Fiber optic for civil engineering and energy applications

When civil or energy assets fail, the resulting costs are astronomical.  It’s no wonder why engineers in these fields consistently maximize safety factors, operators invest heavily in continuous health monitoring solutions such as fiber optic shape sensing, and there is a constant drive for improving inspection tools and procedures.

Fiber optic shape sensing technology is finding applications in these areas and pushing the boundaries of what legacy technologies can offer. 

Laboratories and construction sites are utilizing fiber optic shape sensing to monitor soil displacement resulting from underground tunnel boring, providing a better understanding of how surface structures may be affected. 

Shape sensing technology using optical fiber in oil & gas

Oil & Gas companies embed fiber optic shape sensing technology in their drilling heads to understand where and in which direction they are digging. Nuclear sites are periodically verifying that rod bundles maintain their optimal straight shape to guarantee safe operation. 

Shape sensing is tracking the position of remotely operated vehicles being deployed in environments where GPS, line of sight, and encoders simply can’t get the job done. 

Whether it’s wind energy, oil and gas, nuclear, mining, or any number of civil structures, the unique capabilities of optical fiber 3D shape sensing are solving challenging problems that were once considered impossible to overcome.

Many more applications can benefit from fiber optic shape sensing. Please contact us to discuss your specific requirements.

Case study of optical fiber shape sensing used for bored tunnel soil settlement

1         Introduction

The world’s underground infrastructure is ever growing. Subway and underground train networks are expanding, mining, drilling, and fracking exploration is accelerating, and the vast land beneath us is being increasingly utilized as a safe storage space. These and many other subsurface engineering projects require dirt and rock to be bored and removed, resulting in large excavated voids in the Earth’s crust. These trigger a redistribution of stress and settling of the surrounding soil. It is critical to understand the nature of this settling to confirm the short and long-term integrity of the underground project, to determine reinforcement requirements, and to ensure the safety of nearby underground and surface structures.

The technology was recently utilized to measure the settling movement related to a metro boring project. A fiber optic sensor was placed in a reduced scale boring test setup to measure the soil displacement resulting from a simulated excavation process. The existing methods of Digital Image Correlation (DIC) and electronic-based displacement sensors, due to their inherent limitations, are unable to produce the desired measurements within the test volume. The optical fiber shape sensor acquired an accurate soil displacement distribution along a continuous line of the surrounding soil and provided sub-millimeter validation of their settling models.

2          Experimental Setup and Description

Testing was carried out using a shape sensor that was approximately 3 meters long. A shape sensor consists of a thin, flexible beam with one or more optical fibers bonded to the beam’s top or bottom surfaces. Depending on the length of the sensor, each optical fiber is comprised of hundreds to thousands of fiber optic strain gauges. When the beam is flexed, the measured bending strain distributions are used to obtain a spatially continuous measurement of the beam’s bending radius. Using these values, a 2D displacement profile is derived along the beams’ entire length. The sensor is constructed to be inherently self-compensating for temperature even if temperature gradients are present along the beam’s length. A layer of heat shrink was applied around the beam to protect the fiber installation from damage. The figure on the left illustrates the core capability of the shape sensing solution.

To simulate the post-boring settling process, an acrylic chamber was filled with a sand, soil, and rock mixture which represented a scaled version of underground material. The acrylic volume contained a cylindrical hole near the mid height of the container to allow for a variable-diameter cylinder to be inserted. The soil mixture was filled in around this cylinder to create the bored tunnel. Once the volume was entirely filled, the cylinder was slowly reduced in diameter and removed, thus creating a cylindrical void. The team of researchers utilized the equipment to obtain a soil displacement profile along a horizontal line located 12 cm above the simulated tunnel. To accomplish this, the 2D beam sensor was buried within the soil volume and positioned at the desired height above the cylinder insert. The figure on the right illustrates the experimental setup.

In the figure above, one can see the walls of the acrylic container, one of the holes for the cylindrical insert, the cylindrical insert with radial adjustment, and the 2D beam sensor resting on top of the soil mixture. This image was captured just after the optical fiber sensor was set in place. Once the remaining volume was filled with soil, an initial measurement was taken as a reference before the cylindrical insert was removed to induce settling. Once the insert was removed and enough time transpired to allow for settling to reach equilibrium, a second displacement measurement was obtained for comparison to the original state.

             In this experiment, researchers were interested in the absolute displacement from the original state to the final state. This was calculated as the following:

Bending strain distributions and were streamed over ethernet in real-time to a laboratory PC which calculated 2D shape. The 2D displacement profile was recorded and visualized throughout the experiment. The final 2D absolute displacement measurement is shown below.

The displacement profile was calculated using the equation above. The x-axis represents the length along the fiber optic sensor, and the y-axis is the absolute displacement relative to the reference shape acquired after the sensor was buried, the volume was completely filled with the soil mixture, and before the cylindrical insert had been removed. The final measurement was taken after the insert was reduced in diameter and removed, producing a void in the soil. This distribution agreed well with the expected Gaussian distribution indicated by the researchers’ soil settling models.

3         Summary

The shape sensing system enabled measuring soil settlement due to tunnel boring operations in a way not possible with any other technology. Shape and displacement were measured within a volume where the use of Digital Image Correlation (DIC) is not possible due to line of sight restrictions. The equipment also provided spatially continuous information, filling in the large gaps between LVDTs and other conventional techniques. Furthermore, the solution provided bidirectional information as opposed to displacements in only the vertical or horizontal planes. The measured displacement distributions agreed well with and validated the researchers’ soil settling models, providing them confidence in using them for future work.

Shape sensing technology can be applied to a large variety of media or structures which are undergoing changes in their shape. Aircraft, automotive frames, boat masts, surfaces such as plates or shells, sand, soil, concrete, or many other structures and materials can all be instrumented with these sensors to understand the deformations or movement they see in testing and operation.