Use of Fiber Bragg Grating Strain Gages on a Pipeline ... · Use of Fiber Bragg Grating Strain...

Chapter 16

Use of Fiber Bragg Grating Strain Gages on a Pipeline Specimen

Repaired with a CFRE Composite System

J.L.F. Freire, V.A. Perrut, A.M.B. Braga, R.D. Vieira, A.S.A. Ribeiro, and M.A.P. Rosas

Abstract Re-establishing the maximum operating pressure of a segment of pipeline with metal loss defects, such as erosion

or corrosion defects, can be accomplished either by replacing the damaged segment altogether, or by applying a localized

repair system. The present paper deals with laboratory tests conducted: (1) to understand and describe how the reinforcement

layers of a carbon fiber epoxy composite material can enable a steel line pipe specimen with a metal loss defect to withstand

pressure loading; (2) to compare the test results with those predicted by Mechanics of Materials and by Finite Element

numerical solutions developed previously. Hydrostatic burst tests were performed on three pipe (API 5L X65 ERW)

specimens: one with metal loss defect, one without metal loss defect, and one with metal loss defect but repaired with a

carbon fiber reinforced epoxy composite system CFRE. Fiber Bragg grating FBG strain gages were used to monitor elastic

and plastic strains during the tests of the repaired specimen. The strain gages were bonded either directly on the surface of

the defect, or were inserted in between some of the composite layers in order to show the reinforcement’s effective

contribution to the strength of the repaired pipes. The analytical and numerical results agreed very satisfactorily with

experimentally determined burst pressures and pressure-strain curves, showing that the behavior of composite reinforced

pipelines can be well predicted by using simple Mechanics of Materials or sophisticated Finite Element solutions.

Keywords FBG strain gages • Pipelines • Composite repair • Carbon fiber composite

16.1 Introduction

Re-establishing themaximum operating pressure of a segment of pipeline with metal loss defects, such as erosion or corrosion

defects, can be accomplished either by replacing the damaged segment altogether, or by applying a localized repair system.

Composite repair systems are seen to be more economical than other repair alternatives; since they are ¼ less expensive than

welded steel sleeve repairs and less expensive than the complete replacement of the damaged segment [1]. The present paper

deals with laboratory tests conducted to understand and describe how the reinforcement layers of a composite material made

from carbon fiber reinforced epoxy (CFRE) can enable a steel line pipe specimen with metal loss to withstand pressure

loading, and to compare the experimental results with results derived from two solutions based on Finite Elements and simple

Mechanics ofMaterials [2]. Hydrostatic pressure and burst tests were performed on three pipe specimens: one with metal loss

defect, one without metal loss defect, and one with metal loss defect but repaired with a carbon fiber reinforced epoxy

composite system. Fiber Bragg grating FBG strain gages were used to monitor elastic and plastic strains during the tests.

J.L.F. Freire (*) • A.M.B. Braga • R.D. Vieira • A.S.A. Ribeiro

Pontifical Catholic University of Rio de Janeiro, Rio de Janeiro, Brazil

e-mail: [email protected]

V.A. Perrut

CENPES, PETROBRAS, Rio de Janeiro, Brazil

M.A.P. Rosas

Prima-7S, Rio de Janeiro, Brazil

C.E. Ventura et al. (eds.), Experimental and Applied Mechanics, Volume 4: Proceedings of the 2012 Annual Conferenceon Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series 34,

DOI 10.1007/978-1-4614-4226-4_16, # The Society for Experimental Mechanics, Inc. 2013

133

mailto:[email protected]

16.2 Experimental Methods

The three pipe specimens were made of American Petroleum Institute API 5L X-65 grade steel line pipe [3] produced by the

electric resistance welding process ERW. The nominal specimens’ dimensions were diameter D, thickness t and length A,equal to 457 mm (18 in.), 8.7 mm and 2.7 � 103 mm, respectively. Reinforced flathead caps were welded to the pipe ends so

that the hydrostatic internal pressure tests could be performed. Two of these specimens had long, external metal loss defects

produced by sparking erosion in order to simulate corrosion defects. The defects had a uniform profile as shown in Fig. 16.1.

