Don't open the door if any other valves are open apart from the vent or drain.
Seriously though, pigging is s potentially lethal operation if you do it wrong so the fact you're asking an Internet forum for a risk assessment as though you understand the risks I find very worrying.
This is not a place to find free engineering.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
Out of curiosity, I found this article from the web on "pigging", Link, which indicates the act in the quoted statement is a serious concern on safety. Wish it brings some help to your research.
Should consult with the experienced Contractor for the pigging procedure, safety guidelines, and operation process.
The required assessment may include the process history of the existing system, piping system configuration, proper pigging setup, pigs type and material, potential fire and explosion issue, required PPE, waste treatment, etc.
Don't stand at the end of the pipe where the pig exits.
Don't run the pig backwards.
There are a million things that can go wrong. And, I agree that asking for a risk assessment sounds like the OP is not asking any specific questions but rather is hopeful that someone will just send him/her a sample report.
