Guidance on Safety Risk Assessment for Chemical Transport Operations
*October 2013*
TABLE OF CONTENTS
- Introduction
- Purpose and objectives
- Scope
- General introduction to risk assessment
- Qualitative risk analysis
5.1 Consequence analysis
5.2 Probability analysis 5.3 Risk matrix - Accident scenarios with potential high consequences
6.1 Accident scenarios 6.2 Accident types that can cause scenarios with potential high consequences
6.3 Analysis of scenarios with potential high consequences - Quantitative risk analysis
- Risk mitigation 8.1 Reduction of probability/frequency of occurrence of accidents 8.2 Reduction of potential consequences
Disclaimer
This document is intended for information only and sets out guidance for safety risk assessment for chemical transport operations. The information contained in this guidance is provided in good faith and, while it is accurate as far as the authors are aware, no representations or warranties are made with regard to its completeness. It is not intended to be a comprehensive guide to all the detailed aspects of safety risk assessment for chemical transport operations. No responsibility will be assumed by Cefic in relation to the information contained in this guidance.
1. Introduction
It is of key importance for the chemical industry to ensure the safe transport and handling of its products, in full compliance with regulations and industry best practices. As part of its Responsible Care programme, Cefic is developing and promoting best industry practices (such as SQAS, BBS and ICE Emergency Response) aiming at continuously improving the safety performance of chemical transport activities. Despite all these preventive actions, transport accidents can however still happen. Since these accidents take place in the public domain, they often attract a lot of attention.
Increasing urbanization in combination with higher aversion to risk of the society, may result in more restrictions on the transport of dangerous goods (e.g. restrictions on the transport mode or transport route that can be used or compulsory transportation time windows).
Risk assessment is an important tool that should be used by companies to manage the risks of their transport operations. In order to assist companies in carrying out transport risk assessments, Cefic has developed this guidance note. It provides general advice on how the safety and environmental risks of transport operations can be assessed, taking into account already existing best practices.
2. Purpose and objectives
The purpose of this document is to provide general guidance on safety risk assessment for chemical transport operations. This should allow the identification of transport activities with the highest potential risk towards people, infrastructure and the environment. This guidance should assist chemical and transport companies in carrying out risk assessments of their transport operations. This should help companies in identifying transport activities with high potential risks, to choose the safest transport mode or route and to implement other risk mitigation measures. This document does not aim to provide detailed guidance on risk assessment methods.
3. Scope
The scope of this guidance covers off-site inland transport operations of dangerous goods by road, rail and inland waterways. Transport by pipeline is not included in the scope of this guidance. The focus is on events with high potential impact on people, infrastructure and the environment.
4. General introduction to risk assessment
The following activities are normally undertaken when carrying out a risk assessment (see also Figure 1):
- Risk analysis is the systematic analysis of all available information to identify hazards and their consequences, the potential exposure to these hazards, and the probability of their occurrence, in order to estimate the risk. The outcome of a risk analysis provides information on the risk of the transport operation under consideration. The purpose of the risk analysis is to derive potential consequences connected with specific accident scenarios and the probabilities of their occurrence.
- Both qualitative and quantitative risk analysis methods can be used. Qualitative risk analysis is used as a first step in the overall risk assessment process, so that attention can be focused on higher risk scenarios using quantitative methods of risk analysis if needed.
- Risk evaluation is the evaluation of the acceptability of the identified risk. To allow a systematic risk evaluation, risk criteria need to be defined to determine whether a given risk level is acceptable or not.
- Risk reduction: If the estimated risk of the transport activity under consideration is considered as not acceptable, measures need to be taken to reduce the risk.
