Optical Fiber Technology 20 (2014) 15–23

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Design and experimental study on FBG hoop-strain sensor in pipelinemonitoring

1068-5200/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.yofte.2013.11.004

⇑ Corresponding author.E-mail address: renliang@dlut.edu.cn (L. Ren).

Liang Ren a,⇑, Zi-guang Jia a,b, Hong-nan Li a, Gangbing Song b

a School of Civil and Hydraulic Engineering, Dalian University of Technology, Liaoning 116024, Chinab Department of Mechanical Engineering, University of Houston, Houston, TX 77204, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 29 July 2013Revised 15 October 2013Available online 7 December 2013

Keywords:FBG hoop-strain sensorCircumferential strainCorrosion monitoringLeakage detectionPipeline model experiment

Pipeline monitoring is an important task for the economic and safe operation of pipelines as well as forloss prevention and environmental protection. The circumferential strain is of significance in pipelineintegrity monitoring. In this paper, an indirect pipeline corrosion monitoring method based on the cir-cumferential strain measurement is firstly proposed, with main objectives at designing a circumferentialstrain measuring device. Combined with unique advantages of optical fiber sensing, an FBG hoop-strainsensor was designed and encapsulated. Its enhanced sensitivity mechanism in the circumferential strainmeasurement and manufacturing technique is detailed. The experimental study of the developed FBGhoop-strain sensor is conducted on a PVC model pipeline to investigate its characteristics, including reli-ability and some tentative dynamic tests. Results of model tests show that the FBG hoop-strain sensordemonstrates good performance in the circumferential strain measurement, and can be considered asa practical device for pipeline health monitoring.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

Pipelines are used as one of the most practical and economicallyeffective modes of transport for large volumes of flammable andpotentially dangerous substances, such as the natural gas, forwhich the road or rail transportation are often impractical [1]. Asevident by Sandberg et al., a pipeline rupture will cause not onlyproduct loss, but also serious environmental damage if leaks arenot quickly detected to stop pumping and begin repairs [2]. Corro-sion is one of the major contributors to accidents involving pipe-lines, and thus monitoring of the pipeline corrosion process hasbecome an important research topic [3].

Current corrosion monitoring techniques aim at grasping theprocess of corrosion per se, evaluating the applied circumstanceand controlling the condition of corrosion. Some representativecorrosion monitoring techniques are as follows: electrical resis-tance technique (ER) [4,5], which are based on measurements ofthe thickness reduction due to corrosion, which can be sensitiveto general corrosion rate but its response to localize corrosion islimited; electrochemical impendance spectroscopy (EIS) [6,7],which has been successfully applied to the study of corrosion sys-tems and proven to be a powerful method for measuring corrosionrate of zinc; linear polarization resistance (LPR) [8,9], which isfound to be a powerful tool for monitoring hot corrosion processes

when the controlling-step is by charge transfer; galvanic sensortechniques [10,11], which is developed based on imaging sensorarray to monitor the corrosion rate of steel in the electrolyte solu-tion environment, etc., each with its own advantages and limita-tions. Since corrosion monitoring is a complicated subject, thereis no single monitoring technique suitable for all applications.However, when it comes to corrosion monitoring for the pipeline,all of these above methods are potentially dangerous since they in-volve types of electrical devices. Meanwhile, these off-line detec-tion methods are more suitable for the corrosion rate estimationand needs complicated analysis, which can hardly be applied forpipeline on-line structural health monitoring.

During long-term service, the pipeline is considered to be apressure container with relatively steady inner pressure. Thus, anindirect corrosion monitoring method based on the strain measur-ing can be designed based on measuring the effects of change inthe inner pressure. The occurrence of corrosion, even local corro-sion, will reduce pipeline wall thickness, which in turn will causethe circumferential strain to gradually increase. On the other hand,circumferential strain serves as a detector of pressure change whenemergencies happen. According to these factors, a circumferentialstrain measuring device was designed to estimate pipeline corro-sion, and even abrupt pressure changes.

