Bowtie diagrams were developed in the 1970s as a way of illustrating how risks are managed.  Their use increased significantly after the Piper Alpha disaster and continues to this day.  Although originating in the process industry, other sectors are starting to use Bowtie diagrams.

However, the popularity of Bowtie diagrams is not without its problems.  There has been no definitive guide or standard on how to develop them, or even when they should be used.  People clearly like Bowtie diagrams, but often have inflated opinions of what they can actually achieve and there is a misguided assumption that they can be applied to any activity where there is risk.  Representation of human factors is one particular area where there appears to be a lot of variability and differences of opinion. 

I have written this paper to share my views of how Bowtie diagrams should be used and how human factors should be represented.  I hoped it would start some discussion.  If you have any comments, I would be very happy to receive them.

Bowtie diagrams and human factors (full paper in PDF format)

I hope you find these resources interesting and useful.  I have developed them over the years I have worked as a consultant and they cover some of the key issues I have encountered.

Papers & Publications

A collection of papers I have written over recent years covering topics including human factors, staffing assessments, task risk management, control rooms, shift handover, procedures, training and competence.

Video Animations

Links to some short video animations I have created showing how to perform task analysis, human error analysis and develop causal trees when analysing incidents.


Links to presentations I have given in recent years covering topics including staffing assessments, control rooms, shift handover and accident avoidance.  Also, an introduction to ergonomics for senior school pupils and a two day human factors course presented to companies working in major hazard industries.


This 25 minute presentation looks at the alarms that are received in control rooms typically found in oil refineries, gas plants, chemical sites, power stations and similar.  These may be hard wired alarms to a fixed panel, but most nowadays will be computer based systems.


These alarms should warn operators about something that requires their prompt attention.  Typically an equipment malfunction or control failure.  However, there are lots of problems with alarms in many control rooms, especially with newer computer based systems.

Please note - the videos are hosted on YouTube. if you are looking at this from work your company may have blocked access.



If you don't see the videos above it probably means YouTube has been blocked on your server

Causal Tree Analysis provides a means of analysing the critical human errors and technical failures that have contributed to an incident or accident in order to determine the root causes.  It is a graphical technique that is simple to perform and very flexible, allowing you to map out exactly what you think happened rather than being constrained to accident causation model.  The diagrams developed provide useful summaries to include in incident and accident reports that give people a good overview of the key issues.

Please note - the videos are hosted on YouTube. if you are looking at this from work your company may have blocked access.

Click here to download a copy of the Causal Tree


If you don't see the videos above it probably means YouTube has been blocked on your server

Interlocks provide a means of coordinating the function of different components so that task steps have to be performed in a specified sequence or certain conditions have to be met before a task can proceed. Valves used to create process isolations can be interlocked so that it is physically impossible to manoeuvre them in an incorrect sequence. This is often seen as a method of eliminating the potential for human error.

Advances in technology have allowed more extensive and complex interlocks to be used, which on the face of it, appears to provide the opportunity to make isolations safer than ever before. However, interlocks do not actually eliminate errors; and complexity can be a source of risk. In fact, when all factors are considered there may be an argument to say ‘less is more.’

Whilst there is some guidance available about when interlocks can or should be used; there is very little to say which or how many components should be interlocked. This leaves designers with a dilemma. Do they attempt to apply a ‘sensible’ approach, which may leave them open to criticism because their design is not totally ‘error proof?’ Or do they go to an interlock vendor and ask them to interlock everything,?

One of the problems is that the reason for using interlocks is not always clearly understood or defined. Are they provided to:
• Ensure a ‘spared’ item (e.g. relief valve, filter) remains available at all times and is not interrupted when changing over duty/standby?
• Ensure isolation valves are in the correct position before carrying out a task?
• Ensure the item has been fully isolated and prepared for the task by ensuring valves are manoeuvred in a defined sequence and secured in the correct position;
• All of the above?

Extensive and complex interlock systems are expensive to purchase, install and maintain. They are often only effective for performing one task, and so cause significant problems when other activities have to be performed or if a problem occurs (e.g. valve passes or pipework is blocked). Also, they can create a false sense of security that introduces human factors risks. This paper will discuss these issues using real life examples and suggest that less really can be more.


Presented at Hazards 2017

Whilst Human Factors Engineering (HFE) is starting to be adopted for projects in the oil and gas industry, there is a tendency to leave it until relatively late. This means that opportunities to influence and improve the design are being missed. The reasons for this include a lack of understanding of what HFE can contribute amongst project personnel; and a similar lack of project understanding by the people responsible for integrating human factors. This paper will make the case of doing more HFE earlier in projects, which will improve the way human factors are addressed and result in better design.

