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Research Papers: Pipeline Systems

Effect of Transitions in the Water Table and Soil Moisture Content on the Cathodic Protection of Buried Pipelines

[+] Author and Article Information
Fraser King1

 NOVA Chemicals Research & Technology Centre, Calgary, AB, T2E 7K7, Canadafraser.king@shaw.ca

Russell Given

 NOVA Chemicals Research & Technology Centre, Calgary, AB, T2E 7K7, Canada

Robert G. Worthingham

 TransCanada Pipelines, Calgary, AB, T2P 5H1, Canada

Greg Van Boven

 Duke Energy Gas Transmission, Vancouver, BC, V6E 3K9, Canada

1

Corresponding author. Present address: Integrity Corrosion Consulting Ltd., Nanaimo, BC, V9T 1K2, Canada.

J. Pressure Vessel Technol 133(1), 011703 (Jan 21, 2011) (6 pages) doi:10.1115/1.4002255 History: Received January 10, 2007; Revised August 03, 2009; Published January 21, 2011; Online January 21, 2011

Buried pipelines can be subject to transitional environments due to changes in soil type or moisture content. Changes in the height of the water table, for example, will affect not only the availability of water but also the access of oxygen to the pipe surface. Transitions between different soil types will also result in different exposure conditions for different parts of the pipe. These variations can affect the distribution of potential on the pipe surface and the ability of the CP system to provide adequate protection. A combination of laboratory-scale soil box tests and field measurements on operating pipelines has been used to study the effect of varying moisture content and water level on the level of cathodic protection and on pipe-depth environmental conditions. In both laboratory tests and field trials, the degree of protection was found to depend on the availability of cathodic reactants (O2 and/or H2O). Ingress of O2 results in a positive shift in potential as more current is required to electrochemically reduce the oxidant and the pipe is less easily polarized. Under some circumstances, the ingress of water has the same effect. Although more aerobic conditions lead to more positive potentials, the pipe is not necessarily less well protected. In many dry and/or high resistivity soils, the pipe surface may well be passive because of the high interfacial pH and/or high O2 concentration.

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Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

End-on schematic of the soil box showing the arrangement of asphalt-coated pipe segments, Cott coupons, and NOVAProbes for environmental monitoring. The soil column was 1 m deep and the length of the soil box was 1.5 m. All measurements of water elevation and the location of the pipe segments and the various probes and coupons are referred from the bottom of the soil box. The bottom, middle, upper, and surface NOVAProbes were located at elevations of 15 cm, 32 cm, 56 cm, and 95 cm, respectively, from the bottom of the box.

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Figure 2

Variation in soil moisture content with height from the bottom of the box at the end of the test

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Figure 3

Variation of the soil resistivity at different heights in the soil box in response to changes in the water level (2). The bottom, middle, upper, and surface probes are at elevations of 15 cm, 32 cm, 56 cm, and 95 cm, respectively.

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Figure 4

Variation of the soil redox potential at different heights in the soil box in response to changes in the water level (2). Data for surface probe excluded for clarity. The bottom, middle, and upper probes are at elevations of 15 cm, 32 cm, and 56 cm, respectively.

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Figure 5

Variation of the 5 min depolarized potentials of the asphalt-coated plates in response to changes in the water level (2). The bottom, side, and top pipe segments or plates are centered at elevations of 15 cm, 35 cm, and 60 cm, respectively.

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Figure 6

Pipe-depth soil resistivity for a pipeline located in a dry pasture near Brooks, AB (1)

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Figure 9

Variation of the native coupon potential with soil resistivity from field sites (1)

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Figure 10

Dependence of the corrosion rate of the native coupon on soil resistivity at field sites (1)

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Figure 11

Correlation between the corrosion rate and native coupon potential at field sites (1)

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Figure 8

Cathodic protection potentials for a pipeline located in a dry pasture near Brooks, AB (1)

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Figure 7

Pipe-depth redox potential for a pipeline located in a dry pasture near Brooks, AB (1)

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