Use of Microwaves for the Detection of Water as a Cause of Corrosion Under Insulation

Corrosion under insulation is a significant cause of pipeline failure in the oil and gas industry. This paper describes the initial developmental stages of a technique which uses the structure of an insulated pipeline as a coaxial waveguide to support the propagation of microwaves. These microwaves are used to inspect the insulation layer for the presence of water, which will indicate the regions of the pipe at risk from corrosion. The practical requirements to achieve a low coherent noise level are identified, and the structures and dispersion characteristics of the modes which propagate in coaxial waveguides are examined to determine if these requirements can be met. An antenna array design proposed for pure mode excitation of the TEM mode and its effectiveness is demonstrated experimentally on a 12″ diameter coaxial waveguide. In particular, a signal-to-coherent-noise ratio as high as 39 dB is obtained with this arrangement. The sensitivity of the technique to the presence of water within the waveguide is investigated and it is found that water volumes with a cross-sectional area in the plane perpendicular to the waveguide axis of as little as 5% produce reflections that are readily detectable from the coherent noise floor.