Reliable sizing and monitoring of corrosion in pipelines

By Ralf Ahlbrink, Physicist, and Thomas Beuker, Product Line Manager Inspection and Testing, Rosen

The Australian Pipeliner — October 2009

Corrosion affecting the inside of pipelines constitutes a major risk for pipeline integrity. To optimise the process of monitoring shallow internal corrosion, Rosen has developed a new shallow internal corrosion tool incorporating eddy current technology.

Capable of accurately distinguishing and sizing even minor corrosion features, Rosen’s new shallow internal corrosion (SIC) tool makes it easy to survey and assess the asset degradation process resulting from corrosion– even in so-called unpiggable pipelines.

Overcoming the limits of conventional in-line inspection methods

Internal corrosion, for example top of the line corrosion (TOL) in wet gas lines due to condensation, constitutes a major risk to pipeline operation. 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 offshore pipelines and water pipelines, it is important that internal corrosion be monitored and assessed at an early stage even in challenging conditions.

Since the eddy current (EC) sensor technology incorporated in Rosen’s 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.

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The new SIC tool provides optimal corrosion growth monitoring in situations where inspection is difficult or impossible with conventional in-line inspection methods.

Enhanced corrosion growth monitoring

The EC coil systems of the SIC tool induce and detect currents in conducting materials, such as the pipe wall. Monitoring changes of these ECs enables highly accurate characterisation of surface metal loss defects.

The EC sensors are especially sensitive to shallow features on the internal surface of the pipeline. This property distinguishes it from the magnetic flux leakage (MFL) defect characterisation method, which mainly reacts to volume changes resulting from metal loss, thereby making it more suitable for the detection of deeper features on the outside and the inside surface.

As shown in Figure 1, 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 (blue area in Figure 1). 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 Figure 1). This means that a combined use of the two technologies results in a significant improvement of the overall sizing capabilities.

Making it visible: detection of SIC

To exemplify the precise detection capabilities of the EC versus the MFL method, an inspection of a sample of drill holes was conducted in a 16 inch pipeline. 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 SIC tool clearly shows the separation between the two bore holes shown in Lane 2 of Figure 2.

Figure 3 shows a cluster of pitting in a steel plate with characteristics similar to TOL and bacterial corrosion. The bottom image represents the same feature on the basis of EC sensor data. 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.

Rosen’s EC technology-based SIC tool is specifically designed to facilitate and optimise the process of monitoring shallow internal corrosion. As there is virtually no restriction to the type of tools that can be fitted with EC technology, it can be used even under exceptionally challenging conditions. 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 degradation, for example in the context of limit state design guidelines.

References

CO2 top of the line corrosion in presence of acetic acid: a parametric study. Singer, M., et al. 2009. NACE CORROSION. Paper No. 09292.
Macaw’s pipeline defects. Argent, C., et al. s.l: Yellow Pencil Marketing, 2003. ISBN 0-9544295-0-8.
Fundamentals of eddy current testing. Hagemaier, D. J. Columbus: American Society for NDT, 1990. ISBN 0-931403-90-1.
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.
Submarine pipeline systems. DNV 2007. DNV-OS-F101.

For more information visit the website.

Figure 1: Optimal metal loss depth sizing spectrum of the MFL inspection method (green) and EC technology (blue, incorporated in the SIC tool) in a pipeline with a wall thickness of 15 mm.

Figure 2: Defect patterns (excerpts of C-scans) are shown twice, as EC (top) and MFL (bottom) data. The three vertical lanes differentiate between bore holes of different sizes and distances between them which were inspected in a 16 inch joint with 12.6 mm wall thickness. Whereas the high degree of overlap attests to the reliability of each one of the two methods, the subtle yet significant differences account for the highly beneficial synergetic effect of a combined use of the two technologies.

Figure 3: Pitting corrosion in a steel plate with a photograph of the feature on the left and the EC-based image of the same feature on the right (all values in millimetres).

Image caption:

The MFL and EC combo tool.

Appeared in issue: The Australian Pipeliner - October 2009 The Australian Pipeliner — October 2009