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Publication Date: 2015-Oct-28
Document File: 3 page(s) / 64K

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The invention relates to deploying optical fiber cables via conduits over long distances as required undersea and preventing moisture damage to the cables A distributed temperature sensor is configured to monitor the internal temperature of the conduit along the entire length for any step change in temperature indicative of vaporisation The measurements involve sending laser light along a fiber optic cable and observing the scattering of incident light along the fiber length Raman backscatter distributed temperature sensing interrogator is connected to either end of the optical fiber which on deployment provides temperature readings along every 0 5m of fiber at frequent time intervals as required Using the method indicated temperature of the fiber cable inside the tube is monitored whilst the tube is being expelled of moisture Thereafter if there are no step changes in temperature along the length of the fiber the internal of the tube is considered to be dry

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When deploying fiber-optic cables within a protective tube using the technique of fiber- pumping with fluid, the fluid used to propel the fiber is water, since it is plentiful and cheap. However, it is essential to ensure that once the fiber is successfully deployed the tube is subsequently dried to avoid damage to the fiber. The long intended service life of a fiber optic cable in a subsea flexible pipe makes it essential that all efforts are taken to ensure its longevity. This is important because of the convoluted nature of small diameter subsea flexible pipe used for conveying fiber optic cables. This is especially the case when the flexible pipe is wound onto a storage reel, it may remain for a long period of time.

Evacuating the tube of water and moisture will protect the optical fiber from optical and mechanical damage. Drying is usually carried out by expelling the water from the tube with dry gas and monitoring the end of the tube with a humidity sensor. The humidity sensor method will give a direct indication of the dryness of the exiting gas. This has been demonstrated in EP0734109A1 to Griffioen et al.

Alternatively, a thermocouple to monitor the temperature of the exiting gas can be used to look for a step change in temperature that indicates dryness due to the latent heat of vaporisation. However, in a long and narrow tube of the type used in a subsea flexible pipe, there is a danger that pockets of moisture may remain along the length and not be sensed at the end of the tube.


The proposed invention solves the problem of eliminating moisture along an optical fiber cable by using the deployed optical fiber itself as a distributed temperature sensor. The internal temperature of the tube is monitored along the entire length for any step change in temperature indicative of vaporisation.

Distributed temperature sensing using fiber-optics relies on the use of a standard OM2 multimode optical fiber as the temperature probe itself. A multimode fiber is used for distributed temperature sensing and a pair of fiber bragg gratings (FBGs) are used in each discrete sensing point, partially overlapped in the spectral domain, allowing for temperature- ind...