New distributed optical sensor for detection and localization of liquid leaks Part I. Experimental studies

This work introduces and describes the tests of a new optical distributed sensor. This new sensor is capable of detecting and locating liquid hydrocarbon leaks on long pipelines. It is an improvement over the current fiber bending distributed sensors that use a polymer cable that interacts with an optical fiber by means of a helicoidal wire. When a liquid or gas compatible with the polymer within this type of sensor comes in contact with it, the liquid is absorbed and causes the swelling of the polymer and the concomitant compression of the optical fiber against the helicoidal wire. This phenomenon bends the fiber and increases the optical attenuation of a signal that travels through the fiber. The signal attenuation permits the detection of a specific liquid presence in the vicinity of the sensor and the reflection of the same signal allows to pinpoint the location of this event. The new sensor introduced in this paper has the following advantages over similar devices: (a) the swelling of the polymer is conducted in a preferential direction permitting to concentrate the osmotic pressure towards the optical fiber, which improves significantly the response speed of the sensor. (b) The fiber is placed within a channel located eccentrically in the polymer cable. Therefore, no additional protection is needed to protect the fiber. Of even greater importance, the signal attenuation caused by stress and deformations due to direct contact of the fiber with the helicoidal wire is avoided. (c) An optimal bending period, that considers the polymer nature, was found and employed. Experimental prototypes of this sensor were fabricated using an optical fiber attached to a polymer cable. The experimental results confirm the benefits of this new design. (C) 2002 Elsevier Science B.V. All rights reserved.