Their nominal dimensions were depth d, length L and widthW, equal to 70% of the pipe thickness (6.1 mm), 450 and 85 mm,

respectively. Themetal loss area of one of these specimens was reinforced with a carbon fiber reinforced epoxy (CFRE) repair

system in order to restore it to its original pipe strength. The defect area and superposed repairing composites layers of the

CFRE repair system of this specimen were also instrumented with circumferential optical strain gages based on fiber Bragg

grating sensors. The geometric dimensions, mechanical properties and other relevant data regarding the pipeline specimens,

prediction equations, repair system, experimental setup and test sequence are given in Figs. 16.1 and 16.2, and in Table 16.1.

R2=4mm450mm

6.3mm 8.7mm

R3=4mm85mm

A-A

Corte B-B

B

459mm

R1=10mm

BA A

2.7 x 103 mm

Fig. 16.1 Geometric dimensions of the test specimens and of the machined spark-erosion defect

Time or test event

Tes

t pr

essu

re (

MPa)

X

Defect free specimenRupture at 26MPa

X

Specimen with non-repaired defectRupture at 7.5MPa

X

Rupture test ofrepaired specimenBurst at 27MPa

Specimen with defectGage bonding and application ofCFRP system at pressure of 4.1MPaand 24 hours epoxy curing time

Hydrostatic testPressure at 15.4MP during 4 hours

10 cycles test at maximumoperating pressure of 12.3MPa

1

2

3

3

3

3

Fig. 16.2 Sequence of tests carried out on each of the three test specimens

134 J.L.F. Freire et al.

Table

16.1

Experim

entaldata

Specim

en’sdata

Param

eter

Nominal

data

Specim

en1defect

free

Specim

en2with

defect

Specim

en3withdefectand

repaired

Specim

endim

ensions

D(m

m)

457.2

459

459

459

t(m

m)

8.74

9.0

8.5

8.9

Defectdim

ensions

d(m

m)

6.1

–6.2

6.5

L(m

m)

450

–450

450

Steel

pipeERW

API5LX65mechanical

properties

Yield

strength

(MPa)

SMYS¼

448

Sy¼

512

Sy¼

510

Sy¼

516

Ultim

atestrength

(MPa)

SMUS¼

530

Su¼

630

Su¼

663

Su¼

666

Predictedpressure

Eq.1–3(M

Pa)

Design(1)

12.3

––

–

Hydrostatic

test(2)

15.4

––

–

B31-G

(3)

4.1

––

–

Burstpressure:standard,numerical,analytical,andpresenttest

results(M

Pa)

DNVRP-F101Eq.4

7.5

25(nominal

area)

8.2

26(nominal

area)

Finiteelem

ents[2]

––

–30

Analytical[2]

––

–29

Testresults(present

paper)

–26

7.5

27

Locationofrupture

(testresults)

–Nominal

area

Defect

Nominal

area

Prediction

equations

Designpressure

p d¼

2:t:SMYS:F

D;F

¼0:72

(1)

Hydrostatic

testpressure

p H¼

1:25�p d

(2)

ASMEB31-G

,longdefect,safe

pressure

tooperatewithdefect[4]

p 31G¼

2:t:SMYS:F

D�ð1:1Þ�

1�

d t

�� ;F

¼0:72

(3)

DNVRP-F101(burstpressure

ofspecim

en

withdefect)[5]

p burst;DNV¼

2:t:S

u:

D�t�

1�

d t

1�

d

tffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1þ0

:31L2=D:t

p

0 @

1 A(4)

Repairsystem

Typeofrepairsystem

Carbonfiber

reinforced

polymer

(epoxy)applied

incontinuouslayerswhichadhered

tothepipesurfaceandto

each

other

bymeansofusingafastcuringliquid

epoxy(~3happlication,24hfulluse)adhesive

Number

oflayers,totalthicknessand

totallength

oftherepairsystem

12layers;Minim

um

totalrepairthickness¼

11mm;Maxim

um

totalrepairthickness¼

19mm

(causedbythe

superpositionofadjacentlayers).Theaveragethicknessoftherepairsystem

usedin

calculationswas

15mm

Totallength

¼550mm.Therepairsystem

consisted

ofthreeadjacentsetsof12layersthat

covered

andexceeded

each

end-sideofthedefectlength

by50mm

andhad

about50mm

ofpartial

superpositionoftheiradjacent

boundaries

Mechanical

properties

oftheCFRP(From

[2])

