How To Conduct An AC Attenuation Survey on an Underground Pipeline
AC Attenuation surveys allow you to measure an applied AC signal's current attenuation to determine an underground pipeline's coating condition.
Underground pipelines are always prone to corrosion due to how they react to their surrounding environment. If corrosion is left untreated, the pipeline's integrity and safety is compromised. This could lead to devastating accidents—and, in some cases, loss of lives.
So, to control corrosion on buried pipelines, the main line of defense is applying a suitable coating. This will isolate the pipeline from the surrounding electrolyte—which will help control the buried pipeline's corrosion rate.
Unfortunately, having perfect coating is a pipe dream—any buried and coated pipeline will have several coating imperfections and holidays that will increase as time passes. Therefore, several above-ground survey techniques were developed to assess the pipeline coating's condition and determine the location and size of pipeline coating holidays. (For more on this topic, see: Coating Holidays & Pinholes: Chinks in the Armor.)
In this article, we will present AC attenuation survey and how it can be used as a method for assessing the pipeline coating condition.
An Overview of the AC Attenuation Survey (Electromagnetic) Method
A buried pipeline's coating condition may be assessed by measuring the current attenuation of an applied AC signal. If a coating has a uniform dielectric strength and electrically isolates the pipeline from the surrounding ground at all points, then the strength of the signal produced from the injected current will attenuate logarithmically.
The attenuation rate depends on the coating's conductance in contact with the ground per unit area of pipe and the frequency of the applied AC signal. If there are any coating holidays, the current will attenuate rapidly. The strength of the AC signal remaining on the pipeline is determined at discrete points along the pipeline; and standard mathematical formulas are used to calculate the rate of attenuation for each surveyed section.
What You'll Need
Survey equipment is composed of two main items:
1. A Transmitter
The transmitter is a signal generator conductively attached to the pipeline under survey and to a suitable remote ground. The transmitter injects AC current into the pipeline where an electromagnetic (EM) field is generated with a signal strength proportional to the applied current to the coated pipeline.
The transmitter used in an AC current attenuation survey needs to have have the following characteristics:
- The AC signal frequency must be stable.
- The AC output current must be stable.
- The signal strength must be sufficient to drive the signal a reasonable distance along a well coated pipeline (e.g., 8 kilometers [5 miles] in either direction from the point of signal injection, which depends on the quality of the coating).
2. A Receiver
The receiver is a unit that measures the radiated EM field's strength around the pipeline. Depending on this strength, current flowing through the pipeline is measured and current attenuation of the coated pipeline section is calculated. The receiver should have the following characteristics:
- The receiver must be capable of obtaining accurate readings in the appropriate range and minimize the effect of the magnetic field generated by AC transmission lines in close proximity to the survey area (if available).
- It should be equipped with filters to minimize the effect of interference.
- It must allow the proper recording, downloading and archiving of the collected data.
- Software used to analyze the data gathered in the field must be compatible with the receiver and capable of properly processing the data.
How To Perform an AC Attenuation Survey
Step One: Transmitter setup
Determine a suitable location where the signal transmitter could be connected to the pipeline under the survey. The transmitter could be connected through a CP test lead, through a surface-accessible pipeline feature such as a valve or crossover or at a CP source installation.
If the connections from a T/R unit to the pipeline, and the ground bed are used as a connection method, both cables should be physically disconnected from the T/R unit before being connected to the transmitter.
If you use a a remote temporary ground, it should be placed as far as possible from the pipeline under survey and at right angles to it. If possible, the distance should be more than 50 meters. Moreover, the ground electrical resistance should be kept as low as possible to allow for maximum transmitter output current.
After the transmitter is set up, measure and check the AC signal strength. It should be above the manufacturer’s recommendation. Otherwise, the transmitter's location should be changed until the manufacturer’s minimum AC signal strength is achieved.
Step Two: Receiver Setup
Next, the AC signal's strength may be measured along the pipeline. You must move at least 30 meters (100 feet) from the transmitter and measure the signal on the pipeline. This is done by passing the receiver over the pipeline until the instrument indicates the pipeline's centerline.
Approach the pipe at right angles to its longitudinal axis.
The AC signal current should be measured to ensure normal operation. If the transmitter is located at an electrical insulator, the receiver should measure approximately 100% of the applied signal. If the transmitter is located away from electrical isolating devices, and if the coating condition is similar in both directions, the receiver should measure approximately 50% of the applied signal.
And, if these readings are not obtained, you must determine why before the survey proceeds.
Step Three: Data Collection
The receiver must be positioned directly above the pipeline and, when a reliable reading is measured, record the signal current strength on the pipeline at that location. Several readings should be taken at each survey point until two readings are somehow identical. You can label this "survey point one."
If you measure fluctuating readings, this may suggest the presence of magnetic field distortion. In this case, you have a few options: take steps to repeat the reading, reposition the receiver or ignore it. However, if you encounter large variations in the current readings with a stable and constant depth, this may indicate an indication is nearby.
Next, take another survey point by moving the receiver along the pipeline at a known distance. Record another set of readings, similar manner to the first. You can call this "survey point two".
The distance between survey point one and survey point two may be designated as survey interval one. The attenuation for survey interval one is calculated by obtaining the difference in signal strength between survey points one and two; and the attenuation is expressed in millibels (mB) or other units, as defined by the manufacturer.
Repeat this process until the pipeline segment is surveyed and the current attenuation is determined for all pipeline segments. If the AC signal strength drops below the minimum level (established by the equipment manufacturer) during the survey, reposition the transmitter to complete the coating evaluation of the entire pipeline segment.
Dealing With Magnetic Field Distortion
It is common for surveyors to face a condition where a magnetic field distortion occurs. In this case, be cautious about the reason behind magnetic field distortion.
