How much would an underground connection really cost?

A Cost estimate for an Underground Connection from Bramford to Twinstead

What follows is not conclusive at this point but we felt it worth presenting our initial findings. 

As we report in out October 2010 Newsletter, in the national press we found that there have been reports of the contract signed between civil engineers Costain and National Grid to build a transmission cable tunnel inLondon. The costs are there for all to see and a simple calculation to scale the cost for our local connection tells us that such a tunnel plus cables from Bramford to Twinstead would cost £360 million which is a fraction of the figure National Grid have talked of for such a connection. 

We can however use the published tunnel (only) cost information together with cost estimates for the Siemens GIL cable technology that we advocate to perform a slightly more sophisticated calculation.

We are in contact with Siemens seeking further clarification on transmission capacities for GIL but in the mean time these are our calculations:-

From a Reuters article copied below, we know that 33km of underground electricity tunnel costs £200m when constructed by NG civil engineering contractors Costain in London clay.

From Siemens the German energy infrastructure manufacturing giant, the we know from that GIL to replace several conventional cable circuits costs 6 million Euro/km. (a copy of their email confirming this is also copied below)

From the National Grid “consultation” web site we know the Bramford - Twinstead (B-T) connection is 26-30km long and logically, an underground route can be near straight so we can assume the shorter distance.

26km (length of B-T connection) / 33km (length of NG/Costain London tunnel) * £200million (cost of London Tunnel), = £160 million in round figures for a similar tunnel from Bramford to Twinstead.

26km(length of B-T connection) x 6 million Euro/km(Costof GIL as suggested by Siemens) = 156 million Euro x0.878510611 (to convert price into pounds(1 Euro = 0.878510611 British pounds as at 20/10/10)) = £140 million

Total cost therefore is £160m (tunnel cost) + £140m (cable cost) = £300million for an underground GIL solution from Bramford to Twinstead which is around 3 or 4 times the overhead cost but with much lower maintenance cost (Reliability / maintenance cost data from Siemens’ paper covering assessment of its 25 year old GIL installation in the Black Forest in Germany).

Clearly this is based on the GIL carrying the very high current levels NG say they need. This however is exactly what GIL is designed to do. NB:GIL requires smaller tunnels than the conventional XLPE cables used in the London tunnel from Elstree to St Johns Wood as heat and electromagnetic output is lower.

Below are the data the above is derived from:-

From Siemens:-

Received 19th October

Dear Cllr Holland,

Thank you for your e-mail and please accept our apologiesforthe delay in responding.

GIL is a technology designed for high power AC transmission applications where the alternative would be several underground cables. Each GIL circuit can replace a number of cable circuits, depending on voltage and cable design. The low heat losses in GIL allow it to be more closely spaced than the equivalent cables so it can be a good option where the route corridor is narrow or in tunnels. GIL is not usually competitive with a single or double cable circuit.

Each GIL installation requires detailed design before it can be accurately costed as there are many factors that influence the design and construction of even a short length. For purely order of magnitude guidance a figure of around 6 Million Euro per km for the GIL itself could be considered, to which should be added the necessary civil works. In the case of tunnels these would be very significant. If a specific project is considering using GIL a proper cost assessment would need to be carried out on a consultancy basis.

Best Regards

Lynsey Gray

Senior Communications Executive (North East)

Siemens plc

Communications & Government Affairs

Copied from From Reuters website

Oct 18

British construction firm Costain Group Plc (COSG.L) said it won a tunnel contract of about 200 million pounds ($319.8 million) from utility operator National Grid (NG.L) to upgrade UK's power infrastructure.

As per the contract, Costain is to build a 33-kilometre-long high-voltage electricity cable tunnel.