GoKawiilOne of the most fascinating things I’ve learned about recently is the engineering behind the US Strategic Petroleum Reserve. Here are the key requirements it’s designed to meet:
Store hundreds of millions of barrels of crude oil for long periods of time to respond to disruptions in petroleum supplies.
Keep the oil secure against attacks by our adversaries. This is an especially hard challenge because petroleum has this tendency to light on fire, so a huge concentration of it is especially difficult to keep safe.
Release oil quickly so it can reach the market during a supply disruption.
Keep maintenance costs low and last for decades.
The actual solution, like many great engineering solutions, is incredibly elegant and simple. But before we talk about it, let’s start with how you might go about solving this:
The naive solution: External floating roof tanks
Looking at the requirements, the first thing most people would probably think to do is to just take what we do commercially for storing petroleum and scale it up. External floating roof tanks (EFRTs) are the common solution for storing petroleum. Most tanks are confined to about 50 feet tall and 300 feet in diameter – larger than that and you start to have engineering problems with the foundation and drainage systems. This gives a volume of $V = \pi (150 \mathrm{ft})^2(50 \mathrm{ft})$, or $3{,}532{,}500 \mathrm{ft}^3$, equivalent to about 630,000 barrels of oil.
To hold 714 million barrels of oil (the full capacity of the Strategic Petroleum Reserve), you’d need about 1,130 of these tanks. If you use the standard capacity of the largest commercial petroleum farms (e.g., in Cushing, Oklahoma), you need about 45,000 acres to store these tanks. That’s basically the size of Washington, D.C., which means you’d need to acquire a lot of land.
However, the biggest downside to using tanks is that they’re incredibly vulnerable to attack. Damage can cause spills and fires, and there’s a particular weak point at the seal between a floating roof and the tank shell. Lightning-caused fires have been documented in the seal space of open floating-roof tanks, so a deliberate ignition source (shrapnel, incendiary) could be particularly bad.
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