Layer of Protection Analysis (LOPA) is a method for presenting a risk assessment. LOPA is often used in SIL Target Setting.
Good practice for safety instrumented systems requires compliance with IEC 61508 and IEC 61511. In accordance with these standards we will use risk graph and LOPA for SIL Target Setting
LOPA can be carried out once we have a clear understanding of the hazards, their causes and their consequences.
In a LOPA one recognises the raw cause(s) that can lead to a specified scenario outcome. The frequency of this cause(s) is estimated ignoring all safeguards.
The effect of the safeguards and conditional modifiers are then incrementally taken into account. The result is the "expected" event frequency.
The event frequency is compared to risk tolerability thresholds. The frequency is either intolerable or not. If the frequency is intolerable, or the risk is not ALARP then additional safeguards will be required. If the proposed safeguard is an instrumented trip then a SIL 1 trip will reduce the event frequency by 10 fold, a SIL 2 trip by 100 fold, and a SIL 3 gives a 1000 fold frequency risk reduction.
We carry out LOPA in the manner described by IEC 61511. We follow CCPS guidance on LOPA  and specifically we follow HSE guidelines .
 "Layer of Protection Analysis simplified risk assessment", Centre for Chemical Process Safety, USA, 2001  "Layers of Protection Analysis:Practice and Pitfalls", Health and Safety Laboratory training course 2013, UK
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