Condition Assessment of Cast Iron Mains - IGEM … paper example 03.pdf · Condition Assessment of...

February 2013 IGEM South West Ian Marshall, Young Persons Paper Competition Graduate Engineer, Wales and West Utilities Ltd.

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Condition Assessment of Cast Iron Mains

The need for the development of and requirements for a non -destructive

evaluation tool for use on larger diameter cast iron mains

1 Introduction Upcoming changes to the iron mains replacement programme will place restrictions on the

replacement of large diameter cast iron gas mains. Therefore an onus has been placed on the gas

distribution network operators (GDNs) to innovate in order to ensure the continued integrity of

these mains and manage the risk posed to the public. This paper reviews the need for proactive

development of tools to accurately condition assess larger diameter cast iron mains and the

requirements placed on these tools. The development of such tools will allow for a targeted

approach, helping to minimise the cost of these ageing assets to the consumer whilst ensuring the

continued safety of the public.

2 The Need for Condition Assessment

2.1 The Cast Iron Population The gas distribution industry in the UK dates back over 200 years [1], with cast iron1 the principle

material for the manufacture of gas mains during a significant portion of this period. A large

proportion of the UK gas distribution network is still served by these same mains. Although the

quantity, ages, locations and more recent maintenance histories of these iron mains are generally

well known, very little information is available about their condition other than the fact that it is

deteriorating.

2.1.1 The Failure Modes and Mechanisms of Cast Iron Mains

Without intervention, the condition of the cast iron mains discussed above would continue to

deteriorate until they fail and remedial action is no longer a cost effective or safe option. The many

causes and mechanisms of these failures are generally well known and researched. These include

traffic loading, thermal loading and frost heave, electrochemical corrosion and to a certain extent

manufacturing defects and human error.

However, whilst knowledge of these mechanisms may allow for some prediction of when mains are

more likely to fail, for example trigger temperatures providing an indication of potential frost heave,

they do not provide sufficient information to allow for proactive intervention. On the other hand the

knowledge and ability to detect failure modes could do just that.

1 Please note that all references to ‘iron’ or ‘cast iron’ in this paper refers to both pit cast and the later ‘spun’

or centrifugally cast iron, unless otherwise stated.


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The corrosion of cast iron gas mains is often more complex than simple rusting, with graphitic

corrosion a common mechanism. Graphitic corrosion is a form of selective leaching where the iron

is removed leaving only the graphite flakes, reducing the strength whilst resulting in no difference in

appearance from a non-corroded main [2][3]. Therefore a visual inspection, whilst repairing a

fracture for instance, would provide no indication of the main’s true condition. As a result it is often

difficult to determine the relationship between fractures and the extent of metal loss through

corrosion. However, given the anticipated reduction in beam strength as a result of corrosion and

any pitting acting as potential stress raisers there is a distinct relationship between the corrosion and

stress in a cast iron main.

However, no matter the mode or mechanism, the result of failure is always the same, with an

uncontrolled release of gas. Dependant on location this gas may enter a property with the potential

for ignition and subsequent fatalities.

2.2 The Iron Mains Replacement Programme (IMRP) and Upcoming

Changes Following a number of incidents, in 2002 the Health and Safety Executive instigated the iron mains

replacement programme (IMRP), often referred to as the 30:30 programme. This requires GDNs to

replace all cast iron mains within thirty metres of a building within thirty years in order to remove

the risk to the public [4]. In committing to this programme GDNs ensured their compliance with

relevant legislation requiring pipelines are maintained in an “efficient state, in efficient working

order and in good repair” (Regulation 13, Pipelines Safety Regulations 1996 (PSR))[5].

The authors of a recent independent review of the programme believed the majority of the most at

risk mains had been replaced [6]. This report also proposed that larger diameter mains should be

more resistant to failure through corrosion and fracture than thinner pipes of a smaller diameter,

therefore posing less of a risk [6]. As a result, the report concluded that many serviceable mains

Figure 1: The same section of main after grit blasting. The extent of metal loss previously hidden by the graphite is clearly visible. A portion of the graphite can be seen on the top of the main and was left for illustrative purposes.

Figure 2: Section of main, cleaned of mud prior to grit blasting but otherwise as removed.


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were being prematurely replaced with the cost per life saved excessively high when compared to

other programmes, for instance road improvement works [6].

Consequently, an updated, three tiered approach (now called the Iron Mains Risk Reduction

Programme, IMRRP) to iron mains replacement is to be implemented whereby only Tier 1 mains (8

inches nominal diameter or less) will continue to be replaced in line with the IMRP. A condition and

cost-benefit analysis will be required for other mains in tiers two and three prior to these being

replaced [7]. This approach requires currently unavailable knowledge of actual mains condition and

consequently new techniques are needed to address this gap.

