Distributed fiber-optic sensors can be a comprehensive solution for in-line, real-time monitoring of long-distance pipelines. The technique is based on Brillouin scattering and can measure strain and temperature over distances larger than 150 km with meter resolution using a single instrument. The solution includes ground movement, leakage, and third-party intrusion detection.
Increasing demand for energy and reduction of world oil and gas stocks has delocalized extraction industries to remote places such as the far north. Pipelines, essential components of the energy supply chain, have to be laid in harsh environments where seasonal soil texture changes increase the probability of hazards. Moreover, pipeline routes often cross mountain ranges characterized by unstable grounds. For these reasons, pipeline monitoring systems (PMS) for leakage, ground movement, and intrusion detection are part of new pipeline projects.
A PMS should detect and localize pipeline deformation as well as provide early warning of leakage. Changes in the surrounding ground, including intrusions, are major causes of pipeline failures. Threats can be detected by monitoring ground movement and soil properties along its route. Strain monitoring systems detect ground movement, whereas temperature measurements inform as to soil property change. Another benefit of continuous temperature monitoring is detection of pipeline leaks via the Joules-Thompson effect, i.e., associating temperature change to leakage.
Due to long distances monitored and the linear nature of pipelines, fiber-optic sensors are often considered a technology of choice. Compared to fiber Bragg gratings or long gauge sensors, distributed techniques have clear advantages. For one, the efficiency of the Brillouin-optical time domain analyzer (BOTDA) monitoring system has been demonstrated in several pipeline projects. The technique can measure strain and temperature over several tens of kilometers, with measurement resolution better than 20 me and 1°C respectively. In practice, dedicated strain and temperature cables are commercially available for permanent ground movement and leakage detection.
In monitoring a pipeline, the Omnisens DITEST (distributed strain and temperature) can detect minimum cable displacement of < 0.5 m with meter localization accuracy anywhere along the pipeline over distances > 150 km with a single measuring unit. The possibility of detecting temperature change for leakage detection and localization with a resolution of 0.5°C/bar x DP has been demonstrated, where DP is the pressure variation.
Sensing technology
In BOTDA-based technique, the sensing medium is a standard single-mode optical fiber. The sensing mechanism is based on the stimulated Brillouin scattering (SBS) effect in which a counter-propagating light wave (probe) is amplified at the expense of a pump light wave (Figure 1). The interaction between the pump and probe reaches the maximum when the frequency difference between the two light waves equals the acoustic mode frequency of the fiber, known as the Brillouin frequency nB. Typically, the Brillouin frequency of ITU G.652 fibers is about 10.85 GHz at 1.55 mm. The Brillouin frequency is proportional to strain and temperature variation. Thus, the approach is a method of choice for sensing of mechanical and thermal effects.
The BOTDA technique for geotechnical monitoring is based on measurement of strain along a sensing fiber, or strain measurement cable (SMC). Strain is, effectively, the parameter that can be monitored to detect a landslide. In fact, when a landslide occurs, the shear interface between sections that don’t move and the section of land which slides down is subjected to strain as illustrated in Figure 2. The conversion from lateral displacement to fiber longitudinal strain can be understood as follows: Based on the schematics of Figure 2, we see that the original section d of cable is submitted to a constant strain e whereas the rest of the cable remains strain free.
Thanks to the high sensitivity strain measurement capability of the BOTDA measurement technique, minimum cable displacement as small as 0.3 m can be detected and localized with meter accuracy anywhere along tens of kilometer of SMC. In this particular experiment, an 18 m long section out of a 200 m bare fiber was laterally stressed with an amplitude
of 50 cm. Strain of 700 me is measured, which is much more than an order of magnitude of the strain resolution of the BOTDA technique.
SMC longitudinal displacement induced by ground motion along the pipeline axis can also be detected. Smaller displacement can be detected in the axial direction.
Leakage detection
BOTDA leakage detection relies on accurate distributed temperature monitoring along a temperature measurement cable (TMC) located in the pipeline vicinity. Two approaches need to be distinguished, depending on the type of fluid transported.
