U.S. Department
of Transportation

Pipeline Safety Stakeholder Communications

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Fact Sheet: High Consequence Areas (HCA)

Quick Facts:

  • Consequences of inadvertent releases from pipelines can vary greatly, depending on where the release occurs, and the commodity involved in the release.
  • Releases from pipelines can adversely affect human health and safety, cause environmental degradation, and damage personal or commercial property.
  • Pipeline safety regulations use the concept of “High Consequence Areas” (HCAs), to identify specific locales and areas where a release could have the most significant adverse consequences. Once identified, operators are required to devote additional focus, efforts, and analysis in HCAs to ensure the integrity of pipelines.

What criteria define HCA’s for pipelines?

Because potential consequences of natural gas and hazardous liquid pipeline releases differ, criteria for HCAs also differ. HCAs for natural gas transmission pipelines focus solely on populated areas. (Environmental and ecological consequences are usually minimal for releases involving natural gas.) Identification of HCAs for hazardous liquid pipelines focus on populated areas, drinking water sources, and unusually sensitive ecological resources.

  • Populated areas include both high population areas (called “urbanized areas” by the U.S. Census Bureau) and other populated areas (areas referred to by the Census Bureau as a “designated place”).
  • Drinking water sources include those supplied by surface water or wells and where a secondary source of water supply is not available. The land area in which spilled hazardous liquid could affect the water supply is also treated as an HCA.
  • Unusually sensitive ecological areas include locations where critically imperiled species can be found, areas where multiple examples of federally listed threatened and endangered species are found, and areas where migratory waterbirds concentrate.

HCAs for natural gas transmission pipelines:

  • An equation has been developed based on research and experience that estimates the distance from a potential explosion at which death, injury or significant property damage could occur. This distance is known as the “potential impact radius” (or PIR), and is used to depict potential impact circles.
  • Operators must calculate the potential impact radius for all points along their pipelines and evaluate corresponding impact circles to identify what population is contained within each circle.
  • Potential impact circles that contain 20 or more structures intended for human occupancy;, buildings housing populations of limited mobility; buildings that would be hard to evacuate (e.g., nursing homes, schools); or buildings and outside areas occupied by more than 20 persons on a specified minimum number of days each year, are defined as HCA’s.

How do operators of pipelines know where HCA’s are located?

  • High population areas and other populated areas are identified using maps and data from the U.S. Census bureau.
  • Critical drinking water sources and unusually sensitive ecological areas are identified using information from National Heritage Programs and Conservation Data Centers in each state, in conjunction with The Nature Conservancy.
  • Because of the complexity of HCAs for Hazardous Liquid Pipelines, the Office of Pipeline Safety identifies and maps HCAs for Hazardous Liquids on its National Pipeline Mapping System ( NPMS). These maps are revised periodically by OPS based on new and updated information.
  • Operators of natural gas transmission pipelines must use a specified equation to calculate the radius of “potential impact circles” along their pipeline and compare the structures in those circles to the HCA criteria in the rule.

How do operators determine what pipeline segments require extra integrity protection due to the presence of HCAs?

  • Pipeline operators must determine which segments of their pipeline could affect HCAs in the event of a release. This determination must be made assuming that a release can occur at any point, even though the likelihood of a release at any given point is very small.
  • Hazardous liquid pipelines that pass through an HCA, or that pass near enough that a release could reach the area by flow over land or within a river, stream, lake, or other means, are assumed to have the potential to affect that area.
  • Gas transmission pipelines that pass within any of the HCA potential impact circles are assumed to have the potential to affect that area. (Or, alternatively, operators may choose to treat all of their pipeline segments in Class 3 and 4 areas as HCAs.)

Date of Revision: 12012011