YoungmodulusE(G

Pa)

49(c

¼circumferential);23(l¼

longitudinal);5.5

(r¼

radial)

ShearmodulusG(G

Pa)

0.69(r-l);0.69(l-c);30(r-c)

Maxim

um

elongatione u

(%)

1.6

(circumferential)

Poissoncoefficientm

0.43(r-l);0.0.43(l-c);0.20(r-c)

Epoxy

E¼

1.7

MPa;

m¼

0.45

Experim

ent

details

Testpressure

transducer

Gefranmodel

TK

Volumeofwater

Measurementofvolumeofwater

injected

duringthehydrostatic

testem

ployed

aresistivelevel

transducer

Pressure

device

Pneumatic-airdriven/water

pumpHaskel

BSS-100

Measurementofthecircumferential

strains

inthedefectandrepairareas

Fiber

BragggratingFBGstrain

gages

with2mm

gagelength.Threesensorswereusedin

thedefectsurfaceandwere

bonded

usingcyanoacrylate

adhesive.Fifteen

sensorswereplacedin

betweenthevariousCRFPrepairlayers

sensorsandbonded

tothevariousrepairlayersusingthesameepoxyresinem

ployed

intherepairprocess

Dataconditioners

Pressure

dataandwater

volumedata:

LynxADS2000conditioner.FBGstrain

gages:conditioner

MicronOptics

model

sm125-500,dataacquisitionboardNIUSB-600812bits(N

ational

Instruments)plusopticalfibers

Dataacquisitionrate

1Hz

16 Use of Fiber Bragg Grating Strain Gages on a Pipeline Specimen. . . 135

The three specimens were all tested up to rupture, but before the rupture test, the CFRE repaired specimen was tested

under the following conditions: (1) installing the repair system and bonding the FBG gages while the specimen was

subjected to the maximum allowed operating pressure calculated by the American Society for Mechanical Engineers

ASME B31-G method [4], this calculation taking into consideration the spark-eroded metal loss defect; (2) increasing the

internal pressure to reach the standard hydrostatic pressure test for the repaired specimen, considering in this case that it

recovered the original pipeline strength; (3) decreasing the pressure to zero, followed by ten pressure cycles that varied from

zero pressure to the maximum operating pressure (of the pipeline considered completely restored); (4) increasing the

pressure from zero to the burst pressure. Figure 16.2 shows schematically the sequence of tests performed on each of the

three test specimens.

The specimen that received the repaired system was instrumented with 15 FBG strain gages that had a 2 mm gage length.

The gages were produced in-house and were placed in series of three or six gages along three fibers. In each fiber the gages

were set 500 mm apart from each other. Each fiber, one with three gages and two with six gages, was placed in one of the

three of four channels of the FBG reading device (Micron Optics model sm125-500).

Figure 16.3 shows the gage locations in the repaired test specimen. All the gages were positioned to measure circumfer-

ential strains, and were placed in the following positions: three gages were bonded directly to the defect’s surface (at a

distance of 45 mm from each other, from the center of the defect to the end of the defect area); two gages were positioned

between the first and second layers (one over the center of the defect and another 90� away); two gages were positioned

between the third and fourth repair layers (center and 90� away); two were positioned between the fifth and sixth repair

layers (center and 90� away); two were positioned between the seventh and eighth repair layers (center and 90� away); twowere positioned between the ninth and tenth repair layers (center and 90� away); and lastly, two gages were positioned

between the eleventh and twelfth repair layers (center and 90� away). Figure 16.3 also shows some of the gages bonded

to the defect’s surface, details of the repair system, and the entire specimen showing the three adjacent repair layers used to

cover the full defect length. The fracture area encompassing an originally non-defective area is also shown.

16.3 Pressure Tests and Results

The burst pressures of the defect-free specimen and of the specimen with the unrepaired defect were equal to 26 and

7.5 MPa, respectively. According to the equation of DNV RP-F101 [5], which can be expected to furnish a good

approximation of the burst pressure of the specimen [6, 7], the calculated rupture pressure of the specimen with defect

was 8.2 MPa, which is 9% off from the experimental result.

