Starting from a compositional operational semantics of transition P Systems we have previously defined, we face the problem of developing an axiomatization that is sound and complete with respect to some behavioural equivalence. To achieve this goal, we propose to transform the systems into a normal form with an equivalent semantics. As a first step, we introduce axioms which allow the transformation of membrane structures into flat membranes. We leave as future work the further step that leads to the wanted normal form.