Journal of Loss Prevention in the Process Industries
A matrix-based risk assessment approach for addressing linear hazards such as pipelines
Introduction
The Major Industrial Accident Council of Canada (MIACC, 1997) document “Risk Assessment Guidelines for Municipalities and Industries—An Initial Screening Tool” gives guidance as to reasonable risk thresholds for varying land uses. Further, the screening tool gives guidance as to how to estimate risk levels based on fixed facilities while stating:
Obtaining individual risk contours for a transport corridor is not straightforward. The issue arises though the ‘per unit length’ frequency units that are used in transportation problems.
A second MIACC document “Land Use Planning With Respect To Pipelines—A Guideline for Local Authorities, Developers and Pipeline Operators” (MIACC, 1998) also avoids giving an approach as to how to address the issue of risk calculation with respect to linear risk sources such as pipelines. The document does however suggest that consultation should take place between the various parties when new development occurs based on a separation distance of 200 m. The document then goes on to suggest that this consultation distance should be expanded for high-vapor pressure liquids or sour gas pipelines. The goal of this consultation process is to help local authorities in establishing appropriate setbacks, which are not too restrictive or too conservative.
In the absence of clear guidance as to how to estimate pipeline risk organizations such as the Alberta Industrial Heartland Association (2003) have developed their own risk assessment approaches. These types of approaches suffer based on limited data and do not readily reflect the operation of the reviewed pipeline systems, as the operating companies are not involved in the assessment process. As a result these types of risk assessments tend to be overly conservative and have the potential to misinform the public regarding the potential risk levels. This potential overstatement of risks can be of particular concern when pipeline risks are compared to risks related to fixed facilities where more straightforward risk assessment approaches exist.
The lack of accessible guidance with respect to conducting pipeline risk assessments creates a situation where it is critical that pipeline operators readily assess their pipeline systems in a clear manner and be in a position to share this information when needed as per the approach suggested within the MIACC guidelines. In this way local governments can be provided with appropriate information for land use planning purposes allowing for informed and balanced decision making.
Risk can be defined as the product of the likelihood of an event and its consequence. Within the current risk assessment a collection of matrices has been defined in order to allow this relatively simple risk calculation to be computed numerous times so as to give a measure of risk at both a system level and across the system. The various matrices have been defined based on geography whereby 100 m lengths or 1 ha partitions (100 m×100 m) have been utilized. 100 m was selected as it represents a distance where factors associated with data quality, computational needs, and geometry can be best balanced. At shorter distances data quality and computational needs become an issue. At larger distances errors associated with geometry assumptions can become significant. In addition, assumptions as to the uniformity of the risk levels within the measured cell partitions can also become an issue for larger distances.
Through working with and defining the various matrices key information related to the pipeline system can be readily extracted from the risk assessment:
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Failure rate as a function of location (leak and rupture rates were calculated for each 100 m of the pipeline based on 9 different failure modes).
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Distribution of consequences following a release for each hectare of land within 1 km of the point of failure (multiple scenarios were considered and accounted for day and night conditions, varying wind directions, explosive yields…).
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Geographic risk estimates for each hectare of land within 1 km of the pipeline (this data was also utilized to give risk contours relative to the pipeline).
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Societal risk measures for each hectare of land within 1 km of the pipeline.
The net result is that the company can readily view the risk associated with its pipeline systems from a number of perspectives and is positioned to address those areas representing greatest concern. Further, through an iterative process the pipeline operator can review various risk reduction strategies so as to determine which activities yield the greatest value by balancing cost against the effectiveness of each strategy.
Section snippets
Failure rate determination
As was stated above, failure rates were calculated for each 100 m section of the reviewed pipeline with this calculation being based on the following failure models:
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Internal corrosion
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External corrosion
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Stress corrosion cracking
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Earth movement
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Over-pressure
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Valve/fitting failure
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Construction/material defect
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Third party damage
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Other/miscellaneous failures
With the exception of the stress corrosion-cracking component the various models were developed based on Alberta Energy and Utility Board (EUB) (1998)
Consequence analysis
Data held by the US Department of Transportation—Office of Pipeline Safety (US DOT—OPS) (2003) related to natural gas liquids pipeline releases were reviewed allowing for a determination of the ignition probabilities and explosion probabilities (delayed ignition) as a function of release size. The resulting ignition probabilities were then modified based on the properties of the potentially released materials relative to the “average” natural gas liquids released and for land use types. For
Geographic risk
One common treatment utilized in conjunction with the MIACC methodology is to estimate pipeline risk as per the methodology given for a fixed facility. The resulting distances to the various risk contours estimated in this manner are then applied perpendicular to the pipeline to create risk corridors aligned with the pipeline. Typically people equate the MIACC failure rates with 1 km of pipeline and as such this approach results in situations where the distance to the risk contours is
Modeled outcomes
The MIACC guidelines strive to measure risk acceptability from the perspective of the potentially impacted communities. However, this risk measure does not indicate if an activity is acceptable from the perspective of the operating company. Both of these perspectives need to be reviewed in order for a risk assessment to be considered complete. In this particular case the company utilized a risk scoring methodology that assigns values to various outcomes across several categories when judging
Societal risk
Societal risk based measures, viewed from the perspective of a corporation, attempt to gauge the impact the surrounding community will have on the corporation as a result of the potential impact the corporation has on the community. The non-linear relationship between fatality number and risk value reflects the greater outrage a community will express as related to multiple fatality events compared to individual fatality events. This greater outrage has the potential for greater regulatory
Conclusions
Due to the linear nature of pipelines most risk assessment techniques fail to generate risk values that can be readily compared to fixed facilities. This difficulty in analysis combined with limited agreement on how best to conduct these types of assessments has resulted in a situation where various levels of government are being hampered with respect to land use planning. As a result it is critical that companies involved in pipeline operations conduct their own risk assessments and be in a
References (12)
- Alberta Energy and Utility Board (1998). Pipeline performance in Alberta...
- Alberta Industrial Heartland Association (2003). Regional pipeline corridor and setback...
- Burgess, D. S., & Zabetakis, M. G. (1973). Detonation of a flammable cloud following a propane pipeline break. The...
- CCPS (1996). Guidelines for evaluating process plant buildings for external explosions and fires, New York: American...
- EPA (1999). Final Longhorn Pipeline Environmental...
- European Gas Pipeline Incident Data Group (2002). 5th EGIG Report 1970–2001, Gas Pipeline...
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