Livro Tese Reparo Fibra

8/4/2019 Livro Tese Reparo Fibra

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Reinforcement of existing steel pipes using

composite materials

High temperature applications

Masters Thesis in the International Master Programme in Structural Engineering

ANDERSSON HENRIKVYSOCHINSKIY DMITRY

Department of Civil and Environmental EngineeringDivision of Structural Engineering

Steel and Timber Structures

CHALMERS UNIVERSITY OF TECHNOLOGYGteborg, Sweden 2008Masters Thesis 2008:36


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MASTERS THESIS 2008:

Reinforcement of existing steel pipes using compositematerials.

High temperature applications

Masters Thesis in theInternational Master Programme in Structural Engineering

ANDERSSON HENRIK

VYSOCHINSKIY DMITRY

Department of Civil and Environmental EngineeringDivision of

Structural Engineering


CHALMERS UNIVERSITY OF TECHNOLOGY

Gteborg, Sweden 2008


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Reinforcement of existing steel pipe with composite material

High temperature applicationsMasters Thesis in the International International Master Programme in Structural

Engineering

ANDERSSON HENRIK

VYSOCHINSKIY DMITRY

ANDERSSON HENRIK & VYSOCHINSKIY DMITRY, 2008

Masters Thesis 2008:

Department of Civil and Environmental Engineering

Division of Structural Engineering


Chalmers University of Technology

SE-412 96 Gteborg

Sweden

Telephone: + 46 (0)31-772 1000

Cover:Radial stresses distribution for the pipe with composite repair, ABAQUS model.

Chalmers Repro Service / Department of Civil and Environmental EngineeringGteborg, Sweden 2008


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I

Reinforcement of existing steel pipe with composite material

High temperature applicationsMasters Thesis in the International Masters programme of Structural Engineering

ANDERSSON HENRIK

VYSOCHINSKIY DMITRYDepartment of Civil and Environmental EngineeringDivision of Structural EngineeringSteel and Timber StructuresChalmers University of Technology

ABSTRACT

Nuclear power plants need well functioning cooling systems in order to operatesafely. Nuclear plant cooling system is a system of steel pipes transferring overheatedwater and steam. Like most structures these pipes can suffer from different defects,such as corrosion, cracks or weld defects. Currently used repair methods based on

welding are time consuming and labour extensive. An alternative way of repair cansave time and money.

Fibre reinforced polymers have been used for reinforcing and repairing structureswithin many fields. In the last five years there have been several reports on the use offibre reinforced composite wraps for repair of pipes and pipe work, mostly within theoffshore oil and gas industry.

This thesis concerns the problem of repairing a corroded cooling system pipe usingfibre reinforced composites. Since fibre reinforced composites have not been used forthe repair of such types of pipes a new repair method for this field of application wasdeveloped. The main requirement for the developed repair method was the ability to

significantly reduce the stress in the steel under elastic stage. The case when the hoopstress in the steel turns to negative was considered to be most desired, since itindicates the case when the steel is unloaded and the entire load is carried by thecomposite.

The structure and properties of fibre reinforced polymers are first explained.Commercial use of fibre composites, with special focus on pipe rehabilitation bywrapping, is then investigated. A simple analytical model of a wrapped steel pipe isthen constructed and used to draft an effective reinforcement configuration. Finally, aFE-analysis of this configuration is performed.

The analysis has shown the efficiency of the developed carbon fibre composite based

repair method. The stresses in the steel under the combination of design pressure andtemperature became negative due to thermal effect in the studied case. The thermaleffect appeared to be one of the most critical factors for the design. It was also shownhow the thermal effects depend on such material properties as adhesive curingtemperature, fibre stiffness and thermal expansion coefficient.

Key words: pipe, fibre composites, FRP, CFRP, high temperature application


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II

Frstarkning av stlrr med kompositmaterialHgtemperatur applikationer

Examensarbete inom Civilingenjrsprogrammet Vg- och vattenbyggnad

ANDERSSON HENRIK

VYSOCHINSKIY DMITRY

Institutionen fr bygg- och miljteknikAvdelningen fr konstruktion teknikStl och trbyggnadChalmers tekniska hgskola

SAMMANFATTNING

Drivandet av krnkraftverk krver vl fungerande kylningssystem. Dessa kylsystemtransporterar verhettat vatten och nga genom ett system av stlrr. Dessa rr kan,liksom de flesta strukturer, drabbas av skador ssom korrosion, sprickor ellersvetsdefekter. Nuvarande anvnda metoder fr reparation av dessa typer av skadorinkluderar ofta svetsning, ngot som bde krver kompetent personal och som r

tidskonsumerade.

