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 costs.
Benefits
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 movement.
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.
