SIL determination as a utility-based decision process

• Taking a risk optimisation approach in determining target Safety Integrity Level (SIL).
• Proposing a Cost Benefit Analysis for ALARP demonstration.
• Incorporating the cost impact in allocation of SIL.
• Applying Expected Utility Theory in SIL determination studies.

Using the concept of ALARP (As Low As Reasonably Practicable) and the Expected Utility Theory (EUT), this paper introduces a risk-based optimisation approach to the SIL (Safety Integrity Level) determination process. In the commonly used SIL determination methods the target SIL is determined by comparing the existing level of risk to a pre-set corporate risk target; the gap defines the level of risk that should be reduced by additional layers of protection, such as safety systems. Such methods do not directly factor in the cost impact of the allocated target SILs, nor do they examine the practicability of all SIL alternatives in reducing the risk to as low as possible. The method presented in this paper is based on assigning utility values to different SIL alternatives, in accordance with the cost and benefits of each alternative, and comparing the expected utility values in order to determine the optimum SIL rating as target. A numerical analysis has been developed and applied to the SIL evaluation for a gas turbine over-speed protection to demonstrate the advantages and challenges of the new method.