Safely inerting, opening, and cleaning double wall UST's

[khart]

Is there a standard that someone can give me that's a proven methodology for safely inerting, opening, and cleaning double wall UST's. I am sure it's not much different than single wall UST methods, but we are running on the issue of cutting those 2 walls and not knowing if there was any sort of leak into the interstitial space. We are using LEL meters and nitrogen. Plus cutting two layers of steel. What are people using to cut that much steel?