HomeInfrastructureSevern Tunnel Electrification - Planning logistics and interfaces

Severn Tunnel Electrification – Planning logistics and interfaces

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On Monday 24 October, Great Western Railway introduced a new service between Cardiff and London called, appropriately, “The Welshman”. It consists of 59 daily train services between the two cities and has been introduced to mark the re-opening of the Severn Tunnel, which had been closed from 12 September until 21 October 2016.

The 130-year old, 6.8km long, Severn Tunnel was closed for almost six weeks so that engineers could complete the work to install Overhead Line Equipment (OLE) as part of the £2.8 billion project to electrify the Great Western main line (GWML) route, planned to be completed in 2019.

The reopening of the tunnel was a critical milestone in the project to deliver electric trains for passengers in South Wales. As well as improved journeys, economists expect electrification of the line between South Wales and London to deliver the economic boost for South Wales that will result from better connectivity to London, a critical factor for attracting investment for the electrification of the route in the first place.

Preparatory work

The closure was known as the ‘Severn Tunnel Autumn Disruption’ (STAD) and, to keep the closure of the tunnel to a minimum, a significant amount of preparation work was undertaken in the previous 12 months. Forty tonnes of soot were removed from the tunnel, as well as repairs to the brickwork.

However, the scale of the engineering challenge involved, and the extensive machinery required to electrify the tunnel, meant that the STAD was unavoidable. Network Rail claim that it would have taken up to five years to complete the electrification project if the STAD had not been actioned.

Instead of installing a conventional overhead wire system, Network Rail decided to install a new system, developed by a Swiss company Furrer+Frey, throughout the tunnel. The system is in fact being used in several other tunnels on the GWML and consists of an aluminium rail, held to the tunnel roof by drop tubes and registration arms, with the contact wire that carries the power supply fed into a slot in the base of the rail.

Robust system

This Rigid Overhead Conductor Rail system (ROCS) has several advantages over the usual overhead wire system: the system is more robust than overhead wires and the maintenance required is reduced; the contact with the pantograph is thought to be more reliable and efficient and the system is considered to be more compact than the traditional wired system.

Also, ROCS can be used in tunnels where headroom is constrained, whereas wired solutions need more headroom. This is not easily achieved in a tunnel environment and using the conductor rail system has reduced the amount of work needed to the track levels in order to provide the necessary clearance.

The preparatory work and the work to fit all the equipment for the new system for the electrification of the railway track through the Severn Tunnel was installed by ABC Electrification, comprising Alstom, Babcock and Costain. ABC is responsible for delivering the design, preparatory work and the final fitting of the electrification equipment throughout the tunnel and the surrounding area.

Environmental factors

Some particular problems facing the ABC Electrification team are the environmental factors in the Severn Tunnel, including saline water from the Severn Estuary above it and soot deposition from the freight trains carrying coal that pass through it. As a result, ABC said that the metalwork holding the ROCS to the tunnel structure would be made of high-grade stainless steel, with a lifespan of around 60 years.

Also, although the system is more commonly used in Europe, it is probably the first time that ROCS is to be used above the ballasted track which exists throughout the Severn Tunnel. Previously, on high-speed routes, it was used in conjunction with slab track, where the tracks are concreted into the base of a tunnel.

Local skills

Rail Engineer spoke to Dan Tipper, who is the Network Rail project director for Wales, and John Skentelbery, ABC’s programme director, just after the completion of the work.

They were both keen to point out that, in partnership with principal contractor ABC Electrification and key suppliers AMCO, Keltbray, Arup and Furrer+Frey, Network Rail has been able to source about 75 per cent of the team from Wales. He explained that many of the Severn Tunnel engineers have been recruited from former steel and mining industries, while the team also includes a significant number of former armed forces personnel.

The project team has also completed nearly 3,000 training days this year. Training includes rail competencies, career development, professional and technical training, including specialist overhead line equipment (OLE) qualifications.

Dan explained that the first task was to fix 14km of autotransformer feeder (ATF), a large 65mm diameter cable fixed to the tunnel wall every two metres to 7,000 anchors and cleats that had been fixed in the pre- STAD work programme. This is part of the 50kV autotransformer power system that reduces transmission losses compared with a conventional 25kV system. See David Shirres’ explanation of electrification systems adjacent to this article.

