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Long-distancefiberopticsensingsolutionsforpipelineleakage,intrusionandgroundmovementdetection

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Long-distance fiber optic sensing solutions for pipeline leakage, intrusion and ground movement detection

Marc Nikles

Omnisens S.A., 3 Riond Bosson, CH-1110 Morges, Switzerland

ABSTRACT

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. Keywords: fiber optic sensor, asset integrity monitoring, pipeline integrity monitoring, leak detection, ground movement detection, geohazards, distributed sensing, distributed strain and temperature, Brillouin optical time domain analysis.

1. INTRODUCTION Pipelines are being laid over longer distances in more remote areas affected by geohazards, harsh environmental conditions and possible third party intrusion. Deep water flowlines and arctic pipelines have introduced new challenges in terms of pipeline integrity management as they are submitted to seabed erosion and permafrost thaw settlement or frost heave problems. Pipeline integrity monitoring has often been restricted to visual inspection and mass/volume balance measurements, leading to very limited capabilities to detect and locate pipeline disturbance such as leakages, geohazards or third party’s interferences or intrusions. As a result, pipeline failures are usually noticed only when either the output flow is affected or the surrounding environment is severely affected. It is widely recognized that pipeline failures have huge environmental, cost and image impacts, forcing the oil and gas industry to look for new sensing techniques to perform permanent and real-time integrity monitoring. Fiber optic-based monitoring systems have been proven to be the utmost promising one.

The technique developed by Omnisens S.A. and referred to as DITEST presented in this contribution has been used for the monitoring of onshore and offshore pipelines over the last 6 years and has shown to-date unmatched pipeline integrity monitoring performance. The developed technique uses standard telecommunication grade optical fibers as sensors deployed alongside the pipeline in order to perform a continuous uninterrupted monitoring. Once connected to a measuring unit the optical fibers provide information about temperature and strain conditions with meter resolution along the pipeline. Fully distributed temperature and strain profiles are recorded at regular time interval of a few minutes over up to 40km distance, which can be extended to 100’s km via dedicated repeaters without compromising on the monitoring performances.

*marc.nikles@omnisens.com; phone +41 21 510-2121; fax +41 44 274-2031; www.omnisens.com

Invited Paper

Fiber Optic Sensors and Applications VI, edited by Eric Udd, Henry H. Du, Anbo Wang, Proc. of SPIE Vol. 7316, 731602© 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.818021

Proc. of SPIE Vol. 7316 731602-1

The occurrence and location of leakages is determined by analysis of the temperature profiles and the achievable detection limits are in the 0.01% of the total throughput for oil leaks and even lower for pressurized gas; more than two orders of magnitude lower than that of any conventional mass/volume balance system.

At the same time the fiber optic strain profile is used to detect and locate ground movement and pipeline strain, enabling the early detection of increased stress due to external effects such as geohazards, permafrost thaw settlement or even third party intrusion. Specific fiber optic cables have been developed, demonstrating ground movement sensitivity in the centimeter range. Pipeline strain monitoring can also be performed with sensitivities as low as 10 microstrains provided that the cables are bonded to the pipeline. A variety of cables for either or both leak and ground movement detection is available and can be selected with respect to different soil characteristics and pipeline installation procedures.

2. PIPELINE MONITORING REQUIREMENTS The monitoring is an important part of the pipeline integrity management program defined by pipeline owner/operators. Proper and effective monitoring is aiming at the optimization of the operation and maintenance of company assets towards continuous availability as well as protecting the environment and the population by identifying threats to the pipeline. The requirements of an ideal pipeline integrity monitoring system are:

• Uninterrupted monitoring with no dead zone along the whole pipeline length

• Permanent and continuous 24/7 monitoring regardless of weather and pipeline conditions

• Ability to detect and locate any early signs of geohazards (or ground movements)

• Ability to detect and locate small leaks before they develop into large catastrophic leakages

• High sensitivity to guarantee fast response to any threat to the pipeline

• No false alarm

This paper describes a fiber optic monitoring system which has been develop with the objective to meet the above requirements. The distance range of the monitoring system is compatible with long distance transmission pipeline and is able to cover the typical distance between valves and pump or compressor stations. Since the monitoring is non intrusive, the technique is applicable to any kind of pipelines and the monitoring performance is maintained despite of flow rate and operational changes. The combined information about pipeline temperature and structural conditions is transferred to SCADA systems. The availability in real-time of complete information about the pipeline integrity helps pipeline operators to make the right executive decisions based on actual pipeline operational and structural conditions and not on assumptions.

