Electrically de-icing the Queensferry Crossing cable-stayed bridge.

BBC: “Falling ice causes first Queensferry Crossing closure”

Keeping such bridges open even in icing conditions is really not rocket science. What, to me anyway, is the obvious solution – to pass an electrical heating current through the bridge’s support cables –  doesn’t seem to be “obvious” to other research scientists and engineers whose “Thermal Systems” for melting the ice are reviewed here.

I suggested this simple solution, outlined the calculations required and warned of some dangers in an email to the Queensferry Crossing bridge authorities and contractors in March 2019, but as usual, the authorities ignore solutions until there is a political price to be paid for continuing to ignore solutions in a pig-headed, in-denial kind of way that politicians like to get away with, if they possibly can.

There follows a link to a PDF of the email I sent the bridge authorities last year – hopefully you can click the link and open and / or download the PDF so you can read it.

Queensferry falling ice hazard solution – electrically-heated cable stays

Deicing power for 70km of cables

@ 100W/m = 7MW = household electricity within a 3 mile radius of the bridge.
@ 250W/m = 17.5MW = household electricity within a 5 mile radius of the bridge.

Cable strands

Some strands in the cable are better situated for heating the cable than other strands, depending on their position in the cable as I have labelled them alphabetically, beginning with the label “A” for the centre strand (which is the worst strand for heating the outside of the cable, where the ice would be) and labelling the outer strands last in alphabetical order, which are best for heating the outside of the cable.

The cable strands are by convention named here using the format – “(Number of strands in the cable)-(Letter)”. Thus the centre strand in the 55-strand cable is named as “55-A”, the 6 strands immediately surrounding the sole 55-A are all named of type “55-B”.

For each strand in the cable we can assign a factor of heating capacity.

For the 55-strand cable, the total heating capacity factor assigned is 48.

For the 55-strand cable, there are a total of 24 strands which have utility for heating the cable – 6 of the 55-F type name strands, 6 x 55-Gs and 12 x 55-Hs. The 31 other strands (the 55-A to 55-Es) are not needed for heating per se, though could carry electrical currents whether by design or otherwise.

We can tabulate for each strand label, the heating power fraction and percentage, according to each strand’s heating capacity factor as a fraction of the cable’s total heating capacity factor.

Strands-label Heating power fraction Heating power %
55-F 1/48 2.1%
55-G 2/48 4.2%
55-H 2.5/48 5.2%

 

61-Strand Cable

For the 61-strand cable, the total heating capacity factor assigned is 54.

For the 61-strand cable, there are a total of 24 strands which have utility for heating the cable – 6 x 61-Gs, 12 x 61-Hs and 6 x 61-Is. There are 37 other strands – the 61-A to 61-Fs.

Strands-label Heating power fraction Heating power %
61-G 2/54 3.7%
61-H 2/54 3.7%
61-I 3/54 5.6%

 

For the 73-strand cable, the total heating capacity factor assigned is 54.

For the 73-strand cable, there are a total of 30 strands which have utility for heating the cable – 12 x 73-Hs, 6 x 73-Is and 12 x 73-Js. There are 43 other strands – the 73-A to 73-Gs.

Strands-label Heating power fraction Heating power %
73-H 1/54 1.9%
73-I 2/54 3.7%
73-J 2.5/54 4.6%

 

For the 85-strand cable, the total heating capacity factor is 60.

For the 85-strand cable, there are a total of 30 strands which have utility for heating the cable – 6 x 85-Is, 12 x 85-Js and 12 x 85-Ks. There are 55 other strands – the 85-A to 85-Hs.

Strands-label Heating power fraction Heating power %
85-I 1/60 1.7%
85-J 2/60 3.3%
85-K 2.5/60 4.2%

 


For the 91-strand cable, the total heating capacity factor is 66.

For the 91-strand cable, there are a total of 30 strands which have utility for heating the cable – 12 x 91-Js, 12 x 91-Ks and 6 x 91-Ls. There are 61 other strands – the 91-A to 91-Is.

Strands-label Heating power fraction Heating power %
91-J 2/66 3%
91-K 2/66 3%
91-L 3/66 4.5%

 

For the 109-strand cable, the total heating capacity factor is 66.

For the 109-strand cable, there are a total of 36 strands which have utility for heating the cable – 12 x 109-Ks, 6 x 109-Ls, 6 x 109-Ms and 12 x 109-Ns. There are 73 other strands – the 109-A to 109-Js.

Strands-label Heating power fraction Heating power %
109-K 1/66 1.5%
109-L 2/66 3%
109-M 2/66 3%
109-N 2.5/66 3.8%

VSL SSI 2000 Stay Cable System

VSL Queensferry Bridge

There are a number of options available in the VSL SSI 2000 Stay Cable System so these figures cannot be confirmed without sight of the Queensferry Crossing engineering design specifications (or by actually measuring the cables, which I am unable to do!).

For now, I am assuming for simplicity that the required maximum heating power in Watts/metre of cable length is the same as the stay pipe diameter in mm. This is not far off the maximum heat radiation from the sun on such a stay pipe, square on to the sun, at midday, midsummer, on a cloudless day – or more than enough heat to melt any ice in short order!

