Qualification and Validation of DTS/DSS Technology for Umbilical Monitoring
Authors: Ravet Fabien , Rochat Etienne , Borda Carlos , Nikles Marc Rego 
Published: June 2014

The paper describes first the laboratory and qualifications works that were conducted to implement Distributed Fiber Optic Sensors (DOFS) such as Distributed Temperature and Strain Sensing (DTS/DSS) technologies in umbilicals. It also shows how these technologies have been implemented in the field for umbilical thermal and mechanical stresses monitoring. There is value to use DFOS for offshore structure integrity monitoring such as flow lines, risers and umbilicals. Other applications such as rating of power umbilicals, flow assurance or downhole sensing would help optimize field production as well.

Currently, the offshore full scale implementation of DFOS 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 DFOS. 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 which are detrimental to most of the DFOS. 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 DTS/DSS based on Stimulated Brillouin Scattering (SBS) can overcome the limitations imposed by both WMC and FORJ. The DOFS ability 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 (better than 0.5K) and the resolution (below 0.5K while the measurement time remained unchanged (tens of seconds to minutes). The work also reports the subsequent installation on operational structures as these trials were successful. The DOFS 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.

Introduction

The increasing demand for energy supplies leads operators to explore and exploit remote and deep offshore oil fields. These marine fields often lay in tropical regions where permanent platforms are exposed to severe hurricanes threatening their integrity. Moreover currently exploited marine oil fields become more and more depleted. These conditions cause two major challenges to the operators. First, oil being extracted from an offshore reservoir must be processed at the sea bottom. Processing at the wellhead consists in separating all the phases present in the extracted product. Moreover extended operation of depleted oil reservoirs requires higher pumping powers. Both tasks imply that large powers (&gt;1MW) are supplied to the subsea pumps via the umbilical. Thermal stress is very high, in particular in the splash zone and near the buoy where all umbilical and oil risers are converging, creating a hot spot. This is a particularly critical issue as many offshore fields are located in hot water seas (west coast of Africa and Australia, east coast of Brazil) which strongly limit heat dissipation.

It is common to inspect subsea structures with Remotely Operated Vehicles (ROV's). The method is however expensive and unreliable since visual inspection is only capable of detecting external anomalies and is unable to detect internal structural deterioration. Moreover it does not give any information on how the structure is affected when operational conditions are changed. In particular, hot spot and abnormal temperature changes cannot be detected.