Introduction
Safety Integrity Level Assessment is a functional safety study used to determine the required reliability of Safety Instrumented Functions. It helps confirm whether each safety loop has sufficient risk reduction capability to prevent or mitigate hazardous events in process facilities. The study is commonly performed after HAZOP, where hazardous scenarios, causes and safeguards are already identified.
SIL Assessment Methodology
- Review project documents such as P&IDs, Cause & Effect diagrams, HAZOP worksheets, control philosophy, shutdown philosophy and operating data.
- Identify the Safety Instrumented Functions applicable to the project or facility modification.
- Define the hazardous scenario, initiating causes, consequences and existing safeguards for each SIF.
- Conduct a multidisciplinary workshop with process, operations, instrumentation, control systems, maintenance, HSE and project teams.
- Apply Risk Matrix methodology for initial SIL screening.
- Use Layer of Protection Analysis for higher-risk loops or where additional justification is required.
- Evaluate only valid Independent Protection Layers and avoid double-counting safeguards.
- Review operator actions, alarms and manual interventions carefully before taking credit.
- Assign the target SIL rating and required Risk Reduction Factor for each SIF.
- Record assumptions, basis, workshop remarks and recommendations.
- Prepare SIL worksheets, SIF list, final report and action items for further verification or implementation.
Conclusion
A SIL Assessment provides a clear and technically justified basis for safety instrumented functions. It supports safer design, avoids under-protection or over-design, and helps ensure that each safety loop is aligned with the actual risk reduction requirement of the facility.
1. SIL Assessment: FEED Study for Flare Minimization in Sour Water Strippers
Introduction
This case study involved a SIL Assessment for an offshore facility modification project where an existing safety function was reviewed due to changes in the system configuration. The assessment focused on confirming whether the modified safety loop continued to meet the required functional safety performance.
Project Insights
- The study reviewed one key Safety Instrumented Function associated with pressure protection.
- Existing safety system components, including the transmitter, shutdown system and final element, were considered in the assessment.
- Previous SIL verification information was reviewed to support the revised assessment basis.
- The workshop involved a multidisciplinary team from engineering, operations, instrumentation, control systems, HSE and project disciplines.
- Manual intervention-based safeguards were not credited as independent protection layers.
- Safeguards using the same final element were carefully reviewed to avoid duplicate credit.
- The SIF was assessed using SIL methodology and LOPA principles to determine the required risk reduction.
Outcome
The assessed Safety Instrumented Function was assigned a SIL 2 target with the required Risk Reduction Factor. No additional recommendations were raised by the study team, indicating that the proposed configuration was considered adequate for the assessed scenario. The study provided a documented and auditable basis for proceeding with the modified safety loop.
2. SIL Assessment: EPIC Miscellaneous Works at NFA & WHP3
Introduction
This case study involved a SIL Assessment for a flare minimization project in a sour water and off-gas recovery system at a large LNG processing facility. The project aimed to reduce continuous flaring by recovering off-gas and routing it back into the gas handling system, requiring a detailed review of related safety functions.
Project Insights
- The study covered multiple Safety Instrumented Functions related to pressure, level and temperature protection.
- The SIL Assessment was performed after HAZOP to use the identified causes, consequences and safeguards as the study basis.
- Risk Matrix methodology was used for initial classification of all identified SIFs.
- LOPA was applied to selected loops to confirm the required SIL target and risk reduction.
- Existing and proposed protection layers were reviewed for independence and effectiveness.
- Control loop modifications were considered where they could support risk reduction.
- Additional shutdown isolation and RRF verification were reviewed as part of the recommendations.
- The assessment considered both existing plant configuration and proposed project modifications.
Outcome
A total of 20 Safety Instrumented Functions were assessed by iFluids Engineering & Consultancy WLL. The final classification included non-SIL rated, SIL 1 and SIL 2 functions, with no SIL 3 loops identified. The study generated six recommendations covering control loop improvements, verification of Risk Reduction Factor values and additional isolation requirements. The final SIL Assessment helped strengthen the functional safety basis of the flare minimization project while supporting safer and more reliable operation.
