SIL Determination

FSES are able to us various methods in identifying SIL targets; qualitatively by Risk Graphs, semi-quantitatively by Layers of Protection Analysis (LOPA) or fully quantitatively by Fault Tree Analysis (FTA).

SIL Determination


Safety Instrumented Functions (SIF’s) are important elements within many industries for the protection and mitigation of risks. Therefore, it is important to identify the protection requirements of each SIF in respect to its function. The requirement is often described in terms of Safety Integrity Level (SIL). A SIL can be one of four level, each within a target band known as the Probability of Failure on Demand (PFD), where SIL1 is considered the lowest and SIL4 is considered the highest.


Concept of Risk Tolerability


The purpose of determining the tolerable risk for a specific hazardous event is to state what is deemed reasonable with respect to both the frequency of the hazardous event and its specific consequences.

The tolerable risk will depend on many factors. For example, the severity of the consequences or injury, the number of people exposed to danger, the frequency and the duration of the exposure. Important factors will be the perception and views of those exposed to the hazardous event. Risk reduction is achieved by a combination of all the safety protective features, including any associated SIF. The necessary risk reduction to achieve the specified tolerable risk, from a starting point of the risk presented by the Equipment Under Control (EUC), is shown below.


Safety integrity applies to the Electrical / Electronic / Programmable Electronic (E/E/PE) SIF, other technology safety instrumented systems and external risk reduction facilities and is a measure of the likelihood of those systems satisfactorily achieving the necessary risk reduction. Once the tolerable risk has been set, and the necessary risk reduction estimated, the safety integrity requirements for the SIFs can be allocated in terms of the PFD

SIL4³ 10-5 to < 10-4
SIL3³ 10-4 to < 10-3
SIL2³ 10-3 to < 10-2
SIL1³ 10-2 to < 10-1


Methods of SIL Determination


There are various methods in achieving SIL targets qualitatively by Risk Graphs, semi-quantitatively by Layers of Protection Analysis (LOPA) or fully quantitatively by Fault Tree Analysis (FTA).




What inputs are required by FSES in order to conduct the SIL Determination Study?

As a minimum the following information would be required in order to conduct the workshop:

Existing PHA / HAZOP report


Cause and Effects Diagrams

Facilities Design

Operating Data and Procedures

Maintenance Data and Procedures

Interlock List

Equipment Data Sheets

Based on the project requirements, additional information may be required, which will be highlighted within the Terms of Reference (ToR).

Who is required to attend the SIL Determination Workshop?

As a minimum the following personnel would be required in order to conduct the workshop:

Process Engineer

Controls and Instrumentation Engineer

Process Safety Engineer

Maintenance representative

Operations representative

Based on the project requirements, additional personnel may be required to attend the workshop, which will be highlighted within the ToR.

What is the expected output of the SIL Determination Study?

On award of the study FSES will issue a project ToR, which will highlight the assumptions that shall be made in the study, along with the workshop details, methodology and data sources that will be utilised as well as any further information required from the client.

Upon acceptance of the ToR, FSES will facilitate the SIL Determination study through a workshop providing the facilitator and scribe. Once the workshop has been conducted FSES will prepare a SIL Determination report describing the facility, the scope of work, a detailed methodology, the identified SIF’s, initiating events, protection layers, conditional modifiers, the results of the analysis (i.e. SIL requirements for each SIF) and any recommendations based on the discussions during the workshop.

Based on the results of the analysis, FSES can also conduct a Cost Benefit Analysis (CBA) in order to identify if the further risk reduction can be attained.

FSES highly recommend that following on from the SIL Determination study, a SIL Verification analysis is conducted in order to ensure that the SIF’s meet their required SIL targets as per the requirements of IEC 61508 and IEC 61511. Based on the results of the SIL Determination and Verification studies, FSES are also able to assist in writing detailed Safety Requirement Specifications (SRS) for each of the SIFs.


Why FSES? 


FSES are a globally renowned facilitators of SIL Determination studies having facilitated 100’s of hours of SIL Determination studies, with a team of TUV Rheinland certified Functional Safety Engineers available for facilitation. FSES can provide SIL Determination studies using semi-quantitative and quantitative techniques such as; Risk Graph (including extended Risk Graph), Risk Matrix, Layer Of Protection Analysis (LOPA) and Fault Tree Analysis (FTA). For more information, or to discuss your needs for conducting a SIL Determination study, please contact us using the form below or email [email protected] .


Our Services at FSES

Compliance to the international Standards such as IEC 61508 and IEC 61511 are imperative to meeting the Functional Safety requirements for many processes and have formed part of the quality management systems of many companies. Provision of Functional Safety Services in line with the Phases and Activities detailed in the Safety Life-cycle is an area in which our consultants are able to demonstrated experience and expertise.

Learn more about our services

PHA, HAZID and HAZOP Studies

FSES can provide Process Hazard Analysis (PHA) and HAZard and OPerability (HAZOP) study chairpersons. Our expert PHA facilitators hold certification in conducing HAZOP, HAZAN and CHAZOP.

Functional Safety Training

FSES provide various Functional Training services for all of specific parts of IEC 61508 and its sector derivatives

Consequence Modelling

Discharge and Dispersion is often required to evaluate the consequence of identified hazardous events.

Design Engineering

Functional safety activities are most effective by integration into a well-established and proven quality management system.

Building Risk Assessment

Building Risk Assessment (BRA) is a tool used to assess the risks to people in occupied buildings in the facility.

Escape, Evacuation and Rescue Analysis

Escape, Evacuation and Rescue Analysis ensures that the design of a facility is such that the risk to personnel is kept to within the ALARP region.

Functional Safety Assessment

Independent Management of Functional Safety Assessments, Audits & Support.

Functional Safety Lifecycle and Management

Functional Safety Lifecycle and Management is a key requirement demonstrating how Functional Safety is to be implemented and achieved.

Human Factors

Human factor assessment is a key area in identifying potential hazards which may be instigated by human errors.

Legacy Systems

Legacy Systems Review and Compliance.

Prior Use

Prior Use Assessment.

Proof Testing Procedures

Proof Testing is one of the main requirements to ensure that Functional Safety is maintained throughout the operational phase.

Operation and Maintenance

FSES can carry out review studies and provide consultancy to ensure Functional Safety requirements are maintained during Operation and Maintenance.

Quantitative Risk Assessment

Quantitative of Risk in terms of IRPA, PLL and graphical demonstrations such as FN Plots.

Reliability, Availability and Maintainability Study (RAMS)

Reliability, Availability and Maintainability Study (RAMS) is used as a decision making tool to increase the availability of the system, and thus increase the overall profit as well as reducing the life cycle costs.

Safety Requirements Specification

The Safety Requirements Specification (SRS) is a core document, which is used as the main reference to be followed by designers, installers, and operators of a Safety Instrumented Function (SIF).

SIL Determination

FSES are able to us various methods in identifying SIL targets; qualitatively by Risk Graphs, semi-quantitatively by Layers of Protection Analysis (LOPA) or fully quantitatively by Fault Tree Analysis (FTA).

SIL Verification

Calculation of Random Hardware Reliability and Architectural Assessments for Safety Instrumented Functions (SIFs)

SIS Conceptual Design

Safety Instrumented System (SIS) Conceptual Design