Not only is the Thameslink Programme one of the largest single projects currently being undertaken by Network Rail, but it has also been one of the most successful – promoting collaboration which in turn has engendered a culture that facilitates the development of innovation on the UK rail network. Writes Steve Cox, regional engineering delivery manager – north, Balfour Beatty Rail Projects LTD
One such successful innovation achieved by this programme was the introduction of a reduced depth overhead conductor beam electrification system. This was installed on the Thameslink route between King’s Cross Thameslink (Disused) station and St. Pancras (Low Level) station, approximately 1.7 kilometres of twin-track railway running through a variety of infrastructure types including two stations and a series of tunnels. This special conductor beam, developed by Balfour Beatty Rail, provides the rail industry with a robust, easily constructible, low maintenance electrification system that can be fitted into locations where space is at a premium.
Thameslink connects Bedford, on the Midland main line, with Brighton on the south coast. The 140 mile route crosses London and the Thames, incorporating some of the most challenging infrastructure for the railway electrification engineer. The programme, which is progressing well and is now into its second phase, Key Output 2, will deliver improved journeys and better connections through the capital, tackling overcrowding on one of the UK’s busiest routes.
To achieve the increase in rail capacity on the Thameslink Service, scheduled for 2018 when up to 24 trains per hour will run in each direction through the core area of London, it was necessary for Network Rail to improve the reliability and robustness of the electrification equipment along the route. The area between King’s Cross (Disused) station and St. Pancras (Low Level) station, wired with a Mark IIIB electrification system, had been identified in previous studies to be nearing the end of its normal life expectancy and a weak point in the route.
Network Rail decided that the reduced depth conductor beam solution would provide a robust, low maintenance system and provided Balfour Beatty Rail with a design and build contract to replace the existing MK IIIB equipment. This installation was completed and commissioned at Easter 2013.
Lowest wire heights
The Inner Core Area of the Thameslink route includes a series of low clearance tunnels, bridges and stations into which the overhead line systems must be fitted. This restricted height necessitates, in certain areas, the lowest permissible contact wire heights on the UK rail network of 3925mm.
Consequently, this infrastructure is ideally suited to the reduced depth conductor beam system. At only 80mm in depth, it is 30mm shallower than a conventional conductor beam which is critical where headroom is at a premium as was the case on Thameslink.
The Balfour Beatty reduced depth conductor beam is a hollow extruded aluminium profile supplied in standard 12, 10 or 8 metre sections. These can be cut to finished length and joined together using bolted splice plates, forming a continuous beam into which an un-tensioned conventional contact wire is inserted.
To achieve a constant wear on the pantograph carbon strip, the conductor beam is installed laterally in a sinusoidal wave rather than adopting the usual staggering profile used on wired systems.
Increased asset life
The use of an untensioned contact wire eliminates the need for bulky tensioning devices, such as balance weights or spring tensioners, which are used with conventional electrification systems. As there is no tension in the system, it reduces the loading that is imposed by the electrification equipment onto the surrounding infrastructure.
The reduced depth conductor beam needs less maintenance as it has barely any moving parts to maintain and an increased wear allowance on the contact wire. In a conventional tensioned catenary electrification system, the contact wire life is limited to around 25 – 33% of the cross sectional area of the wire being allowed to wear. At this level, a limit is reached as the tensile stress in the wire caused by the tensioning approaches its maximum allowable limit. With the reduced depth conductor beam system, the contact wire can wear in excess of 35% as there is no tension in the contact wire. This can extend the system life by up to 10%.
In addition to increasing the asset life, with a short circuit rating of 45kA (compared to 6 or 12 kA for a conventional catenary system), the reduced depth conductor beam system provides a more robust solution. Furthermore, the system is more resilient to mechanical damage.
As electrical sectioning in a reduced depth conductor beam system is achieved by creating an air gap between two sections of beam, there is no need for traditional section insulators which are a common source of maintenance issues in conventional overhead line systems.