Fiberkompositmaterial har anvnts vid frstrkning och reparation av mnga olikatyper av strukturer och byggnader. Under de senaste fem ren har ett flertal rapporterrrande frstrkning och rehabilitering av rr blivit publicerade, de flesta inomoffshoreindustrin.

Denna studie rr problemet med att rehabilitera tryckbelastade, korrosionskadade rrmed hjlp av fiberkompositmaterial. D fiberkompositmaterial ej har annvnts frrehabilitering av rr i denna typ av milj tidigare blev en ny metod utvecklat fr attpassa denna typ av applikationer. Det viktigaste kravet p denna metod var attdragspnningarna i det ursprungliga stlrret skulle reduceras till noll. En reduceringav dragspnningarna till under noll, dvs till tryck, ger n bttre resultat d hela lastenbrs av kompositmaterialet och redan existerande sprickor sluts.

Strukturen och egenskaperna hos fiberkompositmaterial gs frst igenom. Vidareundersks kommersiell anvndning av fiberkompositmaterial, med fokus p rr-rehabilitering. En enkel analytisk modell av ett kompositaomslutet rr blirpresenterad. Resultat frn dessa berkningar anvnds fr att skapa en effektivrehabiliteringskonfiguration. Avslutningsvis genomfrs en FE-analys fr att bekrftaerhllna resultat.

Genomfrda analyser har bekrftat effektiviteten hos den utveckladerehabiliteringsmetoden. Spnningspknningen i det ursprungliga stlrret, underbelastning av dimensionerande tryck och temperatur, blev negativ. En viktig orsak till

detta var de termiska spnningar som uppkom p grund av skillnaden ivrmeutvidgningskoefficient mellan stlrret och kompositmaterialet. Det blev irapporten visat hur dessa termiska spnningar beror av val av materialtjocklek,hrdningstemperatur samt styvhet och vrmeutvidgningskoefficient hoskompositmaterialet.

Nyckelord: rr, fiberkomposit, FRP, CFRP, hgtemperatur applikationer


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III


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CHALMERS, Civil and Environmental Engineering, Masters Thesis 2008:IV

Contents

ABSTRACT I

CONTENTS IV

PREFACE VIII

NOTATIONS IX

1 PROJECT INTRODUCTION 1

1.1 Background 1

1.2 Aim 1

1.3 Limitations 2

1.4 Method 2

1.5 Outline of the thesis 2

2 BASICS OF COMPOSITE MATERIALS 3

2.1 Introduction to composite materials 3

2.2 Fibres 32.2.1 Carbon fibres 32.2.2 Glass fibres 42.2.3 Aramid fibers 5

2.3 Polymer resins 52.3.1 Temperature dependence of polymer resins 72.3.2 Curing of polymer resins 7

2.4 Fibre architecture 82.4.1 FRP Laminates 82.4.2 FRP fabrics 8

2.5 Prepegs 10

3 FILAMENT WINDING 11

3.1 Filament wound structures. 11

3.2 High pressure vessels 12

3.3 Prefabricated pipes 14

3.4 Analysis methods 143.4.1 Netting Analysis 153.4.2 Other computational techniques 15

4 EXISTING FRP BASED STRENGTHENING TECHNIQUES 17

4.1 General information 17

4.2 Diamond Wrap 17

4.3 Clock Spring 19


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CHALMERS Civil and Environmental Engineering, Masters Thesis 2008:36 V

4.4 Armor Plate 20

4.5 FACS Group 21

4.6 SNAMs method 23

4.7 Behaviour of repaired structure 24

4.8 Summary 25

5 DESIGN OF COMPOSITE REPAIR 28

5.1 Design procedure 28

5.2 Description of the pipe and loads 29

5.3 Preliminary choice of configuration 305.3.1 Zero stress temperature 325.3.2 Application of the composite repair 345.3.3 Risk of galvanic corrosion 37

