NDT.net • Apr 2006 • Vol. 11 No.4

The Importance of the Nondestructive Technologies for Wooden Poles Network Asset Management

Benoit Y.
CBT, Saint-Sulpice, Switzerland, Email:benoit@cbs-cbt.com

Sandoz J.-L.
CBS, Les Ecorces, France, Email:sandoz@cbs-cbt.com

14th International Symposium on Nondestructive Testing of Wood
May 2005, University of Applied Sciences, Germany, Eberswalde.
Published by Shaker Verlag (ISBN 3-8322-3949-9).


Abstract:

Keywords: wooden poles; NDT; network asset management; life time expectancy, ageing figures

Power and telecom companies are placing increasing emphasis on cost effective processes to extend the lives of existing facilities while maintaining adequate levels of safety and reliability.

In the case of overhead lines, utilities are facing the problem of the wood poles ageing (most of them will reach the end of their "design lives" over the next 10 to 20 years) combined with some high capital and maintenance budgetary requirements, an overloaded network due to an increasing demand and extreme changing climate effect actions on the overhead structural components.

Wooden poles are still widely used and installed in the landscapes: If one assumes that in Western Europe 1 wood pole is employed for every 2 inhabitants, and that this proportion increases in less densely populated countries such as the US and Scandinavia, the economics of optimum use of wood as a resource soon become apparent. In less developed countries, the proportions and the economics vary depending on the natural resources such as wood that they employ

This paper relates a study on more as 60'000 poles checked with a non destructive technology in both Europe and North America. The life time expectancy has been studied taking into account the different species, the different treatments and the different problematic of each country.

The loss of the mechanical performances of the wooden poles has been then clearly identified for each species and each region giving then an accurate view of the life time expectancy of the component in the field.

Because of the material's wide mechanical performances distribution, the application of non destructive technologies on new wooden poles has a big concern as well. This research quantifies the gain of a truncation of the bad new wooden poles in the life time expectancies of the in-service poles. This gain has some huge economic consequences in terms of maintenance management.

The conclusion of this research underlines the importance of the non destructive tests for both the in-service wooden poles and the new ones, and the link between these two components. Each approach has been quantified thanks to a big international database.

1- INTRODUCTION

Power and telecom companies are placing increasing emphasis on cost effective processes to extend the lives of existing facilities while maintaining adequate levels of safety and reliability.

In the case of overhead lines, utilities are facing the problem of the wood poles ageing (most of them will reach the end of their "design lives" over the next 10 to 20 years) combined with some high capital and maintenance budgetary requirements, an overloaded network due to an increasing demand and extreme changing climate effect actions on the overhead structural components.

Based on this multivariate situation (ageing, increasing facilities overloads, and extreme climate change actions) which brings year for year a decreasing network reliability, several companies have started to acquire in-field technical data, based on non destructive tests operated during overhead line visits.

This paper brings some advanced results obtained after six to eight years of data collection. The main results are summarized in order to make an accurate analysis of the fundamental trends got on the international level (Australia, Austria, Canada, France, Germany, Switzerland, USA...) with NDT technical values related to the in-field wooden poles. The sampling of wood poles for these analyses counts about 60'000 in-field wood poles.

Before focusing on the most important NDT results, it is of a basic importance to start by considering the initial wooden poles reliability (including the natural variability) at time t0, basis for the new poles. Then, the nowadays NDT technology like Polux and wooden poles ageing principles will be discussed.

2- NEW WOODEN POLES RELIABILITY

Because wood is a natural material produced by trees located in forests, timber quality has to be defined in a large range of mechanical performances, expressing the natural variability. Each species is concerned about variability for both softwood and hardwood [1].

Based on a large sample of new wooden poles collected in France, Switzerland and Austria, and then tested to failure in Cantilever bending in an accredited research laboratory [2] [3], the MOR probability density function of the most important softwoods (spruce, fir, sylvestris pine) electrician poles (diameter from 20 cm to 35 cm) is given in Figure 1 where the total wooden pole product variability is clearly expressed. A factor of five to eight (500% to 800%) is very common between the ultimate bending strength of the weakest pole and the strongest MOR obtained in the same product class.

Because of an important technical lack of the product grading rules applied to new poles (at t0), a huge difference of bending strengths exists for products apparently perfectly similar (same sizes, same species, same treatment).


Fig. 1: New-wooden-poles MOR variability and standards limits for the allowable bending strengths for Switzerland (15 N/mm2) and France (18.3 N/mm2) [2] [3].

