Structure monitoring
First Claim
1. A method of resonant vibration detection in a subsea riser tethered at opposite ends and a portion thereof free to move therebetween, the method using an optical fiber distributed acoustic sensor (DAS) having an optical fiber, the optical fiber being coupled to the subsea riser at at least two points in the portion such that a known part of the optical fiber is adjacent to, connected to and moves with a known part of the portion of the subsea riser that is free to move, the method comprising:
- detecting backscattered light on the fiber as it moves with the subsea riser, the backscatter being dependent on strain induced in the fiber due to mechanical strain in the subsea riser caused by resonant vibrations occurring in the subsea riser as the portion between the two tethered ends moves;
from the detected backscatter, processing a signal representative thereof to determine a frequency of oscillation of the resonant vibrations of the subsea riser as the portion between the two tethered ends moves; and
frequency filtering the signal detected by the optical fiber DAS to obtain low frequencies, wherein the low frequencies are used for detecting the resonant vibrations of the subsea riser as the portion between the two tethered ends moves,wherein the low frequencies comprise frequencies of less than 50 Hz.
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Accused Products
Abstract
A method and apparatus for monitoring a structure using an optical fiber based distributed acoustic sensor (DAS) extending along the length of the structure. The DAS is able to resolve a separate acoustic signal with a spatial resolution of 1 m along the length of the fibre, and hence is able to operate with an acoustic positioning system to determine the position of the riser with the same spatial resolution. In addition, the fiber can at the same time also detect much lower frequency mechanical vibrations in the riser, for example such as resonant mode vibrations induced by movement in the surrounding medium. By using vibration detection in combination with acoustic positioning then overall structure shape monitoring can be undertaken, which is useful for vortex induced vibration (VIV) visualisation, fatigue analysis, and a variety of other advanced purposes. The structure may be a sub-sea riser.
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Citations
18 Claims
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1. A method of resonant vibration detection in a subsea riser tethered at opposite ends and a portion thereof free to move therebetween, the method using an optical fiber distributed acoustic sensor (DAS) having an optical fiber, the optical fiber being coupled to the subsea riser at at least two points in the portion such that a known part of the optical fiber is adjacent to, connected to and moves with a known part of the portion of the subsea riser that is free to move, the method comprising:
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detecting backscattered light on the fiber as it moves with the subsea riser, the backscatter being dependent on strain induced in the fiber due to mechanical strain in the subsea riser caused by resonant vibrations occurring in the subsea riser as the portion between the two tethered ends moves; from the detected backscatter, processing a signal representative thereof to determine a frequency of oscillation of the resonant vibrations of the subsea riser as the portion between the two tethered ends moves; and frequency filtering the signal detected by the optical fiber DAS to obtain low frequencies, wherein the low frequencies are used for detecting the resonant vibrations of the subsea riser as the portion between the two tethered ends moves, wherein the low frequencies comprise frequencies of less than 50 Hz. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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10. A system for detecting the resonant vibrations of a subsea riser tethered at opposite ends and a portion thereof free to move therebetween, the system comprising:
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an optical fiber distributed acoustic sensor (DAS) system having an optical fiber, the optical fiber being coupled to the subsea riser at at least two points in the portion such that a known part of the optical fiber is adjacent to, connected to and moves with a known part of the portion of the subsea riser that is free to move, the sensor system further comprising; an interferometer arrangement arranged to detect backscattered light on the fiber as it moves with the subsea riser, the backscatter being dependent on strain induced in the fiber due to mechanical strain in the subsea riser caused by resonant vibrations occurring in the subsea riser as the portion between the two tethered ends moves; and a processor arranged, from the detected backscatter, to process a signal representative thereof to determine a frequency of oscillation of the resonant vibrations of the subsea riser as the portion between the two tethered ends moves, wherein the processor is further arranged to frequency filter the signal detected by the optical fibre DAS to determine low frequencies, wherein the low frequencies are used for detecting the resonant vibrations of the subsea riser as the portion between the two tethered ends move, wherein the low frequencies are frequencies less than 50 Hz. - View Dependent Claims (11, 12, 13, 14, 15, 16, 17, 18)
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Specification