Optical Pressure Sensor With Bellows Compensation
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Solution Overview
Problem
Existing pressure sensors for marine seismic exploration face challenges in sensitivity to low-frequency pressure variations, robustness against vibrations and noise, and maintaining accuracy over long periods and varying temperatures, while also requiring a compact design to minimize material and operational costs.
Innovation Solution
The development of an optical pressure sensor device using a Fibre Bragg Grating (FBG) with a chamber filled with a pressure transfer medium and pressure-sensitive mounting assemblies that increase the length of the FBG sensing element with pressure, ensuring mechanical isolation from strain in the cable and incorporating static pressure compensation to differentiate between high-frequency and low-frequency variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If FBG reflectors are used as pressure sensors in streamers, then optical signals can replace electrical signals allowing copper signal lines to be replaced by optical fibres, but the sensors show low sensitivity to low-frequency pressure variations and are sensitive to mechanical vibrations and noise
Solution Approach 1:
A pressure transfer medium (fluid or gel) is introduced as an intermediary between the acoustic pressure waves and the FBG reflector. The medium fills the chamber and transmits pressure variations to the FBG while isolating it from direct mechanical vibrations and noise in the cable, thereby improving sensitivity to acoustic waves while reducing sensitivity to harmful vibrations.
Solution Approach 2:
The patent replaces direct mechanical coupling of the FBG to the cable with an optical measurement system. Instead of using electrical signals from piezo elements that require thick copper lines, the system uses optical signals from the FBG reflector interrogated by light, substituting a mechanical/electrical system with an optical one that is less susceptible to electromagnetic interference and mechanical noise.
2Measurement precision
If the FBG sensing element is made longer to increase sensitivity, then sensitivity to pressure waves improves, but the sensor becomes more susceptible to strain from cable movements and vibrations
Solution Approach 1:
The pressure transfer medium acts as a mediator that decouples the FBG from direct mechanical strain in the cable. The FBG is mounted on the chamber wall rather than being directly attached to the cable, and the pressure medium transmits only pressure variations to the FBG, filtering out cable strain and vibrations.
Solution Approach 2:
The sensor is segmented into distinct functional components: the cable, the mounting assembly, the pressure transfer medium, and the FBG reflector. This segmentation allows the FBG to be optimized for pressure sensitivity while the mounting assembly and pressure medium protect it from cable-induced strain.
3Ease of manufacture
If the sensor cross-sectional area is reduced to minimize material and operational costs, then manufacturing costs decrease, but sensitivity to pressure waves may be compromised
Solution Approach 1:
The chamber is designed as a thin-walled structure that can be made from flexible materials. This allows the sensor to have a small cross-sectional area while maintaining sufficient sensitivity, as the thin chamber wall efficiently transmits pressure variations to the FBG reflector without requiring a large volume.
Solution Approach 2:
The sensitivity of the sensor can be adjusted by changing parameters such as the chamber volume, the type of pressure transfer medium, and the mounting configuration of the FBG, rather than simply increasing the cross-sectional area. This allows optimization of sensitivity while maintaining a compact, cost-effective design.
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
The solution enhances sensitivity to acoustic pressure waves, reduces noise from mechanical vibrations, and maintains accuracy over extended periods and varying temperatures, while minimizing the sensor's cross-sectional area and operational costs.
Implementation Method 1
An FBG reflector consists of an optical fibre wherein, at some location, a series of material modifications is arranged lengthwise in the fibre. Normally, the optical properties of an optical fibre are constant along the length, which optical properties include the refractive index. Such material modification, however, has a slightly different refractive index. A plurality of such material modifications, at mutually the same distance, behaves as a grating, which typically is reflective for a small wavelength band.
Implementation Method 2
Such FBG reflector sensor is typically sensitive to (local) strain. Variations in strain cause variations in length of the fibre, including variations in distance between the successive material modifications of the Bragg grating. This, in turn, translates to variations in the wavelength of the reflected light.
Implementation Method 3
a chamber filled with a pressure transfer medium, preferably oil, the chamber being suitable for immersion in a fluid and having at least one window that is at least partly transparent to pressure waves in such fluid
Data Source
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AI summary
An optical pressure sensor device (1) comprises: - a chamber (2) filled with pressure transfer medium and having at least one window (4) transparent to pressure waves; - an optical fibre (7) with a Fibre Bragg Grating (8); - a first pressure-sensitive mounting assembly (100) arranged within the chamber, holding the optical fibre; - a second pressure-sensitive mounting assembly (200) arranged within the chamber, holding the optical fibre. - The first pressure-sensitive mounting assembly, the second pressure- sensitive mounting assembly, and a static pressure compensation assembly (300) comprise pairs of bellows arranged on opposite sides of the fibre (7). - The bellows (310, 320) of the static pressure compensation assembly have their interior in fluid communication with the pressure transfer medium in the chamber via a choke channel (314, 324), and have very low static stiffness.