Specialised testing and corrosion assessments of select transmission tower structures associated with an overhead High Voltage Alternating Current (HVAC) Electric Power transmission system in Brazil, South America were executed using corrosion prediction modeling. The purpose of the testing, inspection, and analysis for this project was to:
- Perform indirect assessments of selected transmission tower structures
- Assess the feasibility of applying corrosion risk model algorithms for galvanised transmission towers
- Compare and correlate direct inspection results of tower footings with the risk model results
- Demonstrate the validity of the technical approach for a full scale assessment project.
The project integrated corrosion science, field electrochemical and assessment technologies, with field deployable computing hardware in a tool that can provide real time risk ranking in the field.
A degradation classification criterion was developed to assist in characterising the findings of this project. This criterion is shown in Table 2 and is based on the metal loss of the steel members and other visual observations.
3.1 Degree of Degradation by Tower and by Leg
Table 4 provides a summary of the results of the assessment. The table is listed in increasing amounts of degradation and shows the correlation between the predicted and observed corrosion severity. Examples of minor, moderate and severe corrosion are shown in Figure 6-8.
|By Tower||By Leg||Comments|
|Circuit||Avg. of Degr.||Total Risk
|ADCM-2-165||2.3||907||A||221||1||6||no appreciable corrosion|
|D||207||3||minor surface rusting|
|ADCM-2-176||2.5||999||A||216||2||7||up to 50% metal loss on some cross members|
|B||211||4||severe localised attack at the soil/air interface|
|ADCM-2-156||4.0||913||A||263||2||8||up to 100% metal loss with cracking the remaining ligament of the cross|
|ADCM-1-174||4.3||1149||A||291||4||up to 50% metal loss on some cross members|
|B||286||6||up to 100% metal loss on some cross members|
|C||286||5||Over 50% metal loss on some cross members|
|C||263||5||25mm thick corrosion deposits|
The electric utility involved in this study operates over 80,000 transmission towers that are 30-50 years old. In an effort to ensure reliability of power delivery, a methodology was needed to assess the likelihood and severity of corrosion of the underground grillages associated with these structures. Hardware and software were developed to aid in data collection and a risk ranking model was developed using a corrosion severity indexing algorithm. The process was applied to a small sample of structures to evaluate the applicability of the process and to assess the predictive capabilities of the model. The assessment found:
- Two towers had an average Level 2 degradation
- Two towers had an average Level 3 degradation
- Three towers had an average Level 4 degradation
- One tower had an average Level 5 degradation
- No towers had an average Level 1 degradation
In addition, two towers had one footing each classified with Level 6 degradation indicating 50-100% section loss
The predictive capability of the risk ranking model was successful, even for the small population sampled. It is expected that the performance of the model will continue to improve with a larger sample size and as Bayesian updating with observed degradation results enhances the model’s algorithm with observed degradation data. The approach, process and model may now be used for large scale projects to assist in identifying structures that may be at risk due to corrosion.
The process of assessing towers for corrosion risk is typically followed by a direct examination phase of high risk towers (based on predicted corrosion severity, structural loading and operational criticality), which is then followed by the implementation of structure repairs and corrosion mitigation through the application of protective coatings and cathodic protection