Third Menai Crossing (consultation)

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KeithW
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Re: Third Menai Crossing (consultation)

Post by KeithW »

wrinkly wrote:Press release:

http://gov.wales/newsroom/transport/201 ... d/?lang=en
Third Menai Crossing electricity connection to be examined
A feasibility study will be carried out to investigate whether a vital electricity connection could be carried on the proposed third Menai crossing, Economy and Transport Secretary Ken Skates has announced.
I really would need a lot of persuading that carrying high power distribution cables on or under a bridge is a good idea.
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Berk
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Re: Third Menai Crossing (consultation)

Post by Berk »

What would the risks be?? Just to put it into perspective.
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Re: Third Menai Crossing (consultation)

Post by KeithW »

Berk wrote:What would the risks be?? Just to put it into perspective.
Well we are talking about National Grid cables carrying Megawatts of electricity at high voltages

Thinking aloud even if we discount catastrophic scenarios such as melting the bridge, passengers or both consider the implications of failure of a cable.

Scenario 1 - Faulty cable
Step 1 - Isolate the power - hard luck Anglelsey users
Step 2- Close the bridge
Step 3 - Access the cables - possibly involving digging up the road surface
Step 4 - Repair etc

Scenario 2 - Bridge problem such as failed expansion joints
Step 1 - Isolate the power - hard luck Anglelsey users
Step 2- Close the bridge
Step 3 - Repair the Bridge
Step 4 - Test the cables
Step 5 - Reopen the bridge

Now when the Grid bury underground cables they try to AVOID running under roads so as not to run into such problems

Here are the basic guidelines that are followed by National Grid
At the time of installation, equipment is put
in place that monitors the performance of the
cable and its insulation. Over the lifetime of a
cable significant refurbishment and repairs to
ancillary equipment, such as fluid tanks, may
require more significant excavations at joint bays
and stop joints. Vehicular access to strategic
areas of the cable route, such as joint bays,
is required at all times.

If a fault occurs on a 400kV underground cable,
it is on average out of service for a period 25 times
longer than 400kV overhead lines. This is due
principally to the long time taken to locate,
excavate and undertake technically involved
repairs. These maintenance and repairs also
cost significantly more.

The majority of faults on cables are caused by
fluid leaks, faulty joints and accessories, sheath
faults, water cooling failures and, most commonly,
third party damage. Under fault conditions,
between two and six weeks can be required to
locate the fault or fluid leak and repair the cable.
During this period excavations may be required
which can result in road closures and traffic
management measures. In some cases, the
excavations could be in the order of 4m x 30m.

Underground cables are generally matched to the
rating of the overhead line route in which they are
installed; this also determines the cable design
and any necessary cooling. Where an increase
in the rating of an overhead line is required it can
usually be achieved relatively easily by using
different or larger conductors. Where there is an
underground cable installed as part of a route the
up-rating can only be achieved at considerable
expense, for example, by re-excavation and the
installation of larger or more cables or with
additional cooling
So the installation would require 24/7 access, It could require a closure of up to 6 weeks to even locate a fault and a bridge does not meet the criteria of avoiding vibration and shock loads required for underground cable routes.

The real problem though is likely to be electro-magnetic induction. Large power cables produce a significant electromagnetic field. With a pylon installation the conductor is a minimum of 10 metres above the road the field strength is relatively low but stick em under the road and it can be very high. Now move a metal object through a strong magnetic field and you get significant levels of induced electric flow. Given that modern cars have a LOT of electronic sensors this is not likely to end well unless you have at least a couple of metres of gap between cable and road surface.

None of these problems are insoluble but it would cost loads a money and while I can see the attraction of having an underground link given that the existing grid connection is on overhead lines it seems to be a very high price to pay when the pylons march away from the bridge in any case.
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Re: Third Menai Crossing (consultation)

Post by nowster »

Bear in mind that the existing Britannia Bridge has a railway track inside it. If you made Britannia2 be a similar structure, the electricity cables could be on the lower deck, similar to how cables pass through the Woodhead tunnels.
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Re: Third Menai Crossing (consultation)

Post by KeithW »

nowster wrote:Bear in mind that the existing Britannia Bridge has a railway track inside it. If you made Britannia2 be a similar structure, the electricity cables could be on the lower deck, similar to how cables pass through the Woodhead tunnels.
In the case of the Brittania Bridge the power lines span the Strait to the west on overhead lines.
The problems I outlined largely still apply and of course the Woodhead tunnels were closed to traffic before the cables were installed.