3. SENSING PRINCIPLE Developed for telecommunication applications, OTDRs have been the starting point of distributed sensing techniques. They use the Rayleigh scattered light to measure the attenuation profiles of long-haul fibre optic links. In the optical time-domain-coded technique, an optical pulse is launched into the fibre and a photodetector measures the amount of light which is backscattered as the pulse propagates along the fibre. The detected signal, the so-called Rayleigh signature, presents an exponential decay with time which is directly related to the linear attenuation of the fibre. The time information is converted to distance information provided that the speed of light is known, similar to radar or lidar detection techniques. In addition to the information on fibre losses, the OTDR profiles are very useful to localize breaks, to evaluate splices and connectors, and in general to assess the overall quality of a fibre link.

Raman and Brillouin scattering phenomena have been used for distributed sensing applications over the past few years. Raman was first proposed for sensing applications in the 80’s [1], whereas Brillouin was introduced later as a way to enhance the range of OTDR [2] and then for strain and/or temperature monitoring applications [3]. Fig. 1 schematically shows the spectrum of the scattered light from a single wavelength λo in optical fibres. Both Raman and Brillouin scattering effects are associated with different dynamic non-homogeneities in the silica and therefore have completely different spectral characteristics.

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The Raman light scattering is caused by thermally influenced molecular vibrations. Consequently the backscattered light carries the local temperature information at the point where the scattering occurred. The amplitude of the Anti-Stokes component is strongly temperature dependent whereas the amplitude of the Stokes component is not. Raman sensing requires some filtering to isolate the relevant frequency components and is based on the recording and computation of the ratio between Anti-Stokes amplitude and Stokes amplitude, which contains the temperature information. Since the magnitude of the spontaneous Raman backscattered light is quite low (10 dB below spontaneous Brillouin scattering), high numerical aperture multimode fibres are used in order to maximize the guided intensity of the backscattered light. However, the relatively high attenuation characteristics of multimode fibres limit the distance range of Raman-based systems to approximately 10 km, beyond which their decline in usefulness in most practical cases.

Brillouin scattering occurs as a result of an interaction between the propagating optical signal and thermally excited acoustic waves in the GHz range present in the silica fibre giving rise to frequency shifted components. It can be seen as the diffraction of light on a dynamic grating generated by an acoustic wave (an acoustic wave is actually a pressure wave which introduces a modulation of the index of refraction through the elasto-optic effect). The diffracted light experiences a Doppler shift since the grating propagates at the acoustic velocity in the fibre. The acoustic velocity is directly related to the medium density which is temperature and strain dependent. As a result the so-called Brillouin frequency shift carries the information about the local temperature and strain of the fibre as shown in Fig. 2 [4]. The Brillouin frequency shift is an intrinsic parameter of the fiber and its value is independent from the measuring system ensuring long term unbiased measurements with no need of periodic recalibration. Furthermore its perfect linear dependency on temperature and strain allows accurate and straightforward determination of fiber conditions unaffected by connectors or splice losses.

Brillouin-based techniques bring the following advantages over other distributed techniques:

1. The technique makes use of standard low-loss single-mode optical fibre offering several tens of kilometres of distance range and a compatibility with telecommunication components.

2. It is a frequency-based technique as opposed to Raman-based techniques which are intensity based. Brillouin based techniques are consequently inherently more accurate and more stable in the long term, since intensity-based techniques suffer from a higher sensitivity to drifts.

3. Brillouin scattering can be optically stimulated leading to a much greater intensity of the scattering mechanism and consequently an improved signal-to-noise ratio.

