By utilizing a risk-based leakage survey method, SP AusNet, an Australian natural gas transmission and distribution company, has been able to reduce the cost of conducting leakage survey and leak repair operations.
Gas companies worldwide have, for some time, supplemented the traditional “reactive” locate and repair of public reported gas escapes with a more proactive leakage survey approach. SP AusNet’s objective was to find optimum balance between proactive leak management (leakage survey) and reactive leak management (public reported leaks).
By utilizing a risk based leakage survey, SP AusNet has been able to reduce the cost of conducting leakage survey and leak repair, lower environmental effects of gas leaking into the atmosphere and increased public and network safety.
System overview
SP AusNet has 8,500 km of distribution main and 180 km of transmission pressure pipeline supplying gas to over 500,000 customers in western Victoria, which is in the southeast corner of Australia. The company is a subsidiary of Singapore Power Industries who own and operate gas distribution, electricity distribution and electricity transmission in Singapore. The company has successfully utilized reliability centered maintenance (RCM) to reduce costs, but still maintain output performance in fields such as regulator station maintenance, exposed pipe and gas main replacement programs.
Risk-based approach
Leakage survey is a risk-mitigating activity that uses special gas detection equipment to survey transmission and distribution mains. The intent of leakage survey is to detect leaks at an early stage and, by timely repair, avoid gas concentration in air being raised (particularly in confined spaces) to the lower explosive limit (LEL) of 5% gas in air (i.e., 50,000 ppm). At LEL, gas will ignite in the presence of an ignition source (spark or flame) – the consequence being fire or explosion. Gas must be discernible at one-fifth of LEL (i.e. 10,000 ppm).
Leakage survey is also conducted to detect leaks less than 10,000 ppm (generally before the public can smell gas), thus both reducing repair cost and keeping the public perception of safety high. Leakage survey equipment is able to detect gas at concentrations as low as 1,000 ppm.
Additionally, leakage survey is carried out as an environmental tool and used to reduce the volume of gas leaking into the atmosphere and reducing contributions to the “greenhouse” effect. Previously, three survey regimes were used, depending on the area category, to detect and repair leaks (Table 1).
Aims and goals
Gas companies worldwide have, for some time, supplemented the traditional “reactive” locate and repair of public reported gas escapes, with a more proactive leakage survey approach. SP AusNet’s objective was to find optimum balance between proactive leak management (leakage survey) and reactive leak management (public reported leaks).
Review of survey practices
A review of international leakage survey practices was conducted to determine world’s best practice. Twelve gas companies from seven European countries and the USA were reviewed.
The countries reviewed were Belgium, France, Germany, Italy, The Netherlands, Spain, the United Kingdom, and the USA.
Leak detection surveys usually take the form of road-going vehicles fitted with suitable leakage detection equipment, coupled with walking surveys for those pipes not accessible by vehicles.
The types and frequencies of such surveys vary considerably dependant on the type of distribution system that is being operated (i.e. pressure, material, age, any previous leakage history etc.), or through legislative requirements imposed by the country’s safety regulator.
The principle focus of leakage surveying in the UK is to locate and repair “serious” escapes (i.e. those likely to lead to fire/explosion), which in turn will give the safety regulator, and the general public, the confidence that the gas supply system is being managed in a safe and responsible manner.
Within the context of increasing liberalization of the gas markets, and growing competition, gas distribution companies are increasingly finding it necessary to reduce operating costs.
A summary of findings included the following points:
- Legislation (either direct or implied) necessitates leakage surveys in five of the seven countries.
- Frequencies of surveys range from one to five years for iron and steel pipes without corrosion protection, and from one year to seven years for polyethylene pipe and corrosion protected steel pipes.
- Material, pressure and previous leakage history (of the particular section of pipe) were the major factors taken into account when considering the frequency of leakage surveys.
- Safety was the unanimous consideration when each gas company was asked the main reason for undertaking leakage surveys.
- Flame ionization detection is the most commonly used detector.
- The number of leaks found per km during survey ranged from zero to 1.56, reflecting the differing materials and principles of leakage detection in Europe.
- As expected, leakage from ductile and grey cast irons was 10 times the rates of polyethylene pipe and corrosion protected steel.
- Costs ranged from e20 to e121 per km surveyed, averaging at e50.
- Timescales ranged from 0.75 hrs to 4 hrs per km of pipe surveyed depending on whether the pipe was surveyed by vehicle or on foot.
- Leak deferment for non-urgent leaks varied from immediate repair to ongoing monitoring (USA).
It is clear there is a wide range of approaches to leakage surveying techniques throughout the European gas companies. The lack of synergies in the areas of legislation, the choice of what pipes are surveyed and at what frequencies, leakage rates and costs, despite the industries having being established since the mid 1800s and their relatively close proximity to each other, is somewhat surprising.
