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A Taste of Quantitative Risk Assessment

Quantitative Risk Assessment (QRA) is a technique to quantify the risk to personnel within and without

an asset or facility. A QRA study is performed to assign a numerical value to the risk associated with the

operation of a petrochemical facility. The most significant loss to be avoided in the petrochemical

industry is the loss of life. That is why process safety professionals in most gas, chemical, and oil

companies measure risk using units of fatalities per year.

A QRA differs from a quantitative risk assessment in that a quantitative risk assessment provides a

perceived risk ranking of the risk being analysed. It is used to broadly analyse whether risk are tolerable

or not. This type of risk assessment can be carried out based on judgement requiring no specialist skills

or complicated techniques. Examples of tools used to perform a quantitative analysis are Hazard and

Operability Study (HAZOP), Hazard Identification Study (HAZID), and risk tables. A QRA analysis requires

specialist skills and access to information databases to conduct the activity.

For Malaysian upstream oil & gas activities, facilities are remotely located without adjoining third party

assets. Hence, QRAs tend to focus on the risk of personnel present within the facilities.

Some of the expected uses of a QRA are:

Helping to reduce risk by providing more input into risk-based decision making. Quantifying the different risks of different options during the design phase, or changes made

during the operation phase.

Support the demonstration that risk levels are reduced as low as reasonably practicable (ALARP) Define requirements for emergency response planning Not using QRA to support removing protective measures.

An example of the objective of a QRA is:

1. Provide a numerical estimate of the Individual Risk per Annum (IRPA) for the various personnelcategories at the facilities;

2. Provide a numerical estimate of the combined Potential Loss of Life (PLL) per year on therespective facilities;

3. Identify and rank the key risk contributors to personnel working on these facilities;4. Evaluate the acceptability of these risk levels against the agreed risk tolerability criteria; and5. To recommend, if applicable, practical and effective measures to further tolerate the risk within

As Low as Reasonably Practicable (ALARP) levels.

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Methodology

Figure 2 illustrates the classical structure of a risk assessment. It is a very flexible structure, and has been

used to guide the application of risk assessment to many different hazardous activities. With minor

changes to the wording, the structure can be used for qualitative risk assessment as well as for QRA.

The first stage is system definition, defining the installation or the activity whose risks are to be

Hazard

Identification

Frequency

Analysis

Consequence Modelling

Risk Presentation

Assessment

(Risk Acceptable?)

System

Definition

Input to Safety

Management

Risk ReductionNo

Yes

Figure 1

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analysed. The scope of work for the QRA should define the boundaries for the study, identifying which

activities are included and which are excluded, and which phases of the installation's life are to be

addressed.

Then hazard identification consists of a qualitative review of possible accidents that may occur, based on

previous accident experience or judgement where necessary. There are several formal techniques forthis, which are useful in their own right to give a qualitative appreciation of the range and magnitude of

hazards and indicate appropriate mitigation measures, which may be described as hazard assessment.

In a QRA, hazard identification uses similar techniques, but has a more precise purpose - selecting a list

of possible failure cases that are suitable for quantitative modelling.

Once the hazards have been identified, frequency analysis estimates how likely it is for the accidents to

occur. The frequencies are usually obtained from analysis of previous accident experience, or by some

form of theoretical modelling.

In parallel with the frequency analysis, consequence modelling evaluates the resulting effects if the

accidents occur, and their impact on personnel, equipment and structures, the environment or business.

Estimation of the consequences of each possible event often requires some form of computer

modelling, but may be based on accident experience or judgements if appropriate.

When the frequencies and consequences of each modelled event have been estimated, they can be

combined to form measures of overall risk. Various forms of risk presentation may be used. Risk to life is

often expressed in two complementary forms:

Individual risk - the risk experienced by an individual person. This has previously been identifiedas IRPA

Group (or societal) risk - the risk experienced by the whole group of people exposed to thehazard. Where the public or third parties are involved, this risk is usually expressed via the use

of a FN diagram which relates the number of fatalities with the frequency this occurs.

Up to this point, the process has been purely technical, and is known as risk analysis. The next stage is to

introduce criteria, which are yardsticks to indicate whether the risks are acceptable, or to make some

other judgement about their significance. This step begins to introduce non-technical issues of risk

acceptability and decision-making, and the process is then known as risk assessment.

In order to make the risks acceptable, risk reduction measures may be necessary. The benefits from

these measures can be evaluated by repeating the QRA with them in place, thus introducing an iterative

loop into the process. The economic costs of the measures can be compared with their risk benefitsusing cost-benefit analysis.

The result of a QRA is some form of input to the design or on-going safety management of the

installation, depending on the objectives of the study.

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Frequency Analysis

The following section provides an outline of one step of the QRA process, calculating the frequency of

topside releases. In this case, the initiating event is considered loss of containment of hydrocarbons due

to a leak.

Isolatable sectionIsolatable sections define a set of interconnected equipment, piping and ancillaries that can totally

discharge its contents into the environment. The set or system is isolated from other parts of the

process by Emergency Shutdown Valves (ESDV). The inventory within the section and the properties of

the fluid can subsequently be used to estimate consequences of the leak such as jet fire, pool fire, flash

fire, explosion and/or toxic release from that section.

Parts count

Leak of hazardous materials could take place through a hole of any size from a small hole to a large

leakage along a process line or within an isolatable section. It is most likely to occur at any joint and

connection along the process line such as flanges, valves, instrument or equipment. The number of partsor equipment within an isolatable section provides a basis to determine the leak frequency from that

section. The consequences and the frequencies of leak will then provide an estimate of ignited (e.g. fire

and explosion) and unignited (e.g. CO2 release) risks to personnel.

Leak Frequency Data

Leak frequencies for defined nominal hole sizes (i.e. pinhole, small, medium and large) in each

component type are identified from available databases, for example the UK HSE Hydrocarbon Release

Database.

Event treeThe main objective in using event tree analysis is to estimate the frequency of an event to occur if a loss

of containment (LOC) is happening. It is identified in the hazard identification that the outcome events

from LOC, for oil and gas industries, are jet fire, pool fire, explosion, safe dispersion etc. An example of a

truncated event tree is as follows:

Initiating Event

Frequency

Immediate

Ignition (Top Y)

Shutdown Valve

(Isolation)Blowdown Valve Delayed Ignition

Event

E1

E1

E2

E2

Other event

Where the events may be classified as Isolated Jet Fire (E1), Unisolated Jet Fire (E2)

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Further Information

A simplified example of how a QRA is executed is the paper entitled A Simple Problem to Explain and

Clarify the Principles of Risk Calculation, Dennis C. Hendershot, Rohm and Hass Company, Engineering

Division. It is available at

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.124.7084&rep=rep1&type=pdf

A more detailed explanation of QRA for the offshore industry is A Guide To Quantitative Risk Assessment

for Offshore Installations, John Spounge (principal author). It is available at

http://www2.energyinstpubs.org.uk/pdfs/670.pdf

Figure 2

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