Quantitative Risk Assessment
The Major Accident Hazard scenarios as identified in the HAZID are taken forward into QRA for evaluation of the probability of undesired events and the severity of harm or damage being caused, together with the value judgements made concerning the significance of the results.
Major Accident Hazards Screening
On completion of the HAZID study and upon review of other sources of hazard information, the major accident hazard events will be identified.
The definition of a Major Accident is:
An ‘Uncontrolled Occurrence’ in the operation of a site which leads to Catastrophic or Severe consequences to People, Assets, Environment and / or Reputation. The consequences may be immediate or delayed and may occur outside as well as inside the site. There will also be a high potential for escalation.
Note: Examples of ‘Major Accidents’ would include, but are not limited to:
- Loss of containment of flammable and/ or toxic fluids leading to fire, explosion and/ or toxic injury
- Events resulting in structural failure which could lead to further progressive structural collapse
- Other external hazards affecting onshore sites
The definition of ‘Major Accident’ specifically excludes ‘Occupational Accidents’ which have bounded, albeit possibly severe or catastrophic consequences. This means that one or more pedestrian fatalities resulting from a road accident on a site would not be defined as a ‘Major Accident’. Similarly, one or more fatalities resulting from a fall from a scaffolding platform would not be defined as a ‘Major Accident’.
The methodology adopted here for identification of Major Accident Hazards is:
- Identify all the hazards described in the Hazard & Effect Registers that have a severity rating of Severe or Catastrophic to either People, Environment, Assets or Reputation
- Eliminate any Severe or Catastrophic events which do not meet the Company definition of a Major Accident Hazard
- Eliminate any occupational accidents
QRA Rule Set
All the data from the data collection activities will be compiled into a report (rule set) for submittal to COMPANY for approval. The rule set identifies all the parameters and assumptions that form the basis for the QRA.
Quantitative Risk Assessment
The QRA studies shall be conducted in accordance with the COMPANY Technical Guidance for QRA.
The MAH scenarios as described in the previous sections are taken forward into QRA for evaluation of the probability of undesired events and the severity of harm or damage being caused, together with the value judgements made concerning the significance of the results.
The frequency assessment will be carried out :
- Parts count of equipment, to the level of pipeline/ pipe work lengths, flanges, valves, instrument connections etc
- Location assessment of the parts count
- Calculation of the frequency of failure of each of the parts, by location, using internationally recognised and validated failure rate data
- Event Tree Analysis (ETA)
The consequence analysis will be undertaken in a series of steps:
- Identification of all fuel types, as determined by their operating conditions and components. A list of typical fuels inventories will be generated as the basis for consequence analysis
- Assessment of available inventories for release, as defined by isolatable sections of plant, and times to isolation of ESD systems, and failure on demand of ESD systems
- Calculation of release rates
- Estimation of potential pool sizes from liquid releases, based upon release rates and durations, evaporation rates and local topography
- Calculation of potential flammable vapour cloud sizes
- Calculation of the extent of thermal, toxic, and overpressure effects distances
- Time dependant analysis based upon time criteria derived from ESD systems response time
- Suitable sensitivity analyses, including meteorological conditions, topography, surface roughness, operating conditions, isolation times/regimes etc. The sensitivities to be undertaken will be formalised during the data gathering stage and early part of the consequence analysis stage of the project
The calculated impacts shall be presented as contours overlaid on plot plans.
The risk calculation software proposed for the study is DNV SAFETI. The output from the frequency analysis and the consequence analysis will be used to calculate risk, in conjunction with:
- Fatality probabilities
- Escalation criteria
- Weather data
- Equipment layout
- Probit equations (where appropriate)
Risk assessment will be undertaken for the following:
- Location Specific Individual Risk (LSIR)
- Individual Risk Per Annum (IRPA)
- Societal Risk (FN Curve)
- Probability of Loss of Life (PLL)
Individual Risk per Annum (IRPA) will be calculated for the different worker groups. The data provided by the COMPANY regarding occupancy of the personnel around the site will also be used to calculate each worker groups IRPA profile. The resultant IRPA values will be compared to Company’s risk acceptance criteria.
Societal risk shall be calculated for all the potentially impacted population both on and off site, and presented as an FN curve. A typical FN curve is presented in Figure 4‑3 on the next page.
The FN curve shall be compared to Company and other risk acceptance criteria (see below).
Probable Loss of Life (PLL) shall be calculated as derived from the IRPA values and the numbers of each worker group.
Risk Acceptance Criteria
When undertaking a QRA the output from the study has to be assessed for acceptability against a certain acceptability criterion.
The level of risk calculated for employees and the general public is studied in two principal areas. Firstly, Individual Risk, which is the level of risk of fatality for specific people, usually defined as individuals within specific groups. The groups are defined by their activities in or around the facilities. The second area is risk to society, which is a measure of the risk of fatality within the general public.
The process for calculating the levels of risk associated with the facility is:
- Calculate the Location Specific Individual Risk (LSIR) from Major Accident Hazard events using the QRA model
- Define the worker groups and groups of the general public by their location and movements within the effects zone of the facility
- Calculate Individual Risk (IR) figures for each group using the LSIR data
- Use the QRA model to calculate societal risk
Individual Risk values are calculated for each identified group of people and compared to the Individual Risk criteria.
Sensitivity analysis shall be developed to identify which variables have significant impact upon the calculated risk profiles. This will be utilised to identify which variables bring uncertainty into the QRA, for example:
- Meteorological conditions
- Dispersion end points
- Effect of isolation philosophy
- Ignition probabilities
Risk Reduction Measures
The output from the QRA will be used to determine suitable and practicable Risk Reduction Measures to reduce the risk from this aspect of the project to As Low As Reasonably Practicable (ALARP).
The culmination of all the processes discussed above shall be a demonstration that the risks are ‘As Low As Reasonably Practicable’ (ALARP). The essence of the ALARP principle is a demonstration that all practicable Risk Reduction Measures (RRM) have been identified and assessed appropriately (through uncertainty analysis and cost benefit analysis) and implemented (or planned to implemented.