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 (Layers 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.
In essence of SIL classification is to decermine the required integrity of instrumental safeguards.