Composing existing web services for enterprise applications may enable higher level of reuse. However the composition processes are mostly static and lack of support for runtime redesign. In this paper, we describe our approach to the extension of the OWL-S ontology framework for dynamic web service composition. We raise the level of abstraction and propose an abstract service layer so that web services can be composed at the abstract service level instead of the concrete level. Each abstract service is attached with an instance pool including all instances of the abstract service to facilitate fail-over and dynamic compositions.