The electrical strain gage can be naturally chosen as a customsensor applied to circumferential strain measurement with advan-tages of easy disposition, low price and tolerable measurementaccuracy. However, gas or liquid pipelines have many probable

16 L. Ren et al. / Optical Fiber Technology 20 (2014) 15–23

dangers which make electric sensors unsuitable for use. The fiberBragg grating (FBG) with its superior immunity to electromagneticinterference, strong embeddability, high accuracy and reliability[12,13], is an ideal material for developing a strain measuring de-vice. Various linear FBG strain sensors to meet different demandshave been developed and applied to the model tests [14,15] andpractical engineering [16–18]. Compared with the above monitor-ing methods related to electrical devices, the FBG based methodhas its unique merits. First, FBGs are optical devices thus reducingthe chances of safety problems. Second, this indirect method basedon the hoop strain measurement does not influence the pipelinestructure and thus can be called a non-destructive health evalua-tion. Third, FBGs can provide multiple functions in pipeline moni-toring; not only for measurement of the wall thickness reductionto estimate the corrosion but also for emergency prediction likeleakages.

In this paper, an FBG hoop-strain sensor was designed andencapsulated to measure the pipeline circumferential strain varia-tion. The objectives of this work were as follows: (1) to present thesignificance of circumferential strain measurement in pipelinemonitoring, especially regarding pipeline corrosion even the caseof local corrosion; (2) to present the FBG hoop-strain sensor designtheory and to depict its encapsulation technique; (3) to character-ize the performance of FBG hoop-strain sensor in a PVC modelpipeline by comparisons between the response of bare FBG anddeveloped FBG hoop-strain sensor; (4) to investigate the enhancedsensitivity mechanism of FBG hoop-strain sensor by the relevantexperimental study; and (5) to conduct preliminary pipeline leak-age simulation experiments so as to study the dynamic response ofFBG hoop-strain sensor.

2. Significance of circumferential strain monitoring

2.1. Corrosion estimation based on circumferential strain

A pipe expands uniformly under working pressure, causing anormal tension force FN at any cross section along the pipeline axis[19]. A semi-circle truncated pipe with length B is analyzed todetermine the expression of FN as shown in Fig. 1. The resultant

R

p

p

B

(a) Pipe diagram under internal pressure

dϕ ϕ

FR

FN FN

y

xm m n n

(b) Stress analysis of semi-circle

δ

Fig. 1. Pipe stress analysis.

force in the y-direction FR induced by the internal pressure p canbe obtained by integration

FR ¼Z p

0ðpB � R � duÞ sin u ¼ pBR

Z p

0sin udu ¼ 2pBR ð1Þ

Since the pipe thickness d is far less than the radius R, the nor-mal stress of arbitrary point on the section m–m and n–n is esti-mated to be equal. To satisfy the force’s equilibrium condition inthe y-direction

FN ¼FR

2¼ pBR ð2Þ

Consequently the circumferential stress and strain of the pipecan be obtained using the basic theories of material mechanics

r ¼ FN

A¼ pBR

Bd¼ pR

dð3Þ

e ¼ rEp¼ pR

Epdð4Þ

where Ep denotes the Young’s modulus of the pipe. In the above der-ivation, the axial stress is ignored since the pipe can be regarded tobe infinitely long in this direction. As shown in Eq. (4), the relation-ship between the circumferential strain e and the pipeline internalpressure p is established.

In turn, if the pressurized pipeline functions properly, where p,R and Ep in Eq. (4) are considered to be fixed values, the circumfer-ential strain e is inversely proportional to the pipe wall thickness d.Based on this principle, the measured circumferential strain varia-tion can be used to estimate the wall thickness deduction causedby pipeline internal corrosion.

2.2. Case of local corrosion

In most cases of long-term pipeline service without being sub-jected to accidental damage, where the wall thickness deductioninduced by gradual corrosion is circumferentially homogenous atthe same pipe cross-section, any measuring point with strain sen-sors at the same section can be used to reflect the wall thicknessvariation. However, if a local corrosion happens or the pipeline ispartially damaged, the single-point measurement functions ambig-uously supposing that the sensor is located relatively far awayfrom the corrosion area. The following is an example of partial cor-rosion analysis based on finite element method which illustratethe phenomenon mentioned above.

In this finite element pipeline model, the corrosion occurs par-tially on the pipe’s internal wall, shown in Fig. 2, where dcor de-notes the reduced wall thickness caused by the local corrosion.Seven dummy single-point strain sensors are used to representthe circumferential strain variation respectively, in which S1 is

δ

δcorlocal corrosion area

S1S2

S3

S4

S5

S6S7

Fig. 2. Diagram of local corrosion and strain measuring point.