Figure 1. Schematic overview of the main steps of the risk assessment process
flowchart TD
Start --> RISK_ASSESSMENT
RISK_ASSESSMENT --> Definition_of_the_system
Definition_of_the_system --> Hazard_identification
Hazard_identification --> Probability_analysis
Probability_analysis --> Consequence_analysis
Consequence_analysis --> Risk_estimation
Risk_estimation --> Risk_Criteria
Risk_Criteria --> Risk_evaluation
Risk_evaluation --> Decision{Acceptable_Risk?}
Decision -->|Yes| Stop
Decision -->|No| RISK_REDUCTION
RISK_REDUCTION --> RISK_ASSESSMENT
Ref. PIARC Technical Committee C 4 Road Tunnel Operation: Technical Report “Risk Evaluation” Draft Version 5.0 (April 2010)
5. Qualitative risk analysis
Qualitative risk analysis should be used as a first step in the overall risk assessment process. This allows filtering out the lower risk activities so that attention can be focused on higher risk scenarios. Qualitative risk analysis methods do not use precise numeric values. A commonly applied method to support the classification of risks in a qualitative approach is the use of a risk matrix. When carrying out a qualitative risk analysis it is important to maintain consistency in the approach throughout the whole process in order to ensure that the results are based on the same assumptions.
The procedure for qualitative risk analysis consists of the following steps:
- Definition of the transport operation to be analyzed and identification of all relevant hazards involved in the transport operation;
- Consequence analysis: investigation of the potential consequences taking into account product hazards and potential exposure to these hazards;
- Probability analysis: determination of the probabilities of exposure to certain hazards.
5.1. Consequence analysis
A consequence analysis aims to assess the potential consequences of a transport accident by analyzing the hazards of the transported product (hazard severity analysis) and the potential exposure to these hazards in case of an accident (hazard exposure analysis).
5.1.1. Hazard severity analysis
Identification of the potential product hazards and their severity, for example by using the existing UN hazard classification system for dangerous goods transport which is based on the hazard class, the packing group (PG) (see annex 2) and the hazard identification number (HIN) (see annex 3), in combination with the volume of the transport container (i.e. packed or bulk*).
Example of a hazard severity ranking system (see also Figure 2)
- Hazardous goods with a low potential impact: hazardous goods of Packing Group III transported in bulk* and not fulfilling the criteria of very high potential impact goods (see below);
- Hazardous goods with an intermediate potential impact: hazardous goods of Packing Group II transported in bulk* and not fulfilling the criteria of very high potential impact goods (see below);
- Hazardous goods with high potential impact: hazardous goods of Packing Group I transported in bulk* and not fulfilling the criteria of very high potential impact goods (see below);
- Hazardous goods with very high potential impact:
- Goods that are toxic by inhalation (TIH), transported in any quantity.
- Goods transported in bulk* with one of the following ADR/RID hazard identification numbers:
- flammable gases with HIN 23, 263, 238, or 239;
- toxic gases with HIN 26, 265, or 268;
- highly flammable liquids with HIN 33, 333, 336, 338, 339, X323, X333, or X338;
- highly toxic liquids with HIN 66, 663, 664, 665, 668, 669, 886, X88, or 668.
- Bulk means goods transported in tank vehicles, tank containers, rail tank cars or tank barges
Figure 2. Example of hazard severity ranking system
| Hazard severity (potential impact) | Criteria | Score (A) |
|---|---|---|
| Low potential impact | PG III in bulk* | 1 |
| Intermediate potential impact | PG II in bulk* | 2 |
| High potential impact | PG I in bulk* | 3 |
| Very high potential impact | - Toxic by inhalation in any quantity - Flammable gases in bulk* - Toxic gases in bulk* - Highly flammable liquids in bulk* - Highly toxic liquids in bulk* | 4 |
5.1.2. Hazard exposure ranking
Identification of the potential exposure to the transport hazard based on population densities along the transport route and environmental considerations (proximity of drinking water reservoirs, water courses or protected nature areas). The score (B) is based on the most severe ranking based on either population density or proximity of environmental sensitive areas. The scoring should always be used consistently, for example when comparing the hazard exposure ranking of different routes.
