Environmental Risk Management: Fault Tree Analysis - Literature review Example

Name: Instructor: Institution: Date: Environmental Risk Management Introduction The modern world has witnessed a lot of changes especially on the way various systems have been designed and established. Safety for these systems has become one of the top priorities for companies that establish these systems. During high consequence installations such as a nuclear plant installations, there are always concerns as to the probability of accidents occurring. This has led to the development of various models that try to analyze the possibilities of any accident happening. In doing so, these models look at the probable factors that could trigger an accident and the magnitude of the effects caused as a result of the accident. In most instances, these factors are often ignored as they are deemed irrelevant and too small to cause any harm. One of the models used by companies in analyzing the possibilities of any accidents occurring is the Fault Tree Analysis (FTA). FTA is a deductive model meaning that it starts from the top going downwards in trying to determine the factors that could lead to a tragic accident. Therefore, this makes it easier to prove or disprove whether a combination of certain factors have the capacity to cause a system failure resulting to tragic accidents (Javadi, Nobakht and Meskarbashee 46). The other model that is used in delaminating the various factors that could contribute to an accident is the Event Tree Analysis (ETA). ETA, on the other hand, is an inductive model meaning that it starts from the bottom, building upwards to the accident. ETA looks at the occurrence of certain events that could probably lead to an accident happening. It takes into account the various safety barriers that have been installed, and whether such barriers have been installed safely. It also tries to demonstrate the links that exist between various events leading to an accident (Bucci et al 1620) Fault Tree Analysis According to Javadi, Nobakht and Meskarbashee (46) FTA is probably one of the most common models that are used in analyzing risk in high consequence risk installations. This can be attributed to the gravity of damage that could arise from these installations should any accident happen. This model was developed in the year 1962 by the U.S Air Force Ballistics Systems Division contract to analyze the Minuteman Intercontinental Ballistic Missile (ICBM) Launch Control System. Since then, FTA has widely been accepted in most installations that aim to determine the probability of some of the safety hazards that may be present in the installation itself. One of the areas that FTA has been applied is in nuclear plant installations. As earlier stated, FTA starts from the top going downwards meaning that the first step that should be taken when conducting an FTA is to establish top probable event that is likely to happen in any installation. This, in turn, helps in coming up with some of the probable factors that could make it possible for the manifestation of an accident. These factors could either be system failure or human errors. Therefore, with the assistance of FTA it is possible to break down an accident into the basic causes making it possible for any company or organizations to set out preventive measures. These preventive measures are targeted on the basic factors that could cause an accident thus minimizing the probability of the accident happening. FTA Tree Diagram FTA encompasses a tree diagram as illustrated in figure 1, which is used to show the events that are likely to occur in realizing the top event. To demonstrate these events and the relationship that exists between the symbols are used. Each symbol bears a different meaning and relates to a specific event or factor in the FTA tree diagram. The rectangle is the critical part of the tree diagram as it represents the top event, whereby the top event is the most likely accident that a system could experience. However, it could also be located anywhere in the tree to demonstrate further probable events that could occur which do not necessarily have an effect on the top event (Javadi, Nobakht and Meskarbashee 50). Circles are used to represent the basic factor that could lead to the occurrence of the accident. They cannot be broken down further, unlike the rectangle, and therefore, they do not have any input gates. Diamond symbols, on the other hand, are used to demonstrate incomplete event meaning that they are yet to be explored, or event is not developed. Ovals are also, used in the tree diagram to indicate special events that can only materialize under special circumstances. The other symbol used in the FTA tree diagram is the triangle which is used to illustrate a transfer of a fault tree branch to another part of the Fault tree. Figure 1. A simple Fault Tree Diagram Procedure for undertaking Fault Tree Analysis According to Brooke, Phillip and Paige (258-260), in conducting a fault tree analysis there are certain steps that have to be observed to ensure the effectiveness of the model. Failure to observe these steps may lead to omission or exclusion of various factors that could prove vital in the occurrence of any event. The first step in conducting FTA is the identification of the top event. The top event is the most catastrophic event that could affect the system and therefore, a lot of care should be undertaken to minimizing the probability of it happening. The top event could be on the system as a whole, a sub-system, hardware equipment or even a function within the system. Additionally the top event should have the ability to be broken down into various basic factors. After establishing the top event, the next step is to define the overall structure of the model. This step is important as it identifies and establishes some of the basic contributors. These contributors are the ones that should be considered as probable causes to failure which in this case is the top event. Once these factors have been identified, then they should be broken down to the basic factors which if combined are likely to lead to the top event of Fault Tree analysis. Application of Fault Tree analysis A good example where FTA can be utilized is in explaining the probability of a train crash at a crossroad, whereby the rail track crosses a road. The top event in this