L. Ren et al. / Optical Fiber Technology 20 (2014) 15–23 17

the measure point exactly located at the corrosion part while S7

has the longest distance from the corrosion area.Fig. 3 shows the circumferential strain distribution between the

intact pipe and the pipe with a local corrosion based on the finiteelement analysis using ANSYS. In the local corrosion case, theresidual wall thickness dcor is 0.5d (d = 10 mm) and the radian ofthe local corrosion area is 30� along the pipe’s circumference. Bothof the pipes are pressurized with an identical inner pressure inten-sity p = 3 MPa. The length of the pipe is L = 2000 mm, 5 times big-ger than the pipe diameter of 320 mm. Thus the boundarycondition influence to the force computation can be neglectedsince the local corrosion is assumed to exist in the middle alongthe axial direction.

In the intact case, the pipe has an uniform circumferential straineint = 1.96 � 10�4. However, the values are different in the cases oflocal corrosion, in which the maximum strain occurs at the localarea of corrosion. Meanwhile, the strain variation ratio g is also de-fined as follow to represent the degree of strain variation caused bythe local corrosion

g ¼ ecor � eint

eintð5Þ

With this definition, the strain variation ratio is then related tothe circumferential strains of the seven points from the earlier de-picted pipe. As seen in Table 1 below, the strain variation ratio g ofS3 � S7 is rather small, which shows that the circumferential strainat these points is insensitive to local corrosion. Furthermore, in ac-tual pipeline monitoring, if strain sensors are installed at theseinsensitive points, the local corrosion may be undetected. On theother hand, it is also uneconomic to set a large quantity of strainsensors at same pipeline cross-section.

2.3. Indicator in emergency event

Generally, pipeline corrosion is considered to be a gradual alter-ing process, for which the circumferential strain variation can beused to estimate the slow decreasing of pipeline wall thicknessduring long-term operation. However, the significance of circum-ferential strain measurement lies not only in the monitoring ofchanges in the steady state. The dynamic circumferential strain re-sponse induced by some emergency events functions as an indica-tor of pressure sudden reduction.

For instance, when a leakage occurs, a negative pressure wave(NPW) is always formed from the leaky point and spreads outalong the pipeline in a definite velocity [20]. Since the inner pres-sure decreases when the NPW arrives, the circumferential strain ewill decrease, according to Eq. (4). Based on this principle, the

Fig. 3. Circumferential strain distribution comparison between

dynamic signal of circumferential strain can be used to detectthe occurrence of NPW.

Taking into account the above considerations, a circumferentialstrain measuring device was designed, which must meet the fol-lowing requirements. Firstly, the measured circumferential strainof this sensor can reflect the overall circumferential deformationon the same cross-section in order to estimate the pipeline corro-sion even at the local scale. Secondly, the sensor can monitor dy-namic response of the pipeline and the data acquisition systemhas a high enough sampling rate. Thirdly, the sensor needs to beattached on the pipeline surface firmly and reliably to allowlong-term monitoring.

3. FBG hoop-strain sensor design and encapsulation

3.1. FBG hoop-strain sensor design theory

A schematic diagram of an FBG hoop-strain sensor is presentedin Fig. 4. This sensor consists of a fiber Bragg grating, two grippertubes, two gripper blocks, a protective tube, a movable end and afixed end. The fiber in both sides of the FBG is packaged with theepoxy resin in the two gripper tubes. The protective tube is closelymounted onto the external surface of pipeline to be monitored.One of the tube’s basic functions is that it is used to protect thefragile bare fiber from damage. Additionally, the hollow part ofthe protective tube forms an ‘‘orbit’’, ensuring the consistent defor-mation of the FBG hoop-strain sensor system and the monitoredpipeline. The FBG area is not placed in contact with the epoxy resinas this setup then eliminates the multipeaks of reflective light fromthe FBG induced by the nonuniform bonding distribution of theepoxy resin. Both the fixed end and the movable end are installedon the pipeline surface by adhesive or other kinds of mechanicalconnection. The difference is that the griper tube, the protectivetube, and the protective tube are bonded together at the fixedend, and together forms a relative ‘‘fixed-point’’ attached to thepipeline. The movable end shifts position when circumferentialdeformation occurs, which in turn drags the gripper block, causingdeformation in the grating area.

