Case Study 1: The Classical Interference Problem.
Corrosion Engineering International, LLC was called upon in 2016 to provide a series of tests to evaluate abnormal conditions that were reported to one gas distribution operator regarding a crossing with a foreign line. These conditions centered upon a crossing of a pair of high-pressure natural gas pipelines owned by two separate operators taking custody at a shared regulator station from a transmission operator in the region. Corrosion Engineering International, LLC was tasked by Operator A to provide insight as to the cause of the abnormal condition reported to Operator B. Our objective was to validate the findings reported to Operator B and ensure any negative effects attributable to the presence of Operator A’s Cathodic Protection devices were properly mitigated.
Corrosion Engineering International, LLC was able to identify a classic interference condition with negative impacts demonstrated upon Operator B. This was found to be created by the close proximity of the two pipelines and deteriorated coatings present on Operator A’s assets in the vicinity of the crossing. A series of prototype solutions were evaluated for effectiveness in accordance with industry best practices as supported by materials published through NACE International. The most appropriate solutions involved recoating several thousand feet of steel pipeline and possibly implementing a ground bed for the stray current drain. However, a more expedient measure was sought in the meanwhile to limit the continued negative effects known to exist. With a “Time is of the essence” mentality, CEI developed a prototype solution utilizing a temporary bond installed between adjacent test points on each system and a diode to control current direction.
The provided configuration demonstrates a traditional solution with increased cathodic protection current through non-conventional means. The diode, in this case, acted as a half wave rectifier acting upon a natively present alternating current (AC) induced upon Operator B’s line. The solution arrived upon directed a portion of the AC signal previously discharged through the coating to instead divert to ground through the bond to Operator A. The observed effect was a more electronegative shift in the average potential for Operator B, as well as a reduction in AC discharge observed throughout the line. The impact to Operator A owned assets was shown to be negligible as a result.
It was the recommendation of Corrosion Engineering International, LLC that the bond solution outlined above be pursued through the installation of a set of test leads at the crossing, which was not previously installed. The 010 test point owned by Operator B was located directly over the crossing of the two lines which was intended to host the new leads and bond solution outlined herein. The bond condition will stand as a temporary solution as Operator A has scheduled a partial pipeline replacement of the area in question early in 2017. Following this installation, Operator B’s line requires a full inspection with an interrupt to ensure the resulting status does not elevate protection levels into ranges encouraging hydrogen evolution. Such a condition is cautioned as it could prove detrimental to the Fusion Bonded Epoxy coating type employed.
Case Study 2: Custom Design of a Protection System for a Renewed High-Pressure Main
CEI was the primary design driver for a new 12” High pressure system in 2016. The evaluation and design of this system experienced several complications with considerations for configuration of the system, selecting an appropriate Protection system, AC mitigation, DC interference inspection, and the addition of tertiary components added to the protection system at the request of our client.
The result of this design was realized in 2017 when the ICCP Rectifier was energized. The construction cost was estimated to be half that required for the several galvanic systems that would have been required otherwise. Additionally, the current required to protect the line fell within 4% of the estimated current, with 180 mA found to sufficiently protect the line. Additionally, the design resulted in effective mitigation of AC potentials in an inductance corridor and the bonus inclusion of some regulator station piping in our protection strategy.
In 2016 CEI began redesigning one of our clients CP systems as the direct result of our investigation efforts of an entirely separate line. At the time CEI was actively engaged in inspecting a system which was observed to have difficulty in achieving protection. The line was a part of an aging series of line sections that had undergone replacement on portions of the pipeline the previous years. In our efforts to evaluate the line we had determined that an abnormally high AC current existed on the line which had gone unnoticed as the majority of which was draining through portions of the line with poor coating and the impressed current transformer-rectifier, respectively.
As we were providing our observations and recommendations to the operator of this line, it was made known that the corrosion department responsible for the adjacent section upstream was also experiencing some unique AC problems. We were then asked to inspect the adjacent line to provide our insight as to the source and solution to the AC phenomena observed. As we conducted our initial records inspection of the line we realized that the adjacent system had also undergone a substantial replacement of approximately 45% of the existing line. The line was identified as a new 12” .375 wall line with Fusion Bond Epoxy (FBE) type coating. New test points were installed, including new test stations that replaced test points and corresponding lines in the system we were previously working.
