you'll need to know about the gas and oil or condensates at steady state. The do a hold up analysis to see which regime it's in. You'll need to have a profile to see where and how much liquids accumulate. If you are purposely adding oil so the the line is two phase flow, you'll need to know that. You will have to make assumptions on pigging intervals to see how large of a volume will be pushed in with the pig.
If this is getting complicated, its time to seek a professional to help you.