Figure 3. Example of hazard exposure ranking system
| Population density along the transport route | Proximity of environmentally sensitive areas ** | Score (B) |
|---|---|---|
| Low | Very distant | 1 |
| Intermediate | Distant | 2 |
| High | Close | 3 |
| Very high | Very close | 4 |
** Drinking water reservoirs, water courses or protected nature areas
5.1.3. Total consequence ranking
By combining the hazard severity ranking (A) and the hazard exposure ranking (B), the total consequence ranking is obtained. The result should be used to set priorities and to decide whether further steps in transport risk analysis for a transport operation should be undertaken. Total consequence ranking can be done by using a simple scoring approach (see example in Figure 4 below). Based on the total consequence ranking, a selection of transport operations can be made that require further risk analysis.
Figure 4. Example of a total consequence ranking system
| Hazard Severity Ranking Score (A) | Hazard Exposure Ranking Score (B) | Total score |
|---|---|---|
| 4 | 4 | 16 (IV) - Very high |
| 4 | 3 | 12 (IV) - Very high |
| 4 | 2 | 8 (III) - High |
| 4 | 1 | 4 (III) - High |
| 3 | 4 | 12 (IV) - Very high |
| 3 | 3 | 9 (III) - High |
| 3 | 2 | 6 (III) - High |
| 3 | 1 | 3 (II) - Moderate |
| 2 | 4 | 8 (III) - High |
| 2 | 3 | 6 (III) - High |
| 2 | 2 | 4 (III) - High |
| 2 | 1 | 2 (II) - Moderate |
| 1 | 4 | 4 (III) - High |
| 1 | 3 | 3 (II) - Moderate |
| 1 | 2 | 2 (II) - Moderate |
| 1 | 1 | 1 (I) - Low |
5.2. Probability analysis
The probability analysis aims at identifying the probability of occurrence of a transport hazard, taking into account the average accident frequencies for the transport mode being assessed. Data on transport accident frequencies can be difficult to find, in particular data on frequencies of accidents with loss of containment. Transport accident frequencies are normally expressed as the number of accidents per distance driven by the transport vehicle (truck, train, barge).
Annexes 4 and 5 provide examples of accident frequencies mentioned in the Purple Book. More country/product/mode specific accident frequencies can be found in literature.
5.3. Risk matrix
By a combination (multiplication) of the total consequence ranking (see Figure 4) with the probability of accidents (accident frequency), a risk matrix is obtained which allows the classification of individual accident scenarios on a numerical scale (see example in Figure 5 below). Such a risk categorization may be used for the comparison of risks and the identification of scenarios that warrant further investigation and consideration of risk mitigation measures (see Section 8).
Figure 5. Example of a risk matrix
| Total consequence Ranking | Probability of incidents | ... |
|---|---|---|
| Very unlikely | Not likely | |
| Very high consequences (IV) | ... | ... |
| High consequences (III) | ... | ... |
| Moderate consequences (II) | ... | ... |
| Low consequences (I) | ... | ... |
- Risk Category
- Very high risk - 4
- High risk - 3
- Moderate risk - 2
- Low risk - 1
Practical examples illustrating the qualitative risk analysis are given in Annex 6. These examples are based on serious accidents that have happened in the past.
Note: By using the definition of risk as the combination (multiplication) of consequence and probability, one may obtain the same risk value for accidents with high probability and low consequences as for accidents with low probability and high consequences. The risk perception by the general public of these two types of accidents may be completely different. The general public is in general more concerned about accidents with a high impact (e.g. many fatalities in one accident) than about ‘smaller’ accidents happening frequently. To take these different kinds of risk perception into account, an additional factor called ‘risk aversion’ can be used for evaluating the total risk.
6. Accident scenarios with potential high consequences
The following accident scenarios with potential high consequences can be identified for the most commonly transported chemical products. The transport of explosives (class 1) and radioactive materials (class 7) have not been taken into consideration in the selection of these scenarios.
These scenarios can be caused by different accident types (collision, overturned vehicle, derailment etc.) that can create an impact with sufficiently high energy required to damage the containment of the product.
