As long as you're ripping for educational purposes, making free mods, etc. it's generally ok. Don't distribute original game assets, don't try to sell derivative versions, basically don't try to make money off it.
Any 3d model on your GPU can technically be ripped and there's tools for it. Google it if you want more information. Since it's close to piracy and an ethically grey area, don't expect anyone to go much further into detail. There's forum rules here against promoting piracy.