The goal is to help maintain and enhance the integrity of the nation's natural gas infrastructure.
Ophir Corporation – project management and product research
El Paso Pipeline Services Company - input regarding system specifications
WBI Holdings, Inc. – input regarding system specifications and pipeline access for testing
Littleton, CO 80127
The most common type of leak detection equipment used by the natural gas industry is the Flame Ionization Detector (FID). The FID is a simple, easy to use, portable instrument with a 5 to 10 ppm level of sensitivity. A major limitation of FIDs for natural gas leak detection is the fact that they are point sensors capable of sensing only the flammable gases surrounding the device and have poor molecular selectivity. An elevated FID reading may be an indication of a natural gas leak or sewer gas, automobile exhaust, a gasoline spill, a propane gas grill leak, solvents from fresh paint, barbeque lighter fluid, etc. There are also significant costs associated with maintaining the calibration of FIDs and with the hydrogen gas consumables (GRI 1999b).
Airborne, optical remote sensing systems for monitoring both methane and ethane offer an important option and opportunity for the natural gas industry. Such a system will increase the speed of leak detection, improve the leak detection success rate, decrease false alarms and drive down leak survey costs. In turn, this system will improve pipeline safety, reduce operation costs and decrease the rate of natural gas emissions into the atmosphere.
Optical remote sensing over transmission pipelines may provide a time saving and cost efficient method of performing pipeline integrity monitoring without physically excavating the pipe. With the sole exception of “smart pigging,” all currently approved pipeline integrity assessment techniques require excavating at least portions of the pipe being tested. An airborne, optical remote sensing system for natural gas pipeline leak detection will reduce the time and cost of leak detection by providing “real time” information and reducing false alarms and their associated time and cost burden. This will enable the pipeline infrastructure to be inspected on a more frequent basis and provide concrete measurements of leakage. The result will be improved pipeline safety and environmental air quality through reduced greenhouse gas emissions. By enabling both methane and ethane to be independently measured and quantified, the system will reduce the number of false leak detections related to non-natural gas sources of methane.
The potential impacts of the technology include: 1) the effective and efficient airborne evaluation of natural gas pipeline systems for detection and quantification of leaks, 2) reduction in lost product through loss events, 3) enhanced safety to the public, and 4) reduced environmental impact. These goals are achievable because use of optic remote sensing can result in the discovery of leaks in a much timelier manner than is achievable using existing technologies. Early discovery and correction reduces the amount of gas released, thus increasing pipeline safety, enhancing the ultimate amount of gas available for consumer use and reducing environmental impact seen from such emissions.
The method under development relied on optical, infrared absorption and can therefore provide a quantitative measurement of both the concentration and the aerial extent of the leaking natural gas. The contractor demonstrated the sensor’s ability to detect 30 parts-per-billion (ppb) ethane and 50 ppb methane over a 304-meter path-length.
Ophir’s initial performance modeling suggested that a tunable laser approach in the overtone absorption band region would result in superior performance when compared to the company’s existing stationary system operating in the mid-infrared region. The optical path for the transceiver will be of an all fiber design assuring ruggedness and ease of alignment. The infrared light is emitted from the fiber amplifier into the atmosphere using a fiber-coupled telescope and directed toward the region to be monitored. The light reflects off targets such as trees, the earth’s surface or asphalt and passes through the atmosphere a second time, thus doubling the interaction optical path length, and is collected by a second optical telescope. At this point, the collected light is split into two beams: one passing through the blank optical channel and one passing through the target gas channel. A lock-in amplifier may be used to reject stray and background light. A data processor performs the inter-comparison between the optical channels.
In addition to the transceiver design modifications for the airborne application, other design requirements for the airborne application were defined through industry surveys. Two natural gas transmission companies, El Paso Pipeline Services Company and WBI Holdings, Inc., were queried regarding the type of information that would be useful and the approximate sensor flight requirements. Through these survey results, preliminary design requirements including aircraft airspeed, aircraft altitude, weight, size, and minimum detectable concentration for an airborne system were defined.
Signal-to-noise modeling for the airborne remote sensor configuration has shown that appreciable amounts of output power are needed to meet the system requirements for long optical path targets. In addition, little data is available describing the overall signal reflectance that can be expected using an IR source against different background surfaces.
The contractor has investigated the availability of high power sources in the infrared wavelength of interest and has acquired and incorporated the fiber amplifier into the system design. To eliminate as much risk as possible, a number of key tests, including target reflectance measurements, have been conducted.
With the acquisition and incorporation of all equipment required for the optical detection system, the contractor was able to perform initial flight testing of the entire system. The newly ruggedized transceiver, optical breadboard, and the 1550 nm fiber amplifier were flown in November 2004 with good results. Two hours of flight data over various ground surfaces at different altitudes was collected during initial flight testing. The laser return was nearly an order of magnitude higher than the background solar signal even without the pass band blocking filters. Based upon signal strengths used in models, it is believed that there was enough received signal to detect methane at altitudes up to 1000 feet.
Prior to receiving the dual output amplifier, Ophir conducted numerous laboratory and outdoor reflectance tests using an existing 1-W, 1555 nm Raman fiber amplifier. The airborne transceiver was tested out thoroughly as to the collection efficiency with different targets and the collimating properties of the light source transmitter optics. Much was learned about the effects of various surfaces at 150 m (500 ft) optical path length and target-to-receiver angular changes on signal return. The signal-to-noise of the system using the substitute light source was also verified against the predicted models.
The final system was mounted onto an airplane in February 2005, and numerous flight tests were performed over a controlled methane gas leak. The system demonstrated that it was capable of detecting a methane leak of 150 scf/h from an airplane altitude of 150 m and airspeed of 44 m/s (100 mph). The methane leak was confined to a small, low-lying ditch, and the weather conditions were ideal for most of the testing with light to variable winds.
Valuable information about the impacts of dynamically changing flight conditions on active gas correlation analysis was gathered. Issues that need to be resolved include learning more about the impacts of all kinds of surface terrain on the gas correlation ratio, determining the necessary wavelength stability of the amplifier to minimize drift in the gas correlation ratio, and better discriminating between false positives and actual methane hits.
All work on this project is complete and a final report has been submitted.