Helical Optical Fiber Strain Sensing for Riser Monitoring
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Solution Overview
Problem
Current methods for monitoring the dynamic behavior of subsea production risers are costly and complex due to the need for multiple fiber Bragg gratings and fibers to measure various stresses, which also lead to inaccuracies in strain measurement, especially in the resonance frequency range of 10-3-10 Hz.
Innovation Solution
A sensing system utilizing a single optical fiber disposed helically around the structure with a set of fiber Bragg gratings (FBGs) placed on one complete helical turn, capable of generating reflected light indicative of strain values, and a processing system to determine location coordinates and stress values such as bending moment, tensile force, and torsional moment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple fiber Bragg gratings and fibers are used to measure various stresses on the riser pipe, then the measurement coverage is improved, but the system complexity and cost increase
Solution Approach 1:
The patent combines multiple sensing functions into a single optical fiber by wrapping it helically around the riser pipe. Multiple FBGs are embedded within this single fiber, allowing simultaneous measurement of axial strain, hoop strain, and torsional strain through a unified sensing system rather than requiring separate fibers for each measurement type.
Solution Approach 2:
The single helically-wrapped optical fiber serves multiple measurement functions simultaneously. By strategically placing FBGs at different positions and orientations within the fiber, the system can measure various stress components (axial, hoop, torsional) and derive complete stress state information from one universal sensing element.
2Measurement precision
If multiple fibers and gratings are deployed to cover all stress components, then the measurement completeness is improved, but the installation complexity increases
Solution Approach 1:
The patent merges multiple installation tasks into a single operation by using one continuous optical fiber that can be wrapped around the riser pipe in a helical pattern. All FBGs are embedded within this single fiber during one installation process, eliminating the need to separately install multiple fibers and their associated gratings.
Solution Approach 2:
The optical fiber is wrapped in a helical (curved) pattern around the riser pipe, which naturally accommodates the cylindrical geometry of the pipe. This helical configuration allows the fiber to follow the pipe's curvature and provides automatic spatial distribution of FBGs along the pipe length, simplifying the installation process compared to straight-line or radial configurations.
3Adaptability or versatility
If multiple parallel gratings are installed on the pipe, then the stress state coverage is improved, but measurement accuracy deteriorates due to accumulated errors
Solution Approach 1:
The patent combines multiple FBG measurements from a single helically-wrapped fiber to derive complete stress state information. By measuring both axial and hoop strains through FBGs positioned at different orientations within the same fiber, and combining these measurements with the known helical geometry, the system calculates torsional strain and derives all stress components from one integrated sensing system, reducing error accumulation.
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 approach reduces the number of sensors required, simplifies the installation process, and provides accurate stress measurements across the riser pipe, enabling effective monitoring of dynamic behavior without the limitations of existing methods.
Implementation Method 1
Each FBG in the set is configured to generate reflected light that is indicative of strain values at a location of each respective FBG on the optical fiber
Data Source
AI summary
A sensing system that includes an optical fiber disposed helically around an outer surface of a structure along a longitudinal axis of the structure is provided. The optical fiber is disposed such that at least one complete helical turn of the optical fiber covers the length of the structure. Further, the sensing system also includes a fiber Bragg grating (FBG) set comprising a plurality of FBGs in the optical fiber. Each FBG in the set is configured to generate reflected light that is indicative of strain values at a location of each respective FBG on the optical fiber. Furthermore, the system also includes a processing system coupled to the optical fiber. The processing system is configured to determine location coordinates of each FBG and values of one or more of bending moment, tensile force, and torsional moment acting at each FBG location on the optical fiber.