Nowadays more and more complex applications in the Oil & Gas industry, customer tailor made designs and electrification growth present new and tougher rotor dynamic challenges. As a consequence, in addition to lateral, also torsional vibrations need to be monitored and deeply investigated during string test and at site, to validate design or when unexpected issues occur. This paper describes the experiences achieved by the authors on the study and fulfillment of smart investigation methods for torsional vibration analysis at site, where measurements and diagnostics need to be carried out without affecting production and field operation. Several methods have been studied, tested and validated on simulators and on an internal test bench before being finally applied to site troubleshooting with the purpose of avoiding unit shut down, allowing on line monitoring and reducing costs. All studied investigation methods are presented in this paper with results, comparative analysis and an overview of the simulator and of the other systems tested. Possible future developments are described also. It is now possible to protect machineries against unexpected high torsional vibrations leveraging the standard instrumentation normally available for unit protection and monitoring.