Reports detail unusual patterns in some gas leak incidents.
A number of proximate residents detected the odor of gas but only one notified the gas company. At 8:55 a.m. (one hour elapsed time), homes within about 150-240 feet from a compression coupling that had been pulled loose as a result of the hooked line exploded and three people died.
Subsequent tracer studies using R-12 (a freon) indicated dispersion (flow) of gas through backfill, consisting of rock fragments incapable of passing through a 5-inch sieve and around gas, water, and sewer lines which extended to the outer walls of the foundations of the proximate residences. Entry into the residences was through utility connections that penetrated the outer foundation walls of the homes. The fill for utility lines consisted of the following (artificial) horizons:
1. Approximately 8 inches of fine top soil,
2. Approximately 5 to 5.5 feet of large rock fragments (>5 inches in dimension), and,
3. An underlying layer of 8 inches or more of fine soil.
[FIGURE 1 OMITTED]
It was concluded that the fugitive gas could have reached the residences within a period of a half hour or less. In this particular incident, the resistance to gas flow was essentially nonexistent, in other words, the "fill" material possessed such great void ratio that it assisted in the flow of gas from the leak site to the residences.
This example could be classified as construction damage coupled with a man-made conduit for gas transmission. The response time in this incident was excessive. In fact, gas flow was not interrupted for at least one hour after the incident.
Portales, NM, June 28, 1982. On or about May 23, 1982, a contractor excavating a trench to install communication system lines, hooked a 3/4-inch distribution line to a single residence (Figure 1). Originally, the contractor thought the line was abandoned. However, as he attempted to remove the line, he realized that it was an active gas line.
The gas company was notified and a serviceman plugged the line. The serviceman advised the residents that their gas service was interrupted and would be repaired the next day. On May 24, a repair crew replaced the line from the street side of the cut, in other words, the farthest point from the affected residence to the meter set at the residence and, subsequently, relit the pilot lights within the residence.
On June 28, some 36 days after the initial event, the residence which was without gas service on May 23-24 exploded. Six people died.
Later, the gas line from the residence to the distribution main was excavated and it was found that a 65-inch section of pipe between the area in which the 3/4 inch pipe was originally hooked, had been pulled away from a 17-year-old compression coupling as a result of the hooking of the line some 36 days previously. The line pressure was 25 psig. Thus, the "pipe" from the main to the meter set at the involved residence consisted of about 85 feet of pipe plus 65 inches of "dirt pipe," in other words, an area in which gas at 25 psi was transmitted through the void in the soil left by the extracted pipe such that the residence's appliances could still be lit.
Further, it was observed that the soil efficiently adsorbed the gas odorant such that no odor was present to warn of the presence of fugitive gas. The migration pathway from the pipe breach to the meter set was about 60 feet. Bar-hole surveys indicated a large area of gas concentration about the leak site. The migration pathway from the leak to the involved residence was predominantly along the trench that had been cut to repair the damaged pipe from the area of the hooked pipe to the residence.
Subsequent bar hole surveys indicated a major underground gas accumulation of about 270 feet by 170 feet and peripheral gas accumulation of about 380 feet by 220 feet (Figure 2).
[FIGURE 2 OMITTED]
This example is indicative of the fact that damage to pipe is not always immediately apparent. In other words, a significant period of time can transpire between pipe damage and ultimate, untoward results.
Bowie, MD, June 23, 1973. On Jan. 19, 1973, a scorched area (flash fire "signature") was observed in a residence's garage area. The incident was reported to the fire department which investigated and concluded that the flash fire was of unknown origin. On the date of this incident there had been 0.27 inches of rainfall. Between Jan. 19 and June 23, 1973--, a total of 155 days--no gas odors were detected in the area. In the 48-hour period before June 23, a total of 1.62 inches of rain fell in the area. On June 23 the same residence which had experienced the flash fire in January exploded (Figure 3).
