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Cathodic Protection of Water Mains in Ottawa : Analysis and Planning
Cathodic protection of water mains in Ottawa: analysis and planning
Kleiner, Y.; McDonald, S.; Rajani, B.
NRCC-46653
A version of this document is published in / Une version de ce document se trouve dans : Corrosion Control for Enhanced Reliability and Safety, Ottawa, Ontario,
Sept. 15-17, 2003, pp. 1-14
CATHODIC PROTECTION OF WATER MAINS IN
OTTAWA: ANALYSIS AND PLANNING
by
Yehuda Kleiner, Senior Research Officer
and
Shelley McDonald, Technical Officer
and
Balvant Rajani, Senior Research Officer and Group Leader National Research Council of Canada
Institute for Research in Construction
Ottawa, Ontario
CATHODIC PROTECTION OF WATER MAINS IN OTTAWA: ANALYSIS AND PLANNING
ABSTRACT
The inventory within the water distribution system of the City of Ottawa consists of 1750 km of cast and ductile iron pipes, installed between 1874 and 2002. Ottawa started a program of cathodic protection (CP) of its water mains in 1990. The program includes both hot-spot and retrofit CP strategies. Reduction in breakage rate has been observed following the initiation of the CP program but a full economic analysis of its effectiveness was never carried out due to lack of modeling and analytical tools. The Institute for Research in Construction (IRC) of the National Research Council Canada (NRC) has recently developed a software application, WARP, (Water main Renewal Planner) that facilitates economic analysis specifically for water distribution systems. The application of WARP to data from the City of Ottawa is described in the form of a case study.
WARP was developed with the support of 12 Canadian water utilities (including the City of Ottawa) and one anode manufacturer. WARP models deterioration and breakage rates and takes into consideration time-dependent factors such as: temperature, soil moisture and, cathodic protection (CP) strategies, including both hot-spot and methodical retrofit CP. It quantifies the influence of each of these factors on pipe-breakage rates to (a) identify the “true” deterioration rates of buried water mains, and (b) project the economic impact of various operational strategies on future breakage rates. Keywords – water main renewal program, cathodic protection, water mains
BACKGROUND
The City of Ottawa (referred to henceforth as Ottawa) has been a strong
proponent in the use of cathodic protection (CP) as part of their water main rehabilitation program for the last 13 years. Initially its CP program targeted older and undersized water mains but over time it expanded to include water mains with lead services as well as mains with elevated breakage frequency. The CP program is multifaceted and includes hot-spot anode installation and full water main retrofit. The hot-spot CP consists of opportunistically installing a magnesium anode every time a water main is exposed, such as after breakage repair, etc. The retrofit CP consists of systematically protecting water mains by distributed anodes (one per pipe segment) or through anode banks. If a road is to be resurfaced, the anodes are installed one per segment and anode banks are installed in order to minimize the impact on pavement surface. In addition, all new installations of metallic pipes are outfitted with CP protection.
Ottawa has kept records of breakage and repair events in water mains since 1972. These records, encoded electronically, provide the historical data required for the analysis performed by WARP - Water main Renewal Planner.
WARP – Water main Renewal Planner
WARP is a computerized decision support tool that helps to model the deterioration rates of water mains and subsequently plan their renewal. Historical breakage rate patterns of water mains are considered an indicator of the structural condition of the pipes. The analysis of breakage patterns considers time-dependent factors such as temperature (freezing index), soil moisture (rainfall deficit) and cathodic protection strategies, including hot-spot CP as well as methodical retrofit CP. The influence of each of these factors on pipe breakage rate can be quantified to (a) identify the “true” background deterioration rates of buried water mains, and (b) project the impact of various operational strategies on future breakage rates. Static factors affecting breakage rates (e.g., pipe material, diameter, soil) are considered through grouping of water mains into relatively homogeneous groups (Kleiner and Rajani, 2002; Kleiner and Rajani, 2003).
Historical breakage rate patterns are assumed to govern the future behaviour of water mains. Based on this assumption and with the appropriate economic and cost data, the future impact of various operational strategies can be computed, leading to efficient planning of water main assets.