Assuming that the internal tension forces induced throughoutthe whole hoop-strain sensor by the circumferential deformationbe F, the deformation of gripper tube and the FBG are described by

DL1 ¼FL1

EsAsð6Þ

DL2 ¼FL2

EsAsð7Þ

the intact pipe and the pipe with local corrosion in ANSYS.

Table 1Circumferential strain comparison of single-point.

Circumferential strain (�10�4) S1 S2 S3 S4 S5 S6 S7

Intact (eint) 1.96Local corrosion (ecor) 3.63 2.89 1.98 1.90 1.94 1.97 1.98Strain variation ratio (g) 0.85 0.47 0.01 0.03 0.01 0.00 0.01

L1

Lf

L2

bare fiber and FBG

protective tube (d=1.0mm)

epoxy resin

gripper tube (d=0.8mm)

gripper block

fixed end

movable end

Fig. 4. Schematic diagram of an FBG hoop-strain sensor.

Table 2Mechanical properties of the FBG hoop-strain sensor.

Component Fiber core (FBG) Gripper tube

Young’s modulus (Pa) Ef = 72 � 109 Es = 210 � 109

Diameter/thickness (mm) 0.125 0.8/0.1Area (mm2) Af = 0.01226 As = 0.2198Length Lf Ls

18 L. Ren et al. / Optical Fiber Technology 20 (2014) 15–23

DLf ¼FLf

Ef Afð8Þ

where Es, Ef, As and Af denote the Young’s modulus and sectionalarea of the gripper tube and fiber, respectively, the total length ofthe gripper Ls, the total length between the fixed end and the mova-ble end L and the total deformation DL can be expressed as

Ls ¼ L1 þ L2 ð9Þ

L ¼ Ls þ Lf ð10Þ

DL ¼ DL1 þ DL2 þ DLf ¼FLs

EsAsþ FLf

Ef Afð11Þ

In order to study the relationship between the strain variationof the FBG ef and the pipeline e, some basic assumptions are pro-posed to simplify the derivation, (1) the deformation of the FBGhoop-strain sensor system is consistent with the pipeline. In otherwords, the hoop-strain sensor cannot ‘‘resist’’ the inherent defor-mation of the pipeline and (2) the stress transferring loss betweenthe epoxy resin and fiber can be neglected, as has been discussed indetail [21]. Thus ef and e can be expressed as follows:

ef ¼DLf

Lf¼ F

Ef Afð12Þ

e ¼ DLL¼

FLsEsAsþ FLf

Ef Af

Lð13Þ

The strain ratio between the FBG and the pipeline is then ob-tained as:

ef

e¼

FEf Af

FLsEsAsþ

FLfEf Af

L

¼ LEf Af

EsAsLs þ Lf

ð14Þ

The mechanical properties of the FBG hoop-strain sensor arepresented in Table 2. Here, a stainless steel capillary with diameterof 0.8 mm and wall thickness of 0.1 mm is used to make the grip-per tube, which has good flexibility to fit with the circular shape ofpipeline.

To simplify Eq. (14), a stiffness ratio coefficient is applied, whichis defined as

b ¼ Ef Af

EsAsð15Þ

The total length of the hoop-strain sensor L is fixed for a certainpipeline model. Therefore an FBG length coefficient is also defined,that is

a ¼ Lf

Lð16Þ

Thus the strain ratio in Eq. (14), by substituting the stiffness ra-tio coefficient and the FBG length coefficient of Eqs. (15) and (16),can be expressed as

ef ¼L

bð1� aÞLþ aLe ¼ 1

bþ ð1� bÞa e ¼ Ke ð17Þ

K ¼ 1bþ ð1� bÞa ð18Þ

where K is defined as the FBG hoop-strain sensor strain sensitivitycoefficient, representing the scaling relation between the strain var-iation of the FBG ef and the pipeline e. By reference to the parame-ters in Table 1, the stiffness ratio coefficient was calculated to beb = 0.019133. Consequently the numerical expression of FBGhoop-strain sensor strain sensitivity coefficient K can be approxi-mately determined by

