SIL Targeting

HAZARD AND OPERABILITY (HAZOP) STUDY / LAYERS OF PROTECTION ANALYSIS (LOPA)

 FSC has a HAZOP / LOPA software package which significantly simplifies the conducting and recording of the HAZOP meeting and allows the concurrent assessment of the SIL target via LOPA assessment sheets.

Objective of the HAZOP

A HAZOP is a structured brainstorming meeting, set up to identify potential hazards, i.e. risks to personal safety and potential damage to assets, the environment and the reputation. The identified hazards, along with any actions for further investigation and other relevant, supporting information are recorded on HAZOP Worksheets.

HAZOP Study Team

It is important that a HAZOP team is made up of personnel who will bring the best balance of knowledge and experience, of the type of plant being considered, to the study. 

Information Used in the HAZOP

Typical information that needs to be available to view by the HAZOP team are:

Establishing SIL Targets

The LOPA (Layers Of Protection Analysis) considers each hazard  identified by HAZOP and documents the initiating cause and the protection layers  that prevent or mitigate the hazard. The total amount of risk reduction can then be determined and the need for more risk reduction analyzed. If additional protection is to be provided in the form of a SRS, the methodology would allow the determination of the appropriate SIL but for this analysis, LOPA can be used to obtain the demand rate for an assessment matrix.

(Click on the picture for a larger version)

(Click on the picture for a larger version)

Safety System Assessment

SIL Assessment Worksheets

FSC has a SIL assessment software package which significantly simplifies the assessing and recording of proposed SIS (Safety Instrumented System) safety loops.

Each SIL Assessment worksheet provides an analysis of the PFD and performance against architectural constraints for each loop, along with the expected spurious trip rate. The worksheets are laid out in three rows corresponding to the input subsystem, the logic solver and the output subsystem.

The worksheet field headings are described below.

Description

Provides a description of the device selected.

SFF (Safe Failure Fraction)

Shows the calculated SFF for the selected device.

Mean Down Time (MDT)

Enables the MDT (in hours) to be entered into the calculation of PFD.

Partial Stroke Test (days)

If applicable, allows the partial stroke testing period to be entered.

Proof Test Interval (Tp)

Enables the proof test interval (in months) to be entered into the calculation of PFD.

Configuration (MooN)

The configuration of the Input/Output (I/O) subsystems can be entered. For example 1oo1, 2oo3. The configuration of the selected logic solver is presented.

Quantity

Allows the number of I/O subsystems to be entered.

Beta Factor

In the case of redundant configurations of subsystems, displays the contribution assumed for common cause failures.

IS Barriers (ISB)

Allows IS Barriers to be added to the configuration.

Probability of Failure on Demand (PFD)

The PFD for each subsystem is calculated.

SIL Arch

The SIL allowed, based on the architectural performance of the subsystem, is presented.

SIL (PFD)

The SIL allowed, based on the PFD of the subsystem, is presented.

Summaries

The overall PFD, SFF and SIL for the whole loop are presented.

The failure rates were obtained from a combination of sources, e.g. Sintef, Oreda, manufacturer’s data and Technis.

Spurious Trip Rate

The SIL Assessment Worksheets also calculate the spurious trip rate for each loop type. The trip rates of each configuration of sensors, logic and output devices are presented and enable the configurations to be optimised for plant availability.

(Click on the picture for a larger version)