3 Considerations Needed for a Condition Assessment Tool

3.1 The Requirements of a Condition Assessment Tool A non-destructive evaluation (NDE) tool capable of assessing the condition of cast iron mains must

consider the key factors which contribute to failure, providing reliable conclusive measurements. It

must also be practical and cost-effective.

In addition to corrosion the tool must also consider stress, and by association the strain, experienced

by the mains in order to enable an accurate determination of the probability of failure. This is

necessary as, regardless of the immediate cause, these mains will fail when the strength of the

residual metal is less than the sum of the forces acting upon it. The need to understand the strain is

further discussed in Section 3.2.

As cast iron main corrosion and strain can be relatively localised, any technique reliant on sampling

and statistical modelling to assess the overall condition will be less accurate than one which assesses

the whole main. As external examination of the entire length of main is impractical, an internal tool

would be the ideal solution.

3.2 Current NDE Condition Assessment Tools and Their Limitations Condition assessment has not traditionally been performed on cast iron gas mains as appropriate

non-destructive evaluation technology was either non-existent or prohibitively expensive. As the

mains were intended to be replaced, there was no motivation for development of such techniques;

the traditional reactive approach to condition assessment was considered sufficient to enable

prioritising mains for replacement. As a result there are no NDE techniques that meet the exacting

requirements discussed above.

In the absence of such tools a technique that had previously been used on cast iron water mains was

identified and trialled. This external technique utilises a combination of ultrasound and magnetic flux

leakage (MFL) tools to determine the wall thickness as well as the location, size and depth of any

defects. This survey process for this technique is illustrated in Figure 3 to Figure 6 below. Although it

proved successful in determining these attributes for a number of defects on trial mains, these trials

only served to prove the necessity of the requirements, discussed previously; although several of the

fractured pipes were found to have an indication of corrosion, one of the larger diameter mains

showed no signs of corrosion at all.


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Figure 3 [8]: The image above is an example of the surveys performed and shows the MFL portion of a survey being conducted on a 8 inch cast iron main. The survey was performed adjacent to a fracture that is out of the left hand side of this image. Measurements of the soil compression surrounding the main, as well as its location and potential sources of loading, were recorded to allow for an estimation of the strain.

Figure 4 [8]: The image above shows the defects identified as part of the survey (illustrated in Figure 3) highlighted with chalk. A graphical representation of the ten worst (deepest) of these defects is shown in Figure 5 below.


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Figure 5 [8]: The image above is a graphical representation of the ten worst defects found as part of the survey of the main in Figure 3. The average depth of these ten defects was 3.28 mm, with the deepest defect found to be 4.1 mm. For reference the average measured wall thickness was 10.93 mm.

Figure 6 [9]: The image above is of a survey being performed on a 12 inch cast iron main (average measured wall thickness: 12.18 mm). This was found to be the best condition main surveyed with no evidence of any corrosion adjacent to the fracture (to the right hand side of the image) [9]. Therefore the only factor that could have resulted in the failure of this main is excess strain and a plot such as the one shown in Figure 5 was not generated.

The most notable requirement absent from this assessment technique, in common with the majority

of available NDE techniques, is the direct measurement of strain. There are very limited options for

determining strain, with the most common being to attach a strain gauge onto the point of a surface


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where the strain is to be measured. However, for these gauges all strain measurements are relative

to the strain in the material at the point in time the gauge was applied, rather than the true

magnitude. These gauges also only provide local information and even if multiple gauges are used it

is unlikely that these will be successfully attached at the most critical point on the main.

4 Looking Forwards Wales and West Utilities are actively working to develop a technology that that will provide both a

practical and cost effective solution to assessing the condition of cast iron mains. The availability of

such a solution would help to revolutionise the way in which the iron mains population is

maintained; the information provided by this technology would enable the remaining life of a main

to be estimated with an accuracy that has never previously been considered possible.

This solution will provide a clear indication of when intervention is required, allowing for a cost-

benefit analysis to be performed to decide on the best whole life cost option. In place of a

generalised overview of the mains condition, the technology will enable estimates of remaining life

to be made on a metre by metre basis. This will provide the opportunity for innovative and targeted

solutions to be applied in prolonging the life of these assets. Such solutions may be as simple as

digging down on a specific point on a main to relieve the abnormally high strain levels caused

through ground movement or human error. Additionally, the replacement of part of mains as

opposed to the entire length would become a possibility. It is also expected that the point of entry of

a condition assessment tool could be utilised by other equipment, such as a tool for the prevention

of joint leakage (another common failure on certain joint types on cast iron mains), and therefore

making such condition assessments even more cost effective.

