SIL Determination
SIL Determination or Target Assessment
SIL Determination (Target Assessment) defines how reliable each Safety Instrumented Function (SIF) must be in order to control identified risks.
SIL Determination Overview
It sits on the Process Hazard Analysis (PHA) pathway and builds directly on HAZOP outputs.
The work focuses on a set of credible scenarios taken from HAZOP or related studies. For each one, the right targeting method is chosen (LOPA or risk graph), assumptions are made explicit, and a target SIL is assigned to the relevant SIFs. The rationale behind every decision is kept traceable.
Workshops are focused, time-boxed, and easy to follow for both engineering and operational stakeholders. With guidance from our specialists, your team moves from initial scope to a clear, defensible set of SIL targets. Verification and detailed design of the SIFs are then handled under a separate service.
SIL Determination Background & Regulatory Context
IEC 61511 defines the functional safety lifecycle for process sector Safety Instrumented Systems (SIS) and frames how SIFs are specified, implemented, and managed.
In the United States, ISA 84 adopts IEC 61511 as an ANSI standard, so auditors and regulators expect alignment with its lifecycle. LOPA is a recognized method for targeting SILs and is documented by CCPS as good practice for evaluating protection layers and setting risk-based requirements.
HAZOP is an accepted PHA technique under OSHA PSM, which also requires PHA revalidation at least every five years. SIL Target Assessment uses those PHA outputs to set the target SIL for each SIF. Subsequent Verification then demonstrates that the detailed SIS design meets the target; this is covered by guidance such as ISA TR84.00.02 and falls under a separate scope.
How SIL Determination (Target Assessment) Works – Step by Step
- Inputs and Scoping
The engagement begins by clarifying how SIL targeting fits into your broader risk management and functional safety work:
- Aligning on objectives, tolerable risk criteria, and boundaries.
- Taking HAZOP deviations and selecting credible scenarios for targeting.
- Confirming applicable standards and your company practice.
- Agreeing roles, agenda, data sources, and the action tracking approach.
This step ensures that effort is focused on the right scenarios and that criteria are clear from the start.
- Method for Targeting
Next, the targeting method is selected and applied consistently:
- Choosing LOPA for semi-quantitative depth or a risk graph for faster screening.
- Defining consequence severity and initiating event frequency for each scenario.
- Identifying Independent Protection Layers (IPLs) and testing them for independence and effectiveness.
- Comparing mitigated frequency with your risk tolerability criteria.
- Assigning a target SIL to each SIF and recording the logic behind the decision.
The outcome is a set of SIL targets that can be traced back to agreed assumptions, data, and criteria.
- Handover for Design
Once targets are set, the results are prepared for use in design and verification:
- Compiling a clean SIF list with target SILs and initiating scenarios.
- Providing an Assumptions Log and method worksheets.
- Outlining Safety Requirements Specification (SRS) skeleton items needed at the next stage.
- Defining interface points to SIL Verification and to functional safety engineering under IEC 61511.
This handover allows SIS designers and functional safety engineers to continue the lifecycle with a clear, agreed baseline.
Deliverables and Value
Deliverables
- SIL Target Report with scope, criteria, method, and results.
- SIF list with target SILs and scenario references.
- LOPA / risk graph worksheets and Assumptions Log.
- Action register with owners and due dates.
- Executive summary for leadership and auditors.
Client Value
- A defensible link from hazards and scenarios to target SILs.
- Shared understanding across engineering, operations, and HSE.
- Clear handover to SIL Verification and SIS design activities.
- Smoother regulator and third-party reviews.
- Budgets focused on the right protection layers and functions.
Our experts will provide an initial consultation and support your team through SIL Determination — from selecting scenarios and methods to delivering a practical, compliant set of SIL targets.