K ¼ 10:019þ 0:981a

ð19Þ

It can be seen from Eq. (19) that for most cases when L is biggerthan Lf, the mechanical structure of the FBG hoop-strain sensor hasa function of strain sensitivity amplification, since K > 1 from theabove calculation. Meanwhile, a suitable strain sensitivity coeffi-cient of this FBG hoop-strain sensor can be obtained by adjustingthe ratio between Lf and L in accordance with the pipeline defor-mation analysis. It is worth mentioning that a similar sensitivityderivation of a linear type FBG strain sensor has been conductedin Ref. [22]. However, compared with the former analysis, this der-ivation in the FBG hoop-strain sensor is more accurate due to theconsideration of stiffness ratio’s influence which is neglected inRef. [22]. Furthermore, from Eq. (18), the influence of stiffness ratiocoefficient b will be enlarged in the situation that the FBG lengthcoefficient a is rather small. In other words, when the total lengthL is fixed and the length of the bare fiber Lf is relative small, theinfluence of b cannot be entirely ignored and must be taken intoconsideration.

Based on the above principle, the prototype of an FBG hoop-strain sensor with the enhanced sensitivity was formed. However,when compared with the common FBG strain sensor, this

fixed blockpre-tension block

anchorage member

protective tube

Fig. 6. Picture of encapsulated FBG hoop-strain sensor with pre-tension system.

L. Ren et al. / Optical Fiber Technology 20 (2014) 15–23 19

hoop-strain sensor has a bent shape and long gauge, so a specialencapsulation technique with pre-tension system was designed.

3.2. Sensor encapsulation with pre-tension system

Unlike sensors with linear geometry, the mentioned FBG hoop-strain sensor must have good flexibility to fit for the curve of pipesurface. In addition, the seamless connection between the pipe andthe hoop-strain sensor which ensures the consistent deformationmust be considered. Taking into account all these factors, theFBG hoop-strain sensor system with a pre-tension mechanismwas developed, shown in Fig. 5.

At first, the anchorage member was adhered or welded onto thepipe surface, after which the fixed block bonded with the protec-tive tube was installed into it, becoming a fixed support of this sys-tem. Next the pre-tension block connected with the protectivetube was stretched along the pipe circumference in order to havethe protective tube seamlessly in contact with and firmly adheredonto the pipe surface. Note that the pre-tension block used in theabove process cannot be connected to the anchorage member, thusthe protective tube only functioned as an ‘‘orbit’’ for the inner fiberrather than bear the pipe circumferential deformation.

The pre-tension block with the gripper tube was stretched aswell, in turn adding tension to the fiber along the protective tube.The advantage of this approach is that errors of strain measure-ment induced by the fiber deflection may be eliminated. Addition-ally for some practical cases in which the pipeline is already inoperation and pressurized, the pre-tensioning mechanism of strainmeasurement is more suitable for detecting strain reductioncaused by the pressure drop even during pipeline leak. Further-more, it is suggested to exert the pre-tension force slowly andgradually while monitoring the wavelength variation caused bythe tensile force from an optical interrogator. If the fiber is nottightened, there may be no wavelength variation at the initialstages of loading. From the results, the wavelength variationcaused by pre-tension is recommended to be restricted under1 nm so that the FBG will be easily destroyed during testing.

The encapsulated FBG hoop-strain sensor with the pre-tensionsystem is shown in Fig. 6.

4. Experimental study with FBG hoop-strain sensor

The purpose of the following experiment is to test the perfor-mance of the FBG hoop-strain sensor after encapsulation andpre-tensioning. These sensors with different designed sensitivitycoefficients were mounted onto the surface of a PVC pipe to cali-brate and tested for linearity. Meanwhile, since the reliabilityand the stability is a primary issue for pipeline’s long-term health

fixed block anchorage member

pre-tension block

connected with protective tube

connected with gripper tube

Fig. 5. Detailed diagram of pre-tension system.

monitoring, cyclic loading and stability tests were also conducted.Finally, preliminary pipeline leakage tests with different leakagerates were simulated to test the FBG hoop-strain sensors’ dynamicperformance.

4.1. Experimental set-up

The experimental design includes a model pipeline with250 mm diameter, 6.2 mm wall thickness and roughly 10 m longpolyvinyl chloride (PVC) pipes, shown in Fig. 7. An air inlet wasmounted at one end of the model pipeline, connected with theair compressor with maximum pumping pressure of 0.4 MPa.Two diffused silicon pressure sensors are installed to monitorinternal pressure variation. The whole pipeline model was laidon an EPE pad, ensuring that the model was uniformly forced inthe direction of gravity. Some air valves are also set along the pipe-line in order to simulate leakage.

