Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Pipelines <s> An increasing number of pipelines are constructed in remote regions affected by harsh environmental conditions where ::: pipeline routes often cross mountain areas which are characterized by unstable grounds and where soil texture changes ::: between winter and summer increase the probability of hazards. Third party intentional interference or accidental ::: intrusions are a major cause of pipeline failures leading to large leaks or even explosions. Due to the long distances to be ::: monitored and the linear nature of pipelines, distributed fiber optic sensing techniques offer significant advantages and ::: the capability to detect and localize pipeline disturbance with great precision. Furthermore pipeline owner/operators lay ::: fiber optic cable parallel to transmission pipelines for telecommunication purposes and at minimum additional cost ::: monitoring capabilities can be added to the communication system. ::: The Brillouin-based Omnisens DITEST monitoring system has been used in several long distance pipeline projects. The ::: technique is capable of measuring strain and temperature over 100's kilometers with meter spatial resolution. Dedicated ::: fiber optic cables have been developed for continuous strain and temperature monitoring and their deployment along the ::: pipeline has enabled permanent and continuous pipeline ground movement, intrusion and leak detection. This paper ::: presents a description of the fiber optic Brillouin-based DITEST sensing technique, its measurement performance and ::: limits, while addressing future perspectives for pipeline monitoring. The description is supported by case studies and ::: illustrated by field data. <s> BIB001 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Pipelines <s> Casing pipes in oil well constructions may suddenly buckle inward as their inside and outside hydrostatic pressure difference increases. For the safety of construction workers and the steady development of oil industries, it is critically important to measure the stress state of a casing pipe. This study develops a rugged, real-time monitoring, and warning system that combines the distributed Brillouin Scattering Time Domain Reflectometry (BOTDR) and the discrete fiber Bragg grating (FBG) measurement. The BOTDR optical fiber sensors were embedded with no optical fiber splice joints in a fiber-reinforced polymer (FRP) rebar and the FBG sensors were wrapped in epoxy resins and glass clothes, both installed during the segmental construction of casing pipes. In situ tests indicate that the proposed sensing system and installation technique can survive the downhole driving process of casing pipes, withstand a harsh service environment, and remain intact with the casing pipes for compatible strain measurements. The relative error of the measured strains between the distributed and discrete sensors is less than 12%. The FBG sensors successfully measured the maximum horizontal principal stress with a relative error of 6.7% in comparison with a cross multipole array acoustic instrument. <s> BIB002
Monitoring. By using Brillouin scatteringbased fiber distributed techniques, real-time monitoring and early warning systems for liquid and gas pipes have been successfully checked in real installations in a wide set of scenarios. As illustrated in Figure 16 , in buried pipes and in general terms, leaks could be detected through temperature changes on the upper zone (gases) or the bellow zone (liquids) of the pipe; that is, the fiber optics transducercable of the distributed sensor system must be placed over the pipe for gases and below it for liquid leaks detection, respectively. The strain/stress state and safety of casing pipes in oil wells arenas are monitored and evaluated by using BOTDR systems such as in Daqing oilfield BIB002 . The brine (saturated water with salt) is a waste product derived from the mining technique for building underground caverns for gas storage in large rock-salt formation. This brine must be transported by pipelines for its processing or injected back safely into the ground. Because the brine can be harmful to the environment, pipeline small leak detection is a mandatory requirement. As in this case any change in the local temperature indicates a brine leakage in the pipeline section. As an example of this problem, a distributed temperature sensing system was integrated in the brine pipeline placed in the area of Berlin in Germany. The brine is pumped out of the underground caverns and is injected into the pipeline at a temperature of 35 • C to 40 • C. At normal flow rate the temperature gradient along the whole pipeline length is about 8 • C. Since the pipeline is buried at a depth of approximately 2 to 3 meters, the seasonal temperature variations are quite small and the average soil temperature was measured to be around 5 • C. As a result a substantial temperature increase is associated to any pipeline leak even in the case of very small leakages BIB001 . Offshore arctic conditions are a challenge to the safe operation of subsea pipelines exposed to seabed ice gouging, permafrost thaw settlement, strudel scour, and channel migration. The application of fiber optic-based distributed temperature monitoring systems was carried out to monitor the pipeline operational conditions in arctic pipelines  . As visual inspection is impossible, real-time temperature monitoring via optical fibers along the pipeline route can provide an early warning of the development of erosional events, pipeline insulation damages, and seabed soil modifications. In order to monitor two offshore pipelines in the Alaska's Beaufort Sea oil fields a distributed temperature sensor monitoring system was implemented. The pipeline installation is part of Oooguruk oil field developments in the Beaufort Sea. It is composed of 8 km of buried subsea flow-lines transporting the produced fluids from an offshore gravel island/drill site to an onshore above ground pipeline which runs to an existing transmission pipeline. A total of 14 km of pipeline distance is continuously monitored with the fiber optic communication cables installed within the pipeline bundle. The monitoring system was demonstrated to meet the monitoring performance to detect temperature events occurring over just one meter, such as leaks and erosional events. The system has been able to map seabed temperature profiles along the pipeline route and to accurately track temperature excursions as they were occurring with field verified data prior to and during pipeline operation startup. The monitoring system operates permanently and continuously with an active leak detection system based on the detection of local temperature variations  .