Uncertainty aspects in process safety analysis Adam S. Markowski a, * , M. Sam Mannan b,1 , Agata Kotynia (Bigoszewska) a , Dorota Siuta a a Process Safety and Ecological Division, Faculty of Process and Environmental Engineering, Technical University of Lodz, 90-924 Lodz, ul. Wolczanska 213, Poland b Mary Kay O'Connor Process Safety Center, Artie McFerrin Department of Chemical Engineering, Texas A&M University System, 3122 TAMU, College Station, TX 77843-3122, USA article info Article history: Received 1 September 2009 Received in revised form 15 February 2010 Accepted 19 February 2010 Keywords: Process safety analysis Quantitative risk analysis Risk assessment Accident scenario Uncertainty Fuzzy sets abstract Uncertainties of input data as well as of simulation models used in process safety analysis (PSA) are key issues in the application of risk analysis results. Mostly, it is connected with an incomplete and uncertain identification of representative accident scenario (RAS) and other vague and ambiguous information required for the assessment of particular elements of risk, especially for determination of frequency as well as severity of the consequences of RAS. The authors discuss and present the sources and types of uncertainties encountered in PSA and also methods to deal with them. There are different approaches to improve such analysis including sensitivity analysis, expert method, statistics and fuzzy logic. Statistical approach uses probability distribution of the input data and fuzzy logic approach uses fuzzy sets. This paper undertakes the fuzzy approach and presents a proposal for fuzzy risk assessment. It consists of a combination of traditional part, where methods within the process hazard analysis (PHA) are used, and “fuzzy part”, applied quantitatively, where fuzzy logic system (FLS) is involved. It concerns frequency, severity of the consequences of RAS and risk evaluation. In addition, a new element called risk correction index (RCI) is introduced to take into account uncertainty concerned with the identification of RAS. The preliminary tests confirmed that the final results on risk index are more precisely and realistically determined. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Process industry has been continually developing its design methods and operating techniques to overcome the hazards which pose significant risks to life safety, environment, and economy. This development is especially experienced in the refinery and petro- chemical industry, which handles a wide range of flammable and toxic materials that are potentially hazardous. However, recordable losses due to major accidents have indicated a continuing increase over the last few years (Mannan, 2005; Coco, 2003). Therefore, scientific organizations and academia have increased activity in research related to process safety in the chemical industry. Also, local governments and international organizations have established regulations and programs that require manufac- turers to demonstrate to the competent authorities that they have identified existing major accident hazards and adopted appropriate safety measures (SEVESO, SMS-OSHA, and RMP-EPA). The centerpiece of these regulations and programs is the assessment of risk from natural and technological hazards. Such a task can be fulfilled by process hazard analysis (PHA). It requires the application of appropriate risk analysis techniques which should demonstrate that measures both for prevention and miti- gation employed by the establishment result in a level of risk that is as low as reasonably practicable (ALARP). In majority of cases this demonstration will relay on some form of risk analysis and assessment. Chemical process plant as a complex system requires good scientific knowledge on different issues including process safety. When a system becomes complex, Zadeh in his “Principle of Complexity” observed that “as complexity of the system increases, our ability to make precise and yet significant statements about its behavior diminishes until a threshold is reached beyond which precision and significance (or relevance) become almost mutually exclusive characteristics” (Zadeh, 1965). Of particular interest in risk assessment for the complex system is the use of subjective information obtained from experts. This conclusion seems to be the case for present process plants and needs to be taken into account in uncertainty aspects of the process hazard analysis. 2. Sources of uncertainty in process safety analysis Process safety analysis (PSA) is focused on the prediction of future accident scenario of risk related to an unwanted release of * Corresponding author. Tel./fax: þ48 426313745. E-mail addresses: markowski@wipos.p.lodz.pl (A.S. Markowski), mannan@tamu. edu (M.S. Mannan). 1 Tel.: þ1 979 862 3985; fax: þ1 979 458 1493. Contents lists available at ScienceDirect Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp 0950-4230/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jlp.2010.02.005 Journal of Loss Prevention in the Process Industries 23 (2010) 446e454