Distributed strain monitoring of structures using optical fiber sensing is a new technique which has opened new possibilities in subsea pipeline installation survey. Spiecapag and Geostock, for the replacement project of a 1300m long immersed pipeline, asked Cementys to monitor the distortion of the pipeline. To certificate the pipeline, they needed to measure and save the strain conditions of the pipe-line during different steps of towing and laying. In addition, the required system had to offer a real-time control of the pipe curvature so that the marine operations could be driven without exceeding the threshold of the strain level and damaging the pipe. Cementys developed a sensor called SensoluxTM based on Raman and Brillouin Optical Time Domain Reflectometry. The sensor cable contains 4 optical fibers to measure the Brillouin (strain and temperature sensitive) and the Raman (temperature sensitive only) scatterings. The cable-sensing is sensitive at each point of its length, with a spatial resolution of 0.5m, a precision of ±5μdef and a reach of several dozens of kilometres. Three optical cables were glued in three grooves disposed at 120° in the concrete surrounding the metallic pipe. Within a few minutes, the distortion of the 3 optical fibers was measured and the pipe global shape calculated, with a one-meter spatial resolution. The small diameter of the sensor (2mm) made it easy to integrate on pipeline by gluing the optical cable in grooves carved on the concrete. The optical cable was thus protected, with a low level of intrusiveness.

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