2 research outputs found
Synthetic TDR Measurements for TEM and GTEM Cell Characterization
This paper describes the main features of the timedomain reflectometry (TDR) measurement technique and, in particular, the TDR analysis performed using a proper operating mode of the vector network analyzer (VNA), which is called synthetic TDR. Furthermore, some results of reflection measurement, which aim to characterize the impedance behavior of transverse electromagnetic (TEM) and gigahertz TEM cells by means of a commercial VNA in time-domain mode, are presented
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