Analyses and computer simulations during the front-end engineering design of a 167-mile long high-pressure interstate natural gas pipeline predicted that, without suitable mitigation, steady-state AC potentials (referenced to local earth) on the pipeline would easily exceed 100 v. Similarly, the analysis predicted AC coating stress and touch potentials approaching 12,000 v predicted during a power-line-to-ground fault.
The first part of this series (OGJ June 21, 2010, p. 60) discussed mitigation design parameters and results of a network model based on those parameters. The remaining aspects of engineering, presented here, will detail the coordinated design, construction, and commissioning of an AC mitigation and cathodic-protection system.
Coordinated design
Given their overlap in form and function, closely coordinating the AC mitigation and cathodic-protection designs ensures adequate electrical safety and effective corrosion control. The AC mitigation system consists primarily of one steel-cored zinc ribbon placed in the pipe trench adjacent to the pipeline for essentially its entire length.
Zinc ribbon was not installed through the 11 relatively long horizontally directionally drilled (HDD) pipe sections totaling about 5.5 miles and comprising roughly 3% of the pipeline. Four separate sections of the pipeline totaling 12.7 miles required a second zinc ribbon on the opposite side of the trench. Including the second ribbon addressed those few areas where the network analyses predicted pipe potentials above the established upper tolerance limits when only one zinc ribbon was stimulated.
The zinc ribbon used for the AC mitigation connects to the pipeline at cathodic-protection test stations through electronic DC-decouplers. The DC-decouplers will conduct AC steady-state and AC fault current but will block normal levels of DC, which is important in minimizing cathodic-protection current demand. The test stations and DC-decouplers were spaced at nominal 1-mile intervals along the pipeline (typically at road crossings) and at both ends of each HDD section.
AC network simulations showed connections from the parallel mitigation conductor to the pipe every mile or so would be insufficient to reduce pipe potentials effectively during a possible power line fault. As such, between most test stations, at nominal 2,000-3,000 ft intervals, buried galvanic decouplers acted as connections.
Galvanic decouplers function similarly to zinc grounding cells, conducting both AC and DC current to some degree with the magnitude dependent on their electrical impedance. The DC current through the galvanic decouplers also depends on the DC voltage across the device, which is influenced by the cathodic protection. Selection of direct buried and directly connected galvanic decouplers instead of more expensive electronic DC-decouplers eliminated an excessive amount of equipment either aboveground or belowground and the associated additional maintenance and monitoring.
AC mitigation designs often specify pipe-to-mitigation conductor connection intervals of 1,000 ft or less, many times unnecessarily. One advantage of a rigorous network analysis is the ability reliably to predict conditions at different connection intervals, thereby optimizing design and keeping capital costs and future maintenance and monitoring to a minimum. The closer spaced connections are needed primarily for effective distribution of AC current during a possible power line fault.
Another vital function of sufficient predictive modeling is determining how effectively to simulate conditions through HDD sections and find a suitable mitigation scheme when construction may preclude traditional parallel mitigation in open-trench applications. With the many advances in HDD technology and the increasing restrictions on open-trench construction created by wetlands, congested areas, etc., some projects have HDD methods comprising more than 30% of their total lengths.
Other AC safety measures for the pipeline included:
• Prefabricated equipotential gradient control mats at all test stations and around piping at the mainline valves, meter stations, and launcher-receiver areas, to minimize personnel step-and-touch voltages when working in these areas, particularly in the event of a power line fault.
• Dead-front style corrosion control test stations, where there is no normally exposed metallic connection in electrical contact with the pipeline, so that personnel would not be subject to possibly unsafe pipe voltages under steady-state power line operations and in the event of a power line fault.
• Electrical grounding and electrical continuity measures for fencing around various aboveground pipe.
• Nonmetallic casing vents.
The cathodic protection design used impressed current and was based on pipeline company standards. Including a slight increase in the design current requirement accounted for the possible increase in current demand because of resistive connections between the pipe and parallel mitigation conductors through the galvanic decouplers.
The design analysis resulted in 15 rectifiers and associated deep anode groundbeds. Providing each rectifier with a remote monitoring unit (RMU) allows operating parameters and status to be determined via the internet. The design included using magnesium anodes for cathodic protection and AC mitigation grounding for small runs of buried piping at four meter stations.
All corrosion control monitoring test stations included a buried reference electrode. Certain test stations also included current measuring pipe spans and buried steel coupons. Pipe current span test stations facilitate measurement of both AC and DC current in the pipe, which is particularly important for baseline documentation, stray current testing, and troubleshooting. Using Ohm's Law, the current in the pipewall is determined by measuring the AC and DC voltage drop across the pipe span. The current is then calculated by dividing the voltage drop by a field-measured impedance and DC resistance of the span.