The one common thread was that all companies overwhelmingly agreed that safety was the reason for undertaking leakage surveys. However, there was no consistency in approach to focus leakage survey on assets that are most “at risk.” No company ranked either environmental issues or minimizing gas lost to atmosphere higher than safety.
Quantitative risk analysis
Whilst leakage survey is principally undertaken on safety (not economic) grounds, the optimum level of leakage survey is clearly the balance between reasonable expenditure and acceptable risk reduction. There is little point in locating “pin-hole” leakage on steel or polyethylene pipes where the likelihood of subsequent catastrophic failure is very low (as the integrity of the remaining pipe surrounding the hole is sufficient to contain the failure, unlike cast iron pipes), and importantly where the cost of gas lost to atmosphere is negligible compared to the cost of locating and undertaking repairs.
Using quantitative risk analysis, SP AusNet initially reviewed and analysed risks inherent in the current survey and repair processes to establish both acceptable risk and cost data. The risk-based alternative survey regime was then able to be benchmarked to those data sets and deliver substantial on-going annual savings at an acceptable risk.
Methodology
SP AusNet undertook the following steps to consider and move from the current survey regime to the new survey regime:
- Analysis of current survey process and determination of current risk (to establish data sets)
- Analysis consisted of identification of the following key parameters:
- Pipe types
- Area categories
- Leak definition
- Fault tree analysis – probabilities of preconditions
- Outcome analysis – probabilities of consequences.
Pipe types
The following pipe types are used in the SP AusNet network:
- Cast iron
- “Unprotected” steel (i.e. not cathodically protected)
- “Protected” steel (i.e. cathodically protected)
- Polyethylene
- Polyvinylchloride (PVC).
Historical leakage data
Historical leakage data is shown in Table 2. The data has been accumulated from the SP AusNet network over a 10-year period from 1993 to 2002 for the entire length of all mains (in metres) by pipe type. For each pipe type, leakage has been calculated in leaks/km/yr (the system average is also shown in bold).
Area categories
The following area categories were based on Australian Standard AS1697 1985 (SAA Gas Pipeline Code), are applicable for the purpose of leakage survey:
- Category 1 – any location that has 10 or fewer buildings per kilometer (100?m frontage) intended for human occupancy and within 20?m of the pipe.
- Category 2 – any location that has more than 10 but fewer than 100 buildings per kilometre (up to 10 m frontage) for human occupancy and within 20?m of the pipe.
- Category 3 – any location that has more than 100 buildings per kilometer (<10?m frontages) for human occupancy or a building occupancy of 20 persons or more during normal use.
- Category 4 – any location where buildings with four or more storeys above ground are prevalent.
Leak classifications
SP AusNet used two classes of leak:
Class 1. For immediate attention (unplanned repair)
- Public reported
- Presence of gas in or around buildings
- Gas in confined spaces (pits, drains, tunnels, etc).
Class 2. Repair within 6 or 12 months (planned repair)
- Leaks detected on systematic cycle – 48 months
- Leaks detected on annual cycle – 12 months.
A greater cost is incurred for Class 1 compared to Class 2 leaks by virtue of the need for immediate response. Costs include Call Center processing, stand-by emergency response personnel as well as stand-by repair crew (as needed). Hence a reduction in reliance on unplanned repair contributes to both risk reduction and significant cost savings.
Optimizing survey intervals
Figure 1 depicts total pipe leakage costs against survey interval for cast iron pipe in area category 2. The depicted costs are for: leakage survey; repair from leakage survey, public reported leakage repair and the total cost.
As the leakage survey interval is extended then, per annum the:
- Number of public reported leaks increases, thus cost of public reported leak repairs increases.
- Number of leakage survey leaks detected reduces, thus cost of leakage survey detected leak repairs decreases.
- Number of leaks not detected increases, thus cost of unaccounted for gas (UAFG) increases.
- The probability of a leak creating a fire increases, thus the risk cost of a fire increases.
- The probability of a leak creating an explosion increases, thus the risk cost of an explosion increases.
- Cost of surveying is decreased.
The graph in Figure 1 depicts the sum of costs for cast iron pipe, and highlights an optimum (least cost) condition at two years.
Risk-based leakage surveys
As a result of the in-depth analysis within the constraint of maintaining an acceptable risk, the outcomes derived from the first stage constitute a quantum change for the leakage survey process. The changes have resulted in a significant reduction in operating costs, without decreasing network performance.
Although the quantitative risk assessment showed that leakage survey intervals could be vastly increased (leakage survey on polyethylene pipe < 20 years), a more conservative or optimized approach was adopted. Tables 3 and 4 show comparative survey intervals for current and old practices, calculated (theoretical) and proposed (optimized) regimes. The very significant benefits projected come notwithstanding the conservative nature of the changes that were confined to area categories 1 and 2.
