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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 accidentalintrusions are a major cause of pipeline failures leading to large leaks or even explosions. Due to the long distances to bemonitored and the linear nature of pipelines, distributed fiber optic sensing techniques offer significant advantages andthe capability to detect and localize pipeline disturbance with great precision. Furthermore pipeline owner/operators layfiber optic cable parallel to transmission pipelines for telecommunication purposes and at minimum additional costmonitoring capabilities can be added to the communication system.The Brillouin-based Omnisens DITEST monitoring system has been used in several long distance pipeline projects. Thetechnique is capable of measuring strain and temperature over 100s kilometers with meter spatial resolution. Dedicatedfiber 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 andlimits, while addressing future perspectives for pipeline monitoring. The description is supported by case studies andillustrated by field data.

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

1. INTRODUCTIONPipelines 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 challengesin terms of pipeline integrity management as they are submitted to seabed erosion and permafrost thaw settlement orfrost 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 partys interferences or intrusions. As a result, pipeline failures are usually noticed only when eitherthe output flow is affected or the surrounding environment is severely affected. It is widely recognized that pipelinefailures have huge environmental, cost and image impacts, forcing the oil and gas industry to look for new sensingtechniques 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 forthe monitoring of onshore and offshore pipelines over the last 6 years and has shown to-date unmatched pipelineintegrity monitoring performance. The developed technique uses standard telecommunication grade optical fibers assensors deployed alongside the pipeline in order to perform a continuous uninterrupted monitoring. Once connected to ameasuring unit the optical fibers provide information about temperature and strain conditions with meter resolution alongthe pipeline. Fully distributed temperature and strain profiles are recorded at regular time interval of a few minutes overup to 40km distance, which can be extended to 100s km via dedicated repeaters without compromising on themonitoring performances.

*[email protected]; 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


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The occurrence and location of leakages is determined by analysis of the temperature profiles and the achievabledetection limits are in the 0.01% of the total throughput for oil leaks and even lower for pressurized gas; more than twoorders 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, enablingthe early detection of increased stress due to external effects such as geohazards, permafrost thaw settlement or eventhird party intrusion. Specific fiber optic cables have been developed, demonstrating ground movement sensitivity in thecentimeter range. Pipeline strain monitoring can also be performed with sensitivities as low as 10 microstrains providedthat the cables are bonded to the pipeline. A variety of cables for either or both leak and ground movement detection isavailable and can be selected with respect to different soil characteristics and pipeline installation procedures.

2. PIPELINE MONITORING REQUIREMENTSThe 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 assetstowards 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 aboverequirements. The distance range of the monitoring system is compatible with long distance transmission pipeline and isable 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 rateand operational changes. The combined information about pipeline temperature and structural conditions is transferred toSCADA systems. The availability in real-time of complete information about the pipeline integrity helps pipelineoperators to make the right executive decisions based on actual pipeline operational and structural conditions and not onassumptions.

3. SENSING PRINCIPLEDeveloped 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 opticaltime-domain-coded technique, an optical pulse is launched into the fibre and a photodetector measures the amount oflight 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 timeinformation is converted to distance information provided that the speed of light is known, similar to radar or lidardetection 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 80s [1], whereas Brillouin was introduced later as a way toenhance the range of OTDR [2] and then for strain and/or temperature monitoring applications [3]. Fig. 1 schematicallyshows the spectrum of the scattered light from a single wavelength o in optical fibres. Both Raman and Brillouinscattering effects are associated with different dynamic non-homogeneities in the silica and therefore have completelydifferent spectral characteristics.



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The Raman light scattering is caused by thermally influenced molecular vibrations. Consequently the backscattered lightcarries the local temperature information at the point where the scattering occurred. The amplitude of the Anti-Stokescomponent is strongly temperature dependent whereas the amplitude of the Stokes component is not. Raman sensingrequires some filtering to isolate the relevant frequency components and is based on the recording and computation ofthe ratio between Anti-Stokes amplitude and Stokes amplitude, which contains the temperature information. Since themagnitude 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-basedsystems 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 thermallyexcited acoustic waves in the GHz range present in thesilica fibre giving rise to frequency shiftedcomponents. It can be seen as the diffraction of lighton a dynamic grating generated by an acoustic wave(an acoustic wave is actually a pressure wave whichintroduces a modulation of the index of refractionthrough the elasto-optic effect). The diffracted lightexperiences a Doppler shift since the grating

propagates at the acoustic velocity in the fibre. Theacoustic velocity is directly related to the mediumdensity which is temperature and strain dependent. Asa result the so-called Brillouin frequency shift carriesthe information about the local temperature and strainof the fibre as shown in Fig. 2 [4]. The Brillouinfrequency shift is an intrinsic parameter of the fiberand its value is independent from the measuringsystem ensuring long term unbiased measurements with no need of periodic recalibration. Furthermore its perfect lineardependency 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 ofdistance 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 mechanismand consequently an improved signal-to-noise ratio.

Fig. 1: Schematic representation of the scattered light spectrum from a single wavelength signal propagating in opticalfibres. An increase of the fibre temperature has an effect on the Raman and Brillouin components, whereas strainhas an effect on Brillouin components only.

Fig. 2: Strain and temperature dependence of the Brillouinfrequency shift of standard telecommunication opticalfibers.



