PRODUCTION/PROCESSING/HANDLING 47
Tool monitors shallow internal corrosion in pipes
Ralf Ahlbrink and Thomas Beuker outline the benefits of a shallow internal corrosion tool incorporating eddy current technology.
I
nternal 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.
Fig. 2. 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 15mm.
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
Fig. 1. The new SIC tool. The EC sensor carriers are mounted on spring-loaded arms for smooth guidance along the pipe wall4.
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.
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