Event frequency risk assessment

From a human reliability perspective, a number of interesting points arise from this example. A simple calculation shows that the frequency of a major release (3.2 x lO"" per year) is dominated by human errors. The major contribution to this frequency is the frequency of a spill during truck unloading (3 X10" per year). An examination of the fault tree for this event shows that this frequency is dominated by event B15 Insufficient volume in tank to imload truck, and B16 Failure of, or ignoring LIA-1. Of these events, B15 could be due to a prior human error, and B16 would be a combination of instrument failure and human error. (Note however, that we are not necessarily assigning the causes of the errors solely to the operator. The role of management influences on error will be discussed later.) Apart from the dominant sequence discussed above, human-caused failures are likely to occur throughout the fault tree. It is usually the case that human error dominates a risk assessment, if it is properly considered in the analysis. This is illustrated in Bellamy et al. (1986) with an example from the analysis of an offshore lifeboat system. 

All risk assessment techniques, whether qualitative or quantitative, require an estimate of frequency of event occurrence. This frequency, which is extremely site specific, can be influenced by many factors. 

Crawley and Grant (1997) have developed a risk assessment tool for new offshore installations. They have examined typical leak frequencies of equipment items and the ignition probability of these leaks in four pressure bands. With this information it was possible to define leak size and frequency for any piece of equipment and the ignited leak frequency. In off-shore installations gas separation vessels were found to have ten times higher ignited event frequency than oil pumps. 

Quantitative risk assessment (QRA) The systematic development of numerical estimates of the expected frequency and consequence of potential accidents associated with a facility or an operation. Using consequence and probability analyses and other factors such as population density and expected weather conditions, QRA estimates the fatality rate for a given set of events. 

Finding 4. Of the wide range of Program Manager for Chemical Demilitarization risk analyses, the quantitative risk assessments (QRAs) are most closely linked with chemical events. They calculate the frequency and consequences of modeled events, and their analysts study real operational... 

Recommendation 4. The quantitative risk assessment (QRA) for each chemical demilitarization site should be iterative. Actual chemical events should be used routinely to test the completeness of the QRA, which should be routinely utilized to hypothesize the frequency and consequences of chemical events. The Program Manager for Chemical Demilitarization and the U.S. Army Soldier and Biological Chemical Command should use the QRAs to evaluate measures to control future chemical events. The Army should also consider using QRAs to examine scenarios associated with sabotage, terrorism, and war. 

A distinction is made between biomarkers and bioindicators because they can be used in quite different ways and for different purposes in a risk assessment context. As mentioned above, a biomarker is considered to be a surrogate marker of exposure or an early biological marker of effect (e.g., mutations in reporter genes, total chromosome alterations). In contrast, a biological marker of effect that is itself a key event along the pathway from a normal cell to a transformed one is described as a bioindicator (e.g., mutation in critical gene for cancer, cancer-specific chromosome translocation). Biomarkers can be used to inform the dose-response for tumors in a qualitative manner. Bioindicators can be used in a qualitative and quantitative way to inform tnmor dose-response curves. Use of these biomarkers and bioindicators can make it feasible to characterize a dose-response curve at exposure levels below those at which increases in tnmor frequency can be assessed. 

CPQRA A chemical process quantitative risk analysis is the process of hazard identification followed by numerical evaluation of incident consequences and frequencies, and their combination into an overall measure of risk when apphed to the chemical process industry. It is particularly applicable to episodic events. It differs from, but is related to, a probabilistic risk assessment (PRA), a quantitative tool used in the nuclear industry. 

Selection of licensing basis events (LBEs) is based on the probabilistic risk assessment (PRA) performed as part of the Integrated Approach and constitutes the process which establishes the bridge between the engineering approach and the licensing basis for the Standard MHTGR. The use of the PRA for LBE selection provides a basis for judging, in a quantitative manner, the frequency of the entire event sequence and, therefore, the appropriate dose or risk criteria to be applied. 

The applicability in risk assessment and acceptability is that for low-frequency events the probability estimate is not based on a large number of trials and the public evaluation may well be more conditioned on how bad the outcome might be, with little regard for arguments as to how likely it is [p. 12],... 

