Field Test of Airborne Gas Detector ALMA

This article provides a comparative analysis of devices for remote detection of methane leaks. Particular attention is paid to various types of scanning systems and methods for increasing their sensitivity. The results of experimental studies aimed at detecting methane leaks with low concentrations are presented, emphasizing the relevance and practical significance of the work.

By Johan Wictor, Carlos Grijalba Arroyo, Mihai Pelsa, Oleg Ershov, Alexey Klimov, Svyatoslav Kudryavstev & Semen Neverov – Pergam Group (Italy, Switzerland, USA)

The oil and gas industry is a significant contributor to methane emissions. (1) Most methane leaks occur during gas transportation and storage in underground storage facilities. (2)

Since inspections must be carried out at least twice a year – in the spring and autumn (the period of soil movement) – it is necessary to use technologies that will allow the fastest and most effective monitoring of the condition of gas pipelines and the identification of natural gas leaks.

Pergam Group has developed and manufactured portable remote gas detectors ALMA (Airborne Leak Methane Assessment) since 2004. They have been used for remote inspections of pipelines – from helicopters and other aircraft – worldwide for 20 years. Development of ALMA has continued, seeking to improve operation, its usability, the convenience of its data presentation, and more. ALMA Generation 4 (G4) has been used for inspection work since 2018 and was used during the helicopter field test described below. The main goal of this field test was to confirm the detection of an artificial methane leak with a gas flow rate of 17 g/hour, which is accepted as the detection limit for inspections of pipelines in the European Union by airborne gas detectors.

Organization of the Test

The field test of ALMA operation from a helicopter was organized in Italy near the town of Capri in April 24th, 2024. The test flight was made in the vicinity of Capri Aeroclub airfield. A map of the field test is shown in Figure 1.

An artificial gas leak was organized at a distance of approximately 50 meters from the buildings of Capri Aeroclub. Fortunately, an open landfill is located at a distance of approximately 3km from the airfield. Methane concentration is almost always high above all landfills. This additional source of methane was used during test flight for extra confirmation of methane detection by ALMA. These two objects are marked on Figure 1.

ALMA G4 was installed on the nose of a Jet Ranger Bell 206 helicopter with the help of a certified mount (see Figure 2). One can see in Figure 2 an additional scanning system on the base of the rotating flat mirror, which is mounted on the ALMA Optical Unit. This scanning system is used to enlarge the area of the ground surface under investigation during the inspection flight. The exact aim of this test flight was to detect a very narrow gas cloud produced by an artificial gas leak. The laser beam needs to be precisely aimed at this narrow gas cloud. Therefore, this mechanical scanning system was not rotated during test flight; it was in a fixed position.

The arrangement of the artificial gas leak is shown in Figure 3. A high-pressure gas cylinder was used as a source of methane. A pressure reducer diminished the pressure of the gas down to 1-2 bar. A Precise Mass Flow Regulator (MFR) of model MASS-VIEW MV-302 (manufacturer Brankhorst High-Tech BV Company) was used for regulation and measurement of the gas flow rate in the artificial gas leak. The gas flow rate was set at exactly 17 g/hour, which is accepted by regulation rules in the EU as the detection limit for airborne methane detectors.

Gas ran out from the end of a plastic tube with an inner diameter of less than 1cm, allowing for a gas cloud that was very narrow. Gas concentration measured by a hand-held methane detector about 10cm from the end of the tube showed hundreds ppm at a distance of about 10cm. The gas concentration at a distance of 1m from the end of the tube reached the limit of the sensitivity of the handheld methane detector (few ppm). The artificial gas leak was opened 1 hour before the start of the test flight so that a more-or-less static gas cloud was allowed to form.

Results of Gas Detections During Test Flight

The helicopter with the airborne ALMA G4 made a few runs precisely above the artificial gas leak and two runs above the landfill. The route of the helicopter is shown in Figure 4. The duration of the flight was just 15 minutes. One can see that ALMA successfully detected methane three times above the artificial gas leak, as well as more than 100 times above the landfill. The helicopter flew above the artificial gas leak at an altitude of 25-60 meters. The altitude above the landfill was more than 100m. Helicopter speed was in the range of 50-100 km/hour during the test flight.

ALMA measurement data for the test flight was saved. Data processing software allows us to examine the results. Gas concentration graphs from the data processing software for the three detections of the artificial gas leak are down in Figure 5 (a, b, c).

Figure 5 (a), (b), (c): Graphs of methane concentration during 3 gas detections of the artificial gas leak.

Notes to Figure 5:

Upper graphs in Figure 5 (a,b,c) are time dependences of methane concentration in normal units, ppm*m. Lower graphs in Figure 5 are time dependences of the ratio of concentration to measurement sensitivity. Therefore, one can see an excess concentration of methane above the detection limit of the instrument in this graph.

The blue line with points after each 40 msec in these graphs represent the fast measurement of the gas concentration. The red line with points after each 200 msec is the averaged gas concentration. One can see from Figure 5 that the duration of all three spikes of gas concentration was approximately 100 msec, representing the evaluation of the gas cloud width along each helicopter pass.

The differences in indications of gas concentration during these three gas detections are explained by the fact that the laser beam crossed different parts of the gas cloud, and the location of the gas cloud continuously drifted.

If one compares indications of gas concentrations in ppm*m units and indications of normalized concentration (signal-to-noise ratio) for detections ##1 and 2, one can see that for a helicopter altitude of 24 meters normalized concentration is higher, even for lower gas concentration. It confirms that ALMA sensitivity is better at an altitude of 24 meters (Leak 1) than at an altitude of 55 meters (Leak 2).

Conclusion

This field test with airborne gas detector ALMA G4 confirms that the instrument is capable of detecting gas leaks with a flow rate of at least 17 g/hour, from altitudes up to 70 meters. Moreover, measurement results show that this gas detector is capable of lower gas detections down to at least 5 g/hour at altitudes of 25-40 meters, and down to 10 g/hour at altitudes of 50-60 meters. Therefore, ALMA G4 corresponds fully to the regulation rules of the European Union for airborne gas detectors.