Laleham to Twickenham EHV Cable Overlay

Description of Works

Following on from the recently completed Twickenham, Ham, Richmond 33kV Network Reinforcement Project, the EDFe / Murphy Capital Programme Delivery Team were initially tasked with carrying out a detailed feasibility study in order to find an appropriate network reinforcement solution that would provide firm interconnection between Twickenham & Kingston Grid.

Twickenham Grid 132/33 kV Substation is supplied from Laleham 275/132 kV substation via two 12.5 km long 132 kV gas compression circuits, installed in 1965. Recent failures had given rise to concern over their condition and hence long-term reliability.

The criteria & deliverables that the solution had to satisfy were identified as:
• Improve security of supply to 150,000 customers fed from Twickenham, by the removal of unreliable plant.
• Reduce risk of an n-2 failure at Twickenham Grid substation as demand was greater than 100 MVA.
• Reduce costs in association with the maintenance of the circuit and the high cost of repair. Spares and repair resource will diminish over time and become more expensive.
• Project had to minimise impact to the public over other options, only requiring excavations at the joint positions rather than the complete route.
• New innovative cable and processes required.
• Opportunity to demonstrate to other DNO’s and Ofgem about EDF Energy’s continued innovative approach and network developments.
• Project would lead to the removal of the Ham – Richmond FFC, decreasing environmental risk and removing unreliable and high maintenance equipment.

Seeking to utilise the existing pipework as a conduit or medium for the replacement asset the initial design meetings highlighted a necessity for a methodology that would furnish the construction team with the capability to open & close sections of ground at predetermined nodes on multiple occasions over a planned two year single circuit outage. As the existing circuits were installed & jointed in approximately 500Mtr lengths the obvious solution was to utilise these locations in a similar vein.

With the aid of existing strip records & installation data from the works completed in 1963 the team began a desktop study charting both the known position of the 132kV existing circuits & also the location & proximity of additional utilities plant & equipment that had since been installed. Once composite drawings with all known statistics had been produced the team engaged the Local Authorities & Third Party Stake Holders with a view to identifying the most practical & cost beneficial methodology that could be adopted whilst mitigating the impact to the public & ensuring the integrity of the existing supply remained uncompromised.

As the works could only be completed via a single circuit outage during two annual planned 16 Week shutdowns the programme timescales & constraints were extremely prohibitive. During the 16 Week period Murphy required the ability to open & close 26 Joint Bays measuring 12M x 3M x 2.6M multiple times to facilitate both the decommissioning of the existing circuits & also the installation & jointing of the new.

The design team came up with a solution where prior to the planned outages each joint bay location could be excavated in its entirety & a cast in situ chamber constructed. The roof slab of the joint bays would consist of a segmental load bearing steel cover plate that could be removed & replaced in a short time period whilst also providing security for the assets during installation works.

This methodology reduced the volume of excavation from 12.5Km to a little over 350Mtrs thus providing the client with the most cost beneficial solution & also ensuring the impact to the local area & residents was drastically minimised.

With the design & methodology formulated intrusive survey works began in late 2008. Trial holes were completed with a view to validating the correlation between the information provided on the obtained statistics, & what the actual volume & location was of equipment in the ground. At each trial hole location soil samples were also taken to ensure the excavated spoil met the required Waste Acceptance Criteria (WAC) for safe disposal. The trial holes also provided an opportunity to carry out a visual inspection & validate the integrity of the existing pipe work

As survey works progressed it was noted that a large percentage of the existing route had previously been installed on a series of existing railway bridges & structures. This caused concern regarding both the ability to access the asset for the purpose of maintenance or under fault condition & also afforded a lower level of security when compared to a buried circuit.

Following further survey works & close liaison with Network Rail & London Borough of Richmond Upon Thames the team proposed a trenchless solution whereby a Directional Drill could install 2 x 250mm shots for a distance of approximately 420Mtrs launching from a Network Rail Green Zone at the rear of Twickenham Grid under an existing nature reserve & river into Crane Park Recreation Ground. This solution removed the asset from a total of three rail bridges & over 90% of Network Rail Property.

With the route design firmly validated Joint Bay Construction works started in early 2009 & were completed in time for the first planned outage in April 2010. This in itself was an onerous & complex task due to both the existing live 132kV circuits & also the myriad of additional utility plant & equipment that had been installed in close proximity & in certain cases directly adjacent to the system since its installation

Nexan who are global experts in cable & cabling technology furnished EDFe with a replacement cable solution incorporating an optical fibre which is built into the overall construction of the cable. The replacement 132kV “Pipe” type 145kV cable (Type A2XS(FL)2Y+FG 3x1x630RM/167 76/132kV + FO – Multitube Cable in Interstice) is a new type of EHV cable with 3 phases & associated fibres bound in one insulated XLPE sheath with aluminium cores being used for the first time in the UK.

The next challenge that the team needed to address was to design a bespoke mobile cable installation system that could be utilised to both remove the existing cable & also install the new whilst working within a restricted space.

Based on knowledge & experience of installing EHV cable circuits within complex tunnels & deep shafts the team provided the engineering solution resulting in the form of extensive modification of a 20Ton Self Loading Cable Trailer to incorporate a capstan winch drum drive unit which can be utilised to both load & unload the cable drum as required. During the modification process great attention was afforded to the current WAH regulations. Consequently the unit was also modified to include perimeter working platforms, safety hand rails, & stepped access / egress.

The unit was rigorously tested & by early April 2010 & had successfully removed & installed the initial circuits safely & within programme mitigating both cost & necessity for crains & supplementary haulage.

This project has further enhanced our reputation for delivering large and technically challenging projects, often in difficult conditions, on time and to exacting standards. With an emphasis on innovative design & implementation processes the Power Division has a track proven record of furnishing our client base with bespoke turnkey integrated solutions.

Key to the success of the Power Division is its ability to provide a total in-house service with a large multi-discipline workforce. This ensures flexibility, control, and, above all, quality and safety.

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