The pig should never go into the tank - it should to into a pig catcher.
The piping outside the tank should have a branch, bars to guide the pig, a bypass line, pig catcher, instrumentation, etc.
The type of floating roof won't cure bad piping design or inattentive operations.
Then, if the pig does bypass the tank, your problem is potentially a massive amount of high pressure gas entering the tank and damaging the floating roof.
Again, the type of floating roof won't matter much, if at all.
Best option in my opinion is to design a mixed phase liquid/gas diffuser for the inlet pipe and possibly have gas relief vents on the floating roof above the diffuser.