Integral SHM-System for Offshore Wind Turbines Using Smart Wireless Sensors

Currently, wind turbines can incur unforeseen damage up to five times a year. Particularly during bad weather, wind turbines located offshore are difficult to access for visual inspection. As a result, long periods of turbine standstill can result in great economic inefficiencies that undermine the long-term viability of the technology. Hence, the load carrying structure should be monitored continuously in order to minimize the overall cost of maintenance and repair. The end result are turbines defined by extend lifetimes and greater economic viability. For that purpose, an automated monitoring system for early damage detection and damage localisation is currently under development for wind turbines. Most of the techniques existing for global damage detection of structures work by using frequency domain methods. Frequency shifts and mode shape changes are usually used for damage detection of large structures (e.g. bridges, large buildings and towers) [1]. Damage can cause a change in the distribution of structural stiffness which has to be detected by measuring dynamic responses using natural excitation. Even though mode shapes are more sensitive to damage compared to frequency shifts, the use of mode shapes requires a lot of sensors installed so as to reliably detect mode shape changes for early damage detection [2]. The design of our developed structural health monitoring (SHM) system is based on three functional modules that track changes in the global dynamic behaviour of both the turbine tower and blade elements. A key feature of the approach is the need for a minimal number of strain gages and accelerometers necessary to record the structure’s condition. Module 1 analyzes the proportionality of maximum stress and maximum velocity; already small changes in component stiffness can be detected. Afterwards, module 3 is activated for localization and quantization of the damage. The approach of module 3 is based on a numerical model which solves a multi-parameter eigenvalue problem. As a prerequisite, highly resolved eigenfrequencies and a parameterization of a validated structural model are required. Both are provided for the undamaged structure by module 2