Bow tie method

John Baker

Objective

• Explain the bow tie method• Consider whether it is a useful tool

for safety event investigation

Slide 2

Initiating eventSuccess event 1Success event 2

Failure event 2

Failure event 1Success event 3

Failure event 3

AccidentFailure 3Failure 1

Failure 2

Failure 5Failure 4

Fault and event trees

Slide 3

FAULT TREE EVENT TREE

Bow ties use the principles of both fault trees and event trees, forming a ‘bow tie’ shape:

Bow tie principles

Slide 4

Business

upset(point where

control is lost)

Recovery preparedness

Preventivecontrols

CON

SEQU

ENCES

CAU

SES

System vulnerability

System resilience

Bow tie models

Bow ties:• are graphical, not numerical• link to known threats• Provide qualitative indication of

how sufficient a control measure is

Slide 5

Origins

• First developed in late 1970s

• Royal Dutch/Shell first organisation to make full use of technique

• Now widely used in high-hazard sectors

Slide 6

Sector applications• Oil & gas• Aviation• Mining• Marine• Healthcare• Rail (fairly recently)

Slide 7

Creating a bow tie

Slide 8

HAZARD

TOP EVENT

CONTROL 1

CONTROL 2 CONTROL

3

THREAT 1

THREAT 2

ESCALATION FACTOR

1

CONSEQUENCE 1

CONSEQUENCE 2

HAZARD Situation with the potential to cause harm

TOP EVENTThe point where control over the hazard is lost

THREAT Unsafe acts or conditions that would lead to the top event

ESCALATIONFACTOR

Anything that would cause a control barrier to fail

Process1. Identify the hazard and resulting

top event2. Identify the threats that can

cause the top event3. Show the control barriers that

should prevent the threats4. Describe the possible

consequences5. Identify the control barriers that

limit or remove the consequences

6. Identify escalation factors that could weaken or remove the control barriers

7. Identify the controls for the escalation factors

8. Identify people responsible for the controls

9. Link the controls to the safety management system Slide 9

Example – confined space training

Slide 10

Smoke/fumesPressur

e gauge

Insufficient air in BA cylinder Death or

major injury

Discomfort

Insufficient

breathable air for

firefighter to get to

safety

Warning

whistle

Routine

charging

BA cylinder valve

switched offBuddy checks

Moisture in system causes

freezingCompressor filters

Filters blocked

Maintenance

schedule

Safety doors

Safety Officer

Safety doors

Safety Officer

Bow tie – object hazard

Slide 11

Bow tie – fire on train hazard

Slide 12

Bow tie – moving train hazard

Slide 13

Bow tie – station fire hazard

Slide 14

Bow tie in accident investigations• Useful visual aid to

brainstorming possible causal factors and controls

• Useful for understanding possible incident sequences and consequences

• If a bow-tie already exists, helps an investigator understand what controls the organisation believes are in place, and how effective they are Slide 15

Bow tie as a risk assessment tool - strengths• Provides a way of visualising risk

assessments• Gets people involved – promotes

understanding of hazard management and their role in it

• Helps make the distinction between preventive controls and resilience controls

• Includes all credible scenarios *****• Structured approach where quantification

is not possible or desirableSlide 16

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