In the Functional Safety Lifecycle, the next step after hazard and risk assessment is “allocation of safety functions to protection layers”. This is a process whereby suitable control measures are specified for each of the potential hazardous events found during the H&RA Hazard & Risk Assessment (usually a HazOp) . This includes mechanical safeguards such as pressure relief valves, and also Safety Instrumented Systems (SIS).
The process of “allocation” is deciding which protective measures will be put in place, and how reliable they need to be. If a safety instrumented function (SIF) is to be used, the reliability required from it will be expressed as a safety integrity level, or SIL. There are four SILs – 1, 2, 3 and 4, which give increasing levels of reliability, as follows:
| SIL | PFDavg | Risk Reduction Factor (RRF) | ||
| From | From | To | To | |
| 1 | 0.1 | 0.01 | 10 | 100 |
| 2 | 0.01 | 0.001 | 100 | 1,000 |
| 3 | 0.001 | 0.0001 | 1,000 | 10,000 |
| 4 | 0.0001 | 0.000001 | 10,000 | 100,000 |
A key task during “allocation” is therefore to carry out “SIL determination”; in other words to decide what SIL is required from each SIF.
The most common technique used for SIL determination is layer of protection analysis (LOPA). See “what is a LOPA?” for more information.
Layer of Protection Analysis (LOPA) study fora distillery expansion project.
Industry: Brewing and DistillingUpdate of the existing Hazard & Operability (HazOp) and Layer of Protection Analysis (LOPA) Studies of new tanker loading control facilities to be installed to take account of the proposed addition of new tanker loading control valves.
Industry: Brewing and DistillingUpdate to earlier LOPA following review by HSE.
Industry: Mining / Metal / Cement77 LOPA scenarios were reviewed as part of an ongoing functional safety project.
Industry: AgrochemicalA LOPA “Layer of Protection Analysis” study for three events relating to a hydro-power system: flood gates opened not in accordance with procedures, flood gates not opened when required and insufficient flow. The purpose of the LOPA study was to establish the effectiveness of existing and intended protection layers and safety functions. The LOPA study considered four scenarios derived from the three events and identified a set of actions where there were uncertainties in quantifying the initiating causes and frequency modifiers.
Industry: Power
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See also Functional Safety Lifecycle
Process Safety Lifecycle
See also OT Cyber Security Lifecycle