Our field inspection yielded several interesting points. First, the portions of the line now replaced with FBE were demonstrated to have exceptionally high AC potentials. The maximum AC voltages observed on this line were double that shown on the downstream section and exceeding the “Safe Touch” potential of 15 Vac. This was not observed on the portion of the line section that had not been renewed; suggesting more of the poor coating conditions as previously observed on pipeline of this vintage.
Figure 1: AC Profile
To complicate matters, the line section was shown to fail protection criteria with subsequent increases in protective current. Potentials on the new line were shown to rise accordingly, though resulting in potentials known to damage FBE type coating systems. This demonstrated a distinct imbalance between the two portions of the same line in terms of resistance to ground. The existing impressed current system for this line was also at or near it’s maximum rated output, and so more current was as infeasible as it was impractical and damaging for the new coating system.
We returned to our client to recommend that the line section be divided into two parts, effectively isolating the existing pipeline from the newer line for a slew of benefits. The measure would prevent an active galvanic cell from forming should the existing impressed system be disabled. Additionally, it reduced the overall protection current required for the aged line as half of the system could now be removed. The new line could now be protected independently, and likely with a rather low demand in terms of protective current. Through our subsequent testing phase, we determined the specific properties of the line and prototyped protection and mitigation strategies.
With regards to the AC properties, we found that the AC could safely be drained to the retired 12” main that now existed parallel to the live main. To arrive at this solution, we systematically tested drain points to establish the best location and ground structure along the line by charting AC current drain points to existing test points along the route. The highest discharged currents were then tested in test configurations by retesting each point with a ground strategy employed. We found that the AC was best mitigated through an AC drain placed at the midpoint of the inductance corridor attached to the retired main that had been bypassed.
A direct bond to this structure was not suitable as it would greatly increase the current requirements for the structure. Fortunately, the two properties can be treated differently through the use of a capacitor in the circuit. A capacitance of approximately 1mF or better was shown to have the desired effect, reducing the AC potentials to well below any hazard conditions. This was prototyped by wiring a 1 mF capacitor into a test station with a bond to the retired line. We then retested the line with this device in place to show that both AC potentials were sufficiently reduced while leaving polarized potentials in the desired range for protection.
Designs were then constructed to install a Solid State Decoupler (SSD) at the test point previously prototyped. Commercially available devices of this type were specified as they exhibit those properties necessary to achieve the necessary AC impedance and resulting current. They have the added benefit of providing an over-protection feature to prevent potentials to exceed those noted before as potentially damaging to the coating type. Not to mention, such devices were readily available to the client as they stock the devices for similar applications. The device could be mounted directly to the fink-type test station previously referenced.
CP System Design
The low current requirement of the system was in a range expected to be suitable for a passive, or galvanic, protection system. We set out to evaluate the area by collecting soil resistance measurements in the area using the Wenner 4-pin method and evaluating layer conditions using the Barnes’ Layer Analysis. Our analysis of the area determined that the soil resistance was exceptionally high, as was common in our experience in the area. The impact to the design was that it would require up to 4 separate galvanic groundbeds in order to achieve the desired output. Alternatively, we chose to submit a design for an impressed current system; whereby we were able to achieve the desired output with a minimal installation and only 4 anodes placed vertically.
Figure 2: CP Design Strategy
We then began evaluating the potential to include some under protected station piping, at the request of our client. The design for the ground bed coupled with the modest current requirement meant that a significant amount of capacity would be left on the standard model rectifier in use. Coincidentally, the regulator station selected as the location for the ground bed was recently repaired, resulting in some station piping being isolated from its previous CP source(s). It was therefore convenient to add these short lengths of pipeline to the protection system. We knew that we could balance the circuit and control current direction and magnitude to each respective line.
Figure 3: CP Balancing Model
However, similar to our concerns for electrically joining the new FBE line with older sections, we had to take precautions when adding these new lines to the CP system. Firstly, we had to evaluate the resistance to ground of each of the lines individually. From this we could build a theoretical model for the systems to operate, thereby balancing the resistance differential of the systems to balance the current distribution to each respective system. Additionally, we needed a way to control direction of current as the new steel would inherently be more active than the aged steel at the station. This would ensure galvanic cells do not form if the CP system were to go offline for any period of time, as well as prevent recirculating currents in an interrupt survey. Lastly, we needed to know the empirical current demand for each system individually. Current demand tests were then conducted on each line individually to establish current demand and resistance properties.