case would be the collision that is likely to take place between trains and vehicle at the crossroad. This collision could be as a result of the possible events, whereby in one, event the train crosses the crossing point while the bars are not closed and no signal was sent. The other event is whereby the train crosses with bars not closed but a release signal was sent. In these two instances, it is possible to breakdown the reasons as to why the bars never closed which led to collision, which in this case is the top event. Therefore, once these factors are contained then it becomes easier and possible to avoid the top event which represents the collision between trains and vehicles. Advantages of Fault Tree Analysis. It is, therefore, possible to establish some of the benefits that can be associated with FTA in minimizing the probabilities of accidents or catastrophes happening in any given installation. One of the benefits of using FTA is the fact it is Root based model, meaning that it looks deeper into the root causes of any possible accident that may occur. In relation to this FTA makes it almost impossible to leave out any basic factors that may contribute to failure, unlike most models. FTA also, show the links that exist between the various factors in relation to the top event, thus making it possible to tackle them collectively. Additionally, FTA is capable of incorporating or adapting to any changes that may arise in any system thereby ensuring the integrity of the system is not compromised at any instance by new developments. Documentation is also another factor that FTA puts into consideration, in that it is easier to document the results that attained after conducting FTA with ease, unlike most models. The fact that FTA is easy to understand and undertake makes it beneficial to an organization that wish to conduct safety measures on their systems Event Tree Analysis According to Bucci et al (1625) Event Tree Analysis (ETA) is an inductive model unlike Fault Tree Analysis model meaning that it bases its concepts from the bottom moving upwards. The ETA is used to determine the possibilities of any harm that could affect a system. However, in doing so, the model looks at the initiating event which is responsible for the accident. In addition to these, the model also looks at other probable events that are linked to the initiating event and could contribute greatly to the accident. Additionally, the ETA model also tries to establish the effectiveness of the various safety measures that have been put in place to try and ensure that the initiating event never occurred in the first place. The first application of ETA can be traced back to around 1974, where it is believed that it was applied in the WASh-1400 nuclear plant safety study. It is derived from the decision tree which looks at the multiple decision paths that could be undertaken in resolving the problem. During this study, it was discovered that conducting FTA would be strenuous and therefore, another option was considered which would meet the study requirements. It is at this point that ETA was developed and since then it has proved to be an effective model when conducting safety procedures for any system However, to understand the concept behind ETA it is necessary to understand the basic definitions that the model encompasses. One of the definitions that the model tries to establish is the accident scenario. This relates to the manner in which event follow themselves from the initiating event into the main accident. In addition to these, there is the initiating event which in this case relates to the first event that occurred which might have led to the undesired state of events. However, in other instances depending on the effectiveness of the safety measures established accidents may not occur and in this case the ETA model also stipulates some of the possible outcomes that could be witnessed. It is also equally important to understand the pivotal events, which in this case relate to the events that occur between the initiating event and the actual accidents. These events may make it possible to avoid the accident if they are conducted in the proper manner or allow the accident scenario in case they fail. Therefore, pivotal events can be described as the measures that have been undertaken with an aim of ensuring that the initiating event does not lead to any mishap (Hong et al 275) Steps to be followed when conducting Event Tree Analysis Just like when conducting a Fault Tree Analysis there are also steps that have to be adhered to when applying ETA. These steps ensure that the model is effective in tackling the safety issue at hand (Ericson 226). The first procedure when conducting an ETA, is the identification of the relevant initial event which may lead to unexpected tragedies. The initial event can be described as any deviation from the normal function, and if not handled has the ability to lead to unwanted consequences. In determining an initial event, it is necessary to consider what type of event it is where the event is likely to happen and when the event is likely to take place. The next step when conducting an ETA is determining the various measures that have been established to counter the initial event should it occur. These measures could be automatic alarm systems that should go off in case there is a deviation from the normal functions of the system. Once these steps have been undertaken, then the next procedure would involve the creation of ETA tree diagram. The tree diagram portrays the probable outcomes that are associated with the occurrence of the initial event (Meloy122). In most instances, there are two alternatives, whereby the first alternative is positive, and the second alternative is the negative outcome. A positive outcome illustrates the effectiveness of the safety measures established to deal with accidents while the negative outcome portrays the failure of these measures in tackling the accident. The next step then would be to determine the probability of the initial outcome occurring again, while at the same time looking at the probability of the events in the branches recurring. This should be accompanied with a calculation of the probability of the various outcomes connected to the initial event. Once all these steps have been conducted, then the final step of ETA is conducted, which