Installing new supports
The major challenge faced during the design and implementation of this project was the number of interfaces to be considered, and in some cases avoided, within the existing infrastructure.
This was a wired MK IIIB solution with supports spaced at regular intervals of between 10 and 20 metres throughout the route.
The schedule for the construction works required that around 160 new supports for the conductor beam would be installed while the existing wired overhead line system was still in place. Although this de-risked the installation works at Easter 2013, it provided a significant challenge for the design team to position the new supports in a way that would avoid clashes with all existing OLE supporting equipment and other infrastructure.
To add to the complexity of the project, the existing infrastructure throughout the route is not all Network Rail owned and, as such, other asset owners had to be consulted to obtain agreement for proposed attachment locations and support type. Particular attention had to be paid to the new structures provided at King’s Cross Thameslink station as this was proposed as an emergency point of egress for train passengers in perturbed service conditions.
These circumstances led to another innovative solution in the development of pre-cast concrete gravity base foundations for the overhead line support structures. These foundations were brought in on a trailer pulled by a road-rail vehicle and then craned on to the platform, avoiding the need for an excavation or the handling of wet concrete on site. The use of precast concrete foundations provided a safer and more sustainable engineered solution. This technique also facilitated the efficient use of valuable track access.
Wires out, beams in
The transition arrangement between the tensioned overhead line system and the untensioned reduced depth conductor beam system also provided a challenge to the engineering team. The interfacing wired system is a twin contact system in which both the contact wires run at the same parallel height as the in- running beam before terminating separately.
The infrastructure between King’s Cross (Disused) station and St Pancras (Low Level) station is critical to the Thameslink route so once the existing wired system had been removed it was crucial that the new conductor beam system was installed and commissioned within the arranged 104 hour blockade. This was achieved by the project team with four hours to spare.
As could be expected with the first major installation of an innovative product on a critical route, significant and detailed planning was undertaken along with a number of contingency plans being put in place to de-risk the project.
Part of this process involved training staff on installing and maintaining the reduced depth conductor beam equipment to develop familiarity and competence prior to the equipment being installed. This training was undertaken on test sites away from the operational railway prior to the blockade at Easter 2013.
The new supports for the reduced depth conductor beam system were installed using major possession access in April and May 2012, along with reduced access on other weekends. This allowed a significant amount of preparatory works to be undertaken prior to the Easter 2013 blockade when the existing wired MK IIIB overhead line equipment could be removed and the new beam system installed. This process significantly simplified the works.
As the conductor beam product is lightweight and easy to install, it lends itself to a production-line installation process. This was adopted during the installation at Thameslink. An ‘assembly line’ was set up on the Up and Down Moorgate lines with each road having six machines, aligned from north to south, in a ‘factory unit’ comprising one road-rail vehicle with three trailers, an SRS platform, Evo MEWP, SRS wiring unit, Evo MEWP and finally an SRS basket with pan.
Since the majority of supports had been installed prior to the Easter blockade, the first operation in the construction sequence was to remove the existing overhead line equipment and install any new conductor beam supports that had not already been installed. This was followed by the beam installation unit in which the beam sections were cut to length, drilled and installed.
Once a continuous run of beam had been installed, the wire unit was used to fit the contact wire using a special tool that employs a series of rollers to open the conductor beam profile locally and hold the contact wire in the correct position. After the device has passed, the profile closes elastically with the wire inserted into the beam. A protective grease was applied to the wire as part of the process.
The ease of installation provided an advantage when, as on the Thameslink project in the core of London, the possession access is limited.
Throughout the project, a true spirit of collaboration existed between Balfour Beatty Rail and Network Rail with everyone focussed on achieving the project goal of completing the upgraded electrified system by Easter 2013. Without the focus and collaboration shown by the project team, the successful delivery of the project and the associated innovation would not have been possible.