This article looks at what HAZOP studies are, how they fit within a wider hazard study process, and the role of Delta HAZOP studies, which focus on understanding the hazards emerging from creeping changes in a process.
Although ICI’s original Hazard Study process had six steps, modern processes typically have eight, such as in ABB’s Hazard Study methodology (see www.is.gd/deravo):
It should be clear from this list that while the HAZOP study is where the really detailed hazard analysis is carried out, the other studies are also vital to ensure that the HAZOP study is properly planned and subsequently safety measures are properly implemented.
HS 0 - Concept stage: inherent safety workshop. This is about reducing or eliminating the potential for hazard by applying a structured hierarchical approach to the inherent safety of a design.HS 1 - Feasibility stage: process safety information. A checklist-based study is used to ensure everyone involved properly understands the process, its safety implications and what other studies may be needed.HS 2 - FEED stage: HAZID study. This hazardous event study considers preliminary process flow diagrams to identify and remove major hazards.HS 3 - Detailed process design stage: HAZOP study. The hazard and operability study systematically performs a detailed analysis of the process design and operating procedures, leading to a final process design.HS 4 and 5 - Pre-start up safety review. Checklist-based studies carried out before construction hand-over and start-up ensure that safety features identified in previous studies have been properly implemented.HS 6 - Early operational stage. Another checklist-based study which reviews the process early in its operation to ensure that the planned level of safety is reflected by actual experience and enable continuous improvement.HS 7 - Operational stage re-validation. Safety analysis should be periodically reviewed, typically every five-years. This may involve a retrospective HAZOP study using a ‘line-by-line’ deviation analysis. The Delta HAZOP (see below) is another approach at this stage.HAZOP STUDIES
A HAZOP takes a highly structured and systematic approach to examine a process and identify any issues that could risk injury to people or damage to equipment. To properly perform a HAZOP, it is important that a multi-disciplinary team is involved throughout the process. By systematically examining the process, issues can be identified that may be missed in a less-structured assessment. Complex systems are first broken down into simpler sub-systems known as ‘nodes’ which are then reviewed individually. The process for carrying out a HAZOP, as defined in IEC 61882-2001, is shown in the diagram.
Standard guide words are used to prompt an imaginative and qualitative consideration of each node from multiple perspectives (a standard list is included in the box below). When considering each guide word, relevant parameters should also be considered to determine where these may combine to constitute risks. Some common parameters include flow, pressure, temperature, level, time, agitation, reaction, start-up / shut-down, draining / venting and vibrations.
When guide words are combined with parameters to state a hazard condition, it is normal to state this without using the guide word. For example, the combination of ‘More’ and ‘Flow’ would be stated as ‘high flow’.
The Delta HAZOP
When a HAZOP is repeated during the operational life of a process, it is normal for fewer significant issues to be identified. This is an expected and healthy outcome as a process matures. However, it can mean that little value is gained from HAZOP studies after a few iterations. The Delta HAZOP is intended to maintain the usefulness of these studies by focusing on understanding and mitigating risks associated with creeping changes in the process. A Delta HAZOP should therefore be carried out as part of a robust management of change process. By focusing on the changes, a Delta HAZOP has the potential to uncover higher risk levels than a new complete HAZOP.
A new guide to the Delta HAZOP technique was published by the IChemE Safety Centre (ISC), working in collaboration with many partners from industry: see www.is.gd/gewefe.
When deciding whether it is most appropriate to carry out a second complete HAZOP (also known as a ‘re-do’ HAZOP), or a Delta HAZOP, it is important to consider regulations and legislation affecting the process. A Delta HAZOP is not a HAZOP as defined by IEC 61882. It is also important to determine whether sufficient process maturity has been achieved. As a minimum, at least one development stage study or design HAZOP should have been carried out, as well as a utilisation stage or baseline HAZOP characterising the current facility.
A Delta HAZOP can be carried out as many times as required, but each time the selection process should consider whether it will have a greater expected risk discovery than a re-do HAZOP and validation and verification of the process should be carried out.
The Delta HAZOP study follows a four-step process:
- Definitions: Selecting the study type, forming the study team, and setting the scope and objectives for the study
- Preparation: Gather relevant information, review the previous process hazard analysis (PHA) and management of change (MOC) reports, and establish the nodes that require update in this exercise
- Examination:
- a.Structure the examination, reviewing previous PHA findings and MOC documentation to determine if there are any hazard changes, as well as any incident reports
- b.Perform the examination, reviewing the changes that could impact critical systems or increase hazards. Such changes could include operating conditions, human factors, organization changes, external incidents
- Documentation and follow-up, which involves report writing and wrap-up activities.
The focus in a Delta HAZOP is on identifying and then examining areas of change, especially creeping changes that may not have been explicitly considered.
“Delta HAZOP is an alternative approach to a standard HAZOP-type activity. So it’s something a little different; it really focuses you more on subtle changes that can accumulate over time and creating this creeping of the system and the standards, and how things operate,” says Trish Kerin, director of the IChemE Safety Centre.
She continues: “In the past we used to talk a lot about something called a Re-Do HAZOP, and that comes out of the CCPS [US Centre for Chemical Process Safety] revalidation process and methods guidance... Delta HAZOP is a different way to look at it... Your standard HAZOP is guide-word approached, and it prompts you to look through scenarios and you view the node with the guide word checklist. Delta HAZOP focuses on the evidence and the data for what has changed, so it's not using a standard guide word checklist at all. It does use a checklist to prompt you to think about and explore things, but it's looking about what has changed, and it tends to view the system more holistically in itself, rather than focusing only on the specific node. You still do look at the node, but you look at more broadly how that node interacts as well.”
A Delta HAZOP is not a replacement for a conventional HAZOP, which is still essential at the design and early implementation stages. However, at the revalidation stage, a Delta HAZOP is a great tool to have in the kit.
BOX: Standard HAZOP terms
NO OR NOT: Complete negation of the design intentMORE: Quantitative increaseLESS: Quantitative decreaseAS WELL AS: Qualitative modification/increasePART OF: Qualitative modification/decreaseREVERSE: Logical opposite of the design intentOTHER THAN / INSTEAD: Complete substitutionEARLY: Relative to the clock timeLATE: Relative to the clock timeBEFORE: Relating to order or sequenceAFTER: Relating to order or sequenceBOX: CHAZOP
An application of the HAZOP process to control and computer systems is called Control Hazard and Operability: CHAZOP. It can pinpoint vulnerabilities in computer systems, cybersecurity and other elements, according to technical consultancy Engineering Safety Ltd. It says that a ‘what-if’ procedure results in a list of all possible consequences of failure of the control system. The company says that the technique “highlights areas where unwarranted assumptions have been made, training and management are weak, and single points of failure exist that can lead to a process shutdown.”