How to Perform an ACVG Survey
Pipeline coatings are one of the most important parameters for controlling corrosion in buried pipelines. ACVG surveys help keep coatings in tact by allowing operators to find the size and location of coating defects, resulting in improved pipeline coating efficiency.
Pipelines are still considered one of the most cost effective and safest methods of liquid and gas transportation.
Pipelines are usually buried underground to improve land utilization and for safety considerations. However, buried pipelines are prone to corrosion -- and if this corrosion is not controlled, catastrophic pipeline failures could occur. (To learn more about pipeline corrosion, read: 21 Types of Pipe Corrosion & Failure.)
If a pipeline coating is 100% perfect, then pipeline corrosion will be negligible and no risk of corrosion shall be expected. However, no pipeline coating is considered perfect, as coating defects will eventually occur either due to coating defects during pipeline laying and handling or due to its relative movement with the surrounding environment after being buried underground.
Due to the importance of pipeline coating as a method of controlling corrosion and ensuring the integrity of buried pipelines, several above ground survey techniques can be used to locate and size coating defects.
In this article, we will explain how an ACVG survey could be performed as a technique that could detect and size coating defects.
What is an ACVG survey?
An alternating current voltage gradient, or ACVG, survey is a technique that can detect coating defects by measuring the AC voltage gradient created due to AC current flowing to a coating defect at a buried pipeline.
ACVG surveys are highly precise and are able to accurately identify most cable breaks and holidays, including the position of anodes. They can be used as macro and micro tools to locate holidays of all sizes. ACVG surveys provide readings in the form of decibels relative to one millivolt (dBmV), which is used along with other surveys to allow the operator to determine the holiday's severity class (e.g., minor, moderate, severe).
Here are some pros of using this technique:
- ACVG surveys work in diverse soil conditions.
- They are more sensitive on top of pavement than other voltage gradient surveys.
- They are less susceptible to existing or stray direct currents.
- ACVG surveys can identify shorted casings.
- They are highly accurate.
- They are suitable for complex piping arrangements and in congested metropolitan areas.
- They don't involve trailing wires.
- They can be used alongside with other techniques.
- ACVG survey readings are digitally displayed.
- No side drain is necessary.
- In most cases, they require only a single operator.
In ACVG surveys, AC current is generated due to an AC voltage applied to a pipeline -- which will allow current to flow from the current injection source (i.e., the ground) to coating defects in the pipeline through the earth and return back through the pipeline metal to the voltage source connection.
The current pattern at the coating holiday is in an omnidirectional shape. That means it moves more or less equally in all directions, radiating towards the holiday for a short radius (three to five meters) around the holiday before achieving the steady state direction from the transmitter.
The current flowing from the earth towards the coating holiday is usually concentrated around the coating defect. This creates a large potential difference around the coating defect, allowing it to be detected by ACVG surveyor. In this case, the ACVG surveyor measures the potential difference between two special electrodes and a special receiver as shall be shown later. (For more on coating holidays, read: Tools & Tips: Holiday Detection on Coated Steel & Concrete Substrates.)
Instrumentation and Equipment
The instruments needed for an ACVG survey are mainly:
- A signal transmitter.
- A receiver that has a set of electrodes capable of measuring AC voltage gradients in earth.
Each of these pieces of equipment is discussed more in-depth below.
1. A Signal Transmitter
A transmitter, also known as a signal generator, is electrically connected to the pipeline at an above-ground feature or test station and to a remote ground.
If a transmitter is not available, we can rely on the pipeline's cathodic protection (CP) system. This can be done by connecting a series current interrupter to the CP ground bed. The transmitter must be capable of emitting a distinguishable signal that can be detected by the receiver used for the survey. This can be done either by using the transmitter itself or through the current interrupter connected to CP ground bed.
The ideal transmitter includes multiple outputs, with constant current output and a visual indication of the output to assist the operator in determining the suitable current to be injected during the survey. Operators should ensure the transmitter has enough power to allow the AC test signal to be transmitted and detected at a sufficient level for the desired survey length.