5.4 Analytical model 37

5.4.1 Basic model 375.4.2 Obtaining the hoop stress in corroded part 39

5.5 Estimation of behaviour of repaired structure 405.5.1 Stresses in the steel and in the FRP layer for pressure only 405.5.2 Effect of the temperature variation 445.5.3 The compressive E-modulus of FRP 475.5.4 The effect of pre-stressing 48

5.6 Discussions and conclusions 485.6.1 Required composite stiffness 485.6.2 Machine winding against hand lay up 485.6.3 Adhesive deformations 49

5.7 Choice of configuration for further analysis 49

6 FINITE ELEMENT ANALYSIS 54

6.1 Modelling approach 54

6.2 Description of the model 546.2.1 Geometry 546.2.2 Materials 556.2.3 Boundary conditions and loads 566.2.4 Mesh 57

6.3 Performed analysis 59

6.3.1 Positive temperature effect 606.3.2 Negative temperature effect 626.3.3 Combine the pressure and temperature 646.3.4 New set of material properties 666.3.5 Reduction of adhesive stiffness 68

7 DISCUSSIONS 707.1.1 Design temperature 70


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CHALMERS, Civil and Environmental Engineering, Masters Thesis 2008:VI

7.1.2 Shear stress 707.1.3 Use of the high stiffness fibres 717.1.4 Material properties 71

7.2 Recommendations for future research 72

8 SUMMARY AND CONCLUSIONS 73

9 REFERENCES 74

10 APPENDICES 76

10.1 APPENDIX A: Derivation of analytical model 77

10.2 APPENDIX B: Mathcad stress calculations 83


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CHALMERS Civil and Environmental Engineering, Masters Thesis 2008:36 VII


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CHALMERS, Civil and Environmental Engineering, Masters Thesis 2008:VIII

Preface

This master thesis was carried out at Epsilon HighTech, Sweden, and at the Divisionof Structural Engineering at Chalmers University of Technology, Sweden, fromSeptember 2007 to January 2008.

We would like to start with big thanks to our supervisors at Epsilon HighTech, PerLfqvist and Arshad Abosh, for their help and support during the whole project. Wewould also like to thank our examiner Dr. Mohammad Al-Emrani for his valuableadvice and critical guidance.

We would like to give specially thank our families for their appreciation, support andpatience.

Finally, we would like to express our gratitude towards our opponents RichardBerggren and John Lennby for their proofreading and invaluable comments on thisthesis.


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CHALMERS Civil and Environmental Engineering, Masters Thesis 2008:36 IX

Notations

Abbreviations

CFRP Carbon fibre reinforced polymerFE Finite element

FRP Fibre reinforced polymerPhr Parts hardener per 100 parts resin

Roman upper case letters

D Nominal pipe diameterE Youngs modulus

adhesiveE Youngs modulus for adhesive

czE Youngs modulus in the axial direction for the composite layer

cE Youngs modulus in the hoop direction for the composite layer

fibreE Youngs modulus for fibres in the fibre direction

lE Youngs modulus for the steel liner

tN Maximum prestressing force

zN Axial force per meter

N Hoop force per meter

P Internal pressure

maxP Maximum allowable pressure

P Change in internal pressure

prestressP Counter pressure induced by prestressing

T

P Induced pressure caused by thermal loading

( )xRc Radius of the xth composite layer

yieldsteelS . Yield strength of the liner

11S Stress component in radial direction

22S Stress component in hoop direction

T Temperature

gT Glass transition temperature

T Temperature change

Roman lower case lettersb Width of composite band

ch Thickness of adhesive layer

k Nominal thickness of the pipe divided by the reduced thickness

( )xl0 Length of the composite wrapn Number of composite layers

ct Wall thickness for the composite layer


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CHALMERS, Civil and Environmental Engineering, Masters Thesis 2008:X

