The aim of this project is to develop, and to demonstrate, a novel real-time monitoring system to detect in-service degradation in offshore wind turbine support structures using ultrasonic guided waves. This system uses active sonic/ultrasonic waves to cover the whole volume of concern to detect fatigue cracking at welds, and possibly at other locations, in the pile and transition piece. Fatigue performance of current structures is estimated by extrapolation from potentially unrepresentative data, so the possibility exists of cracking before the end of the design life. There may be over 1km of weld in the entire structure and potential sites of cracking may not be easily predicted, so that any monitoring system must be capable of crack detection over a large volume of material. The method will allow cracks to be detected across a large volume of material and is expected to save considerable costs of local examination of welds, especially when these are underwater and/or around the mud line. The project will develop designs of sensors, monitoring procedures and electronics and will be demonstrated on a structure offshore.