The paper will describe the main outcomes of a risk assessment performed by General Electric O&G (GE O&G) in 2013 on a large number of gas turbine rotors that had exceeded their design end of life (EOL).
The assessment involves a large number of medium frames (e.g. MS3002J, MS5002C) plus small industrial gas turbines (e.g. MS1002, PGT10).
The design end of life is the lifespan inside which the risk of uncontained failure is expected to be improbable and this means the failure has a probability of occurrence of 0.12 total events or less during the life of five hundred gas turbines operating 8000 hours per year for 30 years .
GE gas turbine rotors are designed to operate with no need of inspection and maintenance till their design EOL (alias serviceable life).
The gas turbines end users may exceed EOL, provided the rotors have been subjected to a specific investigation that includes teardown and a full inspection.
The disregard of this recommendation may have catastrophic consequences on the gas turbines and on surrounding equipment and personnel.
Such recommendation was provided to customers the first time in 2005 through GETIL 1576 , the recommendation was renewed in 2013 with a new communication NIC 13.16 .
The risk assessment has essentially focused on rotating parts, because rotors (i.e. shaft, wheel, spacer) are the highest gas turbine energy components, that even if designed with high safety margins, they are not immune from time (e.g. creep) and/or cyclic dependent damage (e.g. fatigue).
The assessment has been carried out by following MIL-STD-882C  guidelines and in accordance to the principles defined by ISO 12100-2010 .
The failure risks have been estimated with the help of specific tools by using field data and life calculations performed with physics-based models.
The paper describes the methodology used to support the risk assessment and this means the “rotor life management methodology” that was developed by GE more than ten years ago, and was subsequently improved by including statistical assessments.