ANAHEIM, CA. During this year’s SPIE Defense + Commercial Sensing 2017 conference, panelists from industry, academia, and government demonstrated how miniaturized sensing platforms, and the unmanned aerial vehicles (UAVs) hosting them, can improve the detection of hazardous gas leakage.
The round-table event, held in April, showcased many efforts to quantify methane, a component of natural gas. Aerial monitoring of pipelines, according to many of the presenters, will help to reduce the greenhouse gas contributor and protect the public from explosions and other harmful effects caused by emissions.
Finding the Leaks
The greater discussion of global warming has created additional emphasis on reducing breaks in the U.S. natural gas infrastructure — one that features over two million miles of distribution pipelines. Methane, when emitted into the air, absorbs the sun’s heat and warms the atmosphere.
The natural gas sector has numerous leak points, ranging from the drilling-rig starting sites, known as well pads, to specific parts of a city’s distribution system. Such localized sources create challenges for teams like those led by SPIE panelist Mark Zondlo, who has used UAVs to quantify methane emissions.
At the SPIE Defense + Commercial Sensing event, Zondlo, Associate Professor of Civil and Environmental Engineering at Princeton University, spoke about his team’s experience with sending up UAVs to quantify methane emissions — an effort that began in 2011.
“We're trying to look at leaks on the order of a well pad that is about ten meters by ten meters, or a transmission pipeline that may go 1,000 kilometers,” said Zondlo. “But the leak itself is small, so we have this really wide range of scales that we have to deal with.”
Accurate, precise, and stable gas detection via UAV is difficult, given the vehicle’s small payload, as well as possible measurement interference from the engine’s vibrations or electromagnetic interference (EMI). Even with a theoretically perfect sensor, narrow gas plumes tend to drift, and UAVs are also subject to turbulence, making the localization and quantification of leaks a challenge.
Laser-based sensors, specifically quantum-cascade lasers that emit in the mid-infrared portion of the electromagnetic spectrum, are compatible with small drone platforms due to their high sensitivity and compact footprint. The mid-IR range absorbs a range of molecules in addition to methane, including carbon dioxide and nitrogen.
Zondlo and fellow researchers developed a mid-infrared sensor, equipped with a 3.3-micron antimonide laser, and placed the system on a hexacopter. To address the challenges of in-flight stability, an in-line reference cell accounted for systematic drift during flight.
Flight tests were conducted around a compressor station, a facility that enables transport of natural gas from one location to another. The aerial demonstrations showed an improved detection of methane leaks from areas situated at altitudes of 500 meters.
NASA Methane Tests Take Off
For similar methane-detection purposes, Lance Christensen, senior scientist at NASA’s Jet Propulsion Laboratory (JPL) and another featured panelist at SPIE Defense + Commercial Sensing 2017, placed miniature sensors on aerial robots.
Researchers from JPL and the Mechatronics, Embedded Systems and Automation (MESA) Lab at the University of California, Merced, began flight tests in late February of 2016. The experiments, funded by Pipeline Research Council International (PRCI), took place in central California at the Merced Vernal Pools and Grassland Reserve.
The JPL-led team placed a miniature tunable laser spectrometer on a small aerial robot quadcopter, a fixed-wing UAV, to find and quantify natural gas leaks. In the flight experiment, natural gas was released into the air. Rigged with a small tunable diode laser sensor, at 3.3 microns, the UAV sensed a leak about 35 meters from the site.
“Imagine these leaks in your neighborhood or along a pipeline,” said Christensen. “These are very small leaks that these systems can now detect and find at considerable distances.”
As part of the stochastic, or multi-variable, detection test, the drone flies back and forth to build up a probabilistic picture of the extent and size of a gas plume. In three to four minutes, the picture begins to form.
“You start to feel intuitively what the size of the leak is,” said Christensen.
Christensen believes that the technology has reached “parts per billion” sensitivity and can operate on small quad-copters. The challenge, he said, lies in figuring out how to use the data.
“Making a measurement of a gas plume while flying around tells you there’s a leak there, but you have to invert that measurement into something useful,” said Christensen. “You want to know how much that particular leak is. That’s the nature of the work we're doing now.”
A More ‘Quantitative’ Option
According to many of the researchers on the SPIE panel, lasers provide a quantitative analysis compared to other detection methods like optical gas imaging (OGI). Optical gas imaging cameras use spectral wavelength filtering to visualize the infrared absorption of gas molecules and hydrocarbons.
Although the OGI systems have been considered an Environmental Protection Agency (EPA)-accepted standard for specific applications, such as inspection of large leaks at well heads, Mickey Frish, Manager of Industrial Sensors at Physical Sciences Inc., suggested that the optical gas imager, a passive technology, does not offer the quantitative measurement provided by lasers. Lasers, he said, also find specific gases, compared to OGI techniques that show any and all hydrocarbons.
“When you use a laser, the number that you measure is independent of the background,” said Frish. “You get good, reliable numbers that you can use to measure a leak rate.”
The Remote Methane Leak Detector (RMLD™), manufactured by Houston, TX-headquartered Heath Consultants and transferred from the Andover, MA-based Physical Sciences Inc., provides natural gas leak surveying capabilities. The 15-year-old tunable diode laser spectrometer has also been miniaturized and fixed onto a company-developed quadrotor. The RMLD aims a near-infrared laser beam at a target and measures the amount of methane based on backscatter, or deflection of radiation.
The panel ultimately shared an optimism for a UAV’s ability to detect gases from the air. Princeton University’s Mark Zondlo and other researchers on the SPIE panel — including Tim Day of San Diego, CA-based Daylight Solutions, IBM Research’s William Green, and Ralph Taylor-Smith, General Partner of the technology-focused private equity investment firm Battelle Ventures and Innovation Valley Partners, based in Princeton, NJ — spent much of their presentations showcasing the benefits of miniaturized, drone-ready sensing systems.
“These leaks that we are detecting are going to happen, regardless of whether gas prices are low or high, and it’s a challenge we need to face,” said Zondlo.