A new software tool for web–server clusters development is presented. The tool consist of a set of predefined Hierarchical Timed Coloured Petri Net (HTCPN) structures – patterns. The patterns make it possible to naturally construct typical and experimental server–cluster structures. The preliminary patterns are executable queueing systems. A simulation based methodology of web–server model analysis and validation has been proposed. The paper focuses on presenting the construction of the software tool and the guidelines for applying it in cluster–based web–server development.