(47n) Using HAZOP and LOPA to Define Which Instrumented Functions to Include in Your Safety Instrumented System

Hazard and Operability (HAZOP) and Layer of Protection Analysis (LOPA) are key tools used to identify hazards and help put risks in perspective.  For many design projects, a gap exists in the use of these analysis tools and decisions regarding the application of design standards for Safety Instrumented Systems (SIS; e.g., IEC 61511) to important instrumented functions.  Industry-wide acceptance on guidelines for when to categorize an instrumented function as a Safety Instrumented Function and how to effectively apply HAZOP and LOPA in the decision-making exercise do not exist, and in many cases, may conflict with a Owner-Operator Company's Risk Acceptance Thresholds.  The focus on this paper will be to define a practical strategy for the application of HAZOP and simplified-LOPA to perform an initial screening of which instrumented functions can be non-SIS and which are candidate IEC 61511 Safety Instrumented Functions, and the performance of the next depth of analysis (e.g., a complete LOPA) to validate the need for a Safety Instrumented Function, and its appropriate SIL Assignment.