Hello, I would like to ask you if there is a mathematical theory, that is complete (in the sense of Goedel's theorem) but practically applicable. I know about Robinson arithmetic that is very limited but incomplete already. So, I would like to know if there is some mathematics that could be practically used (expressiveness) and reduced to logics (completeness).

I'm very new to the site and to the maths as well, so please tell me if that's a silly question.