Now, why don't some oil tankers drag the iron chain and how can the static electricity be released?
They haven't got a chain. They're all practical tape
Iron chain is a long time ago, and now switch to anti-static tape, tape containing copper wire, but also on the ground friction, release of static electricity.
Static electricity is generated during the running of the vehicle, and the static electricity generated by the oil tank is easy to cause fire. It is a safe practice to guide the static electricity to the ground through the electrostatic band