SIL study of over-speed modules for gas turbine generators and compressors

MAR Consulting performed a functional safety/SIL study of over-speed safety instrumented functions (SIF) protecting gas turbine generators (GTG) and gas compressors installed in a floating, production, storage and offloading (FPSO) vessel. The SIFs included 2oo3 voting speed sensors and 1oo3 voting valves controlled by over-speed modules.

The work covered the following phases of the safety instrumented system (SIS) lifecycle, as per IEC61511 Ed.1 and the client’s internal guidelines:

  1. Allocation of functions to protection layers (phase 2) using layers of protection analysis (LOPA);
  2. Safety requirements specification (SRS) (phase 3) with general and specific one-pagers SRS datasheets;
  3. Safety integrity level (SIL) compliance (phase 4) of random hardware failures, i.e. the average probability of failure on demand (PFD) and architectural constraints.

In this study, it was found that the achieved PFD was in compliance with the safety requirements, however, extra risk reduction was necessary to achieve the commercial/asset requirements from the operator.


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