How To Relate Resistivity To Cathodic Protection (CP)
How does cathodic protection work?
The reduction oxidation of the electrochemical process is described here Galvanic anode.
Cathodic protection is often used to protect pipelines, bridges and large steel structures from the effects of corrosion (rust).
For small steel structures galvanic anode cathodic protection devices can be employed such as sacrificial anode tapes and ingots attached directly to the structure. For large (and long) steel structures such as pipelines and bridges that have a lot of surface area passive cathodic protection systems are not enough.
Large steel structures, or where the groundbed anode resistance is large, an Impressed Current Cathodic Protection system (ICCP) is required. ICCP works by driving an electric current to a remote sacrificial anode. This anode may be one or an array of anodes buried underground.
What equipment is used?
An Impressed Current Cathodic Protection system (ICCP) consists of a DC power source, often an AC powered transformer rectifier and an anode, or array of anodes buried in the ground. The anode array is often referred to as a groundbed. Typically a 50 Amp DC power source is used.
A groundbed array of anode electrodes are buried below the surface (considered the electrolyte). The groundbed resistance is of utmost importance in ensuring proper cathodic protection as the maximum current output of the ICCP system is proportiaon to the amount of surface area that needs to be protected. It is for this reason that it is important to locate the most conductive subsurface location within the ICCP site.
How do you determine the best location for the anode groundbed?
The spot groundbed resistance is easily found using the resistivity method outlined in the ASTM G57 Soil Test.
Is there an even better way than the ASTM G57 Wenner Soil Test?
Yes there is. Advanced Geosciences, Inc. (AGI) has developed the Electrical Resistivity Imaging (ERI) method of scanning the subsurface for its resistivity profile (or in terms of conductivity which is simply the reciprocal of resistivity) like an MRI or a CAT scan would the human body. Today it is possible, with less labor cost, to get a picture of the subsurface for locating the best groundbed location.
How is the ERI method different from the ASTM G57 Wenner Soil Test?
The ERI method uses the same measurement, but in significantly more automated locations (geometries) to extend the method of the ASTM G57 test and produce a more accurate and complete solution for the same amount of field effort and cost. The ASTM G57 Soil Test is like making a spot measurement while the ERI method scans a complete picture of the subsurface in terms of resistiivty. The fact is that there is a lot of variation in the geology below the surface that is unseen by the eye but isn't missed by the ERI scan. Geologists call this heterogeneity. The subsurface is rarely horizontally layered and uniform.
The ASTM G57 Soil Test employes 4 electrodes that have to be manually moved to many locations. Each of the 4 electrodes used in each measurement are installed in a straight line. The distance between each adjacent electrode is the same. This electrode separation distance can easily be 1000 feet or more. This means a lot of walking back and forth to install the 4 electrodes for each measurement.
The ERI method is identical to the ASTM G57 Soil Test except that the equipment handles the electrode movements by using an AGI multilconductor cable and AGI SwitchBox. This requires only one AGI cable and minimal walking. The AGI SuperSting ERI system automatically scans the ground by reconfiguring the 4 electrodes used in each measurment. It does this many hundreds of times in 5 minutes. The result is that instead of making a single spot measurment you now have hundreds if not thousands of ASTM G57 measurements.
I'm used to seeing apparent resistivy curves and tables from the ASTM G57 Soil Test, what can I expect from the ERI method?
The ERI method produces 2D images of the true resistivity distribution by using modern finite element modeling software called EarthImager (developed at AGI). The mathematics of these calculations are too large and difficult for a human to do by hand so the job is perfect for your laptop. Within seconds, the computer outputs a model of the Earth in terms of resistivity that satisfies our field measurements. This is not an approximation as the apparent resistivity calculated from the ASTM G57 Soil Test. It represent the best fitting resistivity structure of the subsurface possible.
What are the benefits of the ERI method versus that of the ASTM G57 Soil Test?
1. Reduced risk: Using the ERI method reduces risk associated with an improperly designed groundbed which could result in costly redesigns.
2. Energy savings: it costs money to operate a ICCP in terms of electricity needed for the protection of the pipeline or bridge.
3. Less groundbed cost: Reduce the complexity and cost of your groundbed design by locating the groundbed in an optimally conductive location.
How can we reduce design fault risk of our groundbed?
The ICCP system requires the groundbed to have a maximum allowable ground resistance. Simply performing a single ASTM G57 Soil Test may not adequately delineate variations in the subsurface resistivity. Using the 2D ERI method will show you the actual resistivity variations in the subsurface. I.e. you may locate the most conductive location within an area designated for the groundbed array. You may also find that the ground conditions are inadequate to begin with and requires a high power ICCP system instead.
From Ohms law:
Volt = Current * Resistance
Example: A properly designed ICCP system will be able to output ~50 Amp at 50 Volt.
50 Volt = 50 Amp * Resistance
Resistance = 50 Volt / 50 Amp = 1 ohm
We can now see that the ICCP requires the groundbed to achieve a 1 ohm resistance with the ground. The resistivity of the subsurface is therefor very important to ensure proper protection and operation of the ICCP system.
How can I reduce the long term operating cost of the ICCP?
The long term operating cost is dependant on the amount of electricity the system uses continuously. The electricity used is directly proportional to the grounding resistance of the groundbed anodes.
Lets take a look at the equation for Power.
Again from Ohm's law:
Power = Current * Voltage
i.e. if the ICCP system is capable of outputting 50 Amp max at 50 Volt max then running at full power we will have:
Power = 50 Amp * 50 Volt = 2,500 Watt
The average residential kwh (kilo watt hour) cost in the USA is ~$0.12/kwh
Operational Cost = Each ICCP system will cost ~ $7.20/day or $2,563.20 / year to operate
Choosing the optimum (the most conductive) location to install the underground groundbed will reduce costs proportionally.
Explain to me again how a lower groundbed resistance saves the operator money?
The power consumed to output the required 50 Amp is dependent on the voltage applied to the system. The voltage applied to the system is dependent on the groundbed resistance. To summarize the power consumed is dependent on the groundbed resistance.
In the above example the required output power of the ICCP system was 2500 Watt due to the high groundbed resisistance. If the groundbed resistance is lowered because the ERI scan determined a better location with half the resistance then the ICCP power source could operate at 25V to output the same 50 Amp.
Here is the calculation:
Power = Current * Voltage
Power = 50 Amp * 25 Volt = 1,250 Watt
- This system is using half the energy and therefore the cost of operation of the ICCP system is also half.
- The amount of groundbed needed can also be halved since the lower contact resistance make it more efficient. The designer can now weigh the design cost and installation versus the long term operating costs.
- Less complexity will means cost savings in the installation of the ground bed.
Check out this excellent research paper over at the National Institute of Standards and Technology (NIST) covering resistivity and cathoic protection:
Modern ASTM G57 Wenner Equipment
For moderate conditions and "a" spacings smaller than 500 ft consider using the 50 Watt MiniSting.
For all possible conditions and "a" spacings several thousand feet consider using the 200 Watt SuperSting R1/IP/SP with Wi-Fi. This little unit packs a BIG punch. You can control it remotely (from the comfort of your lawn chair of the cool air conditioned cabine of your work truck all from the included Android Tablet. Share your measurement readings with the office right from the field. No more waiting for the data to be cleared before moving to the next location. Its means freedom.