Pipelines transport roughly two-thirds of domestic energy supplies through approximately 2.5 million miles of pipelines throughout the United States. These pipelines carry hazardous liquids and natural gas from producing wells to end users (residences and businesses). Within this nationwide system, there are three main types of pipelines. Gathering pipelines. Gas gathering pipelines collect natural gas from production areas, while hazardous liquid gathering pipelines collect oil and other petroleum products. These pipelines then typically transport the products to processing facilities, which in turn refine and send the products to transmission pipelines. According to PHMSA officials, traditionally, gathering pipelines range in diameter from about 2 to 12 inches and operate at pressures that range from about 5 to 800 pounds per square inch (psi). These pipelines tend to be located in rural areas but can also be located in urban areas. PHMSA estimates there are 200,000 miles of gas gathering pipelines and 30,000 to 40,000 miles of hazardous liquid gathering pipelines. Transmission pipelines. Transmission pipelines carry hazardous liquid or natural gas, sometimes over hundreds of miles, to communities and large-volume users (e.g., factories). For natural gas transmission pipelines, compression stations located periodically along the pipeline maintain product pressure. Similarly, pumping stations along hazardous liquid transmission pipelines maintain product flow. Transmission pipelines tend to have the largest diameters and pressures of any type of pipeline, generally ranging from 12 inches to 42 inches in diameter and operating at pressures ranging from 400 to 1440 psi. PHMSA has estimated there are more than 400,000 miles of gas and hazardous liquid transmission pipelines. Distribution pipelines. Gas distribution pipelines continue to transport natural gas to residential, commercial, and industrial customers, splitting off from transmission pipelines. These pipelines tend to be smaller, sometimes less than 1 inch in diameter, and operate at lower pressures—0.25 to 100 psi. PHMSA has estimated there are roughly 2 million miles of distribution pipelines, most of which are intrastate pipelines. There are no hazardous liquid distribution pipelines. However, some distribution pipelines can be as large as 24 inches in diameter and operate at higher pressures (i.e., over 350 psi). Part 191 (Gas Reporting), Part 192 (Gas), Part 193 (Liquid Natural Gas), Part 194 (Liquid Facility Response Plans), and Part 195 (Hazardous Liquid) of Title 49 of the Code of Federal Regulations. Most natural gas distribution pipelines would generally be considered to be in high-consequence areas, as defined under the transmission pipelines regulations, since they are typically located in highly populated areas. PHMSA regulates hazardous liquid and natural gas gathering pipelines— using uniform, minimum standards—based on their proximity to populated and environmentally sensitive areas. For natural gas gathering pipelines,PHMSA uses class locations—the same classification system used for natural gas transmission and distribution pipelines. (See table 1.) Under this system, PHMSA generally regulates onshore natural gas gathering pipelines in Class 2, 3, or 4 locations. For hazardous liquid gathering pipelines, PHMSA regulates those pipelines in incorporated and unincorporated cities, towns, and villages; pipeline segments that cross a waterway currently used for commercial navigation; and certain rural gathering pipelines within one-quarter mile of environmentally sensitive areas. This includes high-consequence areas, as defined for the hazardous liquid integrity management program. High-consequence areas can also be in Class 1, 2, 3, or 4 locations, which can entail different reporting requirements. For example, gathering pipeline operators in high-consequence areas that are in Class 1 locations are not required to report data on pipeline-related incidents, including fatality, injury, and property damage information. Under the current regulatory system, PHMSA does not regulate most gathering pipelines in the United States based on their location. For example, out of the more than 200,000 estimated miles of natural gas gathering pipelines, PHMSA regulates roughly 20,000 miles. Similarly, of the 30,000 to 40,000 estimated miles of hazardous liquid gathering pipelines, PHMSA regulates about 4,000 miles. However, according to PHMSA officials, the agency has the authority to collect data on all onshore hazardous liquid and gas gathering pipelines, even though it generally does not regulate gas gathering pipelines in Class 1 locations or hazardous liquid gathering pipelines not located in high-consequence areas. Generally, PHMSA retains full responsibility for inspecting and enforcing regulations on interstate pipelines. However, states may be authorized to conduct inspections for interstate pipelines, as well as inspections and associated enforcement for intrastate pipelines. States can also promulgate regulations for intrastate pipelines, including gathering pipelines. PHMSA has arrangements with 48 states, the District of Columbia, and Puerto Rico to assist with overseeing interstate, intrastate, or both interstate and intrastate pipelines. These arrangements, in which states act as “agents” for PHMSA, can cover hazardous liquid pipelines only, gas pipelines only, or both (see fig. 2). State pipeline safety offices are allowed to issue regulations supplementing or extending federal regulations, but these state regulations must be at least as stringent as the minimum federal