Risk-informed plant evaluation is topical because of the recent flood of software tools on the market for managing the mechanical integrity of safety-critical equipment, e.g. pressure systems. These tools are usually presented as risk-based inspection (RBI), maintenance (RBM), or assessment (RBA) tools. The differences between risk-based management practices and risk-based evaluation (or assessment) are detailed in a best practice guide on RBI, which was prepared by TWI for the UK's Health & Safety Executive (HSE).
Risk-based management practices
Risk-based management practices include RBI and RBM. RBI and RBM are normally applied to a group of equipment on a plant. The methodology makes use of systematic assessments to identify all historical, active or potential damage mechanisms in individual equipment within the group. The associated likelihood (probability) of the damage mechanisms leading to failure, and classification of the effects (consequence) of failure of individual equipment on the overall plant, are then determined. The effects of equipment failure normally take account of production losses, health and safety, and environment damage. Once the probabilities and consequences of failure of all feasible damage mechanisms have been identified, a criticality ranking of the equipment within the group can be developed.
This criticality ranking subsequently becomes the basis for developing, for example, risk-based inspection or maintenance plans and schedules for individual equipment. The primary objectives of undertaking RBI and RBM are: (a) to improve evaluation of the mechanical integrity, and therefore safety, of critical plant; and (b) to reduce plant operating costs by extending the intervals between routine inspection or maintenance outages. RBI and RBM must be carefully directed if they are to be cost-effective. These methods can be applied qualitatively, semi-quantitatively or quantitatively. They can be tedious and time-consuming to apply, which leads to the use of database management software for administering these management systems.
Fully quantitative assessment
Risk-informed plant evaluation or assessment (RBA) involves the fully quantitative assessment of the probability of failure of a damage mechanism within a component. In contrast to RBI and RBM, RBA is rarely undertaken because it requires a structural reliability analysis which is applied for a particular damage mechanism, in a particular component, in a particular item of equipment, within a particular plant. The main difficulty in undertaking RBA is in obtaining the appropriate statistical distributions of the loading and resistance variables, as well as the computational time for each assessment.
When RBA is undertaken, detailed justification of the relevance of the underlying input information, as well as the mechanistic damage model, is usually necessary. For example, probabilistic fatigue and fracture analyses depend critically on the assumptions made about the initial distribution of flaws, the scatter in fatigue crack growth data, the spectrum of applied stress cycles, and the spread of fracture toughness data. RBA has traditionally been applied to safety-critical equipment in the nuclear industry. However, RBA is increasingly used in the fossil fuel fired power generation and the oil, gas and petrochemical industries, as a result of the growing acceptance of fitness-for-service (FFS) assessments in these industries.
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