They wind underground beneath homes, across plains and through the state's most populous cities.

And, according to a Houston Chronicle investigation, more than half of the major natural gas transmission lines in Texas were laid more than 40 years ago and now are vulnerable to failure.

Nationwide, the issue of pipeline safety took on more urgency in September, when a natural gas transmission line exploded in San Bruno, Calif., killing eight people and destroying three dozen homes. The National Transportation Safety Board said it has not ruled out pipeline age, or associated problems with welds and corrosion, as potential causes. The Pacific Gas & Electric Co. pipeline in San Bruno was installed in 1956.

In Texas alone, more than 25,000 of nearly 46,000 miles of transmission pipe are older than 1970, some dating to the Great Depression, according to federal records.

Federal regulators warned companies more than 20 years ago to reconsider the use of all pipelines built with lower-quality welding techniques that were widely employed in pipe factories prior to 1970, documents show. Also a potential problem is some aged protective coating on pipes that actually can make them more vulnerable to corrosion, according to a number of pipeline experts.

"Older pipelines have properties that are inherently inferior," said John Kiefner, a pipeline integrity consultant in Ohio who has 43 years of experience.

Jurisdictional issues

One piece of good news for Texans: The Texas Railroad Commission, which regulates pipeline safety, has imposed stiffer inspection requirements on pipeline companies than the federal government or other big states like California. It is currently considering even more regulations.

But there is a shortage of government inspectors and a lot of pipe. Twenty-eight state inspectors oversee the nation's largest complex of intrastate pipelines, each one theoretically responsible for 6,000 miles of state-regulated pipe, and 90 inspections per year, according to commission data.

Among those calling for more pipeline inspectors is David Porter, a conservative Republican oil industry accountant from Midland who was elected earlier this month to the three-member Railroad Commission.

But Texas is crisscrossed by 21,000 miles of interstate natural gas transmission pipelines that are under federal jurisdiction, but not that of the Railroad Commission.

Texas experienced at least two accidents in the past five years where pipeline age appears to have been a factor, according to federal records.

On May 13, 2005, an underground natural gas pipeline exploded near Marshall, spewing nearly six times the amount of gas released in San Bruno, sending a giant fireball into the sky and hurling a 160-foot section of pipe (much bigger than the one in San Bruno) onto the grounds of a nearby electric power generating plant, according to an investigation by the federal Office of Pipeline Safety. The rural location may have spared lives: Only two power plant workers were injured and 40 residents within a one-mile radius were evacuated.

Old coatings vulnerable

No independent investigation was conducted by the NTSB, but the OPS concluded that stress corrosion cracking was the culprit. The interstate pipeline — installed four decades earlier in 1967 - sprang a leak where the metal had rotted. The asphalt enamel coating on the exterior of the pipe was a type "typically associated with and susceptible to stress corrosion cracking," said a report by An-Tech Laboratories in Houston.

Experts say several types of coating on old pipe - including asphalt - are vulnerable to failure. With newer pipe, the industry relies on a superior fusion-bonded epoxy coating, among other improvements.

When the OPS issued a corrective action order to the pipeline owner two weeks after the accident, the first reason it cited was the "age of the pipe," followed by the inherent hazard of natural gas, the high operating pressure of the 3-foot-diameter pipe, and the fact that other sections of the pipeline course through populated areas and next to busy highways.

Pipeline owner Natural Gas Pipeline Company of America, a subsidiary of Houston-based Kinder Morgan Inc. had documented "numerous instances" of stress corrosion cracking along the same pipeline, which crosses Texas and terminates near Chicago, as well as along interconnected systems in Texas, Oklahoma, Arkansas and Iowa, according to the federal report. Also, a massive release of gas with one of the company's 1957-vintage gas pipelines in Oklahoma in August 2003 was blamed on stress corrosion cracking, and in that case, too, the OPS cited pipeline age.

Larry Pierce, Kinder Morgan's vice president for corporate communications, said in an e-mail that the accidents weren't "strictly" age-related, though he acknowledged the type of pipeline coating was a major factor. The cracking "is the result of a combination of many factors, including pipe materials, type of coating applied to the pipeline and the presence of certain soil conditions," he said.

Pierce said the company and its subsidiaries - which control more than 7,500 miles of natural gas transmission lines in Texas - has developed a program to cure its corrosion cracking problem, including an extensive pipeline inspection and replacement effort that exceeds federal guidelines.

In another unpublicized accident that might have been age-related, a ConocoPhillips pipeline split open on Jan. 8, 2008, near Denver City, in West Texas, spilling 1.3 million gallons of crude oil. The OPS focused its investigation - and a $200,000 fine - on the fact that the company failed to detect and stop the leak for more than 24 hours. But it also cited a cause - a weld that split open along the horizontal seam of the pipe due to fatigue - that's often associated with pipe older than 1970. The pipe was made in 1948. ConocoPhillips declined comment. Data wasn't available on the age of ConocoPhillips pipe in Texas.

Congress began imposing federal construction standards on new natural gas pipelines in 1968, resulting in major improvements in welds, coatings and pipeline inspection techniques. But the old pipe remained in the ground.

After deadly and destructive pipeline accidents in the 1980s, the NTSB noticed trouble with the welding techniques that had been used in factories prior to 1970 to knit together the seams of pipelines. The OPS issued two warnings about its findings to pipeline companies in 1988 and 1989, but no widespread replacement or blanket inspection program was ordered.

Replacement carries cost

Congress finally ordered inspections of some big pipelines in 2002, after a pair of deadly pipeline accidents, including one in New Mexico in 2000 that killed 12 campers and was caused by corrosion in a 1950-vintage pipe. But inspections were limited to high-population areas.

The Railroad Commission acted sooner and more thoroughly than Congress, requiring in 2001 that all intrastate high-pressure transmission lines in the state be tested.

After the San Bruno explosion, there were renewed calls to require pipeline companies to replace questionable older transmission pipes with newer, safer ones.

"There's a point where you just want to go to the new technology and go ahead and replace the pipe," said pipeline safety expert Rick Kuprewicz of Redmond, Wash.

But some say that would be both costly and unnecessary.

Charles Yarbrough, vice president of rates and regulatory affairs at Atmos Pipeline-Texas, one of the state's largest pipeline operators, said pipe pulled from the ground in bone-dry West Texas, where corrosion isn't as big an issue, can appear fairly new after three-quarters of a century.

"New pipe going in the ground is around $1 million to $1.5 million a mile," said Yarbrough. "It is not something where you want to just say, 'It is getting old, let's go and replace it all.' "

enalder@hearst.com