6.1 Accident scenarios
- UVCE (Unconfined Vapor Cloud Explosion)
- Hot BLEVE (Boiling Liquid Expanding Vapour Explosion)
- Toxic vapor cloud release
- Pool fire
- Jet fire
- Spillage of substances harmful for the environment
6.2 Accident types that can cause scenarios with potential high consequences
- Energy is required to initiate these scenarios: either kinetic energy (high speed) or potential energy (fall).
- For gaseous substances small leaks are most of the time enough to generate scenarios with a potential high consequence. Gas containers have however a higher shell thickness than liquid containers and can therefore withstand higher energy impacts.
- For liquids large leaks are required to create scenarios with a potential high consequence. To create such large leaks, an accident with sufficient energy to damage the containment is required.
- The direct vicinity of the accident has also an impact:
- Presence of other dangerous goods (e.g. in freight trains)
- Traffic density surrounding the transport vehicle (e.g. on a congested road)
- Population density alongside the transport route
- The proximity of environmentally sensitive areas: river, protected nature area...
- Leaking transport equipment (valves, man-lids etc.) is not included in the scenarios as this is under control of the loaders.
6.3 Analysis of scenarios with potential high consequences
UVCE (Unconfined Vapor Cloud Explosion)
- Flammable cargo (gas or liquid)
- Delayed ignition
- Instantaneous impact
- High impact range
- Nearby populated areas will be impacted
Hot BLEVE (Boiling Liquid Expanding Vapor Explosion)
- Flammable liquid, flammable gas or peroxide
- Heating source or exothermic reaction necessary
- Takes time to develop
- High impact range
- Nearby populated areas will be impacted
Toxic vapor cloud release
- Toxic gas or toxic volatile liquid (toxic by inhalation hazard – TIH)
- Instantaneous impact
- High impact range
- Nearby populated areas will be impacted
Pool Fire
- Flammable liquid
- No instantaneous impact – time needed to have sufficient leaked product
- Ignition source needed
- Impact by thermal radiation or fire propagation
- Impact limited to direct surroundings
- Presence of other Dangerous Goods (e.g. other Rail Tank Cars in same train) could create domino effect (such as a hot BLEVE). A pool fire under the leaking tank could also generate a hot BLEVE.
Jet fire
- Flammable pressurized cargo (gas or liquid)
- Same conditions as UVCE but with instantaneous ignition
- Instantaneous impact
- Fire propagation risk
- Impact limited to direct surroundings
- Presence of other Dangerous Goods (e.g. other Rail Tank Cars in same train) could create domino effect, like hot a BLEVE
Liquid or solid spillage of environmentally hazardous substance
- Low energy accident can be sufficient for creating leak
- Close proximity of a sensitive zone is required
- Instantaneous impact
- Potential high impact range
7. Quantitative risk analysis
After identification of the high risk scenarios using qualitative methods, companies can consider the application of quantitative methods of risk analysis if necessary. Quantitative risk analysis methods should be used primarily for specific transport operations with a very high consequence ranking. Since quantitative risk analysis assessment is based on many assumptions, it is recommended to use quantitative risk analysis only for the relative ranking of different transport options. Calculation of absolute levels of risks is in most cases not meaningful due to the high degree of uncertainty of the available accident frequency data.
8. Risk mitigation
The risk of a transport operation can be reduced by taking measures that either reduce the probability (frequency) of accidents taking place or reduce the potential consequences of an accident. Some examples of risk mitigating measures are listed below (without aiming to be comprehensive). See also Annex 6.
8.1 Reduction of probability/frequency of occurrence of accidents
The following examples are focusing on road transport:
- Reduce the probability of an accident by
- reducing the total volume of transported product
- selection of the mode of transport
- selection of the route of transport
- selection of the carrier (using SQAS etc.)
- training of all people involved in the transportation process (drivers, loaders etc.)
- maintenance and inspection of the transport equipment
- systems increasing the stability of the vehicle
- taking into account weather conditions (postpone transport in case of bad weather conditions)
- taking measures to improve security
- Reduce the probability of leakage in case of an accident by:
- reducing the speed of the vehicle
- improving the quality of the containment (e.g. shell thickness of tanks)
- installing crash-buffers (on rail tank cars
