Distributed Temperature Sensing
Pipeline Temperature Monitoring
Seismic

 

 

Distributed Temperature Sensing
Pipeline Temperature Monitoring

Technology

Distributed Temperature Sensing (DTS) via detection of Brillouin or Raman backscattering can measure a fiber temperature profile to a resolution of <1C at a distance resolution of 1-2m over a fiber up to 30 km in less than 10 minutes.

DTS is implemented using a pulsed laser and detector, and either multimode or single mode optical fiber. DTS excels at obtaining a temperature profile in harsh environments or where strong electromagnetic fields may compromise the safety and performance of electrical point sensors. Additionally, the distributed nature of Raman or Brillouin fiber sensing allows one to cost-effectively monitor long distances where there is no prior knowledge of specific locations for point sensors, such as pipeline leak detection.

Raman backscattering is a characteristic of molecular vibration and is sensitive to temperature. A DTS system implementation using Raman technology measures the intensity ratio of the Stokes peak vs. the anti-Stokes peak to obtain a single point temperature measurement, where the point location is pinpointed through the use of timing measurements. Brillouin backscattering results from vibrations of the bulk of the glass arising from the propagating laser light pulses. This technique can be used to measure both strain and temperature in a fiber sensing system, by measuring the frequency shift of the backscattered signal from the main pulsed signal. A shift of 10-20 GHz is typically found for glass fibers.

DTS has been growing in popularity as a necessary technique to monitor and maintain levels of production in an increasing number of downhole and pipeline applications. The efficacy of Steam-Assisted Gravity Drainage (SAGD) as an enhanced recovery technique is vastly improved as real-time temperature profiles of this production process provide feedback to maximize production. Effective pipeline temperature monitoring also improves the return on investment in these expensive assets, as a DTS system can be installed to monitor flow temperature (to prevent clogging) or as a leak detector in a gas pipeline.

The Lepton Advantage

Lepton’s breakthrough signal/noise performance, combined with superior dynamic range and narrow optical bandwidth, enables extended range and/or improved resolution in both Raman and Brillouin DTS systems. For example, ultra-long fiber systems may be deployed with low launch power into single mode fiber, without the concerns of generating nonlinear optical interference effects. Another way to capitalize on Lepton’s performance benefits is to reduce measurement time for the same level of temperature/distance resolution. And for Brillouin systems, the natural filtering properties of Lepton’s detectors make them a perfect fit to measure the characteristic small shifts from the large-signal Rayleigh peak.

References and Links

“Distributed Fiber-Optic Sensors: Principles and Applications”, by A. Hartog in “Optical Fiber Sensor Technology”, K.T.V. Grattan and B.T. Meggitt, Kluwer: Boston (2000).

Kincade, Kathy. “Optical Sensors Enhance Oil and Gas Yields.” Laser Focus World. September 2006, pp. 76-79.

Pallanich, Jennifer. "Getting Every Last Drop." Offshore engineer. January 2009, vol. 34, no. 1, pp. 25-26.


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Acoustic Sensing Using Fiber Optics
Seismic and Hydrophones

Technology

Seismic

The characterization of Earth-based vibrational activity is used in both the scientific and commercial realms. Seismic data is employed extensively in the Exploration and Production of new oil and gas fields. It is also used by geophysicists to understand and model future seismic activity, in order to better predict earthquakes. Fiber optic acoustic sensing techniques present an attractive technology alternative to more commonly used electrical sensing of these vibrations, due to the higher reliability of optical fiber relative to buried electronics. Another advantage is the opportunity for distributed sensing, or the ability to measure vibrations along the entire length of the fiber cable, as opposed to networks of point sensors used in an electrical system.

An optical fiber is extremely sensitive to small changes in strain caused by Earth’s vibrations. This principle can be exploited for seismic measurement through the use of interferometric techniques. In a simple example, a sensor can be constructed using a pair of fibers at some separation. A vibration will induce a slight difference in optical path length between the fibers, which can be detected as a phase difference between returned laser pulses transmitted through the fibers. Correlation of the phase difference with strain gives a measure of the strength of the vibration.

Hydrophones

Similar technical concepts are used to detect sound waves in undersea applications. Passive acoustic sensing can be achieved through the use of towed fiber optic arrays, which are growing in popularity as they displace piezoelectric or piezoceramic arrays for harbor defense applications. The reliability of fiber as the sensing element also allows for cost-effective permanent installations of fiber arrays on the ocean floor.

The Lepton Advantage

As towed and permanent fiber arrays become larger and more complex, Lepton’s superior signal/noise and dynamic range performance will be able to maintain current optical budgets over much longer fiber spans. These longer spans will provide flexibility to current system designs and will also enable larger area coverage for permanent ocean floor fiber arrays.

References and Links

T.K. Stanton, R.G. Pridham, W.V. McCollough, M.P. Sanguinetti, J. Acous. Soc. Am., 66 (1979), 1893. J.E. Parsons, C.A. Cain, J.B. Fowlkes, J. Acous. Soc. Am., 119 (2006), 1432. G.A. Cranch, R. Crickmore, C.K. Kirkendall, A. Bautista, K. Daley, S. Motley, J. Salzano, J. Latchem, P.J. Nash, J. Acous. Soc. Am., 115 (2004), 2848.

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