Utility uses the pipe-in-pipe strategy to extend the lifetime of its substations.
Electric utility maintenance requires vigilant attention to the condition of equipment, from the smallest wires on transformers to the tall towers carrying transmission lines. In addition to constant exposure to weather conditions and vegetation encroachment, maintenance professionals must consider the durability and longevity of equipment and its material components. When facilities are constructed, a maintenance plan is put in place to ensure that the investment is protected for at least 40 years.
For Georgia Transmission Corp. (GTC), which plans, builds and maintains the high-voltage power infrastructure for 39 of Georgia's 42 electric membership cooperatives, this requires periodic inspections, regularly scheduled maintenance and upgrades to keep the system at peak performance to deliver safe, reliable electric power. This diligence has helped GTC achieve an impeccable reliability record.
In some cases, though, electric grids and reliability are affected by the unexpected. The utility recently turned its thorough approach toward maintenance to a different kind of infrastructure: the web of storm drains underneath bulk transmission substations. In the process, collaboration between two companies led to groundbreaking innovation, all without actually breaking ground.
Sinkhole Brings Aging Infrastructure to Light
GTC oversees roughly 3,000 miles of transmission lines and 600 substations, including upwards of 60 stations that bear the electric grid's bulk load. Many of these bulk-load-serving stations are part of aging infrastructure built decades ago. Beneath the complex network of the electric grid lies an equally complex network of storm drainage pipes. While the substations are continuously monitored and receive regular maintenance, the tens of thousands of feet of storm drain infrastructure beneath them does not.
Evidence in some recent studies indicates that corrugated metal pipes realistically have a lifespan less than the 40 to 50 years expected at the time of installation. Instead, current estimates suggest the lifespan is more likely about 20 years, a milestone for a significant portion of the infrastructure that has already come and gone.
The implications of the compromised stability of some of these pipes became clear when a maintenance team arrived at one of GTC's major substations. A deteriorating corrugated pipe collapsed underneath the wheels of the vehicle and caused a sinkhole.
In order to repair the pipe, heavy equipment was brought in to excavate and replace the damaged section. That's no simple task when near energized substation equipment. Extensive and invasive repair procedures presented safety concerns that had to be addressed, from electrical working clearance issues to deep excavation of drainage structures in close proximity to critical substation equipment.
With thousands of feet of pipe surrounding and running under other crucial substations, GTC considered how widespread the problem might be and how to address it.
Facing the Challenge of Repair and Replacement
Pipe failures introduced safety concerns beyond sinkholes. Collapsed pipes can lead to blockages that result in substation flooding. Though the equipment is grounded, standing water is a definite hazard that threatens system reliability.
The substation design's civil engineering group took on the task of identifying a solution to the problem of deteriorating storm drains. The fix for the initial collapse incident — excavation and replacement — may not work in every instance. Some affected pipes lie under large equipment, breakers and energized transformers. Excavation equipment needed to remove and replace the damaged pipe could experience clearance issues with parts of the substation and initiate soil settling that would affect substation foundations. Deep trenches where piping lay would require reinforcement to prevent collapse.
To resolve these issues using conventional means, a substation would have to be taken offline, and those that support Georgia's 230-kV and 500-kV ranges cannot be taken out of service without careful planning to mitigate major risk to reliability, especially when excavation, repairs and using the appropriate precautionary measures could take several weeks.
With these challenges, the team considered the possibility of abandoning the original infrastructure in places where deterioration had occurred and rerouting with new pipes. That, too, came with a list of concerns, including the possibility of having to regrade the property to ensure proper drainage. In addition to causing certain engineering problems, both excavation and replacement or abandonment and rerouting came with a hefty price tag.
Innovative Technique Allows Trenchless Repair
The most efficient and cost-effective solution would be one that allowed the utility to repair the pipe without excavation. Through a consultant to the utility, GTC was introduced to cured-in-place pipe (CIPP) liner. This trenchless method of pipe repair forms a pipe inside a pipe to repair the damaged section.
