Fiber Optic Strain Sensors Monitor Pipeline Integrity

Strain gauges are very popular for a variety of applications. They are particularly suited to measuring mechanical loads on structures as well as making geotechnical measurements, both applications related to pipeline monitoring in the oil & gas industry. You can also use strain gauge measurement systems to measure a pipeline's operational parameters, such as the pressure of the transported medium, and to prevent failures of a pipeline installed in landslide areas. In this application, you would use strain gauges observe stress on the pipeline in critical areas to detect displacement or deformation that may cause the pipeline to fail. When monitoring pipeline integrity, the measurement system must be completely trustworthy. All business and production processes are vitally important and need to meet all the required regulations. While strain gauges are very sensitive, and provide stable output signals with good long term performance, using them effectively requires knowledgeable personnel with a lot of practical experience. In addition, the instrumentation and cabling can be complex, and each strain gauge must be individually calibrated. To overcome these disadvantages, some users are turning to fiber optic sensors.  HBM, a worldwide manufacturer of sensors and transducers, data acquisition and software, has developed a pipeline monitoring solution using fiber optic strain sensors, called Bragg grating strain sensors. Fiber optic technology has a number of advantages over conventional strain gauge technology, including:

• Simpler wiring
• No calibration
• High resistance to alternating loads and high strain
• Excellent fatigue behavior
• Insensitivity to electromagnetic interference, including lightning strikes and other interference sources that may create a high-potential electric field
• Usable in a potentially explosive environment with no special wiring

These advantages make them a good fit for many applications, including:

• Monitoring pipeline integrity
• Observing structural integrity of bridges and pipelines
• Experimental stress analysis of vessels and tubes
• Materials testing
• Monitoring land movement

The sensors are designed so that the Bragg wavelength is in the C-band, between 1,500 nm and 1,600 nm. The reason HBM engineers determined these wavelengths should be used is that they are also used for telecommunications applications, and optical fibers with very low losses at these frequencies are readily available. With the given grating length, the sensor therefore consists of more than 10,000 grating periods. 

Because it is essential that this system remains operational, it also monitors system availability, sensor availability, power supply voltages, and memory operation, in addition to stresses on the pipeline. The system is also programmed to sound an alarm and send an e-mail in case it detects an abnormality in any of these areas. 

Using fiber optics pays off

Using fiber optic sensors in this application has really paid off for the energy company. For one thing, because each chain of sensors connects to the monitoring system via a single pair of fiber optic cables, cabling costs were far less than they would have been had the system used conventional strain gauge sensors.   Another benefit is that the monitoring system can be located at quite a distance from the sensors. This is because the optical sensors do not require an excitation voltage as do conventional strain gauges. Lastly, the fiber optic cables are safer to use in a potentially explosive environment such as a pipeline. Because fiber optic cables don't carry sufficient energy to cause an explosion, no special precautions are necessary. For more information on fiber optic strain sensors, contact HBM by clicking here or call (800) 578-4260 . 

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