Gas Transmission Networks
National Grid Gas Transmission completed a project under the IFI scheme to evaluate the MTM (Magnetic Tomography Method) for
pipeline inspection. During 2011 and 2012, the MTM system was trialled on13 sections of pipe on the National Transmission System.
Analysis of the results proved that the MTM system has the ability to locate anomalies in a buried pipeline. Comparison of the MTM
data with ILI results has shown that there is a good correlation between anomalies detected by both systems. The trials also included
2 pipelines where existing strain gauges monitoring was fitted to evaluate ground movement in relation to mining activities. A
comparison between the strain gauge measurements and the MTM/SCT data showed that there was a correlation between the SCT
estimated stress in the Stress Concentration Zones and the stress recorded by strain gauges. MTM could identify the location of a
defect but could not report the quantity of stress in the pipe wall caused by the defect. Given further work, it should be possible for
SCT to determine pipe strain and this would remove the need to install strain gauge wires.
The MTM trials in 2011 took place in conjunction with the Russian supplier Transkor. Subsequent to the success of this initial trial,
Speir Hunter (a UK based company) with the University of Leeds, have developed a more advanced version of the technology, the
“SCT inspection system” (The SCT system, was used during the 2012 site trials). The change of supplier from Transkor to Speir
Hunter/University of Leeds, significantly reduced the number of the challenges associated with communication when looking to
progress with the required technology development. The project benefited from additional flexibility and accessibility to the expertise
(i.e. not through an interpreter) of using a technique is available from a UK based source.
The IFI project identified a number of new potential opportunities that the SCT system can deliver as well as recommendations
required to enhance the SCT technique such that it is suitable for field deployment. New opportunities include identification of
pipeline girth welds and pipeline depth measurement.
The current ILI system relies on the identification of girth welds for positioning ILI recorded features. This can lead to variability in
accuracies. This may also be influenced by inaccuracies in as-laid records. The financial impact of this can be significant. Each
dig typically costs in the range of £40-£70k. When a section of pipeline requires repair, the costs of excavation can increase
dramatically if trenches are excavated in the wrong location and need to be extended in one or both directions until the desired
feature is found. The precise identification of the location of a repair site using the SCT technique will eliminate these additional
To develop the SCT inspection system such that it can be used as complementary tool to other pipeline inspection systems such as ILI, DCVG and CIPS.
1. The identification of pipeline girth welds to an accuracy of 90% where the PD/2t >60MPa (a measure of hoop stress where P is pressure, D is diameter and t is the wall thickness). This applies for seamed and seamless pipe but not spiral welded pipe.
2. Obtaining the depth of a pipeline to an accuracy of +/- 100mm with reference to a control point.
3. Obtaining the lateral (central) position of a pipeline using survey grade GNSS (GPS) hardware and software incorporated into the SCT system to an accuracy of +/- 100mm with reference to a control point.
4. Development of the SCT software, to enable a stress profile of the pipeline to be obtained in terms of quantified stress in each stress concentration zone related to hoop stress and SMYS.
5. Identifying the position of Stress Concentration Zones (SCZs) on the pipe circumference.
Progress the characterisation of SCZ features.
The SCT inspection system is a completely novel technique and this project has the potential to deliver significant learning in detailed
understanding of the correlation between Stress Concentration Zones (SCZs) as reported by SCT and defects as reported by the ILI
inspection technique, coupled with in-field real time identification of girth welds for positioning ILI recorded features, pipeline depth
measurement, and strain measurements for pipelines in areas where there is a risk of mining or geological subsidence or ground
For pipelines which cannot be internally inspected, the SCT inspection system has the potential to identify defects and features which
are currently not detectable using existing techniques thereby reducing the risk of failure on such systems. At this stage of
development SCT is unlikely to be a complete equivalent to ILI but would offer a significant improvement over the existing methodology
applied to these particular pipelines.
Learning will be made available via the ENA Smart Portal as well as on www.nationalgrid.com/innovation. It is also anticipated that
learning will be disseminated through industry groups, presentations and publications and through peer review and shared learning
for example through organisations such as PRCI.