ALARP acceptable risk levels

A PRA aims to quantify Equation (9.1). The identification of hazards will always be a qualitative process, but numerical values can be assigned to the consequence and frequency terms. For example, the hazard of overfilling an on-board methanol tank could result in a fire leading to a fatality. If this event occurs maybe once every 5000 years then the risk associated with overflowing a methanol tank is 0.0002 fatalities per year. If the Acceptable Risk Level (ALARP) states that a fatality rate of more than 1 in 10,000 years is unacceptable (the ALARP value discussed in Chapter 1), then the methanol overflow hazard needs to be ameliorated. If the hazard cannot be removed altogether, then the consequence term can be reduced, say by reducing the size of the spill or by using a... 

ALARP is a sound concept. It promotes a management review that should result in achieving acceptable risk levels. Practical economic and risk trade-offs are frequent and necessary in the benefit/cost deliberations that take place when determining whether the costs to reduce risks further can be justified by the resulting decrement in risk. ... 

The concept of As Low as Reasonably Practicable (ALARP) was recognized as a valuable tool in determining acceptable risk levels. However, a word of caution was offered On occasion, achieving risk levels as low as reasonably practicable will not be acceptable. Prior to presenting the following definition, I said that a workable and sound definition of acceptable risk must encompass hazards, risks, probability, severity, and economics ... 

Reduction of all unacceptable risks to an acceptably low level (ALARP—as low as reasonably practicable) by well-recognized and controllable means sustainable in the life cycle of the plant (see lEC standard)... 

IPLs comprising SIS and non-SIS systems reduce the unacceptable risk levels to acceptable risk levels, which should be equal to or below the ALARP level. 

The ALARP concept (as low as reasonably practicable) is discussed with an example of how the concept is applied in achieving an acceptable risk level. 

Where risks are higher that normally acceptable and all reasonable mitigation measures have been examined to find out value and practicality, the principal of risk as low as reasonably practical applies. Where the available risk protection measures have been exhausted and the risk level is still higher that the accepted numerical value, the risk would be considered "As Low As Reasonably Practical" (ALARP). 

Risk acceptance criteria can be defined relative to the risk matrix. The as-low-as-reasonably practicable (ALARP) approach may be chosen, defining three risk levels [4] ... 

Some risk analysts use the term as low as reasonably practical (ALARP) for setting a value for acceptable risk. The basic idea behind this concept is that risk should be reduced to a level that is as low as possible without requiring excessive investment. Boundaries of risk that are definitely acceptable or definitely not acceptable are established as shown in Figure 1.16, which is an FN curve family. Between those boundaries, a balance between risk and benefit must be established. If a facility proposes to take a high level of risk, then the resulting benefit must be very high. 

The concepts inherent in the terms ALARA and ALARP serve as guidelines in determining whether a risk is acceptable, but cannot be used as absolutes in decision-making. It should be understood that in an exceptional situation, even though the risk level attained is as low as reasonably achievable or practicable, a decision will be made that the residual risk is not acceptable and that the operation should not proceed. 

Contemporary risk management follows a maturing path to the establishment, acceptance and management of a level of risk that is deemed tolerable and as low as reasonably practicable (ALARP). The recent issue of military standards [MoD 2004] describes six processes for risk management hazard identification, hazard analysis, risk estimation, risk and ALARP evaluation, risk reduction and risk acceptance. Whilst these are not the universal descriptions of the processes involved, the underlying principles are consistent with other procedures and handbooks, for example lEC 61 SOS, JSP 4S4 and Mil Stan 882D. 

ZlO-2005 tersely states its purpose in Section 1.2 as follows The primary purpose of this standard is to provide a management tool to reduce the risk of occupational injuries, illnesses, and fatalities. This question logically follows. What risk reduction level is to be achieved This chapter Establishes that achieving a zero risk level is unattainable Discusses the great variations in cultural and situational aspects of risk acceptance and Combines the elements of risk (probability and severity) with ALARP (as low as reasonably practicable) to arrive at a definition of acceptable risk, the operational goal. 