Fig. 1: Schematic representation of the scattered light spectrum from a single wavelength signal propagating in optical

fibres. An increase of the fibre temperature has an effect on the Raman and Brillouin components, whereas strain has an effect on Brillouin components only.

Fig. 2: Strain and temperature dependence of the Brillouin frequency shift of standard telecommunication optical fibers.

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4. The stimulation mechanism involves two counter-propagating lightwaves which can be controlled individually providing a very valuable way to adjust the measurement parameters with respect to the application requirements in terms of resolution, distance range, acquisition time while offering large optical budget.

The active stimulation of Brillouin scattering can be achieved by using two optical lightwaves [5]. In addition to the optical pulse usually called the pump, a continuous wave (CW) optical signal, the so-called probe signal is used to probe the Brillouin frequency profile of the fibre. A stimulation of the Brillouin scattering process occurs when the frequency difference (or wavelength separation) of the pulse and the CW signal corresponds to the Brillouin shift (resonance condition) and provided that both optical signals are counter-propagating in the fibre. The interaction leads to a larger scattering efficiency resulting in an energy transfer from the pulse to the probe signal, and an amplification of the probe signal. The frequency difference between pulse and probe can be scanned for precise and global mapping of the Brillouin shift along the sensing fibre (Fig. 3). Lastly at every location, the maximum of the Brillouin gain is computed and the information translated to temperature or strain using the calibration coefficients in Fig.2. The probe signal intensity can be adjusted to acceptable levels for low-noise fast acquisition whatever the measurement conditions and fibre layout, thus solving the small signal-to-noise ratio issues which are generally associated with distributed sensing based on spontaneous light scattering.

The localization of the temperature or strain information along the fibre is possible using a pulsed pump signal. The interaction of the probe with the pump is recorded as a function of time and the time information can be converted into distance. An actual temperature profile of the fibre can be computed using calibration curves (Fig. 2). Thanks to the high speed of light, fibre lengths of several kilometres can be scanned within a fraction of second, yielding several thousands of measurement points. Fig. 3 shows the identification of 2 hot spots along a 30km fiber.

Fig.3: Effect of 2 hot spot on the Brillouin gain spectrum along a 30 km fiber; the frequency difference between pump and probe

signal giving rise to the maximum Brillouin gain corresponds to the local Brillouin frequency shift. The local temperature or strain information is then computed using calibration curves as the one shown in Fig. 2.

The systems based on stimulated Brillouin scattering are often referred to as Brillouin Optical Time Domain Analysis (BOTDA) in the literature and the DITEST monitoring technique is based on the BOTDA measuring technique. Typically, the DITEST technique can achieve temperature and strain measurement performance such as 10 με strain resolution and 0.5°C temperature resolution (defined as 2 times the standard deviation on repetitive measurements) over distance up to 30 km with spatial resolution of 2 meters. The acquisition time (time to get one complete profile) may vary from a few seconds to 10 minutes depending on the distance and the measurement performance requirements [6]. The DITEST technique offers flexibility that makes possible the development of regeneration or repeater modules that provide either an extension of the distance range to 100’s of km without compromising on the measurement performances or remote sensing capabilities as described in section 5.

30 km fiberoptic cable

Ambient +50°C

Ambient +20°C

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4. FIBER OPTIC SENSING FOR PIPELINE INTEGRITY MONITORING Fiber optic sensing fulfills pipeline integrity monitoring by offering ground movement detection, leak detection, subsea pipeline monitoring and soil property change monitoring. These features are addressed in the following sections. 4.1 Geohazards - ground movement detection

