Easier from Above

How do you monitor a natural gas pipeline stretching from Texas to the heartland for leaks? Take the high road.
By Rulon Simmons

Natural gas production in the United States is booming. Each day in August 2008, wells produced 5.4 billion cubic feet more than they did a year earlier. To meet the demand for natural gas, suppliers must build longer and more complex pipelines.

Natural gas transmission pipelines typically vary in size from 6 to 48 inches in diameter. They also vary in material type and integrity. Newer pipes are usually made from steel, while older ones are often iron and tend to be more susceptible to leaking. Nevertheless, given the volatile nature of the product, gas transmission pipelines comprise one of the nation's safest modes of freight transportation. Pipeline operators have to make sure they stay that way.

To enhance the safety of gas transmission pipelines, federal regulations require that operators develop programs to assess and mitigate safety threats, such as leaks or ruptures due to corrosion. Efforts must focus specifically on pipeline segments located in highly populated and frequently used areas, such as parks. Of course, it's one thing to develop programs to enhance safety. But to actually find a leak - and find it fast -that's a different ballgame.

Northern Natural Gas (NNG) recently faced that challenge. The Nebraska-based company operates a natural gas pipeline extending from the Permian Basin in Texas to the upper Midwest. The NNG system currently includes over 15,000 miles of transmission pipeline, 5.3 billion cubic feet per day of peak capacity, and five natural gas storage facilities. NNG provides transportation and storage services to about 75 utilities and numerous end-use customers.

Recently, NNG worked voluntarily with the U.S. Department of Transportation (DOT) to prove the integrity of one of its main pipelines. This DOT collaboration qualified as a high-priority challenge for the company. The accelerated timeline increased the level of difficulty. Rick Loveless, a senior environmental specialist at NNG, quickly realized that ground-based pipeline survey techniques wouldn't get the job done in time.

The project would require an innovative approach using geographic information system (GIS) methodologies. Newly developed GIS data management techniques use a combination of technologies to monitor a pipeline's integrity. The system pulls together an in-depth comprehension of a region's geography; adds elements of computer science, cartography, and information science; and blends in sophisticated remote control techniques.

Based on his reading of the DOT's requirements and his prior experience, Loveless recommended that NNG call on ITT Corporation's Airborne Natural Gas Emission Lidar (ANGEL) Services. "I knew the service worked because of a trial survey we had done in 2006 on our Texas and Oklahoma pipeline," Loveless recalls. In that survey, ANGEL Services discovered various pipeline and facility emissions, including an underground pipeline leak.  

Safety Fast

Airborne scanning makes it possible for a company to inspect natural gas pipelines up to 100 times faster than traditional methods, and a company can detect and quantify leaks with a high degree of accuracy. It also surveys, maps, and images pipelines, providing detailed reports and covering up to 1,000 miles a day flying at 120 mph.

Differential absorption lidar (DIAL) lies at the technological heart of ANGEL Services. DIAL is a spectral laser technology that uses a set of lasers tuned to detect a specific gas absorption feature. In this case, the lasers are tuned to detect a narrow mid-wave infrared methane absorption feature. A minimum of two lasers are required, one with a wavelength tuned to a unique "on-line" absorption feature and another tuned to a nearby adjacent "off-line" wavelength where there is no gas absorption. The ratio of the reflected returns of the "on-line" and "off-line" lasers indicates the amount of methane present. DIAL lasers are neither dependent on nor affected by sunlight conditions, sun angle, seasonality, or daily temperature fluctuations. DIAL differs significantly from conventional lidar, which uses a single laser for 3D mapping. Conventional lidar does not consider spectral aspects of a target but rather uses the timed return of a pulse to accurately determine distance.

While the technology used by ANGEL Services is new to the natural gas industry, it has been proven through the years by ITT Corporation. The DIAL technology has been used to collect information and images with a wide range of remote-sensing components and systems, from weather satellites to high-altitude overflights.

Installed aboard a low-flying aircraft, ANGEL Services scans a 70-foot-wide ground track along the pipeline right of way, collecting up to 180,000 laser measurements per minute and remotely sensing methane emissions. It then images and maps both gas concentrations and plume sizes.

The aircraft typically flies at 750 feet above ground level. At that altitude, the lasers have sufficient light energy to travel to the ground, be reflected, and be detected back at the aircraft. Furthermore, the system is sensitive enough to detect any methane absorption in the laser path. Since the atmosphere itself contains some methane, flying much higher than 1,500 feet would be impractical. At higher altitudes, naturally occurring methane makes it much more difficult to detect the fugitive methane.

