Millennium Main Rehabilitation Project (2025)
VIP-WECO hybrid liner installed within the large-diameter GRP pipes - Courtesy of Morrison Water Services (now M Group)
The existing large-diameter trunk main running from Nunhead Reservoir to Deptford is one of the key network assets supplying water to the Greenwich Peninsula. Tunnel sections of the pipeline had experienced failures in the past and the glass reinforced plastic pipes had deteriorated with extensive blistering, leading to concerns that the pipes are no longer serviceable and DWI compliant. Morrison Water Services (now M Group) was engaged by Thames Water Utilities Ltd to carry out site investigations, identify a cost-effective pipeline rehabilitation solution, and subsequently to deliver a £21m project to rehabilitate this trunk main using a hybrid lining system.
Background
The Millennium Main is a large-diameter trunk main conveying potable water from Nunhead Reservoir to Deptford to support the Greenwich Peninsula and the Millennium Dome. The main is approximately 3km in length and it was built in 1999.
The water main is a combination of ductile iron (DI) and glass reinforced plastic (GRP) pipes; the latter being installed within concrete jacking pipes where trenchless methods were used to cross three railway lines and avoid open-cut trenching in some roads.
The existing GRP pipes sleeved in concrete jacking pipes (approximately 1.82km in length) can only be accessed through existing tunnel shafts deep below ground level; with access requiring extensive traffic management due to their locations.
In 2012, GRP pipes failed within one of the trenchless sections on two separate occasions. The site investigation revealed that the tunnel annulus grouting between the GRP pipes and the host concrete sleeve pipes at the failure locations was either entirely missing or incomplete, causing excessive tensile forces in the pipe wall and resulting in pipe failures. Subsequently, all the voids detected in this section were filled with grout. The GRP pipes within this trenchless section were relined with steel pipes (185m in length) and the main was returned to service.
In 2015, another failure occurred along another trenchless section. The post incident investigation again found that annulus grouting above the failed GRP pipe was incomplete, leaving air pockets between the GRP pipe and the concrete sleeve pipe. During AMP6, further investigation into the annulus voids and annulus grouting were carried out along the section that failed in 2015 and the adjoining trenchless section (total 270m in length). However, the main was not put back into service due to the risk of pipe failure that might occur along the remaining GRP pipe sections which had not been surveyed and repaired (approximately 1.36km).
Existing tunnel shaft – Courtesy of Morrison Water Services (now M Group)
PPR survey & annulus grouting
In 2020, Thames Water appointed Morrison Water Services (now M Group) to carry out pipe penetrating radar (PPR) surveys to identify the locations of annular voids and grout the identified voids along the remaining GRP pipe sections (approximately 1.36km in length), in order to recommission the Millennium Main.
As part of the site investigations, pipe wall samples were taken from GRP pipes to test pipe strength and analyse their composition. The results indicated that the GRP pipes should have more than 30 years remaining asset life if the grout annulus were fully reinstated. The existing GRP pipes were first surveyed using PPR method to identify annular voids and the survey results were verified by an endoscope camera survey on targeted areas to confirm the highlighted voids.
On completion of the surveys, all the identified annular voids were filled with a special free-flowing grout to ensure complete void filling. The grouting work was completed in March 2022.
Osmotic blistering in GRP pipe internal surface
During the inspections of the GRP pipes, extensive osmotic blistering in the GRP gel coats was observed with many of the blisters ruptured, exposing the resin liner underneath. Further blisters were likely to form in the presence of water during operation and ruptures of the blisters was also expected.
Osmotic blistering in GRP pipe internal surface – Courtesy of Morrison Water Services (now M Group)
The exposed resin liner is not DWI Regulation 31 approved material, hence the ruptures of the osmotic blisters can cause potable water coming into contact with non-DWI approved resin which is a breach of DWI Regulations and Thames Water Standards.
In addition, the underneath GRP laminate would hydrolyse with time, weakening pipe structure and reducing the effective thickness of the pipe if this osmotic blistering issue were not addressed. Considering this, Thames Water made a decision to line all the GRP pipes within the trenchless sections.
