Acceptable risk matrices

Risk indices are single numbers or a tabulation of numbers that are correlated to the magnitude of the risk to people. Some risk indices are relative values with no specific units. The limitations on the use of indices are that they may not be an absolute criteria for accepting or rejecting the risk. Risk indices also do not communicate the same information as individual or societal risk measures. An example of risk indices is a risk ranking matrix. Table 6-4 (modified from CCPS, 1992) shows how severity and likelihood are combined to obtain risk indices. An example risk matrix is shown in Figure 6-3 (RRS, 2002). 

Figure 10.11 Risk matrix adapted from the IEC 61511 standard, indicating the accepted and non-accepted risks, as well as an intermediate field. The numbers represent the number of required IPLs together with the required SILs.

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] ... 

Determine the consequence and likelihood of the hazard scenario. This evaluation should include an examination of safety, environmental, and economic losses (including the requirements associated with safety and environmental regulations). Based on an assessment of the overall risk associated with the identified hazard, decide if additional safeguards or Layers of Protection/lndependent Protection Layers (IPL) are required. The criterion for acceptable risk could be single numerical value, or it could be determined through use of a risk matrix. 

The purpose of a risk management matrix is to (a) provide a logical framework for hazard analysis and risk assessment and (b) assist risk decision makers in arriving at their risk reduction and risk acceptance or declination conclusions. The implicit goal is to achieve acceptable risk levels. Several standards and guidelines now include the concepts of residual risk and acceptable or tolerable risk (e.g., ANSl/Bll TR3, ISO/IEC Guide 51, SEMI SIO—see references for full titles). 

Earlier in this chapter, reference was made to a speaker who reviewed the hazard analysis and risk assessment methods used in his company, which relied on typical risk assessment and decision-making matrices, to achieve acceptable risk levels. Use of such matrices is a method some organizations apply to arrive at acceptable risk levels. Table 15.1 is an example of such a risk assessment matrix. Using the results from Table 15.1, levels of remedial action or risk acceptance for individual risk categories can be established, as in Table 15.2. 

Table 2.3 shows the hazard risk matrix, which incorporates the elements of the hazard severity table and the hazard probability table to provide an effective tool for approximating acceptable and unacceptable levels or degrees of risk. By establishing an alphanumeric weighting system for risk occurrence in each severity category and level of probability, one can further classify and assess risk by degree of acceptance. Obviously, from a systems standpoint, use of such a matrix facilitates the risk assessment process. 

Identify the levels of acceptable risk approved for the project that appear in the risk assessment matrix. 

Firstly the chapter approaches risk management in a general sense, including the phases of risk assessment (risk identification, risk analysis and risk evaluation), risk control (risk reduction or mitigation, and risk acceptance), risk documentation and communication, and risk review. Then some methods for risk assessment are explored further, such as matrix type and Failure Mode Effect Analysis (FMEA) using risk priority numbers (RPN). 

Many standards, including lEC 61S08, suggest the familiar risk matrix approach, such as in Table 1, for deciding whether risks are tolerable or not. The Roman numerals I to IV represent risk classes, where risk class I is intolerable and IV is acceptable. Although lEC 61508 is clear that this is an example only, we have observed many projects which have attempted to apply this matrix directly without question. 

The use of risk matrix has been depicted in Fig. 1/3.3.3-1 through a simple example of single risk target pertinent to one cement plant. In this example, it is seen that for the same cause there could be two different risk levels as shown by differences in hatch lines. In the example, the same likelihood has been considered for both the cases. Since the risk level is medium, it is accepted with suggestion of installing additional devices to reduce the risk level. 

A risk assessment matrix for use in determining acceptable risk levels is offered as an example. 

All personnel involved in the risk assessment processes must understand the definitions used for occurrence probability and severity and for risk levels in the risk assessment matrix chosen. An example of a risk assessment matrix is shown in Chapter 6 under Acceptable Risk. It includes examples of ... 

Although not mandatory in the HAZOP method, this study comprised a risk assessment to all deviations detected. Risk assessment was performed through the use of a risk matrix already used in similar industries which included the combination of probability (possibility that the event occurs) and severity (as a consequence of the event). For the risk assessment criteria, four levels of probability were defined very unlikely/remote, likely/possible, probable and frequent. For severity were defined also four levels reduced, moderate, high, very high/catastrophic. The designation of risk parameters took into account the probability of occurrence, the measures implemented, historical events, potential injury to persons, to materials, to the environment. The combination between the four levels lead to four types of risk, grouped in two levels, acceptable/not significant, which even... 