At this maximum heating power and after the cable cores warm up, they will emit 1000÷π = 318 Watts of heat energy per metre-squared of stay pipe surface area.

Number of strands
in the cable
Stay Pipe Diameter
(mm)
Heating power
(Watts/metre)
55 200 200
61 225 225
73 250 250
85 250 250
91 280 280
109 315 315

It is now possible to tabulate for each cable-label strand, the maximum heating power per metre and assuming a strand resistance of 0.001137 ohms per metre, what the maximum strand current and voltage potential per metre would be.

Strands-label Maximum strand power (Watts/metre) Maximum strand current (Amps) Maximum strand voltage (milliVolts/metre)
55-F 4.2 61 69
55-G 8.3 86 97
55-H 10.4 96 109
61-G 8.3 86 97
61-H 8.3 86 97
61-I 12.5 105 119
73-H 4.6 64 73
73-I 9.3 90 103
73-J 11.6 101 115
Strands-label Maximum strand power (Watts/metre) Maximum strand current (Amps) Maximum strand voltage (milliVolts/metre)
85-I 4.2 61 69
85-J 8.3 86 97
85-K 10.4 96 109
91-J 8.5 86 98
91-K 8.5 86 98
91-L 12.7 106 120
109-K 4.8 65 74
109-L 9.5 92 104
109-M 9.5 92 104
109-N 11.9 102 116

Cable voltages and power

To calculate the cable voltages and power and to calculate the total maximum power to heat all the cables of the Queensferry Crossing accurately, I will need to know how many of each size of cable and their lengths.

Direct Current Heating

Those theoretical differences between strand situations only matter for direct current heating if it is possible electrically to isolate strands from each other. The strands are attached via steel wedges to a steel anchor head, which, for now, effectively connects all the strands together electrically.

Electrically isolating heating and signal strands

Cable anchorages

Teflon/PTFE-coated glass fibre fabric sheaths to electrically isolate the strands from the anchor head. The outer strands are for heating. The inner strands are for signals.

It should be possible to insert Teflon/PTFE-coated glass fibre fabric sheaths between the wedges which grip the strands we wish to insulate and to isolate from the anchor head and from each other, unless and until they are connected to electrical heating or signal circuits.

The signal circuits could be used to report to the power supply control electronics at one end of the cable, the output of heating current sensors at the other end of the cable, to help to detect current leakage faults in the cable strands’ insulation, to implement a residual current device, to trigger safety power-cut-outs or circuit-breakers, most notably.

Teflon is a good insulator and is used for thread seal tape illustrating the properties of lubrication of the wedge to its housing cone required. The glass fibre fabric should provide strength under compression and a superior dimensional stability versus creep under load that a pure Teflon sheath may suffer from.

Clearly the sheath would have to remain thick enough to insulate against the highest voltage difference which might appear between the heating strands and the anchor head.

Such sheaths would likely not be available as an off-the-shelf product in the required dimensions (though general purpose PTFE-coated fibre glass cloth is commonly available) and would likely require to be custom manufactured, tested and proved in the laboratory.

So isolating the strands for DC heating purposes presents technical challenges. It would be very convenient if the outer strands could be preferentially used for heating purposes without having to isolate the strands electrically etc. but to achieve that we must consider using not direct current but alternating current instead.

Alternating Current Heating

The skin effect observed with alternating current changes matters in that with increasing frequency the heating current will tend to distribute towards strands nearer the surface of a cable. However if too great a frequency is used then the skin effect will increase the resistance of even the most superficial strands so much that inappropriately high and difficult to insulate against voltages would be required to obtain the required heating power.

Assuming that the appropriate AC frequency can be determined for preferentially heating the superficial strands of the Queensferry Crossing stay cables, although there would be no need to isolate the strands from the anchor head, there then presents the challenge of isolating the anchor heads and anchorages so that the current is not dissipated through the bridge instead of heating the cables as required.

Having isolated the cables for heating purposes, one may then wish later to reconnect the cables electrically to the rest of the bridge and disconnect the heating power supplies for lightning protection purposes. Certainly, one would not wish to encourage a lightning strike to find its way to ground via the bridge’s cable deicing power supplies!

Tower ice

To prevent the bridge piers or towers (with non-conducting concrete surfaces) from icing up, they could be surface fitted with new electrical heating trace cables which are then appropriately electrically-powered for deicing when necessary.

Ideally, such additional heating elements would have been embedded into the surface of the piers at construction time. Too late for that now.

Another option to consider is heating the hollow piers from within. However, considering the considerable mass and thickness of the piers their surfaces would have to be kept above freezing temperature all winter long. Heating the piers from within, there simply wouldn’t be time to allow the piers to get freezing cold because there was no icing then suddenly heat them from the inside to deice a sudden incidence of icing.

So heating from within bridge piers would use more electricity, though the cost shouldn’t be prohibitive – surplus grid electricity is a common occurrence at times of high wind power generation, so the electricity grid managers should offer a very low price for such electricity (just the grid connection charge) – plus it should be a lot safer upgrade from the point of view of bridge users – far less chance of things falling onto the road during the fitting of the piers’ internal heating elements.

Leave a comment

Filed under Uncategorized

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s