5 Conclusion This paper has presented the requirement for innovation in developing a practical and cost effective

condition assessment tool for cast iron mains. In doing the necessity for this tool to measure strain

directly alongside the level of corrosion has also been demonstrated. The discussion has shown

investment in the development and future use of such a tool would represent a proactive approach

to ensuring compliance with relevant legislation and potential future regulator challenges. The

availability of accurate information about the condition of larger diameter cast iron mains will

enable cost benefit analysis, with a precision previously unknown, to determine the best time and

method for remedial action to be undertaken.


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6 References [1] C. Murray, “Celebrating 200 Years of the Gas Industry”, Presented at the IGEM Annual

Conference and Engineering Update, Nottingham, 4th September 2012

[2] WebCor Corrosion Consulting Services, “Different Types of Corrosion – Recognition, Mechanisms

& Prevention: Dealloying, Selective Leaching & Graphitic Corrosion”, [Online] Available at:

http://www.corrosionclinic.com/types_of_corrosion/dealloying_selective_leaching_graphitic_corros

ion.htm, Accessed: 13th January 2013

[3]Flowserve (December 2002), “Materials Newsletter – Graphitic Corrosion”, Vol. 2, No. 2, [Online],

Available at:

http://www.flowserve.com/files/Files/Literature/Foundry/newsletters/mn_2002_12.pdf, Accessed:

13th January 2013

[4] The Health and Safety Executive, “Enforcement Policy for the replacement of iron gas mains

2006-2013 – December 2005”, [Online] Available at:

http://www.hse.gov.uk/gas/supply/mainsreplacement/irongasmain.htm, Accessed: 21st December

2012

[5] The Health and Safety Executive, “The Pipeline Safety Regulations 1996”, London, April 1996,

[Online]. Available: http://www.legislation.gov.uk/uksi/1996/825/made [Accessed: 11 December

2012]

[6] The Health and Safety Executive – Gas and Pipelines Unit (May 2012), “10 Year Review of the

Health and Safety Executive’s enforcement policy for the replacement of iron gas mains”, [Online],

Available at: http://www.hse.gov.uk/gas/supply/mainsreplacement/10-year-review.htm, Accessed:

21st December 2012

[7] The Health and Safety Executive (May 2012), “The iron mains risk reduction programme 2013-

2021”, [Online], Available at: http://www.hse.gov.uk/gas/supply/mainsreplacement/enforcement-

policy-2013-2021.htm , Accessed: 21st December 2012

[8] R.J. Treece, P. Bailes and A. Robson, “Condition Assessment near Fracture Locations, 8 Inch Cast

Iron Main, Park road, Silverton”, Advanced Engineering Solutions Limited, Cramlington,

Northumberland, RP3929, March 2012 (Produced for Synthotech Limited, Harrogate, North

Yorkshire on behalf of Wales and West Utilities Limited)

[9] R.J. Treece, P. Bailes and A. Robson, “Condition Assessment near Fracture Locations, 12 Inch Cast

Iron Main, Moor lane, Porthcawl”, Advanced Engineering Solutions Limited, Cramlington,

Northumberland, RP3930, March 2012 (Produced for Synthotech Limited, Harrogate, North

Yorkshire on behalf of Wales and West Utilities Limited)

7 Bibliography Cambridge Economic Policy Associates Ltd, “HSE/Ofgem: 10 year review of the Iron Mains

Replacement Programme”, Rep. RR888, 2009, [Online], Available at:

http://www.hse.gov.uk/research/rrpdf/rr888.pdf, Accessed: 21st December 2012

http://www.corrosionclinic.com/types_of_corrosion/dealloying_selective_leaching_graphitic_corrosion.htm

http://www.corrosionclinic.com/types_of_corrosion/dealloying_selective_leaching_graphitic_corrosion.htm

http://www.flowserve.com/files/Files/Literature/Foundry/newsletters/mn_2002_12.pdf

http://www.hse.gov.uk/gas/supply/mainsreplacement/irongasmain.htm

http://www.legislation.gov.uk/uksi/1996/825/made

http://www.hse.gov.uk/gas/supply/mainsreplacement/10-year-review.htm

http://www.hse.gov.uk/gas/supply/mainsreplacement/enforcement-policy-2013-2021.htm

http://www.hse.gov.uk/gas/supply/mainsreplacement/enforcement-policy-2013-2021.htm

http://www.hse.gov.uk/research/rrpdf/rr888.pdf


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R.J. Treece, “Summary of Cast Iron Gas Main Failure Investigation”, Advanced Engineering Solutions

Limited, Cramlington, Northumberland, RP3937, March 2012 (Produced for Synthotech Limited,

Harrogate, North Yorkshire on behalf of Wales and West Utilities Limited)

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