The objectives of the QRA are to determine the location specific individual risk (LSIR) associated with the process facility in order to aid the decision making process. It is aimed
SP-1258 Specification: Quantitative Risk Assessment & Physical Effects Modelling
The methodology adopted for the same has been described below
Figure 1 – QRA Methodology
Hazard identification means the identification of undesired events that lead to the materialization of the hazard and the mechanism by which those undesired events could occur.
The main hazard from process facilities is that of an uncontrolled release of flammable material which may subsequently be ignited and toxic material. The causes and consequences of the major hazards with respect to the plant would be studied under this task.
A technique commonly used to generate an incident list is to consider potential leaks and major releases from fractures of all process pipelines and vessels. This compilation includes all pipe work and vessels in direct communication, as these may share a significant inventory that cannot be isolated in an emergency. The following data were collected to envisage scenarios:
The consequence modelling evaluates the resulting effects if the accidents occur, and assessing their impacts. Estimation of the consequences of each possible event is performed using QRA software tool.
Following consequence outcome events are considered for process releases:
For each hazardous scenario, consequence modelling is conducted to determine the area impacted by each consequence event. For each consequence event, the extent of the impact of the effects depends on the conditions specified for the hazardous scenario. These include:
An important component of risk analysis is the estimation of the likelihood or frequency of each failure case or release scenario. None of the events considered in this analysis are common and major catastrophic events are very rare. Leak frequencies were developed using a parts count, and event tree analysis was used to evaluate likelihood of success or failure of release mitigation safeguards onsite, specifically the potential for detection and isolation of leaks.
This section presents a brief description of the methodology and approach adopted for the frequency analysis. To calculate the risk associated with a LOC scenario, it is necessary to estimate the failure frequency. The frequency of occurrence of such an event is based on the probability of the LOC scenario and the presence of constraints that influence the development of the event. In case of leakage in the pipeline systems, flammable substances may be released into the atmosphere. This can occur in the form of a small gasket failure in a flanged joint, a bleeding valve, inadvertently left open valves, failure of pipeline, corrosion, or any other external factors.
The risk criteria are based on the HSE Corporate risk of the company. The IRPA has been compared against the individual risk tolerability criteria as defined in the HSE corporate risk is used to determine whether the total IRPA is considered broadly acceptable, tolerable or intolerable.