Magnetic field distortion could be caused by a coating fault, a pipe derivation branch, electrical connection or a metallic contact where a drop in the AC signal magnitude occurs. When the magnetic field distortion is produced by something besides those factors, the AC signal magnitude drops and then regains to the magnitude level it had before being distorted. (For more on this topic, see: AC Induced Corrosion of Buried Pipelines.)
How to Calculate AC Attenuation Survey Results
After collecting your data, you need to calculate your survey results. The data acquired during the survey should include:
- Current magnitude, in milliamperes (mA), at each survey point.
- Depth-to-pipeline centerline or depth of cover above the pipe.
- Distance between each survey point.
- Current attenuation for each survey section.
Survey results could be presented in several different formats, for example:
X-Y (Scatter) Graphs
These are useful with survey sections of varying lengths and in evaluating whether the EM field is distorted. However, this plot is not commonly used in assessing the coating condition for underground pipelines. This is shown in the graph below.
Because current attenuates logarithmically, plotting gain/loss on a log Y-axis provides a consistent survey report that allows you to compare different sections along the pipeline under evaluation. As an example, Figure 2 (below) presents current in mA (Line A) and the current logarithm (Line B).
Interpreting Line A may suggest a higher current attenuation at the beginning of the line (sections two and four), which is not true. When the data is plotted on a semi-logarithmic scale, it shows a constant current attenuation.
Current Attenuation Plots
In this plot, the current loss is represented in current attenuation in mB per unit length (mB/m or mB/ft) as shown in Figure 3 below. We can’t compare current attenuation rates for different transmitter frequencies in this plot, as the attenuation rate is related to the transmitter frequency used.
As shown in Figure 3 above, the x-axis represents the pipeline survey intervals in meters. It can be clearly shown that the pipeline was surveyed at different lengths. The y-axis represents the current attenuation in mB/m for each section. This plot has the advantage of comparing the coating quality for different sections of pipelines surveyed by comparing the current attenuation values. The sections having lower current attenuation values have a better coating quality than other sections with higher current attenuation values.
Coating Conductance Plots
This plot is done by processing attenuation data from Figure 3 in combination with pipeline diameter and wall thickness data. (For more on this topic, see: 8 Methods of Coating Thickness Measurement.)
The x-axis still represents the distance between each survey point. However, here, the y-axis represents the conductance value.
The only assumption adopted to present this plot is a uniform soil resistivity of 1000 Ω-cm. If different soil resistivities are found, a correction for the conductance value shall be made with the aid of the formulas in NACE TM0102-2002.
Coating conductance plot
How to Analyze Data From An AC Attenuation Survey
By analyzing the current attenuation plot, you can compare coating quality for different survey sections.
With reference to Figure 3 and Figure 4, you could clearly conclude the pipeline's coating quality—with a current attenuation value of five, seven and two millibels per meter—is worse than other pipeline sections. These same sections are the one with higher coating conductance values of 8079,15830 and 1293 µS/m2 .
Further assessment for the coating quality could be determined with the aid of NACE TM0102-2002 tables presented below, where the values of coating conductance for the surveyed pipelines are compared with values in NACE TM0102-2002 tables and an assessment for the coating quality is done.
Table of Specific Coating Conductance vs. Coating Quality for 1,000 Ω-cm Soil
For the above surveyed pipeline's coating conductance values, and the values in Table 1, we can conclude the section with 1293 µS/m2 has fair coating quality; however the other two sections—with coating conductance values of 15830 µS/m2 and 8079 µS/m2—have poor coating quality.
It should also be noted that AC attenuation surveys are considered a screening technique that compare coating quality between different sections of pipelines. If an area is assessed to have a poor coating quality, further analysis should be done using an Alternating Current Voltage Gradient (ACVG) or Direct Current Voltage Gradient (DCVG) technique.
Several above-ground survey techniques can assess coating condition for buried pipelines. One of these techniques is the AC current attenuation survey.
AC current attenuation surveys are considered a screening technique in order to compare the coating condition for several pipeline sections in a fraction of time compared to other survey techniques. The technique could be used to prioritize the sections in need of immediate attention, while other (more detailed) techniques—such as DCVG or ACVG techniques—could pinpoint, size and locate coating defects in sections with poor coating conditions.
A total of nine technologies have been evaluated:
- Cathodic Protection Close Interval Surveys.
- Direct Current Voltage Gradient Surveys.
- Alternating Current Voltage Gradient Surveys.
- Alternating Current Attenuation Surveys.
- Electromagnetic Acoustic Transducer In-line Inspections.
- Cathodic Protection Current Mapper In-line Inspections.
- Metal Loss In-line Inspections.
- Visual Examinations.
- High-Voltage Holiday Detections.
All of these technologies (except number eight) involve electrical, electromagnetic, and/or ultrasonic measurements that indirectly indicate the existence of coating damage.
Cathodically protected structures receive electrical current through the environment surrounding the structure. The collection of electrical current along a linear structure (such as a pipeline) results in an axial current profile that depends on the current density the structure receives—which depends on the external coating system's performance. When we record current measurements at regular intervals, the generalized coating performance is checked by examining the rate at which electrical current is collected along the pipeline. (For more on this topic, see: Troubleshooting Cathodic Protection Systems and Function Systems.)
An Alternating Current Attenuation Survey is basically the application of a unique AC signal to energize the pipeline. This unique signal is easier to detect than the cathodic protection current (which is DC), and the attenuation characteristics of AC current is more strongly affected by coating performance than DC current—making this more sensitive than a DC current attenuation survey, Alternating Current Attenuation Surveys are not meant for locating areas of coating damage; they are instead done to screen pipeline regions (lengths of 150 to 300 feet) for generalized coating performance variation. It is usually performed in conjunction with ACVG.