Re-HAZOP and Modification HAZOP are two different methodologies used in Hazard and Operability (HAZOP) studies that serve distinct purposes. Distinguishing when each is required is important to effectively identify and mitigate hazards and operability issues.
What is a Modification HAZOP?
The main objective of a Modification HAZOP is to identify new hazards or if existing hazards has changed (increased severity or likelihood) as a result from a proposed change to a process or system. This is to ensure that the system can be operated safely and efficiently with the proposed changes. A Modification HAZOP could be required when changes are introduced to and can affect:
Design conditions of existing equipment/systems;
Composition or physical properties of the fluid;
Safety systems or process control systems;
Operations and Maintenance.
Changes that normally do not require a Modification HAZOP are:
Changes that are identical to the original design, with the same conditions and properties, no impact on other systems;
Minor changes to P&ID or routine maintenance procedures that have been reviewed by subject matter experts.
Pitfall with modification HAZOP
If multiple small modifications are made over a period of time, the narrow focus of individual Modification HAZOPs may result in missing important hazard scenarios that arise from the collective changes. In such cases, it may be necessary to perform a Re-HAZOP. In this case, it is important to consider the company or asset Management of Change (MoC) procedures.
What is a Re-HAZOP?
A Re-HAZOP or Revalidation HAZOP is typically conducted after an asset (or system) has been in operation for a certain period or after significant changes have been made. The purpose of a Re-HAZOP is to evaluate the design assumptions and to capture lessons learned from operational experience. To conduct a Re-HAZOP, one can either approach it with a fresh perspective or use previous Process Hazard Analysis (PHA) as a basis. Some industries require a Re-HAZOP periodically such as every 5 or 6 years, while others require it based on the size of a modification or the cumulative effect of multiple minor changes.
What triggers a Re-HAZOP?
The following list can be used as a guideline to understand when a Re-HAZOP is necessary:
A defined period has elapsed since the previous PHA of the system;
The cumulative impact of multiple minor modifications to a system or interrelated/dependent systems;
Shortcomings or gaps identified in previous HAZOP assessments;
New information, gained from operational experience, supplier, or similar designs in the industry such as near misses’ incidents, accidents, and operability issues;
Changes in regulatory or corporate guidelines that cause a process design to deviate from latest standards.
Necessary information to conduct a Re-HAZOP
In addition to the normally required input for HAZOP assessments, the following items are useful to achieve a successful Re-HAZOP:
An overview of near misses, incidents, and accidents from the system or from similar systems in the same industry;
A structured overview/register of all modifications made since the last Re-HAZOP study;
Copies of previous HAZOP reports including modification HAZOPs related to the process under examination and associated closeout report;
Asset data regarding proof test results, corrective maintenance, etc. for the critical components and safety instrumented functions under review.
Read also our article about: "HAZOP vs HAZID – when is one more useful than the other?"