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Calculating and Tracking Soil Resistivity Change in High AC Corridors

This paper will look a data derived from sites that demonstrate this evaluation of soil resistivity and how this data may be applied in other aspects of cathodic protection and pipeline integrity management.

Product Number: 51317--8984-SG
ISBN: 8984 2017 CP
Author: Jamey Hilleary
Publication Date: 2017
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Over the last decade increased attention has been directed toward the corrosion effects of high AC current density on pipeline structures collocated in high voltage power line right of ways. Efforts directed at identifying areas of concern evaluating the magnitude of the problem in those areas and applying mitigation systems to reduce high AC levels have flourished. Subsequent monitoring of AC voltage and AC current density has also increased dramatically as organizations seek to protect buried pipelines from this relatively new threat. Historically AC current density has been calculated using measured AC voltage soil resistivity and holiday size (diameter). Soil resistivity is generally measured at the area or areas of concern using the Wenner 4-point or similar method and expressed in ohms per centimeter or ohms per meter. Very often the soil resistivity testing is done primarily for sizing the electrode grounding system for draining AC from the pipeline. In subsequent pipeline AC current density calculations this soil resistivity measurement is used as a constant in the calculation formula. Many factors including moisture content temperature and salt content can affect soil resistivity. In many regions the soil resistivity may change frequently and significantly. Because of this in many if not most instances soil resistivity is actually a variable though it is generally applied as a constant. It should be taken into account in many aspects of corrosion control. With increased use of coupons and monitoring equipment direct measurement of AC current density can be derived by evaluating the current flow to ground from a coupon bonded to the pipeline structure. These measurements typically are taken frequently (one or more per hour) and the values retained for continual analysis of the effectiveness of the AC mitigation system. At the same time AC voltage on the pipeline structure is typically measured and retained as well. This combination of measurements provides a unique opportunity to track the changes in soil resistivity in real time. By using the soil resistivity “constant” derived from the Wenner method the holiday diameter size equivalent to the exposed area of the coupon and the real-time measurement of the AC voltage from the pipeline a “benchmark” AC current density measurement can be calculated. This calculation where the only numeric variable is the AC voltage measurement can be compared with the real-time current density measurement from the bonded coupon. The changes in soil resistivity will be reflected in the difference between the current density measurement calculated using the measured soil resistivity as a constant and the direct measurement of current density. As soil resistivity lowers for instance the directly measured current density value will increase. The change in soil resistivity can then be calculated using the difference between the two current density values. This paper will look a data derived from several sites that demonstrate this real-time evaluation of soil resistivity and explore how this data may be used in evaluation of other aspects of the overall cathodic protection system and pipeline integrity management.

Key words: AC voltage, induced AC, soil resistivity, high voltage transmission

Over the last decade increased attention has been directed toward the corrosion effects of high AC current density on pipeline structures collocated in high voltage power line right of ways. Efforts directed at identifying areas of concern evaluating the magnitude of the problem in those areas and applying mitigation systems to reduce high AC levels have flourished. Subsequent monitoring of AC voltage and AC current density has also increased dramatically as organizations seek to protect buried pipelines from this relatively new threat. Historically AC current density has been calculated using measured AC voltage soil resistivity and holiday size (diameter). Soil resistivity is generally measured at the area or areas of concern using the Wenner 4-point or similar method and expressed in ohms per centimeter or ohms per meter. Very often the soil resistivity testing is done primarily for sizing the electrode grounding system for draining AC from the pipeline. In subsequent pipeline AC current density calculations this soil resistivity measurement is used as a constant in the calculation formula. Many factors including moisture content temperature and salt content can affect soil resistivity. In many regions the soil resistivity may change frequently and significantly. Because of this in many if not most instances soil resistivity is actually a variable though it is generally applied as a constant. It should be taken into account in many aspects of corrosion control. With increased use of coupons and monitoring equipment direct measurement of AC current density can be derived by evaluating the current flow to ground from a coupon bonded to the pipeline structure. These measurements typically are taken frequently (one or more per hour) and the values retained for continual analysis of the effectiveness of the AC mitigation system. At the same time AC voltage on the pipeline structure is typically measured and retained as well. This combination of measurements provides a unique opportunity to track the changes in soil resistivity in real time. By using the soil resistivity “constant” derived from the Wenner method the holiday diameter size equivalent to the exposed area of the coupon and the real-time measurement of the AC voltage from the pipeline a “benchmark” AC current density measurement can be calculated. This calculation where the only numeric variable is the AC voltage measurement can be compared with the real-time current density measurement from the bonded coupon. The changes in soil resistivity will be reflected in the difference between the current density measurement calculated using the measured soil resistivity as a constant and the direct measurement of current density. As soil resistivity lowers for instance the directly measured current density value will increase. The change in soil resistivity can then be calculated using the difference between the two current density values. This paper will look a data derived from several sites that demonstrate this real-time evaluation of soil resistivity and explore how this data may be used in evaluation of other aspects of the overall cathodic protection system and pipeline integrity management.

Key words: AC voltage, induced AC, soil resistivity, high voltage transmission

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