The test procedure and results are shown and described herein. Figure 16.2 helps to illustrate the sequence of the tests.

The remaining tubular specimen that also contained the defect was pressurized up to 4.1 Mpa (safe operating pressure for the

specimen with an unrepaired defect with depth d equal to 70% of thickness t, according to ASME B31-G [4]). This pressure

level was kept constant during the application of the repair system (layers of bi-directional carbon fiber fabric and liquid

epoxy adhesive) and of the process of bonding the fiber optic strain gages. Subsequently the specimen was maintained at the

same pressure for 24 h, which was the time needed for the epoxy resin to cure completely. The initial measurement readings

(zeroing process) of the fiber optic strain gages were at the 4.1 MPa pressure. Next, the specimen was pressurized at a rate of

0.4 MPa up to 15.4 MPa (hydrostatic test pressure of the pipeline without defect, which is 25% above the design operating

pressure of a API 5L X65 oil or gas pipeline, class 1 division 2, without defect) and it was kept at this pressure for 4 h.

Finally, the pressure was decreased back to zero at a rate of 10 MPa/min. The strain values measured by the gages located on

the defect’s surface and in the third and ninth repair layers are presented in Fig. 16.4.

Circumferential strains measured during the pressure test (4.1–15.4 MPa) by the gages positioned in the third and ninth

layers of the repair system, located immediately above the repair and in a position 90� away from the defect’s center, are

presented in Fig. 16.5. One can see that strains are larger in the locations right above the defect than in the locations outside

(90� away) the defect.The non-linear distribution of strains along the thickness of the repair system and above the defect’s center area can

be observed in the graph in Fig. 16.6. One can see that the strain’s distribution is not uniform along the repair thickness.

The strains are larger in locations closer to the inner layers, as one would expect if a thick theory of tubular specimens under

internal pressure were applied. One can also see in the graph that strains in the metal surface start reaching elasto-plastic

conditions when total test pressure is equal to 12 MPa (note that the zero measured strain corresponds to the initial test

pressure of 4.1 MPa). This can be stated because the definition of yield strength in API 5L [3] considers a total strain of 0.5%.

After reaching the maximum hydrostatic test pressure of 15.4 MPa, the specimen was submitted to 10 cycles from zero up

to maximum operating pressure. In other words, the internal pressure was increased from zero MPa to 12.3 MPa. The


pressure increase and decrease rates both varied between 10 and 15MPa/min. The measured strain-pressure cycles presented

very repetitive results, as shown in Fig. 16.7, which shows the superposition of the 10 strains cycles measured with the gage

positioned in the ninth layer above the defect location.

Fig. 16.3 Fiber Bragg grating FBG strain gage locations and test specimen: (a) general location of the fiber optic strain gages; (b) three strain

gages, belonging to the same fiber, bonded inside the defect on the specimen’s surface; (c) location of the three strain gages bonded on the

specimen’s surface; (d) epoxy filling of the defect’s recess; (e) exit of optical fibers from the repair system; (f) tubular-repaired specimen after burst

test – rupture location outside the defect-repaired area


After the tenth cycle of pressurization, the strain gages were reset to zero and the internal pressure was again increased to

the point that the specimen burst after reaching the rupture pressure of 27 MPa. Figures 16.8 and 16.9 show the increasing

strain gage readings with increasing test pressures for the gages mounted in the third and ninth layers above the defect.

Although the gage mounted in the third layer lasted throughout the test, the gage in the ninth layer failed when the total

measured strain approached 3,200 me and the pressure was around 20 MPa. The gages mounted on the defect’s surface failed

due to strains higher than 5,000 me, which occurred during the 10 test cycles. The graphs in Fig. 16.9 help to illustrate the

behavior sensed by the gages during the first hydrostatic test and its unloading, and during the final burst test. Regarding the

latter, linear behavior was present under loading conditions up to the point that the pressure of 15.4 MPa was reached. This

happened because the 15.4 MPa pressure was the maximum pressure reached during the hydrostatic test. From this point and

higher, the angular slope of the curve depicting the burst test changed. This can be explained by the fact that since the steel

pipe material was under plastic behavior, most of the remaining rigidity presented by the curve was due to the repair system,

which continued to display elastic behavior.

