HAZOP vs HAZID?

Many people get confused between these two similar, but different safety risk assessment techniques: HAZOP (HAZard and OPerability study ) and HAZID (HAZard IDentification study).

• What is the difference between them?
• Which one should we use?
• When should we use them?

A HAZOP uses guidewords to identify where a process exceeds it normal operating or design parameters.  The guidewords such as “More Pressure”, “Less Temperature” or “No Flow” are prompts to the study team to identify causes for the process to deviate from its design and operating envelope (deviations).  It may also be applied to steps in a batch process, or even steps in a manual procedure. 

A HAZID also uses guidewords, but these are usually in the form of Hazards that have the potential to cause harm.  These tend to be much higher level, covering a complete unit, process block or operating facility.  This is often referred to as a “10,000-foot view” and is frequently assessed in conjunction with a plant layout plan, a map of the local environment, information on consumable and waste materials, and environmental emissions.  We frequently use Google Earth as the key tool for a HAZID.

A HAZOP study generally considers upsets, failures and errors in an automated process with operator control.  It is concerned with keeping the process materials “inside the pipe” without loss of containment or damage to the assets.  The impact or consequences of concern are health and safety when the process material is released (loss of containment), and potential fires, explosions, toxic impact or exposure of personnel to very hot or cold fluids.  It also considers the potential environmental impact, and sometimes includes asset damage, loss of production and reputational damage.  In essence it considers failures in the control system or from operator intervention, and the impact on the process inside the pipe.

A HAZID study on the other hand, is primarily looking at the hazards outside the pipe – not process failures, but all the associated activities required for the production of the end product.  This may include transport and logistics hazards – both onshore and offshore; emissions to air, water or land; construction and maintenance hazards; and the impacts to neighbouring facilities or communities of an accident on the site and vice-versa.  Essentially, it is concerned with the site activities outside the process, and the impact of the process on the surrounding environment and vice-versa.

Which study type should I use, and when?

If you are developing a new project, it is best to plan a HAZID study early, during the concept or options phase of the project.  This will allow you to identify significant hazards early, to feed into the design options and budgeting process.  The HAZID will also indicate any additional studies or investigations required at that stage, and help with early permitting.  It is important to keep the study very high level, and not try do a “mini-HAZOP” or to recommend actions that are part of the normal design process.  The purpose of the HAZID study is to identify any hazards and associated risks (likelihoods) that are significant or unusual, as part of the initial options development.  This should take less than 1 day at this early stage, depending on the complexity.

As the project develops, it is normally recommended to conduct a more detailed HAZID in the early FEED (Front-end Engineering Design) phase.  By this time, the main design and planning options should have been decided, and more information should be available to identify areas of interest and further study required during the FEED engineering.  A reasonable site plot plan and block flow diagram should be available, with a basis of design showing the critical materials and processes.  It is very helpful to have some emissions data at this time.  This may be a slightly longer workshop, particularly if it includes construction hazards.

Towards the end of the FEED phase, once detailed P&IDs and control information is available, an initial HAZOP would normally be considered.  At this time, you are unlikely to have detailed vendor information for any packaged equipment.  The purpose of this HAZOP is to provide a design tool to allow any safety and environmental recommendations to be incorporated into the design, and to firm up the budgetary pricing for control and safety systems.

In the EPC or Detailed Design phase of the project, another HAZID would normally be planned.  This would consider the outcomes, recommendations and conclusions of specialist health, safety and environmental studies that should have been part of the previous FEED design or recommended in the earlier HAZIDs.  It may also be able to consider emergency response, evacuations and other procedures that should be in draft by this time.  This HAZID needs to be quite early in this phase, otherwise significant findings may be too late/difficult/expensive to implement, increasing the residual risk and impacting the permitting/licensing process.

As soon as packaged equipment vendors have been contracted and have prepared drawings, Vendor HAZOPs should be undertaken.  These would lean on the previous FEED HAZOP for the process consequences but would focus on the packaged equipment and interfaces.

Finally, a detailed design HAZOP should be planned for just before “Approved for Construction” drawings are released.  This forms the final safety check on the design, to confirm that it is safe to startup, operate, maintain and shutdown.  It is important to consider all of these (and any other) operating modes at this time, as draft operating procedures, and startup plans should be available.  Note that the previous FEED HAZOP was essentially a design tool, while this one is the safety risk assessment before turning the key to start up the plant.

For an operating facility, it is important to do a form of HAZOP (or associated SWIFT/What-if study) for all major changes in the operation of the plant.  These would normally be part of the MoC (Management of Change) process.  The facility should also have a periodic site-wide HAZID and HAZOP with a suitable frequency (say 5 years), to identify any incremental changes to the design, operation or activities on the facility that have occurred since the last full study.  This should also take account of any incidents or recorded failures of equipment or procedures during that time.

If you require a non-standard risk assessment that doesn’t fit the standard HAZOP or HAZID process, I always recommend to start with identifying what you are assessing: is it an automatic process, where we are trying to keep the hazardous materials in the pipe?  Or is it an activity (or series of activities) where the hazard is outside of the process, or after the hazardous material has been released? For the former, use a HAZOP (or SWIFT/What-if), while for the latter use a HAZID.  For some cases, such as pre-commissioning/commissioning work, you may need both.

For more information, please do contact us to discuss.