system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do oil rigs work?

And are they stationary platforms or do they move around?
gpt
There are various kinds of oil rigs. It is important to position the drill rig, the drilling machinery over the hole it is drilling.

A common practice is to build the oil platform somewhere, tow it to the drilling location, then anchor it by lowering the platforms legs to the bottom of the sea.

Alternatively the rig can be positioned precisely using several propellers powered by engines to keep it exactly in place.

Drilling rigs are built so that they can be reused elsewhere.

The legs do not actually sink down to rock which might be miles down. They generally sink into mod deeply enough to be good anchors for the platform.

New Orleans bridges are built with their foundation sunk into mud. The amount required to keep the bridge floating is calculated. The foundations have that much floatation. The bridges can rise and fall.