Cathodic Protection (CP) is applied to buried steel pipelines to complement the protection provided by pipe coating. If the coating is damaged causing holidays, or if there is an electrical contact between the pipeline and another metal structure, it is necessary to increase the CP current to compensate for the resulting losses. Note, this is not achievable using Sacrificial Anode Cathodic Protection (SACP).
Excessive current can be detrimental to the pipe. The engineer responsible for the pipeline integrity must therefore achieve a fine balance between effective protection and acceptable current consumption.
If the current becomes excessive, the cathodic protection ineffective or a new pipeline has been constructed and the coating quality needs to be assessed, then the pipeline must be surveyed in an endeavour to locate all types of major faults.
The Pipe Current Mapper (PCM) has been specifically designed to perform this function, and is covered under ANSI/NACE SP0502 “Pipeline External Corrosion Direct Assessment Methodology“.
Table of Contents
1 Shortcomings Of Other Techniques
A brief description of the shortcomings of commonly used traditional methods of surveying for pipeline current faults are detailed below :
1.1 The Line Drop Method
The potential gradient between two access points is measured and the current derived through calculation. This method is lengthy, complicated, costly and presents a significant risk of error in measurements and calculation.
1.2 The Pearson Method
Two electrodes are placed on the ground and used to detect the fault current. This method is personnel intensive and impractical in very dry soil conditions; it does not measure current losses, so cannot differentiate between minor and major faults
1.3 The Current Gradient Method
A traditional pipe locator is used to detect an ac current (typically 1 kHz) applied to the pipeline. Large signal losses are assumed to be due to large faults. In fact the losses are also due to capacitance losses along the pipeline, so faults are very often masked by the natural decay of the signal. This current does not exactly mirror the CP dc current. The effect can be particularly misleading at pipeline branches. Capacitance losses also limit the length of pipeline that can be surveyed in one operation.
2 Benefits of Using The PCM Equipment
The PCM has been designed for the pipeline industry with support from Gas Industry leaders. It addresses problems associated with traditional methods of pipeline surveying by providing the following benefits:
An accurate method of evaluating and mapping cathodic protection current losses.
Current magnitude and direction measurements are made above surface without the need for connections to the ground or pipe.
Near dc signal mirrors closely the CP current.
For use in all weather and ground conditions.
One person operation.
Indication of fault magnitude, hence avoiding unnecessary and costly excavations.
High-powered transmitter enables analysis from one injection point to the next.
Transmitter can be powered from 220V or batteries and is therefore flexible
Integral data logging.
USB interface allows uploading of data for graphical representation of records on site.
An A-Frame (ACVG) accessory for accurately pinpointing and sizing coating defects
3 General Operation and Equipment
The PCM has been designed to overcome the shortcomings of the existing techniques and now provides pipeline engineers with an accurate, cost-effective product that can be used in all weather and ground conditions. An extremely low, “near dc” frequency (4 Hz) is used to mirror as closely as possible the dc current generated by the cathodic protection. It also avoids signal losses caused by capacitance so nearly all the signal loss is due to coating faults or shorts to other structures. Conventional pipe locators cannot practically detect such a low frequency, so the receiver is fitted with a precision, high performance magnetometer. This detects and measures the 4 Hz magnetic field. Advanced signal processing techniques filter and amplify the signal, so that push button current measurements (magnitude and direction) of the “near dc” signal can be made. Integral data logging functions enable the storage of this data so that current loss versus distance can be plotted
Figure 1 : Typical PCM Transmitter Setup at TRU
The PCM system’s high-powered transmitter makes long range signal detection possible up to 15 km. Therefore, fewer hookups are required. The transmitters flexible power requirements enable it to utilize the variety of supplies available. The transmitter has been designed to be compatible with both distribution and transmission networks. The hand-held receiver unit first locates the pipeline, even in heavily congested areas, and then provides the operator with a measurement of current magnitude and direction of the near dc signal as applied by the system’s transmitter.
The receiver makes the required calculations and displays the results at a single button press. The measurement can be stored, with a further button press, in the integral catalogue. This provides the operator with a method that quickly troubleshoots the CP system by pinpointing metallic contacts and locating areas of coating defects, as detailed in Figure 2 below. Additional cable location modes also allow the unit to be used as a traditional cable and pipe locator.
Figure 2 : PCM Location of coating fault
PCM Attachment (magfoot) contains the magnetometer (mag foot) that detects the near DC mapping current. The PCM magnetometer foot is switched on when the PCM current key is pressed to take a PCM measurement. When the PCM foot is fitted to the PCM receiver there will be a confirming “beep” from the loudspeaker. The PCM magfoot also stores the pipeline current mapping results in the built in data logger, and it must be attached to the receiver to upload results to PC
Careful use of the PCM will give the location of coating faults to within just a few metres. The addition of the A-Frame attachment enables the operator to pinpoint the fault to within a few centimetres, keeping excavation costs to a minimum.
A signal entering the ground is detected through the A-Frame spikes (ACVG) and amplified by the receiver electronics. An arrow on the display points to the direction of the fault, enabling quick and easy pinpointing as detailed in Figure 3 below.
Figure 3 : A-Frame used to pin-point coating defect
The PCM is provided with a simple to operate, Windows based upload software package. The “Icon” operated software enables the uploading of the PCM data and automatic conversion to a text format. This is compatible with commercially available spreadsheet software packages. A typical example relating to the pipe graphics is illustrated below in figure 4.
Figure 4 : Example of data output.
4 Surveying Methodology
The PCM surveying methodology is detailed below :
The PCM transmitter (PCM Tx) is set up as follows. Connect the output leads as follows: WHITE to pipeline and GREEN to anode groundbed.
Select Mapping frequency: ELF was selected in order to ensure that the current and direction can be mapped.
Connect the PCM Tx to the battery or ac power source.
Switch on PCM Tx transmitter.
Set current output level.
Slide on and connect PCM Magnetometer foot
Check Battery level, replace if necessary.
Set to same mapping frequency as transmitter.
Set to Peak mode.
Good local knowledge, a map and a notepad are essential to obtaining the correct information.
Locate the pipeline and move at least 15m from the transmitter.
Use the receiver and verify that the PEAK and NULL positions are within 15 cm
Obtain depth reading. A sensible reading indicates a site free from interference.
Press mapping key to obtain PCM current reading. Press the key to save result and note log number on notepad and map.
Use the GPS to select next mapping location, or continue along the pipeline for predetermined distance. 50 – 100m are adequate.
Select Null mode and trace pipeline to next mapping location.
Repeat procedures 1 – 6 until pipeline survey is complete.
Use Accessory A Frame (ACVG) to pinpoint fault
Upload results to PC, and prepare graph of current against distance.
Categorise areas for excavation and repair (if required).
5 Interpretation of PCM Site Data
The AC signal current applied to a coated pipeline in perfect condition, will be lost by capacitance as distance increases from the point where the signal is applied. The resulting graph drawn using A/mA will have an exponential slope, because there is a greater current loss rate closes to the transmitter.
The PCM uses near DC signal (4Hz) for current measurement, so capacitative losses are minimal.
Example; There was a coated pipeline which had three defects of equal electrical resistance, one near the beginning, the next in the middle and one at the end. If the current loss graph is prepared using mA on the vertical scale the second and third faults would be shown as progressively smaller steps in the graph. This is due to Ohms law, and the loss of current at the previous fault.
We are launching courses in Pipeline Design and Business & Project Development soon. Sign up and will notify you.