The steel coupons assist in determining polarized "instant-off" DC pipe-to-soil potentials. They also measure AC current densities, one metric in determining AC corrosion's effects.
Construction
AC mitigation system installation went well and was closely coordinated with other aspects of pipeline construction. Extensive specialized additional materials, equipment, and installation requirements, and working near high-voltage power lines, yielded a number of lessons:
• The added costs of the AC mitigation system installation and commissioning are best defined early in the design and budgeting process.
• AC mitigation measures should be adequately conveyed during the pipeline permitting process.
• Job-site safety and installation training-orientation for inspection and construction personnel are essential and should be done at the onset of work and periodically throughout construction as a refresher, particularly when personnel change.
• The mitigation system design process should identify special construction procedures-equipment not normally needed, e.g., while parallel mitigation conductors are often designed to be placed at the bottom of the pipe trench. This can require special trench shoring requirements for safety and OSHA compliance. Placing mitigation at a shallower depth may be just as effective (as determined through comprehensive network analyses and design engineering) and not require special measures.
• Maintaining adequate physical and electrical separation between the parallel mitigation and pipe is essential.
• When test station posts are also used as pipeline markers, installation procedures must place them directly over the pipe.
• Given the somewhat extensive amount of additional wiring and other hardware associated with the test stations to facilitate effective monitoring of AC effects, installation of aboveground wiring and terminations is best performed by experienced personnel.
System commissioning
Total cathodic-protection current for the entire pipeline at start-up was 86 amp, or an average cathodic-protection current demand of 16 μamp/sq ft of pipe surface, neglecting the influence of the AC mitigation equipment connected to the pipeline. The actual cathodic-protection current demand for the pipeline alone is less. The low cathodic protection current demand shows quality installation of the pipe coating, the cathodic protection system, and the AC mitigation system.
An AC and DC close interval potential survey occurred several weeks after continuous operation of the cathodic protection system (Fig. 1). Five channels of simultaneous data collection were logged every 2.5 ft while crossing the pipeline rights-of-way:
• Over-the-line DC pipe-to-soil potential.
• "Left" DC side drain potential.
• "Right" DC side drain potential.
• "Left" AC side drain potential.
• "Right" AC side drain potential.
DC pipe-to-soil potential data provides close-spaced documentation on cathodic protection performance and regulatory requirements relating to pipe potentials. As with direct-current voltage gradient (DCVG) and ACVG survey techniques, side drain potential measurements provide insight regarding coating quality. No coating irregularities were detected.
Unlike ACVG (and DCVG) procedures requiring an external source of current directly connected to the pipe, electromagnetic induction resulting from operation of overhead power lines causes measured AC side drain potentials. Analyzing the AC earth voltage gradient measurements (side drains) coupled with soil resistivity measurements can determine a relative index for identifying pipe sections possibly experiencing higher AC current densities. Side-drain potentials are measured between a copper/copper-sulfate reference electrode over the pipe and a copper/copper-sulfate reference electrode 10 ft on either side of the pipe.
Measurement of a planned sixth channel of data, over-the-line AC pipe-to-soil potential, did not occur accurately because of considerable electromagnetic interference on the data collection system. Induced AC affected the trailing wire connecting the datalogger system to the test station. The AC voltage induced in the wire masked the AC pipe-to-soil potential.
While electronic filtering of the other channels of data resolved the influence of otherwise erroneous induced AC on these channels, filtering of the over-the-line AC pipe-to-soil potential would have precluded an accurate measurement of this parameter between test stations. Experience shows AC over-the-line pipe-to-soil potentials can be accurately measured under many conditions some distance from a test station in the presence of overhead high-voltage AC power lines. Primary influencing factors include soil surface moisture and soil resistivity.
Normal variations in AC current through the power line conductors, separation distance, and the AC potential induced onto the pipe appear less important. Appropriate electrical safety precautions should be employed when making measurements at test stations and particularly when extending test wires away from the test station, because of the possibly high and unsafe induced AC voltage on the wire.
Technicians directly measured AC pipe-to-soil potential at the test stations. The algebraic sum of the maximum measured pipe-to-soil potential at the upstream or downstream test station, and the maximum AC side drain potential between the test stations, provided an estimate of the maximum AC pipe-to-soil potential halfway between test stations. Fig. 2 shows the AC pipe-to-soil potential vs. distance profile.
Fig. 2 also shows the AC potential profile predicted from design analysis. Good agreement generally exists between actual potentials and predicted potentials. Most measured AC potentials are <5 v, showing proper functioning of the AC mitigation system.
Maximum AC potentials of 10 and 13 v occur near mileposts MP-23 and MP-117 respectively. Maximum potentials typically occur near where power lines enter or leave the pipeline right-of-way. These potentials and all others stated in this article are referenced to a copper/copper sulfate electrode.