0

2

4

6

8

10

12

14

16

0 500 1000 1500 2000 2500 3000

9th repairlayer, 90apart

Strain µe

Pre

ssur

e (M

Pa)

3rd repairlayer, 90apart

3rd repairlayer, abovethe defect

9th repairlayer, abovethe defect

Fig. 16.5 Circumferential strains measured in the third and ninth layers in locations above the defect’s center and 90� away

0

2

4

6

8

10

12

14

16

18

0 2000 4000 6000 8000

defectsurface

9th repair layer

Srain µe

Pre

ssur

e (M

Pa)

3rd repairlayer

Fig. 16.4 Circumferential strains measured by the fiber optic strain gages located on the defect’s surface, and in the third and ninth layers of the

repair system, all above the defect’s center position


16.4 Numerical and Analytical Results

The results of the experiment can enable one to understand the structural and reinforcing behavior of the repair system.

It may be considered as a benchmark to be used in comparisons with results generated by a numerical model and by an

analytical model developed to accurately simulate the integrity assessment of pipelines with composite repair systems.

Reference [2] developed the two model solutions for the present case. The solutions used a Finite Element FE model and a

simple Mechanics of Materials model. The FE simulation employed solid elasto-plastic 3D elements and the Ansys 11.0

software. The epoxy resin used to fill the defect gap was also included in both the numerical and the analytical models.

Pressure = 6MPa

Stra

in µε

Radial position mm

Radial position mmSt

rain

µε

CFRE layers

Steel pipe

Epoxy filled gap

Radial position and symmetry line

Fig. 16.6 Circumferential strain distributions across the repair thickness and their variation with test pressure (zero strain corresponds to the

starting test pressure, which is equal to 4.1 MPa)

0

2

4

6

8

10

12

14

16

18

0 500 1000 1500 2000 2500 3000 3500

Hydrostatictest

10 cycles atoperatingpressure

Strain µe

Pre

ssur

e (M

Pa)

Fig. 16.7 Circumferential strains measured by the fiber optic strain gage located in the ninth layer. The graph shows the hydrostatic test increasing

from 4.1 MPa to maximum pressure of 15.4 MPa and 10 superposed and very repetitive pressure cycles from approximately zero pressure up to the

maximum operating pressure of 12.3 MPa


The epoxy filling and the composite CFRE material were treated as linear elastic materials. It is beyond the scope of this

article to give more details on both solutions, which are fully described in reference [2]. Two results from these simulations

are presented in Figs. 16.10 and 16.11. Figure 16.10 shows the variation in the circumferential strain caused by increased

pressure at points belonging to the numerical solution and located at depths equivalent to the third and ninth layers of the

repair system. These results are presented together with the strain gage responses given by the gages located in the same

positions of the repair system during the final burst test. One can see that the numerical solution curves agree quite well with

the actual results.

Figure 16.11 shows the comparison between the numerical solution and the simple solution derived from Mechanics of

Materials. In the latter, the structure is composed of three concentric and contacting pipes formed by the steel, epoxy filler

and composite materials. These pipes interfere with and react to the application of internal pressure to the steel pipe walls.

The steel pipe material is modeled by an elasto-plastic bi-linear material, where the yield strength is reached at 0.5% total

strain and the ultimate strength is reached at 9% total strain. The epoxy filler and the composite materials are considered as

presenting a linear behavior, although maximum total (ultimate) strains can be imposed to limit their contribution in adding

strength and/or rigidity to the entire structure response. In the present solution, the maximum pressure to be calculated is the

one that makes one of the three materials reach its ultimate strength first, or, as was the case in the present analysis (see in

Fig. 16.3f that fracture occurred outside the defect area), the ultimate strength of the steel of the pipe without defect. As one

can see from the comparison in Fig. 16.11, both solutions agree satisfactorily, and as a result, the analytical solution agrees

satisfactorily with the actual results of the experiment. This successful comparison is important in terms of opening an

opportunity for predicting the behavior of composite reinforced pipelines using simple Mechanics of Materials solutions.

