Qualification of a Fiber-Optic Strain Monitoring System for Deep-Water Risers

The evaluation of the fatigue performance of deep-water risers requires the monitoring of the dynamic and static strain levels undergone during the whole lifetime. ENI E&P, SMARTEC and Tecnomare have developed and qualified a comprehensive solution for riser strain monitoring based on fiber optic technology. The system is composed of: a) underwater part, b) interconnection system and c) surface equipment. In the underwater part, riser segments are instrumented with four strain sensors, installed parallel to the riser axis and arranged at 90° angles around the riser circumference. The sensors measure the average strain over a measurement basis between 0.5m and 5m, typically 2 m. This disposition allows the evaluation of axial and bending strains in both orthogonal directions. This information is available also in the case of failure of a single sensor. Through a junction box, sensor signals are transferred to a standard underwater cable (the interconnection system), up to 5 Km long. The surface equipment, located in a convenient location in the platform control room, consists of an interferometer that allows both dynamic and static strain measurements. ENI E&P has developed and tested a prototype version on a shallow water riser in the Adriatic Sea for two years [1]. Based on this experience new sensors have been developed together with special technical solutions for deep-water applications, by ENI E&P, SMARTEC and Tecnomare. Between 2002 and 2003 the system has been qualified on a riser mock-up in hyperbaric chamber (up to 360 bar) and on a full-scale riser section in controlled laboratory conditions. This paper presents the design and performances of the components and the results of the qualification tests.