Soil moisture content is the main factor that controls how far and at what concentration natural gas spreads from a leaking pipeline underground, according to a new study.
Pipeline operators should consider how the amount of water found in the surrounding soil affects gas movement when trying to determine potential hazards posed by a pipeline leak, said SMU’s Kathleen M. Smits , who led the study recently published in the journal. Elementa which examined the soil properties of 77 locations across the country where a gas leak had occurred.
“We need look no further than Dallas or Georgetown, Texas, to see where underground pipeline leaks can have catastrophic consequences,” said SMU Chair of Civil and Environmental Engineering Smits. Lyle School of Engineering and Solomon Professor for Global Development. “We often find that such incidents are the result of a lack of clear protocols for detecting leaks or assessing damage. That is why more emphasis needs to be placed on the importance of environmental factors such as the humidity of the soil and how to properly account for them in leak scenarios.”
In general, the team, co-led by Colorado State University Energy Institute researcher Younki Cho, found that methane gas escaping from a pipeline does not not spread as far as soil moisture content increases. This translates to a higher concentration of methane gas near the site of the leak in wetter soil, the study found.
The reverse was true with drier soil.
But Smits pointed out that simply knowing how wet the soil is at the time of the leak is not enough to draw conclusions about the impact of soil moisture content on gas movement. Soil moisture – or lack thereof – at the time of the leak triggers different complex behaviors in the soil as methane gas infiltrates the same spaces as water and oxygen in soil pores. Soil moisture content can also change over time due to weather conditions and other factors such as seasonal groundwater levels.
“You have to understand how humidity controls both movement and focus,” Smits said. “It’s something we can help [pipeline owners] to move forward in resolving leak incidents. »
The research team examined more than 300 soil samples from leak sites across the country. The samples – which were taken at the time of the leak and again after the leak was repaired – were weighed when wet. They were also weighed a second time after being dried in an oven.
“The difference between dry and wet weights, together with knowing the volume of the soil sample, allowed us to calculate soil moisture,” Smits explained.
Other soil qualities like its texture and permeability were also examined by the team, but did not show as much impact on how natural gas moved underground.
Other co-authors were Nathaniel L. Steadman, Department of Civil Engineering, University of Texas at Arlington; Bridget A. Ulrich, University of Minnesota Duluth Natural Resources Research Institute; Clay S. Bell, CSU Energy Institute; and Daniel J. Zimmerle, director and senior director of the Methane Emissions Technology Assessment Center at CSU.
Foot surveys are more effective at detecting pipeline leaks than mobile detection in some circumstances
In another study aimed at improving gas leak detection, Smits and researchers at CSU’s Energy Institute found that there are instances where using a mobile detection unit from the front or roof of a car is not as effective as walkers carrying a portable detection device.
In a mobile survey, the detection unit measures high levels of methane in the air while the car is driving. Higher travel speed or wind speed outside the car reduced the likelihood of leak detection compared to a foot patrol.
“For example, if you just isolate the speed of travel – comparing a person walking at 2-3 miles per hour to a car traveling at a slow speed of 20-30 mph – the probability of detecting a leak drops by 85 % for a walk survey at 25% for a car,” Smits said.
The study, published in the journal Environmental pollution, showed that atmospheric stability also had an effect on mobile surveys.
Atmospheric stability essentially determines whether the air will rise, fall, or do nothing. Warmer, less dense air rises (unstable), while cooler, denser air sinks (stable). Air remaining at the same altitude is considered neutral.
The researchers found that mobile surveys conducted at speeds between 2 and 11 miles per hour became progressively less efficient (by 85% to 60%) in finding a leak as atmospheric stability changed from extremely unstable to extremely stable conditions. . Foot surveys conducted under these same conditions did not reflect the variability.
“On-foot surveys find by far the most leaks, but they are labor intensive and cost a lot of money,” Smits noted. “This study shows that if operators want to use an alternative method such as a mobile survey, they must wisely choose an appropriate survey speed under different weather conditions to achieve a detection probability equivalent to the traditional on-foot survey.”
The findings could apply to any type of buried pipeline, Smits said.
CSU’s Cho, Bell, Cho and Zimmerle participated in this study, as did Energy Institute researcher Stuart N. Riddick. Shanru Tian, a Ph.D. student in the Department of Civil Engineering at the University of Texas at Arlington, was the lead author of this study.
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