Monitoring of welded seams on the foundations of offshore wind turbines
Offshore foundations for wind turbines are expected to increase in significance over the coming years. Various wind parks are already producing energy and there are many more planned or currently being built. The maintenance of these offshore wind turbines is challenging due to the limited accessibility and expensive logistics. Currently, 25% of these structures have to be evaluated each year regarding their condition and stability. For this task, welded seams are the focus of interest. At the moment, they are visually inspected by divers; in the oil and gas industry, the alternating current field measurement (ACFM) technique is used as well. None of these are applicable on the inside of jackets, for example, due to safety issues. Visible inspection is often limited by the sight conditions and ACFM can only measure directly under the sensor. The Fraunhofer IKTS has therefore developed a transducer ring to be placed permanently on the foundations of offshore wind turbines. It is particularly well suited for the monitoring of jackets. The measurement device, CoMoSeam, has been successfully tested underwater. Furthermore, an artificially-initialised crack could be detected and located correctly. The investigation is realised by guided waves, which have a lower frequency range than the commonly used ultrasound techniques. The advantage lies in the reduced number of sensors compared to NDT ultrasound techniques, which also leads to a lower resolution. Nevertheless, the resolution is decreased but is still far better than that achieved by the currently used inspection methods. This paper presents the hardware used for building the sensor ring as well as the measurement technique. The lamination required to ensure that the equipment is waterproof is especially challenging due to the large diameters demanded for offshore platforms. To detect cracks correctly, even in harsh environments, sophisticated data processing is necessary to automatically eliminate all obviously incorrect data. The method is just being introduced in the regulations and will be adapted to a diver-free installation and operating regime.