Every year in the US natural gas pipelines are damaged by construction activities, even though warning signs often mark where a pipe has been laid.
Now, a new fibre optic cable system being tested in a joint US Department of Energy/Gas Technology Institute project could give gas pipeline companies a remote method for detecting encroaching construction activity and help prevent accidents.
This month, in the first field test of the system, crews from ANR Pipeline, a subsidiary of El Paso Corporation of Houston, Texas, buried fibre optic cables over one mile of active pipeline in Northwest Indiana.
During the next several months, construction equipment will create vibrations in the ground, causing compressions in the soil surrounding the pipeline. If the vibrations and soil compressions are sufficiently close to the pipeline, the fibre optic cable will bend ever so slightly. Even a minute deformation in the cable will change its light transmission and reflection properties and send an early warning of a potentially hazardous encroachment.
‘Third-party damage’ is the major cause of damage to natural gas transmission pipelines. As urban areas expand and the demand for natural gas increases, buildings are being constructed increasingly closer to pipelines. More construction near pipelines increases the probability and consequence of damage.
Several approaches have been suggested or are being studied for detecting potentially harmful impacts on the hundreds of thousands of miles of underground gas pipelines that criss-cross the United States.
Satellite, ground-based visual and global positioning system monitoring systems are among the new technologies being studied to help protect the integrity of the nation’s pipeline system.
Fibre optic technology, however, is projected to be one of the least costly options. Moreover, it can help prevent pipeline damage, not simply detect it after it happens.
Four different fibre optic cables were used in the testing. Three multi-mode fibres (Hergalite, Corning 62.5/125 and Corning 50/125) and one single-mode fibre (Fibrecore Limited’s SM600), were buried above the pipeline approximately 15-50 centimetres below the surface of the ground.
Construction equipment does not have to break the fibre to be detected. When intrusion is detected, an alarm sounds, alerting the pipeline company of equipment encroaching on its pipe. This would enable pipeline companies in the future to take immediate action to stop unauthorised excavation and prevent potential damage to the pipeline.
The system can not only warn of approaching construction activity, it can also help operators pinpoint where the potential hazard is developing. Stresses in the fibre optic cable will cause some of the light to be reflected back to its source. Because the velocity of light in the fibre will be known, the location of the encroaching equipment can be determined by measuring the time for the reflected light pulse to return.
The largest technical barrier to perfecting fibre optic cable technology for pipelines is in developing methods to distinguish between potentially hazardous and benign intrusions into the right-of-way.
Compared to benign activities and noises, such as pedestrian traffic and thunder, construction equipment will be large and have characteristic signals. One goal of the project is to minimise or eliminate false readings from the cables through all seasons and soil conditions.
Results of the project are expected in the autumn of 2003.