To ensure the AC signal transmitted is capable of being transmitted along the desired survey length, the surveyor must make sure the transmitter connection has sufficiently low impedance so enough AC current can be injected for it to be transmitted along the survey length. (Signal transmitters are also used in AC attenuation surveys. To learn more, read: How To Conduct An AC Attenuation Survey on an Underground Pipeline.)
2. A Receiver
The receiver is used to measure the strength of voltage gradient around the pipeline and determine the direction to the nearest indication.
The ideal receiver includes a visual display showing the magnitude of the voltage gradient. It should also include a directional indicator to the ground fault or coating holiday. The receiver measures the voltage gradient using a pair of probes that have good ground contact and are connected to the receiver.
The voltage gradient should be displayed using the following formula:
dB µV. dB μV = 20 log (probe A) – 20 log (probe B)
This value can be calculated by the receiver and displayed automatically in the receiver's display on a scale from 0 to 100 dB. The receiver should have an indication to confirm that it is connected to the probes and in the proper mode. This will avoid unnecessary re-work if the survey is done whilst the receiver is not connected to the probes or in a different mode of operation.
ACVG Survey Procedure
It should be noted that manufacturers' instructions must be followed in order to ensure a successful survey. The surveyor must be familiar with the device used and have proper training on the equipment to be used. The survey procedure is as follows:
- Connect the transmitter to a power supply. This is to provide necessary power to inject current into the pipeline under test.
- Connect the transmitter terminals to the pipeline under test and to an electrically isolated ground. Check that low resistance to ground is achieved to inject a strong signal. If this is not possible, search for another location to place your grounding connection or connect several stakes or ground points in parallel to achieve a lower grounding resistance. If the pipeline CP system is used as a transmitter, the current interrupter must be connected to the DC output side between the rectifier and anode bed or between rectifier and pipeline under test.
- Turn the rectifier and current interrupter on.
- Adjust the CP current output to establish an adequate output signal level. The surveyor must ensure that an adequate signal level is achieved on the section of the pipeline under test. If the adequate signal is not achievable, the surveyor must move the transmitter to ensure adequate coverage of the pipeline section.
- Go to the distant test post that marks the end of the section. There, create a temporary fault with a short length of wire with approximately 30 mm (1.2 in) of bare wire in contact with the earth. The surveyor should ensure that the fault is detectable by the receiver by performing a short (e.g., 10-meter [33 ft]) survey centered on the wire, and a survey off to the side by the same distance as the depth of cover over the pipe. The minimum reading should be 50 dB μV. This verification is necessary to ensure that adequate signal is sensed at the end of the pipeline under test, and to ensure that the pipeline is isolated from foreign pipelines and structures.
- Start walking along the pipeline route. Measure the voltage gradient at consistent survey intervals.
Best Practices for an ACVG Survey
When taking readings, the surveyor should walk in line with the pipeline being surveyed. They should place the probes in a direction parallel to the pipeline axis, and preferably over the pipeline centerline. The readings should be taken every one and a half to three meters. If walking directly over the pipeline is not possible, walking parallel and to the side can be sufficient to detect indications. However, in this case, the surveyor must record the distance and direction off the pipeline's center line. In all cases, the probe's alignment must be checked -- both forward and backward. The voltage gradient magnitude and direction must be observed to help the surveyor in detecting indications along the underground pipelines that may be holidays. A very good practice is to pour a small amount of water on the surface to achieve good ground contact.
If the magnitude reading is less than 30 dB μV, it is unlikely that there is an indication nearby. An arrow indication is usually available in the receiver display to point towards the most significant indication as long as there is a sufficient signal in the area. If no arrows are displayed then there are no detectable indications in the area or there is too little current in ground to activate the direction arrows.
When the surveyor passes an indication, the indication arrow reverses direction indication that an indication has passed. If this situation occurs then the surveyor must return back and continue to take measurements at shorter intervals until he can pinpoint an indication with an accuracy of approximately 150 mm.
How Can a Surveyor Determine the Indication Location?
The surveyor must note the numerical dB μV value as they are surveying the line. The numerical dB μV rises as the surveyor approaches an indication; it then briefly falls when they are directly over the indication; then it rises again when they pass the indication.