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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 applicationrequirements 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 probethe Brillouin frequency profile of the fibre. A stimulation of the Brillouin scattering process occurs when the frequencydifference (or wavelength separation) of the pulse and the CW signal corresponds to the Brillouin shift (resonancecondition) and provided that both optical signals are counter-propagating in the fibre. The interaction leads to a largerscattering efficiency resulting in an energy transfer from the pulse to the probe signal, and an amplification of the probesignal. The frequency difference between pulse and probe can be scanned for precise and global mapping of the Brillouinshift along the sensing fibre (Fig. 3). Lastly at every location, the maximum of the Brillouin gain is computed and theinformation 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 onspontaneous light scattering.

The localization of the temperature or strain information along the fibre is possible using a pulsed pump signal. Theinteraction of the probe with the pump is recorded as a function of time and the time information can be converted intodistance. An actual temperature profile of the fibre can be computed using calibration curves (Fig. 2). Thanks to the highspeed of light, fibre lengths of several kilometres can be scanned within a fraction of second, yielding several thousandsof 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 probesignal 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 strainresolution and 0.5C temperature resolution (defined as 2 times the standard deviation on repetitive measurements) overdistance up to 30 km with spatial resolution of 2 meters. The acquisition time (time to get one complete profile) mayvary 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 100s of km without compromising on the measurement performances or remote sensing capabilities as described in section 5.

30 km fiberoptic cable

Ambient +50C

Ambient +20C



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4. FIBER OPTIC SENSING FOR PIPELINE INTEGRITY MONITORINGFiber 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 ofnatural geohazards can significantly affect the integrity of pipelines; they range from geotechnical, hydrotechnical andtectonic hazards [7]. Fiber optic sensing for pipeline ground movement detection is based on the measurement of strainalong a sensing fiber integrated in a dedicated Strain Measurement Cable (SMC). Unlike telecommunication fiber opticcable, 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 alandslide. In fact, when a landslide occurs, the shear interface between the sections which dont move and the section ofland which slides down is submitted to strain as illustrated in Figure 4. The conversion from lateral displacement to fiberlongitudinal strain can be understood as follows [8, 9]. Based on the schematics of Figure 4, it can be seen that theoriginal section d of cable is submitted to a constant strain , whereas the rest of the cable remains strain free. The cableelongation d depends on the lateral displacement L and the strain is simply given by:

= d/d

It can be shown that the fiber strain can beexpressed as:

( ) 11 2 +== d Ld

d

where the ratio L/d provides informationabout the magnitude of the cabledisplacement.

Thanks to the high sensitivity strain measurement capability, small cable displacement be detected and localized withmeter accuracy anywhere along tens of kilometer of SMC. Typical results are presented in Figure 5 which shows 2examples 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, a1.5m 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 TemperatureMeasurement Cable (TMC) located in the vicinity of the pipeline [10]. Two approaches need to be distinguisheddepending on the type of fluid that is transported by the pipeline.

As illustrated in the schematic of Figure 6, thesurrounding of the pipeline is cooled when the fluid iscompressed. Leakage detection is then based on theJoules-Thompson effect. The fluid being in adiabaticregime, any pressure change, as caused by a leak forinstance, induces a temperature drop which affects theTMC. The interrogator detects then the temperaturechange leading to the leakage detection andlocalization. Typical figures are 0.5 oC/bar x P whichindicates that a small pressure change would inducesignificant temperature variations.

Transported liquids such as crude oil, brine or heating

system fluids are at a temperature higher than thesurrounding soil temperature. Any small leak thenleads to an increase of temperature in the vicinity of the

pipeline. The occurrence of a local hot spot along thesensing 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 pipelineenvironment, 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 additionalcooling 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 greaterwhen 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, shorecrossing, offshore buried pipeline sections. Being able to monitor seabed erosion helps identify and remediate erosionconditions similar to those that may have contributed to shallow water or river crossing pipeline failures. If necessary thefiber 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 beused 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 pipelineroute may be exposed to seabed ice gouging and permafrost thaw settlement. If the pipeline route is located near themouth of a river, it may become exposed due to erosion of the seabed from springtime river overflood draining throughholes in the ice sheet (strudel scours) or river channel flows. The thermal influence of the pipeline on the permafrostneeds 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 pipelinesurrounding temperature.

High pressure Gas pipelinesLNG or LPG pipelines

Pipeline

Leak

Temperature effectsTemperature effects

warming coolingOil or fuel pipelines,Heating pipelinesBrine pipeline

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

Continuous temperature sensing enables the monitoring of the permafrost conditions and to locate potential erosionevents. The example of a buried subsea pipeline operating at warmer temperatures than seawater temperatures asillustrated in Figure 7. Seabed erosion and possible exposure of a pipeline may be detected and located throughtemperature 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 thesurrounding soil and environmental stresses.

5. COMPREHENSIVE FIBER OPTIC PIPELINE MONITORING SOLUTION5.1 Generic System Overview

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

strain and temperature monitoring units,including combinations of measuringunits, remote signal regenerationmodules and optical switches; each ofthese units constitutes an optical nodelocated in a pipeline node such as a

pumping or compressor station;

strain and temperature measurementcables (respectively SMC and TMC)connecting two stations;

data communication interface betweenmonitoring units and the control station;

Measurement control, visualization andconfiguration 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. Althoughlow loss optical fibers are available (typical fiber propagation loss < 0.25 dB/km), the attenuation of the fiber still setslimits to the measurement range. Furthermore the performances in terms of spatial resolution and temperature/strainaccuracy are also related to the distance range, since the optical waves are being affected by the fiber attenuation. On

Pipeline(s)FiberOptic Communications &Temperature Monitoring Cable

Thermal Influence followingstartup t oThermal Influence t o + x monthsThermal Influence t o + 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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