Risk Reduction and SIL-Analysis (Risk Graph, LOPA)

Safety mitigations to minimize the risk of hazards often include Safety Instrumented Functions (SIF) that instigate process trips or shutdowns via an independent safety system. The SIFs identified during HRA need to be designed according to the required risk reduction or SIL target, in accordance with acceptable Risk Criteria.

The SIL target can be calculated using Layer of Protection Analysis (LOPA), which is one of several methods detailed in IEC 61511-3. This semi-quantitative method involves evaluation of all demand sources, protection layers and mitigation measures, in order to avoid ‘over’ or ‘under’ design. The LOPA approach, as with the HAZOP studies, requires a team of experienced discipline engineers with process, operations and maintenance backgrounds to work through the LOPA steps for each demand scenario. LOPA often follows directly after HAZOP with the same Team in order to provide a fully aligned analysis.

As for HAZOP, it is important that an experienced and independent chairman is used to manage the LOPA, as the chair’s experience will ensure a robust study is conducted that will provide proper input for the design team and stand up to later scrutiny by third party auditor.

It is recognized that LOPA may not be appropriate for all projects and an alternative approach such as calibrated Risk Graph as per VDI/VDE 2180 may be used. A combination of methods may also be used, with Risk Graph acting as a screening tool to quickly identify SIL targets and LOPA or Fault/Event Tree analysis providing more detailed evaluation for SIL 2 targets and above.

Depending on the project, PSC uses proprietary software such as LOPA Works, TRAC, ExSILentia, or in-house excel templates. Failure rate data is taken from PSC in-house database for various industries (based on sources such as OREDA, NFPA, API 581, FRED, EGIG), but client-specified software and reliability data can also be used if preferred.