AC Interference: The Unseen Problem

Metallic objects in the vicinity of electric transmission circuits may become energized as a result of the magnetic fields created by the current flow.  AC interference can affect metallic structures such as pipelines or railroads.  This can be a safety hazard to personnel who come in contact, can damage the pipeline coating, or can cause corrosion of the pipeline itself.  Lethal voltages may be present on metallic structures during typical operation of overhead transmission circuits, or may be caused during abnormal conditions such as a transmission circuit fault or lightning strike.  Pipeline coatings may disbond or be damaged due to conductive coupling.  AC corrosion is the phenomenon of metal wall loss at a coating holiday on a coated pipeline in low resistivity soil.  This is a great concern to operators as the integrity of the pipeline can be severely impacted.  Above ground piping locations such as meter stations, valve sites, and test stations are places where personnel are most likely to be in contact with the pipeline.  These areas need to be studied to ensure touch and step voltages are not above IEEE Standard 80 allowable safe levels of permissible body current.

Modeling Software: Engineered Approach

EIU uses Safe Engineering Services CDEGS modeling software to build a computer model of the 3-Dimensional pipeline and transmission circuit layout.   The model calculates steady state induced voltages and AC current densities along the pipeline.  Simulated transmission circuit faults are calculated along the pipeline to evaluate coating stress voltages and to ensure touch and step voltages around exposed appurtenances are safe.  Soil resistivity data is measured and multilayer soil models are used in the calculations of induced and conductive interference effects.  EIU will design a mitigation system to achieve the operator’s specifications in terms of induced buried/above grade touch voltage, current density, coating stress voltage, and preferred materials.

AC Mitigation Design

EIU will design a cost effective mitigation system that ensures all of the AC interference effects are reduced to safe levels.  This is done by strategically grounding the pipeline in specific areas.  This tailored approach requires less materials, reduced cost of installation, reduced system maintenance and record keeping.  By controlling where AC voltage and current are removed from the pipeline the system can be optimized for the operator.  EIU will supply a full set of construction drawings and specify materials for installation.  Typical grounding systems may consist of bare linear grounding conductor and dc decoupling devices, ground loops, and gradient control mats.

AC Mitigation Installation

EIU can supply the required materials and install the mitigation system as shown in the construction drawings.  The process can include land owner notifications, permitting, utility locating, potholing, plowing, trenching, coating inspection, repair, backfilling, and restoration.  Daily construction records are kept along with submeter GPS survey data to create a complete set of construction as builts.

Electrical Safety Inspectors

EIU can provide electrical safety inspection services for all contractors working on pipeline projects.  Piping being strung out can easily have over 1,000 Volts of AC capacitive charge if not properly discharged.  Parked vehicles, construction equipment arcing, overhead circuits, pipeline tie-ins, and substations are all safety hazards that should be actively monitored while construction is underway.

AC Mitigation Inspectors

EIU can provide trained mitigation installation inspectors to perform or verify proper installation of the entire mitigation system.  We can also ensure proper excavation techniques, exothermic weld connections to pipelines, coating repairs, DC decoupler wiring, and final tie-ins.

Testing and Commissioning

EIU can provide final testing and commissioning of the completed mitigation system to verify proper installation and operation of the system.   The entire pipeline will be measured and recorded with the mitigation system disconnected and connected and then compared to the design limits, and AC interference model results.

Site Survey

EIU will perform a site survey on every project to gather GPS information, pipeline locations, facility locations, dimensions, above ground piping layout, insulating joints, existing mitigation systems, cathodic protection test station locations, AC / DC readings, foreign crossings, and constructability issues.  We will take photos to document these features.  We will perform nondestructive soil resistivity tests at pre-approved locations and communicate daily with a designee of the operator.  We will also gather transmission circuit tower locations, circuit configuration, substation information, and operator information for power data collection.

Additional field services

AC-CIS, DCVG, coating inspection, surface installation of test stations, and RMU installations.