Geohazards or ground movements are recognized by the pipeline industry as major threats to pipelines. A variety of natural geohazards can significantly affect the integrity of pipelines; they range from geotechnical, hydrotechnical and tectonic hazards [7]. Fiber optic sensing for pipeline ground movement detection is based on the measurement of strain along a sensing fiber integrated in a dedicated Strain Measurement Cable (SMC). Unlike telecommunication fiber optic cable, the SMC design allows the cable strain to be transferred to the fiber which in turns can be detected and monitored. Strain introduced by ground movement effectively is the parameter that can be monitored to detect the development of a landslide. In fact, when a landslide occurs, the shear interface between the sections which don’t move and the section of land which slides down is submitted to strain as illustrated in Figure 4. The conversion from lateral displacement to fiber longitudinal strain can be understood as follows [8, 9]. Based on the schematics of Figure 4, it can be seen that the original section d of cable is submitted to a constant strain ε, whereas the rest of the cable remains strain free. The cable elongation Δd depends on the lateral displacement L and the strain ε is simply given by:

ε = Δd/d

It can be shown that the fiber strain ε can be expressed as:

( ) 11 2 −+=Δ

= dLddε

where the ratio L/d provides information about the magnitude of the cable displacement.

Thanks to the high sensitivity strain measurement capability, small cable displacement be detected and localized with meter accuracy anywhere along tens of kilometer of SMC. Typical results are presented in Figure 5 which shows 2 examples simulating long (20m) and short (2m) ground movement transition zones.

Fig. 5: (a) Detection of 50cm lateral displacement over 20m; (b) Detection of 5cm lateral displacement over 2m. In both cases, a

1.5m spatial resolution was used to perform the measurements.

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4.2 Pipeline leak detection

Pipeline leak detection relies on continuous accurate distributed temperature monitoring along a Temperature Measurement Cable (TMC) located in the vicinity of the pipeline [10]. Two approaches need to be distinguished depending on the type of fluid that is transported by the pipeline.

As illustrated in the schematic of Figure 6, the surrounding of the pipeline is cooled when the fluid is compressed. Leakage detection is then based on the Joules-Thompson effect. The fluid being in adiabatic regime, any pressure change, as caused by a leak for instance, induces a temperature drop which affects the TMC. The interrogator detects then the temperature change leading to the leakage detection and localization. Typical figures are 0.5oC/bar x ΔP which indicates that a small pressure change would induce significant temperature variations.

Transported liquids such as crude oil, brine or heating system fluids are at a temperature higher than the surrounding soil temperature. Any small leak then leads to an increase of temperature in the vicinity of the pipeline. The occurrence of a local hot spot along the sensing cable is the signature of a leakage.

4.3 Offshore pipeline integrity monitoring

The challenges associated to the design and the operation of subsea pipeline or flowlines varies depending on the pipeline type and route; but the failure risks are in most cases associated to [11]: the modification of the pipeline environment, seabed topology, as well as pipeline crossing and dropped objects (such as ship anchors or fishing gears). A modification of the pipeline direct surrounding due to seabed erosion or seabed migration can lead to additional cooling of the exposed pipeline section and possible hydrates and wax plugging [12]. The extent of hydrate or wax formation problem increases with pipeline length through the effects of cooling and the challenge is significantly greater when assuring flows in deep water and remote subsea locations, emphasizing the need of pipeline permanent monitoring [13, 14].

Additionally subsea migrating bedforms submit the pipeline to large strain with eventually the risk of pipeline upheaval buckling. Anomalous event, which could expose the pipelines, can be detected based on the differential temperature between a pipeline and its environment. Whereas visual pipeline ROV inspections are difficult or even impossible, standard subsea fiber-optic cable laid along the pipeline has proven effective to provide an early warning of such events before they develop into catastrophic pipeline failures. Examples include erosion monitoring of shallow water, shore crossing, offshore buried pipeline sections. Being able to monitor seabed erosion helps identify and remediate erosion conditions similar to those that may have contributed to shallow water or river crossing pipeline failures. If necessary the fiber optic temperature monitoring system can be combined with fiber optic strain measurements in order to map in real-time bedform migration and to detect and localize pipeline strain. Last but not least, temperature based fiber optic can be used to detect and localize pipeline leaks through the associated temperature change.

