Distributed Strain Sensing for Pipeline Safety Against Fault Moving and Landslide

Overview

Main Objective

The project developed a robust distributed fiber optic strain sensing system for long-term monitoring of buried gas pipelines that are potentially vulnerable to ground deformation at fault crossing and landslide sites; Tested the developed system at a PG&E site; and Developed a commercialization plan for the system for pipeline monitoring.

Public Abstract

Many pipeline systems traverse for hundreds of miles over terrain with varied environmental and geotechnical characteristics, and therefore it is not always possible to avoid passing through zones where permanent ground deformation is likely to occur. If the movement is sufficiently large, the stresses can cause a wide variety of failure mechanisms, including tensile failure, buckling, wrinkling and joint failure. Differential ground movements can instigate deformations that may impact serviceability requirements or prompt failure mechanisms that may exceed limit states. This project examines the feasibility of the new distributed fiber optic strain measurement system developed at UC Berkeley for proactive long-term monitoring of buried gas pipelines that are potentially vulnerable to ground deformation across faults and landslides. The system has the following unique characteristics (i) low cost (<$20k) using an advanced digital architecture replacing the conventional microwave synthesizer, (ii) a special pulse and modulation to reduce measurement time for real-time dynamic sensing, and (iii) use of small gain stimulated Brillouin scattering with fast data management. This invention allows strain data to be captured within short data acquisition time of less than one minute. The current prototype system achieves a 2cm readout interval, 2m spatial resolution and about 20µε accuracy. Fiber optic material itself (silica-based) is relatively inert for long term monitoring. Still, there is no currently available solution to ensure that the cable is firmly attached to the pipeline for long-term pipe strain monitoring. Furthermore, the attachment of fiber optic cable to the pipeline is time-consuming. In this project, different fiber optic cable attachment methods will be tested in the laboratory. The development includes specifications of fiber optic material, installation methods, and repair strategies. The attachment methods selected based on the laboratory test results will be deployed at a PG&E field site in Gilroy, CA. The pipeline will be installed in 80' segments with an open trench that will not exceed 160' long at any given time. This poses a challenge for on-the-job workflow and the trial will evaluate the technology's robustness for real pipeline deployments. The pipeline deformation at the segment joints is essential to monitor, as a failure at the weld location is a concern. This requires strong coupling at a location that will be a discontinuity prior to welding in the trench. Overcoming these difficulties in the field trial will provide advancement toward the industry's acceptance and adoption of this innovative technology. After installation, long-term monitoring of the instrumented pipeline will be performed during the duration of the project. Three-dimensional finite element modeling of the soil-pipeline interaction that is expected to occur at the field site will also be performed in order to assess the value of the distributed strain data in evaluating the actual condition of the pipeline. Based on the findings of the laboratory tests and field trial, a commercialization plan for the distributed fiber optic strain-based pipeline monitoring system will be developed.

Summary and Conclusions

The project is to examine the feasibility of a distributed strain sensing (DSS) for long-term monitoring of buried gas pipelines that are potentially vulnerable to ground deformation across faults and landslides.

Laboratory four-point bending tests were conducted to examine different fiber optic cable attachment methods for monitoring strain development with buried pipelines. The experimental results demonstrate that the method to attach fiber optic cables on pipelines (using an appropriate adhesive, followed by wax tape and an outer wrap) ensures excellent deformation coordination with the monitored pipeline, allowing the fiber optic sensor to capture the complete strain distribution at all locations during loading process without damaging the pipeline.

A field test was conducted on a replaced steel gas pipeline at PG&E site using the installation method validated in laboratory tests. Three sensing cables were attached along the pipeline at three clock positions (12, 1:30 and 10:30) respectively. Each strain cable is 400 ft long. The monitored section includes two welded pipe joints with one weld at 220ft and the other at 280ft. One strain cable and one temperature cable were laid on the ground along the pipeline. The length of the strain cable and temperature cable is 1000ft respectively.

The collected field test results showed the strain changes with an error margin of less than 20µ. The minor changes of strain corresponded to thermal expansion and contraction of the pipe steel due to temperature fluctuations throughout the monitoring period. The noted variations in strain were attributed to the substantial rains experienced in the season. The field test is ongoing after the project was completed and the project team continues to collect field test data.

A 3D continuum finite element (FE) model was developed to conduct numerical simulation of the field conditions under fault movement. The simulation results indicate that the peak strain location shifts with increasing fault displacement. For a long continuous pipeline, the affected and curved region during fault displacement could extend to a considerable length, such as the central 50-meter section. These findings are important when examining the distributed strain profiles obtained in the field to identify the most vulnerable locations where damage of pipelines could occur due to land movement.
An advanced soil constitutive model was developed to simulate soil-pipeline interaction and analyze the distributed strain profiles obtained from the fiber optic strain sensing cables installed on the pipelines under various conditions of land movement.

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