Distributed Pressure Sensing Cable Using FBG Cavities
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Solution Overview
Problem
Conventional down-hole sensing techniques for pressure measurement in environments like geothermal wells and oil wells are limited by the need for separate packaging and splicing of sensors, which increases cost, complexity, and reduces resolution.
Innovation Solution
A pressure sensing cable with polarization maintaining optic fibers and Fiber Bragg Grating (FBG) pairs is used, where each FBG pair defines a reflective cavity with overlapping wavelength ranges, allowing for distributed pressure sensing without separate packaging or splicing, and a processing unit identifies pressure readings based on the return signal's wavelength-specific reflections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If separately packaged sensors are spliced along a cable for distributed pressure sensing, then pressure measurement capability is achieved, but device complexity and cost increase
Solution Approach 1:
Multiple pressure sensing functions are merged into a single continuous optic fiber through integrated FBG pairs. The FBG pairs are directly formed in the optic fiber rather than being separate components, eliminating the need for individual sensor packaging and splicing operations while maintaining distributed pressure sensing capability along the cable length.
Solution Approach 2:
The optic fiber serves multiple functions simultaneously: it acts as both the transmission medium for optical signals and the sensing element for pressure measurement. The FBG pairs embedded in the fiber enable the same fiber to function as both waveguide and sensor, reducing overall system complexity compared to separate sensing components.
2Measurement precision
If separately packaged sensors are used for distributed pressure sensing, then pressure measurement is enabled, but manufacturing cost increases
Solution Approach 1:
The sensing elements (FBG pairs) are integrated directly into the optic fiber during manufacturing, eliminating the need for separate sensor production, packaging, and assembly operations. This consolidation reduces material costs and manufacturing complexity while enabling distributed pressure sensing throughout the cable length.
Solution Approach 2:
The optic fiber itself serves as the sensing element through the integrated FBG pairs, eliminating the need for separate sensor components. The fiber structure provides both signal transmission and pressure sensing functions, reducing overall system cost by removing redundant components and assembly processes.
3Measurement precision
If conventional sensor splicing is used, then distributed sensing is achieved, but resolution is reduced
Solution Approach 1:
The FBG pairs are integrated continuously along the optic fiber without discrete splicing points, creating a seamless sensing distribution. This continuous integration maintains high spatial resolution by eliminating gaps or discontinuities that would occur with traditional spliced sensor arrangements, while also reducing the complexity associated with multiple splicing operations.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enables accurate, simplified, and cost-effective distributed pressure measurement along the length of the cable, providing a pressure map and reducing the need for separate sensor packaging and splicing, while withstanding harsh environmental conditions.
Implementation Method 1
The first FBG defines a first wavelength range at which the first FBG is reflective. The second FBG defines a second wavelength range at which the second FBG is reflective.
Implementation Method 2
The first wavelength range of the first FBG and the second wavelength range of the second FBG at least partially overlap, defining a FBG cavity extending between the first FBG and the second FBG.
Implementation Method 3
a polarization maintaining (PM) optic fiber having a length and Fiber Bragg Grating (FBG) pairs formed in the PM optic fiber
Data Source
AI summary
A pressure sensing cable in one embodiment includes a polarization maintaining (PM) optic fiber having a length; and a Fiber Bragg Grating (FBG) pair formed in the PM optic fiber. The FBG pair includes a first FBG formed at a location along the length of the optic fiber. The first FBG defines a first wavelength range at which the first FBG is reflective. The FBG pair also includes a second FBG spaced a distance from the first FBG to define a FBG cavity extending between the first FBG and the second FBG. The second FBG defines a second wavelength range at which the second FBG is reflective. The first wavelength range of the first FBG and the second wavelength range of the second FBG at least partially overlap.