Introduction

Brazil has a large-scale electricity generation and transmission system, with hydroelectric power plants predominating. The National Interconnected System (SIN) is responsible for interconnecting electrical systems through a transmission grid. Power transmission towers are fundamental components of this system, supporting the high-voltage cables that carry electricity to the distribution substations in urban and industrial centers.

These transmission towers are designed to support high-voltage cables, as well as lightning cables (over long distances) and are sized to withstand loads such as their own weight, the wind and the cables, in the event of a break.

One of our clients requested a structural assessment of five power transmission towers, due to pathological manifestations. It should also be noted that the transmission lines supported by these towers are crucial to the client's production process, as they transport energy to the substation of its industrial plant. Figure 1 shows the general arrangement of one of the assets in question.

Figure 1: Transmission tower arrangement. SOURCE: Kot Collection.

Tendo isso em vista, a Kot propôs a realização de uma avaliação completa da integridade estrutural das estruturas metálicas, desde o levantamento de campo inicial até a avaliação estrutural, com proposição de reforços estruturais. Veja, a seguir, algumas das etapas seguidas no estudo.

Field survey

Initially, a field inspection was carried out to assess the actual structural condition of the towers. This service aimed to identify non-conformities and pathological manifestations in these lattice structures. During the inspection, it was possible to map points of corrosion and oxidation, as well as plastic deformations in some areas of the metal profiles. Some of the pathological manifestations identified can be seen in Figure 2.

Figure 2: Corrosion (left) and plastic deformation (right). SOURCE: Kot Collection.

In addition, Kot checked the dimensions of the structures in order to verify any discrepancies between the design conditions and those actually found in the field. At the end of the field work, the data collected enabled the visual inspection report to be drawn up, which included a map of the pathological manifestations and non-conformities observed, as well as an assessment of their risks and the actions required to restore the towers.

Finite element method (FEM) for structural analysis

The study began with the structural analysis of the towers using the finite element method (FEM). In order to apply this methodology, it was necessary to evaluate the design documentation, as well as those collected in the field. In this sense, the model generated took into account the pathological manifestations identified, as can be seen in Figure 3.

Figure 3: Representation of the plastic deformation identified in the field in the computer model. SOURCE: Kot Collection.

Structural analysis

The static analysis was the first of the checks carried out, in accordance with the applicable standards. During this study, the highest utilization indexes (UIs) were found in elements that were already compromised, either by corrosion or even warping. The results obtained were presented visually using a color scale. As can be seen in Figure 4, bar elements were found to be under greater stress than admissible (elements in red), showing the need for structural reinforcements.

Figure 4: Utilization rates in tower bar elements. SOURCE: Kot Collection.

As análises dos elementos de barra foram realizadas com o auxílio do software próprio da Kot, o Procal 3D. Visite o LinkedIn ou Instagram da Kot para conferir, nas palavras do Diretor Técnico e responsável pelo Procal, Frederico Mol, um vídeo sobre a principal ferramenta de engenharia utilizada aqui na Kot.

The connections were analyzed according to current normative criteria, using an analytical method and taking into account the various types of connection present in the asset. It was found that for storm wind conditions (VT), the tower's connections are susceptible to exceptionally high stresses, resulting in the possibility of bolt shear. In the case of normal operating winds (V0), the tower meets engineering standards. Figure 5 shows the elements with failed connections.

Figure 5: Elements with failed connections. SOURCE: Kot Collection.

The structure's natural vibration modes were evaluated during the modal analysis, checking for possible coupling and resonance of the towers with the wind. The results showed no risk of coupling for longitudinal and transverse winds. Figure 6 below shows one of the natural vibration modes identified.

Figure 6: Natural vibration mode identified. SOURCE: Kot Collection.

Proposed solutions

Kot proposed changes to the upper part of the tower in order to comply with regulatory criteria and mitigate structural risks, since non-conformities were observed during the course of the work. After defining the reinforcements to be proposed, a structural check was carried out in order to validate the recommended modifications. Figure 7 shows some of the suggested solutions.

Figure 7: Proposed changes. SOURCE: Kot Collection.

Conclusion

During the study, it was possible to identify several components in critical condition in the assets, with accentuated corrosion and plastic deformations, for which immediate replacement was recommended. In addition, the static analysis failed bar elements due to the UIs obtained, prompting the proposed structural reinforcements. Finally, Kot also proposed modifications to adapt the rest of the structures to the storm wind scenario.

A Kot conta com um time de profissionais qualificados em integridade estrutural pronto para desenvolver as melhores soluções de engenharia para o seu negócio. Entre em contato com nossos especialistas para mais informações!

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