Maybe trying to flush the tank without dropping it? Who knows..
My boss on the offshore rigs Robert needed to weld a gas tank, what and why I don't remember.
He ran a big hose from the exhaust pipe into the tank, the exhaust displaced the oxygen while he welded it. Everybody got way back away from what they thought was going to be a huge fire and man burned alive. It worked out, no fire extinguisher needed.
I've thermite welded many anode bags onto steel fuel storage tanks both above and below ground. In the case above it makes no difference, but when welding tanks we had them full to capacity to eliminate all vapors. I saw pipeline guys weld on hot lines often when I worked at the Amoco pipeline terminal.
