The goal is of this project is to develop a system employing an optical fiber as a distributed sensor system to detect encroachment into a pipeline right-of-way and alert pipeline operators before damage occurs. The project will: (1) develop the necessary hardware and demonstrate its ability to detect construction equipment near underground pipelines and, (2) develop methods for distinguishing between potentially hazardous and benign intrusions into the right-of-way.
Gas Technology Institute (GTI) – overall project management and research products
ANR Pipeline – use of transmission pipeline site for tests and access to a range of construction equipment
Des Plaines, Illinois 60018
Optical fiber cables can transmit light for many miles; however, each segment of the optical fiber scatters a small amount of the light in all directions and part of the scattered light travels back to the light source. The light scattering can be used to test the integrity of the fiber with a device called an optical time domain reflectometer (OTDR). Attached to one end of the fiber, the OTDR contains an optical coupling device that permits the launching of light pulses into the fiber and detection of returning scattered light. A series of light pulses is sent into the fiber and the amount of light returning to the launch point is measured. Normally the returned light intensity decreases uniformly with distance along the fiber. A flaw in the fiber will either reflect more light back to the source or scatter more light into the cladding. In either case the amount of light detected by the photodetector changes.
In this application, an optical fiber is buried above a pipeline and light pulses are periodically sent down the fiber. If heavy equipment operating on the ground above compresses or vibrates the fiber, changes in its optical properties will cause only a small portion of the light to be reflected back to the OTDR where the variation in reflectivity is detected. The location of the disturbance can be determined by measuring the time it takes for the reflected light pulse to return. The velocity of light in the optical fiber is known and if the time it takes for the pulse to travel down the fiber and return is measured, the location of the scattering point or flaw can be determined. When light pulses 10 nanoseconds in duration are used, the distance resolution is seven feet. Telecommunication companies use an OTDR to measure miles of optical fiber, to insure the light attenuation matches the fiber specifications, and to find large flaws such as bad splices.
Because of the extra sensitivity and signal processing requirements, a custom OTDR is required to detect the variation in reflectivity and to collect and manipulate the data to discriminate among possible causes. Because potentially harmful encroachment is rare, a method for distinguishing potentially harmful unauthorized equipment incursions from benign interferences such as pedestrians or equipment moving nearby is critical. The OTDR includes a laser diode driver and a high-speed amplifier to create and detect light pulses, and a high-speed digital oscilloscope programmed to collect and process data. A 10-nanosecond light pulse is injected into the optical fiber and the returning signal is digitized at a high rate. Conceptually, this divides the fiber into two-meter long segments. The resulting time histories from individual segments are used to pinpoint encroachment, characterize signals created by construction equipment versus benign background noise, and discriminate between the signal sources.
Commercially available fibers were evaluated for use as the distributed sensor and four fibers were selected as the best candidates. ANR Pipeline Division of El Paso Energy Corporation provided the optical fiber testing site where the fibers have been installed. The fibers were buried at four depths, ranging from 6 to 24 inches, to provide sensitivity tests over a range of depths. The sensors were installed parallel and perpendicular to two pipelines.
Natural gas transmission companies mark the right-of-way areas where pipelines are buried with warning signs to prevent accidental third-party damage. Nevertheless, pipelines are sometimes damaged by unauthorized construction equipment. A single incident can be devastating, causing death and millions of dollars of property loss. The industry currently monitors pipelines with weekly over flights with small aircraft or walking patrols. An optical fiber intrusion detection device could be used to detect such unauthorized construction equipment. Alerting a pipeline company that construction equipment is moving close to its pipe permits immediate action to stop unapproved excavation and potential damage to the pipeline. The proposed system would provide real-time policing of pipelines 24 hours a day, seven days a week. Prevention of third-party damage will reduce public and utility injuries, service interruptions, and repair costs, resulting in a safer, more reliable transmission infrastructure. This same technology could be adapted to monitor critical perimeters at compressor stations and custody transfer points.
A high-sensitivity, rapidly responding custom optical time domain reflectometer (OTDR) was designed and built to generate, collect, and analyze the light signals created by encroaching construction equipment. During the evolution of this instrument, the sensitivity was increased by a factor of roughly 1,000. In combination with the most sensitive optical fiber, the custom OTDR can detect static loads on the fiber as small as 0.2 pounds. The spatial resolution of the custom OTDR was + 2 meters or + 10 meters depending on the diode laser used. The spatial resolution did not affect the proof-of-concept.
A method of collecting a time series of light intensity variations from a selected 2 or 10-meter section of fiber was developed and implemented. This capability is required to distinguish benign from hazardous encroachments. The custom OTDR is capable of detecting changes in the light intensity at frequencies up to 5 Hz. The upper limit on frequency response was caused by limitations in the software package used. Other methods of measuring the frequency response of the technique show that light fluctuations in excess of 50 Hz can be detected.
An advantage of the OTDR method is its potential for independently monitoring simultaneously occurring encroachment. This capability was demonstrated.
Optical fibers were evaluated to identify those most sensitive to stress and vibrations. Hergalite® proved to be the most sensitive fiber. Hergalite is made from a standard 50/125-micron multi-mode communication optical fiber that is spirally wrapped with a fine plastic line. The plastic line increases the microbending in the optical fiber when soil stress and vibrations are present. The increased microbending causes greater changes in the light returning to the detector. Selected fibers, including Hergalite were installed along an operating ANR Pipeline (El Paso Gas) transmission pipeline. The fibers were installed at depths ranging from 6 to 24 inches in a 1700-foot long loop. Part of the loop passes over the pipeline.
The method of burying the fiber impacts the sensitivity of the technique. Detection of a small vehicle with the fiber buried four inches deep was achieved. Projections show larger vehicles would be detected at greater depth. Ongoing improvements in the method of burial should improve the sensitivity.
The following general conclusions can be drawn:
The project has been completed.
Final Report [PDF-100KB] - November 2004
Fossil Energy Techline Experimental Fiber Optic Cables to Warn of Potential Pipeline Damage
Right-of-Way Enchroachment Detection, GTI FOCUS Factsheet (June 2003)
Quarterly Report - March 2002
Status Assessment [PDF 42KB]
"Status of Detection of Unauthorized Construction Equipment In Pipeline Right-Of-Ways", Huebler, J. E. and Givens, M., Proceedings of the Natural Gas Technologies 2005, Orlando, FL, January 31-February 2, 2005.
"Detection of Unauthorized Construction Equipment In Pipeline Right-Of-Ways", Huebler, J. E. and Givens, M., Proceedings of the Natural Gas Technologies II Conference, Phoenix, AZ, February 8-11, 2004.