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RE: Bragg grating strain sensor

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Mat:

You might be interested in reading the following two papers, available
at your local technical library:

1. 	Duggan, D. M., Wallace, E. R., and Caldwell, S.R., "Measured and
Predicted Vibrational Behavior of Gulf of Mexico Platforms, OTC 3864."
Twelfth Annual Offshore Technology Conference, Houston, Texas, May 1980.

2. 	Duggan, D. M., Wallace, E. R., and Caldwell, S. R., "Measured
Vibrational Behavior of a Gulf of Mexico Platform, OTC 4137."
Thirteenth Annual Offshore Technology Conference, Houston, Texas, May
1981.

One problem with using accelerometers is that you will probably find
yourself trying to extract data from the noise range.

Regards,

Stan R. Caldwell, P.E.
Dallas, Texas 

-----Original Message-----
From: Mateusz Podskarbi [mailto:podskarm(--nospam--at)2hoffshoreinc.com] 
Sent: Wednesday, May 19, 2004 10:39 AM
To: seaint(--nospam--at)seaint.org
Subject: Bragg grating strain sensor


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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