Presented at EHF 2017

Effective emergency procedures that support the people who have to detect, diagnose and respond to hazardous situations can reduce the likelihood that minor incidents will escalate.  Unfortunately, procedures often fail to support the people who have to deal with the early stages of an incident. This paper examines the reasons why emergency procedures may not provide adequate support, and sets out some guidelines to help in writing more effective ones.

Published in the Loss Prevention Bulletin April 2017

The double block and bleed method of valve isolation has become almost the default method of isolation in the process industry. However, there are limitations and misunderstandings in the methods of proving integrity.  This paper highlights several ways in which double block and bleed isolations can fail, resulting in hazards with major accident potential. Key learning points include:

• Implementing an isolation involves more than simply closing some valves;
• Multiple failures can and do occur — and because valves are often of the same type and in the same service, common cause failures are an issue;
• Valve integrity must be proven and this requires pressure. There will be times when no pressure is available from the process, or it is only available from the wrong direction;
• Reducing the risk to personnel carrying out maintenance will often be transferred to those implementing the isolation.


Published in the Loss Prevention Bulletin August 2016

Here are some of the presentations I have given over the years. They are all hosted on Slideshare.


2016 - Irish Ergonomics Society


2015 - Trinity College Dublin TOSCA project


 2014 - IChemE Hazards conference


2012 - Young Generation Network


2012 - Young Generation Network


2012 - IEHF North West Branch


2012 - IEHF Annual Conference


2011 - Society of Petroleum Engineers Europe


2010 - Ysgol Aberconwy Secondary School GCSE Technology


2010 - IBC Control Rooms course


2010 - Ergonomics Society Annual Conference (now CIEHF)


2008 - IChemE Hazards conference


2008 - Ergonomics Society Annual Conference (now CIEHF)


2007 - North Wales Occupational Health and Safety Group


2007 - Ergonomics Society Annual Conference (now CIEHF)


2006 - IChemE Manchester Branch


 2006 - IChemE Hazards conference


2005 - IBC Control Rooms course

2008 - European Process Safety Centre

This paper is a development of one I presented at Hazards 24 [Ref 1].  I believe that human factors can make a great contribution to the way the risks of major accidents are managed.  However, whilst its use in industry is growing it is failing to reach its potential because it is not properly integrated into other safety studies.  Task analysis is arguably the human factors technique that has the greatest potential for overcoming this hurdle.

Linking Human Factors to Safety Studies (download full paper in PDF format)

In 2001 a document Contract Research Report (CRR) 348/2001 was published by the Health and Safety Executive (HSE) that introduced a method of assessing staffing arrangements for process operations in the chemical and allied industries.  I’ve lost count, but over the last 10 years I have been involved in at least 30 staffing assessment projects for more than 15 different clients.  Also, even where the method is not formally used I often refer to elements of it as guidance for my other human factors and risk consultancy work.  Having spent 10 years using the method I decided it was a good time to stand back and reflect.  In general, although I can point to some flaws in the method, I have found it to be a very good framework for assessing human and organisational factors.  It prompts you to ask challenging questions and to be objective in your analysis.  Also, I have found that the observations and recommendations I have made as a result of using the method have been very well received by my clients.

Download my Christmas 2011 Paper - 10 Years of Staffing Assessments

Hierarchical Task Analysis is an excellent method of capturing how tasks are performed in a systematic and structured fashion. It is a graphical technique that allows groups to work on the analysis together.  This is useful for identifying different practices and for developing a consensus on what constitutes best practice.

Hierarchical Task Analysis is particularly useful when developing procedures, training plans and competency systems.  Also, being hierarchical it is particularly suited to new projects as it can be developed in conjunction with the design.

This article titled Task Risk Management will give you a context in which HTA is particularly valuable. The following video clips should you how to carry out HTA in practice.

Please note - the videos are hosted on YouTube. if you are looking at this from work your company may have blocked access.


The video is in two parts.      Click here to download a copy of the completed hierarchical task analysis

Part 1

Part 2


If you don't see the videos above it probably means YouTube has been blocked on your server

All organisations involve people in some way.  One issue that this brings is that all people make mistakes, forget things, get distracted, break rules and generally fail.  Human Factors helps us understand how people fail, the potential consequences of failure and how the associated risks can be reduced.

