HAZOP vs HAZID – when is one more useful than the other?

Introduction to HAZOP and HAZID

Two common ways of identifying and analysing potential risks are the Hazard and Operability (HAZOP) and Hazard Identification (HAZID) studies. Apart from the acronyms being quite similar, both analyses are often conducted through multidisciplinary workshops with participants from relevant disciplines. The HAZOP and HAZID methodologies also follow a similar structure and are both qualitative risk assessments. Therefore it is not uncommon that the two safety-related studies are mixed up. So how are the two studies different, and when is one more suitable than the other?

What is a Hazard Identification (HAZID) study?

A Hazard Identification (HAZID) study is a structured examination of a facility to identify potential hazards early on in the project design. The HAZID focuses mainly on hazards arising due to the external environment (i.e., not the technical/electrical components and design), planned activities, or inherent dangerous properties of the equipment/process. Guidewords are used to aid in identifying hazardous scenarios. The HAZID study is often conducted at a layout drawing or 3D-model level.

What is a Hazard And Operability (HAZOP) study?

The HAZOP study is a detailed, structured, systematic, and qualitative methodology for the analysis of a planned or existing process or operation. The main principle is the definition of the intention of the design or activity, and the use of suitable predetermined parameters and guide words to identify possible deviations from this intention. The design intent defines how a part of the process or system under analysis is expected to operate and its purpose. This includes the design and operating conditions (temperature, pressure, flow, etc.), and other relevant details, as well as an explanation of the control loops and related logic included to ensure that the process is kept within the operating envelope. HAZOP studies are often conducted at a Piping and Instrumentation Diagram (P&ID) level.

Similarities between HAZID and HAZOP

Now, let us look at similarities with regard to methodology. For both HAZOP studies and HAZID studies, the following outline applies:

• The system under review is divided into smaller parts called nodes (one example of a node being a separator, including inlets and outlets).

• For each node, the systematic application of guidewords is performed to stimulate creative thinking of potential causes and consequences for hazards related to the guidewords.

• The consequences are assessed without safeguards in place, typically focusing on the final impact on personnel (safety), environment, finances, and reputation/community.

• Once the scenario and consequences have been identified, an identification of safeguards is made. A safeguard is a device, system, or action that will prevent a hazardous event from reaching its potential. This includes both preventative or mitigating safeguards. For operating assets, the focus is on existing safeguards. For projects, the focus is on safeguards already implemented in the design.

• In case it is deemed necessary, actions and recommendations are raised in the HAZID and HAZOP. These are raised when the safeguards are unlikely to prevent or mitigate a consequence or there is a shortfall in compliance with a regulation or company standard. The process is then repeated for the next guideword until all guidewords have been walked through. The process is then repeated for the node. In this way, various scenarios are identified.

• Both studies are carried out in a workshop setting by a multidisciplinary team.
  

Differences between HAZID and HAZOP – when is one more useful than the other?

Even though the methodologies and approaches are similar, there will be practical differences between the two analyses. Here is a quick overview of the key differences:

• A Hazard Identification study focuses on identifying potential hazards. A HAZID can be viewed as a more general risk analysis tool designed to identify the inherent hazards at a plant, due to e.g., its location, the planned activities (loading, offloading, processing, etc.) the amounts of substances and fluids present, and their inherent (hazardous) properties, as well as other external events (fires, floods, lightning, etc.) that may affect the plant (i.e., the identification itself being in focus). The HAZID is normally the starting point for all other assessments of inherent risks and should normally be done as early as possible.

• For a HAZOP (Hazard and Operability) study however, the focus is on identifying potential hazards related to how the fluids at the plant are processed, and how the plant is intended to be operated (e.g. the intended temperatures, pressures, flows, etc.), Hence, the main goal is to identify potential deviations from design intention, and how the process is designed to keep operating conditions within safe limits and prevent excursion of these limits to be realized as hazardous consequences.

• A HAZOP node is defined as part of a process system, e.g. from a pump discharge to a separator. A HAZID node is normally defined as an area, e.g. a module or deck level.

• Parameters/guidewords for a HAZOP are standardized for process systems, e.g. Pressure (More/less), Temperature (More/less), while guidewords for HAZID should cover a wider range of hazards (Loss of containment, Collision, Dropped objects, Extreme weather);

• The output from a HAZOP is used to further develop a process system and add safeguards. The HAZOP is also oftentimes the starting point for a deeper analysis of various instrumented safety functions in e.g., a LOPA (Layers of Protection Analysis (LOPA): Importance, Methodology, and Application in Hazardous Scenario). Output from a HAZID can be used for design improvements e.g., in the layout, input to barrier management (How to perform successful Bow Tie workshops), safety strategies, and the basis for a hazard register.

Conclusion and Summary

To summarize, the risk assessment methodologies are similar but serve different purposes, are possible to do at different stages of the project and focus on different aspects. Both studies follow a similar methodology and are conducted by a multidisciplinary team.

The HAZID is mainly used for identifying hazards, regardless of their origin (external factors, layout choice, or inherently dangerous media), whereas a HAZOP focuses on hazards arising due to the process design and how it can deviate from the design intention. Therefore, the guide words used in the studies will vary, where HAZID guidewords may include more general guidewords such as "fire and explosion", "dropped objects" etc. A HAZOP will primarily have guidewords related to the process such as "high temperature", "no flow", "low level" etc. A HAZID is recommended at an early stage, as input to later phases and design developments, while a HAZOP is performed when the process design is matured to a P&ID level.

Finally, the output from the studies can be used for different purposes. The HAZID may serve as input to improvements in general layout, and safety strategies and may work as a foundation for further barrier management by e.g., conducting a Bow Tie study. A HAZOP on the other hand, may be used to further develop a process system e.g., to increase operability or add safeguards. Furthermore, the HAZOP is oftentimes used as a starting point for further analysis of safety instrumented functions (e.g., via a LOPA).

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