Ralf Ahlbrink and Thomas Beuker outline the benefits of a shallow internal corrosion tool incorporating eddy current technology.
Internal corrosion, for example top-of-the-line corrosion (TOL) in wet gas lines due to condensation2, constitutes a major risk to pipeline operation1. Under certain circumstances, the internal corrosion growth rates can be as high as several millimetres per year. Since internal corrosion is prevalent in all sorts of assets including off-shore and notably water pipelines, it is important that internal corrosion can be monitored and assessed even in challenging conditions.
Since the eddy current (EC) sensor technology incorporated in Rosen's shallow internal corrosion (SIC) tool can be used in bi-directional and robotic inspection tools, it is a suitable inspection method even in the presence of such challenges as high wall thickness, high product flow rates, tight geometrical constraints and in cases where the gas product prohibits ultrasonic technology (UT) measurements.
The new SIC tool thus provides optimal corrosion growth monitoring in situations where inspection is difficult or impossible with conventional in-line inspection methods.
The EC coil systems of the SIC tool (Fig.1) induce and detect currents in conducting materials, ie in the pipe walls. Monitoring changes of these eddy currents enables highly accurate characterisation of surface metal loss defects.
The EC sensors are especially sensitive to shallow features. This property distinguishes it from the MFL defect characterisation method which mainly reacts to volume changes resulting from metal loss and is therefore more suitable for the detection of deeper features.
As shown in Fig.2, magnetic flux leakage (MFL) is best suited for deeper and, generally, more substantial metal loss defects.
However, it can be used within the optimal sizing spectrum of the EC inspection method (the blue area in Fig.2).
Sizing capabilities
Conversely, because the EC inspection method provides maximum signal indication and better feature width sizing resolution, measurements taken with the SIC tool can assist MFL feature depth sizing algorithms in the MFL range (green area in Fig.2). This means that a combined use of the two technologies results in a significant improvement of the overall sizing capabilities.
To exemplify the precise detection capabilities of the EC versus the MFL method, an inspection of a sample of bore holes was conducted in a 16-in line.
Fig.3 illustrates both the difference in signal characteristics between EC and MFL and the higher lateral resolution in defect surface measurement of the former method.
The high lateral resolution of the EC technique leads to a more accurate distinction of individual pits in dense clusters: in contrast to MFL, the EC data furnished by the shallow internal corrosion tool clearly shows the separation between the two bore holes shown in Lane 2.
Bacterial corrosion
Fig. 4 shows a photograph (left) of a cluster of pitting in a steel plate with characteristics similar to top-of-line (TOL) and bacterial corrosion. The image (right) represents the same feature on the basis of EC sensor data.
A specific pseudo-lift-off conversion formula was used that showed excellent depth sizing performance even for pitting defects with unfavourable surface-to-depth ratios, ie even where the pit diameter is larger than the maximum depth only by a factor of two to three.
Tight coil spacing in circumferential direction not only ensures high repeatability of spatial dimension measurements but also means that EC technology is very suitable for high-resolution mapping.
Conclusion
Rosen's eddy current technology-based SIC tool is specifically designed to facilitate and optimise the process of monitoring shallow internal corrosion in pipelines.
As there is virtually no restriction to the type of tools that can be fitted with eddy current technology, it can be used even under exceptionally challenging conditions which are prevalent in the oil and gas industries.
Due to its high lateral resolution of defect surface measurements, EC technology not only accurately distinguishes individual pits in dense clusters but detects and sizes even marginal corrosion features with great accuracy, thereby providing invaluable information on asset degradation3.
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PhD Ralf Ahlbrink, physicist, and Thomas Beuker, product line manager inspection and testing, ROSEN Technology & Research Center, Lingen, Germany. www.roseninspection.net
REFERENCES:
1. Argent, C, et al. Macaw's Pipeline Defects. sl: Yellow Pencil Marketing, 2003. ISBN 0-9544295-0-8;
2. CO2 Top of the Line Corrosion in Presence of Acetic Acid: A Parametric Study. Singer, M, et al. 2009. NACE CORROSION. Paper No. 09292;
3. DNV. Submarine Pipeline Systems. 2007. DNV-OS-F101;
4. In-Line Inspection of Dents and Corrosion Using 'High Quality' Multi-Purpose Smart-Pig Inspection Data. Beuker, T, Brown, B and Paeper, S, 2006. International Pipeline Conference.
5. Hagemaier, D J, Fundamentals of Eddy Current Testing. Columbus: American Society for NDT, 1990. ISBN 0-931403-90-1.
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