According to the IEC 61508 / 61511 standards a risk analysis has to be performed for Loss Of Containment scenarios from a HAZOP study which are considered potentially dangerous. The method for this analysis is free to choose. Some companies use a risk matrix, some a risk graph and others the method LOPA (Layer of Protection Analysis). Each method has its pros and cons.
In order to determine the risks in objective way, some factors are important:
It is essential to document the risk assessment (SIL classification) properly, the choice of the various parameters must be clear and explained in detail.
Using LOPA, it is essential to know which incident frequences the company tolerates. What is the risk reduction of independent protection layers and frequency modifiers?
A pitfall is to think too optimistic about certain risk reduction. For example, the risk reduction of an alarm.
The LOPA methodology is based on the LOPA standards of CCPS.