It was necessary to retrofit induced AC mitigation on a newly installed 30 inch natural gas pipeline due to an inadequate pre-construction study. This paper presents details of the study and of the novel mitigation approach taken.
Mitigation of induced AC voltages on pipelines paralleling high voltage alternating current (HVAC) power lines is a serious matter and is not to be taken lightly. A Texas pipeline company proposed to build a 9.8 mile 30" natural gas pipeline to supply fuel gas to a merchant power plant (MPP) under construction. Approximately two miles of the proposed pipeline would parallel a dual 345kV power line circuit supported on galvanized steel lattice work towers. The dual shield wires are bonded to each structure.
The gas pipeline company retained a consulting firm to evaluate AC induction on the proposed parallel pipeline. The "Induction Analysis Study" basically said that AC induction from steady state or fault conditions was a problem. No mitigation grounding scheme was proposed. The recommendations consisted essentially of:
1. Advise the power company of the pipeline activity on their right-of-way (R/W).
2. All personnel should use eleven inch low voltage lineman's rubber gloves protected with lineman's leather gauntlet gloves.
3. Measure the line voltage and ground the pipeline if it exceeds fifteen volts. Or if pipe potential exceeded fifteen volts AC, employ ground mats.
4. Ground pipe up on skids at necessary intervals to reduce the voltage to fifteen volts. = Curtail work if an electrical storm is in the area and the supervisor in charge feels it poses an additional hazard.
Based on the assumptions and recommendations contained in the report, the pipeline was constructed in the spring and early summer of 1999 during the worst drought conditions encountered in that part of Texas in the prior twenty years. This was fortunate since the dry soil conditions greatly increased the tolerable steady state step and touch voltages. The pipeline was constructed without the benefit of any form of electrical grounding. We understand that some workers did complain of the discomfort of electrical shock during construction of the pipeline. However, there were no injuries reported. If the pipeline had been construction under wet conditions typical of early spring in Texas, the consequences would have been much more serious.
INVESTIGATION
After the pipeline was constructed and placed into operation, the pipeline operator contacted ELK Engineering Associates, Inc. (ELK) to perform an induced AC mitigation study and make recommendations to control the observed detrimental induced AC effects on the pipeline. After we gathered all necessary field data and contacted the power company for the appropriate electrical parameters, Electro Sciences, Inc. constructed a mathematical model of the R/W and exercised their proprietary AC induction program to predict the peak voltages under maximum line loading conditions. This computer run determined that steady state voltages as high as 160 VAC could be experienced on the pipeline under emergency level load current conditions. Peaks as high as 40 VAC would occur under normal daily load profile conditions.
Computer modeling various mitigation schemes clearly showed that installation of a single zinc ribbon mitigation wire would have provided effective grounding of the pipeline in the very low electrical resistivity (800 ohm/centimeter) soil conditions. However, since the pipeline was already constructed, this was no longer an economically viable alternative. It also was necessary to install supplemental grounding at the meter station serving the MPP. Stray AC flowing in the pip
