Service enterprise engineering often involves designing systems that perform routine decision-making, as in business processes. However, exceptional events, which require effective decision-making to coordinate the response, are not routine. This paper presents a systematic, scenario-based methodology for designing such a decision-making system. The approach is based on the operational procedure methodology that has been used for developing avionics systems. The paper discusses an application to the case of medical disaster mutual aid, in which multiple hospitals must coordinate activities to respond to a mass casualty incident.