lt Wall thickness of the metal liner, nominal wall thickness of the pipe

mint Minimum wall thickness of the pipe

redt Reduced wall thickness of the corroded part of the pipe

Greek lower case letters

Coefficient of thermal expansion

cz Coefficient of thermal expansion for composite in the axial direction

cCoefficient of thermal expansion for composite in the hoop direction

fibre Coefficient of thermal expansion for fibres

l Coefficient of thermal expansion for steel

cz Strain in the composite in the axial direction

c Strain in the composite in the hoop direction

lz Strain in the liner in the axial direction

l Strain in the liner in the hoop direction Reduction factor

cz Poissons ratio in the axial direction for the composite layer

c Poissons ration in the hoop direction for the composite layer

l Poissons ratio in the elastic regime for the steel liner

Stress

comph Hoop stress in the composite induced by internal pressure and temperature

loading

cz Axial stress in the liner

c Hoop stress in the composite

hl Hoops stress in the liner induced by internal pressure and temperature

loading

ifiedhl mod Hoop stress in the corroded part of the liner

otemph ln Hoop stress in liner induced by internal pressure

AFRPhoop .. Hoop stresses in the composite induced by internal pressure

BFRPhoop .. Hoop stresses in the composite induced by internal pressure and thermal

loading

CFRPhoop .. Hoop stresses in the composite induced by internal pressure, thermal

loading ad prestressingAlinerhoop ... Hoop stresses in the liner induced by internal pressure

Blinerhoop ... Hoop stresses in the liner induced by internal pressure and thermal loading

Clinerhoop ... Hoop stresses in the liner induced by internal pressure, thermal loading

and prestresing

Alinerhoop %... Hoop stresses in the liner induced by internal pressure divided by

AnoFRPliner ..


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CHALMERS Civil and Environmental Engineering, Masters Thesis 2008:36 XI

Blinerhoop %... Hoop stresses in the liner induced by internal pressure and thermal loading

divided by AnoFRPliner ..

AnoFRPliner .. Hoop stress in the liner before application of composite

lz Axial stress in the liner

l Hoop stress in the liner

max Maximum allowable stress in steel

modmax Modified maximum allowable stress

r Radial stress in the adhesive

new Hoop stress in the corroded part of the linerl

T Hoop stress induced in the liner due to temperature change


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CHALMERS, Civil and Environmental Engineering, Masters Thesis 2008:36 1

1 Project Introduction

1.1 Background

Nuclear plant cooling system is a system of steel pipes transferring overheated water

and steam. There are Class 1, 2, 3 pipes and non nuclear systems according to the rateof responsibility according to nuclear code. Since the pipes transfer overheated waterand steam they are continuously operated under temperatures over 100C.Temperature drop to the ambient is expected only in case of shut down of the wholesystem or its parts.

Like most structural elements parts of the cooling system can be damaged. Forexample, cracks can appear in the steel or in welds. Corrosion can also take place,especially in the secondary parts of the system (Class 3 or lower) which are lessprotected. Parts suffering from these types of damage require repair or replacementsooner or later. The most common repair techniques are filling a defect by depositingweld filler metal in the cavities and placing a sealed pressure containment device

sleeve or clamp over the defect. These operations are time consuming and labourextensive. They also require highly qualified welders to be involved. Alternativemethods can save time and money.

FRP composites have been successfully used for repair in the other fields. In the lastfive years, there have been several reports on the use of fibre reinforced compositewraps for repair of pipe work mostly in offshore oil and gas industry. The most part ofrepaired pipes operates under ambient temperatures. No examples on the use ofcomposites at power plant piping systems could be found.

The cooling system is operated under elevated temperatures and subjected to anumber of load combinations. No plastic deformations are allowed under the primary

loads such as pressure. This puts specific requirement on the composite repair. Therepair should provide a reduction of the stress in the steel under elastic stage. Thematerials used in the repair have to withstand high temperature. Since the steel andcomposites have different thermal expansion coefficients temperature induced stresseswill appear. These stresses need to be taken into account.

1.2 Aim

The aim of this study is to investigate the possibility to use FRP reinforcement torepair defected pipes for a power plant cooling system. The defects can be corrosion;

crack or mechanical damage. A FRP-based repair method, with the purpose ofreducing stresses in the original steel pipe as much as possible, should be developed,analyzed and presented. This method should include information regarding suitablestrengthening materials and the way to apply the composite strengthening system tothe pipe.