Fig. 3 shows an AC pipe-to-soil potential versus time trace obtained using a battery-powered digital datalogger at one of the test stations. The potentials vary cyclically over time depending on power line current demand. For this particular pipeline and measurement location, the maximum potentials occur during the late afternoon and early evening hours when the power line current is the greatest. Similar pipe potential measurements on other pipelines show a similar cyclic pattern, but with maximum potentials sometimes occurring late at night and into the early morning hours (~10 p.m. to 7 a.m.).
Monitoring AC potentials must occur over a sufficient time to capture the range in values. In one instance, potentials measured during the day were below the generally accepted 15-v safe upper limit; at night it was above this limit. Had dataloggers not been used, the condition would have remained undetected and incorrect conclusions regarding the significance of the AC interference may have occurred.
Fig. 4 normalizes AC side drain potential measurements accounting for variation in soil resistivity. The higher normalized potentials show a greater influence of AC current. These data along with the other AC and DC measurements establish a good baseline and aid in identifying and prioritizing pipeline sections potentially more susceptible to AC corrosion.
Maximum normalized AC side drain potential occurs near milepost MP-82.5 and is in good agreement with the maximum AC current measured through the DC-decoupler at this location. This condition is related to the pipeline leaving the power line rights-of-way at MP-80 then returning at MP-82.5, with the combined divergence and convergence over a relatively short distance as the primary cause. This area also has lower soil resistivities when compared to adjacent areas, likely resulting in a lower resistance-to-earth of the parallel mitigation system and contributing to the higher AC current through the DC-decoupler (55 amp).
Fig. 5 summarizes AC current densities. The heavier line shows measured AC current densities using the 20 coupon test stations. Coupon current densities range from less than 1 amp/sq m to 14 amp/sq m (MP-0, insulating flange at end of pipeline). AC current densities less than 20-30 amp/sq m are typically of no corrosion consequence.1 2
The lighter line in Fig. 5 shows theoretically calculated AC current densities along the entire pipeline, assuming no mitigation. Since the pipeline includes effective AC mitigation measures, actual AC current densities will likely be less than the calculated values, as coupon data infer.
Ohm's Law provides the means for deriving theoretical current densities based on the AC pipe-to-soil potential, the soil resistivity in the area of the measurement, and an assumed circular coating defect (holiday) having an exposed pipe area of 1 sq cm. The calculation involved is one common method for estimating AC current density and the influence of this current on pipe corrosion rates.3
While the calculated values are worst case, they help identify and prioritize sections along the pipeline potentially more susceptible to AC corrosion. Typically, for a given coating quality, these sections correspond to sites having higher AC potentials in lower resistivity soils.
Fig. 6 summarizes the AC current measured through the electronic DC-decouplers at each test station between the pipe and the AC mitigation system. Maximum measured current equaled 55 amp at MP-82.5. Most currents measured less than 10 amp. The algebraic sum of the AC current through the individual DC-decouplers is about 820 amp.
AC current through the DC-decoupler illustrates proper functioning of the AC mitigation system. Magnitudes and distribution of current along the entire pipeline reinforce the need for suitable electrical safety precautions, particularly for aboveground facilities such as mainline valves, meter stations, and the cathodic protection test stations.
Fig. 7 summarizes DC pipe-to-soil potential measurements based on the close interval survey data. Also shown are coupon "instant-off" potentials measured at the 20 test stations along the pipeline with coupons. Momentarily disconnecting each coupon from the pipe connection at the test station and measuring the potential of the coupon obtained these data.
Between rectifiers, operating ("on") DC pipe-to-soil potentials measured –1.8 to –1.2 v. Peak values greater than –2 v occurred near the rectifiers. All rectifiers were operating at the time of the measurements. Baseline DC data demonstrate proper operation of the cathodic-protection system and an effective level of corrosion protection and compliance with applicable corrosion control sections of 49 CFR Part 192.4
Stray current interference tests to determine the effect of the pipeline cathodic protection current on power line tower grounds and nearby generating-substation facilities occurred as part of cathodic protection commissioning. No adverse conditions were detected.
References
1. PRCI Project PR-262-9809 Report, "Cathodic Protection Considerations for Pipelines with AC Mitigation Facilities," pp. 3-25, Pipeline Research Council International Inc., Houston, 1999.
2. European Standard CEN/TS 15280:2006, "Evaluation of A.C. Corrosion Likelihood of Buried Pipelines—Application to Cathodically Protected Pipelines," 2006.
3. PRCI Project PR-262-9809 Report, "Cathodic Protection Considerations for Pipelines with AC Mitigation Facilities," p. 21. Pipeline Research Council International Inc., Houston, 1999.
4. Pipeline and Hazardous Materials Safety Administration, US Department of Transportation, US Title 49 CFR Part 192—"Transportation of Natural Gas by Pipeline: Minimum Federal Safety Standards," 2005.
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