Benefits of the new leakage survey regime were:
- Reduced risk of liability to senior management.
- Cost of leakage survey is reduced.
- Higher proportion of leaks are undertaken in a planned manner.
- Reduced water ingress and subsequent consumer outages.
- Improved reliability key performance indicators (KPI).
- Reduced cost to sub contractors.
- Reduced maintenance cost.
- Improved rapport with technical regulator.
- Consumer confidence in gas safety will be increased as fewer leaks will be detected by the public.
SP AusNet introduced the first stage changes to leakage management in 2002.
However, we believed further improvements could be made and that a united approach with the other two gas distribution companies in Victoria was required. SP AusNet had theoretically evidenced that further changes could be made. The quantitative risk assessment showed that leakage survey on polyethylene pipe could be extended beyond 20 years. A wider statistical analysis with, superior and broader data, was needed.
Leak management second stage
Recent changes to the Australian Standard have allowed gas distribution businesses in Australia to introduce further innovation in the management of gas networks. With the assistance of the other two distribution companies in Victoria, an analysis of Victorian leakage data was conducted.
Gas assets with high probability of failure that are in high hazard locations have the greatest potential for future failure from breaks. Worldwide experience indicates that it is the release of large volumes of gas from breaks that has the potential to create an explosive mixture in a property.
Joint leakage or pinhole corrosion failures do not release significant volumes of gas and have shown elsewhere internationally not to be a failure type that leads to serious incidents such as explosions in properties.
Data analysis
Victorian mains leakage data from 2001 to 2004 was analyzed for each company and as combined data, then subcategorized by:
- Mains leakage type – broken mains, corrosion, joint leak
- Mains material type
- Leak identification – leakage survey or public reported leak
- Leaks per 1,000 km of material
- Diameter – small (< 150mm), medium (150mm), large (> 150mm)
- Month and year
- Breakage zone – low (<.0 2), medium (0.2 to 0.4) and high (>0.4) (breaks/km/year)
- Breakage zone versus future failures.
The higher risk leaks (breaks) tend to be found by public reported leak rather than leakage survey by about a factor of 10. This can be expected, as these failures tend to be a significant release of gas that is easily identified by the public and reported. Figure 2 shows the average number of broken mains by material type over 12 months. There are seasonal peaks in broken mains between February and April – at the end of a long dry summer, when maximum tensile ground movement affects cast iron mains. There is also a peak in July – which occurs after seasonal rain.
The public reported leak response process is critical in identifying higher risk cast iron breaks that generally release larger gas volumes when compared to joint and corrosion leaks. The chance of identifying breaks at the time of the leakage survey is much less. The small number breaks identified by leakage survey are frequently hairline breaks that are not releasing significant amounts of gas and hence not found by public reported leak response.
Leakage survey identified five times the number of joint leaks than by public reported leak response, suggesting that the majority of leaks found by leakage survey are small, low risk leaks that are not easily identified by smell. These tend to be joint leakage in low breakage zones. Leakage survey is not focused at times of maximum ground movement.
Leakage survey on low risk polyethylene pipe and protected steel assets is providing little if any benefit. Leakage survey frequencies can be extended on medium risk assets (medium and low breakage zone cast iron, PVC, non protected steel), taking account of each categories failure/breakage rate and their location (e.g. hazard locations). For each main, a breakage zone has been calculated that depends on the breakage rate per km of all other cast iron main within 500 m of the main.
There is a general pattern of medium breakage zones between high and low breakage zones, which are a good sense, check of the concept. High breakage zones align with clay type soils, while low breakage zones align with sandy soils. The linear patterns confirm there is a relationship between the high breakage zones and geophysical features across Melbourne.
Cast iron high breakage zones are readily identified and have a high likelihood of future breaks. These are potential targets to increase the effectiveness of leakage survey. In scheduling the repair of leakage survey indications, priority should be given to small diameter cast iron mains in high breakage zones in high hazard locations. These are most likely to fail as a break.
Leakage survey indications that are not classified for investigation and repair should be deferred and monitored on subsequent leakage surveys and/or replaced on the block renewal program. Operators should focus their leakage surveys on higher-risk cast iron assets at times of higher ground movement.
Conclusion
By utilizing a risk-based leakage survey SP AusNet has been able to reduce the cost of conducting leakage survey and leak repair, lower environmental effects of gas leaking into the atmosphere and increased public and network safety.
Acknowledgments
The author wishes to acknowledge the contributions of officials with Alinta Gas Services, Origin Energy, and GTL Business International. Based on a paper presented at the International Gas Union’s 23rd World Gas Conference held in Amsterdam, the Netherlands.
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