4.4 Soil property changes, frost heave,

Offshore arctic conditions pose additional challenges to the safe operation of subsea pipelines [15, 16, 17]. The pipeline route may be exposed to seabed ice gouging and permafrost thaw settlement. If the pipeline route is located near the mouth of a river, it may become exposed due to erosion of the seabed from springtime river overflood draining through holes in the ice sheet (strudel scours) or river channel flows. The thermal influence of the pipeline on the permafrost needs to be taken into account since the heat generated by the pipeline melts the backfill and warms up the surrounding

Fig. 6: Effect of different types of leakages on pipeline surrounding temperature.

High pressure Gas pipelines LNG or LPG pipelines

Pipeline

Leak

Temperature effectsTemperature effects

warming coolingOil or fuel pipelines,Heating pipelinesBrine pipeline

T/ °C

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soil causing permafrost thaw settlement. These arctic conditions can apply significant loads on a subsea pipeline or leave it exposed above the seabed to other applied loads.

Continuous temperature sensing enables the monitoring of the permafrost conditions and to locate potential erosion events. The example of a buried subsea pipeline operating at warmer temperatures than seawater temperatures as illustrated in Figure 7. Seabed erosion and possible exposure of a pipeline may be detected and located through temperature changes observed along the TMC [17]. Cable only needs to be installed in close proximity of the buried pipeline.

Fig. 7: Seabed erosion and permafrost thaw settlement due to combined thermal influences gradually melting the backfill and the

surrounding soil and environmental stresses.

5. COMPREHENSIVE FIBER OPTIC PIPELINE MONITORING SOLUTION 5.1 Generic System Overview

The DITEST comprehensive long range pipeline monitoring system is schematically composed of the following components (Fig. 8) [10]:

• strain and temperature monitoring units, including combinations of measuring units, remote signal regeneration modules and optical switches; each of these units constitutes an optical node located in a pipeline node such as a pumping or compressor station;

• strain and temperature measurement cables (respectively SMC and TMC) connecting two stations;

• data communication interface between monitoring units and the control station;

• Measurement control, visualization and configuration software

5.2 Strain and temperature interrogator

Scenarii have been developed to multiply the monitored distance range for the monitoring of long distance transmission pipelines and to support remotely interrogating a sensor deployed over long distances from the control station. Although low loss optical fibers are available (typical fiber propagation loss < 0.25 dB/km), the attenuation of the fiber still sets limits to the measurement range. Furthermore the performances in terms of spatial resolution and temperature/strain accuracy are also related to the distance range, since the optical waves are being affected by the fiber attenuation. On

Pipeline(s)Fiber Optic Communications & Temperature Monitoring Cable

Thermal Influence following start up toThermal Influence to + x monthsThermal Influence to + y months

Backfill

Seabed and Trench Boundaries

Fig. 8: Schematic of complete pipeline monitoring solution. Location

of the monitoring unit is arbitrary.

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one hand, the decreasing pulse intensity generates a smaller interaction and on the other hand a weaker signal on the photo-detector is associated to a lower signal-to-noise ratio that requires longer averaging times. The distance range of this technique is therefore in practice limited to some 30 km with meter spatial resolution. However, the pump-and-probe technique offers flexibility that makes possible the development of regeneration or repeater modules that provide either an extension of the distance range or remote sensing capabilities [18].

Fig. 9: Extending Monitoring Distance with Remote Modules.

An extended range concept is shown in Figure 9. A standard fiber optic telecommunication cable is used to bring the pump and probe signals to a DITEST Remote Module (DRM) that includes optical signal processing for optical power control and signal routing. The module performs active signal regeneration by using optical amplification techniques similar to those extensively used in optical telecommunications. The modules can be cascaded leading to remote distances in excess of hundreds of kilometers. However, implementing the DRM’s and for submerged offshore applications will require submersible housings and subsea power supply to the DRM’s. Qualification tests have been successfully performed with fiber lengths up to 125 km from one measuring unit or 250km with a centrally located measuring unit monitoring 2 sections of 125km [18].

