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Helicopter LIDAR speeds up pipeline inspection

15 May 2006

Adlares, E.ON Ruhrgas and the German Aerospace Centre (DLR) think that their airborne LIDAR system could help to revolutionize gas pipeline monitoring, increasing network inspection rates from 2 to 80km/h. James Tyrrell catches up with Adlares' Matthias Ulbricht to find out more.

Gas pipeline inspection is a huge task. Just looking at the figures for Germany alone, with a network of more than 40,000 km of long-distance gas transport pipelines, the need for a fast and remote monitoring system soon becomes apparent. If you then consider the many transit pipelines running through the country from Eastern Europe into France, Belgium and Holland, and the various metropolitan gas distribution networks, you begin to identify a real market opportunity.

Currently, gas companies such as E.ON Ruhrgas, with a network of approximately 12,000 km, rely on monthly airborne visual inspections backed up by walking surveys. The two-person walking teams ensure a high level of pipeline safety, but with inspection speeds of just 2 km/h, it is an expensive task for the industry.

The ideal solution of course would be to combine a remote sensing technique with the routine aerial surveillance. However, for the system to be attractive to gas companies, the apparatus must be able to detect tiny gas leaks of just 100-150 l/h, which makes the exercise much more complicated.

Getting started

Undeterred by the difficulty of high-speed pipeline inspection, Adlares, a four-person company located in Berlin, Germany, was founded in 2001 to develop mobile LIDAR systems for methane monitoring. Facing an increasingly competitive market, gas firms were anxious to get their hands on the technology. Adlares co-founder, Matthias Ulbricht, was well-equipped for the task as he was previously general manager of Berlin-based Elight Laser Systems, a developer of LIDAR instruments for air pollution monitoring.

"What counts is the cost per kilometre and [with our airborne LIDAR system] we can offer not just better prices, but also a higher density of measurement points," Ulbricht told OLE. "The figures are still under discussion as it depends on the size of the network, but there will be sufficient savings for the gas companies."

Dubbed CHARM (CH4 airborne remote monitor), the gas-sensing programme is a joint effort between Adlares, the DLR and E.ON Ruhrgas. Energy firm E.ON Ruhrgas provided project funding and DLR staff in Oberpfaffenhofen and Stuttgart supplied expertise in atmospheric physics together with a wealth of knowledge in laser and detection systems.

Absorption spectrum

One of the key technical decisions facing the CHARM scientists was the choice of operating wavelength. "When you look at the absorption spectrum of methane you see that it has four major absorption bands," Ulbricht explained. "From the point of view of the available technology, 1.6 μm is the most appealing region - you have fast detectors, the optical materials are standard and you can build OPOs [optical parametric oscillators] with sufficient output."

However, the requirement to detect very small leaks forced the designers from the DLR to consider other factors. "The problem with 1.6 μm is that the absorption cross-section of methane is fairly low," said Ulbricht. "Moving up to the 2.4 μm region doesn't give you any major benefits in terms of sensitivity, but it does make the technology more complicated. At 7.8 μm, methane is hard to detect because of the interference with other gases."

With its focus on device sensitivity, the group decided to take advantage of the strong absorption band around 3.3 μm and in return was forced to develop custom hardware.

Customized LIDAR

At the heart of the system is a two pulse differential LIDAR set-up. A seeded Nd:YAG laser emitting at 1064 nm feeds its 100 Hz dual pulse output (15 mJ/pulse) into a custom OPO, which is operated in ring cavity mode and emits in the 3 μm region. The need for mobile operation placed certain constraints on CHARM's engineering team.

"Diode pumping was mandatory for us because we have limited electrical power on the helicopter," said Ulbricht. "If you have a lamp pumped [device], the total efficiency is much lower." According to Ulbricht, the final output energy of 0.8-0.9 mJ/pulse is sufficient to collect high-quality data.

"We developed the apparatus in such a way so that we can make a single pulse pair evaluation," he revealed. "We transmit two laser pulses, the first pulse is tuned precisely on a methane absorption line in the 3 μm region and the second pulse is tuned close-by to the absorption line."

Both probe and reference pulses are sent from the helicopter, which typically flies at an altitude of 120 m. The expanded beam travels through the atmosphere towards the buried pipeline and is fully eyesafe on the ground. A small fraction of the laser light is then scattered back to the system where it is collected by a telescope coupled to a fast analogue detector. Finally, the signal is digitized, ready for analysis.

"As long as we have no methane in the air, both laser pulses will behave similarly, which means that the back scatter intensity will be the same for both laser pulses," Ulbricht explained. "We measure the laser power of each spot so that we can normalize the signals and a methane reference cell ensures that the probe beam is tuned exactly to the right wavelength."

If methane is present in the beam path, then the first laser pulse is slightly attenuated as the gas molecules absorb the infrared light.

In practise, the system computes a ratio of the two signals, correcting for the difference in absorption cross-section and the optical path length to determine the corresponding methane concentration.

Field trials

Working with a benchtop set-up, the first task for the airborne LIDAR team was to test its apparatus on the ground. "We started with field trials where we measured out of the window and looked at a target 150 m away just to see if it was possible to detect methane," said Ulbricht. "Then we switched to a small helicopter, a so-called MD500, and made our first flights over artificial leaks. We opened gas bottles slightly and again were able to successfully detect the methane." Today, the system is flown in a BO105 helicopter, which gives the option of longer flight times.

Initially, the team attempted to guide its LIDAR unit manually, steering the beam via a joystick to follow a video-camera image of the pipeline. However, this required a high level of operator skill and was felt to be unsatisfactory for routine inspection. In 2004, the group began to devise an automatic beam-guiding system based on a rotating mirror and position-sensing electronics.

"We have developed and patented a scanner that distributes the measurement points not only precisely on the pipeline, but also along either side of it," said Ulbricht. "This means that even if the leak comes out of the ground, say 2 m away from the pipeline, we can still identify it." The beam-guiding system is also useful when the helicopter is unable to fly directly over the pipeline.

Thanks to the unit's inertial measurement capability, the scanner compensates for left and right (roll) movement along the helicopter's flight axis. As Ulbricht explains, uncorrected roll movements are more disruptive to the scan profile than front and back (pitch) deviations from the flight path.

The set-up is fitted with a differential GPS, which gives a positioning accuracy of better than 50 cm, and linked up to a computer databank of gas pipelines. With the newly automated apparatus, the operator simply has to select the pipeline on-screen and then the scanning system will do the rest. According to Ulbricht, the precision for pinpointing the laser is now better than 2 m.

However, there is more to the set-up than just pinpoint accuracy. Adlares has also developed a suite of "intelligent" data algorithms that can distinguish between genuine and false gas alerts - methane is naturally present in the atmosphere at levels of 1.7-2 ppm. The software can also compensate for other methane sources in the area such as waste disposal sites.

Stamp of approval

With the system now very much up and running, the CHARM team is busy gaining experience on a real gas pipeline and has started evaluating the E.ON Ruhrgas network. In fact, the plan is to monitor the entire pipeline twice this year.

Meanwhile, the German gas and water association is evaluating the group's LIDAR method against its gas inspection norms to confirm that the airborne measurement can be used officially in place of a walking survey. In terms of air safety, the German Federal Aviation Authority is expected to formally approve the technique later this year.

• The company presented this work at Laser Optik Berlin, which took place in Germany on 23-24 March.

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