0

5

10

15

20

25

30

0 1000 2000 3000 4000 5000 6000

Sensor failure

Strain µe

Pre

ssur

e (M

Pa)

9th

repairlayer 3rd repair

layer

Fig. 16.8 Circumferential strains in the third and ninth layers of the repair system (above the defect) during the burst test

0

5

10

15

20

25

30

0

5

10

15

20

25

0 1000 2000 3000 4000 5000 6000 70000 500 1000 1500 2000 2500 3000 3500 4000 4500

Hydrostatictest

Strain µeStrain µe

Pre

ssur

e (M

Pa)

9th repair layer 3rd repair layer

Burst test

10 cycles at operatingpressure

Hydrostatic testBurst test

Fig. 16.9 History of strain data collected for both gages positioned in the third and ninth layers of the repair system (above the defect) during the

hydrostatic test, 10 test cycles (only gage in the ninth layer) and burst test


In the present case, the burst test pressure was 27 MPa (nominal non-defective pipe specimen section), while the FE solution

also displayed fracture at a nominal section for a pressure equal to 30 MPa. The simple analytical solution for the nominal

pipe section is 29 MPa.

16.5 Conclusions

This paper has presented the hydrostatic test results of experiments conducted on three pipeline specimens made of API 5L

X65 ERW steel line pipe to demonstrate how the reinforcement layers of a carbon fiber epoxy composite material can enable

a steel line pipe specimen with a metal loss defect to withstand pressure loading. Fiber Bragg grating strain gages were

0

100

200

300

400

500

600

0 5 10 15 20 25 30 35

Analytic solution atdefect of steel pipe

FE solution for surface defect

FE solution forinternal wall of repair

FE solution for epoxy filler

Pressure (MPa)

von

Mises

equi

vale

nt s

tres

s (M

Pa)

Analytic solution forinternal layer of repair

FE solution for externalwall of repair

Analytic solution forepoxy filler

Fig. 16.11 Comparison between the experimental and the numerical (FE) solutions for points located in the steel surface defect, in the epoxy filler

and in the composite repair system. Calculations were stopped when the pressure reached 30MPa, which is the burst pressure for a nominal section

of a specimen without defect

0

5

10

15

20

25

30

35

0 1000 2000 3000 4000 5000 6000 7000

9th repair layerFE numerical solution

Strain µε

Pre

ssur

e (M

Pa) 3rd repair layer

FE numerical solution

3rd repair layerStrain gage

9th repair layerStrain gage

Fig. 16.10 Comparison between the experimental and the numerical (FE) solutions for point located above the defect in the third and ninth layers

of the repair system


successfully used to monitor the elastic and small plastic strains during the tests of the repaired specimen. The strain gages

were either bonded directly to the surface of the defect or were inserted in between some of the composite layers, and they

demonstrated the reinforcement’s effective contribution to the strength of the repaired pipes. The test results compared

satisfactorily to results predicted by a Mechanics of Materials analytic solution and by a Finite Element numerical solution

developed previously, showing that the behavior of composite reinforced pipelines can be satisfactorily predicted by using

simple Mechanics of Materials or sophisticated Finite Element solutions.

References

1. Duel J, Wilson J, Kessler M (2008) Analysis of a carbon composite overwrap pipeline repair system. Int J Press Vessel Pip 85:782–788

2. Rosas MAP (2010) Evaluation of sleeve type repair models for pipelines with thickness loss defects. Doctoral thesis (in Portuguese),

Mechanical Engineering Department, Pontifical Catholic University of Rio de Janeiro, PUC-Rio, September 2010

3. API Specification 5L (2009) Specification for line pipe, 44th edn. American Petroleum Institute, Washington, DC

4. ASME (2009) ASME-B31G manual for determining the remaining strength of corroded pipelines – a supplement to ANSI/ASME B31 code for

pressure piping. The American Society of Mechanical Engineers, New York

5. DNV (1999) Corroded pipelines, recommended practice RP-F101. Det Norske Veritas, Høvik

6. Freire JLF, Vieira RD, Castro JTP, Benjamin AC (2006) Burst tests of pipeline with extensive longitudinal metal loss. Exp Tech 30(6):60–65,

November–December

7. Freire JLF, Vieira RD, Diniz JC (2007) Effectiveness of composite repairs applied to damaged pipeline. Exp Tech 31(5):59–66,

September–October


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