Quay cranes are particular transportation devices for which operation's safety and CRAMP parameters (Cost, Reliability, Availability, Maintainability, and Productivity) should be fulfilled with regard to a harbor maintenance strategy. The maintenance process is first considered within a holistic modeling framework in order to cope with the current practices of treating strategic, operational and engineering maintenance issues independently without taking into account their interactions within an entire Enterprise System. Proactive maintenance is then highlighted as a new model aiming to globally optimize the components operation parameters throughout three interacting prognosis, diagnosis and monitoring processes. Technical issues related to Intelligent Maintenance System are finally proposed in order to support proactive maintenance operations at the enterprise field level and applied to quay cranes in a particular site within the frame of the European Eureka ‘Robcrane' project.