A natural gas high pressure pipeline with a diameter of 20" (508 mm), a wall thickness of 8 mm subject to a pressure of pi = 70 bar is planned in the vicinity of a residential area. For the section passing close by this residential area a risk assessment is to be performed. This is a so-caUed risk-based analysis, since the expected frequency of the undesired event (rupture of the pipeline and gas release) is directly taken from statistical material (actuarial approach) and not determined by a detailed analysis of the engineered systems involved. 

As far as methods for risk assessment at workplace are concerned. Risk Assessment Matrix method is the most popnlar one. Risk estimation entails evaluating both the severity and frequency of hazardous events. 

LOPA is a simplified risk assessment to determine if there are sufficient IPLs against an accident scenario. As illustrated in Fig. 1, many types of protection layers can be considered against an unwanted accident. The thickness of the arrows represented in Fig. 1 indicates the frequency of the specified consequence for the initiating event. The results of LOPA can be used for the decision-making for numerical criteria and the number of IPL credits althou LOPA does not suggest which IPLs to add or which design to choose [William G. Bridges. 2001]. 

Acceptable risk is strongly related with the acceptable probability of failure and the acceptable amount of losses. There is general agreement in the literature and in regulatory circles that risk should at least be judged from two points of view in relation to inundation consequences. The first point of view concerns the risk assessment by society on a national level which relates to the number of casualties due to a certain hazardous event. Risk is defined as the relation between frequency and the number of people suffering from a specified level of harm in a given population from the realisation of specified hazards . If the specified level of harm is limited to loss of life, the societal risk may be modelled by the frequency of exceedance curve of the niunber of deaths, FN-curve. Secondly, the... 

Societal risk concerns the risk assessment by society on a national level related to the number of casualties due to a certain hazardous event (Vrijling et ah, 1998). The societal risk can be modelled by the frequency of exceedance curve of the number of fatalities, a FN-curve. The FN curve can be described on a double logarithmic scale in the following form (Jonkman, 2007). 

Risk assessment of nuclear power plants is based on evaluation of core damage frequency (CDF). Thus we consider 1st and 2nd task category. Task category 1 defines all initiating events, which damage the reactor core. Task category 2 is focused to assess initiating events occurrence probability and to assess safety related systems malfunction probability. 

When conducting a quantitative risk assessment the analyst(s) must define its probabilistic basis, and in most cases this means either to use the classical frequency approach or the Bayesian approach. The classical statistical approach is and has been the most commonly used probabilistic basis in health care (Schneider 2006). This approach interprets a probability as the relative fraction of times the event considered occurs if the situation analyzed were hypothetically repeated an infinite munber of times. The imder-lying probability is unknown, and is estimated in the risk analysis. In the alternative, the Bayesian perspective, a probability is a measure of imcertainty about future events and outcomes (consequences), as seen through the eyes ofthe assessor(s) and based on his/her backgroimd information and knowledge at the time of the analysis. Probability is a subjective measure of imcertainty. 

In this paper, risk is defined as usual, i.e., it is a combination of the frequency F of an (undesired) event and its consequence C. This definition follows the ISO standards in risk management terminology in general (ISO 2002) as well as specifically in the Code (ISO 2005). The Code references to the management standard in all terms related to risk, i.e., risk, risk analysis, risk assessment, risk evaluation, andriskmanagement. 

Frequency of historical events related to cargo operations is presented in (Vanem et.al. 2008), and has been outlined on available data as 9.5 x 10 per ship per year. Risk model for the spiU event (one of mayor problems during unloading/loading) is presented at figure 4. This risk assessment procedure do not include sources of spUl event that might be ... 

E q)losives storage risk assessment requires the specification of both event consequences and event probabilities. Accidents in explosives are low frequency events, with few realisations, so defining occurrence probabilities for major incidents in explosive storage is problematic. 

Risk assessment All personnel transfers are considered stand-alone events and require a formal risk assessment before the operation is started. Any risk assessment should consider such issues as the necessity and frequency of the transfers, environmental conditions (weather, wind speed, wave height, etc.), types of vessels used, availability of personal protective equipment, simultaneous operations, and qualifications of the personnel involved. If issues or conditions change during an operation, proper management of change procedures should be in place to track concerns of risk. 

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