Retrofit Solutions for Steel Pole Corrosion Degradation
Corrosion of steel poles and lattice towers is a pervasive challenge for many utilities, especially those that find themselves in very wet or humid climates with lots of dense vegetation present around lines and structures. Soil conditions can further contribute to these risks depending on your service territory. Coastal areas represent particularly corrosive environments that can wreak havoc on steel assets if proactive countermeasures are not in place. And with so many other critical maintenance priorities consuming limited resources and already constrained O&M budgets, it is understandable why asset owners sometimes default to a reactive strategy when it comes to corrosion. But this approach always proves costly in the long run.
One of Exo’s long-time clients in Florida is facing this same challenge with one of their older transmission lines. While most of the tangent structures are concrete and thus do not face the same risk of corrosion degradation, many of the large angle and dead-end structures were galvanized steel monopoles installed back in the early 70’s. These steel poles and towers are approaching the end of their 50-year design life, but it is not uncommon for utilities to stretch the duty cycle of their structures an extra 10, 20, or 30 years through proactive inspection and maintenance. This is where Exo got involved.
We were asked to perform a structural evaluation of these assets to determine their overall health and assess whether they could remain in service and offer reliable performance. While there is a visual component to our inspections, non-destructive testing (NDT inspection) is a critical element of the process which is necessary to diagnose structural issues resulting from corrosion. What we discovered on these steel poles is something we come across quite frequently. Almost all of the structures inspected had severe corrosion emanating from the pole’s interior, where the protective zinc galvanizing layer had failed. This was measured using ultrasonic thickness measurement gauges. In all but one case, the poles appeared relatively sound from the outside. However, most had lost at least half of their shaft wall cross-section. In one case, the corrosion had penetrated the entire thickness of the pole, leaving a large hole in the shaft.
Exo engineers took those wall thickness measurements and calculated the remaining strength of towers, and determined that much of their design strength and load capacity was gone. While remediation was certainly an option to consider, the unique design of the structures, the age of the line, and the cost to make repairs all pointed the client to the decision of replacing the steel poles. But that’s not the end of the story.
As is often the case, outside of emergency actions, it can take time to gather permitting, budget, and schedule restoration or construction teams to execute the work. And our client realized that it could likely be 1, and possibly 2 more hurricane seasons before the poles could be replaced.
Exo was asked to design, fabricate, and install a solution that would temporarily brace the poles and restore their lost strength to prevent catastrophic failures before the scheduled replacements. These structural modification and life extension solutions are one of Exo’s core competencies in the utility space. Because the poles in question were guyed, the biggest concern was compressive loading that could cause the pole shaft to collapse onto itself, like how you might crush a soda can. A modular pole splint/collar was designed to fit around the pole shaft, covering the area of severe corrosion near the base of the structure. It was important to get a snug but not overly tight fit that would transfer load but also not crush the shaft wall itself. With a successful installation of the braces, the line owner could rest easy knowing their line was structurally sound until the point in time where replacements could be made.
Asset Management for Proactive Solutions
Exo believes in taking a proactive and pragmatic approach to asset management of steel transmission poles and lattice towers. It’s how we deliver maximum value for each and every client. With the right inspection and maintenance programs in place, including steel pole corrosion assessment and remediation, asset owners will find that they have more repair options at their disposal and alternatives to an ultimate and costly replacement strategy.
Exo offers alternatives to steel pole and tower replacement whenever possible. And when it is time to replace, we help you customize a plan for a smooth transition that optimizes cost savings. If you think you may be facing similar challenges with corrosion or other structural deficiencies, don’t wait. Act now!