Some Safety Case reports contain an as low as reasonably practicable (ALARP) demonstration section that shows how the identified major hazards have been managed such that the associated level of risk is acceptable. The vexed topics of ALARP and acceptable risk are discussed in Chapter 1. 

IPLs reduce the frequency of occurrences severity to bring risk level from unacceptable level to Acceptable level < ALARP... 

With initial assessment, all risks are listed and suitable IPLs are deployed. After this the entire system is reassessed. Because we are interested in SIS, in the final assessment only SIS is shown. After each assessment the IPLs and SIS are validated. After final assessment of overall safety requirements it is ensured that the risk level is at an acceptable level, as shown in the figure. This shall be a level equal to or below the ALARP level. In principle, what has been discussed here is more or less the same as discussed earlier, the only difference is that here the basic implementation process is shown. Similarly, SIS in the design phase is shown in Fig. XII/1.0-3. The reason for showing the figure is to recapitulate further details about SIS design. 

The approach that is being widely promulgated in the UK is that the general form or framework for acceptability criteria should be represented as a three-tier system with (a) an upper bound on individual or societal risk levels, beyond which risks are deemed unacceptable (b) a lower bound on individual or societal risk levels, below which risks are deemed not to warrant concern and (c) an intermediate region between the previous two, where further individual and societal risk reduction are required to achieve a level deemed as low as reasonably practicable, the so-called ALARP principle. To achieve a wide consensus on the acceptable flood risk, it is indispensable that the various methods, rules, and tools to be developed in the advanced ALARP framework are robust and transparent. ... 

Ethically and legally, it is expected that the risk of any undertaking is mitigated/controlled to an acceptable level. In the UK in particular, the acceptable level of risk is defined as ALARP. This requirement calls for the identification of the controls of hazards on an individual basis (for each hazard) including consideration of all possible controls. It is only when there is nothing else practicable that can be done to reduce the risk level further, that the safety risk emerging from the system can be considered as ALARP. 

To support the argument that the safety risk emerging from the system is ALARP, it is also necessary to demonstrate the practicability argument by carrying out a cost-benefit analysis to assess and compare the costs of any proposed changes to the system against the reduction in risk levels inclusive of the acceptance/rejection criteria. 

The accident scenarios may consist of the hazards, intermediate events, additional events and consequences. It is important to define the initial events (hazards). The measure of safety of ships in damage conditions is the risk level. The risk acceptance criteria can be the risk matrix or ALARP concept. 

Figure 21.1 shows the relation between the acceptance criterion and the SHE goal. Those combinations of frequency and consequence that were denoted high represent a risk that falls above the acceptance criterion. A medium risk of accidents falls within the ALARP region, i.e. between the acceptance criterion and the goal. ALARP expresses that the risk level has been reduced as far as reasonably practicable and that no further cost-effective measures can be identified. Only an accident risk below the SHE goal is denoted low . 

Once the level of concern is understood, the FMEA team needs to appraise its likelihood. Remote and rare events may be acceptable without further action. The term ALARP is often used for such events it means reduce risk as low as is reasonably practical. Higher likelihood hazards will demand more attention, as indicated in Figure 8.10. The acceptance of a hazard without control needs to be justified. It is important to note that the FDA does not include likelihood in its guidance... 

On this basis the concept of so far as is reasonably practicable (SFAIRP) was bom and subsequently enshrined in the 1974 UK Health and Safety at Work Act. SFAIRP, whilst a significant step forward, fails however to precisely acknowledge the notion of risk and its relationship to practicability. As such the slightly modified term as low as reasonably practicable (ALARP) has been widely adopted in the UK and some other countries as the basis for risk acceptability. The ALARP principle can be defined as that level of risk which can be further lowered only by an increment in resource expeuditure that is disproportionate in relation to the resulting decrement of risk [2],... 