Once the AMCO work team had become familiar with the process of installing the ATF cable, they were achieving 800 metres in a 10- hour shift, a significant achievement.

Meticulous planning

The work that followed had to be planned like a military operation. The devil was in the detail and one minor flaw in the plan would jeopardise the whole process. Meetings with main suppliers AMCO and Keltbray were held daily and, as pointed out by both Dan and John, you could not distinguish one company from another as everyone was working as one team. It was an essential requirement if the work was going to be completed successfully and in time.

The phases of work that were meticulously planned, using equipment and platforms especially designed by AMCO/Foulston Forge and Keltbray for the Severn Tunnel, were as follows:

  • Install 857 anchors using a specially designed rig designed to drill three holes, place resin and then fix bolts and anchor;
  • Fix 1,700 vertical drop tubes weighing 45 to 60kg, setting them high then lowering into final position;
  • Fix pre-assembled registration cross arm to the drop tube;
  • Install the aluminium conductor beam in 12-metre lengths, completing 480 metres per shift;
  • Install 14km of copper contact wire within the aluminium profile beam;
  • Install 14km of earth wire;
  • Fix a total of 22 expansion joints into the conductor beam – these expansion joints, a bit like fish-plated joints, are made from aluminium, weigh 100kg, and are approximately two metres long with a sliding mechanism inside.

The work was planned to be undertaken in three 10-hour overlapping shifts. All the work was orchestrated from a central depot at Filton where they operated 24/7 ‘Silver Command control’ across the entirety of the STAD possession, of which the Severn tunnel was one major element, and had the expertise on hand to address any technical issue that arose.

Myriad of interfaces

Plans had to be amended on a daily basis to ensure that the work was completed without incident and the successful management of the myriad of interfaces became a critical factor. Not only was there a complex operation to install the equipment for electrification within the Severn Tunnel taking place, but the STAD S&C alliance team also completed the renewal of 750 metres of track in the Down line within the tunnel.

In nearby Patchway New Tunnel, the STAD IP Track/Babcock team lowered 385 metres of track, with design and tunnel stability monitoring and guidance provided by ABC/Arup. The ABC/ AMCO team then installed 1.6km of ATF cable as well as a significant amount of OLE anchors and registration arms working toward an OLE completion date in that tunnel in the new year.

In addition, the STAD IP Track/Babcock team lowered 770 metres of track in Patchway Old tunnel, with design and tunnel stability monitoring and guidance provided by ABC/Arup. Whilst this work was in progress, they came across an unrecorded old drain and large cross tunnel supports that had to be dealt with, eating up precious time.

Both track lowers were excavated in 20-metre lengths as this was the optimum logistical method for these single-road Victorian tunnels. By monitoring tunnel movement, and by reviewing the tunnel stability model with specialist advice from ABC/Arup/AECOM, the Babcock track-lower method and length of excavation was reduced, in some cases, to just six-metre lengths to ensure that the structural integrity of the tunnel was maintained.

Despite all these challenges, such good progress was made that two additional designs and a final 90 metres of additional track position were achieved through tremendous teamwork and focus. The track lower and OLE will be progressed in this tunnel during 2017.

Hidden beams

At another STAD site in Stoke Gifford, a set of switches was replaced. Also, 72 augured concrete pile foundations were constructed in the area ready to receive masts, 21 masts erected and a signal gantry was replaced and raised to enable the Series 1 catenary wire OLE to be installed. This work was close to another track lowering site at Little Stoke Farm where they came across a cluster of concrete beams dating back to 1920s. The find had to be investigated but it was decided to leave them where they were discovered and design a track alignment over the top.

Both Dan and John are very proud of what they, their integrated team and other STAD delivery teams have achieved, not only because of the work that was successfully completed on time but also because there were no lost-time accidents. They also recorded 1,680 ‘close calls’, a clear indication of the safety culture that prevailed; an attitude that can only work if everyone is working together.

Written by Collin Carr


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