Detection of Water Through Steel-Lined Grout Using a Stepped-Frequency Continuous Wave Radar

Monopiles are one of the most common foundation types for offshore wind turbines (OWT). They are subject to large bending moments which can lead to crack formation and ingress of water in the grout that connects the monopile foundation to the rest of the OWT via a transition piece. It is therefore of great interest to be able to continuously inspect the grout. To characterize the grout material used in this paper in terms of its dielectric properties an open-ended coaxial probe is utilized. In addition, a simplified laboratory demonstrator is designed and manufactured to enable first radar measurements. Grout-filled TEM horn antennas and a stepped-frequency continuous wave (SFCW) radar are purpose-built to enable measurements from 100 MHz to 2 GHz. The antennas are characterized and compared to the simulation. Finally it is shown that water can be detected through one meter of steel-lined grout by analyzing differential signals.