Distributed Fibre-Optic Sensing For Long-Range Monitoring Of Pipelines
Distributed fiber-optic sensing presents unique features that have no match in conventional sensing techniques. The ability to measure temperatures and strain at thousands of points along a single fiber is particularly interesting for the monitoring of elongated structures such as pipelines, flow lines, oil wells and coiled tubing. Sensing systems based on Brillouin and Raman scattering are used for example to detect pipeline leakages, verify pipeline operational parameters, and prevent failure of pipelines installed in landslide areas, optimize oil production from wells and detect hot-spots in high-power cables. Recent developments in distributed fiber sensing technology allow the monitoring of 60 km of pipeline from a single instrument and of up to 300 km with the use of optical amplifiers.

New application opportunities have demonstrated that the design and production of sensing cables is a critical element for the success of any distributed sensing instrumentation project. Although some telecommunication cables can be effectively used for sensing ordinary temperatures, monitoring high and low temperatures or distributed strain presents unique challenges that require specific cable designs. This contribution presents advances in long range distributed sensing and in novel sensing cable designs for distributed temperature and strain sensing. The paper also reports a number of significant field application examples of this technology.

Flow lines, pipelines or gas-lines often cross hazardous environmental areas,  from the point of view of natural exposures such as landslides and earthquakes, and from the point of view of third-party influences such as vandalism or obstruction. These hazards can significantly change the original structural functioning of the flow line, leading to damage, leakage and failure with serious economic and ecologic consequences. Furthermore, the operational conditions of the pipeline itself can induce additional wearing or even damage.

The structural and functional monitoring can significantly improve pipeline management and safety. Providing regularly with parameters featuring the structural and functional condition of the flow line, monitoring can help
(1)  prevent the failure, (2) in time, detect the problem  and its position and
(3) undertake maintenance and repair activities in time.

Thus the safety is increased, maintenance cost optimized and economic losses decreased. Typical structural parameters to be monitored are strain and curvature while the most interesting functional parameters are temperature distribution, leakage and third-party intrusion. Since the flow lines are usually tubular structures with kilometric lengths, structural monitoring of full extent is an issue itself. The use of discrete sensors, short- or long gauge is practically impossible, because it requires installation of thousands of sensors and very complex cabling and data acquisition systems raising the monitoring costs. Therefore, the applicability of discrete sensors is rather limited to some chosen cross-sections or segments of flow line, but not extended to full-length monitoring.
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