Hydrocarbon Processing - November 2021 - 61

Environment
and Safety
H. J. PATEL, Fluor, New Delhi, India
Assessment of independent protection layers
in an LOPA study-Part 1
Industrial facilities, especially those operating in the chemical,
oil and gas and petroleum industries, contain inherent risks in
operations due to the processing of materials that are hazardous
in nature. Hazards, operability issues, associated risks and their
consequences must be accurately identified and analyzed to ensure
safe operations. Safety instrumented systems (SISs) are deployed
to reduce risk to tolerable or acceptable levels to achieve
safe operations.1
The reliability of safety functions implemented
in an SIS is determined by the magnitude of risk reduction required
and is expressed in terms of safety integrity level (SIL).
A layer of protection analysis (LOPA) is one of the methods
used to determine the SIL of the safety instrumented function
(SIF). During the LOPA study, the design is thoroughly examined
for one or more independent protection layers (IPLs) in
the design to assess whether the required risk reduction has
been achieved. The success of the LOPA study depends on
proper assessment of the protection layers and their contribution
to risk reduction.
This article reviews the attributes of IPLs for their effective
consideration in an LOPA.
If used improperly, any SIL selection method may lead to
an inappropriate SIL target with a potentially intolerable level
of risk. Qualitative methods like a safety layer matrix and risk
graphs are simple, easy-to-use, less time-consuming and can be
used in a project's early stages to screen a large number of SIFs.
However, this tends to be more conservative and may result in
a higher SIL requirement, eventually leading to increased costs.
Semi-quantitative methods like calibrated risk graphs and
LOPA methods are more quantitative in nature and, therefore,
more precise than qualitative methods. They can be used during
the detailed engineering stage of the project, or when it is necessary
to validate/review previous results from qualitative/semiqualitative
methods. However, this is more time-consuming and
requires more resources than a qualitative method.
Among fully-quantitative methods, quantitative risk analysis
(QRA) is the most resource intensive. QRA is uncommon in
the process industries, but has been used to analyze cases where
the risk is extremely high. Fault tree analysis (FTA) or event
tree analysis (ETA) methodologies are used to evaluate the scenarios
in detail and provide more exact results [e.g., a minimum
value of risk reduction factor (RRF) that is required for SIF].
It is important to select the most appropriate method for an
SIL study. A LOPA is a widely accepted method for SIL determination
in the process industries-within the LOPA study, it
is vital to select and carefully evaluate IPLs to achieve the most
appropriate result for SIL assignment.
LOPA PROCESS
The starting point for a LOPA is to utilize data collected and
developed in a hazard and operability (HAZOP) analysis. FIG.
1 shows different data from a HAZOP that will be used directly
for the LOPA study.
The first step in a LOPA study is to define the impact event,
which is taken from the " consequence " data field of the HAZOP.
In the second step, consequence severities or severity levels [i.e.,
minor (M), serious (S) or extensive (E)] are designated for the
selected impact event. In Step 3, all applicable initiating causes
of the impact event are listed. Step 4 determines the likelihood
values of the initiating causes, in the number of events per year.
The team's experience is vital in determining the frequency of
initiating causes. The next step in the LOPA process is identifying
all possible protection layers with their probability of failure
on demand average (PFDavg) values. The LOPA team must be
very careful while selecting the appropriate protection layers-
protection layers that perform their function with a high degree
of reliability may only qualify as IPLs. These selected IPLs will
drastically reduce the risk to the process and may impact the
overall result if not selected properly. FIG. 2 is part of a worksheet
for conducting an LOPA study.
The intermediate event likelihood is computed by multiplying
the initiating likelihood (Column 4) by the PFD values of
the protection and mitigation layers in the following step (Columns
5, 6 and 7). This calculated result is the number of events
per year and is entered into Column 8. If the intermediate event
likelihood is greater than the corporate criteria for events of this
severity level, additional mitigation is required. Inherently safer
methods and solutions should be considered before additional
FIG. 1. LOPA-required information from a HAZOP.
Hydrocarbon Processing | NOVEMBER 2021 61
Hydrocarbon Processing - November 2021

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