Use of microwaves for the detection of corrosion under insulation

Corrosion Under Insulation (CUI) is a widespread problem throughout the oil and gas industry, and is a major cause of pipeline failure. CUI occurs on pipelines fitted with thermal insulation; the insulation itself is protected from the environment by a layer of metallic cladding and sealed to prevent water ingress. This cladding can deteriorate from age or become damaged, allowing the ingress of water into the insulation, which allows corrosion of the external pipe surface to initiate. This corrosion can proceed at an accelerated rate due to the elevated process temperature of the pipe, compromising the integrity of the pipeline. The detection of this type of corrosion is an ongoing problem for the oil and gas industry, as the insulation system conceals the condition of the pipe. Therefore, there is a requirement for a long-range, screening inspection technique which is sensitive to the first ingress of water into the insulation, in order to provide an early warning of areas of a pipeline at risk from CUI. This thesis describes the development of a new inspection technique which employs guided microwaves as the interrogating signal. Such guided microwaves provide a means of screening the length of a pipeline for wet insulation, by using the structure of a clad and insulated pipeline as a coaxial waveguide to support the propagation of electromagnetic waves. Areas of wet insulation will create impedance discontinuities in the waveguide, causing reflections of the incident microwave signal, allowing the water patches to be detected and located. The performance of such a guided wave inspection system is intrinsically linked to the signal-to-coherent-noise ratio (SCNR) that can be achieved. Therefore, the value of the SCNR that the technique is capable of achieving is of central importance to this thesis. The excitation system is optimised to maximise the SCNR, whilst the effect of typical pipeline features such as bends, pipe supports and the various types of insulation which can be used, are studied to quantify the effect on the SCNR. A wide variety of methods are employed throughout the development of the guided microwave technique described in this thesis. Theoretical methods are employed in the initial stages to enable the development of a model to describe electromagnetic wave propagation in the large coaxial waveguides formed by pipelines. Numerical simulation techniques are employed when there are too many parameters to study for experimentation to be a viable option, and to study complex problems for which no analytical solution exists. Experiments are conducted in the laboratory using a model setup which employs metallic ducting to represent an insulated pipeline. These experiments are performed to demonstrate the practical feasibility of the technique, and to study pipeline features in a controlled environment. Finally, experiments are performed in the field on a section of real industrial pipeline, in order to validate the accuracy of the model experimental setup in representing conditions which exist on real pipelines. The main findings of the thesis are that it is possible to excite a guided microwave signal in a large coaxial waveguide with a high SCNR. Experiments revealed that the technique is highly sensitive to the presence of water in the waveguide. Measurements of the effect of different types of insulation demonstrated that rockwool causes a very low attenuation of the microwave signal, while polyurethane foam insulation has a slightly higher attenuation coefficient. An investigation into the effect of bends determined that, whilst significant mode conversion occurs at a bend, the transmission coefficient of the TEM mode is high for typical bend angles and bend radii in small diameter pipes. The behaviour of the signal at a typical pipe support was also examined; the reflection from the support was minimal, whilst the transmission beyond the support remained relatively high. Whilst there is still further work to be done before this technique can be applied in the field, the major aspects of practical implementation that could affect the technique have been investigated here, and the results consistently indicate the feasibility of the technique for long-range screening of insulated pipelines for water