SIL Allocation Study (LOPA / Risk Graph)
We allocate the IEC 61511 Clause 9 SIL band for each safety instrumented function using LOPA or a calibrated risk graph on a risk reduction factor basis that holds up under audit
SIL Allocation Study (LOPA
/ Risk Graph)
SIL allocation is the hazard study step where each identified safety instrumented function is converted into a target safety integrity level, which is the input the functional safety engineering team needs to design the safety instrumented system. The work sits firmly in the risk assessment workflow, because HAZOP identifies the candidate function, then LOPA or a calibrated risk graph quantifies the residual risk gap, and the allocation step assigns SIL 1, 2 or 3 under IEC 61511 Clause 9. The discipline is about defensible calibration rather than the arithmetic of the calculation. We are honest about IPL eligibility against the CCPS tests, we apply conditional modifier discretion that survives audit, we make conservative initiating event frequency assumptions where data is thin, and we flag SIL 3 and higher scenarios for inherent safety review rather than engineering them to ever higher integrity. The output is a SIL allocation register that hands cleanly to the functional safety engineering team for safety requirements specification development, FMEDA processing and verification of the probability of failure on demand and per hour, with that verification workflow living in our functional safety service line.
How the study is executed
A structured, facilitated process that runs from scope definition through close out and produces defensible, actionable outputs.
Identify Safety Instrumented Functions from HAZOP action register, Bow Tie barrier register, and P&ID review, document hazard scenario (initiating event, consequence, existing safeguards) for each SIF candidate, classify demand mode per IEC 61511 Cl.3.5.
Select SIL allocation method per scenario complexity and regulatory expectation, LOPA (most defensible), calibrated risk graph (ISA TR84.00.04 Annex), risk matrix, or LOPA matrix, document method choice and calibration.
Apply CCPS four part IPL test to existing safeguards, calculate Required Risk Reduction Factor against site tolerable frequency per consequence tier, allocate residual risk reduction to the SIF being assessed.
Assign SIL band per IEC 61511 Cl.9 / Table 4, SIL 1 (RRF 10, 100), SIL 2 (100, 1000), SIL 3 (1000, 10000), flag SIL 3+ scenarios for inherent safety review per CCPS guidance, document allocation rationale per SIF.
Apply conditional modifiers honestly, ignition probability, occupancy, mitigation, per CCPS LOPA Cookbook, conduct sensitivity analysis on TF, IEF, and IPL PFD assumptions, defend any non conservative choices.
Issue SIL Allocation Register per SIF with method, RRF, allocated SIL, and assumption log, transmit to functional safety engineering team for SRS development and lifecycle verification, integrate with PHA action close out.
What the study covers in full
Outcomes of SIL Allocation Study (LOPA / Risk Graph)
- We calibrate the risk reduction of each function to its specific demand scenario rather than to a rule of thumb
- We identify the small subset of functions that disproportionately drive your site risk
- We prevent the silent function degradation pattern seen in legacy DCS and SIS installations
- We anchor your layer of protection logic with quantitative integrity
- We give you full IEC 61511 Ed.2 lifecycle defence including the new cyber security cross references
- We make the work defensible under FSA Stage 1 through 5 examination
- We meet the OSHA PSM mechanical integrity expectation for safety critical instrumentation
- We satisfy the demonstrate ALARP requirement in your COMAH safety case for instrumented protection layers
- We set rational proof test intervals balanced against the probability of failure on demand and operating cost
- We engineer the spurious trip rate into mean time to spurious failure targets so you avoid 1oo1 trip exposure
- We build bypass governance and impairment management into your safety manual
- We give you a defensible MOC pathway for instrumented modifications
- We help you avoid the SIL inflation cost trap of conservative allocation
- We right size the architecture across 1oo1, 1oo2D and 2oo3 to the integrity you actually need
- We reduce the production losses that spurious trips drive on continuous units
- We strengthen your underwriter dialogue with manufacturer FMEDA grade evidence
Codes & standards we work to
Triggers that signal the need
Where SIL Allocation Study (LOPA / Risk Graph) applies
Wellheads, separators, gas compression, FPSO topsides, produced water systems.
Distillation columns, reactors, heat exchangers, storage spheres, LPG handling.
Cryogenic exchangers, liquefaction trains, BOG compressors, storage and sendout.
Reactive systems, batch reactors, solvent handling, runaway reaction scenarios.
Boilers, HRSGs, steam headers, hydrogen systems, ammonia SCR units.
Sterile vessels, CIP/SIP, pressure fermenters, solvent recovery, spray dryers.
Tangible deliverables
- SIL allocation register per function with method, risk reduction factor and allocated band
- LOPA worksheets and calibrated risk graph records per scenario
- IPL eligibility assessment per the CCPS four part test
- Conditional modifier rationale log for ignition, occupancy and mitigation
- Function inventory with HAZOP and Bow Tie traceability
- Escalation register for SIL 3 and higher scenarios with inherent safety options
- Sensitivity analysis report on tolerable frequency, initiating event frequency and IPL assumptions
- Hand off package for the functional safety engineering team covering downstream specification and verification
Ready to start your project?
Speak with our team to scope an engagement tailored to your facility, regulatory context, and lifecycle stage.