The operational modes

The exceptional situations are defined by a combination of modes belonging to various mode sets :

• The primary mode set: the operation was for energy production, maintenance, etc.. Therefore, the operation mode was a member of the mode set (Production, Maintenance, ...)
• Sub-system mode sets: describing the operation of sub-systems. The subsystem relevant to the analysis are those of the safety cooling backup systems.
• Safety control mode sets: describing the operation of specific controls. The safety control relevant to the analysis was of manual control of specific cooling cycles.

  In this accident, the system did not prevent closing the safety backup system while in production, and did not inhibit manual turning off of the auxiliary pumps.

  The failure in this case study is attributed to scenario ambiguity, due to implicit setting of the primary modes.

  To enable identification of scenario mismatch, the modes should be defined explicitly, and the Scenario analyzershould verify that they match.

  Guidelines about mode setting

Updated on 18 Apr 2016.