Risk acceptance criteria are the limits above which an operator will not tolerate risk on the installation. These criteria must be defined for each type of risk to be assessed. Similar to the traditional pipeline RBI, the subsea equipment RBI quantifies risk from the aspects of Safety, environment, and economy. Most importantly, the safety level depends on product, manned condition, and location class. If the product is toxic or the location is in a sensitive area, then the safety class should be considered to be high.
The risk acceptance criteria are used to derive the time of inspection, which is carried out prior to the acceptance limit being breached. This would allow either the reassessment of the risk level based on better information, a detailed evaluation of any damage, or the timely repair or replacement of the degraded component.
The acceptance criteria are defined for each of the different consequence categories. Acceptance criteria may be based on previous experience, design code requirements, national legislation, or risk analysis. The acceptance criteria for a function may be “broken down” into acceptance criteria for the performance of the individual items comprising the function.
Generally, due to the quick reaction ability of extensive valves and sensors, the main risk is the economic loss of the subsea tree or manifold. For a pipeline and riser, however, safety, environmental, and economic risks should be considered.
For each type of item and for each deterioration process, the acceptance criteria with regard to personnel risk, environmental impact, or economic risk may be represented by a risk matrix as illustrated in Figure 11.3.
Figure 11.3. Illustration of the Principle of Acceptance Criteria and Risk Matrix [2].
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Risk Assessment Methodology
Yong Bai, Wei-Liang Jin, in Marine Structural Design [Second Edition], 2016
38.3 Risk Acceptance Criteria
38.3.1 General
How safe is safe enough? Risk acceptance criterion defines the overall risk level that is considered acceptable, with respect to a defined activity period. The criteria are a reference for the evaluation of the need for risk reducing measures, and therefore need to be defined prior to initiating the risk analysis. Additionally, the risk acceptance criteria must reflect the safety objectives and the distinctive characteristics of the activity.
The risk acceptance criteria may be defined in either qualitative or quantitative terms, depending on the expression for risk. The basis for their definition includes:
•Governmental legislation applicable to the safety in the activity
•Recognized industry standards for the activity
•Knowledge of accidental events and their effects
•Experience from now and past activities
According to the purpose and the level of detail for the risk analysis, the acceptance criteria may be:
•High-level criteria for quantitative studies
•Risk matrices and the ALARP principle
•Risk comparison criteria
Fischhoff et al. [1981] identified and characterized various methods for the selection of risk acceptance criteria. They indicated that values, beliefs, and other factors all influence the selection of risk acceptance criteria. The complexity of defining risk acceptance criteria should be explicitly recognized, due to the uncertainty of their definition, lack of relevant facts, conflicting social values, and disagreements between technical experts and the public. The selection of risk acceptance criteria is subject to a rigorous critique, in terms of philosophical presuppositions, technical feasibility, political acceptability, and the validity of underlying assumptions made about human factors.
38.3.2 Risk Matrices
The arrangement of accident frequency and the corresponding consequences in a matrix [see Figure 38.3] may be a suitable expression of risk where many accidental events are involved or where single value calculations are difficult. The matrix is separated into three regions, including:
Figure 38.3. Risk matrix.
•Unacceptable risk
•Acceptable risk
•A region between acceptable and unacceptable risk, where evaluations have to be carried out in order to determine whether further risk reduction is required or whether more detailed studies should be conducted.
The limits of acceptability are set by defining regions in the matrix, which represent unacceptable and acceptable risk. The risk matrix may be used for qualitative and quantitative studies. If frequency is classified in broader categories such as rare and frequent, and consequences are classified as small, medium, and catastrophic, the results from a qualitative study can be shown in the risk matrix. The definition of the categories is particularly important in the case of qualitative use.
The categories and the boxes in the risk matrix can be replaced by continuous variables, implying a full quantification. An illustration of this is shown in Figure 38.4.
Figure 38.4. Risk matrix in terms of continuous variables.
The following are examples of situations where the use of a risk matrix is natural:
Evaluation of personnel risk for different solutions such as integrated versus separate quarters.
•Evaluation of risk in relation to operations such as exploration drilling.
•Evaluation of risk in relation to a particular system such as mechanical pipe handling.
•Evaluation of environmental risk.