WATER MAIN DATA AND PIPE GROUP DEFINITIONS
The data required for the analysis comprises pipe inventory data, break history data and climatic data (monthly precipitation and daily temperatures) for the analysis period. The Ottawa pipe inventory data included 16,383 pipes (a total of 2,200 km) each with material type, diameter, installation year and systematic CP (year of installation). Soil data were not available for each pipe.
Break history data included year and month of each breakage event from 1972 to 2001. Breakage type data were not available.
Climatic data were obtained from Environment Canada and included average monthly precipitation and average daily temperatures for the period 1971 to 2002. In WARP, the daily temperatures are converted to freezing index (FI) in units of degree-days, which is a surrogate measure for the severity of a winter in a given year. The temperature and the precipitation data are converted to rain deficit (RD) in units of length (mm or inch), which is a surrogate measure for soil moisture. Two types for RD values are used. One is cumulative RD, which depicts the average soil moisture during any given year. The other is a snapshot RD, which depicts the rain deficit at the beginning of the freezing season. Detailed explanations on how FI and RD are calculated, along with the rationale for their usage are provided in Kleiner and Rajani (2002).
For the case studies presented here, two groups of pipes were established. Group 1 consisted of lined and unlined cast iron (CI) pipes, 150 – 300 mm in diameter, installed between years 1961 and 1971. Service connection data were not available although it was assumed that the vast majority of the pipes in Group 1 had copper service connections. The total length of the water mains in Group 1 was 318 km and the total number of breaks recorded was 1,539. Group 2 consisted of ductile iron (DI) pipes, 150 – 300 mm in diameter, installed between years 1972 and 1977. Service connection data were also not available and it was also assumed that the vast majority of the pipes in
Group 2 had copper service connections. The total length of the water mains in Group 2 was 152 km and the total number of breaks recorded was 509.
In Group 1, 38 km of water mains have been CP-retrofitted through the period 1996-2001. These retrofitted mains were denoted Group 1b. The water mains in Group 1 that have not been retrofitted with CP (280 km) were denoted Group 1a. Groups 1a and 1b have been hot-spot-CP-protected since 1990.
In Group 2, 27 km of water mains have been CP-retrofitted through the period 1996-2001. These retrofitted mains were denoted Group 2b. The water mains in Group 2 that have not been retrofitted with CP (126 km) were denoted Group 2a. Both Group 2a and Group 2b have been hot-spot-CP-protected since 1990.
BREAKAGE RATE ANALYSIS and PLANNING Breakage rate analysis: Group 1 - cast iron mains
A break history analysis was conducted on Group 1a, to determine the influence of climatic conditions as well as hot-spot CP on its breakage pattern. Figure 1 illustrates the results. The coefficients (shown to the right of the chart in Figures) corresponding to each of the covariates applied in this analysis indicate their roles and influence in the response of the breakage pattern to environmental and operational conditions. A similar break history analysis conducted on Group 1c is shown in Figure 2.
• The ageing rate of 0.046 of Group 1a indicates that the breakage rate (breaks/km) increases about 4.6% annually. This ‘background ageing’ is depicted by the smooth curve in Figure 1.
• The freezing index (FI) at a value of 0.091 has some significance in influencing the breakage pattern. The rain deficit (RD) coefficients with values of –0.01 and 0.014, however, play an insignificant role. A plausible reason for FI having such a minor influence on breakage rate in Ottawa is that the typical depth of water mains is 2.4 m (8 feet), which is below much of the frost penetration. Ottawa receives a high precipitation rate throughout the year and therefore the soil moisture is rarely depleted which explains why RD has no significance on the breakage pattern of Group 1a.
• The hot-spot CP has a notable influence in decreasing break rates (coefficient value of -0.125), as is evident from the downturn of the breakage rate starting in the early 1990s.
• The quality of the model ‘fit’ in the break history analysis of Group 1a (adjusted coefficient of determination r2 = 0.631) is quite high.
In order to determine the impact of retrofit CP on the breakage rate of Group 1, a break history analysis of Group 1b is required. However, due the relative small size of Group 1b, and due to the fact that retrofit CP was implemented only in the last 5 years of the group’s history, there were insufficient data to perform a credible break history analysis. Instead, an alternative method was applied, in which breakage data were pooled so as to reflect breakage rates at years relative to the year of retrofit, rather than along an actual time line, as is illustrated in Figure 2.