Lining options investigated
Thames Water and Morrison Water Services (now M Group) reviewed a number of lining solutions including:
- Steel liner.
- PE liner.
- Cementitious coating.
- Sprayed in place pipe (SIPP) liner.
- Cured in place pipe (CIPP) liner.
- Hybrid lining system.
Approximately 185m of a steel liner was installed within GRP pipes when GRP pipes failed in 2012. The steel liner option provides a robust solution, however, it was estimated that it would cost approximately £45m to reline the whole length of GRP pipe whereas the estimated cost for the hybrid liner option was approximately £17m.
Both steel liner and PE liner options require a longer installation period due to difficulties in installing pipes from restricted tunnel access shafts. The hydraulic modelling work also confirmed that the reduced pipe bore by these options would cause additional pipe head loss, affecting pressures in the downstream network.
Cementitious coating and SIPP options were discounted primarily due to the risk of the cementitious coating and spray lining not being able to properly adhere to the surface of the GRP pipes. CIPP liner option was also discounted due to the absence of DWI Regulation 31 approval at the time of optioneering.
Based on the results of the optioneering, Thames Water made a decision to reline the GRP pipes using a hybrid liner, considering the whole life cost, pipeline hydraulic performance, DWI approval, construction period and other risk factors.
Development of GRP pipe lining solution
Morrison Water Services (now M Group) partnered with VIP Polymers Ltd to develop a bespoke lining system, VIP-WECO hybrid liner, to line the internal surface of the GRP pipes for this project.
VIP-WECO Hybrid Liner system – Courtesy of VIP Polymers Ltd/Thames Water
Each hybrid liner is 1.7m long and comprises of one EPDM centre sleeve and two EPDM end seals. The EPDM rubber sections are then covered with stainless steel sheet and fixed to the GRP pipe, using stainless steel tensioned rings. The liners are butted up against each other to achieve full coverage of the GRP sections (approximately 1.63km).
The structural assessment of the hybrid liner confirmed that the lining system can withstand internal pressure and resist possible buckling when the GRP pipes are left empty and the liner is exposed to the full head of groundwater through cracks and voids in the GRP and host concrete pipes, in the worst case scenario.
The components of the hybrid liner system can provide durability for at least 50 years, which is longer than the remaining asset life of the GRP pipes. VIP Polymers Ltd obtained DWI Regulation 31(4)(a) approval for the hybrid liner they produced.
During the development of the hybrid liner, an above-ground mock-up of the lining system was built to refine the detailed arrangement of the hybrid liner, to test the installation of the lining system, and to check if the installation work could be carried out safely during the actual installation.
Hybrid liner mock-up – Courtesy of Morrison Water Services (now M Group)
Millenium Main: Supply chain – key participants
- Main contractor: Morrison Water Services (now M Group)
- Designer: AECOM
- Subcontractor: DWG Infraco
- Key material supplier (hybrid liner): VIP Polymers Ltd
Construction
Morrison Water Services (now M Group) commenced the installation of the hybrid liner in the GRP pipes in June 2023. The works within the GRP pipes and tunnel shafts were carried out by operatives who were qualified for high risk confined-space work with provisions of health, safety and rescue support, and periodic rescue training was conducted during the site investigation and construction phases.
A total of 973 hybrid liner units were installed in the GRP pipes and each unit was tested by air pressure to check leak tightness of the liner unit. The installation of the hybrid liner was successfully completed in October 2024.
Currently, existing DI pipe sections (approximately 1.2km) are being surveyed to check the conditions of the pipes and any identified defects are being repaired.
Progress
This project successfully restored the tunnel annulus grouting and the integrity of the internal pipe surface of the GRP pipes to comply with DWI regulations. It is currently planned to return the main to service in August 2025.
VIP-WECO hybrid liner installed within the large diameter GRP pipes - Courtesy of Morrison Water Services
