TVA tackles below-grade steel corrosion with a robust inspection, mitigation and repair program.
Tennessee Valley Authority operates and maintains one of the largest single-owner transmission systems in the United States, with 15,900 circuit miles (255,886 km) of line and over 102,200 structures, of which more than 40% are steel. Although once thought to be maintenance-free, steel has proven that it does indeed require inspection, maintenance and repair. Investigation of below-grade corrosion is one key to supporting the safety and reliability of steel transmission structures.
In 2004, Tennessee Valley Authority (TVA) initiated a project with the Electric Power Research Institute (EPRI) to evaluate a new device designed to assess the condition of below-grade steel without excavation. After being tested with the new device, 140 towers received a full excavation to determine the accuracy of this new technology.
TVA's excavation revealed that one-third of the 140 towers had heavy corrosion and were in need of repair — a far worse condition than the device had indicated. To ensure the findings were not an isolated anomaly, an additional 100 towers were selected for inspection. Again, one-third of those structures were found in need of repair due to heavy corrosion.
With one-third of its towers in need of repair, and little faith in detection devices, TVA began investigating more traditional options for inspection and repair. Predictive inspections, which include a partial excavation of each tower to a depth of 12 inches to 18 inches (305 mm to 457 mm) on one side of each leg were considered. This partial inspection methodology would allow TVA to identify the “worst of the worst” in a cost-effective manner; however, it was not a long-term solution.
TVA determined that a comprehensive inspection program with full excavation of each tower to a depth of 18 inches to 24 inches (457 mm to 607 mm) on all sides of each leg provided the greatest long-term benefit. Not only would full excavation allow TVA to ascertain accurate tower conditions, it also would present the opportunity to apply protective coatings and mitigate corrosion to extend the useful service lives of these high-dollar assets.
Using a rating system based on specific predetermined criteria and a corrosive soil map, TVA began assessing the condition of each of its steel towers. At the onset of the program, the oldest towers were inspected first, but a transition was made as it became clear that inspecting the most system-critical towers first would have a greater impact on system reliability.
Inspection and Repair
Towers are prioritized for inspection based on the importance of the line. Factors that determine line importance include the load it serves, the number of customers served by the line and the line's overall importance to the system for bulk power transmission. Other factors such as the impact on nuclear facilities also are considered. Once selected for inspection, towers — whether direct buried or secured by concrete foundation in soil or water — are inspected and evaluated individually.
A visual inspection is performed to detect any above-ground defects and potential safety concerns. The potential for corrosion of direct-buried members is determined through a below-grade visual assessment and measurement to determine actual cross-section loss. Each leg is excavated to a depth of 18 inches to 24 inches becasue most damage occurs between the ground line and 24 inches below grade. During excavation, several indirect tests are performed on the surrounding soil including structure-to-soil potential measurements, soil resistivity, soil pH and soil oxygen content (redox), all of which serve as environmental indicators of the likelihood and severity of corrosion.
Once excavated, each leg is cleaned, carefully inspected for corrosion and measured to identify areas of deterioration and evidence of mechanical damage. Structures are then categorized according to the specific corrosion rating system. Tower legs that are in Condition A (good condition) and Condition B (fair condition) receive a protective coating to mitigate further corrosion, and proper grade is returned by backfilling. Tower legs in Condition C (needing repair) and Condition D (requiring immediate attention) are repaired on a second pass. Following the repair, these tower legs also receive protective coatings.
Once a tower has been identified as needing repair, the real challenge begins. TVA's system features towers that range from 1910s to 2000s vintage. Even on a single line, there are often different tower types and vintages, each of which requires a different repair solution. Using internal resources and independent engineering firms, TVA developed a programmatic process for repairing towers, reducing the need for costly heavy equipment at each repair site.
Each tower type has a specific repair kit featuring a solution specifically engineered to that tower type. The repair solutions, which are designed and constructed to meet ASCE 10 standards, restore original strength and are actually capable of increasing capacity when greater strength is required. The standard repair kit works approximately 95% of the time. On the remaining towers, such as those with concrete foundations, structural engineers are consulted to develop a repair solution specific to the individual tower.
Benefits of a Cyclical Program
Executed correctly and on a cyclical basis, a below-grade inspection and repair program can improve system safety and reliability, and extend the service life of steel transmission structures, effectively reducing repair costs. This effort also helps to alleviate premature tower replacements and subsequent capital expenditures.
The return on investment for below-grade inspection and repair of towers can be measured in several ways. However, the value of outages that have yet to occur and the value of towers that have not failed is difficult to establish. Likewise, the reduction in costs related to emergencies that have not occurred is also difficult to measure, even though there are very real costs attached to all of these. What can be measured are the costs that are avoided as a result of precluding below-grade corrosion with a cyclical program.
In 2009, TVA's average cost to inspect, coat and seal the footings of one tower was 15% of the cost to repair it, or six-and-a-half times less expensive than repairing it. Of the 1,133 TVA steel towers inspected in 2009, 998 towers were coated and required no further attention. This included 593 towers listed as Condition B (moderate corrosion), which, without inspection and coating, would have migrated into Condition C (heavy corrosion and 10% to 25% section loss) and required repair. Inspecting and coating 998 towers in 2009 versus repairing 593 of them at a later date helped TVA avoid more than US$2.8 million in repair costs.
TVA's average cost to repair one tower was 7.5% of the cost to replace it, or 13 times less expensive than replacing it. In 2009, 16 Condition C and 119 Condition D towers were repaired. Had those 135 towers not been repaired, they would have continued to deteriorate, eventually requiring replacement. Repairing those towers in 2009 versus replacing them at a later date helped TVA avoid more than $10.8 million in replacement costs.
Adding years of dependable service life to steel structures defers repairs and replacements, reducing operations, maintenance and capital spending, and helping to increase earnings and future cash flow. This is most easily illustrated by comparing the cost savings of inspection versus repair and repair versus replacement.
Budget and Standards
At TVA's current rate of inspection, it would take 48 years to complete an entire inspection cycle. The recommended program cycle is 20 years, and TVA is working diligently toward that goal. Unfortunately, the steel-is-forever mentality has led many utilities to view towers as a lifetime asset, requiring no maintenance and, therefore, no maintenance budget. Proper maintenance of these steel transmission structures will require significant growth of the current maintenance budget.
While there are no current standards or mandates regarding below-grade corrosion inspection and maintenance, NACE International and IEEE have recognized the importance of these issues and created a joint committee to develop standards to address them. The publication of these standards could help to hasten the growth of maintenance budgets across the country.
In the meantime, with the help of contract resources that offer both project management and engineering expertise, TVA is effectively reducing tower failures and improving the overall safety and reliability of its transmission system.
John E. Peckinpaugh (firstname.lastname@example.org) started working at the Tennessee Valley Authority in the transmission construction organization building transmission lines and substations almost 35 years ago. He has developed both construction management and maintenance procedures. He also has specialized in the development of lightning-mitigation methods to improve poor-performing transmission lines. He is now the manager of TVA's transmission line maintenance program.
Corrosion Rating System
|Condition||Corrosion and Section Loss||Action|
|A: Good condition||Minor corrosion||Clean and apply protective coating|
|B: Fair condition||Moderate corrosion Minor section loss||Clean and apply protective coating|
|C*: Poor condition||Heavy corrosion 10% to 25% section loss||Repairs needed|
|D*: Bad condition||Major corrosion >25% section loss||Immediate repairs needed|
|*No longer meets NESC standards|
NACE International www.nace.org
Tennessee Valley Authority ww.tva.gov