The pipeline surrounding is cooled when the fluid is compressed. Leakage detection is then based on the Joules-Thompson effect. The fluid being in adiabatic regime, any pressure change, as caused by a leak for instance, induces a temperature drop which affects the TMC. The interrogator detects the temperature change, leading to leakage detection and localization. Typical figures are 0.5°C/bar x DP which indicates that a small pressure change would induce a significant temperature variation.
Transported liquids such as crude oil, brine, or heating system fluids are at a temperature higher than that of the environment. Leakage leads to a temperature increase in the pipeline vicinity. The hot spot is the signature of a leakage.
Buried pipelines can be subjected to hazards provoked by third-party intrusions, intentional or accidental. Soil removal around the pipe is easily detected by monitoring temperature change along the TMC, as temperature of the pipeline and its vicinity is different from that of the environment. Intruders can pull the SMC by accident, which is easily detected and located by the BOTDA.
Seasonal change also affects buried pipelines and is a source of hazards. The temperature sensing system allows operators to monitor pipeline condition and locate potential erosion events, especially for buried subsea pipeline operating at warmer temperatures than seawater. Seabed erosion and possible pipeline exposure may be detected and located through temperature changes observed along the TMC. Cable only needs to be installed in close proximity to a buried pipeline. This can also be a means to monitor permafrost thaw settlement, a source of upheaval buckling.
When an SMC is tightly attached to the pipeline, any deformation such as buckling or pipe deformation can be detected. In fact, induced strain is transferred from the pipe to the cable. When the SMC is bonded to the pipeline, crack formation also can be detected. An advanced post-processing software tool can be used to extract the relevant information.
Monitoring solution
The proposed solution for long range pipeline monitoring referred to as Omnisens DITEST-AIM, as illustrated in Figure 4, includes the following elements:
• Strain and temperature monitoring units, combining interrogator (DITEST), remote signal regeneration modules, and optical switch — each of these monitoring units constitutes an optical node located in a pipeline node such as a pumping station;
• Strain and temperature measurement cables — SMC and TMC respectively — connecting two stations;
• Data communication interface between monitoring units and control station; and
Measurement control, visualization, and configuration software.
The strain and temperature interrogator system, or DITEST, is a long-range laser-based monitoring system based on SBS. Inherent stability comes from use of a single laser source and high-speed electro-optic modulator for generation of both pump and probe signals. Intensity of both optical signals can be controlled for the highest possible signal-to-noise ratio and reduced acquisition time. Frequency difference between pump and probe signal is precisely controlled by the modulation frequency applied to the electro-optic modulator, leading to 10-5 precision on the frequency determination.
Typically, the DITEST system performs strain profile measurement with a 20 me resolution (defined as 2 times the standard deviation on repetitive measurements) and a spatial resolution of 1 m over the first 10 km and 1.5 m up to 30 km. Smaller spatial resolution or longer distances can be achieved with similar performance and high dynamic range (up to 20 me of optical budget). 50’000 distance points can be measured with a minimum sampling interval of 0.1 m. The acquisition time (i.e., to get one complete profile) may vary from one second to 10 minutes depending on the application requirements. Furthermore, the DITEST system was developed for automatic and unattended monitoring over long periods of time.
Scenarios have been developed to multiply the monitored distance range for longer pipelines and to support remotely interrogating a sensor placed a long distance from the control station. Although low-loss optical fibers are available, attenuation of the fiber still sets limits to the SBS measurement range. Furthermore, performance in terms of spatial resolution and temperature/strain accuracy is also related to distance range, since the optical waves are affected by fiber attenuation. On one hand, decreasing pulse intensity generates a smaller interaction. On the other, a weaker signal on the photo-detector is associated with lower signal-to-noise ratio, which requires longer averaging times. The distance range of this technique is therefore limited to some 30 km with meter spatial resolution. However, the pump-and-probe technique offers flexibility that makes possible development of regeneration or repeater modules to provide either an extension of the distance range or remote sensing capabilities.
