The hazard and operability study or HAZOP is a well-known and widely used technique for process hazard analysis. It is one stage of the original six-stage hazard study process developed by Imperial Chemical Industries in the 1960s . The technique has been refined over the years and the International Electrotechnical Commission, Geneva, in 2016 issued a comprehensive standard .
HAZOP is a structured process that delivers a thorough risk analysis when undertaken with sufficient rigor. However, due to its structured guide-word nature, repeating a HAZOP on the same node again and again can lead to diminishing returns in identifying risks. So, do we need a new way to look at old risks? Many companies, like ExxonMobil, have been asking this question and searching for ways to better identify risks. They have found that concentrating on changes made since the last HAZOP — via a so-called Delta HAZOP — not only can save time in assessment but also produce a superior risk-discovery process.
As our facilities continue to operate over many years, creeping changes occur that potentially can impact our risk profile. To try to deal with this, we use management of change (MoC) processes. A robust MoC process is a key element in every safety management system. It is designed to provide a structured assessment of the change to ensure all hazards are understood and adequately managed. The challenge, though, is that some changes take place without going through the MoC process. While sometimes this happens because someone forgets or ignores the MoC procedure, other times it occurs because a change, such as one resulting from wear and tear, goes unnoticed. For example, a trip system might activate more often due to a subtle change in the equipment. A traditional or ReDo HAZOP would not detect this as a change and adequately consider it in the assessment. In contrast, Delta HAZOP focuses the assessment team on the changes since the last HAZOP, however subtle they may be.
The foundation for the Delta HAZOP methodology is the Center for Chemical Process Safety revalidation process . This then is built upon to focus on the subtle or creeping changes. The method requires at least one prior utilization-stage HAZOP to feed into the review. The IChemE Safety Centre (ISC) has released a guidance document on how to undertake a Delta HAZOP . ExxonMobil introduced this process at the Hazards 29 conference and later in an article . It then worked with the ISC to bring other companies together to refine the process into the guidance document. As the working group progressed, it became apparent that many organizations were using some form of review like this. The ISC document provides a common platform for companies to work from.
The process follows the same four steps defined in IEC61882 :
3. examination; and
4. documentation and follow-up.
Preparation for a Delta HAZOP may take longer than that for a ReDo HAZOP, as there is more information to gather, but the actual assessment process usually is shorter. Overall, the assessment takes less time. The reduced workshop time can save quite a bit of money but the true value of the process is its ability to uncover higher-order risks than via a ReDo HAZOP. The assessment still requires the same type of team. However, it is engaged in its review for less time, meaning a Delta HAZOP may produce a more useful outcome for less effort. This makes the Delta HAZOP process very appealing. However, you must make sure it really suits your circumstances — it is not a shortcut to save money.
Is It Right For You?
To determine if a Delta HAZOP is suitable in your circumstance, you must review a range of inputs and make a judgement call on whether it is the most-appropriate tool. There is no right or wrong answer; it is about understanding the inputs to arrive at a balanced decision. Table 1 lists 11 key questions you must consider, with explanations of what they entail (download the PDF here).
Once you review these 11 items, you can make a balanced judgement on whether a Delta HAZOP is right to use in your current circumstances.
If you determine it is, then you must assemble the information and data necessary to properly perform a workshop.
The majority of the information required for a Delta HAZOP are the same as for a ReDo HAZOP. However, you also must gather and consider some additional information:
• all relevant MoC documents;
• details for all changes not captured in the MoC process;
• cumulative change impacts;
• staffing changes;
• regulatory or company-standard changes;
• other relevant risk-assessment reports; and
• pertinent inspection and audit reports.
The Delta HAZOP process focuses on examining the changes that have occurred, so it requires any information relevant to a change.
Once all the necessary information has been assembled, the Delta HAZOP workshop proceeds in six sequential steps:
1. Provide an overview of the plant and any changes that have taken place since the last HAZOP.
2. Review the status of the findings from the last HAZOP, focusing on whether the recommendations have been incorporated as intended.
3. Check the list of MoC items, both temporary (even if closed) and permanent, to see if any require a deeper dive.
4. Go over the incident database for applicable events and near-misses to spot any trends.
5. Assess if any alterations or mark-ups to piping and instrumentation diagrams (P&IDs) indicate a change in the way the plant works.
6. Look for any subtle or creeping changes in:
• plot plans;
• new material information;
• interlocks and safety instrumented systems;
• inspection/test practices;
• design standards;
• safe work practices;
• detection and suppression systems;
• hazardous area classification; and
In each of these steps, the workshop team discusses the information and what it means. This then may lead to the need to seek more details on a particular item as the team examines the implications. During the workshop, the team may suggest specific actions to undertake to manage an identified risk.
Once complete, the workshop must be documented and any actions managed within the company’s systems. You should keep and maintain all documentation as you would for a ReDo HAZOP.
The Delta HAZOP mainly focuses on spotting subtle or creeping changes that occur over time as well as looking at documented changes. It does not follow the structured HAZOP guide-word approach but rather views the system more holistically. It can be a more-effective process compared to a ReDo HAZOP because it concentrates on changes in operations. However, you need the right type of inputs into the workshop and a knowledgeable team to ensure its quality.
You can get more details on the information required and how to conduct a Delta HAZOP in the ISC guidance document , which is free to download here. IChemE also will be providing an online training module for Delta HAZOP leaders. For information on this, please contact [email protected].
This information has been developed on the best current knowledge as assessed by a range of companies and facilitated by the ISC. The information contained in this article and the guidance document is provided in good faith but without any liability on the part of IChemE, the ISC and ExxonMobil.
The ISC would like to thank ExxonMobil for sharing its initial process with us. We also would like to acknowledge the following companies for their work in the development of the guidance document: Chevron, DEKRA, EnQuest, ExxonMobil, HIMA, ioMosaic, IRESC, Jemena, Orica, PSRG, R4Risk, Rio Tinto, Safety Solutions Ltd., Santos, Sherpa, Todd Corp., Woodside, and Worley.
1. Crawley, F., and Tyler, B., “HAZOP: Guide to Best Practice,” 3rd ed., IChemE/Elsevier, London (2015).
2. “Hazard and operability studies (HAZOP studies) —Application Guide,” IEC 61882:2016, IEC, Geneva, Switz. (2016).
3. Frank, W.L., and Whittle, D.K., “Revalidating Process Hazard Analyses,” CCPS/Wiley, Hoboken, N.J. (2001).
4. “Effective revalidation of risk assessments — Delta HAZOP,” IChemE Safety Centre, Rugby, U.K. (2021). Downloadable here.
5. Kenny, P., “Delta HAZOP: Revalidation and Focus on Major Accident Hazards,” p. 22, The Chemical Engineer (Dec. 2019).