Development of a technique for inspecting the foundations of offshore wind turbines

One design for the foundation of an offshore wind turbine consists of partially overlapping cylinders (the monopile and the transition piece) with a layer of grout between the two. Unfortunately, slippage of the transition piece over the monopile has been observed in many foundations, threatening the long-term stability of the structure. Several utilities have arrested the slippage by welding restraints inside the foundation to transfer the axial loads from the tower to the monopile foundation; however, bending moments are still transferred through the grouted section. If the grout degrades in service then the turbine could still be at risk. An inspection technique is therefore necessary so that foundations can be prioritised for repair. A novel solution is required as conventional NDT techniques either cannot find the conditions of concern or deployment in an offshore environment is hazardous. The approach taken has been to use a swept frequency ultrasonic technique operating below 100 kHz, so that resonances characteristic of the structure and its various fault conditions can be mapped. Laboratory trials supported by mathematical modelling have demonstrated the viability of the approach and a method for delivering the sensor to the regions of interest in a foundation has been developed. Offshore trials have been carried out at an operating wind farm, producing excellent results and a practical inspection system