Within reliability engineering, different time periods are being used to determine time-related reliability parameters. For example, when determining the mean (expected) time to failure (MTTF) of downhole pumps, typically the operating times or running times are used. But, in some cases, also periods where the pump is available (in an up state), and idle for whatever reason (e.g. surface facilities are down), could be included. ISO 14224:2016 and IEC 60050-192:2015 are key documents guiding reliability data collection and analysis in the petroleum, petrochemical and natural gas industries. However, not much guidance is provided in these documents on what circumstances one should use one time period or another. Furthermore, the concept of running time is lacking a formal definition, and the way it differs from operating time could be given more focus in these documents.
In this paper, we discuss the perceived gaps in international standards and how further guidance could be provided to achieve consistent use of reliability parameters and associated terminology across the industries addressed, focusing on the MTTF. We refer to a downhole pump example, and cross-reference also the ISO/TR 12489:2013, to clarify implications for reliability modelling and calculations. One suggestion is the introduction of the terms running time to failure and operating time to failure, which would be consistent with existing terminology, while also clarify the time periods being referred to.