With the national design code values (allowable bending stress) located on the Figure 1, it clearly appears that few new poles don't even respect any security coefficient from the first day of their lives while some others will have a security coefficient up to four to six in the same overhead line. Consequently, it means that each overhead network contains some weak points from t0, and then the weak point number will increase versus time, if no maintenance works are done. The weakest poles are totally healthy (without decay) and their weaknesses are basically explained by a lower fiber density.

Moreover, wooden poles have always been treated by chemicals in order to prevent the bio-degradation process. Nevertheless, the chemical protection is weakening itself versus time, and at a certain time ti, each pole will initiate a decay process.

The decay process reduces the pole strength from 100% to around 0% in only few years (from two to eight years in function of physical system conditions such as species, treatment, climate, soil, pole size etc...).

Considering these basic figures at a time ti (20 years, 30 years etc…) a technical NDT use to measure the in-field poles residual strength will have to be able to integrate both the initial wooden poles variability and the individual pole status related to the decay process. These two objectives have been integrated into the Polux NDT device which is presented in the next section.

3- THE POLUX TECHNOLOGY

The Polux technology has been developed in a large R&D project supported by EDF (Electricité de France), the Swiss Federal Institute of Technology located in Lausanne and the CBS-CBT timber engineering group [4]. The physical principle of the Polux device is the measurement of two physical basic wood properties at the pole's ground line (GL) level:

    1. The GL local compression strength F, translating the residual wood density, directly correlated to the pole's residual Cantilever bending strength
    2. The internal wood moisture content (MC) which relates the decay process (active or non active) obtained through the wood bio-degradation equation (eq. 1) giving COs and HsO.

Wood + micro-organisms + Os --> COs + HsO       (1)

Figure 2 shows the Polux metrology part based on two isolated electrodes which are manually introduced into the wood. The penetration compression load is measured by a load sensor located in the mechanical part of the device. The two electrodes are driven at 40mm into the wood. At this position, the internal MC is measured between the electrodes tops (non isolated areas) by electrical resistivity readings.


Fig. 2: Metrology system of the Polux device in order to get the local density (residual bending strength) and the internal moisture content.

Then, the Polux decision software gives the non destructive evaluation (NDE) of the pole for both characteristics, the residual bending strength by pondering mainly the F value, and the remaining residual life by analysing the associated MC to the residual strength.

3.1 The pole security index [5]

In order to calibrate the Polux decision software which gives the in-field pole security index at time ti, several large samples of in service wooden poles have been measured by Polux, then removed from the field and transported to the laboratory in order to be tested in Cantilever bending until failure. The Polux calibration equations have been obtained by fitting the data with a regression model like eq. (2):

MORres = a1F + b1·MC + e       (2)

Where:
MORres : Residual bending strength at time ti [N/mm2]
a1 b1 e : Calibration model parameters
F, MC: NDT physical variables, F: local density variable at GL, MC: internal moisture content at GL.

Figure 3 shows an example of NDE made by Polux for a group of North American softwood poles (Red Pine, Western Red Cedar, Douglas Fir, Southern Pine, Jack Pine) treated by pentachlorophenol. It appears that several different species can be evaluated by Polux while the key point will be the local wood density.


Fig. 3: Polux NDE for a group of North American in-service softwood poles (Polux vs. residual bending strength obtained by failure tests, in PSI) [6].

For this example, it appears that Polux can evaluate the individual residual strength, at time ti, with an accuracy of ±16%. This performance is extremely significant compared to the pole strength differences reaching 500% to 1000% for timber applied in engineering.

Based on the residual strength evaluation, Polux will then communicate to the operator the pole's security level displayed by a scale of four colours:

- Red: No more security coefficient
- Flashing red: Security coefficient located between 1.1 and 1.3 of the designed value
- Flashing green: Security coefficient evaluated from 1.3 to 1.8 of the designed value
- Green: Security coefficient higher than 1.8 of the designed value.

After the security index evaluation, Polux will then evaluate the residual life time expectancy.

3.2 The life time expectancy [7]

Based on both the density index and the decay process status, it becomes possible to evaluate life time expectancy for each measured pole. This time evaluation will suggest the priorities in terms of poles replacements for the utility maintenance forecasts. For the time function, Polux has been calibrated by pondering the residual strength (security index) with MC giving the following empirical model eq. (3):

      (3)

Where:
LTEres : Life time expectancy, in years (maximum eight years before next control)
MORini : Nominal bending strength for a new pole [N/mm2]
MORres : Residual bending strength obtained in eq. (2) [N/mm2]
MC: Polux NDT variable for moisture content, according to the statistical distribution as shown in Figure 4.
K1, K2 : Decision model parameters


Fig. 4: Statistical distribution of the moisture content. Example on Western Red Cedar, Canada, 13'504 samples, showing the trimodal distribution shape: healthy poles from 15% to 50%, decay processing poles from 50% to 120% and poles saturated by wet soil from 120% to 180%.

