During the past 10 years, the oil and gas industry has gained confidence in the capabilities and the reliability of Fiber Optic Distributed Sensing (OFDS). Real world implementation can be found in both downstream and upstream branches. For example, at one end of the industry spectrum, service companies have started to use Distributed Temperature Sensing (DTS) for in-well temperature monitoring to optimize oil recovery processes. At the other end, pipeline operators have installed pipeline integrity monitoring systems based on Distributed Temperature and Strain sensing (DITEST) to fulfill realtime monitoring of soil stability, pipeline 3D deformation and leak detection.
The use of OFDS for offshore structure integrity monitoring such as flow lines, risers and umbilicals has been qualified and field tested already whereas other applications such as performance rating of power umbilicals and enhanced flow assurance system using OFDS are being evaluated.
Currently, the offshore full scale implementation of OFDS technologies still faces challenges of its own. In particular, sensor integration into the structure to be monitored is a minimum requirement which applies to any OFDS. If fiber optic is a common mean of data transmission, subsea conditions imply the use of specific components such as Wet Mate Connectors (WMC) and Fiber Optic Rotary Joints (FORJ). These components present large insertion and return loss characteristics for which OFDS require special attention to the OFDS system to comply with such characteristics. The effect of these components is twofold. First it impacts the sensor optical budget limiting its measurement range. Second, sensor sections remain completely blind due to the high reflection levels leaving the structure without status information over distances that can be as large as several kilometers.
The present works describes how the DITEST based on Stimulated Brillouin Scattering (SBS) can overcome the limitations imposed by both WMC and FORJ components and fully comply with SURF monitoring requirements. The ability of the DITEST is justified theoretically and demonstrated experimentally through qualification trials involving hotspot detection while WMC and FORJ are part of the sensor path. Their effects are quantified through the determination of the measurement dead zone (shorter than 4m), the temperature uncertainty and the resolution. The work also reports the subsequent installation on operational structures as these trials were successful. The DITEST has been installed to continuously monitor the temperature of a 3km long power umbilical and control the heating system of subsea rigid flowlines whose length can be as large a 45km.