The resultant risk matrix can be used to provide and develop an action plan which may also be assigned numbers so that priorities can be identified. Risk matrices are often colour coded to provide a visual concept of whether or not the residual risk is tolerable or acceptable. 

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. 

The criteria within the method is to achieve an adequate level of risk using the risk acceptance criteria, risk matrix or ALARP concept, Gerigk (2010). 

This is the person, or persons, that have the authority to accept potential mishap risk. Typically, the mishap risk for a hazard falls into one of several different possible levels based on the risk matrix, and a person of higher rank or authority is required to accept the corresponding level of risk. DoDINST 5000.2 states that high, serious, medium, and low risk must be accepted by the CAE, PEO, PM, and PM, respectively. In theory, the risk level should be pushed down, through design measures, to the lowest level in order that the risk can be accepted by the lowest level decision authority. In reality, this is not always possible due to various system constraints, thus requiring a decision authority hierarchy. 

Frequency of occupational accidents. The frequency of occupational accidents must not be higher than for comparable platforms in operation. Norskoil also uses a risk matrix similar to the one shown in Table 22.4 in determining the acceptability of occupational accidents. 

The combination of the hazard severity and the hazard probability defines the hazard risk classes. These classes are listed in Table 4 with different levels of tolerability Class A forms the intolerable area of the risk matrix, Class B and C the tolerable area and class D means acceptable risk. 

The Risk matrix helps evaluate if a risk given by an estimated probability of occurrence (frequency) and a classification of the damage (consequence) is acceptable or not Fig. 1 shows an example for a risk matrix. 

The second major issue raised in this paragraph is to do with the word unacceptable . If risks are to be categorized for acceptability then some objective criterion is needed. Most companies use a simple risk matrix, such as that shown in Chapter 1 to determine which hazards are acceptable, and which are not. A more sophisticated approach is to use the ALARP concept. 

During 1999, the European Commission ARIBA project attempted to build an accident risk tolerability matrix for air traffic operations on UK Health and Safety Executive lines. The main reason for this was due to the fact that UK industry safety assessments usually use the HSE studies and guidelines about tolerable and acceptable risk , with the following (simplified) HSE definitions ... 

The rationale described above leads to the building of defences against all those parts of the risk matrix that are judged to be acceptable to the system. 

For each q and q risk value and the Severity Rating (S), a level of design acceptability is determined from where these values intersect on the Conformability Map. The symbols, relating to the levels of design acceptability, are then placed in the nodes of the Conformability Matrix for each variability risk which the failure mode is directly dependent on for the failure to occur. Once the level of design acceptability has been determined, it can then be written on the Conformability Matrix in the Comments section. Cpi values predicted or comments for suppliers can be added too, although predicted Cp values can also be written in the variability risks results table. 

Each cell in tlie matrix (Table 18.4.2) is assigned a risk ranking as indicated by the letters. In this approach, an A level risk corresponds to a very severe consequence with a high likelihood of occurrence. Action must be taken, and it must be taken promptly. At tlie other end of the scale, a E level risk is of little or no consequence witli a low likelihood of occurrence, and no action is needed or justified. For example, a level C risk might warrant mitigation witli engineering and/or administrative controls or may represent risks tliat are acceptable with controls and procedures. 

Workplace safety has been taken care of by the reworking of some classes of additives into more environmentally acceptable forms. Some trends are the increased use of additive concentrates or masterbatches and the replacement of powder versions by uniform pellets or pastilles which release less dust and flow more easily. Moreover, the current move to multicomponent formulations of stabilisers and processing aids in a low- or nondusting product also takes away the risk of operator error, aids quality control, ISO protocols and good housekeeping. An additional benefit is more homogeneous incorporation of the additives in the polymeric matrix. 

It was assumed that the production of most obvious CRMs (i.e. simple matrix CRMs for classical parameters that were already produced in the past) could be tackled by commercial companies. The role of the European Commission is felt to be necessary when the technological risks for CRM production are significant. The development of CRMs is not a normal market since it involves very specialised manufacturing procedures. No commercial company can afford to develop CRMs in support of many purposes. Quality is the key word commercial materials might not be of sufficient quality to respond to the demand. The EC should act where national initiatives cannot comply with the demand. Participants considered that reference materials should be certified at the EC level, not at the national level, and that commercial products should refer to primary (certified) materials. A scheme should be developed for the mutual acceptance of materials, e.g. establishing a kind of EC label of quality in this context, accreditation of RM producers should become mandatory. 

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