Ask any gas network to name their number one priority and the answer will be the same: safety. Gas networks invest millions each year to keep customers, colleagues and the public safe from harm, resulting in a safety record that the sector is rightly proud of. But safety aside, the industry is also committed to ensuring our world class gas network is a force for good when it comes to its environmental impact, both in the short and long term.
Few nationwide endeavours encapsulate this twin commitment to safety and the environment better than the Iron Mains Risk Reduction Programme (IMRRP). But what is this programme and why does it matter?
What is the IMRRP?
The IMRRP, often called the ‘mains replacement programme’, was introduced in 2002 under a mandate from the Health and Safety Executive (HSE) for gas distribution networks to replace the majority of iron gas pipes within 30 metres of buildings with modern plastic pipes by 2032.
The primary purpose of the programme is to improve the safety of the gas distribution network. The programme was introduced to address growing concerns about the effectiveness of aging cast iron gas mains, with leaks caused by structural failures posing serious risks to people and buildings. The new polyethylene pipes are far more resilient as they are corrosion-resistant, less prone to cracking and have a longer lifespan.
Three tiers of pipe diameter are included in the programme:
- Tier 1 covers pipes that are 8 inches and under (approximately 80% of all ‘at risk’ iron pipes)
- Tier 2 refers to pipes over 8 inches but under 18 inches (approximately 15% of all ‘at risk’ iron pipes)
- Tier 3 covers pipes sized 18 inches and above (approximately 5% of all ‘at risk’ iron pipes)
It’s the pipes that fall under Tier 1 that must be replaced by 2032, with replacement still taking place across the other tiers but bound by less firm deadlines.
What has the impact of the IMRRP been?
Over the past 23 years, the IMRRP has fundamentally changed the nature of our gas distribution networks for the better. The scale of the change has been very significant, with an average of over 3,000km of pipes replaced each year, meaning that the Tier 1 element of the programme is now around three quarters complete. To take the year 2024/25 alone, 3,738km of iron pipes were replaced with safer, more environmentally friendly plastic pipes. If laid out in a straight line, that’s enough to reach from the UK to the east coast of Canada!
A review of IMRRP was carried out in 2010-2011 by the HSE and Ofgem, which was a relatively early indicator of the effectiveness of the programme. This review confirmed that the programme had kept pace with network deterioration and reduced emissions, leading to significant financial savings and greater reliability, alongside the positive safety and environmental impacts.
Fast-forward to 2025 and statistics show that methane emissions from gas leakage in the network have fallen by 50% since 2000, delivering significant environmental benefits through replacing iron pipes with plastic. There is always more that the gas networks can be doing to tackle leakage, such as through the use of things like smart pressure control, sensors and other smart technology, and our members are absolutely committed to minimising leakage as much as possible. But the IMRRP has given us the fantastic platform of a modern, resilient network which is fit-for-purpose to tackle the many challenges we’ll face as the energy system decarbonises.
What next for the IMRRP?
In part thanks to this major programme, Britain’s gas network is world leading; transporting energy in a way that’s reliable (gas network reliability currently sits at 99.996%), safe and efficient. Considering the billions invested in the programme and the huge infrastructure changes, it is doubly important that our gas networks are utilised well into the future.
The IMRRP has prepared the gas network to play a leading role in meeting our climate ambitions. This world class infrastructure is ready to be used as part of a whole systems approach to hitting Net Zero, combining green gases, electrification and other decarbonisation measures.
Ofgem’s recent RIIO-3 Draft Determinations (covering the period 2026 – 2031) saw the funding for the final stages of the programme remain at a similar level, with £5.9bn committed to replacing 16,978km of iron mains. However, there has still been a substantial funding cut from Ofgem between business plans and Draft Determinations, despite the fact that macro drivers of costs in the economy are putting pressure on costs to deliver and the workload is likely to get more complex as the GDNs approach the end of the programme. Ofgem’s decision to tighten the non-mandatory (Tiers 2 and 3) funding criteria through refining how they assess, model, and depreciate new assets also means that GDNs will be expected to do far more with a not dissimilar level of funding, despite the clear needs case (both for safety and environmental reasons) for appropriate funding in this area.
Conclusion: the importance of getting mains replacement right
The lesson of this trail blazing programme is clear: consistent investment prevents decline, avoids higher costs later, and protects public trust. While this proactive and targeted approach means that gas networks have avoided the consequences of the underinvestment seen in other industries, we mustn’t lose sight of the finish line. As we approach the final chapter of the IMRRP, it’s imperative that GDNs are supported with the level of investment required to successfully finish the job.