If you wanted a cable that didn't go overhead and a have a few million to spare the way to do it is to lay a subsea cable in an excavated trench as was done between Vancouver Island and mainland BC and the Interconnect to France.
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Re: Third Menai Crossing (consultation)

Post by Chris_533976 »

I just think given the situation with Menai, that building an identical second Brittania right next to the first would be the right thing to do. Would look far better and probably would be far more accepted than another modern structure.
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Re: Third Menai Crossing (consultation)

Post by Berk »

Chris_533976 wrote:I just think given the situation with Menai, that building an identical second Brittania right next to the first would be the right thing to do. Would look far better and probably would be far more accepted than another modern structure.
That would actually make a lot of sense. Not only would you be duplicating the S2 roadway but you would be duplicating the S1 railway (it was singled after the fire, partly due to the downturn in railway traffic, but also to facilitate the road conversion and maintenance works).
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Re: Third Menai Crossing (consultation)

Post by nowster »

I've just been reminded that the current plan is for the new electricity cables to go through a tunnel under the Menai strait.

http://www.bbc.co.uk/news/uk-wales-nort ... s-37495774

The crossing study is also looking at putting the power cables through the bridge.

http://www.bbc.co.uk/news/uk-wales-politics-44022405
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Re: Third Menai Crossing (consultation)

Post by Berk »

Is something wrong with the old
Bridge that people don’t want to run cables cross it?? Why not wait until after the new bridge is ready, then you can run them at rail level.

No need to close the bridge to motor traffic then.
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Re: Third Menai Crossing (consultation)

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Berk wrote:Is something wrong with the old Bridge that people don’t want to run cables cross it?? Why not wait until after the new bridge is ready, then you can run them at rail level.

No need to close the bridge to motor traffic then.
Just rail traffic?
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Re: Third Menai Crossing (consultation)

Post by Berk »

I think it could be neatly time to coincide with rail possessions. Though if you do run cables along the rail deck, I guess it would have to remain singled...
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Re: Third Menai Crossing (consultation)

Post by AndyB »

Water and gas travels across the present Britannia Bridge - slight obstacle to double track!
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Re: Third Menai Crossing (consultation)

Post by wrinkly »

Apropos of nothing, is it still possible for maintenance vehicles to cross the bridge alongside the rail track, or do the pipes take up too much space?
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Re: Third Menai Crossing (consultation)

Post by Berk »

I can’t definitely say. I believe heavy maintenance requires a full occupation, so is done routinely as far as possible.
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Re: Third Menai Crossing (consultation)

Post by AndyB »

Photographs suggest it's tight but possible. I would imagine it's done under full possession when the line is closed.
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Re: Third Menai Crossing (consultation)

Post by KeithW »

Berk wrote:Is something wrong with the old
Bridge that people don’t want to run cables cross it?? Why not wait until after the new bridge is ready, then you can run them at rail level.

No need to close the bridge to motor traffic then.
The rail crossing is rather an important link and running 400kV lines next to a live rail track is a spectacularly bad idea. The plan of using a submerged cable is simple and not that expensive.
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Re: Third Menai Crossing (consultation)

Post by nowster »

Not submerged, but tunnelled.

This will be interesting as the strait is a geological fault.
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Re: Third Menai Crossing (consultation)

Post by KeithW »

nowster wrote:Not submerged, but tunnelled.

This will be interesting as the strait is a geological fault.

Usually they lay a cable in a subsea trench in such geology but a quick look at the literature suggests the last significant movement of the fault was around 60 million years ago and most of it happened during the period when the Great Glen was formed in Scotland and Edinburgh was a major centre of volcanic activity.
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Re: Third Menai Crossing (consultation)

Post by Chris Bertram »

They could always consider reinstating the ferry at Caernarfon, re-joining the two parts of B4419.
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From the SABRE Wiki: B4419 :

he B4419 is a road in two halves in North Wales. It is described in the 1922 Road Lists as Carnarvon - Llangaffo - Pentre-berw, crossing the Menai Strait via a ferry that was discontinued in the 1950s, thus leaving two unconnected sections which remain to this day. The ferry was only really suitable for foot passengers, which raises the question of why the road was numbered as a through route; the A5 was always the main road on and off the island.

As such, B4419 may

... Read More
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Re: Third Menai Crossing (consultation)

Post by vlad »

Chris Bertram wrote:They could always consider reinstating the ferry at Caernarfon, re-joining the two parts of B4419.
I'm not sure the original ferry was ever suitable for anything more substantial than a bike, so a new ferry would need quite a bit of infrastructure. I can't help thinking it'd be easier just to build another bridge.
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From the SABRE Wiki: B4419 :

he B4419 is a road in two halves in North Wales. It is described in the 1922 Road Lists as Carnarvon - Llangaffo - Pentre-berw, crossing the Menai Strait via a ferry that was discontinued in the 1950s, thus leaving two unconnected sections which remain to this day. The ferry was only really suitable for foot passengers, which raises the question of why the road was numbered as a through route; the A5 was always the main road on and off the island.

As such, B4419 may

... Read More
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