The fundamental difficulty with the concept of ALARP is that the term is inherently circular and self-referential. For example, the phrase best available technology used in the list above can be defined as that level of technology which reduces risk to an acceptable level— in other words to the ALARP level. Terms such as best operations and high standard are equally questionbegging. 

Realistically, it has to be concluded that the term ALARP really does not provide much help to risk management professionals and facility managers in defining what levels of risk are acceptable. It may be for this reason that the U.K. HSE chose in the year 2006 to minimize its emphasis to do with ALARP requirements from the Safety Case Regime for offshore facilities. Other major companies have also elected to move away from ALARP toward a continuous risk reduction model (Broadribb, 2008). 

The technique does require that companies put forward a value for the level of risk that they will accept ( How safe is safe enough ). As noted with the discussion to do with ALARP in Chapter 1, the selection of such a value is fraught with difficulties. 

The TR integrity requirement is stated in HSE (2006b), and requires a demonstration that the frequency with which accidental events will result in a loss of TR integrity, within the minimum stated endurance time, does not exceed the order of 1 in 1000 per year. No ALARP objective is stated, but this frequency is required to be reduced to a lower level wherever reasonably practicable. Where the frequency is close to the limit of 1 in 1000 per year, acceptance that further risk reduction measures are grossly disproportionate should be only on the basis of a very rigorous demonstration. 

In Figure 8 the acceptance criteria for the SR are shown. The range between the upper and lower tolerable level is the ALARP zone the risks falling in this region are considered as acceptable only if their reduction is not possible or cost effective. 

The process of determining what level of protection and safety makes exposures, and the probability and magnitude of potential exposures, as low as reasonably achievable, taking into account economic (e.g., cost, effect on production) and social factors. It is typically utilized in the nuclear industry. See also Acceptable Level of Risk As Low as Reasonably Practical (ALARP). 

See Acceptable Level of Risk As Low as Reasonably Practical (ALARP). 

The issue of sufficiency with regard to confidence in an assertion is complex. The notion of reducing risk to a level that is As Low As Reasonably Practicable (ALARP) has become widely accepted (HSE 2001). Risk is a quantifiable entity (i.e. the expected loss). Confidence is also quantifiable (i.e. the probable truth of a claim). However, except where purely statistical evidence is used, to reason about confidence quantitatively requires first qualitative reasoning about the sources of uncertainty in arguments. In our approach wc focus on these qualitative arguments, and leave the subsequent encoding and quantification of these to those who believe that quantification can reap further utility. 

Management has to decide if the calculated level of risk is acceptable, and, if it is not, what actions need to be taken to reduce it. Some Safety Management Systems—in particular many Safety Cases—are built around the concept of a numerical value for an ALARP (As Low as Reasonably Practicable) value, as already discussed. If the risk lies above or below the predetermined ALARP value then corrective actions must be taken. 

The results of the PRA show that the AP1000 has significantly less dependence on operator action to reduce plant risk to acceptable levels than current plants. This was shown through the sensitivity analyses and the operator action contributions from both the risk decrease and risk increase measures. Almost all operator actions credited in this PRA are performed in the control room there are very few local actions outside the control room. Further, the human actions modelled in the AP 1000 PRA are generally simple. Thus, the tasks for AP 1000 operators are easier and less likely to fail. If it were assumed that the operators never perform any actions credited in the PRA, the plant events CDF would still be lower than the result obtained for many eurrent pressurised water reactors including operator actions.This low dependence on operator aetion is therefore ALARP. 

Despite the widely published figures for Maximum Tolerable Risk (e.g.. Table 2.2), the UK HSE sometimes press for a Maximum Tolerable Risk to be targeted at a lower level nearer to the Broadly Acceptable level (e.g., an order of magnitude). This, however, is a controversial area. In the authors opinion, whatever may the starting point be, the ALARP calculation will, in any case, cause the risk to be reduced to an appropriate level. 

Thus any expenditure within a budget of 3800 which might reduce the risk to the Broadly Acceptable level should be considered. If no realistic risk reduction can be obtained within this sum it might be argued that ALARP is satisfied. 

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