The continuous demand for energy supplies leads operators to explore and exploit more remote offshore oil fields characterized by deeper deployments, longer tiebacks and subsea completion. These offshore developments either lie in tropical regions subject to stormy weather or in extreme arctic conditions. They imply new requirements in terms of operational efficiency and predictive maintenance for cost optimization as well as environmental footprint minimization.
Consequently, leak detection and flow assurance are required over distances exceeding hundreds of kilometers. And the increasing trend of building all electric completion and heated flowlines impose the use of equally long high voltage power cables.
Distributed temperature sensing (DTS) is a widely adopted technology for leak detection and localization, for heated flow line monitoring and for power cable thermal rating. Optical fiber cables are installed alongside the structure providing detection with meter accuracy localization and degree thermal variation sensitivity. However, there is currently no commercially available DTS instrument capable of reaching a 100km single span without any means of repeating the signal, let alone longer range. Thus, a 300km pipeline would normally require marinized equipment, which is often not a practical solution.
The Omnisens DITEST, a DTS instrument based on Stimulated Brillouin Scattering (SBS) is intrinsically capable of the longest available sensing length, now reaching a sensing distance of around 70km at once; in addition, it is compatible with optical amplifier technology as found in any submarine network. In the present work, we show that, by combining 5 spans of slightly above 65km and 4 optical repeaters together, it is possible to achieve a total of 330km sensing from a single DITEST. Measurement time over the full distance was typically 100minutes, providing a temperature resolution of typically 1K for a spatial resolution of 3m at the end of the 330km long sensing fiber. Shorter measurement time, more suitable with the application could be achieved using optical coding.
As the optical repeater technology is similar to that of telecommunication, efficient cost reduction could be achieved by sharing the repeaters between the sensing system and the needed communication system. At the end of each sensing span, the sensing cable is connected to the repeater for amplification purposes and then goes back to the pipeline for monitoring the next span. The repeaters themselves are electrically powered by conductors embedded in the communication infrastructure.
A viable and cost effective solution to meet the requirements of subsea long distance leak monitoring is thus demonstrated.