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1.3 Limitations

The goal of the FRP repair method is to reduce the stresses in the steel part of therepaired pipe. This stress reduction in the corroded region is an indirect way to dealwith problems caused by various stress concentrators such as cracks and corrosionrelated damages. The influence of such stress concentrators is not investigated for this

reason. The influence of fatigue and time effects such as creep and materialdeterioration are not investigated. Full interaction between materials is assumed i.e.no cracks or boundary slips are modelled.

1.4 Method

In order to get the necessary background information a literature study wereperformed. This literature study has mainly focused on FRP materials and pipereinforcement techniques. A simple analytical model of a wrapped steel pipe ispresented. Knowledge gained in the literature study, together with results from the

analytical model, were then used to draft a reinforcement configuration that deliver asubstantial stress reduction in the steel part of the pipe. A FE-analysis of the chosenconfiguration was performed in order to get information about aspects that are notcovered by the simplified analytical model. Results from the FE-analysis were thenused to highlight the weak points in the configuration and to pick up the most efficientrepair technique.

FEM-calculations are performed in the finite element program ABAQUS version 6.5-1.

1.5 Outline of the thesis

The first part of the project, chapter 2-4, treats the literature study regarding FRPcomposite materials and previous use of FRP for pipe strengthening.

In chapter 5 a simple analytical model is presented together with a description ofloads and pipe geometry. Results from the analytical model is displayed andcommented. Based on these results the choice of reinforcement material andconfiguration is discussed.

In chapter 6 the FE-modelling is described and the result are displayed andcommented.

In chapter 7 the final conclusions are presented along with topics for further research.


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2 Basics of composite materials

The following chapter gives a short introduction and background to compositematerials. As this field of knowledge is quite wide focus is on general informationonly. Listed material properties are for the most common types of FRP materials only.

2.1 Introduction to composite materials

Composite materials or composites are engineered materials made from two or moreconstituent materials. These constituents often have significantly different physical orchemical properties and remain separate and distinct on a macroscopic level withinthe finished structure.

Fibre reinforcement polymer (FRP) is a composite material made out of twoconstituents: the fibre phase and the binding polymer matrix phase. FRP are

anisotropic materials and their properties highly depend on fibre arrangement. Thefibres act as the reinforcing part of the composite. The polymeric matrix both protectsthe fibres from damage and transfer force between fibres by interfacial shear. Thefibre phase is often several times stronger and stiffer than the matrix phase but muchless tough or ductile. To get a well functioning composite the right combination ofcarbon fibre and matrix must be found.

The properties of unidirectional fibre reinforced composites are strongly anisotropic.Properties measured parallel to the reinforcing fibres are quite different from the onesmeasured at a right angle.

2.2 Fibres

The most common types of fibres used in FRP are fibreglass, aramid and carbonfibres. Carbon and aramid fibres are the most expensive as they posses higher strengthand stiffness than glass fibres. Glass fibres on the other hand, due to being relativeinexpensive, is used on a much wider scale [1].

2.2.1 Carbon fibres

Carbon fibre is a solid semicrystalline organic material consisting on the atomic levelof planar two-dimensional arrays of carbon atoms [2]. A good alignment of the basalplanes parallel to the fibre axis is required to give the fibres their outstandinglongitudinal properties. The planar alignment also makes the fibres themselvesanisotropic. Carbon fibres have a diameter of 5-10m [2].

Carbon fibres are very durable and perform very well in hot and moist environments;the fibres do not absorb moisture and have showed good performance well above2000 C [3]. Fatigue behaviour is excellent [2] and as can be seen in table 2.1 the


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material has a very low or negative factor of thermal expansion. On top of being verystiff and strong carbon fibres are also very brittle. Carbon fibre based composites isgenerally modelled as linearly elastic until failure [4]. As carbon fibres conductselectricity very well the fibres must be isolated from all metallic components toprevent galvanic corrosion. In the case of strengthening metallic structures with CFRPthe adhesive layer often functions as an adequate insulating layer.