is the compilation and presentation of the results of the analysis (Meloy124) Practical application of Event tree Analysis Event Tree Analysis can be conducted in nuclear plant to avoid any accidents that may arise within the plant during operation time. Nuclear reactors require cool environments for them to conduct their operations effectively. Therefore, in case the nuclear reactor overheats then this could lead to disaster; however, if effective safety measures are developed to prevent overheating then it becomes possible to avert disaster. This can be explained using the ETA tree diagram as illustrated in figure 2. It is, therefore, possible for the nuclear plant based on the ETA analysis to come up with probable solutions should they encounter such a case in their operations. Advantages of Using ETA It can be applied on varying design systems meaning that it can be incorporated in any kind of design that an organization has adopted in running its system, unlike other models. The model also simplifies the complex system relationships, thereby making it possible to understand and undertake the necessary steps in ensuring that the probabilities of an accident happening are minimized. In addition to these, the models also incorporates hardware, software and human error in evaluating the various factors that may contribute to the occurrence of a disaster. Therefore, ETA cover almost all areas that may be affected or be the cause of the unwanted disaster (Ferdous et al 291) Disadvantages Despite all the benefits associated with the use of ETA, there are certain aspects of ETA that make it unfavorable. One of the shortcoming of ETA is the fact that ETA only covers a single initial event, meaning that for multiple event then one has to formulate various ETA system. Therefore, this makes ETA a tiresome process in determining the probability of various accidents that may occur. Additionally, ETA requires skilled personnel to analyze the results of the model thereby hindering its application to only a few individuals within an organization (Meloy 126) Fig. 2 ETA tree diagram Application of FTA and ETA in Bhopal disaster. The Bhopal disaster is a tragedy that occurred in the year 1984 involving Union Carbide India Limited (UCIL) in Bhopal, India. The accident was characterized by a leak of very toxic gases, which led to the death of many people who lived around the company. However, in trying to understand the importance of safety precautions the paper introduces both Fault Tree Analysis and the Event Tree Analysis. The paper tries to demonstrate the effectiveness of each of the model in preventing such a calamity from ever happening again (Jasanoff 50) Fault Tree Analysis The top event in this case would be a gas leak of very toxic gases and therefore, the model tries to look at the various factors that could lead to disaster. Using FTA it is then possible to try and establish some of the factors that if combined are likely to cause a gas leak. One of the factors that could be a contributing factor is the alarm system, in that the fact that it does not work efficiently makes it a contributing factor. Additionally, the installed system did not have the capacity to treat the gas to a safe level meaning that most of the toxic gas managed to escape. These are some of the basic factors that led to the Bhopal gas disaster in India. Therefore, if FTA model was used to conduct safety measures then it would have been easier to identify and determine the probability of the disaster happening (Brooke, Phillip and Paige 255), Event Tree Analysis Before the disaster in 1984 there were other gas leaks that took place but were contained. These gas leaks should be considered as the initial events in determining the probability of the disaster happening. The next step would be to look at the safety measures the company had set up to counter any gas leaks. If the safety measures were working properly, then it would be easier to deal with the problem at hand. However, the alarm system was not functioning meaning that it was practically impossible to inform operators of any gas leaks. Additionally, the piping system that was to supply water to try and diffuse the gas was malfunctioning and little amount of water was sprayed. This was coupled with the fact that copper was used for the piping system despite the fact that it corrodes if exposed to MIC. Therefore using ETA the company would have been capable of understanding and knowing the faulty safety measures that were installed. (Ferdous et al 286). Based on these it would have been capable of making the necessary changes to its safety measure thus reducing the probability of the disaster from happening. Therefore, the Bhopal disaster could have been avoided if the company had adopted the ETA model to analyze the possibilities of a gas leak ever happening. References Bucci, Paolo, Kirschenbaum, Jason, Mangan, Anthony L, Aldemir, Tunc, Smith, Curtis and Wood, Ted. “Construction of event-tree /fault-tree models from a Markov approach to dynamic system reliability.” Reliability Engineering and System Safety, 93 (2008): 1616 – 1627. Print. Brooke, Phillip J. and Paige, Richard F. “Fault trees for security system design and analysis.” Computers & Security 22.3 (2003): 256-264. Print. Ericson, Clifton A. Hazard Analysis Techniques for System Safety. New Jersey: John Wiley & Sons, 2005. Print. Ferdous, Refaul, Khan, Faisal, Sadiq, Rehan, Amyotte, Paul and Veitch, Brian. “Handling data uncertainties in event tree analysis.” Process Safety and Environmental Protection 87 (2009): 283–292. Print. Hong, Eun-Soo, Lee, In-Mo, Shin, Hee-Soon, Nam, Seok-Woo and Kong, Jung-Sik. “Quantitative risk evaluation based on event tree analysis technique: Application to the design of shield TBM.” Tunnelling and Underground Space Technology 24 (2009): 269–277. Print. Jasanoff, Shelia Learning from Disaster: Risk Management After Bhopal. Pennsylvania: University of Pennsylvania Press, 1994. Print. Javadi, Mohammad S, Npbakht, Azim, Meskarbashee, Ali. “Fault Tree Analysis Approach in Reliability Assessment of Power System.” International Journal Of Multidisciplinary Sciences And Engineering 20.6 (2011): 46-50. Print. Meloy, Anthony F. “Arenal-type pyroclastic flows: A probabilistic event tree risk analysis.” Journal of Volcanology and Geothermal Research 157 (2006): 121 – 134. Print. Read More