A model based diagnostic and health-monitoring method has been developed for blowout preventer (BOP) annulars. The method is based on system identification techniques. Given experimental fi eld data, a reduced order model is developed and then identified and adapted to the data. The change detected in real-time in the reduced order model coefficients not only does it give an idea about the health of the system but also permits to identify the current operating stage of the system. The method has been tested to a Blow Out Preventer annular system and has provided information on the current and forecasted health of the system as open and closure cycles are executed. The method provides a. direction in the model parameters space in which the system ages so that a diagnostic and prognostic decision can be made.