Performance obtained with the remote sensing modules is similar to that available directly from the instrument in terms of repeatability and temperature/ strain accuracy. Every monitoring unit is connected to the DITEST server in the control room through a LAN network using TCP/IP protocol. Optical fibers in the TMC can be used to build the LAN.
The control, operation, and configuration of the DITEST AIM system include the following software modules:
• Measurement-control routine: runs on each monitoring measuring unit to control optical-signals generation and acquisition and optical-switch module; process the detected signals; generate alarms; activate relays; store data; and transfer data to separated computers.
• Configuration interface featuring graphical interface to define measurement settings, permanent monitoring agenda, data storage, alarm management, etc.; It also provides monitoring unit real-time status information as well as historical events stored in an event list.
• Visualization software displaying strain and temperature profiles, events, and alarms at the control room interface.
Example application
In 2002, the construction of a natural-gas storage facility some 1500 m under the ground surface was started in the area of Berlin, Germany. Building underground caverns for gas storage in large rock-salt formation using mining equipment requires hot water and produces large quantities of salt-saturated water, so-called brine. In most cases, brine cannot be processed on-site and must be transported by pipeline to where it can be used for chemical processes or injected back into the ground. Because brine can be harmful to the environment, the pipeline should be monitored by a leakage detection system. In the Berlin project, a 55 km pipeline was built. GESO was selected to provide the leakage detection system. To cover the distance, it was decided to use two DITEST analyzers, although one instrument could have theoretically covered the distance with its two channels. However, fiber-cable installation would have required some 60 splices (corresponding to an additional loss of up to 3 dB), reducing the instrument range. This justified use of two instruments.
During construction, the fiber cable was first placed in the trench and buried in sand some 10 cm beneath the pipeline. Cable position with respect to the pipeline is important to guarantee all leakages are detected. Sensing cable position is a trade-off between the maximum contrast in the event of leakage and assurance to detect leakages occurring from every point of tube circumference.
Brine is pumped out from the underground caverns and injected into the pipeline at a temperature of about 35°C. At normal flow rate the temperature gradient along the whole pipeline length is about 8°C. Since the pipeline is buried in the ground at a depth of approximately 2 to 3 m, seasonal temperature variations are quite small and soil temperature was measured to be around 5°C. As a result, substantial temperature increase is associated to every leakage even at low rates. Pipeline construction was completed in November 2002 and the pipeline put into operation in January 2003. In July 2003, a first leakage was detected by the monitoring system, caused by excavation work in the vicinity of the pipeline. By using the 0.927 MHz/deg temperature coefficient, the local temperature increase due to leakage was 8°C. An alarm was immediately and automatically triggered and the flow was eventually stopped.
Offshore arctic conditions pose unique design challenges to safe operation of subsea pipelines exposed to seabed ice gouging, permafrost thaw settlement, strudel scour, and channel migration. Application of fiber optic-based distributed temperature monitoring systems has demonstrated the ability to monitor pipeline operating conditions and to achieve efficient flow assurance monitoring. As visual inspection is impossible, real-time temperature monitoring via optical fibers along the pipeline route provides early warning of erosion events, insulation damages, seabed soil modifications and allows the operator to take timely and appropriate actions to ensure pipeline integrity.
The Omnisens DITEST-AIM monitoring system successfully monitored a 14-km pipeline bundle prior to startup in late 2007 and during operation since then. The pipeline installation is part of Pioneer Natural Resources Inc. developments in the Ooguruk Oil Field in Alaska’s Beaufort Sea. Eight km of buried subsea flowlines transport produced fluids from an offshore gravel island/drill site to an onshore above-ground pipeline which runs to an existing pipeline used by another operator. A total of 14 km of pipeline distance is continuously monitored with fiber optic communication cables installed within the bundle. The Omnisens DITEST-AIM monitoring system was selected for its performance over long distances, including the ability to detect temperature events occurring over just one meter. The system maps seabed temperature profiles along the pipeline route and accurately tracked temperature excursions with field-verified data prior to and during pipeline operation startup. n
Acknowledgment
Based on a paper presented at ASME’s 7th International Pipeline Conference, held September 29-October 3, 2008, in Calgary, Alberta, Canada.
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