In-situ wireless monitoring of offshore wind towers

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The project addresses the need to monitor blades and support structures for offshore wind generators. The composite blades can suffer fatigue failure and structures are susceptible to corrosion/fatigue. Blades and structures require regular inspection to avoid catestrophic failure risk. Current inspection methods can only be performed offline through human intervention, and they are limited to a few square cm. These inspections are cumbersome, time consuming, hazardous and expensive. The project will develop novel technology to monitor offshore wind tower structures and turbine blades continuously, using combined passive embedded inductive displacement sensors and acoustic emission (AE), and active (long range ultrasonics) methods. The data will be transmitted and the system controlled by wireless communication with the shore base. This system will drastically reduce the cost of inspection, whilst also making it more reliable. The project will use inductive displacement sensors for monitoring blades, whereas the team at the University of Warwick will be responsible primarily for sensor and transducer development for structural health monitoring of the towers. This will use a new design of ultrasonic transducer, based on the use of a fabrication technology known as micro-stereolithography (MSL). This rapid prototyping method allows transducers of different shapes to be manufactured, by building them up layer by layer. Our current system, one of the few such facilities in the country, has a resolution of about 10 microns, but we are working on a new generation of machines that will increase this by hopefully an order of magnitude. The aim is to investigate two types of transducer for possible use in this application. The first of these will be a flexible piezoelectric array, capable of being mounted on the surface of the tower itself, or embedded within the composite material. It is imperative that these devices be cost-effective, adaptable to different geometries, and capable of operation in hostile environments. The MSL system will thus be used to produce a hinged array substrate, within which individual piezoelectric elements will be embedded. This both gives environmental protection, and allows them to be more easily attached to the curved cylindrical tower. The research will investigate various geometries and operation frequencies, for both active and passive ultrasonic use at frequencies of up to a few MHz. The second device is one pioneered at the University of Warwick, and works on electrostatic principles. This uses a thin flexible polymer membrane above an air gap to snese displacements due to propagating ultrasonic signals. We have made the first examples of these transducers using MSL, and have demonstrated their operation as detectors of signals on solid materials. Due to their relatively low frequency of operation (typically 100-500 kHz) they are ideal as AE sensors, useful for listening for crack initiation and propagation (Note that they will not be used as transmitters due to their lower sensitivity in this mode). In addition to the above, the research team will contribute to signal processing techniques, required to treat received ultrasonic signals and to extract the required information (e.g. location of defect) from the types of signal originating from the sensor technologies developed during this project. The outputs will be interfaced to wireless transmission modules, provided by other members of the project team.