (17), the thrust force is calculated from the bending moments at the instrumented section and applied to the FE model.…”

“…The SG-based correction values can be solved using (4), ( 5), (8), and (9). The value of key parameters can be calculated as follows.…”

“…The less frequent pitch angle of 0-40° also brings high damage indices. These pitch angles are related to Region 3 of the power curve, with wind speeds between the rated (11.5 m/s) and the cut-out (25 m/s) wind speeds [9]. The SCADA data shows that the blade pitch was 23.8 • , an average over the 10-min window of data, whereas the blades' pitch angles were 11.9 • and 35.7 • before and after this event, respectively.…”

“…The fatigue demand on the foundation has been studied using a full year of data. FE models of the B2 turbine have been built in SAP2000, OpenSees, and OpenFAST tools, and verified in the authors' previous studies [9,12]. Strain at several jacket joints is estimated using virtual sensing based on the FE model of the turbine.…”

“…The process of model updating or the integration of digital twinning is to update the digital twin based on the measured response of the structure, in this case, an OWT that is continuously monitored by sensors. The authors have experience in system identification and digital twinning of the 6 MW Block Island Wind Farm OWT [9][10][11][12][13] and an operational 6 MW monopile [14,15]. They also completed the Block Island Structural Monitoring Project for the U.S. Bureau of Safety and Environmental Enforcement [16].…”