As we were conducting this battery of tests we noted some abnormal results. The resistance to ground for each of these lines was abnormally low. More so, the current demand for each line was shown to be exceptionally high. We began applying additional test methods to better understand the properties we were observing. Through application of coating holiday tests, such as ACVG and DCVG we noted some rather sizeable indications centered upon the valves inside and immediately outside of the yard.
Figure 4: AVCG Indications
The DCVG portion of the test became difficult to interpret. With the complex routing of the various lines the direction of gradients became difficult to map underground direction. We altered our test method slightly to allow a more agnostic view of the lines. Using an adjacent CP system rectifier, we applied a test current with a fast interrupt period. To reference current direction, we placed a remote reference well outside of the gradient which we determined by evaluating for remote earth condition. We then took measurements in a grid fashion about the station yard. The result was interpreted by magnitude of shift at each location within the yard. This was visualized by creating a contour of those shifts in a 3d format.
Figure 5: DCVG Surface Gradients
Figure 6: Results of CP Surface Gradient Indications
The information that this contour map provided allowed for a prioritization of the holidays previously indicated that could then be pursued. Based on this prioritization, several locations of the station were scheduled for excavation. The result was the discovery of bare sections of pipe and fittings within the station, resulting in lower resistance to ground and elevated current demands. These areas were repaired with wax tape coatings and backfilled.
The result of this test sequence was the addition of one of the three steel lines in the station to the circuit. One of the lines in the station was found to be shorted through its isolating joint to the adjacent CP system. The current requirement of this system made it infeasible to protect with the new system, and so it was recommended to evaluate it as part of the adjacent line section. The remaining 6” line transitioned to plastic at the adjacent road and increased the current load by only 60mA. This line was balanced against the high-pressure line by way of a 1ohm resistor, as determined in the circuit model of the two lines. The two lines could then be isolated on the DC, or time domain, with the insertion of a simple diode. The diode would act to isolate the two lines and prevent recirculating currents in the event of current interruption such as an unplanned power outage.
Commissioning of the CP system:
Following the installation of the CP components, we resumed work on this line to commission the new protection system and ensure its effective implementation. In our evaluation of the system we conducted both an interrupted survey of the line as well as depolarization survey in order to assess effective polarization levels on the line. In this sequence we realized that the line would not pass the desired criteria. To complicate matters, the line was found in some areas to exhibit “native” potentials more electronegative than the desired -900mV CSE criteria to be employed.
These points led us to evaluate the line for foreign or unintentionally applied protective currents. The locality of the highest “native” potentials coincided with some repairs in asphalt over the line. This led us to believe that a short to a service might be to blame as an intermediate pressure distribution line routes in parallel to the high-pressure line; servicing the houses in this neighborhood. We set out to test this hypothesis by way of some indirect inspection techniques: namely Current Mapping and Close Interval Survey. Both tests gave negative indications for anodes or foreign contacts.
Figure 7: CIS Results: In Search of Anodes/Shorts
As observed in our results, there was not definitive indication for an anode despite the variations in the profile. Of note: the peaks observed with the ON-OFF survey did not align to peaks observed in the Native profile. Should a directly connected exist there, we would expect to see the potentials spike in both profiles. Additionally, the ON-OFF would have demonstrated a lower resistance to ground with the increased current traveling to the effective coating defect. In the shown results, no major drops in the calculated resistance to ground (Rvg) were observed.
Next, we evaluated the line for indications of stray current. By systematically testing the line and sequencing adjacent or geographically close rectifiers, we were able to identify interference effects on the line from three separate rectifiers. Of the three rectifiers, the closest rectifier had the most impact as it influenced the native potentials on the line and demonstrated a discharge condition at the most cathodic points on the line.
We then set out to evaluate the effects of the three systems and determine if a new mitigation strategy was warranted. In order to observe a true depolarization, we shut off all 4 CP systems aforementioned for a period of 48 hours and resurveyed. Similarly, we energized all systems for a second 48-hour period. The line was resurveyed with synchronous interrupters installed at each of the implicated rectifiers. The outcome of this survey demonstrated the line to be effectively protected with sufficient polarized potentials observed in a fully interrupted condition.
Figure 8: Satisfactory Results
This sequence represents the best cross section for complex designs. Many times, as field technicians we find that conditions change in the progress towards protecting our assets from corrosion. Recognizing the process is dynamic allows for adaptability in the analysis.