Retrofit
Transition
Post-retrofit No retrofit CP
The three phases of the retrofit model, pre-retrofit breakage increase, post-retrofit transition period and post retrofit breakage increase are clearly shown. The coefficients of this model appear to the left of the chart. There are insufficient post-retrofit data for a credible post-retrofit breakage rate.
Discounted life-cycle cost analysis: Group 1a - cast iron mains
The life-cycle costs of existing water mains include breakage repair and failure related costs, cathodic protection measures, pipe replacement as well as breakage costs that are associated with the replacement pipe when it starts to deteriorate. As a pipe ages and deteriorates, its breakage frequency increases and thus breakage repair costs increase. At the same time, the more pipe replacement is deferred, the discounted (present value) cost of the replacement reduces. Consequently the total cost typically forms a convex curve, whose minimum point depicts the optimal time of pipe replacement. An effective cathodic protection program will shift this total cost curve so as to delay the optimal time of replacement as well as reduce the total cost (Kleiner and Rajani, 2003). For Group 1 this is illustrated in Figure 3 (unit cost data for the life-cycle analysis are shown in Figure 4)
0
Begin retrofit after
years 100,000 200,000 300,000 400,000 500,000 600,000 700,000 800,000 900,000 0 50 100 150 200 250 Year of replacement D isco u n te d l if e-cy cl e c o st ($ /k m ) 0 70 100 150 No Retrofit 30 (Optimal)
Figure 3 shows that without CP the optimal time for replacing a pipe is at the age of 90, and its total discounted life-cycle cost if replaced at that age is about $145,000/km. However, if the pipe is retrofitted at age 30, the optimal time for replacement is deferred to beyond 200 years of age and its total discounted life-cycle cost is reduced to about $55,000/km. Figure 3 also shows that retrofit CP will be beneficial (deferring
replacement and reducing life-cycle costs) even if implemented at a pipe age as late as 100 years. Since Group 1 comprises water mains installed between 1961 and 1971, the present time is the optimal time for starting their retrofit.
Planning: Group 1a - cast iron mains
WARP allows the comprehensive planning of rehabilitation and replacement of water mains through examination of various scenarios. The definition of each user-defined scenario requires information on:
• Economic data (direct breakage cost, indirect breakage cost, pipe replacement cost, cathodic protection (hot-spot and retrofit), and discount rate) as illustrated in Figure 4.
• Schedules for pipe replacement and/or pipe CP retrofit for the planning horizon, as well as the beginning year of a planned hot-spot CP program.
• Ageing (deterioration) rate of new (replacement) pipe.
Figure 4: Economic data for planning and life-cycle costs of Groups 1 and 2. Table 1 shows 6 different planning scenarios for Group 1a, with a planning horizon of 30 years:
• Scenario 1 depicts the current Ottawa strategy, in which an average of 2 km of pipe are replaced and an average of 4 km of pipe are retrofitted annually (Raymond 2003). Hot-spot CP is continued.
• Scenario 3 depicts a ‘hot-spot CP only’ strategy (no retrofit, no replacements).
• Scenario 4 depicts Ottawa’s current strategy, but without replacement (only retrofit).
• Scenario 5 depicts Ottawa’s current strategy, but without retrofit (only replacement).
• Scenario 6 depicts an ‘aggressive retrofit’ strategy (no replacement). The rightmost column in Table 1 depicts the average breakage rate over the planning horizon (30 years). It can clearly be seen that scenarios 1, 2 and 5 are inferior because they are significantly more expensive than the others and they even do not reduce the average breakage rates to lower levels than the others.
Scenarios 3, 4 and 6 are roughly equal with some tradeoffs between a slightly higher cost for a reduced average breakage rate (Scenario 6 has the highest cost among the three but achieves the lowest multi-year average breakage rate).
Table 1: Planning scenarios for Group 1a.