Based on the above mentioned techniques, some FBG hoop-strain sensors with different sensitivities were manufactured andapplied in this experimental study. Meanwhile, bare FBGs were ad-hered to different points of a pipe cross-section, and served as ref-erence circumferential strain values, shown in Fig. 7(c). Sb-1, Sb-2

and Sb-3 were three single circumferential strain measuring pointsusing the bare FBGs, while Sb-H is a bare FBG wound around thepipe surface without pre-stretching. It is noted that due to the ex-act value of the PVC Young’s modulus (about 3000 MPa) being un-known, this have made calculations of circumferential strainsomewhat inaccurate for this model. As a result, the measured cir-cumferential strain value of the bare FBGs is approximately consid-ered to represent the real deformation of this model.

4.2. Comparison between bare FBG and hoop-strain sensor

A stepwise pressuring test is conducted, from 20 kPa to 150 kPaby 10 kPa for each load step. The wavelength variations of an FBGhoop-strain sensor and four bare FBGs induced by pressure in-creases are shown in Fig. 8.

Likely as a result of different loading conditions in the directionof gravity and discontinuity of PVC material, the response magni-tudes of bare FBGs differed from that of the hoop-strain sensors.Like the FBG hoop-strain sensor, the bare FBG Sb-H wound aroundthe model surface can also reflect the overall circumferential strainvariation. However, since the bare FBG had no pre-tension totightly adhere itself to the model surface, the wavelength variationmagnitude was relatively smaller than the other single-point FBGs.All of these sensors have good linearity, as presented in Table 3.

pressure sensor

simulative leakage valve

bare FBGs and

FBG hoop-strain sensors air inlet with a compressor

(a) Picture of pipeline model

bare FBG with

protective layer

FBG hoop-strain sensor

(b) Bare FBG and FBG hoop-strain sensor

Sb-1

Sb-2

Sb-3

Sb-H

(c) Bare FBGsat the same cross-section

Fig. 7. Picture of model pipeline and FBG sensors.

20 L. Ren et al. / Optical Fiber Technology 20 (2014) 15–23

Note that the linear coefficient of FBG hoop-strain sensor reached0.9988, demonstrating that the encapsulation technique did nothave any adverse effect to the circumferential strain measurement.

It is then quite obvious on the other hand that the wavelengthvariation magnitude of FBG hoop-strain sensor was much biggerthan the bare FBGs due to its enhanced sensitivity mechanism asmentioned above. The FBG hoop-strain sensors with different sen-sitivity coefficients were encapsulated and their working perfor-mances are detailed in the following section.

4.3. FBG hoop-strain sensors with different sensitivity coefficient

In accordance with the principle presented in Section 2.2, theFBG hoop-strain sensors of different lengths were each

encapsulated with a protective tube. In order to obtain the sensors’experimental sensitivity coefficients, the calibration tests wereconducted by a pressurization method as mentioned in the abovesection. The response of a bare FBG was also used as a circumfer-ential strain variation reference. Three of the measuring resultsare graphically illustrated in Fig. 9, showing different wavelengthvariation magnitudes under the same stepwise internal pressure.

To further clarify this enhanced sensitivity effect further, sensi-tivity coefficients based on both theoretical computation andexperimental study are presented in Table 4. Additionally, the geo-metric parameters Lf and L are also listed in Table 4. Note again thatL is a fixed value since all tests were conducted in the same pipemodel, approximate to the pipe’s circumference (�785 mm). Thetheoretical K was calculated by substituting geometric parameters

20 40 60 80 100 120 140 160

0

500

1000

1500

2000

2500

FBG hoop-strain sensor

Sb-1

Sb-2

Sb-3

Sb-H

Linear Fit of FBG hoop-strain sensor

wavele

ngth

variation

(pm

)

pressure (kPa)

Fig. 8. Strain variation comparison between bare FBGs and FBG hoop-strain sensor.

Table 3Comparison of linear coefficients.

Sb-1 Sb-2 Sb-3 Sb-H FBG hoop-strainsensor

Linearcoefficients

0.9899 0.9919 0.9867 0.9927 0.9988

20 40 60 80 100 120 140 1600

400

800

1200

1600

2000

2400

hoop-strain sensor 1

hoop-strain sensor 2

hoop-strain sensor 3

wavele

ngth

variation

(pm

)

pressure (kPa)

Fig. 9. Comparison of FBG hoop-strain sensors with different sensitivity.

