Quantitative Risk Assessment (QRA) Study

What are the Objectives ?

• • quantifying the overall risk associated with a facility or activity.
• • identifying the main contributors to the overall risk.
• • identifying measures to reduce the overall risk and analysing their effectiveness.
• • where a number of alternative options are available, determining the relative safety and cost effectiveness of the alternatives.
• • assisting in the identification of new alternatives.
• • demonstrating compliance with selected risk tolerability criteria and assisting in the demonstration and achievement of ALARP to all stakeholders.
• • as an aid to communication to the workforce and third parties of their impact on, and exposure to, risk.

However, client’s order of precedence in the project will be governing in the end and if any client specific standards available will be followed, based on client’s advise.

Governing standards

Majority of the Operators developed on the basis of methodologies and values based on the following accepted methods that are laid out in:

• • Control of Major Accident Hazards Regulations, 2015 (COMAH)
• • Reducing Risks, Protecting People: HSE’s Decision Making Process’, HSE, 2001.
• • A Guide to Quantitative Risk Assessment for Offshore Installations, 1999

However, client’s order of precedence in the project will be governing in the end and if any client specific standards available will be followed, based on client’s advise.

When to perform QRA ?

The QRA is an integral part of the ALARP process for both project activities and operations.

QRA process starts at concept stage. In some cases, coarse/preliminary QRA may be carried out during concept stage especially where several options have been identified. This allows a comparison of the relative risk levels from the different options to support the decision-making process.

Comprehensive QRA is performed during the FEED Stage of the project. This QRA shall be later updated during the EPC (Detailed Engineering) Stage based on newly established information/data. EPC QRA also forms the basis for the operations and forms part of COMAH dossier.

For existing facilities, where QRA is not available, it shall be carried out at the first available opportunity (earliest) and update the COMAH Dossier as per COMAH, 2015 regulations.

As part of COMAH update, QRA shall be reviewed to determine if significant changes to facilities, barriers in place, manning, building functionality or occupancy, etc. are observed or carried out. QRA study shall be updated based on these changes to ensure assessment reflect the updated changes.

Integrated QRA shall be developed as part of 5-year COMAH/HSEIA update to ensure integrated risk from all existing facilities including modification and brownfield projects is established.

Where no significant changes are identified over five years and outcome of previous QRA report is still applicable and technically robust, no update of QRA is required. In such case, organisations shall develop technical note justifying the same with relevant supporting documents (such as HAZID, risk register review, risk assessments, MOC, etc.) and shall submit for Regulatory Approval as part of COMAH process for regulatory approval.

How it is performed?

Step 1: Sectionalisation of -Step 1: Sectionalisation of plant under study. It is called as marking Isolatable section – plant inventory bound by location of the Emergency Shut-Down Valves (ESDVs) and Emergency Depressurisation Valves (EDPVs) within piping and equipments. Isolatable sections will be further divided into various failure cases based on the locations, process parameters, operating modes, etc. to capture appropriate risk profile and release events. These isolatable sections and failure cases will be clearly marked on the P&ID and attached to QRA Report and shall be reviewed and approved as part of milestone requirements.

Step 2: Deriving/identification of Potential Consequences- development of the top event into a serious incident depends on the effect of safety systems, prevailing environmental conditions, actions by personnel and presence of ignition sources. Development of credible accident scenarios using event trees thus provides a structure to the conceptual and physical escalation scenario analysis. Minimum following events and consequences such as Jet fire, pool fire, toxic gas dispersion, Vapour Clour Explosion (VCE), Boiling liquid Expanding Vapour Explosion (BLEVE), flammable gas dispersion etc. shall be identified. Simple example of event tree is provided in image below,

Step 3: Inventory Estimation – The material mass accumulated in all isolatable section (both static and dynamic) inventories will be calculated;

Step 4: Failure Frequency Estimation Failure frequencies are frequency of potentially hazardous events estimated from historical databases (generic failure frequencies) combined with part count information for associated isolatable section and failure scenarios (e.g. length of pipelines, number of valves/flanges, etc.). Part count methodology shall be adopted for determining failure frequencies.

Step 5: Source Term modelling An important input to physical effects calculations is the source term, i.e. the rate at which hazardous material reaches the environment and the conditions of the material (e.g. temperature, composition). For leaks from equipment, the source term refers to the mass release (or leakage) rate, calculated based on process parameters (pressure, temperature, composition), inventory and release profile A coefficient of discharge is used to account for the hole shape/type and friction loss as the material passes through the leak hole.

Step 6: Physical Effects aka Consequence Modelling The term 'effect' refers to the possible consequences from releases of hydrocarbons and toxic gases. For example, this may be the extent of a gas cloud's flammability or toxicity or it may be a measure of thermal radiation or explosion overpressure. In brief, the estimation of extent to which the released and ignited gases/liquid would affect and spread out when it is not ignited for all expected consequence outcome.

The following image shows the graph of a jet fire release,

Step 7: Impact assessment - Exposure to various consequence identified as part of physical effect modelling can lead to major injury or fatalities. As part of impact assessment, vulnerability of humans to the consequences of major hazard events at onshore and offshore installations, primarily those producing and/or processing hazardous fluids are established. The impact assessment is generally expressed in terms of lethality which is fraction/percentage of the exposed population who would suffer fatality on exposure to a given consequence level.

Step 8: Risk Modelling – Involves providing field data of Ignition probabilities for ignition sources present in and out of plant under study, on site and off-site population, building details and assigning vulnerabilities, point of interests where we require risk values basically risk receptors in software.

Step 9: Risk calculation & Presentation Risk is estimated in per year basis for any particular individual/group present in the particular place for the exposed duration in a year. It is presented in terms of Location Specific Individual Risk (LSIR), Individual Risk Per Annum (IRPA), Potential Loss of Life (PLL) and Societal risk. LSIR contours shall be generated from software to show the geographical limits of different levels of risk present in plant on top of the layout for reader’s understanding. A typical LSIR contour overlaid on a layout is shown in the following figure.

Step 10: ALARP Demonstration - ALARP demonstration is required when the calculated Individual Risk Per Annum (IRPA) for various worker group lies within tolerable region as per UK HSE risk acceptability criteria. The QRA shall provide a clear demonstration that the risk is or will be reduced to ALARP in accordance with UK HSE. ALARP process starts by identification of the Major Risk Contributors to given worker group or at a location. These major risk contributors are further drilled down to determine the top contributors to risk. Risk reduction measures are identified against these contributors to reduce the risk.

Action plan:

How Aura Veritas can help achieve your objective?

Aura Veritas QRA Experts are independent and experienced in performing more than 200+ QRAs in their professional carrier, who understands your business and the challenges that comes within, shall perform quality assessments as a third party auditor or HSE specialists to achieve the aim of improving safety and ensuring compliance to relevant laws, codes and standards.