Risk Matrix Methods

Risk matrices provide a framework for an explicit examination of the frequency and consequences of hazards. The method may be used to rank the hazards in order of significance, screen out insignificant ones, or evaluate the need for risk reduction of each hazard. A risk matrix separates the dimensions of probability (POP) and consequence (COF) into typically three to six categories (A to E in Fig. 12.9). 

There is little standardization in matters such as the size of the matrix or the labeling of the axes. 

Risk matrices may use quantitative definitions of the frequency and consequence categories or some numerical indices of frequency and consequence (e.g., one to five) before adding the frequency and consequence pairs to rank the risks of each hazard or each box on the risk matrix. The strengths of the risk matrix approach are [10] 

However, there are several problems with this approach, which are less apparent  

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For each hazard, there is potentially a wide range of accident severities. This is difficult to represent properly by traditional risk matrix methods. 

The risk matrix method can provide a quick understanding of the risk profile of the facility and can be based on judgment or be further investigated using more detailed information. This method has limitations also. It is not easy to incorporate the effects of risk reduction measures within the risk matrix, and it cannot be used for cumulative risks. 

A common method employed in qualitative safety artalysis is the use of a risk matrix method (Halebsky (1989), Tummsda and Leung (1995)). The two parartreters that are considered are the occurrence likelihood of the failure event and the severity of its possible consequences. Upon identifying all the hazards within the system under consideration, each hazard is evaluated in terms of these two parameters. The severity of all the failure events could be assessed in terms of the four categories (i.e. Negligible, Marginal, Critical and Catastrophic) as shown in Table 3.1. 

Figure 37. Risk Matrix for Swiss Risk Assessment Methods. (Ref. 42)...

Some companies are now using Risk-Based Inspections (RBI), especially for the costly preparation efforts for internal inspections. RBI is a method that is gaining credibility and popularity. Organizations calculate a proposed inspection frequency by considering the consequences of failure (as described previously) and the likelihood of failure based on the anticipated internal corrosion rates. The method employs a risk matrix. 

To complete this analysis a corporate risk management representative came to the facility to lead a local team through a semi-quantitative risk analysis methodology. The interdisciphnary team, which had received background information on the risk matrix and the semi-quantitative method that will be used, consists of... 

Once a set of recommendations has been developed, the options must be analyzed to determine the benefits, or essentially the level of risk reduction. The risk analysis method utilized to assess the baseline operational risks should be the same method used to analyze each of the potential risk mitigation options. Therefore, if a semi-quantitative technique was used to assess the baseline risk, then the same assumptions, techniques, and risk evaluation criteria (risk matrix, risk index, etc.) should be used to evaluate the effectiveness of the risk reduction options. 

First, the importance of learning lessons from past process safety incidents is highlighted in Section 3.2. The subsequent section presents preliminary hazard review procedure, risk matrix, what-if method, plot plan and layout review, pressure relief system review and fire safety design aspects. Section 3.4 presents PHA techniques and procedures hazards and operability analysis (HAZOP), failure modes and effects analysis (FMEA), instrumented protective system (IPS) design, fault trees, event trees, layer of protection analysis (LOPA) and finally SIS life eyele. The importanee of revision of PSI is highlighted in Seetion 3.5. 

Based on any unacceptable and unmitigated risk identified during hazard analysis, further risk assessment and risk mitigation techniques need to be applied. LORA and conceptual SIS designs based on Risk Matrix can be employed if a qualitative to semi-quantitative method is preferred. Fault tree and event tree analyses with a robust LOPA can be applied if a quantitative method is essential... 

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. 

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. 

The amoimt of detail and effort required increases from quahtative (Q) approach to semi-quantitative (SQ) one to quantified risk assessment (QRA). For the Q or SQ approaches, a risk matrix is a convenient method of ranking and presenting the results. It is important that the risk matrix used should be capable of... 

To a certain degree it is a matter of preference which risk analysis method to choose. If it decided to use [ISO 13849-1] for the complete design of the E/E/PE-based safety functions, maybe it is more natural to use the risk graph within that standard. Either way, these risk analysis chapters in [ISO 13849-1] and [lEC 62061] are informative and allow you to use the risk matrix in [lEC 62061] to identify which SIL is required and then transform this SIL to a PLr by using table 4 in [ISO 13849-1] (see Figure 4) and then use [ISO 13849-1] for the remaining design steps of the E/E/PE-based safety functions. 