What started above as an AC evaluation led to a total redesign of the protective measures on a new system. Through subsequent tests, the line could be mitigated of its AC problems and tertiary systems could be added in close proximity. Finally, when all pieces arrive in place the outcome is not as expected. By applying the same open observations and corrosion principles, new conditions can be identified to allow for proper interpretation of these results.
Case Study 3: An Interference Condition Resolved by Isolating Transient Short on Foreign Line.
In 2015, CEI was in the process of conducting a series of routine inspections of Cathodic Protection (CP) Systems onsite at one of our clients’ facilities. During the course of this survey, one point was found to be significantly below the protection criteria standardized by NACE International, and significantly worse than previous years. The system was referred by our field testing personnel to our senior staff for diagnosis.
The system historically performed well, though something was recently different. This line section was expected to pass as the current demand of the line was typically less than 500mA provided by way of an Impressed Current Cathodic Protection Transformer-Rectifier (Rectifier) unit. To confirm the issue, we began by repeating the last survey and ensuring to evaluate every point on the line. A singular point was found to fail once again; this time more significantly than previously observed.
The point in question was known to cross a foreign line, though it had no history of poor performance and was geographically close to the Rectifier installed on this system. No work had been performed in the area since the previous year, and the operator of the foreign line had not made any changes to its CP or protected structure. By all accounts, there were no deliberate changes to the two systems.
Upon further inspection of the design and installation of the crossing facilities it was found that an interference current drain was originally installed at this location, comprised of a string of magnesium anodes. However, the age and use of the drain facility made it ineffective as the anodes had been consumed or passivated; effectively beyond their lifespan. The test point also contained a lead for the foreign line. A casing was known to have been installed on the foreign line at the crossing, though only one lead was present at the crossing test point. The indication from client records suggested that the casing was the point represented.
The area was inspected again using standard Structure-to-Electrolyte Potential measurement against a known reference to confirm the issue. On that date, the tested structure was found to be worse than any other day tested previously. A Close Interval Survey was conducted in the immediate proximity of the crossing to evaluate the severity and extents of the phenomenon. The affected area was shown to be dramatically pronounced directly over the crossing.
Figure 1: Confirmation through CIS
The operator of the foreign line was contacted to coordinate cooperative testing in order to identify the source of the problem. Access to the foreign operator’s Two Impressed Current Sources was granted and information was provided as to the location of test points on the foreign line. Both of those Impressed Current Sources were interrupted with our client’s CP source while we re-evaluated the area. The foreign sources were interrupted synchronously while our client’s Rectifier was left interrupting with a faster, overlapping interrupt sequence. The results indicated that a classic interference discharge condition was present.
Figure 2: Interrupt Visualization
Both of the Foreign Impressed Current systems were traced on the lines to identify their area of influence on both systems. The results of which indicated that the geographically closer of the two sources to the crossing had the majority of influence, while the more distant was shown to attenuate before reaching the point in question. In the case of the affecting CP source: the signal was shown to carry a significant portion on our client’s line and discharging at the crossing.
At this point we had demonstrated that the interference condition did exist. Additionally, we were able to show the net effect of the condition was not negatively affecting the line while the systems were energized. This was evident in the observation that a full interrupt of both parties demonstrated an instant-off reading more electronegative than -850 mV CSE, as prescribed in NACE SP0169-2013. This, however, did not satisfy the “How” of the situation. A case could also be made that a discharge condition was still possible should the domestic Rectifier go offline for any period of time.
Close Interval Potential Survey (CIS or CIPS) was applied to the foreign line through the crossing. Through this inspection, an epicenter was determined to exist at the crossing of the two lines. Of note here: we were able to positively ID the test lead in the crossing test station to be common with the foreign line. As we noted previously, the fact that this lead represented the casing led to the conclusion that the casing on the foreign line was shorted to the main and required remediation by the foreign operator.
Figure 3: CIS of Foreign Line with Domestic Overlay
As the described condition was unfavorable to the foreign pipeline operator, an excavation team was dispatched quickly to investigate the issue. At the time the cause of the shorted casing was unknown. For this reason, it was not certain whether the condition could be remedied. While excavating the location, it was revealed that a test station previously existed in this vicinity. It was made known by the excavation team that they would seek to re-install the test station, at the very least, to ensure they could perform future tests.
