Guided wave monitoring of offshore wind turbine monopiles
Many existing offshore wind farms (OWF) in European waters are coming to the end of their original design life in the next decade. Life extension offers the potential to continue their operation and to contribute to renewable energy generation and thus sustainability targets. Typically, these offshore wind turbines were constructed with a monopile embedded in sandy soil as the most efficient support structure in low to medium water depths. Fatigue due to variable wind and wave loading and corrosion due to the maritime conditions are the typical damage mechanisms that need to be considered. Critical weld locations below the mudline experience high and varying bending loads but are inaccessible for manual or visual inspection. The feasibility of employing guided ultrasonic waves to detect corrosion and fatigue defects in the inaccessible, submerged part of the monopiles was investigated from Finite Element (FE) simulations and experiments on a laboratory scaled prototype. A prototype was manufactured from welded sections to approximate the monopile of a 5 MW offshore wind turbine on a scale of 1:25 with the correct ratios of length, diameter, and wall thickness. The tank was filled with 1.2 m of sand and 0.8 m of water from the bottom (saturated sand). Experiments were conducted employing the T(0,1) torsional guided wave mode excited at the top end above the water level to monitor the wave propagation and sensitivity for defects. The effects of embedment in water and sand on the attenuation of the guided waves were quantified and compared to theoretical predictions to ascertain that the complete length of the monopile can be monitored. An artificial defect (notch) approximately 0.25 m below the mudline was introduced and increased in severity in stages and the sensitivity for the detection of defects at a circumferential weld verified. FE calculations using the ABAQUS software were conducted to aid the selection of suitable guided wave modes and excitation frequencies, and the guided wave propagation across the multiple welded parts with thickness changes simulated. Good agreement with the experimental results was achieved. The guided ultrasonic wave data could be combined with load and stress predictions to facilitate fatigue reliability analysis based on Structural Health Monitoring (SHM).