Oxton Trunk Main Recommissioning (2021) – DRAFT
Newark is currently supplied from boreholes in North Nottinghamshire. The Environment Agency strategy is to reduce borehole abstraction licence capacity. Pressure from significant residential growth and development across Newark drove the need for an alternative source. The Oxton Project would provide immediate relief by increasing capacity into the catchment area by 5 ML. A future additional scheme is planned for a new trunk main to connect other control groups for future reinforcement of the distribution system to secure a long-term supply and resilience. The proposed solution was to enable capability to transfer water from a river treatment works to supply parts of south and east Nottingham in place of borehole water, improving sustainability and freeing capacity up to cope with housing growth around Newark.
The outline design proposed 1.4km of new 450mm trunk main to avoid pressure loss and 3km of 600mm diameter to provide connectivity for the rezone to take place. The route of the 450mm main followed the River Trent as well as crossing another smaller main river, posing ecological and water management issues for the team to overcome during detailed design and construction. The 600mm main would pass through pasture and agricultural fields adjacent to a dual carriageway but was to prove even more problematic than the smaller main with significant constraints along even the most viable routes between the two critical points in the water network. This would push the team to dive into the watery world of leak detection rather than digging in and progressing the default option of open cut main laying.
During the early stages of design progression, it was identified that the pasture and agricultural fields between the points in the existing network requiring connectivity held significant high potential for archaeological remains which would result in escalating costs and timescale, as well as impacting on environmental and historic features. Alternative options were revisited in an effort to manage the impact to the project. Open cut works within a major arterial route into Nottingham were quickly discounted. Moving the route to the other side of the highway was confirmed as unsuitable for efficient construction due to flooded former gravel extraction pits and the risk of future extraction licences being granted in the area.
The answer had been lying abandoned and all but forgotten for over a decade. The 24” cast iron water main which had previously served the area had been valved off and left abandoned. The perception of risk in the ST Operations Team was that use of the main was too high risk, and the valves in question were not to be touched for fear of leaks in multiple locations and a high chance of discolouration leading to unsatisfactory drinking water for thousands of residents in the area.
The abandoned main section was located under a major dual carriageway for large parts of its length, crossed the carriageway in several other locations and had 14 bends to navigate. Initial ground investigation data indicated a high-water table and poor strength granular deposits around the abandoned main. The crown of the main was also found to be unusually deep at between 2.4 and 2.6m at several locations. Excavations would be required within the highway verge and lay-bys at entry and exit points for the HDD CCTV and acoustic survey to install a series of hydrants and valves.
Undaunted, feasibility discussions were held with Severn Trent stakeholders including Public Health and Standards and Water Quality Departments to agree parameters for acceptance criteria prior to the abandoned section being considered for recommissioning. The trade-off would be a rigorous scheme of investigation, verification and cleansing which would guarantee the continued quality of water supply. The team would have to bring to bear a wide range of technology and techniques in order to bring the vision to reality.
The investigation undertaken commenced with routine techniques widely adopted in the industry. These included CCTV, acoustic leakage detection hydrophone surveys. These were supplemented by non-destructive testing as well as destructive investigation on the sections of the main removed in order to gain access for surveys.
The results, although encouraging, threw up suspected leaks in several locations. This was not the definite proof the team needed to satisfy the success criteria on leakage, which were particularly onerous given the area of supply and the critical nature of the asset once in service. The main would, in effect, have to pass the similar tests as the brand-new pipeline the team hoped to avoid. Further investigation would be required to give the client the reassurance required to accept the solution.
Drawing on expertise within the team, an emerging technology was identified as the potential lifeline the study needed. The latest in acoustic leak detection undertaken from within the pipe itself. The project team engaged Advanced Pipe Inspection (API), the approved supplier for PIPA Ltd leak detection equipment. PIPA specialise in providing the latest innovative technology to enable HD CCTV for visual pipe condition, pressure drop and leakage detection, and identifying risks on low temperature pipe zones.
Working with API, the PLUTO S system was selected as the tool of choice to give a watertight guarantee of the integrity of the abandoned main. The detection head would be retracted through the main during the survey using a motorized drum which offered design flexible enough to operate on a large diameter 24” main. Furthermore, it would be able to navigate around numerous bends and obstacles if required.
