Semi-quantitative risk assessment methods

What is LOPA. LOPA is a simplified semi-quantitative risk assessment technique and a hazard analysis tool, mainly deployed to assess the adequacy of IPLs (a requirement set forth in CCPS document). As LOPA is a semi-quantitative method, it uses numerical quantities to estimate the parameters needed to... 

Taking up the authors preliminary studies (Leksin et al. 2013, Mock et al. 2012) and linking up the indexing method and semi-quantitative risk assessment approaches result in an enhanced QRA audit methodology. Within the scope of this methodological and operational framework, the paper is structured as follows ... 

Specialized techniques known as risk assessment are created as part of the developing science of safety. They are based on the qualitative, quantitative, semi-quantitative and simulation methods, using control and data acquisition as well as computer databases, operating system based on GIS. 

LOPA is a semi-quantitative tool for analyzing and assessing risk. This method includes simplified methods to characterize the consequences and estimate the frequencies. Various layers of protection are added to a process, for example, to lower the frequency of the undesired consequences. The protection layers may include inherently safer concepts the basic process control system safety instrumented functions passive devices, such as dikes or blast walls active devices, such as relief valves and human intervention. This concept of layers of protection is illustrated in Figure 11-16. The combined effects of the protection layers and the consequences are then compared against some risk tolerance criteria. 

Consequence assessment for the purposes of establishing design basis differs from consequence assessment in the context of a risk analysis study (see Qualitative and Quantitative Methods, below). A qualitative, or semi-quantitative (order of magnitude) consequence severity estimate may suffice for the latter. 

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. 

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... 

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... 

We suggest a semi-quantitative approach where the initial part of the risk assessment process is carried out in a workshop by the use of expert elicitation. The information gathered in the workshop is then refined by the risk analysts. The alternative safety measures are categorised by cost-effectiveness to provide support for decision-making. In the proposed method evaluation of cost-effectiveness is based on calculated expected values as in a traditional cost-effectiveness analysis (e.g. expected cost per ejqiected nmnber of lives saved), as well as uncertainties. 

Quahtative risk assessments also have their limitations - these are well-known. Without quantification it is difficult to discern what is important and what can be removed from further evaluation. The semi-quantitative approach recognizes this by seeking a balance between the different types of methods. 

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. Oiganizations calculate a proposed inspection frequency by considering the consequences of failure and the likelihood of failure based upon specific factors, which affect internal corrosion rates. Practitioners of RBI look at the effects of operating temperature and pressure, metalluigy, and the aggressive (or passive) nature of the process, possible contaminants, and recent inspection results in this semi-quantitative assessment. The method employs a risk matrix. 

SESSION II Risk Theory and Risk Analysis for Landslides. [1] Landslide Risk Management concepts and framework and examples (2.5 h) [2] Deterministic and Probabilistic models for slope stability evaluation (2 h) [3] Introduction to modelling of catastrophic landslide events (2 h) [4] Empirical models for travel distance (1.5 h) [5] Application examples of probabilistic methods and semi quantitative methods for landslide hazard zonation (2h) [6] Landslide Frequency Assessment (1.5 h) [7] Different components of vulnerability to landslides. Prevention and long term management of landslides (3.5 h) [8] Case Studies coal waste dump risk assessment, example from motorway in La Reunion Island, Aknes Rock slope in Norway (2 h) [9] Application of QRA to other geotechnical problems Internal erosion of dams, crater lake hazard (1.5 h) [10] Advanced numerical models initiation of landslides, propagation of sediments/climate change effects (3.5 h). 

This procedure relies on a semi-quantitative method of assessing the potential risk by looking at the health aspects. It is based on five parameters ... 

The first definition begs the question What does several mean In practice it is likely to be very difficult to assess SIL requirements unless there is a set of agreed definitions of the parameter values, almost inevitably in terms of quantitative ranges. These may or may not have been calibrated against the assessing organisation s risk criteria, but the method then becomes semi-quantitative (or is it semi-qualitative It is certainly somewhere between the extremities of the qualitative / quantitative scale.)... 

L. D. Radu, Qualitative, Semi Quantitative and Quantitative Methods for Risk Assessment ... 

Many authors (Kentel and Aral, 2004, Faber and Stewart, 2003, Nilsen and Aven, 2003, Tixier et al., 2002, Pender, 2001, Harms-Ringdahl, 2001, Wang and Ruxton, 1997) have discussed the limitations of traditional semi-quantitative methods for Occupational Safety Risk Assessment (OSRA) and state that the kinds of uncertainties include data scarce or incomplete, measurement error, data obtained from expert judgment, or subjective interpretation of available information which cannot be treated solely by traditional statistical or probabilistic methods. So, probabilistic OSRA... 

There are a number of methods for evaluating risk. The method applied for any particular risk will depend on a number of factors, such as the complexity of the activities carried out and the type and nature of the workplace. For many of the day-to-day risks that people in the workplace are exposed to, including fire, a simple qualitative assessment will suffice, for more complex risks a quantitative or semi-quantitative assessment may be needed. 

The use of semi-quantitative techniques for risk assessment is widespread and it is often referred to as a quantitative method however, it is easiiy seen that risk is expressed as a numericai vaiue, the estimation of the magnitude of the risk is in fact subjective and therefore quaiitative. A semi-quantitative evaiuation of risk aiiows numericai vaiues to be assigned to both severity and iikeiihood in the absence of data. 

The use of the combination of indexing and semi-quantitative methods in audits for individual risk assessment in the field of fire and explosion protection... 

Semi-quantitative methods use quahtative as well as quantitative evaluation procedures. Methods of indexing and ranging of risks also belong to this method. From the practical point of view, it is difficult to appraise the value of indexing methods for risk analysis purposes. Unlike quahtative methods it gives a quantitative assessment of a risk level, but differs in straightforwardness demanding for computer recourses. It also uses a small number of parameters to assess the efficacy of the FEP measurements. 

The combination of system susceptibility and threat properties for attack probability is influenced by the DREAD Risk assessment model [12] and the OWASP Likelihood assessment method. Like in the DREAD approach, we estimate the probability by summing up system susceptibility and threat property values. In combination, the four properties allow a semi-quantitative assessment of the probability. 

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