Fig. 10: Temperature measurements performed 116.4 km away from the measuring unit using DRM. The test setup is composed of

different fiber sections (0.5, 1, 2, 5, and 10 m respectively) placed in a temperature controlled bath. The result demonstrates the ability to perform high resolution temperature measurement over extend distance range with no compromise on spatial resolution and

temperature measurement accuracy.

The DRM enables to maintain the monitoring performance over extended distances while the monitoring performance in terms of temperature and strain accuracy obtained with the DRM’s is equivalent to the performance directly available from the instrument. In the example shown in Figure 10, a test setup is connected to the DRM located 100 km away from the measuring unit. The test setup comprises 5 sections of different fiber lengths (ranging from 0.5 m to 10 m sections) connected via a 16.4 km fiber spool to the DRM. The five different test sections are placed in a temperature controlled bath so that their temperature can be varied. Temperature measurements with a 1.5 m spatial resolution are repeated for different bath temperatures (from 15oC to 45oC). The measured profiles are shown in Figure 10. The average measurement time is about 5 minutes per profile. Although the spatial resolution is set to 1.5 m, the 0.5 m section is clearly identified as a consequence of spatial over-sampling (interleaving). Full temperature accuracy is obtained for

125km of monitoring range along the pipeline

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6. CASE STUDIES 6.1 Brine pipeline leak detection

In 2002 the construction of a natural gas storage facility some 1500m under the ground surface was started in the area of Berlin in Germany [19]. Using mining technology the building of underground caverns for gas storage in large rock-salt formation requires hot water and produces large quantities of water saturated with salt, so-called brine. In most cases the brine cannot be processed on-site and must be transported by a pipeline to the location where it can either be processed, or injected back safely into the ground. Because the brine can be harmful to the environment, pipeline small leak detection was a mandatory requirement. A 55km pipeline was built and a DITEST-based fiber optic leak detection system was design to permanently and continuously monitor the pipeline. The installation of the pipeline and the fiber cable had to deal with several road and river crossings which required horizontal drilling and several cable junctions. Some 60 splices (that correspond to an additional loss of up to 3 dB) had to be done to complete the cable installation. The high optical budget of the DITEST interogator can accommodate such losses and since the frequency-based Brillouin technique is insensitive to attenuation the calibration is straightforward and stable over time leading to long term, maintenance free operation of the leak detection system.

During the pipeline construction phase the fiber cable was first placed in the trench and buried in the sand some 10 cm underneath the pipeline. The cable positioning with respect to the pipeline is important in order to optimize the leak detection sensitivity of the system. The positioning eventually is a trade-off between the maximum contrast in the case of a leak and the detection response time of leaks occurring from every point of the tube circumference. The pictures in Figure 13 show the pipeline construction before backfill.

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. The pipeline construction was completed in November 2002 and the pipeline was put into operation in January 2003. In July 2003, a first leak was detected by the monitoring system. It was later found that the leak was accidentally caused by excavation work in the vicinity of the pipeline. Figure 14 shows the occurrence of the leakage and its effect on the temperature profiles as

Fig. 13: Brine pipeline construction in the Berlin area n Germany.

Figure 14: Detection and localization of a leakage provoked by

excavation works in the vicinity of the pipeline.

Leakage

Junction box 30

Junction box 31

Temperature profile before leakage

Temperature profile after leakage

Junction box 30

Junction box 31

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they were immediately displayed on the central PC display in the control room. The graphs in Figure 14 correspond to measured raw data, i.e. Brillouin frequency shifts, as a function of distance and the figure shows a zoom in view of the section with the leak. The local temperature increase associated to the brine leak is measured to be around 8°C. An alarm was immediately and automatically triggered which could stop the flow and eventually could avoid large pipeline spills.

6.2 Arctic Alaskan offshore pipeline monitoring

Offshore arctic conditions pose unique design challenges to the safe operation of subsea pipelines exposed to seabed ice gouging, permafrost thaw settlement, strudel scour, and channel migration [15, 16, 17]. The application of fiber optic-based distributed temperature monitoring systems has demonstrated the ability to monitor the pipeline operational conditions and to achieve efficient flow assurance monitoring [17]. 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, seabed soil modifications as shown in Fig. 8. It allows the operator to take timely and appropriate actions to ensure the integrity of the pipeline.

