Meticulous coordination, skill and teamwork, were key to facilitating an underwater penstock replacement at the Queen Mother Reservoir. A remarkable project that required precise planning.
The Queen Mother Reservoir is four miles west of Heathrow Airport and stretches across just about 2 km2. It’s a massive inland body of water which forms a critical part of London’s drinking water infrastructure. Established in 1976 and operated by Thames Water, the reservoir is constructed from compacted London Clay core embankments and draws water from the River Thames through tunnels which are 20 metres below ground level.
From the surface, it is a stable and calm environment. But, when a 3.6m x 1.8m draw-off penstock, a vital piece of submerged infrastructure, failed at a depth of 24 metres, precision engineering, underwater operations, and a thorough project plan was required.
The Underwater Problem
During an engineering inspection in 2023, the lowest of the reservoir’s ten penstocks, which are mounted on the walls of the outlet tower, had failed in the closed position. These penstocks play an essential role in regulating water levels and enabling emergency drawdown. To access the failed unit, at a depth of 24 metres, presented a formidable challenge.
“Emptying the reservoir, which holds around 37 million metres3 of water, simply wasn’t an option,” explains Glenfield Invicta’s Engineering Services Manager for Dams, Reservoirs and Hydropower - Ken Ottley. “It was clear that the work would have to be carried out by divers, and that meant we needed a strategy that engineered out as much risk as possible.”
Images above: Planning sketches. Failed penstock in situ underwater and viewing via monitor.
From Survey to Commissioning
The project began in August 2023, and Glenfield Invicta were commissioned to survey the submerged penstock and formulate a viable solution for the safe removal and replacement. A deep-water survey was conducted to map the existing penstock, frame, and operating components. The data collected was used to create a detailed 3D model, which allowed divers and engineers to visualise and plan every step of the operation.
Ottley developed an approach to de-risk the final installation and simplify the most critical stage of the work: alignment and fixing. His solution was to bolt a custom-designed backplate directly to the tower wall when the old penstock was removed. The new penstock could then be securely fastened to the backplate, which removed the need for precise underwater alignment under challenging visibility and physical constraints.
“Given the restricted movement in diving suits and helmets, simplicity was paramount. The backplate allowed the divers to position and fix the new unit without spending excessive time or making complex adjustments at depth,” Ottley adds.
The team conducted a trial installation on a similar penstock located just 6 metres below the surface before tackling the 24m-deep penstock. This dry run allowed them to test the underwater cutting and drilling procedures, the work pontoon and lifting frame, giving the client confidence to proceed with the full operation.
Images above: Engineers and diving team on way to the platform. Penstock arriving on site and pre-install assessment.
Installation and Execution
To prepare for the final replacement, the reservoir level was lowered by 10 metres to improve dive conditions. The divers could safely work for 98 minutes at a time at 14 metres depth. If the full depth of 24 metres had been required, dive time would have been limited to just 29 minutes, which would significantly increase both risk and project duration.
Removing the old penstock involved precision cutting using thermal lances, a process monitored in real time from the surface.
“Watching on the monitors as the divers worked with thermal lances to remove the penstock was incredible. It was a true demonstration of teamwork, skill, and coordination,” says Ottley.
When the original penstock was removed, the replacement unit, which was supplied by RRR (Treble R) in Redditch, was installed using the backplate system. The existing operating gear and refurbished spindles were able to be retained, but the actuators were renewed to the latest Rotork IQ models, which offer remote operation capability and greater reliability.
Commissioning of the new equipment was completed and finalised in October 2024, and complete documentation provided.
Above images: The platform mid-reservoir and pre-dive checks.
A Model of Collaborative Engineering
This project is a prime example of Glenfield Invicta’s ability to deliver complex and high-risk infrastructure solutions through innovation, collaboration, and technical expertise.
“I was very fortunate to be part of such a fascinating project and to work alongside such a committed team of engineers and divers. The success of the installation speaks volumes about the planning and professionalism involved,” concludes Ottley.
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