1. For the deep ocean parts of the route, cables and associated equipment (such as repeaters) are simply spooled out from the back of the cable laying ship, to settle on the ocean floor.
2. For shallow waters, the cable is buried. This is done by dragging a plow along the bottom which cuts a furrow and puts the cable into it. The plow has an altitude control and a camera so that an operator on the ship can control it, and a magnetometer to check if the cable is properly buried behind it.
3. For areas where burying isn't practical but they anticipate ships will anchor, they use armored cable.
For #1, the costs are going to be the cost to operate the ship while it slowly spools out the cable and the cost of the cable. For #3, same thing, but with more expensive cable. For #2 I'd expect it is similar, except the ship goes a lot slower (about 0.5 knots when using the plow, compared to about 5 knots when laying surface cable).
Finally, there is this.
#4. At the shores, they need to avoid damaging reefs and other habitats, not wreck the beach, and things like that. The cable needs to be in conduits that are buried or anchored. And building those conduits needs to be done in a way that does not mess up the environment.
So what you've got then for a long cable project is two ends that present underwater construction projects, the shallow waters near the two ends where you have to bury the cable, and then the long deep ocean stretch where you are just spooling the cable out.
This suggests the costs are going to have a component that doesn't really depend on how long the thing is (the two ends and the shallow waters near the ends where burial is needed) and a component that is proportional to length (the long run between the two shallow waters near the ends).
At 5 knots, it would take about 1000 hours to lay the deep sea part of the cable. If the ship costs $50k/hour to operate, that would be about $40 million. (I have no idea what it costs to operate these ships, but Google tells me that big cruise ships cost about that much to operate, and I'd guess that a cable laying ship is cheaper).
Assuming the underwater cable itself is 10 times as expensive as regular cable, its about $150 million for 9000 km.
That's brings us to about $200 million for the deep ocean part.
Still sounds really inexpensive when I consider it contains a large number of repeaters and is meant to stay at the bottom of the ocean.
Edit: Forgot to write, I haven't run the numbers myself but I enjoyed your reasoning here, you put a smile on my face :
> At 5 knots, it would take about 1000 hours to lay the deep sea part of the cable. If the ship costs $50k/hour to operate, that would be about $40 million. (I have no idea what it costs to operate these ships, but Google tells me that big cruise ships cost about that much to operate, and I'd guess that a cable laying ship is cheaper).
Repeaters aren't terrible expensive, so they only add a few million to the total cost.
Looking at what I can find, it looks like way more than 10 times the cost.
The Google project was $33k/kilometer, so I don't think I could have been too far off on the cable itself. Looking at other undersea fiber projects, that seem about typical. For example, this one [2] estimated $27k/kilometer [1].
Here's an Alibaba seller with submarine fiber for $2000-9000/kilometer [2].
The submarine cables have an aluminum or copper tube around the fiber optics, an aluminum water barrier, and a sheath of stranded steel wires, and an outer polyetylene layer, with various other layers of mylar, polycarbonate, and petroleum jelly in between.
I'd expect the metal layers to be the most expensive parts. Looking at the cost of tubes or cables of those materials, it looks like each of those would be in the $1000-2000/kilometer range.
[1] http://infrastructureafrica.opendataforafrica.org/ettzplb/co...
[2] https://www.alibaba.com/product-detail/Submarine-Fiber-Optic...
Source: I've laid subsea cable.
The fun thing was the company hand book had a whole other section of T&C allowances etc if you worked on a ship.