38.3.3 The ALARP Principle
The ALARP [“as low as reasonably practicable”; see Figure 38.5] principle is sometimes used in the oil and gas industry [UK HSE, 1992]. The use of the ALARP principle may be interpreted as, satisfying a requirement to keep the risk level “as low as possible” provided that the ALARP evaluations are extensively documented. In the ALARP region [between “lower tolerable limit” and “upper tolerable limit”], the risk is tolerable, only if risk reduction is impracticable or if its cost is grossly disproportionate to the improvement gained. The common way to determine what is practicable is to use cost–benefit evaluations as a basis for the decision on whether certain risk reducing measures should be implemented. A risk may not be justified in any ordinary circumstance, if it is higher than the “upper tolerable limit.” The “upper tolerable limit” is usually defined, whereas the “lower tolerable limit” may sometimes be left undefined. This will not prohibit effective use of the approach, as it implies that ALARP evaluations of risk reducing measures will always be required. The ALARP principle used for risk acceptance is applicable to risks regarding personnel, the environment, and assets. Trbojevic [2002] illustrated the use of the ALARP principle for a design.
Figure 38.5. The ALARP principle.
38.3.4 Comparison Criteria
This type of criteria is suitable for more limited studies that aim at comparing certain concepts or solutions for a particular purpose with established or accepted practices. The criteria are suitable in relation to operations that are often repeated such as drilling and well interventions, heavy lift operations, and diving. The use of the comparison criteria requires that the basis of the comparison be expressed precisely.
The formulation of the acceptance criterion in this context may be that the new solution cannot represent any increase in risk, in relation to current practices.
Examples of comparison criteria are:
•Alternative design [or use of new technology] for a fire water system can be at least as safe as conventional technology
•The risk level for the environment cannot be higher than with the existing solution
•Alternative solutions can be at least as cost-effective as the established practice
This type of risk acceptance criteria is also suitable for risk regarding personnel, the environment, and assets.
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Probability of ship collision and grounding
Shengming Zhang, ... Richard Villavicencio, in Probability and Mechanics of Ship Collision and Grounding, 2019
1.1.2 Risk acceptance criteria
For a rational reduction of risk related to hazards, such as ship collisions and grounding events, it is necessary to establish a risk acceptance criterion. Without a generally agreed acceptance criterion, it is not possible to find the balance between safety in terms of risk reduction and the cost to the stakeholders.
Usually, risk acceptance criteria must be established for three main types of risks:
•Fatalities
•Pollution of the environment
•The loss of property or financial exposure
The acceptance criteria for fatalities related to shipping accidents are normally based on two principles:
•The individual fatality risk shall be approximately the same as typical for other occupational hazards.
•The frequency of accidents with several fatalities, that is, the societal fatality risk, shall not exceed a level defined as unconditionally intolerable, and moreover, the general as low as reasonably practicable [ALARP] risk management shall be applied. Fig. 1.1 illustrates the principle of the ALARP criterion.
Fig. 1.1. Typical risk acceptance criterion, F-N diagram.
The latter criterion is introduced because society is often more concerned about single accidents with many fatalities than many accidents with few fatalities per accident.
If the risk is estimated to be in the ‘intolerable’ region in Fig. 1.1, then the activity should not be allowed. For risks that are not in the negligible region, the general ALARP risk management shall be applied. That is, in principle for all nonnegligible risks, it is required that measures for risk reduction are to be identified and analysed and their societal value assessed. In this ALARP region, an economic criterion can be applied to consider the effectiveness of safety measures or risk control options; see Eq. [1.2].
Similarly, ALARP criteria have been used by authorities to reduce accidental oil spills from tankers. Here, the cost for preventing an oil spill accident must be based on the cost of oil, the clean-up cost, the environmental damage cost, etc. Shipping and grounding accident costs are often dominated by clean-up costs due to oil spillage.
Authorities like International Maritime Organization [IMO], International Association of Classification Societies [IACS], and national administrations normally focus on these types of risks one after the other. That is, so far, the ALARP principle has been applied separately for fatalities and for environmental impacts when considering new rules. The general economic costs associated with severe accidents have not been considered very often.
With a significant percentage of all ships involved annually in a serious and costly accident, the economic loss will have a significant influence on the outcome of expressions such as Eq. [1.2]. This indicates that it is often possible to introduce risk mitigation measures that can be cost-effective. One example is improved navigational safety measures that influence all three risk categories at the same time.
Ideally, the additional cost of risk-reducing measures in the form of construction cost plus present value of operational costs is evaluated against the effect of the risk in the ALARP region. The condition for a decision to introduce risk-reducing measures could be.
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