Fiber Optic Sensors for Monitoring Pipe Bending Due to Ground Movement

This paper describes an installation of fiber optic sensors designed to measure pipe bending due to ground movement at three sites on a 16” gas transmission line. The sour gas pipeline had failed in December, 2004 from excessive forces related to ground movement. As temporary mitigation the pipeline was daylighted to reduce the soil traction forces but a comprehensive monitoring program had to be developed before placing the pipeline back into conventional service. During the time when the section of pipeline was daylighted, three linear and three coiled fiber optic sensors were installed at each of three sites selected as part of a system designed to measure bending strains in the 0.01–1.0% range. The two types of sensors were placed in pairs approximately at the 12, 4, and 8 o’clock positions. Conventional vibrating wire (VW) strain gauges were also installed at the fiber optic sensor locations for comparison purposes. Slope inclinometers were installed at each of the instrumentation sites to correlate ground movement to pipe bending. Following pipeline re-coating, and back-filling, visits to the site were made at approximately monthly intervals to gather data at conveniently placed break-out boxes. The complete fiber optic sensor system functionality is described and results are presented that show how the raw strain data are transformed into bending using software that also serves as a secure database.