bow tie analysis

Bow Tie Analysis

Bowties - History

Early Stages

The exact starting point of the Bowtie Methodology has been lost in time but it is

believed that they were originally called “Butterfly diagrams” and evolved from the Cause

Consequence Diagram of the 1970s.

It is then thought that David Gill of ICI plc developed the methodology and called them

bowties in the late 70’s. It is generally accepted that the earliest mention of the bowtie

methodology appears in the ICI Hazan Course Notes 1979, presented by The University

of Queensland, Australia.

Early 1990's

The technique was given a huge boost in the early to mid 90’s when the Royal

Dutch/Shell Group developed the technique as a result of the Piper Alpha disaster. The

team started working with Shell in the late 90s to improve the quality and effectiveness

of the bowties being developed and allow the company to get the most out of the

technique.

Shell is acknowledged as the first major company to integrate the whole bowtie

methodology fully into its business practices but as the 1990's grew to an end the

approach became a standard method within many other companies.

The structured approach of the bowtie methodology was particularly popular in risk

analysis within safety cases where quantification is not possible or desirable.

Bowtie analysis can be used for assessing any type of risk, e.g. environmental, safety,

business, political, security, etc. and is currently used in a wide range of companies,

industries, countries and regulators.

The bowtie diagram provides a powerful graphical representation of the risk assessment

process which is readily understood by the ‘non-specialist’.

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What is a bowtie diagram?

The bowtie has become popular as a structured method to assess risk where a

qualitative approach may not be possible or desirable. The success of the diagram is

that it is simple and easy for the non- specialist to understand. The idea is a simple one

of combining the cause (fault tree) and the consequence (event tree). When the fault

tree is drawn on the left hand side and the event tree is drawn on the right hand side

with the hazard drawn as a "knot" in the middle the diagram looks a bit like a bowtie, as

shown.

bowties - The Process

A bowtie diagram can easily be created by defining the:

Event to be prevented Threats that could cause the event to occur Consequences of the event occurring Controls to prevent the event occurring Controls to mitigate against the consequences

This is shown in the example below where we have a tiger escaping from a cage.

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The threats that will cause the

tiger to escape are identified and

listed to the left, i.e. it could

escape if the cage is not strong

enough or the gate is left open.

The consequences of the tiger

escaping have been identified and

displayed to the right, i.e. it could

maul/kill a member of the public or

could itself be killed.

The level of risk associated with each consequence can be assigned according to the

organization’s risk matrix and displayed on the diagram.

Now that the hazard has been defined, we can put threat controls in place which will

stop the hazard from occurring and consequence controls which will prevent the

outcome. (click on the image to see a larger version)

Controls can be linked to specific sections or procedures of the organization’s safety

management system, can have responsibilities and inspection frequency assigned, and

be ranked and colour coded for effectiveness. The diagram below represents a simple

bowtie diagram.

On each control there is the ability to identify:

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the task

who is responsible for the task

classify the control as appropriate

Any documents that may be applicable

How the task/control will be validated

The controls can be categorised by Basic Risk Factors (BRFs) / General Failure Types

(GFTs) as described by James Reason.

The control can also have "Threats to the Controls" (sometimes called escalation

factors) which weaken the effectiveness as shown below. Each threat to the control can

in turn have barriers to prevent the threat from occurring.

bowties - The Uses

The bowtie methodology can be used for any type of hazard analysis, from major

accidents, through occupational and environmental to business, IT and security risks.

Typical examples are:

Demonstrating control of Risks

The bowtie methodology is an effective way of demonstrating that an organization’s risks

are reduced to ALARP (As Low As Reasonably Practicable) without the over reliance on

qualitative risk assessments that has been apparent in the past. The bowtie

methodology is accepted by safety case regulators as a demonstration of ALARP.

Controls on the diagrams can be categorised by Safety Integrity Levels (SIL),

Effectiveness and other types of control that may be required.

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Communication and Training

As the bowtie is a graphical representation of the issue and is readily understood by a

wide audience it is commonly used in communication. Bowtie diagrams have been

printed out in anything from full colour A1 size posters to A5 pocket booklets as an aid in

communicating HSE issues. As bowtie diagrams are easy to understand, all members of

the workforce can participate in the process.

Organizational Improvements

It is possible to use bowties in conjunction with numerous techniques to identify the

branches which are weak. The use of BowTie allows ownership and critical tasks to be

given to controls ensuring that critical controls do not get over looked. In a similar way

BowTie also allows the controls to be categorised by Basic Risk Factors (BRFs) /

General Failure Types (GFTs) as described by James Reason and so linked into an

accident investigation.

Identifying Risks

Bowtie diagrams are an effective means of systematically identifying hazards, assessing

the controls and promoting discussion. The bowtie diagram graphically demonstrates

that controls are in place to reduce the risk to ALARP and document the HAZID results.

Maintaining a Risk Register

Bowtie diagrams provide a simple, easily understood and readily updated representation

of the hazards involved in an organization’s activities and the measures taken to control

the risk.

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bow tie analysis

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