L. Ren et al. / Optical Fiber Technology 20 (2014) 15–23 21

in Eq. (19). The experimental K was the maximum wavelength var-iation ratio of FBG hoop-strain sensor to the bare FBG. As can beseen from the table, the experimental results agree well with thetheoretical calculations in most cases, from hoop-2 to hoop-6.However, the K value based on the experiment was less than thetheoretical value potentially due to the influence of the epoxy resin

Table 4Sensitivity coefficient comparisons between theory and experiment.

Sensor-1 Sensor-2 Sen

Lf (mm) 30 112 15L (mm) 764a 0.039 0.147 0.1K (theoretical) 17.385 6.142 4.7K (experimental) 5.915 5.665 4.1

used to adhere gripper tube on the strain transfer, a factor which isnot taken into consideration during theoretical analysis. However,this error can be acceptable in practical pipeline monitoring.

It should be noted that this enhanced sensitivity mechanismseems to fail particularly in the case that the gripper tube is toolong, just as seen with the sensor-1. Unlike the linearly shapedstrain sensors, the gripper tube in the FBG hoop-strain sensor mustbe bent from a straight tube in order to fit the curved pipe surface.The bending of gripper tube caused friction between itself and theprotective tube which blocks its sliding within the protective tube.From the experimental results, this frictional effect did not influ-ence the sensitivity mechanism where the length of the grippertube is relatively small as seen in sensor-2 through sensor-6. Yet,owing to the longer gripper tube in sensor-1, a large friction forceblocks the gripper tube’s movement, thus preventing the sensorfrom having high-sensitivity. According to the results listed inTable 4, the highest sensitivity achieved was 5.665. Theoretically,it is possible to reach higher levels of sensitivity, but the tradeoffwith the physical integrity of the sensor may lead to early failureof the sensor. On the other hand, it is expected that the highestsensitivity reported in this paper will be adequate for many appli-cations, thus reducing the need to design for higher sensitivities.

4.4. Cyclic loading and stability test

Reliability is an important aspect of any sensor [23], especiallyin terms of long-term monitoring of lifeline engineering, such as along distance pipeline. Regarding the developed FBG hoop-strainsensor, the anchorage that can provide a strong and stable grippingforce plays a decisive role in the functioning of circumferentialstrain measurements.

In order to investigate the reliability, the cyclic loading test wasconducted on the above mentioned model pipeline. The pipelinewas pressurized repeatedly from 30 kPa to 70 kPa, during whichthe peak and valley wavelength variations of each loop were re-corded and presented in Fig. 10. Since the cyclic pressurizing wasa time-consuming process, the temperature influence on the wave-length variation of FBG hoop-strain sensor was compensated by aFBG temperature sensor [24]. It was then noted that the tempera-ture effect on the structural deformation was much greater than tothe FBG wavelength variation itself. When the temperature rose,the circumferential deformation increased. In consequence, bothpeak and valley series showed the same tendency with the temper-ature variation. However, it is worth mentioning that differencesbetween each pair of peak and valley as listed in Fig. 10 remainessentially flat during each pressurizing cycle.

The pressure holding test was then implemented, during whichthe temperature and pressure variation were synchronously ac-quired as references, as shown in Fig. 11. The model was initiallyloaded with an approximate 80 kPa internal pressure and re-mained valve-closed for several hours. Similar to the cyclic loadingtest, the time history response of FBG hoop-strain sensor agreedwell with the dramatic temperature fluctuation owing to its pre-vailing influence to structural deformation in the first 6 h. How-ever, the pressure declined continuously due to the poorairtightness of the model pipeline. Although the temperature

sor-3 Sensor-4 Sensor-5 Sensor-6

0 226 384 604

96 0.296 0.503 0.79126 3.234 1.953 1.25993 2.757 1.754 0.870

0 3 6 9 12 15

-9

-8

-7

-6

-5

-4

-3

100

200

300

400

500

600

700

800

100

200

300

400

500

Tem

pera

ture

(°C

)

Cycle

TemperaturePeakValleyΔ wavelength

wavele

ngth

variation

(pm

)

Δw

ave

length

(pm

)

Fig. 10. Cyclic loading test of FBG hoop-strain sensor.