CEI offered a temporary solution to our clients and the foreign operator that might alleviate the interference issue between the two parties. Within the limited timeline, an interference mitigating groundbed was designed to allow interference currents to discharge from one party to the other without damaging either party. The model called for six magnesium anodes placed in close proximity. Three of the anodes were placed in a line, side by side, with their leads spliced to a singular wire to be routed to the domestic operator’s existing test station. The other three anodes were then placed side by side and in parallel to the other three with their leads spliced and routed to the foreign test point that was to be reinstated.
The design was executed on the day of the excavation while the excavation team was daylighting the casing. As the outcome of the day was still undetermined, the decision to execute the interference mitigation was made with the anodes placed horizontally, parallel to the foreign casing. The first string was laid with 4’ of un-compacted backfill on top. A layer of rockshield was laid in this layer to prevent any future shorts to the two anode strings that would effectively result in a short or continuous bond between the two structures. The remaining three anodes were then laid on top in the same configuration with their wires spliced and routed opposite of the first string.
Figure 4: Grounding Cell Solution
Once the anodes were laid, the excavation crew resumed their work, searching for the end of the casing. As the crew worked back, they noted some leads in the soil. These leads were expected to represent the structure leads previously lost in the down test station. The leads were tested at that point and were noted to be electrically isolated form the casing. A retest of all of the structures represented revealed that all potentials had returned to normal: the interference condition and short to the casing had both cleared.
Upon further inspection the short to the foreign casing had been the result of the test leads shorting against the casing. The leads were noted to have been slightly worn and the copper conductors exposed through the insulating jackets. The worn areas were determined to align with the area they had rested against the casing. As a result, the leads were coming in contact with the casing, though not in any permanent way. This explained the transient nature of the short and the varying degrees of severity noted from day to day.
The test leads were refurbished, and the previous test point was established on the foreign main. The interference mitigation strategy was left in as a precaution, should an interference problem present at another time in the future. The leads to each anode string was terminated at each test point, though left disconnected from the respective structures. Subsequent tests on both lines following backfill showed that all conditions had been resolved.
Case Study 4: The Evaluation of Cathodic Protection Effectiveness
Corrosion Engineering International, LLC was requested in 2014 to evaluate the effectiveness of a Cathodic Protection system protecting a 12” coated steel pipeline, which supplied high-pressure natural gas to various regulator stations feeding the surrounding metropolitan area. Corrosion Engineering International, LLC was tasked to identify the cause of the ailing cathodic protection system and provide design recommendations to meet criteria established in Code of Federal Regulations; Title 49 Part 192 Subpart I.
Upon review of client historical information, CEI personnel proceeded to conduct a baseline interrupted survey to determine the present cathodic protection system condition. This initial evaluation confirmed the established cathodic protection system boundaries, rectifier current and voltage output, and pipeline polarization at various test station locations. The field data indicated that 90% of the pipeline polarization was below required criteria with several test locations near native steel potentials. By comparing theoretical current calculations and rectifier current output, Corrosion Engineering International determined the rectifier current output would normally be sufficient to protect a 12” pipeline with less than 10% coating loss. However, the present polarization indicated the system had greater than anticipated coating loss or direct contact with a foreign structure. CEI personnel performed a Pipeline Current Mapping test to evaluate CP current distribution and Alternating Current Voltage Gradient to evaluate the coating condition. The combined test data indicated that two large coating holidays were present within 50 feet of the anode bed which was causing an ineffective current distribution across the entire pipeline.
Corrosion Engineering International recommended a direct examination of the coating deficiencies conducted and any coating repairs performed to improve the coating condition. The direct examination identified two uncoated 12” pipeline tees, which supplies natural gas to a regulator station, were the culprit. The pipeline tees were coated, which caused the cathodic protection current distribution to normalize, and all but a few locations now meet the required criteria. However, the platinum impressed current anode bed was also determined to be near depletion and required replacement.
Corrosion Engineering International was directed to provide a cathodic protection design for the replacement anode bed with design calculations, anode bed specification, installation drawings, and anode bed placement. While evaluating the present location, CEI determined soil resistivities would cause unnecessarily high installation costs due to the presence of rock and be ineffective in providing the required current output to protect 100% of the 12” natural gas pipeline. CEI personnel evaluated various locations along the pipeline and determined that three viable locations were suitable for the new impressed current mix metal oxide anode bed. Corrosion Engineering International provided the necessary design calculations, anode bed specifications, installation drawings, with a recommendation for two anode bed placement locations for improved current distribution.