It started with an unrealistic wish. In 1998, as a member of Entergy's Transmission Technology Delivery Department (TTDD), I met with Entergy Transmission grid managers and Bennie Daigle, then director of Entergy's Asset Management. During the meeting, Daigle posed the question, “If we could provide any one thing to enhance system reliability, what would that be?”
After some discussion, the grid managers agreed that they wanted a way to accurately and rapidly test the strength of wood crossarms using airborne methods. They also wanted to do it without men performing climbing and sounding inspections. In addition, they wanted to do it routinely in a much safer manner on a cycle that drastically reduced or eliminated crossarm failures while in service.
Clearly this was a dream. It was easy to grasp many of the benefits of this dream, but also the difficulties in making it a reality. Wood is a variable element whose structural properties change over time from weather exposure. Additionally, even if a system could be developed to measure the relevant properties, adaptation to an airborne delivery method would have considerable challenges.
Arms Fail at Worst Possible Time
The concern about failing crossarms is not unique to Entergy's transmission system. Utilities rely on wood transmission line structures and crossarms to support 50% or more of the transmission line miles in the United States and Canada. Many of these structures are 30 to 50 years old and are in varying stages of degradation. The most frequently used means of testing for integrity is to have a lineman climb the structure and “sound” it with a hammer in search of voids or rot. This is an expensive method of testing that involves the subjective judgment of a lineman. It is also increasingly difficult for utilities to maintain a consistent program of testing due to the complexity of scheduling inspections with experienced linemen with the same relative sounding techniques. Inspections are always competing with other budget priorities; therefore, cost-effective inspection programs are at a premium.
When crossarms fail, they remove lines from service, usually during extreme weather events when the line is needed the most, and repair crews are on overtime and stretched thin. The costs in monetary terms and customer ire are both high. A reliable means of reducing this risk is timely and necessary — particularly in light of the ages of these structures and the increased demands being imposed by regulators and customers for improved reliability.
Georgia Tech NEETRAC Assists
Entergy's TTDD took on the task of developing such a system and engaged Georgia Tech's NEETRAC research facility to help. AIR2, a transmission line contractor for Entergy that uses helicopters and embraces innovation, was also recruited to assist in the effort. NEETRAC had done a considerable amount of work with neural networks and surmised that if the vibration of a crossarm, generated by the sound of helicopter blades, could be captured and analyzed by a neural network, it would be possible to discern the “elements” of strength from the vibration reading. Originally, a laser vibrometer was employed to capture the vibration signature. Laboratory testing of the system proved quite promising. However, difficulty was encountered when the system was transferred to the field. Ultimately, it was determined that laser vibrometers were too sensitive to withstand the rigors of field use on helicopters.
This realization was a crushing blow to the team after more than two years of effort and considerable expense. However, all was not lost. By this time, AIR2 had hired Dr. James Mahaffey, formerly with the Georgia Tech Research Corp., to direct the development efforts. In the course of the initial work with lasers, much was learned about the properties of wood and how to discern important structural information from its vibration. Further, the AIR2 effort benefited from previous testing and documentation of wood strength by the U.S. Department of Agriculture Forest Service. This work relied heavily on the time it takes induced vibration to propagate through a wood member. Mahaffey, in conjunction with Dr. David Salmon of AIR2, conducted further research into the correlation of wood property measurements with wood strength.
Field and Laboratory Testing
After laboratory testing of hundreds of wooden crossarms, which compared and correlated measured strength to actual strength by breaking the arms, it was determined that three measurements provided reliable indicators of wood strength. The measurement categories selected were shock wave travel time, crossarm density and wood hardness. The challenge was to determine how to evaluate and weigh each measurement to most accurately predict the strength of the wood. Dozens of mathematical formulas were evaluated until an analytical tool was determined. The final result is a set of three independent measurements that, when evaluated together, provide an accurate determination of strength.
A set of spar arm measurements was performed in May 2003. A set was 21 spar arms taken from transmission lines of both Entergy and Florida Power & Light and subjected to testing using the THOR system at the NEETRAC facility in Atlanta, Georgia. The arms were then stressed to failure under mechanical load to measure mechanical strength. The arms ranged in condition from brand new to badly deteriorated. Since some of the arms were 30 ft (9 m), while others were 35 ft (10.7 m), the strength was recorded in foot-pounds of breaking moment. This enabled the strength of a 30-ft beam to be directly compared to the strength of a 35-ft beam.
Figure 1 shows the actual beam strength versus the estimated strength based on shock wave speed, wood hardness and percentage of gamma ray absorption. The arms from Entergy and Florida Power & Light were different in appearance and data clustered at different strengths.
