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

How do Bragg grating sensors work?

Bragg grating sensors consist of an optical fiber that contains a distributed Bragg reflector. The most frequently-used technology involves inscribing nano-structured Bragg gratings in the form of periodic variations of the optical refractive index into the core of the optical fibers, as shown in Figure 1.

The length of the Bragg grating is approximately 4 – 6 mm. The Bragg grating acts like a filter, reflecting a particular wavelength of light and transmitting all others.

The dimensions of the grating determine the light frequency that it reflects. The reflected wavelength (λB), called the Bragg wavelength, is defined by the equation,  λB = 2n · Λ  where n is the effective refractive index of the grating in the fiber core and Λ is the grating period.  As the sensor is compressed or stretched, n and Λ change and the value of  λB changes.

Figure 1

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. 

Optical sensors monitor pipeline integrity

An energy company recently installed an HBM measurement system using fiber optic sensors to monitor a section of gas pipeline in Germany. Because the pipeline runs through a river valley, regulations called for the pipeline to be monitored 24 hours a day, seven days a week. The system monitors local stresses, ensuring pipeline integrity even if there is some movement caused by geological activity.

The fiber optic sensors are installed on the pipeline as shown in Figure 2. When installed in this way, the system monitors several crucial pipeline parameters, including how far the pipeline actually moves and the rate of change of the movement.

For this particular application, there are eight HBM Optimet fiber optic sensor chains with six sensors in each chain. Each sensor in the chain operates at a slightly different frequency in the 1,500 – 1,600 nm band. Because of this, the entire chain can be connected to a data acquisition system with only two fiber optic cables. One supplies the light source, while the second provides the input signal to the interrogator.

Figure 2. Each fiber optic chain on this pipeline includes six different sensors. A single fiber optic cable connects the six sensors to the monitoring system.

The system, as shown in Figure 3 consists of:

  • HBM DI-410 Interrogator. This instrument supplies the optical signals for the fiber optic sensors and measures their responses. The DI-410 is a four-channel device that can make up to 1,000 measurements/second. It connects to the data acquisition system and router via an Ethernet interface.
  • Multiplexer (not shown). The multiplexer connects all eight chains to the four-channel DI-410 Interrogator.
  • HBM CX22B-W Data Recorder. This instrument records the readings from the DI-410 Interrogator, as well as the power supply voltages.
  • TK704U UMTS Router. The TK704U UMTS router is a machine-to-machine (M2M) industrial cellular router. It is compatible with 2G or 3G cellular networks and provides reliable wireless connections. In the event that there is a fault condition, the system sends an e-mail to the operator via the connection provided by this router.
  • An uninterruptible power supply system. The system is designed to operate from a 230 VAC mains supply. In the event that the AC supply is interrupted for whatever reason, however, the system switches over to a battery backup supply. This ensures that the system can continuously monitor pipeline stresses. 
Figure 3

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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