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How to speed up retrospective HAZOPs whilst achieving effective risk reduction

15 August 2018

This article by Graeme Ellis of ABB describes an approach for retrospective hazard reviews on existing facilities that combines Hazard and Operability (HAZOP) Studies  and Process Hazard Review (PHR) in a flexible and efficient manner, optimising the time required for busy operations-based teams.

Companies operating high hazard facilities have a responsibility to identify potential process safety hazards, implement robust risk controls, and ensure that barriers are maintained throughout the lifecycle. Design stage risk assessments, including Hazard and Operability (HAZOP) Studies, only address the first step in this process, and there is a need for periodic reviews to identify weaknesses in barriers and make recommendations to demonstrate continuous improvement.

Best practice requires reviews every 5 years, driven by global regulations and a recognition of the extent of change and new learning on a facility over this timescale. Many companies use retrospective HAZOP studies requiring a ‘line-by-line’ assessment of the process using deviation guidewords. HAZOP studies are therefore very time consuming and present a major resourcing challenge for an operations based team.

The well-proven Process Hazard Review (PHR) methodology provides an alternative approach, using a higher level ‘system-by-system’ approach with hazardous event guidewords. PHR is more efficient and typically 4-5 times faster than HAZOP on the same facility. Whilst PHR offers clear benefits in terms of resource requirements, some companies have continued to use HAZOP due to corporate requirements, meeting perceived regulator demands, or the need for an in-depth study where there were serious deficiencies in the original process design.

Why carry out Retrospective Hazard Reviews?

During the operational stage of a facility there is a need to periodically review and update the Process Hazard Analysis (PHA) to ensure that key barriers are working effectively, take account of new information that has been gained on the process, and ensure continuous improvement in reducing risks. These reviews need to take account of changes including:

* Creeping change caused by many small modifications;

* Loss of experienced staff;

* Ageing or obsolete equipment;

* New understanding of hazards from inside or outside the company.

In the EU, retrospective hazard reviews are routinely carried out during the preparation of on-shore Safety Reports or off-shore Safety Cases in order to identify all Major Accident Hazards (MAHs) and the associated risk control barriers. For major hazard installations in the US, to comply with the Process Safety Management (PSM) Standard, the PHA need to be revalidated every 5 years. A number of methods are stated in the standard, including; What-if, What-if / Checklist, HAZOP, FMEA, Fault tree analysis, or an appropriate equivalent methodology, such as PHR.

How are HAZOP and PHR different?

The objective of HAZOP studies is to ensure that the process design as shown on the Piping and Instrument Diagrams (P&IDs) is fit for purpose and meets the applicable standards. The process is split into Nodes as individual lines on the P&ID (continuous processes) or steps in the operating sequence (batch processes), and a series of guidewords such a ‘No Flow’ or ‘High Pressure’ used to identify the causes of any significant deviations from the design intent.

The PHR methodology was developed by ICI Ltd in the early 1990s for the periodic review of existing facilities, to address the following difficulties experienced during trials using HAZOP studies.

• Excess time required involving busy operations staff

• Excess number of actions with many related to operational issues

• Failure to identify the bigger picture related to process safety events

• Difficulties applying HAZOP with out-of-date P&IDs.

PHR splits the process into larger Nodes than for HAZOP, at the system level, and uses guidewords directly related to hazardous events, with guidewords based on loss of containment or release of energy, such as ‘Internal Explosion’ or ‘Puncture’. The differences between the HAZOP and PHR approaches are shown on the diagram above.

Why use PHR rather than HAZOP?

The choice of HAZOP or PHR often depends on meeting regulatory requirements, client corporate preferences or previous practices on the site. The table below provides a number of factors that should be considered before embarking on a PHA revalidation programme, providing an improved justification for the choice of HAZOP or PHR.



Company standards/ Regulator requirement

In some companies there is a corporate requirement for either HAZOP or a PHR approach written into corporate standards, in which case this method is mandatory.

Regulators have directed companies towards HAZOP based on their preferences or perceived compliance with the US-PSM standard which explicitly mentions HAZOP.

Recent Process Safety Incidents / No or poor design stage HAZOP

The higher level approach for PHR works well if there are no serious deficiencies in the basic process design.

The structured and thorough approach offered by HAZOP may be required if there have been serious process safety incidents, or where there are concerns about the basic process design due to lack of design stage HAZOP or evidence of poor quality in this HAZOP.

