This paper highlights the coordinated AC mitigation and cathodic protection strategy for a 167 mile (269 km) long high-pressure interstate natural gas pipeline. Ninety-five percent of the pipeline is located adjacent and parallel to high voltage AC power transmission lines ranging to 500 kV. Analyses and computer simulations during the front-end engineering design predicted that, without suitable mitigation, steady-state AC potentials (referenced to local earth) on the pipeline would easily exceed 100 V; AC coating stress and touch potentials approaching 12,000 V were predicted during a power line to ground fault. The AC mitigation and cathodic protection designs were closely coordinated with operations personnel to assure systems that were maintenance-friendly. Corrosion control commissioning included a multi-channel AC and DC close interval potential survey protocol. The baseline data document the success of the AC mitigation and cathodic protection in safely and cost effectively controlling soil corrosion and AC interference. Long-term surveillance includes state-of-the-art remote monitoring and the use of coupon technology.
The subject 167 mile (269 Km) long pipeline parallels and crosses various overhead 3-phase high voltage AC power transmission lines with phase-to-ground voltages ranging from 115 to 500 KV. Four power companies are involved. The limits of each power line circuit along with their steady-state line currents and maximum estimated fault currents are shown in Table 1. The photograph in Figure 1 shows some of the power lines along a portion of the right-of-way. The lateral separation distance between the pipeline and the closest power line varies from approximately 75 to 150 feet (23 to 46 m). The pipeline crosses under one or more of the power lines twenty-eight times. There were no power line phase transpositions.
Recognizing the complexity of the right-of-way relative to electrical interference, the pipeline owner authorized an engineering evaluation of the anticipated AC effects as part of the project design. This included estimating the AC impacts on the pipeline without mitigation and developing a mitigation design strategy to safely and reliably operate the pipeline without excessive pipe potentials or unnecessary concern regarding AC corrosion. The AC evaluation and mitigation design were effectively coordinated with the cathodic protection and other aspects of the design to assure a total systems approach. Key aspects to the success of the design included routine information exchange between all parties involved: the pipeline owner's project manager, the pipeline designer, the AC mitigation and corrosion control engineer, the pipeline corrosion control staff, and the four power companies. This was particularly critical when pipe alignment changes were necessary during the design and as the construction of the pipeline got underway.
Soil resistivity data were collected along the pipeline right-of-way at nominal 1 mile (1.6 Km) intervals and at power line crossings. The Wenner 4-pin measurement technique (ASTM G57) was used at each test location to determine apparent soil resistivities at different depths ranging to 100 feet (30 m). The soil resistivity data along with the relative geometry between the pipeline and the different power lines.