[PlasmaKrab]

Right, 250 square meters would be one hex, else it is a 250 meter-wide square...
Anyway a one-hex oil rig, may be even easier to simulate, e.g. with a large industrial-type building. Since you probably can't place buildings on water hexes, that would require one hex of just any terrain type (impassable to make it landing-proof?), maybe at a large positive altitude?

Also, from the map you posted, there seems to be at least two large oil fields with several platforms close together in each. It could be interesting to have such a "swarm" of oil rigs, roughly 1km apart on the same map. Some could be highjacked by rebels, some not, the player would have to find out the hard way...
You can add to the challenge by placing some alloying AA units on a few of the platforms, e.g. a light AAG on the deck, or some MANPADS.