Investigation revealed a circumferential crack in a 1/2-inch 20 psig service line some 120 feet from the involved structure. Geological survey of the area indicated horizons of clay, gravel, and hardpan clay, from the surface to below the depth of the involved pipe. It was concluded that gas--trapped within the gravel layer--was forced upward by the rising water table during the previous day's rain, thereby introducing sufficient gas into the structure so that ignition occurred.
Subsequent to the accident, the gas accumulation area was determined to measure about 520 feet by 170 feet (Figure 4).
This example is indicative that gas can escape from a pipe, occupy a region of (relative) void space, and be displaced laterally by a rising water table to escape into an area which is not "capped" by a water-sealed (rained upon) surface. Such escape routes would include residences with breaches in slabs or residences possessing pilings or other direct contacts with underlying porous soil. As a result of this incident three people died and one was seriously injured.
Allentown, PA, June 9, 1994. On May 23, 1994, in an attempt to remove an abandoned fuel oil tank, soil was excavated from beneath a 2-inch, 55-psig natural gas supply line. This line was visually observed to "sag."
On June 9,--some 18 days later--gas odors were noted in the area and, shortly thereafter, an explosion occurred in a retirement residence. Subsequent to the event it was discovered that a compression coupling downstream of the unsupported pipe had pulled loose as a result of mechanical stresses induced on the coupling by the sagging of the unsupported pipe. Fugitive gas entered in breaches in the foundation wall of the incident structure and was transmitted to various levels of the structure by elevator shafts and utility chases prior to ignition.
The leak was most probably of short duration, in other words, an hour or less. This particular example is indicative that seemingly "unimportant" observations, for example, mechanical stresses induced by removal of or subsidence of support of a pipe, may result in untoward results. The leak path length in this incident was less than five feet. As a result of this incident one person died, 66 were injured and more than $5 million in property damage occurred.
These incidents reveal the following valuable information:
1. Leak durations before incidents may be short (minutes, hours) or long (weeks, months),
2. Area geology (and local, man-made geology in, for example, fill materials) may, and often does, dictate fugitive gas migration pathways, i.e., fugitive gas will follow the path of least resistance,
3. Parameters of weather, e.g., barometric pressure changes and/or rainfall, may alter local geology in terms of sealing ground surfaces, altering water tables, or inducing volume changes, with the concomitant result that fugitive gas is displaced laterally and/or upward in a relatively short period of time to produce explosive atmospheres within structures, and,
4. Path lengths for migration of fugitive gas may vary markedly, i.e., they are unpredictable, and will depend on local geology as well as length of time that a leak has existed.
What Can You Do?
Explosions, fires and personal injuries resulting from underground gas leaks almost always result in costly and lengthy litigation. Plaintiff's attorneys frequently cite such items as inappropriate response time, inadequate emergency procedures (or failure to follow proper procedures), improper or inaccurate locating and marking of pipeline facilities, failure to adequately train personnel in emergency procedures, failure to stop the flow of gas in a timely manner, and an insufficiency of odorization agent. This list should not be construed as all inclusive.
[FIGURE 3 OMITTED]
Gas utilities can refine emergency response procedures by a number of means, including studying and appreciating NTSB Pipeline Accident Reports, communicating within the gas industry and sharing lessons learned, setting up internal practices for periodic review of all emergency plans and ensuring that personnel are adequately trained to carry out these procedures.
It would be nearly impossible for emergency plans to address every potential incident in minute detail; however, general procedures should recognize each segment of operations and system architecture, such as multiple pressure levels and piping material. Such plans should clearly address proper call-out procedures, availability of personnel and equipment, system sectionalization procedures in case a segment of a system is shut down, procedures for requesting assistance from police and fire personnel, and other details appropriate or unique to each utility's needs. Here again are opportunities for the gas utilities to share information to enhance the overall safety of the industry.
Finally, it is virtually impossible to define the local geology of a transmission, distribution, or service pipeline system such that the migratory consequences of a pipeline leak can be predicted. Any "minor" event such as damage to any pipeline should be treated with great respect. Always expect the unexpected.
