> There is a legal floor of ~250M, after which it is reserved for government national defense.

There is no "government national defense floor". As far as I can tell, this originates from someone misreading the SPR drawdown laws and then a bunch of people parroting it. The ~250M "floor" (really 252.4M) only applies for SPR drawdowns that aren't required by a severe energy supply interruption or to meet US obligations under the international energy program. Deciding what qualifies as a "severe energy supply interruption" is left up to the president. Essentially there's nothing stopping the president from emptying the reserve entirely. See sections d and h here: https://uscode.house.gov/view.xhtml?req=(title%3A42+section%...

> Public analyses have said that the cavern integrity becomes threatened once levels fall below 300M.

This isn't how the SPR works. They withdraw oil from it by pumping in water (oil floats on water, so the oil goes to the top). This means that the caverns won't cave in or something if there's too little oil, since they'll have the same volume inside them. What does wear out the SPR is drawing down at all, since the water erodes the caverns. The problem is cumulative, not caused by levels going below a certain point. There's a good article that goes over how the SPR works and its current status here if you're interested: https://files.gao.gov/reports/GAO-26-106918/index.html

The problem with drawdowns is they can and have compromised the structural integrity of the caverns used in the SPR. Like oil, they're a finite resource and should be used judiciously.

https://www.osti.gov/servlets/purl/2585591

Thanks for the link, I didn't know very much about how this is implemented and now it's much clearer. I fully expect to see the same things I've just learned on a Youtube video or cable TV news some time from now when stuff goes badly wrong and it doesn't mesh with the simple "Huge metal tank somewhere" picture of such a reserve I'd imagine the average voter has.

Like Britain's ageing gas mains this feels like a measure which should have become irrelevant just as it would otherwise need renewing, a huge saving - but of course we (as a species) will always procrastinate so that was never going to happen. For the next 5-10 years Britain will continue replacing pipelines to deliver a fossil fuel (natural gas, methane, for heating) we should have long since stopped using but didn't.

Either way it seems the US has somewhere from 1 to 8 months before accounting for reductions due to national considerations. That's concerning no matter how you frame it.

What's the outlook for the other major economies?

The caverns have salt walls. The water erodes the salt caverns.