Abstract Summary
Most offshore wind turbines rely on monopiles as the foundation concept. Monopile foundations are sensitive to scour which can alter their dynamic response. Scour has a negative impact on the entire offshore wind turbine system by reducing natural frequencies, which can increase resonant behavior and thus ultimately reduce the life expectancy. To monitor the onsets of scour operators mostly rely on bathymetric surveys that are executed at regular intervals. However, this only offers discrete information in time, e.g. once a year, at a considerable cost. As an alternative for detecting scour on operating turbines relies on monitoring data from accelerometers to track the changes in eigenfrequency continuously over time Scour is inferred when the resonance frequencies drop unexpectedly, with detection limits currently set at around 2 to 5% change in natural frequencies. However, even when a drop in frequency is identified there is no way to guarantee this variation is solely related to the development of scour, nor can the amount of scour be quantified without an updated model of the turbine. This contribution studies the feasibility of detecting scour by monitoring changes in sub-soil bending moments inferred from strains sensors installed on the monopile. For this purpose, an experimental campaign was conducted on a small-scale model of a driven monopile foundation founded in sand. The monopile set-up consists of strain sensors above and below the ground level based on two different technologies namely, Fiber Bragg Grating (FBG) technology and Optical Frequency Domain Reflectometry (OFDR). Temperature and displacement sensors above the ground level were installed as well. Monotonic lateral tests were performed for different scour configurations to investigate whether it is possible to detect scour formation by means of the measured pile bending moments. Experimental results showed that strain measurements below the ground are sensitive to scour even in the case of shallow scour hole formation.
