Monitoring Pipeline Integrity with Fiber Optic Strain Sensors

Strain gauges are widely used for many applications, and in particular are perfect for making geotechnical measurements and determining mechanical loads on structures.

Strain gauge measurement units can also be used for measuring a pipeline's operational parameters for instance, the pressure of the transported medium, and for preventing failures in pipelines installed in landslide areas.

This article explains how strain gauges observe stress on the pipeline in vital areas so as to detect deformation or displacement which might lead to the failure in the pipeline.

Monitoring Pipeline Integrity

The measurement unit must feature high reliability while monitoring the integrity of the pipeline. All business and production methods are highly important and have to comply with all the required regulations.

Although strain gauges are highly sensitive and provide constant output signals with high-quality long term performance, these units should be handled only by experts with sufficient experience. Moreover, the cabling and instrumentation are quite complicated and each strain gauge has to be separately calibrated.

In order to overcome these hurdles, some users are beginning to use fiber optic sensors. HBM, a global manufacturer of sensors and transducers, data acquisition and software, has designed a pipeline monitoring solution implementing fiber optic strain sensors known as Bragg grating strain sensors.

Benefits of Fiber Optic Technology

Fiber optic technology has many benefits over traditional strain gauge technology, including:

• Simple wiring
• Superior fatigue behavior
• Does not require calibration
• High resistance to irregular loads and high strain
• Usable in a potentially explosive environment without the need for special wiring
• Insensitivity to electromagnetic interference such as lightning strikes and other interference sources that may generate a high-potential electric field

Applications of Fiber Optic Technology

These benefits enable them to be suitable for several applications such as:

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

Bragg Grating Sensor Operation

Bragg grating sensors comprise an optical fiber containing a distributed Bragg reflector. The technology most commonly used involves inscribing nano- structured Bragg gratings as periodic variations of the optical refractive index into the optical fiber core(Figure 1).

The Bragg grating measures about 4-6mm in length. The Bragg grating behaves as a filter, and reflects a specific wavelength of light while transmitting the remaining light. The grating’s dimensions establish the light frequency that it reflects. The reflected wavelength (λ_B), called the Bragg wavelength, is illustrated by the equation, λ_B = 2n • ∧ where ‘n’ is the effective refractive index of the grating in the fiber core and ‘∧’ is the grating period. When the sensor is stretched or compressed, n and ∧ changes and the value of λ_B also changes.

Figure 1. Image credit: Hottinger Baldwin Messtechnik GmbH

The sensors are built such that the Bragg wavelength is in the C- band between 1,500nm and 1,600nm. HBM engineers had a particular reason for designing the sensors with specific wavelengths so that they can also be applied in telecommunications applications, and optical fibers with minimal losses at these frequencies are easily available on the market. Due to the given grating length, the sensor comprises more than 10,000 grating periods.

Optical Sensors Monitor Pipeline Integrity

Recently an HBM measurement unit using fiber optic sensors was deployed by an energy company to monitor a section of gas pipeline in Germany. The pipeline required constant monitoring as per regulations as it was laid next to a river valley.

The unit regulates local stresses, thereby ensuring the integrity of the pipeline even in the event of movement caused by geological activity. The fiber optic sensors are fitted on the pipeline (Figure 2). This enables the unit to monitor many important pipeline parameters such as actual distance moved by pipeline and the rate at which the movement changes.

In this specific case, eight HBM Optimet fiber optic sensor chains with six sensors in each chain were used. Each of these sensors was programmed to operate at a slightly different frequency in the 1,500–1,600nm band. Due to this feature, it is possible to connect the whole chain to a data acquisition system using just two fiber optic cables. One is used for supplying the light source and the other for providing 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. Image credit: Hottinger Baldwin Messtechnik GmbH

The system (Figure 3) includes the following:

• The HBM DI-410 Interrogator transmits optical signals for the fiber optic sensors and measures their responses. It is a four-channel instrument capable of performing up to 1,000 measurements/second. It connects to the data acquisition system and router via an Ethernet interface.
• The multiplexer connects all eight chains to the four-channel DI-410 Interrogator.
• The HBM CX22W data recorder records the readings from the DI-410 Interrogator and the power supply voltages.
• The TK704U UMTS router is a machine-to-machine (M2M) industrial cellular router. It is well-suited for 2G or 3G cellular networks and provides dependable wireless connections. In case of faults, the system sends an e-mail to the operator via the connection provided by this router.
• The power supply system is uninterruptible, and can operate from a 230 VAC mains supply. In case there is an interruption in the AC supply, the system switches over to a battery backup supply, thus ensuring that the system can constantly monitor pipeline stresses.

Figure 3. Image credit: Hottinger Baldwin Messtechnik GmbH

As it is vital that this system always remains operational, it also monitors unit availability, power supply voltages, sensor availability and memory operation along with stresses on the pipeline. The unit is also designed to raise an alarm and send an e-mail in the event of any irregularity in any of these areas.

Benefits of Using Fiber Optics

The energy company greatly benefitted from the usage of fiber optic sensors. The sensors could be connected to the monitoring unit via a single pair of fiber optic cables, thus greatly reducing cabling costs compared to if the company had to use traditional strain gauge sensors. Another plus point is that the monitoring unit can be placed far from the sensors as optical sensors do not require an excitation voltage as traditional strain gauges.

Finally, fiber optic cables are a safer option for use in a potentially explosive environment like a pipeline. As fiber optic cables do not carry enough energy to cause an explosion, there is no need for any special safety measures.

This information has been sourced, reviewed and adapted from materials provided by HBM, Inc.

For more information on this source, please visit HBM, Inc.