Advantages Disadvantages
Allows the foresight of the outcome in an accurate manner
Allows the decrease of risks
Reduces time used for decision making process
Real data can be used for analysing different future scenarios
Decisions can be taken in a more accurate manner
Continuous improvement is promoted
Only few tools offer “what-if” capabilities
Model is only as strong as the team behind
Relatively unstructured approach
Team needs to be composed by experts
Only as strong as the team behind the model
Evaluations are mostly subjective
Scenario assessments are time consuming
Deep understanding of the system to be analysed is required
Data and information need to be collected from different sources
Many efforts are required to prove the reliability of the simulation model
Needs to be updated regularly
Table 24 – Summary of Qualitative Risk Assessment Methodologies
3.3.3 Hybrid Risk Assessment Methodologies Bow Tie Model
The Bow-Tie Model (BTM), also known as ‘Barrier Diagram’, is used by companies world- wide across several industry sectors (Lewis & Smith, 2010). The BTM is a combination of two complementary techniques, namely the Fault Tree Analysis (FTA) and the Event Tree Analysis (ETA). In other words, the BTM uses both techniques, while they focus on opposite sides of a given undesired event. As their names suggest, the FTA helps to identify eventual causes for faults or risks of a particular system, while the ETA helps to identify eventual events, that is to say undesired outcomes and consequences of possible failures. Both are systematic methods conceived to find out how a particular situation can arise and what may ensure from a critical event (Clifton, 1990). This graphical technique uses logic diagrams to exercise the identification of risks’ causes and effects. The FTA has first been introduced in 1961 (Lee et al. 1985) and can be shown as follows:
Multiple Causes Multiple Consequences
Preventive Measures Alleviation Measures
Focus of FTA Focus of ETA
While Fault Trees (FT) move backwards, starting from a given failure via all possible causes, the Event Trees (ET) move forwards, starting from a failure showing the accident’s sequences. The FTA-ETA Method gives managers qualitative insights, so that sequences can be quantified and hence, probabilities of particular situations or events can be approximated (Takaragi et al, 1983). In most cases a disaster does not appear because of one single failure, since they normally take place because of a combination several incidents. Howbeit, the FTA – ETA approach assumes that each branch of a tree embeds mutually exclusive events which are independent of one another. As FT and ET include many judgements, analysts have to ascertain the problem’s structure as well as the importance of the various branches.
The fact of illustrating the hazard as well as its causes and consequences in a visual manner fosters common understanding and clear communication at all levels of a company, i.e. members from senior management as well as operations personnel, as well as controlling institutions. The big picture can be guaranteed and the sequence of events and previous incidents are kept. As a picture is worth a thousand words, unnecessary or lower importance barriers may be reduced (Lewis & Smith, 2010). In addition, the communication between the different stakeholders is encouraged and stimulated. In fact, people will take responsibilities in case their proposed action is taken. Besides, the issue of lack of ownership can be avoided. Another point to allude is the fact that the BTM is less labour intensive than other traditional techniques, which can also be seen as a gain of efficiency (Treytl & Himmelbauer, 1996). Furthermore, the volume of safety analysis may be reduced since the spots where resources should be focused for risk reduction can be identified (Hack, 2004) and since the workforce involvement allows every participant to see why their tasks are critical for risk control (Lewis and Smith, 2010).
The systems’ simplification via the BTM helps discovering pertinent information
Initial Undesired
Event
Figure 54 – Bow Tie Model: Fault Tree Analysis & Event Tree Analysis
model fails in identifying common cause failures, partial failures or time delays (Bell, 1989;
Steward, 1990). Moreover, the BTM is not able to quantify the level of risk in absolute terms. In fact, it cannot be used for complex inter-relationships analysis. Further, Lewis and Smith (2010) explained that if a company wants to identify individual protections for every line of every section of every unit of the process facility, then the BTM will not be helpful.
Fishbone Diagram
The Fishbone Diagram is generally used to evaluate the causes and sub-causes of a given event, i.e. to identify potential risks of a business or process. Those diagrams are usually used to illustrate and communicate the relationships between several causes which may lead to an undesired event. A conditio sine qua non is that a team, including all relevant experts is composed. This team analyses the given problem via a research of the main cause variables. To do so, they use the “6M”, namely (1) Management, (2) Machine, (3) Material, (4) Man (in the sense of human being), (5) Methodology, and (6) Milieu (in the sense of working environment) (Mahto & Kumar, 2008; Syska, 2006). Within those six main influencing factors, sub factors need to be ascertained. The latter will be scrutinised through the five “why” questions (Syska, 2006).
For each “why?” an answer needs to be found. By this mean, it can be ensured that a problem will be considered in detail. By focusing on an escrow issue the problem’s real cause, which is frequently hidden by other indications or symptoms, may de facto be revealed (Suske, 2010).
Upon completion of the diagram the team adopts the problem’s main cause, which will be discussed, analysed and tested. If the given main cause turns out being not as relevant, the next possible cause needs to be treated in the same way (Syska, 2006).
One of the model’s main advantages is that it is, because of its visual representation, easy to understand and to apply (Syska, 2006). In addition, it allows a thoughtful analysis which avoids that any possible main cause might be missed. The team is focused on the big picture while it may search for possible influencing factors leading to an undesired event. Moreover, it shows sub-causes and fields of vulnerabilities before they lead to major difficulties. On the other hand, the simplicity of a fishbone diagram, which is often seen as an advantage, can also be interpreted being a drawback. In fact, because of this simplicity and since reality is usually very complex, the analysing team may have difficulties to represent the really interrelated nature of the problems’ causes and effects. Also, because of the reality’s complexity, the diagram may become extremely large in space. Otherwise, the team may not be able to investigate the relationships between the different causes and effects as detailed as intended. The major drawback is probably that the model is quite time consuming since it is very likely that the team expends a great amount of time and energy in theorising potential causes while many of them have no significant impact on the problem to be solved. In addition, the model treats the potential causes in an equiprobable manner, while in reality, their probabilities and hence their priorities may differ flagrantly (Kanti Bose, 2012).
Corollary
The Bow-Tie Model (BTM), a combination of the Fault Tree Analysis and the Event Tree Analysis, helps to identify the eventual causes for faults or risks of a particular system and the eventual outcomes or consequences of an undesired event. The BTM can be shown as logic diagram, which enables common understanding and fosters clear communication not only at all levels of the company but also between all the different stakeholders. Since the BTM is less labour intensive than other traditional techniques, a gain of efficiency can be guaranteed and a
mind that a simplification always imports a certain loss of data or information. Another negative aspect is that the model is based on strong assumptions of mutually exclusive and independent events for each branch of the considered tree.
The Fishbone Diagram is easy to understand and to apply since it simplifies reality.
As stated above, this simplification may also be considered being a drawback because of its consequential loss of information. Since the undesired event’s sub-causes are shown, the situation can be viewed as overall picture, notifying the different fields of vulnerabilities. The model’s inherent concern is that it may be difficult to represent the interrelated nature of the defined issues’ causes and effects and that the potential causes are usually treated in an equiprobable way. A summary of the different hybrid risk assessment methodologies is given in Table 25.