August 2010 Vol. 237 No. 8

Features

Airborne Leak Detector Certified In Germany For Urban Gas Grids

The first German airborne technology for detecting very small natural gas pipeline leakages in built-up or rural areas was recently tested and certified by the DVGW, the German Technical and Scientific Association for Gas and Water.

Germany is among the leaders when it comes to gas supply safety in Europe. All German gas companies belong to an association known as the DVGW, a technical organization which develops and publishes engineering standards laying down rules and procedures for work in the gas and water industries, including gas transmission.

German legislation governing the surveillance of natural gas transmission pipelines requires routine leak testing, and the relevant DVGW regulations (e.g. G465; G466) ensure that high pipeline safety levels are maintained.

In recent years, active and passive remote gas detection methods have become an increasingly important pipeline inspection tool. These methods – which utilize the fact that methane absorbs certain infrared wavelengths – promise to significantly accelerate the survey procedure.

The first European gas grids were established about a century ago and grew only by degrees. For this and other reasons (e.g. landscape) the German pipeline topology is quite complex, especially in urban areas. Visual airborne pipeline monitoring is best conducted by helicopter, making this the aircraft of choice for deploying remote monitoring systems.

The European natural gas supplier E.ON Ruhrgas initiated a project with the objective of combining visual inspection by helicopter with the detection of pipeline leaks using an airborne remote methane detector.

The gas supplier teamed up with the laser company Adlares and the German Aerospace Centre (Deutsches Zentrum für Luft- und Raumfahrt, DLR) to develop a remote natural gas detector based on infrared laser technology and which enables the pipeline operator to comply with his statutory leak testing obligations by monitoring gas pipelines from helicopters. The detector was intended primarily for use in built-up areas and for occasional leak testing in rural areas as well.

From the outset the system has been developed to meet the requirements of European gas regulators as well as the very strict German gas regulations so as to ensure the same degree of safety as is achieved by walked surveys. The fruit of this project is an innovative methane detector known as CHARM, which stands for CH4 Airborne Remote Monitoring. The detector is mounted on a BO 105 helicopter and used to check pipelines for gas leaks. A customized box contains the entire laser system, the optics and the detection system plus a powerful PC network capable of handling the demanding computational requirements.

Figure 2: A large box contains the laser system, the optics and the detection system as well as a computer network.

Figure 3: Methane detection is automatically activated on the pipeline and three cameras also take georeferenced photos of the pipeline corridor.

CHARM is based on the Differential Absorption LIDAR (DIAL) principle, an established active remote sensing method for detecting different gases in the atmosphere.

An essential component of the system is an injection-seeded pulsed Nd:YAG laser that emits pulse pairs with a repetition rate of 100 Hz1 and a temporal spacing of less than 100 ?s between the pulses of a pair. A nonlinear frequency converter shifts the emitted wavelength in the infrared wavelength range (approximately 3.3 µm) to achieve the maximum gas detection sensitivity.

The wavelength of the first pulse is tuned to a specific absorption line of methane, while the wavelength of the second pulse – which is only slightly different – is not absorbed by the gas. Careful wavelength selection by CHARM ensures that other gases like water vapor or propane do not interfere.

The laser pulses are directed to the ground where the light is scattered in all directions. The small fraction of the emitted light that is scattered back to the system is collected by a telescope and focused on the detector. The integrated gas concentration can be directly calculated by comparing the signals of both pulses.

Combining Relevant Information
Along with sensitive methane detection, pipeline remote leak surveying requires precise guidance of the beam on the pipeline and a mechanism to monitor a safety sector around the pipeline. Therefore, CHARM is equipped with high-precision automatic beam steering equipment that allows aerial scanning of a pipeline corridor.

The laser light pulses are transmitted via a patented scanner which distributes the measurement spots on the ground. An inclined off-axis rotating mirror distributes most laser spots over a width of 7 to 12 m (23–40 feet) within the scanning corridor. The diameter of each defocused eye-safe laser spot is approximately 1 meter (3.3 feet).1

Figure 4: The distribution of measurement laser spots can be analyzed by PC. Red spots show increased methane concentration.