regulations. If a state wants to issue regulations that apply to pipelines that PHMSA does not regulate, such as unregulated gathering pipelines, it must do so under its own (state) authority. While gathering pipelines generally pose lower safety risks than other types of pipelines, PHMSA does not collect comprehensive data on safety risks associated with gathering pipelines. In response to GAO’s survey, state pipeline safety agencies cited construction quality, maintenance practices, unknown or uncertain locations, and limited or no information on current pipeline integrity as safety risks for federally unregulated gathering pipelines. Operators of unregulated gathering pipelines are not required by federal law to report information on such risk factors. Consequently, federal and state pipeline safety officials do not know the extent to which individual operators collect such information and use it to monitor the safety of their pipelines. In our survey of 52 state agencies, 39 agencies—10 monitoring hazardous liquid and 29 monitoring natural gas—responded that they had onshore gathering pipelines that PHMSA does not regulate located in their state. (See app. II for a summary of our survey results.) For these 39 agencies, four of the five top responses cited the following risk factors for onshore unregulated gathering pipelines as among the highest public safety risks. Construction quality. Eighteen state agencies reported that the quality of installation procedures and construction materials is a moderate or high safety risk for unregulated gathering pipelines. The construction phase of pipeline installation is critical to ensure the long-term integrity of the pipeline because the installation methods and materials used in pipeline construction affect the pipeline’s resistance to deterioration over time. For example, one inspection requirement for regulated pipelines is that they may not be installed unless they have been visually inspected at the site of installation to ensure that they are not damaged in a manner that could impair their strength or reduce their serviceability. This requirement does not currently apply to unregulated gathering pipelines. Maintenance practices. Sixteen state agencies reported that the extent to which pipeline operators maintain their pipelines is a moderate or high safety risk for unregulated gathering pipelines. According to agency officials, after a pipeline is installed and operational, periodic maintenance—such as inspecting and testing equipment—is important to prevent leaks and ruptures and could extend the operating life of a pipeline. Furthermore, preventive measures and repairs conducted on unregulated gathering pipelines, as well as a record of such activities, could provide useful information on the safety and history of a given gathering pipeline. Location. Sixteen state agencies reported that the unknown or uncertain location of unregulated gathering pipelines presents a moderate or high safety risk. Although individual operators may know the locations of unregulated pipelines, state and local safety agencies may not know or may be uncertain about the locations and mileage of unregulated pipeline infrastructure in their communities. This information is particularly useful for “Call Before You Dig” programs operated by states and localities. If unregulated gathering pipelines are unmarked and program officials do not know the location of the pipelines, businesses and citizens may damage a pipeline during excavation, which could result in an incident—including fatalities, injuries, or damage to property or the environment—as well as the shutting down of the pipeline for repair. Pipeline integrity. Sixteen state agencies reported that not knowing or having limited knowledge about the integrity—the current condition— of unregulated gathering pipelines is a moderate or high safety risk. Factors that affect the integrity of all pipelines—such as excavation damage and corrosion—also affect gathering pipelines. For example, excavation damage to a pipeline from nearby digging activities (see fig. 3) is the leading cause of pipeline incidents and, as previously noted, the uncertain location of unregulated gathering pipelines may increase the potential for such damage. Furthermore, corrosion can occur on the inside and outside of metal pipelines and is not easily identified without appropriate pipeline assessments. From 2004 through 2010, corrosion was reported as the cause of about 60 percent—or nine incidents—of regulated gas gathering pipeline incidents. Generally, pipeline experts we spoke with said limited information on the integrity of unregulated gathering pipelines prevents analysis to assess the internal and external condition of these pipelines. According to responses to our survey and interviews with industry officials and representatives, land-use changes and the increased extraction of oil and natural gas from shale deposits are two changes in the operating environments that could increase the safety risks for unregulated gathering pipelines. Land-use changes. The fifth top response reported by state pipeline safety agencies we surveyed was that increased urbanization has caused rural areas to become more densely populated and, in some cases, developments have encroached on existing pipeline rights-of- way. (See fig. 4.) Nineteen state agencies reported land-use changes as a moderate or high risk for federally unregulated gathering pipelines. Federal and state pipeline safety officials we spoke with are concerned about the safety and proximity of people who work and live near pipeline rights-of-way.