Thomas Solberg Fylking | Principal Consultant
- Jun 16, 2021
- 5 min read

Process HAZOP - The essentials that you need to understand

Updated: Jun 12, 2023

Process HAZOPs can be traced back to the 1960s to analyze complex chemical process systems. The process HAZOP was further recognized as a suitable risk-reducing tool after disasters such as Flixborough in 1974. Industries with a similar process complexity with risk of major accidental hazards adopted the methodology as well.

Today, sectors such as the chemical industry, petrochemical, oil and gas, renewables, and low-carbon industries all use process HAZOPs.

This insight will take a closer look at the essentials for understanding a process HAZOP.

What is a process HAZOP?

A HAZOP (Hazard and Operability Study) is a methodical examination of process systems, focusing on identifying process-related deviations that can lead to accidents or severe operability concerns. For a process system involving flammable or toxic liquids or gases, the HAZOP is a fundamental tool to ensure a safe and robust system design.

The overall intention with the Hazard and Operability Study is to:

To stimulate creative thinking and imaginative examination of the design under review to ensure that various viewpoints are expressed;
The workshop intends to use the experience and knowledge of the workshop participants;
Identify risks, evaluate deviations, and recommend areas of improvement.

It is standard to conduct the process HAZOP with participants from relevant disciplines in a multi-discipline workshop. To ensure a structured approach, the processing system is divided into manageable nodes. Each of the nodes has a specific design intent and uniform process conditions. In the Hazard and Operability Study, a set of parameters and guidewords is used as a starting point for the identification of deviations. An example is the parameter “Pressure”, with the guide word “More” which is a typical starting point for overpressure scenarios. For each of the scenarios identified, the HAZOP team is assessing the cause of the deviation, the worst credible consequence as well as the safeguards in design to prevent or mitigate the scenario. They then use this information to evaluate the need for further risk-reducing measures, such as due to shortfall in compliance with requirements and standards, high-risk scenarios, or severe operability concerns.

The Hazard and Operability Study scope is planned under specific company-internal guidelines and IEC 61882 (The International Electrotechnical Commission (IEC) is a worldwide organization for standardization. The standard describes the principles and approach for HAZOP studies.).

What is not a process HAZOP?

A process HAZOP is not the best tool to catch all kinds of hazards. It is important to understand the limitations of a process Hazard and Operability Study. The following 4 points serve as a guideline to help you select the correct tool:

A process HAZOP considers process-related hazards, i.e., hazards that origins from a process-related deviation. Examples are overpressure, extreme temperatures, overfilling of vessels, or malfunction of critical equipment;
For hazards related to non-process related events, such as earthquakes, dropped objects, impacts from vehicles, etc., a HAZID (Hazard Identification study) is more suitable (read more about HAZOP vs. HAZID in this insight);
For hazards related to a working environment and work-related accidents, and for non-routine tasks and operations, an SJA (Safety Job Analysis) is more suitable;
Lastly, for a thorough investigation of various specific failure modes of equipment, an FMECA (Failure Mode, Effects & Criticality Analysis) is more suitable.

Why perform a process HAZOP?

A process Hazard and Operability Study intends to do four things; (1) reduce risk related to the processing system, (2) identify and limit operational issues, (3) serve as input to other important interfacing processes, and (4) avoid expensive modifications after the processing system is designed.

The HAZOP is an important validation tool to ensure that the design is robust concerning safety and operability. Therefore, you typically apply it once per main project phase. Doing so is one of the key project milestones.

Furthermore, if you have not reviewed a process design as part of a HAZOP assessment, it can in the worst case lead to accidents or incidents.

In short, perform a process Hazard and Operability Study to:

Ensure that the process design allows for efficient and safe operations in all relevant operational modes to avoid accidents and incidents;
Ensure that the application of the safety barriers is such that hazards are properly eliminated or mitigated;
Identify deviations from related standards and project requirements;
Identify design improvement.

A correctly executed process HAZOP will be a central input to other risk and safety assessments and processes such as:

LOPA (Layers Of Protection Assessment);
SIL allocation (Safety Integrity Level);
QRA (Quantitative Risk Assessment);
Performance standards;
Barrier management activities;
Operational and Maintenance procedures;
Emergency preparedness.

When to perform a process HAZOP?

A process Hazard and Operability Study is an investment of time and resources. This means that timing is of the essence when performing a process HAZOP.

It's important that you do not arrange the HAZOP too late in the project. A HAZOP conducted after fabrication can lead to expensive modifications and delays in the overall project schedule if the Hazard and Operability Study identifies scenarios requiring re-design.

However, it is advisable to not arrange the HAZOP too soon either. The process design will most likely be immature at an early stage in the project phase. In addition to this, there will likely be a limited amount of information available. Consequently, this can lead to multiple assumptions and guesswork. Therefore, you would need a new HAZOP at a later stage. Also, it is important that the process design has undergone a design review, to ensure that the discussions do not drift towards design review.

When planning the process HAZOP?

As a minimum, you should plan a process Hazard and Operability Study as part of the detailed design phase (execute phase). The process HAZOP serves as a milestone for the completion of the detailed design phase and is a tool for confirming that the process system design is robust and minimizes the potential for process-related accidents and incidents.

For projects conducted in several phases, performing the HAZOP as part of Concept and/or Front End Engineering Design is advisable. The benefit is to identify hazards that may have a significant impact on process design.

It's also important for you to have a Management of Change process. This ensures the systematic evaluation of any post-HAZOP design changes. If a design change affects the outcome or assumptions made in the HAZOP, the HAZOP may no longer be valid.

You can also perform a process HAZOP of existing plants and facilities during normal operations:

In case of modifications that may affect the operating envelope of the process design;
Re-HAZOP to capture lessons learned from operational experience and evaluate if the design assumptions are still valid. Learn more about Re-HAZOP and Modification HAZOP from one of our articles: "Re-HAZOP and Modification HAZOP: Understanding the differences and when to use each methodology."