To answer the question "what is HAZOP?" you first need to know that it is an abbreviation of HAZard and OPerability; a brainstorming study technique looking at how industrial process plant can go wrong.  A HAZOP study is a team analysis exercise, often used in the design stage of new or novel processes, but also used at regular intervals to discover operability issues with existing processes.

What is HAZOP?

A HAZOP study is a structured technique for determining hazard and operability issues. Using guide words such as "low or less" and applying these to process parameters such as flow and temperature, the HAZOP study systematically looks for hazard causes, consequences and safeguards.

A HAZOP study can be carried out on practically any design, but only if there is documentation that represents the system in sufficient detail. HAZOP typically requires having Piping and Instrumentation Diagrams (P&ID's) developed to a level where they represent the existing plant accurately, or for new designs, the drawings must have reached a good level of maturity.

Any plant undergoing major modification can use HAZOP to determine if process design changes will introduce or reduce additional hazards or operability issues.

HAZOP is commonly applied on operating chemical and energy industry facilities at intervals of three to five years to "revalidate" original assumptions and capture any operational changes that may have crept in over time.

Any company operating processes or machinery with the potential for loss of life or significant environmental damage must carry out hazard identification and risk assessment. For most process facilities, this involves looking for events that could result in the “loss of containment” (LOC) of hazardous materials.

There are many ways of approaching process hazard analysis (PHA), often using methods like HAZOP - Hazard and Operability studies. Whichever method is adopted, the key is to use a systematic approach with a multi-discipline team.

If approached methodically, a HAZOP should provide:

  • Credible information on possible causes of hazards.
  • Estimated consequences of hazardous events.
  • Safeguards or protection layers that can prevent event occurrence or mitigate escalation.
  • Actions for safety and operational improvement.

Preparing for HAZOP

A HAZOP should be prepared with appropriate drawings, segregation of nodes for study and agreed-upon guide words.

If using HAZOP for preliminary risk assessment, a risk matrix will be needed to estimate initial severity and frequency of consequences for further consideration. The risk matrix must be defined or agreed by the organization that will operate the system or process.

It is also important to define how the HAZOP will be recorded. Companies often attempt to use regular word-processing software or spreadsheets, but in practice these are usually inadequate, even for small studies. A specialist HAZOP software is highly recommended.

Before convening a HAZOP study, a trained facilitator must be appointed. A senior and independent facilitator is recommended to make sure the team keeps on track and does not stray into design or re-design.

Depending on the complexity and size of the process, the facilitator may need a scribe to record the HAZOP findings. However, good HAZOP recording software can make the need for a separate scribe unnecessary.

The HAZOP team members should always be as experienced as possible with the system or process being studied. The best approach is to pick team members that thoroughly understand the system or process design, how it is operated in practice, and to supplement this with additional personnel with expertise in process safety, electrical & instrumentation, maintenance and any other specialisms to support major packages provided by sub-suppliers.

The HAZOP Study & Follow-Up

Once a suitable team is assembled, a HAZOP study will be guided by the facilitator in a systematic way, recording results and recommendations as the study progresses.

It is traditional that HAZOP is held in face-to-face meetings over several days or weeks, but more recent "global" studies are conducted by leveraging remote internet meeting software. The former has the obvious advantage of focus and personal contact, but the latter can be far more productive and cost-effective at involving the right people.

Most HAZOP studies throw up follow-up actions that cannot be resolved immediately, so it is crucial that an action tracker is available to team members after meetings are concluded.

Notes on doing HAZOP with LOPA

Some companies look to combine HAZOP and Layer Of Protection Analysis (LOPA) into the same study session, which can have some benefits, but also presents some additional challenges.

The LOPA technique identifies selected high-risk scenarios from the PHA/HAZOP stage, assigns numeric event frequencies to causes and probabilities of failure to independent protection layers (IPL). In this way, a LOPA can provide a semi-quantitative view of multiple scenario risks which can then be used for SIL determination.

If the HAZOP team is large, combining the HAZOP and LOPA into one team is not efficient. It is far better to complete HAZOP and solve major HAZOP actions before tackling LOPA.

When completing LOPA as a separate activity to HAZOP, and by looking at only the high-risk events, the team completing LOPA will focus on a smaller number of scenarios than the HAZOP team. This approach allows more focused time to discuss the sequence of events and credits for protection layers. 



HAZard and OPerability study - a team brainstorming technique originally invented at ICI (Imperial Chemical Industries) in the UK for identifying hazards and operability issues. A HAZOP involves a muti-discipline group systematically applying guide-words to each part (or node) of a process. The goal of HAZOP is to identify possible deviations from design intent, and to document causes, consequences, safeguards and recommendations for improvement.


LOPA is a semi-quantitative technique that is often applied after HAZOP to help make decisions about the sufficiency of protection layers. The technique was first defined by the Center for Chemical Process Safety - CCPS in the USA. LOPA can be applied in a number of different ways and can even result in determining required Safety Integrity Levels (SIL) for instrumented protection functions.


Process Hazards Analysis, a general term that gets used mostly in N.America for multiple different methods, including HAZOP, LOPA, What-If?, FMEA (failure modes and effects analysis), FTA (fault tree analysis), ETA (event tree analysis).


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Jon Keswick, CFSE

Jon Keswick is a Certified Functional Safety Expert (CFSE) and founder of eFunctionalSafety. Feel free to make contact via Linked-In or comment on any of the eFunctionalSafety blog pages.

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