This paper proposes Task Risk Management as a means of integrating the principles of task analysis into a wider risk management process. The paper describes methods and approaches that I have used and found to be very effective and practical.

I have used the term Task Risk Management to show the benefits of taking a task based approach prioritised around process safety risks. I believe that done properly, the way human factors and process risks are understood and managed can be improved significantly.

Task Risk Management (full paper in PDF format)

Task Risk Management job aid

Recent accidents at Buncefield and Texas City have illustrated how poor shift handover can contribute to major accidents.  This is not a new discovery, but given the ever greater interest in human factors, it is one that is finally receiving attention.

Shift handover is a complex, high risk activity that is performed very frequently.  Normally we would try to ‘engineer out’ high risk frequent tasks, or at least automate them to minimise the likelihood of error.  However, this is not an option for shift handover.

Co-author Brian Pacitti of Infotechnics

Shift handover paper (PDF format)

The design of modern control rooms has benefited a great deal from ergonomics and resulted in working environment, furniture and human-machine interfaces that are more consistent with the needs of the people who work in them.  However, I feel that many people involved in the design of control rooms assume that using the latest technology and following the most up to date standards will result in a successful outcome.  They are reassured that what they have developed looks like a control should, but fail to understand that they are not simple objects that can be defined by their physical arrangements.  A control room is actually a component of a complex system where people and equipment come together to control that system.

Control room human factors paper (PDF format)

Companies have invested a great deal of time and effort into training over the years, and it is not the intention here to say that this has all been wasted.  However, unless training is closely linked to a competence system the chance are that the fundamental requirements of the business may not be met because the training provided may not be what is required and/or the cost of that training may be greater than the benefit achieved.

Moving from training to competence (PDF format)


Tug of war


There are a lot of issues that relate to staffing levels and how individuals work as teams.  However, it can be difficult to discuss them as abstract ideas.

Using an analogy based on 'tug of war,' a number of staffing and teamwork scenarios are discussed. Can bigger teams always achieve more than smaller ones?  Does everyone have to be hands on?  How do technical and engineering solutions fit in?

Download a PDF version

I think everyone is familiar with procedures, but do we really know what they are?  Dictionary definitions vary, but they typically suggest a procedure is:

  • A manner of proceeding; a way of performing or effecting something.
  • A series of steps taken to accomplish an end.
  • A set of established forms or methods for conducting the affairs of an organised body such as a business, club, or government.

Interestingly none of the definitions refer to written documents. However, in practice it is generally accepted that a procedure is written in a way that describes a task method.

Although people often want to eliminate risk, this is simply not possible.  Instead it is important we understand the risk we undertake, put sensible controls in place and then make a conscious decision about whether we are happy to accept the risks that remain.  This is what we call risk management.


Special Report for Indicator Tips & Advice - Health & Safety

Available to purchase here


Special Report for Indicator Tips & Advice - Health & Safety

Available to purchase here


Presented at seminar titled 'Workload, organisational change and stress'

Energy Institute, London


Guidance Document -available here

Energy Institute

Co-authors P Waite and A Gait


Health and Safety Executive Research Report RR292

Co-authors A Gait and P Waite


Presented at conference 'Control Rooms: Operation and Design'

IBC, London



Loss Prevention Bulletin issue 204

Also, IChemE Hazards XX Conference, Manchester(April 2008)

Co-author B Pacitti of Infotechnics



Health and Safety Executive Research Report RR174

Co-authors R Lancaster, R Ward, P Talbot

2004 - Energy Institute

The Energy Institute (formerly the Institute of Petroleum) had identified that the staffing assessment methodology (CRR 348/2001) developed by Entec for the Health and Safety Executive was a very valuable tool, but that some companies were not using it because they perceived it to be too difficult to learn and use. Also, the Institute had received feedback from its members saying that they did not feel the methodology was suitable for automated plant. Working closely with the Institute's human factors working party, Andy developed a 'User Guide' that explained the practical aspects of conducting an assessment using the methodology. As well as explanations about the underlying principles and terminology used, the guide provided practical advice and forms that could be used to collect information during assessment workshops. Also, it explained how companies should use the methodology in managing organisational change, including the assessment of risks associated with existing staffing arrangements and the impact of proposed changes. An extension to the methodology was developed to provide users with a method of assessing the impact to the operator of implementing technological change. The report and associated material is freely available from the Energy Institute's website.


Health and Safety Executive Contract Research Report CRR 2002/432

Co-authors J Henderson, K Wright


PhD Thesis

Edinburgh University