A New Functional Systems Theory based Methodology for Process Hazards Analysis
Introduction
Process plants (refineries, chemical plants, petrochemical, pharmaceutical, etc.) deal with a large amount of potentially dangerous materials (toxic, inflammable, explosive, etc.) and many times in extreme conditions (such as high temperatures and pressures). This can lead to equipment failures, plant shutdowns or even worse accidents with catastrophic consequences. In spite of the safety layers of protection (basic process control system, alarms, safety instrumented systems, protective systems, etc.) there are accidents every day with losses up to 1,000 million $ each year only in the US refineries. The existence of accidents is due mainly to the increasing complexity of the process plants. This complexity appears because of two factors. The first one is a more complex process structure (energy integration, minimum waste, higher demands on yield and production, environmental constraints), the second one is a more complex control system (systems that performs many more tasks than before with a non- predictable software). This complexity problem is even worse because both factors are not independent but they are highly interrelated. In order to have safer and more robust plants Process Hazard Analyses (PHA) are carried out to identify the potential problems and also to propose possible solutions such as process changes. Traditional PHA techniques are HAZOP, What-If, FMEA, Checklists etc. This traditional approach is based on a chain of events (failures) analysis, being a loss the consequence of all these failures. The proposed solution is to protect the weakest or most dangerous elements in that chain. This approach has serious limitations: they do not consider systemic failures (due to the interaction between components), they simplify or even do not take into account some factors such as the human factor or the importance of software failures or the company’s safety culture. Leveson (2012, Leveson 2014) has developed a methodology based on systems theory called STPA (Systems Theoretic Process Analysis) in order to consider interactions and to overcome the limitations of the traditional methods. This approach considers safety as an emergent property and treats it as a control problem. Thus, the methodology is oriented to enforce that the safety control constraints are met in the design and operation of the plant. The procedure has been applied to some domains (aeronautics, trains, etc.) but not to chemical processes. In this paper we present and apply STPA to a chemical process and then take its systems theory foundations into D-higraphs (De la Mata and Rodriguez, 2010), our functional modelling based hazards analysis methodology. The rest of the paper is organised as follows: section two presents the functional modelling methodology D-higraphs. Section three presents the STPA methodology. Section four applies STPA alone and integrated with D-higraphs to a case study. Finally, last section draws some conclusions and discusses the results.
Section snippets
From Higraphs to D-higraphs
D-higraphs are an adaptation (dualization) of Higraphs, a general kind of diagramming objects well suited to the behavioural specification of complex concurrent systems (Harel, 1987). They consist of blobs, representing transitions, and edges, representing states. They were first presented in Rodríguez and Sanz (2009) as a functional modelling technique that merges functional and structural information of the system modelled.
Blobs and Edges
Blobs and their basic constituents are depicted in Fig. 1 (above)
STAMP (Systems-Theoretic Accident Model and Processes)
STAMP is a new type of accident model based on systems theory rather than the traditional analytic reduction using chains and reliability theory. In the STAMP model safety appears as an emergent property that arises when system components interact with each other. This property is enforced by a set of safety constraints that have to be guaranteed.
STPA
STPA is a hazard analysis technique that builds upon the STAMP accident causality model. As such, it is based on control and system theory rather than
Integrating STPA in functional models: a case study
In this section we use the systemic approach introduced by the STPA methodology with the functional model developed. The information available in this model is enough to conduct the STPA steps identifying hazardous scenarios and proposing corrective alternatives to enforce safety constraints. In order to illustrate the procedure a simple process has been selected. The process is the polymerization reaction in a batch reactor. This reactor has two different feeds, one corresponds to the monomer
Conclusions
Traditional Hazard analysis techniques have shown limitations to deal with new existing complex systems. This work has presented a systems-based theory, STPA, that can deal with systemic failures when analysing a system. The methodology has been applied on a chemical process showing that it can provide the same safety recommendations as other techniques (like HAZOP) but also considering other factors out of the scope of those techniques. As a result STPA can be considered a valid alternative to
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The past and present of System-Theoretic Accident Model And Processes (STAMP) and its associated techniques: A scoping review
2022, Safety ScienceCitation Excerpt :Besides publications based on the application and testing of the original versions of the STAMP model and techniques, researchers have investigated their combinations with other approaches and adaptations to specific contexts. For instance, Rodriguez and Diaz (2014) examined the integration of STPA with functional models for performing process hazard analysis, while Kondo et al. (2018) modified the terminology used in STAMP to introduce risk analysis for safety and cybersecurity for industrial control systems. Furthermore, STAMP, STPA and CAST have been occasionally included in books, suggesting their gradual acceptance and consideration as supplementary to other models and techniques.
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