Risk Quantification with Extended HAZOP Study
- Rainer Semmler (A notified expert under Art. 29a BImSchG) TÜV SÜD Chemie Service GmbH

The HAZOP study has been applied successfully to the systematic safety assessment of chemical plants and industrial processes for decades. However, automated production processes and modern materials now give rise to new demands concerning risk management. This article explains how conventional HAZOP study can be made fit for the future by adding modern risk-assessment methods.

For over 40 years, companies in the chemical industry have used the HAZOP study to make production processes safer. The method is aimed at identifying potential risks in operations and processes, analysing their causes and effects, and developing suitable corrective actions to prevent incidents and increase plant safety.

The HAZOP (Hazard and Operability) method was developed in the early 1970s by engineers at the UK chemical company Imperial Chemical Industries (ICI) in response to the rapidly growing process industry and the associated rise in serious accidents. The Seveso disaster of 1976, where an unidentified amount of the highly toxic dioxin TCDD was released at a chemical company north of Milan, was one of the most severe chemical incidents in Europe. In the year following the disaster, ICI and the Chemical Industries Association published the HAZOP method under the eponymous title “A Guide to Hazard and Operability Studies”. The European Seveso I Directive came into force in 1982. This directive introduced the requirement of a systematic safety assessment for plants of a certain hazard potential and/or substance inventory. EC Directive 96/82/EC replaced the Seveso I Directive in 1997 and a further revision – the “Seveso III Directive”, also referred to as the Major Accident Directive – will come into force in 2015.

Discussion of the Qualification of Safety Risks
The HAZOP study starts from the assumption that major accidents are caused by unintended deviations of safety-relevant operating factors such as temperature, pressure or filling levels. Where and why these deviations occur, which consequences can be derived therefrom and which preventive actions have to be taken to respond to potential hazards, are all issues that are addressed in a facilitated expert discussion. Here, a multi-disciplinary team of internal and external experts addresses each individual component of a production process, its target function and possible deviations. In a direct dialogue facilitated by an experienced study leader, safety and process engineers, technicians and designers systematically assess all plant components and process steps for potential risks.

For this purpose, the method assigns a specified function to every process step that is assessed and uses a set of guide words (eg, ‘no’, ‘less’, ‘more’, ‘earlier’, ‘later’ etc) to systematically check the relevant parameters of this target function (eg, temperature, pressure etc.) for resulting deviations. Special lists are available that can be adjusted to the specific terms of reference and assist with the selection of suitable parameters and guide words. The guide-word method thus supports the experts in their qualified and comprehensive examination of the subject matter and in ensuring a constructive discussion. Once all possible risks have been identified, the team goes on to assess the causes and possible consequences of a deviation and subsequently identifies possible preventive and/or corrective actions. The recorded brainstorming and its results are fully documented in a report.

Technological Progress Makes New Demands
The HAZOP method has reliably identified and minimised risks for decades. It has contributed considerably to improving the operation of technical facilities and continuously increasing the safety level of production processes. By systematically harnessing the expertise and experience of the discussion participants, the method enables customised safety concepts to be developed and adjusted effectively to specific needs. At the same time, as a risk management tool, the HAZOP study can be applied almost universally to each system and throughout the entire life cycle of a plant or process.

However, the conventional HAZOP study also has its limits. Against the backdrop of breath-taking technological advancements in the chemical industry, plant owners and experts increasingly face new, complex and rapidly changing problems. Today, fully automated and computerised process control systems are used to control, regulate and ensure the safety of plants in the process industry. New materials and operating fluids are developed and used in increasingly short cycles. This trend brings in its wake new risks that are difficult to assess and that require immediate anticipation and response. Empirical information about operations in practice are needed to adequately support the qualitative approach of the conventional HAZOP study.

Modern Risk Management Complements Conventional Approach
According to the experts from TÜV SÜD Chemie Service, one feasible solution is to complement the qualitative HAZOP study by introducing quantitative and probabilistic modern risk management methods. Defined parameters and analytical models additionally permit quantitative forecasts of safety-relevant consequences.

Identification of the risk rating, which can be determined from the ratio of the failure probability of a system and plant to its potential consequences, is fundamental for systematic risk quantification. The probability of failure of a plant is difficult to predict. However, findings on the failure frequency of individual components can be related to each other to arrive at initial estimates. The comparison of these estimates with empirical statistics then makes it possible to calculate the parameters of probability of occurrence and consequences of damage. Embedded in a company-specific risk matrix, the risk rating provides information as to whether, and to what extent, preventive and corrective actions must be initiated. It also assists with evaluating the effectiveness of various actions so that the best possible solution can be chosen.

Even the conventional HAZOP study requires precise planning, implementation and follow-up. Probabilistic and quantitative methods also often prove complex and work-intensive. The method can be simplified by using general statistics for the probability of occurrence of human, mechanical and electrical errors. These empirical data from risk research ensure efficient risk traceability and quantification.

Expertise for Implementation in Compliance with Legal Requirements
The HAZOP study provides companies with an effective tool for the assessment of technical risks. At the same time, extended systematic risk assessment assists company managers in maintaining their plants and systems in compliance with the legal requirements and the state of the art and in keeping liability risks to a minimum. Planning, operation and maintenance in the chemical industry must take into account the relevant directives and regulatory acts (eg, the Federal Air Pollution Control Act, the Occupational Safety and Product Safety Acts and the Hazardous Substances and Major Accidents Regulation) and verify that it is in compliance with these regulations. TÜV SÜD Chemie Service supports company managers in quickly implementing all legal requirements and reaching legal compliance. Independent team managers or study leaders, secretaries and technical consultants pool their expertise and experience to ensure efficient realisation of systematic safety assessments in accordance with the HAZOP method. Working with the internal experts in the companies, the study team thus can identify, evaluate and document process-related weaknesses.