(Yes, I know the answer: It would cost more and oil execs and investors never go to jail for polluting.)
The area between the double-walled pipes is called an interstice, and that's how leaks are detected. Additionally the interstice drains to a containment sump, so if any liquid is present, the leak is obvious, but there are sensors there too.
Fuel is more corrosive than crude oil. However the sludge in crude can cause pitting corrosion over time. These systems are built such that corrosion and leaks are expected and inevitable. Gas stations and places like data centers, or anywhere with fuel storage or dispensers must be inspected monthly for this reason.
Source: I work for an environmental compliance company, and built software to model and report on these types of fuel storage, transport, and dispensing systems.
(3) the volume and speed of material in pipes is immense. The void space between pipe layers would have to run continuously for the length of the pipe. Any blockage or constriction would produce a water hammer effect, turning one leak into many. A cracked pipe is bad, but a double-layered pipe exploding because of a crack miles upstream is horrible.
What fills this void space? Air? Air plus hydrocarbon in an enclosed/hot environment?
It's not like this space has to be particularly small. Double walled pipes are relatively common in many forms. Long-distance heating uses double walled stainless steel piping with insulation between the walls. HV systems sometimes use double walled conduits to have a second layer for the insulating gas (e.g. SF6) in case the inner layer leaks. Double walled pipes are used for lines containing especially hazardous materials.
The inter-wall space is often monitored for ingress of "stuff", triggering a controlled shutdown if it is flooded.
> In burried sections this may be a non-issue but wrapping all the valves and above-ground facilities in layers would make inspection problematic.
This is about risk mitigation, so total mitigation is not required. Protecting, say, 80-90 % of a pipeline would reduce spill risk a lot. Also, the facilities you mention may be suitable to other forms of mitigation (e.g. walls, flood basins).
> That would be very hard to engineer.
I don't think engineering is the real problem here. Cost and incentives are.
They are fixed together in some places, and able to slide relative to each other in most places. Fixed points are located in the middle of straight sections and there are C-shaped “expansion loops” that allow the inner pipe to expand or contract relative to the outer pipe.
The outer pipe has a vacuum applied by a vacuum pump, and a greater than nominal vacuum leak indicates an issue.
Of course, it’s expensive compared to a single pipe.
But actually, scratch that. We're in the modern era now. Send a drone into the outer pipe, and use some sensors (perhaps also video) to detect excessive moisture, or corrosion, or whatever. So it's no longer expensive.
Most new pipelines are inspected currently with "drones", called smart pipeline inspection gauges, or smart PIGs, that use ultrasonic or magnetic flux to measure the wall thickness over the entire length of the pipe [1]. The PIGs are propelled through the pipe via the working fluid in the pipeline.
I think creating a similar device to travel in the annulus of cased pipe would be much more difficult. The centralizers used to keep the inner pipe centered in the outer pipe would pose a pretty big obstacle. Plus, there would be nothing propelling it forward, and communicating with the device through a steel pipe would be very difficult.
[1] https://primis.phmsa.dot.gov/comm/FactSheets/FSSmartPig.htm?... [2] https://kwikzip.com/ (example)