Fibre optic strain sensors are being increasingly deployed subsea due to significant improvements in reliability and ease of deployment brought about by embedding the optical fibre within a composite carrier that simply clamps to the structure to be monitored. There is significant experience in deploying these sensors on a variety of risers by a number of different installation methods as part of project integrity management strategies. Building on experience from monitoring risers, the technology is now being applied to other subsea infrastructure including manifolds and flowlines for monitoring parameters including pressure, axial load, shape and temperature. The same technology has even been deployed downhole for measuring loads in production tubing. Two different types of instruments have been designed to meet the needs of the industry, namely single point sensors and sensors that measure the profile of parameters over an extended distance. Point sensors have been developed to measure axial load, internal pressure, curvature and temperature. Each sensor mechanically attaches to the pipeline to transfer strains and temperatures in the pipeline. The instrument has been demonstrated to detect pressure changes from quasi-static to hundreds of hertz as required for conditions such as flow induced vibration and detecting slug flow. By locating a number of pressure sensors along a flowline the onset of waxing or hydrate build-up can be detected. The flexibility of the technology enables deployment in the yard, on deck and even retrofit installation by ROV. Profile sensors have been developed to measure bending profiles of flowline buckle regions, flowline free spans and riser touchdown zones. The profile sensors are elongated instruments that cover the entire length of the flowline to be monitored. Fibre optic sensors are embedded within a composite carrier during a continuous manufacturing process such as filament winding or pultrusion. Instruments with as many as 280 embedded sensors have been manufactured and deployed to date. Instruments have been developed to measure upheaval and lateral buckling strain profiles in HPHT flowlines. Deployment methods have been developed for retrofit installation and for installation on deck. The paper will present the current status of subsea pipeline monitoring using fibre optic strain sensors including results of both point and profile sensors. In addition further applications for the technology will be discussed.
Guidelines for the characterization and use of fibre optic sensors: basic definitions and a proposed standard for FBG-based strain sensors