The short answer
- HAZID (hazard identification) is a broad, early screening of the hazards of a facility or project, often at the concept stage with little design detail.
- HAZOP (hazard and operability study) is a detailed, systematic examination of the design – usually P&IDs – node by node with guide words.
- They complement each other: HAZID early to shape the concept, HAZOP once the detailed design is available.
- Neither replaces the other, and neither ranks risk on its own; scenarios with severe consequences often continue into LOPA or SIL determination.
HAZOP vs HAZID at a glance
| HAZID | HAZOP | |
|---|---|---|
| Purpose | Identify the major hazards of a facility or project | Identify deviations from the design intent, their causes and consequences |
| Project stage | Concept and early design | Detailed design, before construction; changes; revalidation |
| Input | Layout, process concept, site information | P&IDs, process description, operating data |
| Method | Checklist of hazard categories and HAZID guide words | Guide words × parameters applied node by node |
| Granularity | Facility, area or system level | Node level: lines, vessels, procedure steps |
| Typical duration | Hours to a few days | Days to weeks, depending on the number of nodes |
| Output | Hazard register, early design and siting decisions | HAZOP worksheet with safeguards and recommendations |
| Standard reference | ISO 17776 (offshore) includes HAZID guidance | IEC 61882 application guide |
What is a HAZID?
A HAZID is a structured study to identify the possible hazards of a system. It works at a high level: a multidisciplinary team walks through hazard categories – such as flammable and toxic substances, high pressure, dropped objects, natural hazards, interfaces with neighbouring facilities or security – and records where they arise, what could happen and which design or siting measures are needed.
Because it needs little design detail, a HAZID is typically done at the concept stage, when layout and process choices can still be changed at low cost. ISO 17776 for offshore installations contains an annex with HAZID guide words.
What is a HAZOP?
A HAZOP examines the detailed design systematically. The P&IDs are divided into nodes; for each node the team states the design intent and applies guide words such as NO, MORE or REVERSE to parameters such as flow, pressure and temperature. For each credible deviation it records causes, consequences, existing safeguards and recommendations.
Because it depends on detailed design information, a HAZOP is done once the P&IDs are mature – typically before construction – and again for significant changes and in periodic revalidation of operating plants.
How HAZID and HAZOP work together
- Concept stage – HAZID: screen the major hazards and influence layout, siting and process selection.
- Detailed design – HAZOP: examine the P&IDs systematically for deviations and check the safeguards.
- Risk assessment – LOPA or SIL determination: assess scenarios with severe consequences semi-quantitatively and define the required safety instrumented functions.
- Operation – revalidation and management of change: revisit the studies after changes and at regular intervals.
HAZOP vs What-If, FMEA, LOPA and bow-tie
| Method | What it does | Relation to HAZOP |
|---|---|---|
| What-If | Brainstorming with “what if …?” questions | Faster but less systematic; results depend heavily on the team’s experience |
| Checklist | Checks the design against known hazards | Efficient for standard equipment; only finds what the checklist contains |
| FMEA | Failure modes of individual components and their effects | Component-oriented; HAZOP is process-oriented and covers interactions |
| LOPA | Semi-quantitative assessment of single scenarios and protection layers | Uses scenarios identified in the HAZOP |
| Bow-tie | Diagram of threats, top event, consequences and barriers | Visualises and manages barriers for selected major events |
| Fault tree analysis | Deductive analysis from an undesired event back to its causes | Often follows when a HAZOP scenario needs deeper analysis |
Frequently asked questions
Is HAZID done before HAZOP?
Usually yes. A HAZID screens the major hazards at the concept stage; the HAZOP follows once detailed P&IDs are available.
Can a HAZID replace a HAZOP?
No. A HAZID works at a high level and does not examine the detailed design line by line. Where a systematic examination of the P&IDs is needed, a HAZOP or an equivalent method is required.
Does HAZOP rank risk?
Classic HAZOP identifies deviations, causes, consequences and safeguards. Risk ranking with a risk matrix is a common addition, and LOPA is used for scenarios with severe consequences.
What is the difference between HAZOP and What-If?
What-If is a less structured brainstorming method. HAZOP applies a fixed set of guide words to every node, which makes it more systematic but also more time-consuming.
Sources and further reading
- IEC 61882:2016 – Hazard and operability studies (HAZOP studies) – Application guide
- ISO 17776:2016 – Petroleum and natural gas industries – Offshore production installations – Major accident hazard management during the design of new installations
- US EPA: Risk Management Program Guidance, Chapter 7 (hazard evaluation techniques)
- IChemE Safety Centre: Effective revalidation of risk assessments – Delta HAZOP (2024)