Recommended practice for ECDA exhibits some confusion.
In its ECDA tool selection matrix, the RP 0502-2002 illustrates confusion as it considers six CP/Coating Survey Methods--Close Interval Survey, DCVG, ACVG, Pearson, Electromagnetic and AC Current Attenuation--and presents a distorted view of the practical application of the methods.
First, there is confusion between AC and DC methods and between CP evaluation techniques and coating fault delineation methods. Within the AC techniques there is a lack of realization that all AC techniques use the Pearson Technique in various forms to delineate fault locations. For example, the ACVG has the two ground-contacting electrodes mounted on an A frame instead of two surveyors, and uses a meter indication instead of an audible signal. It is the Pearson Technique, but in a different format. All AC techniques are relatively useless in city streets and when in the locality of overhead power lines and many pipelines are paralleled by high voltage lines in a common right-of-way.
Electromagnetic techniques cannot be used where pulsing DC is being used from CIPS or DCVG surveys. Similar comments apply to Electromagnetic Soil Resistivity Measurements. Also, all EM techniques lose discrimination when soil resistivity is high, e.g. greater than 100,000 ohm cm.
There exists a major problem with all AC and EM techniques which is that their data has no direct relationship with the external corrosion control techniques applied to a buried pipeline. Therefore, data obtained by these techniques cannot be specifically correlated with the historical records and the ongoing operation and control of the pipeline's cathodic protection system.
The best available combination is two separate but compatible techniques that have a direct correlation with the data from the operation of the pipeline's CP system. The two techniques are:
* CIPS to monitor the pipeline's CP profile and interference.
* DCVG to locate coating faults.
The RP identifies that DCVG is not applicable to some areas such as city streets and river crossings. This is a total distortion of the scientific facts.
Another distortion is the confusion between the two CIPS variations (Lateral and Trailing) and true (Mulvany) DCVG. In Europe, the lateral CIPS technique has been used for more than 25 years. It is called the Intensive Method and is more prevalent in areas under German technical influence. In the last two to three years some equipment purveyors have fraudulently called this technique combined CIPS/DCVG where the lateral CIPS is thought to be DCVG. Lateral CIPS is not the same as the analog (Mulvany) DCVG technique. Even worse, the recommended lateral half-cell is at a distance of only two to three meters from the pipeline.
Lateral gradients stretch many meters depending on soil resistivity and the CP current flowing to individual faults. Two meters distance would represent only a fraction of the total gradient to remote earth so any lateral data is useless for analysis for the Direct Assessment step in ECDA. Even worse, in the UK the use of one half-cell trailing behind the other at a distance of several meters is also called combined CIPS/DCVG. Both of these variations are very poor representations of the correct methodology which consists of conventional CIPS used in conjunction with analog (Mulvany) DCVG.
The NACE External Corrosion Direct Assessment Specification RP2002--by calling for two survey techniques--is incomplete. To provide the best information for subsequent analysis, four complementary techniques are suggested, including analog DCVG, close-interval pipe to soil potential survey, soil information (including resistivity) and sub-meter accuracy differential geographic positioning system (DGPS) information.
1. Analog DCVG is required to accurately locate and assess coating faults. This technique was chosen because of its simplicity and undisputed accuracy at locating and determining the characteristics of coating faults. Analog DCVG has no attachment to the pipeline and should not be confused with Lateral CIPS or Trailing CIPS. Lateral CIPS does not provide the same data as analog DCVG.
2. Close Interval Pipe to Soil Potential Survey, to assess the pipeline's cathodic protection system and DC Interference. In this case, CIPS equipment is modified to operate at the DCVG ON/OFF sequence of 0.45 seconds ON, 0.8 seconds OFF so the two techniques--DCVG and CIPS--can be run as a one-pass survey.
3. Soil information including resistivity to assess the soil corrosivity at coating fault locations. Because of this relationship, soil resistivity should be mandatory in any ECDA study.
4. Sub-meter accuracy DGPS for coating fault location and distance measurement. Accurate distance measurement is, in fact, the most difficult parameter to record.
The quality of the data collected will depend not only on surveyor training but also on the type and quality of the survey equipment. Not all survey equipment is easy to set up and use. A number of CIPS units use modifications of laptop computers or generally used data loggers. These usually have limited memory and/or battery life and require a certain competence in using computers and are often of limited capability of synchronizing with satellite interrupters. Ideally, for field surveying, equipment should be simple to set up and use.
Significant differences also exist among analog DCVG units. Several manufacturers have designed their equipment with a press button automatic return of the needle to the center zero position. This type of circuit design limits the flexibility of the instrument in complex pipeline networks and also prevents determination of the corrosion status. Variations also exist in instruments.
