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.