After reviewing a presentation on the technology that had never been used on behalf of an electric utility, GTC engaged Southeast Pipe Survey to make the repairs. The Patterson, Georgia-based company specializes in maintaining and repairing sewer and water lines with services ranging from inspection, line cleaning, rehabilitation and preventive strategies.
Since 1985, Southeast Pipe Survey has employed the CIPP technique in hundreds of thousands of linear feet of piping on behalf of municipal utilities and the private sector. For the first GTC project, Southeast Pipe installed 640 linear feet of cured-in-place lining and two catch basins in the Adamsville, Georgia, substation. Georgia Transmission is the first electric utility for which Southeast Pipe Survey has provided CIPP services.
With CIPP, a custom-made flexible sleeve of polyethylene mat and fiberglass strand is created and saturated with a thermosetting resin coating. Prior to installation, a camera with a closed-circuit video feed is inserted into the pipe to verify the diameter, length, the exact condition of the pipe and active service connections and their locations. The pipe is cleaned to remove debris and any protruding taps.
During installation, an overhead rig is used to insert the inverted tube into the damaged pipe through manhole access. A boiler truck is used to heat the pipe and hot water is forced through it, turning the inverted sleeve right side out. The flexible material easily navigates bends in the pipe.
A consistent circulated flow of 180°F cures the resin. Slowly, the water is cooled to a temperature of 100°F and the patch hardens, restoring the structural integrity of the pipe. The liner fully cures in about eight hours.
Success and Ongoing Infrastructure Restoration
CIPP resolved many of the issues presented by invasive excavation and replacement. First, CIPP offered a safer way to address pipe repair that eliminates the need for heavy equipment or extensive digging that can unsettle substation foundations. The overhead rig used during installation maintains a safe distance from power lines and other sensitive equipment, and minimizes or eliminates environmental concerns. Before starting the project, GTC collaborated with Southeast Pipe Survey to ensure all safety concerns were identified and a detailed plan was in place.
Use of CIPP represents a significant cost savings, as well. In the first project Southeast Pipe Survey undertook for GTC, pipe repairs cost 27% less than the estimated cost of pipe excavation and replacement. Those costs did not include necessary grading and replacement of the ground grid and cable trays or the gravel dressing that would need to be replaced after repair and grading.
Ultimately, the cost savings could have been as much as 50%. In the four projects Southeast Pipe Survey completed, GTC has benefited from an estimated $500,000 in cost savings. For a cooperative like GTC, cost savings for the utility translates into cost savings for its members and their customers.
CIPP also minimizes the risk to reliability. In Adamsville, work was performed during a planned substation shutdown. To protect system reliability, repairs had to be completed before the station was re-energized. By preparing a thorough plan, the repairs were made successfully within the allotted time. Most CIPP installations can be completed in a day.
Lastly, CIPP reinforces the structural integrity of the pipe for about 50 years. While the liner reduces the diameter of the pipe by about ⅛ inch, its smooth interior increases the water flow in comparison to corrugated metal.
Georgia Transmission's mission is to deliver safe, reliable and affordable electric power to its members. In the case of the CIPP liner repairs, GTC and partner Southeast Pipe Survey were able to reinforce that by reinforcing the integrity of the underlying infrastructure without a threat to reliability and at a significant cost savings.
To date, Southeast Pipe Survey has installed more than 10,500 linear feet of cured-in-place pipe in storm drains at three substations for GTC. Southeast Pipe Survey is now using a new technology that uses ultraviolet light to cure the liner. This new method provides an even faster installation and a better, stronger finished product. It is being used at the fourth substation project currently underway and repairs will continue as GTC identifies the need.
Chip Buttrill (email@example.com), a design services manager for GTC, manages the engineering department responsible for transmission line, substation and civil site designs. Buttrill is a professional engineer.
Randy Wise (firstname.lastname@example.org), a senior civil engineer for GTC, leads the civil site design group in developing and implementing design standards and project designs, conducting quality reviews of project designs, and investigating process improvement opportunities with new technologies. Wise is a professional engineer.
Georgia Transmission Corp. | www.gatrans.com
Southeast Pipe Survey | www.southeastpipe.com