[system]

“On the subject of the underwater welding, how do you actually obviate, or stop, the fire risk? If you've got inflammable stuff in that pipepline, I know you said you'd put an argon atmosphere in there, but, is that enough?”
Asked by Nick in Cambridge

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[CliffordK]

One of the things about a fire risk is that you need to have a mix of fuel + oxidizer.

Take away either, and there is little risk of fire.

With that in mind, underwater welding would seem like an excellent environment for welding due to the lack of air.  One could essentially weld in 100% methane (Natural Gas) with no risk of explosion.