Discounted Costs ($1,000) Replacement Retrofit
Scenario length / year (km)
Breaks HS CP anodes Retrofit CP Replace m ent Total Breakage rate / 100 k m / year 1 2 4 6,695 79 2,410 30,119 39,303 16 2 Nil Nil 22,800 0 0 0 22,800 59 3 Nil Nil 9,470 120 0 0 9,590 23 4 Nil 4 7,902 94 2,410 0 10,406 19 5 2 Nil 8,413 106 0 30,119 38,638 21 6 Nil 28 3,432 20 7,367 0 10,819 7
A note of caution is warranted here. Group 1a consists of 280 km of pipes with the same material, size and vintage. In this type of statistical analysis there are two implicit assumptions; (1) the group of pipe is more or less homogeneous with respect to its breakage pattern response to the climatic and operational conditions, and (2) the breakage rate is more or less uniformly distributed along the water mains in the group. One should be aware that despite these assumptions, in a large group of 280 km there are likely to be some significant deviations from the average values represented by the
coefficients of the model. For example, the water mains in area of Crystal Beach
(12.3 km) belong to Group 1 (same material, size and vintage). However, they have been observed to have a relatively high breakage rate, and consequently most have been retrofitted with CP. Figure 5 illustrates the break history analysis that was performed on the 9 km of retrofitted water mains in Crystal Beach. It is evident that the ageing rate is almost twice the average of Group 1., however, due to the small sample, the statistical significance is considerably lower. It is very important to perform this type of analysis while applying sound engineering judgment. If there are significant deviations from the average pattern, these should be analyzed separately, while recognizing the limitations of a small statistical sample.
Figure 5: Crystal beach – a subset of Group 1 with a significantly higher ageing rate.
Breakage rate analysis: Group 2 - ductile iron mains
A break history analysis was conducted on Group 2a, to determine the influence of climatic conditions as well as hot-spot CP on its breakage pattern. Figure 6 illustrates the results.
• The ageing rate of 0.037 indicates an annual breakage rate (breaks/km) increases of about 3.7%.
• The freezing index (FI) and the cumulative rainfall deficit (RD) turned out to be completely insignificant for Group 2a, and were thus omitted from the analysis (omitting insignificant covariates from a regression analysis may improve the coefficient of determination because less degrees of freedom are lost). The
snapshot RD at a value of 0.112 has some significance in influencing the breakage pattern.
• The hot-spot CP has a notable influence in decreasing break rates (coefficient value of -0.097). This influence however, is less than that observed for Group 1a.
• It can be seen in Figure 6 that the break history analysis was performed starting from the year 1976, although breakage records were available starting from 1972. Ottawa started using ductile iron mains in 1972. It is believed (Raymond, 2003) that the exceptionally high breakage rates observed in 1972-3 are due to
inconsistencies in reporting. As well, the relatively high breakage rates (relative to a new pipe) observed in years 1974-5 were attributed to teething problems of dealing with a new type of pipe. Consequently, the break history analysis was performed starting from 1976.
• The quality of the model ‘fit’ in the break history analysis of Group 2a (adjusted coefficient of determination r2 = 0.242) is rather low compared to that of
Group 1a. This low quality fit could be explained by the very high breakage rate recorded for 1998, which could not be explained by FI or RD.
Similar to Group 1b, due the relative small size of Group 2b, and due to the fact that retrofit CP was implemented only in the last 5 years of the group’s history, there were insufficient data to perform a credible break history analysis on Group 2b. As a consequence, a pooled data analysis was carried out here too to ascertain the coefficients of the retrofit CP programs. The results are illustrated in Figure 7. Here too there are insufficient post-retrofit data for a credible post-retrofit breakage rate.
Figure 7: Group 2b, pooled data analysis.
Discounted life-cycle cost analysis: Group 2 - ductile iron mains
The discounted life-cycle cost analysis for Group 2 is illustrated in Figure 8. Unit cost data for the life-cycle analysis were taken the same as in Group 1 analysis (shown in Figure 4).
Figure 8 shows that without CP the optimal time for replacing a pipe is at the age of 110, and its total discounted life-cycle cost if replaced at that age is approximately $100,000/km. However, if the pipe is retrofitted at age 40, the optimal time for
replacement is deferred to beyond 200 years of age and its total discounted life-cycle cost is reduced to approximately $55,000/km. Figure 8 also shows that retrofit CP will be beneficial (deferring replacement and reducing life-cycle costs) even if implemented at a pipe age as late as 100 years. Since Group 2 comprises water mains installed between 1972 and 1977, the next 10-15 years is the optimal time for starting their retrofit.