-150

-100

-50

0

50

0 5 10 15 20 25

60

80

100

120

140

Pre

ssure

(kP

a)

Time (s)

Pressure sensor

FBG hoop-strain sensor

suspected leakage occurrence time

Wavele

ngth

variation

(pm

)

0 5 10 15 20 25 30

-160

-140

-120

-100

-80

-60

-40

-20

0

20

leakage rate

Wavele

ngth

variation

(pm

)

Time (s)

small

medium

large

(b) Detection of different leakage amount by FBG hoop-strain sensor

(a) Leakage detection by pressure sensor and FBG hoop-strain sensor

Fig. 12. Dynamic responses induced by leakage.

22 L. Ren et al. / Optical Fiber Technology 20 (2014) 15–23

remained stable during the last 4 h, the wavelength still decreasedalong with the air leakage.

Essentially, from results of the above two reliability tests, thedesigned FBG hoop-strain sensor was demonstrated to work stea-dily and reliably. The absence of abrupt variation in these testsdemonstrated that the anchorage member supported the sensorwith an adequate and stable gripping force.

4.5. Dynamic response monitoring

Trial tests of leakage simulation were conducted by using anopening control gas valve shown in Fig. 7(a) presented earlier inthis paper. Following these tests, the FBG hoop-strain sensor wasacquired in real-time by using the FBG interrogator with the sam-pling rate of 1000 Hz. The pressure variation was synchronouslysampled during leakage process. The response comparison be-tween the pressure sensor and FBG hoop-strain sensor in the sameleakage simulation is shown in Fig. 12(a). It is noted that the wave-length in the normal operation was made to be null so as to ob-serve their negative variation discernibly. The sudden drop ofwavelength detected by the FBG hoop-strain sensor was consid-ered to be an effective indicator of a leakage event. By implement-ing some signal change detection methods, the arrival time of NPWcan be also estimated. Provided that two FBG hoop-strain sensorsare installed at both ends of a pipeline section, the leaking pointcan be inferred by the NPW arrival time difference when theNPW velocity is known. Experimental results of the leveled leakage

0 2 4 6 8 10200

400

600

800

1000

0

5

10

15

20

0

20

40

60

80

100

pressure-induced

FB

Gsensor

hoop

(pm

)

Time (h)

FBG hoop-strain sensor

Temperature

Pressure

temperature-induced

Tem

pera

ture

(°C

)

Pre

ssure

(kP

a)

Fig. 11. Pressure holding test of FBG hoop-strain sensor.

amount are shown in Fig. 12(b). The leakage rates can be inferredaccording to the wavelength changes to some extent.

5. Conclusion

A novel configuration of FBG hoop-strain sensor was designedand encapsulated to measure the circumferential strain of pipeline.The corrosion monitoring principle using this sensor was intro-duced and its sensing mechanism was detailed. This paper mainlyfocused on the experimental study of a model pipeline to investi-gate its performance in the circumferential strain measurement.From the above investigation, the following conclusions can beobtained:

� The FBG hoop-strain sensor can be applied to measure thecircumferential strain of pipeline. Compared with the bareFBG, it has advantages of higher sensitivity and good protec-tiveness. Meanwhile, the encapsulation technique does notinfluence the linearity of FBG strain measurement.

� The FBG hoop-strain sensor can be designed according tothe practical monitoring requirements to be with anenhanced sensitivity.

� The FBG hoop-strain sensor has good stability and reliabilityaccording to experimental results, showing a promisingpotential in the practical pipeline monitoring.

L. Ren et al. / Optical Fiber Technology 20 (2014) 15–23 23

� The FBG hoop-strain sensor can also be used to monitor thedynamic circumferential strain response induced by suddenevents, such as a pipeline leakage.

Further investigation may move onto the monitoring of pipelineuniform corrosion and localization of local corrosion and pipelineleakage based on the NPW detection by application of the FBGhoop-strain sensor.

Acknowledgments

This work was supported by the Science Fund for CreativeResearch Groups of the National Natural Science Foundation ofChina (Nos. 51121005 and 51261120375), the National Key BasicResearch and Development Program (973 Program) (No.2011CB013605), the National Key Technology R&D Program ofChina (No. 2006BAJ03B05), the Natural Science Foundation ofChina (Nos. 51108059 and 10702012), the Open Foundation ofKey Lab of Liaoning Province (JG-200909) and China ScholarshipCouncil (No. 201206060081). These grants are greatly appreciated.

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