In the context of Knowledge Management, we carry out a Corporate Memories (CM) project for the Company CIRTIL. Oour purpose is to focus on the modelling of the application domain. It is built as a domain ontology with a structure supporting a semantic model based on ontological relationships. In this paper we, present our S3 model which permits to model knowledge and to index documents. We also show how semantic indexing of technical documents can be improved by mean of the domain ontology. We show finally the interest of our model with the implementation of a prototype.