Comment by KaiserPro
1 day ago
So the initial premise is the bit that gets me.
For the glass to be the insulator we need, I'm assuming the author envisions a solid tube, with no airgaps (can't do fibre braid as that would allow gaps which means loss of insulation, or you'd need oil to fill the gaps.)
This means huge bend radius in the order of hundreds of meters. Not only that but laying it on the ocean bed would require trenching and full support to stop localised bending.
Now to the manufacture:
> The cable is then quenched in water to surface harden it, before it moves out of the back of the ship and falls to the ocean floor over a length of many kilometers (due to very low curve radius).
So that'll cause the tube to break. Glass builds up hige amounts of stresses when it cools down quickly (see prince ruperts drop) so needs an annealing step. ( https://en.wikipedia.org/wiki/Annealing_(glass) )
Moreover changes in temperature mean that using aluminum is probably going to cause the glass to shatter when the temperature changes. which means that you either need https://en.wikipedia.org/wiki/Kovar or somehow make expansion joints every n meters.
Finally that cable is going to be heavy, so unless you make it around the same densisty as salt water, it'll have so much weight it'll snap as soon as you try and dump it into the sea.
apart from that, looks good. well apart from the units are wrong to start with.
TLDR:
you'd need 5x the width of Polyethylene to achieve the same level of insulation at high voltages. but as silica tube doesn't bend and shatters really easily, cant be transported and has a slow extrusion rate, it seems logical to just use PE.
PE doesn't work as well as you imagine. As well as needing waaaay more of it, due to the power of 2 in the volume of a cylinder formula, and it being much more expensive, it also can't withstand high temperatures, which means the current carrying capacity of the core is lowered.
> As well as needing waaaay more of it
Have you done an accounting of how many kilometers you can fit on a 200,000+ tonne boat? Seems to me you could cost-effectively carry nearly 20x as much cable weight as current cable layers. You need 25x the volume of polyethylene, but that's only 10x the weight and it isn't even counting the weight of the conductor.
> Glass builds up hige amounts of stresses when it cools down quickly (see prince ruperts drop)
That internal stress is deliberate. It counterintuitively makes the cable have more tensile strength since glass tends to only fail when a crack propagates from the outside.
The bend radius is huge yes.
But it can span ~64 meter gaps without support, so the need for trenching should be minimal.
During the laying process in deep water, one can use buoys along the length to gradually lay the heavy cable on the seafloor so the tension isn't in the cable.
> So that'll cause the tube to break. Glass builds up hige amounts of stresses when it cools down quickly (see prince ruperts drop) so needs an annealing step.
Did you miss that the prestress is the point? There also could still be an annealing step- a continuous oven just like glass fiber manufacturing. Annealing time for prestressed fibers is very short, although I am very skeptical you could actually get something like this to work in practice.
> Moreover changes in temperature mean that using aluminum is probably going to cause the glass to shatter when the temperature changes.
Does temperature change at the bottom of the ocean? I suspect the heat per meter from resistive losses will be very, very low, but it is a missing point.
> Finally that cable is going to be heavy, so unless you make it around the same densisty as salt water, it'll have so much weight it'll snap as soon as you try and dump it into the sea.
That is addressed in the post- balloons to keep the bend angle low as it descends.
> it seems logical to just use PE.
MSC Irina has a deadweight tonnage (cargo+fuel etc) of 240,000 tonnes. PE would be ~15 cm thickness and weigh ~66 tonnes per km, so you'd get somewhere in the region of 3600 km of cable per trip. Atlantic submarine cables are <7200 km, so yeah- it seems very hard to make the case that glass is worth it.
NB: I do not believe that 14 MV cables could be 30 cm in width, but it doesn't matter much. If you make 8 trips instead of 2, it's still hard to justify. Current cable-laying ships are pretty small, despite cables still being decently big- cargo ships are way bigger. Not scaling up the ships would be very silly when they already exist.