These are operational procedures or method statements and procedures issued by operations or the contractor.
hot work needs to follow the hot work procedure and permits applicable to your site, enforced by the owner / operator.
It would help if you decided which main design and installation code you wish to follow ( usually one of the ASME B31 series), but could be many others depending on your location and jurisdiction.
Before cutting or welding you need to remove and purge the pipe of hydrocarbons or any material which can cause you harm. Then check and monitor during the work.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
If you are torch cutting a length of enclosed pipe and the torch extinguishes, wait a minute for gasses sucked into the cooling pipe to dissipate before re-lighting or else you will get a flashback. What else do you need to know?
My research on a project somewhat similar in scope but in my case it was the removal of an empty gasoline tank that require some flame cutting operation; I came across an older NFPA code that listed several gases to inert the tank. One method the NFPA stated was to use the exhaust fumes from an internal combustion engine which may no longer be recommended. Nonetheless research the NFPA codes as I don't remember the code number as a starting point.