The cable was floated down the already pressurized pipeline and captured at the exit point. A minimal number of insertion and exit points were required along the route thanks to the long-range cable of up to 2000m. The was an essential benefit over other more widely available instrumentation because it allowed fewer lane closures spaced further apart on the busy road. In doing so, undertaking some of the works at night, and by working collaboratively with Highways England and Nottinghamshire County Council, nmcn PLC were able to significantly reduce disruption to the public, commuters and local residents around one of the main arterial routes into Nottingham. The PLUTO S system would be able to pinpoint the location of leaks to millimetre accuracy, which would be of vital importance in the event of needing to dig down to make repairs in the carriageway of an ‘A’ road.
Unlike other floating CCTV systems that disturb sediment, the PLUTO S works against the flow of water to give clear images whilst remaining tethered at two points. This was a key feature to ensure the main passed drinking water sample criteria, any disturbance to sediment or the internal pipe walls may have caused water quality complications on completion of the survey.
The survey was a triumph and the acoustic leakage sensor confirmed that no leaks were present and the high-definition CCTV footage showed joints and welding to be of good condition. All four samples taken from cut out sections of the main were found to be of good condition, showed low tuberculation and following non-destructive testing demonstrated >50 years life span remaining.
With the green light in place (and a sigh of relief), the project team could move on with mains cleaning and water sampling, before being able to bring the main back into commission once more.
With confirmation in place that the solution the team had invested so much investigative cost into was accepted, attention turned to completing the rezone necessary to enable the network resilience that was at the heart of the project. The water transfer involved rezoning an area of approximately 2700 homes into an adjacent control group, thereby freeing up capacity in the catchment which would supply housing growth in Newark in the short term. This would require careful planning to ensure continuity of network operations and quality of supply to the homes of the always discerning general public.
The project team agreed with Severn Trent’s Public Health and Standards and Water Quality Departments to implement a programme of cleaning the main, and adjacent in-service pipes, with flushing techniques in several phases. Due to flow reversal in the existing network the rezone carried a high risk of discoloration which would be readily identified by Severn Trent customers that can lead to complaints. The risk was mitigated by gaining early warning of such issues using several live loggers with trigger level criteria to monitor at several locations and allow controlled flushing and valve operations for the rezone preventing discoloration became an issue for customers.
Early public engagement was critical in managing the high number of customers affected. Each customer receiving a letter prior to the work to notify them of the change in water and potential changes in taste and discoloration risks.
The recommissioning and supply rezone was a success. The abandoned main was brought back into service once more and no customer complaints were received throughout the process.
Following the success of the feasibility investigation the project and design team were able to recommission the existing main for the water transfer rezone. By avoiding lifting and laying techniques in highway and or adjacent fields this provided a number of significant environmental, customer, pubic, carbon and financial cost, and time saving benefits were realised for the client. In this instance, the estimated project saving enabled through the use of the PIPA PLUTO S system is thought to be in the region of £1.5m, with a delivery data circa 9-months earlier than otherwise possible.
The laying of new water mains in rural areas is unavoidably disruptive to the land owners, user and the environment. Fields along the comprised a horse-riding school, crop and pasture land that would have had large swathes sterilized for not only for the main laying but also up to 1 year after to allow reinstatement and regeneration to take place. It also prevented the removal of 16 hedgerows, large areas of vegetation and many large trees, thus preserving the visual aesthetic of the environment and preventing disturbance to wildlife habitat.
Beyond the environmental impact visible on the surface, in this instance extensive archaeological interest was protected. The mitigation for potential archaeological remains would have required extensive time-consuming strip mapping causing substantial increase in costs for the client and nmcn PLC and potentially 9 months delays to the project end date.
For nmcn PLC, the benefits would not stop there. Buoyed by the successful implementation of the PIPA pipe survey equipment the project team began to look beyond to the next application. The technology provided by PIPA Ltd has been proven to successfully pinpoint even tiny leaks in pressurised pipes regardless of whether water is flowing. If the technique could be used to confirm the suitability of an abandoned pipe, why not a new one?
The pressure testing of new water mains infrastructure, particularly large diameter trunk mains, requires the use of excessive quantities of water. This water is drawn from the network, and is then disposed of, wasting the energy that it has taken to treat and distribute it. If the PIPA system could be employed to verify leak tightness of new mains it could effectively replace traditional pressure testing procedures thereby saving large volumes of water.
The nmcn PLC team are actively progressing through the approvals process to utilise this technology on the next trunk main project which would see the testing of 6.3km of 600mm diameter pipework. Several other water companies are now implementing these techniques to become more cost and time efficient and save wasting megalitres of water and capacity from the network.