Complexity of process

PHR at system level works effectively for relatively simple processes such as storage systems and continuous processes. The more thorough approach to identifying initiating causes offered by HAZOP may be appropriate with more complex processes, such as batch reactors with multiple steps or at HP/LP interfaces on Oil & Gas facilities.

Availability of Operations based staff

Retrospective Hazard Reviews place a great demand on busy operations based staff, yet input from knowledgeable plant based staff is key to success whether using PHR or HAZOP. For some companies the use of the more time efficient PHR method is the only viable option given limited staff availability. There are examples where reviews using HAZOP have not been completed due to the excess time required.

Quality of design data including P&ID’s

On many operating plants the accuracy of the P&ID’s is poor, especially those beyond 10 years old where many modifications have been carried out. The nature of HAZOP using structured line-by-line approach based on the P&ID’s can lead to problems on existing facilities where the P&ID’s are out-of-date. PHR provides an increased flexibility with the higher system level approach, and allows progress on risk reduction without the need to wait for P&ID updates to be carried out.

Need for quick results

The increased time for HAZOP can result in delays between the start of the study and the results being finalised and improvement actions taken. Based on ABB experience, an Offshore platform may take a total of 6-9 months to complete, compared with 1-2 months for a PHR of the same facility. Some companies with concerns about current performance may be looking for quicker identification of improvement actions in order to reduce risks as offered by PHR.

Availability of skilled Facilitators

PHR is a more demanding technique for the Leader, requiring hazardous events at a system level to be identified using a ‘helicopter view’ of the process, with the experience of knowing when to dive deeper into specific issues. By comparison HAZOP provides a more structured approach for less experienced Leaders, and may be the preferred route if such internal resources are to be used.

Limit number of actions / Focus on major accidents

The additional time required for HAZOP often results in a proportionate increase in the number of actions or recommendations raised, which can present challenges for the company in effectively closing out with an adequate justification. Experience has shown that the PHR approach with greater focus on major accident hazards generally results in less recommendations focussed on high risk issues.

Using PHR and HAZOP flexibly

The decision on whether to use HAZOP or PHR for a retrospective review is normally applied to the complete facility, including all the process systems and related utility systems. The latest tools developed allow a more flexible approach where both HAZOP and PHR approaches can be used for a single facility, with Node sizes chosen appropriately and the choice of guidewords for each Node varied between those for HAZOP (deviation based) or PHR (hazardous event based).

When both HAZOP and PHR approaches are being used on a facility, a decision is made on a system by system basis and the following are typical factors that should be taken into account.

* Simple systems use PHR approach, e.g. raw material storage, simple continuous process units (e.g. distillation unit), utility systems;

* Complex systems use HAZOP approach, e.g. batch reactor with multiple steps, complex continuous process units; (e.g. at a, HP/LP interface)

* HAZOP approach used where significant incidents or near misses have occurred, or where there are concerns about the adequacy of the basic process design.


When planning retrospective hazard reviews a key decision is required on the use of either a HAZOP or PHR approach, and this can have a major impact on the time required, with PHR being typically 4-5 times faster than HAZOP. Key factors in the choice of method are PHR requiring more experienced leaders to ensure that hazardous events are identified effectively, and HAZOP preferred where there are concerns about the basic design of the process or quality of the previous PHA.

Suitable software allows a flexible approach with both PHR (system level) and HAZOP (line/step level) approaches to be used on a single facility. This flexible approach allows the efficiency of the overall process to be optimised, with PHR used to make faster progress where possible, and more detailed HAZOP used where required to provide greater thoroughness. Ultimately, the decision on approach can have a major impact on the resources required and likelihood that the review can achieve the required reduction in process safety risks.

About the Author

Graeme Ellis is a Principal Lead Consultant with ABB with 37 years’ experience in the process industry, now specialising in Process Safety for major hazard installations. He is a Fellow of the IChemE and initially worked as a Process Engineer in design for MW Kellogg and Hercules before gaining operational experience and training as a hazard study leader with ICI Ltd.

Since 1994 Graeme has provided PSM consultancy services in all sectors of the process industry, specialising in PHA revalidation for existing operations. He is a member of the UK Energy Institute Process Safety Committee, and completed an update of EI guidance on Inherent Safety in Design in 2014.

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