Figure 1 is an illustration of the predictive capability of the three data elements of strength acquired by the THOR system. The strengths indicated by the three elements are reduced to a single strength rating per beam. The relative strength ratings of each beam in the group were plotted in the scatter diagram and then compared to the actual breaking strength of each beam when stressed to destruction. A best-fit line is established from the scatter-plotted values. The correlation coefficient is then calculated using statistical standard deviation equations. The correlation coefficient of 0.67 is indicative of the high correlation of predicted strength with actual strength.
Similar testing performed on hundreds of other crossarms yielded consistently high correlation coefficients. The conclusion drawn from these tests is that a high degree of reliability can be achieved by replacing crossarms given a weak strength prediction by THOR.
Entergy has gained sufficient confidence in the test results to commence use of the THOR system on in-service crossarms. Central Hudson Gas & Electric also participated in testing and determined that, first, the THOR system yielded more accurate identification of weak crossarms than traditional methods and, secondly, the system would save budget dollars by not replacing strong arms that appeared to be bad using other methods.
Various pod designs have been developed that address the wide variety of shapes and dimensions of crossarms in service. Additionally, accessories have been constructed to address crossarm access issues that result from different transmission structure configurations.
Ideally, transmission line managers would test their entire system on three- to five-year cycles. The test would accurately identify weak crossarms needing replacement, as well as register strength readings of stronger arms. The measurements of the stronger arms on subsequent cycle readings could then be compared with the previous cycle(s) to evaluate degradation over time, estimated remaining life and needed testing frequency.
In its current form, the THOR measurement system composes the following elements and procedures.
A self-contained pod, equipped with an accelerometer and a gamma-ray densitometer, is temporarily attached to one end of the crossarm being assessed. This is accomplished by a lineman sitting on a platform attached to the side of a helicopter.
The helicopter flies to the other end of the crossarm and hovers in position so that the lineman on the platform can strike the end of the crossarm several times with a hammer instrumented with an accelerometer.
A data acquisition system in the helicopter records the acceleration of the hammer and the response of the crossarm where the pod is attached. The accelerometer signal is transmitted from the pod to the data acquisition system via a radio link.
The gamma-ray densitometer consists of a weak source of gamma rays and a detector of gamma rays. The pod is designed to position the source and detector on opposite sides of the crossarm. The instrument counts the number of gamma ray particles that pass through the arm and reach the detector. This count provides information on the density of the crossarm — the more dense the crossarm, the more gamma rays are absorbed by the wood and the fewer reach the detector.
The reading of the gamma-ray densitometer is also recorded by the data acquisition system.
A global positioning system is employed to record the precise location of the structure being measured.
In a post-processing phase, three discriminants of crossarm strength are calculated:
The shock wave propagation speed.
The beam density at the location of the pod.
The hardness of the wood at the location of the hammer blows.
The three discriminants are the processed to generate an estimate of crossarm strength. While a helicopter is assessing a crossarm, it is also desirable to perform a visual inspection of the arm to note any obvious visual signs of decay such as large cracks, visible pockets of rot or the growth of vegetation. Additionally, a high-resolution photograph may be taken for future reference.
The result of the analysis is a ranking from the arm predicted to be weakest to the arm predicted to be strongest. Predicted breaking strengths, together with upper and lower bounds of expectation, can be provided for each beam. The utility may set thresholds based on predicted breaking strengths and replace arms predicted to be weaker than those thresholds. Alternatively, if the budget allows for only a few replacements, beams predicted to be weakest can be replaced.
The THOR system will be of most benefit if it is used as part of an ongoing program involving multiple inspection and replacement cycles over many years. The data pertaining to each crossarm can be compared from cycle to cycle to determine how quickly the strength of each arm is diminishing as the years of service accumulate. Crossarm replacement policies can be developed to achieve a cost-effective balance between the expense of replacing crossarms and the expense of catastrophic transmission line failure. Using THOR, it will be possible to make confident judgments as to which arms should be replaced on any given inspection cycle.
The THOR system was the name given to this new process by the AIR2 development team. Comic book fans of the 1960s and students of Norse mythology will recall that THOR was the magic hammer wielding the God of Thunder. THOR could fly with his hammer that was often drawn with emanating lighting bolts. The name stuck. The THOR system is a proprietary system owned by AIR2, LLC with patent pending.
Ian Barras has worked in Entergy Transmission for nearly five years as an R&D project manager in Transmission System Engineering. Prior to that, he attended Tulane University in New Orleans, Louisiana, U.S., where he received both his BSEE and MSEE degrees. Barras is currently working as a distribution asset planner in the New Orleans metro area. He also attends law school at Loyola University of New Orleans.