In the field, Polux is communicating through two simultaneous results. Besides the security scale, this NDT device gives the life time expectancy by lighting the coloured diodes as follow:

- Red: No more time, the pole is dangerous and has to be changed as soon as possible.
- Flashing red: Estimated life time expectancy is from two to three extra years.
- Flashing green: Estimated life time expectancy is about five more years.
- Green: Estimated life time expectancy is about eight more years before next inspection.

Furthermore, after having displayed in real time the pole NDE, Polux stores all the data. The operator will have initially chosen both the wood species and the treatment type. Then, Polux will be able to run in using the appropriate calibration model and will register the pole number and the NDE results.

Linked to Polux, a handheld computer running with the specific software Picus can download the measured data before displaying them in both summarizing figures, the statistical reliability level of the tested poles set and the statistical life time expectancy of this same set. Examples of data reduction are given in Figure 5.

a) Statistical reliability level of the tested poles
b) Statistical life time expectancy of the tested poles
Fig. 5: Summarization of results given by the software Picus after wooden poles Polux NDT inspection, for both residual safety index and life time expectancy.

In the field, the operator has the opportunity to set up a security sign (a red, orange, light green or dark green tag) on each measured pole with a nail dated of the year of the inspection as shown on Figure 6.


Fig. 6: Sign on the measured pole relating the Polux NDT result with a coloured tag and a nail with the year of the inspection.

4- AGEING FIGURES

4.1 Consecutive controls

By using a NDT like Polux, it is possible to print the data giving for example the relationship between Polux NDE and time.

Moreover, after six or eight years, a new inspection acquires some new measurements. By comparing together the data files of the first visit to the next one, wood poles ageing is able to be observed.

This in-field research program has been supported by Swisscom, the Swiss telecom operator. A sample of 1'451 telecom poles, with a diameter of 18 cm at the GL and a length of 9m has been measured a first time in 1996 and a second time in 2000 (923 poles), 2001 (248 poles) and 2002 (280 poles). The second visit has been split between 2000, 2001 and 2002 and has always been made at the same period of the year and at a similar temperature (±3°C). The average age of the sampling was 22 years old in 1996, 29 years old in 2000, 28 years old in 2001 and 23 years old in 2002.

Figure 7 presents the trend of F, the compression load, evaluation for all the three samples. By plotting F versus pole age, the ageing effect can be expressed.

The Figure 7 relates that for small diameter telecom poles, the ageing can be quite fast (four to six years). This trend is speaking for a control each five years, able to detect next weakest poles.

Furthermore, it can be observed on Figure 7, that the third sub-samples, controlled after 6 years, was a set of younger poles. Consequently, the decreasing residual strength was a bit slower. Figure 8 shows the ageing slope versus mean age of the controlled poles.

These figures confirm that the decay process is increasing when the pole set is becoming older and older. Consequently, one can assume that the pole ageing is non linear. Starting with a quiet low slope, the loss of bending strength will reach a maximal value at the end of life of the pole set. When is the expected end of life?

This question is the most important one from the asset management point of view. Some fundamental ageing trends are discussed in the following section.




Fig. 7: Polux NDT results after two successive inspections on the same poles set, in the same conditions. First inspection in 1996 (mean age: 22 years old), second inspections applied in 2000 (mean age: 29 years old), 2001 (mean age: 28 years old) and 2002 (mean age: 23 years old). Swisscom poles, 1'451 samples from the same network section.


Fig. 8: Ageing slopes vs. mean age of the poles from three sub-samples of the same Swiss telecom network station. Label: Year of measurement: mean age of the sampling, ageing slope.

4.2 Residual strength versus time

Based on the NDT data obtained in several countries by using Polux, some other figures can be observed where F is plotted versus time (or pole age). Data files can be selected by countries in accordance to the wood species and wood treatments which are different from one case to another one.

On the Figure 9, 22'531 poles have been measured in North America, with a diameter from 30 cm to 45 cm and treated by pentachlorophenol. The species are Western Red Cedar, Red Pine, Larch, Southern Pine and Spruce with an age from two to 70 years old. While Figure 9 displays the species all together, Figure 10 splits the data for each species. It can be observed that the ageing can be strongly different from one species to another one.


Fig. 9: Polux NDT results vs. time on 22'531 Northern American wooden poles, all types of species, pentachlorophenol treated.

Scientifically, the natural wood variability linked to each species affects the ageing model very much for all other parameters the same. For example, Lodge Pine and Western Red Cedar which are well known for their natural durabilities (Western Red Cedar is used worldwide in the construction domain as clothing for the walls) shows the slowest weakening process versus time.