The surveyor must realize that there is a change in arrow direction on either side of indication location and take measurements at shorter intervals before and after the indication until the position at which the arrow changes direction is found.
To pinpoint exactly the indication location, the surveyor must realize the location at which the dB μV reading is lowest when both probes are of equal distance on either side of the indication. This confirms the indication is directly under the center of the electrode array at the Y axis.
After determining the indication location at the Y axis, the surveyor should turn the electrode array at 90 degrees so that the arrays are perpendicular to the pipeline. The previous steps are repeated and the indication location at the X axis is determined.
The indication location should then be exactly at the center of the electrode array. The indication location should then be marked using flags, stakes or GPS coordinates.
If further indications are sought to be found, the surveyor should continue moving along the pipeline section to be surveyed while ensuring that both probes are in good contact with the ground.
If the surveyor finds that the indication arrow stops pointing backwards to the previous indication, then the signal from the previous indication might be so small to detect or the signal from the next indication might be larger than the previous indication.
Data Analysis in ACVG Surveys
Data analysis in ACVG is important to determine repair priorities and determine indications that might affect the pipeline integrity.
To determine the severity of an indication, the surveyor must record the maximum indication level of each detected indication.
The surveyor must take into consideration while analyzing the maximum values that soil/ground cover can have an effect on these values, however a good indication could still be achieved if the previous survey procedures are followed. It should be noted that the maximum fault value is measured when the surveyor places one of the electrode array probes directly above the pipeline indication and the other probe perpendicular to the pipeline.
The surveyor should start at the indication position, testing at 0.3 m (1 ft) (or smaller) intervals while recording the highest dB μV reading and, ideally, the log number.
When a large section of poorly coated pipeline is present, the surveyor may not be able to pinpoint a discrete indication. In this case, the receiver may indicate high dB values while the current direction arrow will be fluctuating in directions (i.e., changing directions continuously back and forth). In the event of a fluctuating direction arrow, the surveyor must walk along the pipe and take several more readings until the arrow finally locks onto the signal again and gives a directional indication in the reverse direction. The entire area between the two indications is suspect, and further investigation may be necessary to confirm or deny the existence of poor coating.
The indication locations should be measured and located with respect to a permanent aboveground reference point. GPS data loggers, as well as maps and geographical information systems (GIS), could be used to store indication locations.
Data Analysis for ACVG Surveys
The surveyor should analyze ACVG results while considering all other known information. If there is a possibility of plotting the results of a current attenuation survey along with ACVG survey, it would be beneficial to help correlate and validate survey results.
It would also be beneficial to use a graphical representation including the electrode array dB indications and the distance from signal generation point with pipeline information to facilitate data interpretation.
ACVG results must be interpreted carefully. The voltage gradient is a function of the current, which can be affected by soil resistivity around the pipe and homogeneity of soil. (For more on soil's role in corrosion, read: 5 Key Factors Present in Corrosive Soils.)
The larger the current flowing to the pipeline, the larger the voltage gradient reading. Therefore, having some idea of soil resistivity, pipe size and other data will help in interpretation of ACVG data. For example, a coated pipe in high-resistivity soil may generate a small indication when, in fact, the actual indication size (as defined by bare surface area in contact with the electrolyte) may be of a larger size than a higher indication in another area with low-resistivity soil.
Having a perfect pipeline coating is considered one of the utmost important parameters that can control corrosion of buried pipelines. (For more on pipeline coatings, read: The Growing Disconnect in the Pipe Coating Industry.)
However, in real life, this is practically impossible. Thus, aboveground surveys were developed to determine damaged areas of pipeline coating. One of these techniques, called AC voltage gradient (ACVG) survey, has the advantage of being able to determine the defect location and calculate its size. Having an approximate way of sizing the defects can help the pipeline operator in prioritizing defect repair and scheduling which defects are in need of repair first -- and thus increase the pipeline coating efficiency.
Having a high pipeline coating efficiency, in addition to a well monitored cathodic protection system, can increase the pipeline reliability and safety of operation.