The navigation system is composed of a Differential GPS and an Inertial Measurement System (IMS), which provides navigation and motion data. The helicopter position is ascertained to within 0.5 meters (1.5 feet). For automatic pipeline tracking the data are merged with the pipeline position stored in an onboard databank. Fast real-time-processing of this data provides a tracking accuracy of the pipeline of better than 1 m (3 ft) even with complex pipeline topologies, while the pilot flies in a sector about 100 m (330 ft) wide along the pipeline.

During the inspection the system takes three high resolution photos of the pipeline every second. The central camera focuses on the pipeline itself, while two additional cameras expand the range of vision and provide a broad overview of the surroundings of a pipeline section. As a result, the gas supplier receives up-to-date photo documentation of the pipeline topology as well as information on the current status of the pipeline in terms of leak safety.

Online data evaluation not only provides alert messages in the case of detected gas clouds, it also determines the degree of measurement spot density around the pipeline to verify if the pipeline is sufficiently monitored. The operator can view this data displayed on a digital map.

CHARM has been shown to be capable of detecting very small leakages ranging from as little as 100 l/h (0.06 scfm) depending on wind conditions. It has proven that–even for wind speeds as fast as 6 m/s (13 mph)–leakages of 500 l/h (0.3 scfm) can be positively detected. The system also gathers information that provides intelligence on the general condition of the pipeline sections (e.g. photo series).

To prove the reliability of CHARM in detecting even the smallest traces of methane, a multi-factor experiment was conducted at a specially constructed test facility. This experiment involved numerous test flights over the facility where specified subsurface leakages were simulated. The parameters that are varied in this experiment are the amount of released natural gas (leakage), weather conditions, aircraft altitude and aircraft speed. All experiments were supervised by the German Gas Association (DVGW).

The DVGW has defined and published a new technical standard of Airborne Remote Monitoring (G501) and CHARM was the first airborne remote monitoring method to be certified by the DVGW. The system has demonstrated that its remote leak inspection achieves results comparable to walked surveys.

The combination of CHARM and its carrier aircraft, a BO105 helicopter, has also been certified by the European Aviation Safety Agency (EASA) and has obtained unlimited permission to operate even below the minimum height over ground limit of 150 m (500 ft).

Summary
Most traditionally deployed gas detection devices have been continuously optimized but have now reached their limits in terms of accuracy and reliability. In an evolutionary step, remote detection methods have become an essential tool in expediting inspection procedures. Active laser remote detection methods based on the absorption of infrared radiation by methane molecules can provide information on methane propagation in a pipeline section.

In Germany, the DVGW devises technical rules that enjoy quasi-legal status and guarantee safety standards in the transportation and distribution of natural gas.

Airborne methane detection devices for natural gas pipeline leak testing have been in use in Central Europe since 2005 as a complement to walked leakage surveys. The DVGW recently adopted a new technical standard (G501) specifying the functional and procedural requirements of Methane Airborne Remote Monitoring. The first system in Germany to meet these new requirements and be certified by the DVGW is known as CHARM, an infrared laser system mounted on a helicopter.

Multi-factor tests have demonstrated that CHARM – the most enhanced system on the European market – has the capability to survey pipelines beneath paved surfaces of urban areas (e.g. asphalt) or in rural areas and to reliably detect very minor leakages.

Unlike fixed laser systems, CHARM is able to target the pipeline accurately thanks to an automatic tracking system that combines relevant flight movement information with pipeline position data. The system distributes laser spots within an adjustable corridor by means of an inclined off-axis rotating mirror.

The newly accredited airborne remote monitoring system should help reduce costs and come to supersede conventional walked surveys.

Author footnote
(1) The present CHARM system operates at a measurement rate of 100 Hz. A new system with a pulse rate of 1,000 Hz is under construction and will help to increase the survey speed and the width of the inspected sector.

Authors
Dr. Axel Scherello is with the Gas Technology Competence Center of E.ON Ruhrgas AG, Essen, Germany. He can be reached at 49 201 184 8704, e-mail: axel.scherello@eon-ruhrgas.com.

Matthias Ulbricht is Managing Director of ADLARES GmbH, Teltow, Germany. He can be reached at 49 3328 3306 11, e-mail: ulbricht@adlares.com.

Thomas Kern is with the Gas Technology Competence Center, E.ON Ruhrgas AG, Essen, Germany. He can be reached at 49 201 184 8591, e-mail: thomas.kern@eon-ruhrgas.com.