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Bragg grating strain sensor

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I am working on the project for monitoring the deepwater offshore riser
vibrations.
The purpose of monitoring is to measure the fatigue damage accumulated
during riser service.

There are several options considered as to what monitor and how.

The old-school approach is to use accelerometers and angle rate sensors
attached to the riser at several locations. This data is later used in
combination with FE model to assess riser response -> stresses -> fatigue.
This is however criticized for not directly measuring riser stress/strain
and relaying on two many assumptions (FE model and nature of the riser
response) and two much data processing (from accelerations to displacement
timetraces).

The other approach is to measure riser strain. The typical strain gauges are
not favorable due to its poor long time reliability especially in deepwater
environment. The potential system is so-called smart-rod system offered by
several companies. The system is based on measuring strain with light. The
Bragg grating is used to measure the change in light color due to stretching
of the grating attached to the riser.  The change in color is than
transformed to the amount of stretch of the optic fiber. Having two of these
fibers at different distances from the bending axis and obtaining the
differential strain allows calculating the strain in the riser.

>From what I know the Bragg grating strain sensors are quite widely used in
onshore civil and structural engineering. Does anybody have any experience
with them? Are there any other systems to measure the strain that could be
used for subsea application?


Best Regards,
Mat Podskarbi


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