[FIGURE 4 OMITTED]
Note: This report was originally presented at the Gas Technology Institute symposium entitled "Emergency Response Planning--Best Practices" in Orlando, FL. The available, legible National Transportation Safety Board Pipeline Accident Reports have been compiled and indexed in CD format by subject, word, date and place. This compilation is available through GTI.
The more recent accident reports at NTSB (dating back to only 1996) are available in full text at http://www.ntsb.gov/Publictn/P_Acc.htm. Other, older NTSB pipeline accident reports are available from the National Technical Information Service, Springfield, VA in hard copy. Summaries of the reports can be found at http:/biblioline.nisc.com under NTISRECS for a fee. A complete set of the NTSB Pipeline Accident Reports (in hard copy) costs in excess of $2,000. An annotated compilation of these reports, capable of being printed in their original format, is available through GTI.
Anon., 49 CFR 192, Subparts L and M.
Hotz, D. H., "Response to Odor-Related Incidents," Odorization III, Institute of Gas Technology, (IGT) Chicago, 1993.
Jacobus, J., "A New Look at Warning Agents for Fuel Gases", IGT Odorization Symposium, Chicago, 1994.
Jacobus, L, and J. S. Roberts, "The Adequacy of Odorization of Fuel Gases," IGT Odorization Symposium, Chicago, 1995.
Jacobus, J., and J. S. Roberts, "The Relationship Between Gas Composition and Odor Intensity," IGT Odorization Symposium, Chicago, 1996.
Kiefner, J. E, and E. B. Clark, "History of Line Pipe Manufacturing in North America," American Society of Mechanical Engineers, New York, 1996.
Little, R.W., "Utility Odorization Practices and Response to Odor Related Incidents," Odorization III, IGT, Chicago, 1993., p. 653.
Southard, J. S., "Investigation of Incidents in Which Odorization is an Issue: A Litigation Oriented Approach," Odorization III, IGT, Chicago, 1993.
National Transportation Safety Board PARs Are Rich Source Of Information
One of the principal sources of information concerning pipeline accidents is the National Transportation Safety Board's Pipeline Accident Reports (PARs). These reports, approximately 100 in number, have been issued since 1970 on an interim basis. Until recently these reports, comprising approximately 4,000 pages total, had not been compiled. The compiled, annotated reports are now available.
Natural gas emissions ("leaks") in transmission and distribution systems can occur for a variety of reasons, including, but not limited to:
1. Damage to pipe during construction activities with concurrent line or connector failure,
2. Damage to pipe during construction activities with subsequent line or connector failure,
3. Defective pipe,
4. Thermal effects, and
(Authors Note: It is instructive to analyze the reports concerning gas leaks and subsequent migration of the gas, including some of the more interesting and instructive reports discussed below. Perhaps the most important lesson for those who respond to leak reports is to always expect the unexpected.)
John Jacobus has a bachelor of science degree in chemistry from Southwestern at Memphis and a doctor of philosophy degree in organic chemistry from the University of Tennessee. After a 25-year teaching career (Princeton, Clemson, Tulane) he entered the consulting profession. He resides" in Atlanta where he provides consulting and expert witness services in a broad range of fuel gas issues involving gas odorants and gas related incidents.
Arthur G. (Buddy) Yaeger, Jr. has a bachelor of science degree in chemical engineering from Tulane University and retired from Entergy Corp. with nearly 37 years experience in operations and engineering management in the natural gas' distribution business in New Orleans. He is president of Natural Gas Distribution Consulting, inc. where he provides consulting and expert witness services in a broad range of gas distribution operations, practices and procedures.
By John Jacobus, Jacobus and Associates Corp., Atlanta, GA, and Arthur G. (Buddy) Yaeger, Jr., Natural Gas Distribution Consulting, Inc., Georgetown, TX
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|Title Annotation:||Gas Leak Migration|
|Author:||Jacobus, John; Yaeger, Arthur G., Jr.|
|Publication:||Pipeline & Gas Journal|
|Date:||Oct 1, 2007|
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