Monitoring and Damage Detection in Structural Parts of Wind Turbines

Abstract

Structural Health Monitoring (SHM) is known as the process of in-service damage detection for aerospace, civil and mechanical engineering objects and is a key element of strategies for condition based maintenance and damage prognosis. It has been proven as especially well suited for the monitoring of large infrastructure objects like buildings, bridges or wind turbines. Recently, more attention has been drawn to the transfer of SHMmethods to practical applications, including issues of system integration. In the field of wind turbines and within this field, especially for turbines erected off-shore, monitoring systems could help to reduce maintenance costs. Off-shore turbines have a limited access, particularly in times of strong winds with high production rates. Therefore, it is desirable to be able to plan maintenance not only on a periodic schedule including visual inspections but depending on the health state of the turbine’s components which are monitored automatically. While the monitoring of rotating parts and power train components of wind turbines (known as Condition Monitoring) is common practice, the methods described in this paper are of use for monitoring the integrity of structural parts. Due to several reasons, such a monitoring is not common practice. Most of the systems proposed in the literature rely only on one damage detection method, which might not be the best choice for all possible damage. Within structural parts, the monitoring tasks cover the detection of cracks, monitoring of fatigue and exceptional loads, and the detection of global damage. For each of these tasks, at least one special monitoring method is available and described within this work: Acousto Ultrasonics, Load Monitoring, and vibration analysis, respectively. Farrar & Doebling (1997) describe four consecutive levels of monitoring proposed by Rytter (1993). Starting with „Level 1: Determination that damage is present in the structure“, the complexity of the monitoring task increases by adding the need for localising the damage (level two) and the „quantification of the severity of the damaged“ for level three. Level four is reached when a „prediction of the remaining service life of the structure“ is possible. By using the monitoring systems described above, in our opinion only level 1 or in special cases level 2 can be attained. For most customers, the expected results do not justify the efforts that have to be made to install such a monitoring system. In general, our work aims at developing a monitoring system that is able to perform monitoring up to level 4. Therefore, we think it is necessary to combine different methods. Even though the different monitoring approaches described in this paper differ in the type 9