For example, on the 10 mV range, all the push-button center instruments are calibrated + or - 10 mV about the center rest position. The manual bias instruments on their 10 mV range are calibrated + or - 5 mV about the center rest position, making such instruments two times as sensitive. This means they can operate at lower pipeline DCVG signal strengths or survey at greater pipe depths. This has proved correct in comparison field trials.
Attempts have been made to produce DCVG instruments with a digital display instead of an analog meter. The problem with digital instruments is that the response indicator picks up all fluctuations in voltage noise from the rectifier, making it very difficult for the surveyor to be certain of what the instrument is indicating, particularly at low voltage ranges.
A variety of GPS equipment has been used, with the best being the Trimble Pro XRS or equivalent. These instruments are expensive. Therefore, cheap, less accurate handheld units often are employed. Unfortunately, these are really insufficient for accurate data comparison work. Older techniques -such as wire dispensers--are worse at measuring distance.
The only cost-effective way to measure soil resistivity of a pipeline right-of-way is by using electromagnetic techniques to obtain a continuous profile at rates up to 15 miles per day, see Figure 1. However, as with all soil resistivity-measuring methods, the CP must not be pulsing. Therefore, an EM survey has to be run as a stand-alone technique, logging coating fault and right-of-way features into the EM data logger together with DGPS locations.
[FIGURE 1 OMITTED]
Another problem also has to be recognized. The survey route has to be plotted to the side of the pipeline and not the actual pipeline trench location itself. Otherwise, the resistivity of the pipeline steel can significantly dominate the data.
Future ECDA articles will deal with data quality, analysis, excavations and post-assessment.
Summary Of Type Of Survey Data Collected By The Four Recommended Techniques.
The following information can be collected by the DCVG Technique:
* Fault location to within a 15-cm circle.
* Fault %IR severity. This is related to the physical size of the fault but soil pH effects can modify this relationship, see Figure 2.
* Fault corrosivity factor. A new factor currently under development and related to active corrosion site prediction. (Initial studies gave an 83% correct prediction).
* Fault corrosion status (net current flow to or from a coating fault which is a NACE Criteria for Protection). Determines if a fault is receiving adequate CP for protection, see Figure 3.
* Individual coating fault CP current demand, see Figure 4.
* Fault approximate shape and orientation on the pipeline.
* Attenuation of the cathodic protection from rate of DCVG signal decay.
* Effective range of a rectifier influence.
* Determine % efficiency of insulation of gaskets, separation of casing and any other foreign structure on the pipeline right-of-way.
[FIGURES 2-4 OMITTED]
The CIPS technique can collect the following information:
* CIPS ON Potential, uncorrected and corrected for attenuation step.
* CIPS OFF Potential, uncorrected and corrected for attenuation step.
* Large coating fault indication by CIPS (Misses all small faults (below 20%IR).
* Effective range of CP by potential decay.
* Weak areas of CP when ON and OFF come together.
* Interference effects from AC and other DC Sources and structures.
* Soil composition voltage variations due to changes in soil chemistry.
The soil monitoring techniques can collect the following information:
* Soil resistivity measurements, see Figure 5
* Change in soil type, (clay to sand)
* Composition and moisture content
* Soil pH at fault locations
* Rock or stones present in soil (major source of damage to all coatings), and
* Location of vegetation at coating faults (major source of failure in some coatings).
[FIGURE 5 OMITTED]
The sub-meter DGPS can collect the following information:
* Location of all survey data points, pipeline features and right-of-way furniture
* Terrain altitude
* Distance for all survey techniques including ILI tools
* Time of day of measurement
* Date of measurement, and
* Cross-reference information for different types of survey data.
The following information can be collected from historical records:
* Past survey data typically CIPS, Pearson, CP Records, inline inspection records, AC and DC interferences. Most data will not be coating fault specific with generally poor distance measurement.
* Past history of pipeline operation, excavations and leak or third-party interference reports.
J.M. Leeds, Ph.D., Pipeline Integrity Management Ltd, Wigan, UK
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||cost-effective way to measure soil resistivity of a pipeline|
|Comment:||Recommended practice for ECDA exhibits some confusion.(cost-effective way to measure soil resistivity of a pipeline)|
|Publication:||Pipeline & Gas Journal|
|Date:||Jun 1, 2006|
|Previous Article:||Three tools illuminate engine performance analysis.|
|Next Article:||Pipelines and light rail transit: a delicate balance: keeping stray-current corrosion at bay.|