Collocation discontinuities also factor into the risk. Locations where the pipeline enters or deviates from the transmission line right-of-way have an increased risk for AC-induced corrosion due to disruptions in the electromagnetic field caused by the changing route orientation.
The transmission line structure can also provide clues as to its likelihood of causing interference. Lines supported by large, steel structures are likely to have higher steady‑state current, whereas small, wooden structures, which commonly support distribution circuits, generally have lower steady-state current. Circuits with one or multiple (bundled) large conductors associated with each phase also tend to have higher steady-state currents. When observing the tower structure, note the size and length of the insulators between the tower and phase conductor. Longer insulators generally translate to higher voltage circuits. One way to roughly estimate transmission line voltage is to multiply the number of insulator disks by 15.
Pipeline Characteristics
Several characteristics of the pipeline can influence the degree of AC interference.
High-quality pipeline coatings are among the main variables that contribute to elevated AC voltages because of the coatings’ inability to self-ground. Most of the induced voltage is retained by the pipeline because of the absence of coating defects. Therefore, old coal tar-coated pipelines typically experience fewer AC interference issues than superior fusion-bonded epoxy-coated lines.
The presence of pipeline insulators can have a beneficial or adverse effect on the degree of AC interference. Electrically isolating segments of pipe can reduce AC voltages (due to a reduction in continuous, parallel pipe length); however, locations where electrical isolation exists can also experience a spike in AC voltage because the induced current is unable to flow past the insulator and attenuate farther down the pipeline, causing a charge buildup.
Factors as simple as diameter and depth of the pipeline can make a difference. As pipeline depth and diameter increase, the magnitude of AC interference is reduced, assuming all other variables remain consistent.
Coupon Test Stations
Test stations used to collect AC voltage measurements can provide valuable insight regarding the degree of interference. In accordance with the standard from NACE International on mitigation of AC and lightning effects on metallic structures (SP0177-2014), a steady-state touch voltage of 15 VAC or more with respect to local ground is considered a shock hazard, and the installation of AC grounding systems is necessary.
After determining that the pipeline is safe to work around, one can focus on the corrosion risks. When current is induced onto the pipeline and begins to flow longitudinally, it is going to look for a path to return to its source. The path of least resistance is most likely through a pipeline coating defect, and the subsequent discharge results in accelerated corrosion. The probability of AC-induced corrosion can be predicted based on current density levels. NACE International provides the following guidelines: