THE ANALYSIS OF FAILURE DATA IN THE PRESENCE OF CRITICAL AND DEGRADEDFAILURES

Reported failures are often classified into severity classes, e.g., as critical or degraded. The critical failures correspond to loss of fun ction(s) and are those of main concern. The rate of critical failures is usually estimated by the number of observed critical failures divid ed by the exposure time, thus ignoring the observed degraded failures. In the present paper failure data are analyzed, applying an alternati ve estimate for the critical failure rate, also taking the number of o bserved degraded failures into account. The model includes two alterna tive failure mechanisms, one being of the shock type, immediately lead ing to a critical failure, another resulting in a gradual deterioratio n, leading to a degraded failure before the critical failure occurs. F ailure data on safety valves from the OREDA (Offshore REliability DAta ) data base are analyzed using this model. The estimate for the critic al failure rate is obtained and compared with the standard estimate. ( C) 1997 Elsevier Science Limited.