More than Lasers

In addition to DIAL data, ANGEL Services captures high-resolution geo-referenced digital imagery and video of the entire ground track. The imagery and the DIAL data are geo-referenced based on a sophisticated onboard system that merges GPS data with data from the aircraft's inertial measurement unit (IMU). The high-resolution imagery is used to show context for the DIAL leak indications, and the video provides right-of-way observance from a customer's desktop computer.

All of the collected data is currently shipped overnight to ITT's ground data processing facility in Rochester, New York, where it is typically processed within 24 hours of receipt. Customers are immediately notified of all significant leak indications including strength, extent, and precise location coordinates. The data is provided in an ArcGIS format, as most pipeline operators are familiar with this. ArcGIS is also used during mission planning to ingest customer-provided data regarding pipeline locations; mile markers; compressor, metering, and valve stations; areas of high consequence; and structures near right-of-ways. These GIS datasets can be overlaid with maps and other database information, and then they are seamlessly integrated into a client's existing integrity management systems. All the data collected is reported in GIS format and includes location, time, and date information for reports and recordkeeping. In the near future, ANGEL Services plans to enhance delivery of results to customers through an online application. Through this service, anyone with appropriate permission access will be able to view the results quickly from
anywhere in the world.

The ANGEL Services aircraft is a Cessna Grand Caravan 208B. It stages its operations out of smaller regional airports near the survey areas. The plane carries a pilot, co-pilot, and sensor operator. The pilots follow a route developed based on a GIS database of pipeline segment locations. The sensor operator monitors the health of the DIAL system during the mission, particularly making sure the lasers remain locked at the proper wavelengths.

Aerial scanning crews don't have to deal with obstacles that often slow down ground crews. Difficult terrain and property access issues are a thing of the past. Jeff Cullison, NNG's regional pipeline specialist, said the decision to use aerial scanning was "based on proven accuracy as well as the short time duration to cover a long distance."

Several Leaks Found

Based on ANGEL Services' experience, a failed-infrastructure leak is found at about one out of every 16 facilities. An underground pipeline leak is found every few hundred miles. The pipeline leaks are typically underground. This is not a problem for aerial scanning, however, since methane migrates up through the soil to the surface, where it is detectable from overhead.

NNG's survey covered 423 miles, and it found 11 leaks. But Cullison admitted that at first he wasn't convinced that all 11 were legitimate. "We were skeptical. So, we dismissed two of the leaks after we dug down three or four feet, and hand-held gas detection failed to locate them," he says.

NNG employed ANGEL Services again, reflying the original route. This time, a new - and major - leak was found. A pipe had cracked since the initial survey. "This crack probably wouldn't have been discovered for another year when the pipeline was next scheduled for inspection. Or it would have split open," Cullison reports.

The two leak indications initially reviewed by NNG showed up again. "The airborne laser technology showed us the full extent of the leaks. The gas wasn't coming out directly on top of the pipeline. We could have gone over the leaks several times with normal detection methods and not found them," Cullison says. Digging revealed underground flange connections where bolts had loosened. "These kinds of leaks only get worse over time and can turn into significant incidents." NNG crews responded quickly to fix the leaks and restore pipeline integrity.

"This technology's speed and degree of accuracy is well above anything else we have done," Loveless states. "So many times when we did our work, whether it was flying an airplane or using a helicopter, we relied on visuals and there was room for mistakes."

In addition to finding leaks in the NNG lines, cartographer Jeff Bradley noted that his company's use of aerial scanning provided additional safety benefits to other companies and to the public. "A couple of leaks belonged to other companies adjacent to our pipelines, and we let them know so they could take corrective action," he says.

NNG has used ANGEL Services to fly more than 3,000 miles of pipeline leak surveys and corridor mapping imagery from the Permian Basin in Texas to the upper Midwest. More than 30 unaccounted-for pipeline leaks and facility emissions have been detected, including multiple underground pipeline leaks.

Since the DOT requirements were met, NNG has contracted with ITT to perform pipeline leak surveys and corridor monitoring work in New Mexico, Texas, Wisconsin, Minnesota, Kansas, and Michigan. The contract also includes a new application of the service: aerial leak surveys and mapping of NNG's 30,000-acre storage field in Cunningham, Kansas. With natural gas use increasing in this country, along with the miles of pipeline to transport it, the DIAL technology stands poised to pay even bigger dividends.


 

Rulon Simmons is a senior scientist with ITT Space Systems Division, focusing on Airborne Natural Gas Emission Lidar (ANGEL) Services. He has extensive experience in imaging and remote sensing, including prediction of image quality and exploitation of multispectral, hyperspectral, and lidar imagery.

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