Improvements in or relating to CUI monitoring

Monitoring for evidence of corrosion under insulation (CUI). At least one range sensor 28 is mounted on an insulated metallic generally cylindrical structure such as a pipeline 12, each range sensor having a transmitter 28a and receiver 28b that are spaced apart by a monitoring length A. A plurality of point sensors 34a-l are mounted on the structure within the monitoring length. Operating the at least one range sensor and point sensors over a first period and identifying a first pattern by correlating a parameter measured by each sensor type. Removing the at least one range sensor and using the correlated data from the point sensors to monitor for evidence over the monitoring length to warn of potential corrosion. Also disclosed is an apparatus for monitoring for evidence of corrosion, which includes a signal generator to provide a signal to at least one range sensor transmitter to be transmitted axially along the insulation of the structure.