The DITEST fiber optic monitoring system is used to monitor 2 offshore pipelines in the Alaska’s Beaufort Sea oil fields. The example described herein shows the temperature monitoring of a 14km pipeline bundle prior to its startup in late 2007 (Fig. 15). The pipeline installation is part of Oooguruk oil field developments in the Beaufort Sea and is composed of 8km of buried subsea flowlines transporting the produced fluids from an offshore gravel island/drillsite 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 DITEST fiber optic communication cables installed within the pipeline bundle. The monitoring system 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 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.

Fig. 15: Example of temperature monitoring prior to pipeline start-up showing typical erosional events.

6.3 Geotechnical and leak detection system for Peru LNG

In the Andes mountains, pipeline failures caused by geohazards reach 50% of the total number of incidents while ground movement remain a minor threat (<1%) in geologically stable regions such as Western Europe [7]. The first section of

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the Peru LNG pipeline route crosses the mountains from East to West where geohazards are frequently found. The project consists of a new 408 km, high pressure 34 inch gas pipeline system to transport natural gas from a take-off point on the existing TGP Pipeline System at approximately km 211 to a new LNG facility constructed approximately 170 km south of Lima on the coast of Peru, at Pampa Melchorita.

The first 60 km pipeline section is equipped with a DITEST fiber optic-based pipeline monitoring system. The monitoring system includes:

• Geotechnical monitoring including detection and localization of ground movements;

• Leak detection and localization.

The complete monitoring system includes the monitoring equipment which is located in a shelter and both TMC and SMC sensing cables installed in the trench some 0.6 to 1.5m in the ground and above the pipeline in soft backfill material (Fig. 16). The complete monitoring system is composed of a multiple channel DITEST strain and temperature interrogator, SMC for geotechnical monitoring and TMC for leak detection, which is part of the telecommunication optical fiber cable. The latter is used for the communication between the interrogator and the control station as schematically illustrated in Figure 10. The monitoring requirements are permanent and continuous leak and ground movement monitoring over the complete distance with immediate alarm in the case of detected threats for the pipeline integrity.

7. CONCLUSIONS The monitoring technique presented in this article and referred to as the DITEST fiber optic pipeline integrity monitoring system was developed to address the requirements of pipeline owner/operators in terms of integrity monitoring, aiming at the early detection of the major threats to pipeline, such as ground movement, leak, soil property changes, permafrost thaw settlement and frost heave, seabed migrations, scouring, etc. and even intrusion via temperature and/or strain changes.

The monitoring technique has been used for the monitoring of onshore and offshore pipelines over the last 6 years and has shown to-date unmatched pipeline integrity monitoring performance. The developed technique uses standard telecommunication grade optical fibers as sensors deployed alongside the pipeline in order to perform a continuous uninterrupted monitoring. The availability in real-time of the information about the pipeline structural conditions is an important enhancement of pipeline owner/operators’ integrity management program, resulting in potential reduction in maintenance, surveillance costs and health-safety-and-environment risks.

The distance range of the DITEST monitoring system was developed with the objective to be compatible with long distance transmission pipeline and is therfore able to cover the typical distance between valves and pump or compressor stations. Since the monitoring is non intrusive, the technique is applicable to any kind of pipelines and the monitoring performance is maintained despite of flow rate and operational changes. The combined information about pipeline temperature, leak occurrence and structural conditions is transferred to SCADA systems. The availability in real-time of complete information about the pipeline integrity helps pipeline operators to make the right executive decisions based on actual pipeline operational and structural conditions and not on assumptions.

Acknowledgements

The author would like to warmly thank Fabien Ravet and Fabien Briffod from Omnisens SA for their contribution to this article as well as Dana DuToit from Omnisens North America for kindly accepting to present the paper at the conference.

Fig.16: Cross-section of a pipeline trench with pipeline, TMC and SMC lay-out in the backfill.

Pipe

Trench partially filled with soft materials (sand…)

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