On the presented graphs, it can be observed that the faster decay process is for Spruce (and Southern Spruce, but in this case, the sampling is too small to confirm a trend). Actually, Spruce is well known as a species which very difficult to treat. For the European species (spruce and fir), Figure 11 shows 35'103 poles used by the Swiss electricity companies with the following dimensions: diameter from 22 cm to 32 cm, length from 9 to 16 m and treated by CCB salts (copper, chrome and bore). In this particular case of softwoods quiet difficult to treat, even in high pressure facilities, the observed ageing is more accelerated in comparison to the figures obtained in some other countries. Based on this experience, the inspections are set up each five years in order to cover the ageing trend and to be efficient with the maintenance service.

The ageing figure for European spruce and fir has the same trend as the North American one, confirming of an accelerated decay process for softwood with less natural durability.


Fig. 12:
Fig. 10: Polux NDT results vs time on 22'531 Northern American wooden poles, species by species, all pentachlorophenol treated.


Fig. 11: Polux NDT results vs. time on 35'103 Swiss wooden poles (power companies). Species: Spruce and Fir (mixed together), CCB treated.

5- MAINTENANCE SERVICE

Based on both, the new wooden poles reliability and the ageing figures, it appears very important for the utilities to support a service including the two objectives:

1- Increasing the network reliability by replacing the weakest poles what increases by the same way the global factor of safety (FOS) of the line.
2- Saving the maintenance costs by extending the life time expectancy (LTE) of most of the poles, because age is a very poor NDT variable versus residual strength.

5.1 Global factor of safety (FOS)

As explained in part 2, the natural variability of new poles is strongly affecting the whole network reliability by implanting new supports having a very weak FOS (reaching sometimes for two or three percents of them no security at all!).

Because of these particular characteristics, the maintenance controlling visits have to be done with a NDT like Polux able to measure, at any time ti, the own residual pole strength. The fact of just checking if there is an internal decay or not is too limited because some poles are weak from the beginning, without any decay, while some others have internal decay although the remaining wood is strong enough to extend the pole's life from about three to eight extra years.

Furthermore, because of the system effect occurring in the overhead lines when a first pole breaks leading to a domino effect breaking all the other successive good poles (Figure 12), any single pole has to be pondered with, for example, its four other neighbours (n-1, n-2, n+1, n+2) .


Fig. 12: Example of a domino effect in a wooden-poles network. A weak pole breaks and leads the other ones to failure because of an extra non expected load.

Figure 12 This is an additional control relating that sometimes three successive flashing red poles are more dangerous regarding the domino effect probability rather than a red one located between four green poles.

Based on the final data file, the Picus software computes the pole system ponderation by using eq.4:

      (2)

Where:
MORres,is : Residual strength taking into account of the system effect for the pole i
MORres,i : Residual strength of the pole i
MORres,i+1 : Residual strength of the pole i+1
MORres,i+2 : Residual strength of the pole i+2
MORres,i-1 : Residual strength of the pole i-1
MORres,i-2 : Residual strength of the pole i-2

Based on this statistical information, the maintenance priorities in terms of pole replacement are clearly highlighted and the weakest poles regarding the whole line effect will be changed in priority.

In order to increase the control visit return, some other information can be taken by the operator during his line inspection.

5.2 Complementary in-field data collection

In the field, Polux can be linked to a data processing peripheral tool offering the operator the possibility to answer to any other questions concerning the utilities.

Administrative line data, environmental description, overhead lines components status and GPS (Global Positioning System) coordinates can be then collected.

Picus software has been developed to be supported by any handheld computer running with Windows CE, Windows.net or Epoc. The technical NDT control on wood supports joint to the line data collection are of an extreme efficiency in order to allow the best network asset management.

6- CONCLUSIONS

All the data now collected on the international stage concerning the in-service wooden poles thanks to a non destructive technology like Polux and the knowledge about new components allow the explanations of the wood pole ageing and ensure a high security and reliability level for the overhead lines.

In the field, Polux can be linked to a data processing peripheral tool offering the operator the possibility to answer to any other questions concerning the utilities. Administrative line data, environmental description, overhead lines components status and GPS (Global Positioning System) coordinates can be then collected. Picus software has been developed to be supported by any handheld computer running with Windows CE, Windows.net or Epoc.

The technical NDT control on wood supports joint to the line data collection is of an extreme efficiency in order to allow the best network asset management.

Literature:

  1. Variability of softwood and hardwood
  2. and
  3. Accredited laboratories for cantilever bending tests
  4. CBS-CBT group
  5. The pole security index
  6. Canadian correlation
  7. Life time expectancy

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