Semi-quantitative techniques, consisting of index and matrix methods. Examples are Dow Fire and Explosion Index, Mond Index, and Pipeline Index for risk ranking of different facilities of the same type, and the matrix method for risk ranking of potential hazardous events that a given facility or organization can face. 

A safety design review process will not be effective until the participants in the review team have acquired knowledge of hazards and risks and agreed on the risk assessment methods and risk assessment matrix to be used. Chapter 7, A Primer on Hazard Analysis and Risk Assessment, serves as a basis for the education of team members. Also, a safety design review process can be successfiilly applied only if senior management has been convinced of its value. 

Comprehensive identification of hazards requires brainstorming method. It shall consider 3 Ps person, plant, and procedure. All hazards must be specifically identified for which frequency and consequence in a risk matrix (as discussed in Clause 3.0) should be considered. For better understanding, see Fig. 1/4.3.5-1. 

HERE METHOD USING RISK MATRIX IS SHOWN ONLY. OTHER METHODS ALSO DISCUSSED IN WRITE UP... 

Apply a qualitative risk-based analysis to the event to determine the event potential severity and likelihood, o A risk matrix o A risk questionnaire o Another method of assessing risk Assess the PSSR event in regard to its complexity using the risk-based analysis results. 

The Standard provides guidance and examples of methods of undertaking risk estimation. These methods include risk matrix, risk graph, numerical scoring, and hybrid approaches. Figure 9.3 shows the general elements of risk associated with a particular hazardous situation. 

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... 

In the example application the risk of lack of water supply using the matrix method, due to the possibility of the occurrence of the accident on the distribution network was determined. The one asset of risk—lack of water—was considered with respect to a water consumer as the main water recipient. 

In quantitative matrix methods for all parameters of risk a point weight is assigned. In this way it is possible to make an analysis for various undesirable events. 

The process of risk assessment consists of determining (estimation) its numerical value and comparing it with the accepted criteria values. The criteria values for the individual levels of scale depend on many factors, expert assessments, as well as accepted method of risk assessment, e.g. 2, 3,4, or 5 parametric matrix method. 

In quantitative matrix methods all the risk parameters have the relevant point weight assigned (depending on the adopted scale). To estimate the level of risk of the undesirable event occurrence, the criteria for the parameter showing the probability of the undesirable events occurrence (failure of water pipes) depending on the determined level of failure rate indexes, were proposed in Table 3. In Table 4 the values for parameter of losses caused by the undesirable event, depending on the value of the loss factor for the loss of water consumers safety, were proposed. The point scale and the criteria for the parameter V (vulnerability of the system to the undesirable event occurrence) were adopted according to the criteria available in the literature (Tchorzewska-Cieslak 2011). 

The calculation procedure presented above concerns a general characteristics of matrix methods used for risk assessment (Rak Tchdrzewska-... 

Rak X Tchdrzewska-Cieslak, B., 2006. Review of matrix methods for risk assessment in water supply system. Journal of Konbin, 1 67-76. 

The definition of water supply system failure risk should take into account both the possibility of the loss of water consumer safety as a result of lack of water or interruptions in water supply and consumption of poor quality water. Using the matrix method for Quantitative Risk Analysis (QRA), the following formula for determining the water supply system failure risk was proposed (Michaud Apostolakis 2006, Tchorzewska-Cieslak Rak 2006, Johanson 2008) ... 

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). 

With more than two branches, that is, if the given parameter consists of three options (e.g. probability of undesirable event occurrence P is defined by values P, P2, and P3), the graph becomes more complicated and difficult to read. That is why, under these circumstances, a combination of a risk graph and a risk matrix is utilized (combined method). This procedure takes into account more possible parameters of hazard effects however, the division into individual groups depends on the subjective evaluation by company s risk assessment specialist. 

This chapter presented a disruption risk assessment method for managing the supply disruptions in a global supply chain. The assessment can help practitioners to quantify risks in their supply chains based on hazards, vulnerability, and risk management practices. The disruption risk scores of suppliers facilities and transportation links can lead a company to proactively manage its suppliers. They then can use the disruption risk matrix to visualize the relative risk of all idenfified hazards. We presented a case study of a global distribution company to illustrate the application of this framework in assessing disruption risks for facilities and transportation links. This framework can be used to develop a company disruption risk profile, which in turn can be used to identify the critical network components that are prone to disruptions and to prioritize the risk mitigation activities. 

Two methods for determining the risk level in a risk assessment are commonly used in safety standards Risk matrix and Risk Priority Number. 

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