Summary In the current state of development, it is very hard to remotely evaluate the actual condition of wind turbines structures. This makes it impossible for a wind farm owner to correctly deliberate about the need of retrofitting the current wind turbine structure, as well as to decide about the possible extension of its operating lifetime. Taking into account this problem, this paper introduces the main aspects in the development of a vibration‐based monitoring system for an onshore 2.0‐MW wind turbine based on the identification of the modal properties of the most important vibration modes. Initially, the wind turbine is introduced, as well as the implemented monitoring system. Then, the main steps of the monitoring system are briefly described. A detailed attention is given to the statistical procedure based on regression models used to minimize the influence of operational and environmental effects over the features used to detect structural changes in the wind turbine. Lastly, the suitability of the system to detect damages is thus assessed, through the analysis of three common damage scenarios: onshore foundation damage; damage by scour on an offshore foundation; and blade damage. It is concluded that the system is capable of detecting damage in onshore and offshore foundations, as well as in rotor blades. The most important contribution of the paper is the definition of monitoring framework for damage detection, its implementation, and validation with monitoring data collected during more than 1 year in a utility‐scale wind turbine.

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Summary The analysis of the dynamic behavior of wind turbines is a complex task. These structures present diverse configurations, dependent on the wind conditions. Consequently, the dynamic behavior is highly dependent on their operating conditions. This paper analyzes the dynamic behavior of a 2.0 MW wind turbine during 1 year of operation. In that sense, post‐processing tools aiming to decompose the measured vibration response of structures into modal responses, originally developed to be applied with the SSI‐DATA algorithm, are implemented and applied. Initially, an original data processing that permits the application of these tools to two additional output‐only modal identification algorithms is introduced. Then, these tools are applied for the first time to data collected in a wind turbine during a period of 1 year. The obtained results evidenced the effectiveness of the proposed post‐processing procedure and the distinct dynamic response of the wind turbine according to its operating conditions.

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Abstract. The paper presents recent results from a research project based on a continuous dynamic monitoring of a wind turbine. The monitoring system was developed with the capacity to detect structural abnormalities based on the continuous tracking of the modal properties of wind turbines (natural frequencies, modal damping ratios and mode shapes INTRODUCTIONWind power exploitation has shown a consistent growth both in onshore [1] and offshore installations [2], with increasingly competitive costs. This growth has been reflected in the installation of wind turbines composed by very flexible support structures and with large rotors. Due to these characteristics, wind turbines became very susceptible to vibration problems and, consequently, to fatigue damage problems.This paper presents a continuous dynamic monitoring system, under development by the Laboratory of Vibrations and Structural Monitoring (ViBest, www.fe.up.pt/vibest) of FEUP, with the purpose of identifying structural changes (i.e. damage) and estimating the fatigue damage condition of a wind turbine. The system is based on the continuous tracking of modal properties of the structure (frequency values, modal damping ratios and mode shapes), identified during the different operating conditions of a wind turbine. The strategy adopted for the monitoring system has already proved to be suitable to be installed both in onshore and offshore wind turbines [3].The modal identification of large structures excited by operational conditions, an approach usually named as Operational Modal Analysis (OMA), is a technique widely used in several applications [4]. Some examples of implementation of monitoring systems based on OMA in wind turbines are presented in [5,6].The assessment of fatigue condition through acceleration data records was already investigated by some researchers [7,8]. However, in these works, the authors used the results from modal identification tests to tune a finite element model in order to estimate the stress condition of the structure. This paper presents some results already achieved with the developed dynamic monitoring. Initially, the results obtained with the continuous tracking of the modal properties of the wind turbine during one year are presented. In the second part, an improved methodology to estimate the stress time history of the tower structure is presented, where only the acceleration data, alongside with a simple stiffness matrix of the tower, is used. This methodology is applied to a numerical example using the HAWC2 aeroelastic code [9].

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