Introduction
In the planning of a safety study programme for any upstream facility, two studies almost always appear on the schedule: a HAZID and a HAZOP. On fast-track projects — and in conversations with clients who are new to the process safety requirements — there is often pressure to run one instead of the other, or to combine them into a single compressed workshop.
That pressure comes from a misunderstanding of what each study does. A HAZID and a HAZOP are not interchangeable. They address different questions, require different inputs, produce different outputs, and are conducted at different stages of the project lifecycle. Understanding the distinction is a prerequisite for planning a study programme that delivers genuine safety value.
What Is a HAZID?
A Hazard Identification study is a structured, high-level review of the major hazards associated with a project or facility. It is typically conducted at concept stage, pre-FEED, or early FEED — before the engineering is sufficiently developed to support a node-by-node examination of piping and instrumentation.
The HAZID team works through a structured set of hazard categories:
- Fire and explosion hazards
- Toxic and asphyxiant releases
- Structural and mechanical failure
- Dropped objects and lifting operations
- Marine hazards (for offshore projects)
- Environmental impact
- Transportation and logistics hazards
- External events (vessel collision, extreme weather)
For each category, the team asks: does this hazard exist on this project? What could cause it? What are the potential consequences? Are there gross gaps in the safeguarding concept?
The output is a hazard register — a structured log of identified hazards with their causes, consequences, and initial assessment of whether the proposed safeguarding concept is adequate at a high level. The HAZID does not examine individual control loops, instrument failure modes, or the adequacy of specific relief valve sizing. It operates at the facility level, not the P&ID level.
The HAZID answers: What are the major hazards on this project, and is the design concept heading in the right direction?
What Is a HAZOP?
A Hazard and Operability study is a detailed, systematic examination of a specific process design — conducted node by node on issued P&IDs, using structured guidewords applied to each measurable parameter at each point in the process.
The guidewords — No, Less, More, Reverse, As Well As, Part Of, Other Than — are applied to parameters such as flow, pressure, temperature, level, and composition at each node. For each guideword-parameter combination, the team identifies causes, traces consequences, and assesses whether the safeguards shown on the P&ID are adequate.
A HAZOP cannot be conducted on a block flow diagram or on early process flow diagrams. It requires a P&ID that is sufficiently developed to show actual control loops, instrumentation, relief devices, isolation points, and interlocks. The standard is typically "Issued for HAZOP" — a formal issue status confirming that the P&ID is representative of the final design intent and has been accepted by the discipline lead.
The output is a HAZOP action register and completed HAZOP worksheets — a detailed, nodated log of every significant finding, with recommended actions, responsible engineers, and target dates.
The HAZOP answers: For this specific process design, is every credible deviation from normal conditions adequately safeguarded?
The Key Differences
| HAZID | HAZOP | |
|---|---|---|
| Timing | Concept / Pre-FEED / Early FEED | Late FEED / Early Detailed Engineering |
| Input required | PFDs, plot plan concept, facility description | Developed P&IDs at IFR or IFH status |
| Method | Hazard category checklists and structured brainstorming | Guideword-driven node-by-node deviation analysis |
| Level of detail | Facility and concept level | Process line, instrument, and safeguard level |
| Typical duration | Half a day to two days | Two to eight days (complexity dependent) |
| Team composition | Broad multidisciplinary — process, structural, HSE, operations | Process, instrument, operations, independent facilitator |
| Primary output | Hazard register | HAZOP worksheets and action register |
| Safeguard review | Is the concept broadly safeguarded? | Is each specific safeguard adequate for this deviation? |
Can One Replace the Other?
No — and this is the most consequential mistake made on fast-track projects.
Running only a HAZID and omitting the HAZOP leaves the detailed process safety picture entirely unexamined. Instrument failure modes, control loop interactions, safeguard adequacy for specific scenarios, and operability during start-up and shutdown are not addressed at HAZID level. A facility that has passed only a HAZID is a facility whose P&ID-level safety has not been examined.
Running only a HAZOP and omitting the HAZID means the study team examines process deviations without having first established whether the overall facility concept, layout, and design basis are sound. You may spend four days examining separator high-pressure scenarios in detail, without anyone having formally established whether the facility siting, flare stack height, and emergency response concept are appropriate for the location and hazard profile.
The two studies are complementary precisely because they operate at different levels of engineering resolution. A HAZID shapes the project concept and design basis. A HAZOP verifies the detailed design once it exists.
Timing in the Project Lifecycle
The most common scheduling error is running both studies at the same point in the project. This happens when a project team decides to "do the safety studies" as a single block of work in early FEED, producing a compressed HAZID-HAZOP hybrid that is insufficiently detailed for either purpose.
The correct scheduling is:
HAZID: At concept gate or early FEED, as a direct input to the design basis. Findings from the HAZID should influence facility layout, equipment selection, concept safeguarding philosophy, and emergency response planning. The hazard register becomes a living document, updated as the design develops.
HAZOP: At P&ID freeze, after the instrumentation and control philosophy is established. This is typically at the end of FEED or at the FEED-to-detailed transition. The HAZOP action register must be incorporated into the detailed engineering scope, so timing it at this transition maximises its influence at minimum cost.
Practical Guidance
Greenfield facility, full project lifecycle: Both studies are required and should be formally scheduled with defined input deliverables. The HAZID is a milestone at concept gate; the HAZOP is a milestone at P&ID freeze.
Brownfield modification: A targeted HAZOP on the affected P&ID nodes. A supplementary HAZID is warranted if the modification substantially changes the hazard profile — for example, adding a new process fluid, a new pressure tier, or a new area classification boundary.
Concept or pre-FEED scope only: A HAZID is the correct study. The engineering detail required for a HAZOP does not yet exist.
Fast-track early production facility: Both studies, compressed but not eliminated. A focused one-day HAZID at concept. A two-to-three-day HAZOP once the P&IDs are at a defined review status. Removing either study from the schedule does not save time — it defers the identification of problems to construction or commissioning, where they cost an order of magnitude more to resolve.
A Final Note on Independence
Both studies benefit from independent facilitation — a facilitator who did not design the facility and has no stake in the findings. This is not a procedural nicety. It is the mechanism by which genuine problems get recorded rather than explained away.
The most dangerous HAZOP is one facilitated by the lead process engineer who designed the P&ID. Their instinct, entirely natural, is to explain deviations rather than record them as hazards. An independent facilitator maintains the discipline of the guideword method and ensures that findings which feel uncomfortable are documented with the same rigour as those that are straightforward.
At FEEC, we provide independent HAZOP and HAZID facilitation as a distinct service — separate from our design engineering scope — precisely because that independence is what makes the study valuable.
