This article aims to present an approach based on a combination of life cycle analysis methods (LCA) and problem solving by constraint satisfaction (CSP). This original approach makes it possible to vary the design of the different dimensions of the functional units of a complex system and thus to make it easier to identify the best architecture along with the best functional definition of the system. Our approach (EcoCSP) presented allows to negotiate performance to move towards an environmental optimum corresponding to a set of acceptable specifications. The EcoCSP approach is implemented to define the functional performance of a green passengers' ferry.