0
Begin retrofit after
years 100,000 200,000 300,000 400,000 500,000 600,000 700,000 800,000 900,000 0 50 100 150 200 250 Year of replacement D is count ed l if e-cycl e c o st ( $/ km ) 0 70 100 150 No Retrofit 40 (Optimal)
Figure 8: Life-cycle cost of pipes in Group 2.
Planning: Group 2a - ductile iron mains
Table 2 shows 5 different planning scenarios for Group 2a, with a planning horizon of 30 years:
• Scenario 1 depicts the current Ottawa strategy, in which an average of 6 km of pipe are retrofitted annually (Raymond, 2003). Hot-spot CP is continued.
• Scenario 2 depicts a ‘do nothing’ strategy.
• Scenario 3 depicts a ‘hot-spot CP only’ strategy (no retrofit, no replacements).
• Scenario 4 depicts a moderate strategy consisting of half the retrofit rate of Ottawa’s current strategy, i.e., 3 km/year
• Scenario 5 depicts a super aggressive strategy consisting of double the retrofit rate of Ottawa’s current strategy, i.e., 12 km/year
It can clearly be seen that the ‘do nothing’ strategy (scenario 2) is inferior, being the most expensive and resulting in the highest average breakage rates.
Scenarios 1, 4 and 5 are roughly equal with some tradeoffs between a slightly higher cost for a reduced breakage. Scenario 3 is the least expensive but results in a higher average breakage rate.
Table 2: Planning scenarios for Group 2a. Discounted Costs ($1,000) Scenario Retrofit length / year (km)
Hot Spot CP Breaks HS CP anodes
Retrofit CP Total Breakage rate / 100 k m / year 1 6 Continue 1,462 14 2,834 4,310 7 2 Nil Stop 5,080 0 0 5,080 29 3 Nil Continue 3,090 39 0 3,129 17 4 3 Continue 2,240 25 1,807 4,072 12 5 12 Continue 1,105 7 3,273 4,385 5 SUMMARY
Two case studies have been presented to illustrate how WARP – Water Mains Renewal Planner can be used to analyze the effectiveness of an existing cathodic
protection program and subsequently make cost-efficient plans for future pipe protection and replacement. The planning process includes the analysis of breakage history to decipher the coefficients governing the behaviour of the breakage pattern. Subsequently a life-cycle cost analysis can be done, followed by scenario generation for optimal
strategies that are commensurate with budgetary constraints and acceptable performance criteria. The case studies also demonstrate that even with a powerful analysis tool such as WARP, engineering judgment is imperative in order to obtain credible results.
Break history analysis assumes that the population of pipes is relatively
homogeneous with respect to breakage patterns. Further, it implicitly assumes that breaks are uniformly distributed along the water mains in the group under analysis. This need for homogeneity motivates the analyst to partition the water main population into many small (unique) groups. At the same time, it is often not practical to apply the analysis to very small, groups because of the loss of statistical significance. Engineering judgment is required to balance these two conflicting needs, as well as to identify sub-groups that may need special attention.
ACKNOWLEDGEMENT
This case study was based on valuable data made available by Mr. D. Raymond of the City of Ottawa for which the authors are extremely appreciative.
REFERENCES
Kleiner, Y.; Rajani, B.B “Forecasting variations and trends in water main breaks,” Journal of Infrastructure Systems, 8, (4), December, pp. 122-131, December 01, 2002.
Kleiner, Y.; Rajani, B.B. “Water main assets: from deterioration to renewal”, AWWA Annual Conference, Anaheim, Ca., June 15-19, 2003, pp. 1-12
Kleiner, Y.; Rajani, B.B. “Quantifying Effectiveness of Cathodic Protection in Water Mains: Theory," Submitted for publication in Journal of Infrastructure Systems, 2003. Raymond, D. 2003. Personal Communication, City of Ottawa, Ontario.