Table 2.1 Approximate properties for common grades of carbon fibres [1] and

[2]

Type of fibre Density

[g/cm3]

Youngsmodulus

[GPa]

Tensilestrength

[MPa]

Maxelongation

[%]

Thermalexpansioncoefficient

[10-6 /C]

Standard

High strength

High modulus

Ultrahighmodulus

1.7

1.8

1.9

2.1

250

500

500

800

3700

4800

3000

2400

1.2

1.4

0.5

0.2

-0.5 to -1.6

2.2.2 Glass fibres

Most glass fibres are based on silica (SiO2) Additions of oxides of calcium, boron,sodium, iron and aluminium [3] are added for their contribution to glass propertiesand manufacturing capability.

For structural composites three commonly used types of glass fibres are E-glass, C-glass and S-glass. The E-glass was originally designated for electrical use, howevertoday is the most wide used glass fibre for multi-use civil infrastructure applications.S-glass was designated for high strength and C-glass is for very high corrosionresistance [A4 or www.eFunda.com].The diameter of an individual glass fibre variesbetween 3 to 24m [2]. Glass fibres are sensitive to moisture in alkaline environments

[3]. Glass fibres are also susceptible to creep rupture and lose strength under sustainedstresses [2]. The endurance limit of glass fibres is generally lower than 60% of theultimate strength [2]. Bulk silica glasses have softening temperatures around 850C,but the strength and modulus of E-glass decreases rapidly above 250 C [3].


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Table 2.2 Approximate properties for E-, C- and S-glass fibres [1] and [2]

Type offibre

Density

[g/cm3]

Youngsmodulus

[GPa]

Tensilestrength

[MPa]

Maxelongation

[%]


[10-6 /C]

E-glass

C-glass

S-glass

2.57

2.46

2.47

73

73

74

3400

2350

4600

2.5

2.5

3.0

5

-

2.9

2.2.3 Aramid fibers

Aramid fibres are synthetic organic fibres consisting of aromatic polyamides. The twothe most common types of these fibres are Kevlar 29 and Kevlar 49. Aramidfibres can have difficulty achieving a chemical or mechanical bond with the resin.Aramid fibres are not as brittle as glass or carbon fibre and can be readily woven onconventional fabric looms.

The disadvantages of aramid fibres are high moisture absorption (up to 6% by weight)and low melting temperatures (around 425 C) [2].

Table 2.3 Approximate properties for Kevlar fibres [1] and [4]

Type of fibre Density

[g/cm3]

Youngsmodulus

[GPa]

Tensilestrength

[MPa]

Maxelongation

[%]


[10-6 /C]

Kevlar 29

Kevlar 49

1.44

1.44

83

124

2760

3620

3

5

-2

-2

2.3 Polymer resins

Polymer resins are commonly used to form both matrix materials as well as adhesives.This is an important property as it is necessary for the matrix to be compatible withthe adhesive. In some engineering applications the FRP fibres are applied clean of anymatrix. The fibres are then impregnated with a polymer resin that bonds the fibres tothe adhered surface. Other main advantages are low cost, easy processibility, good


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chemical resistance and low cure shrinkage. They do however degrade by prolongedexposure for ultraviolet light and some solvents [1].

Polymer resins can be divided into two broad subgroups; thermoplastic resins andthermosetting resins. Thermoplastic resins can soften or melt when heated. Meltingand solidification of these polymers are reversible and they can be reshaped by

applying heat and pressure. Thermosetting resins on the other hand do not soften butdecompose if subjected to high temperatures. These types of resins cure even ifsubjected to high temperatures. From Table 2.3 it can be seen that thermosettingresins have a much lower strain to failure as well as generally higher stiffness thanthermoplastic resins.

Table 2.3 Selected properties for different types of resins [3]

Resin Youngsmodulus

[GPa]

Poissonsratio

Tensilestrength

[MPa]

Failurestrain [%]

Coefficientof thermal

expansion( )

[10-6K-1]

Thermosets

Epoxy

Polyester

3-6

2.0-4.5

0.38-0.4

0.37-0.39

0.035-0.l1

0.04-0.09

1-6

2

60

100-200

Thermoplastics

Nylon 6.6

Polypropylene

PEEK

1.4-2.8

1.0-1.4

3.6

0.3

0.3

0.3

0.06-0.07

0.02-0.04

0.17

40-80

300

50

90

110

47

When mixing a matrix material or an adhesive there are often other constituentsinvolved except of the main polymer resin. The most common form is a two-component

Hardeners (including curing agents, activators or catalysts) are added in quantitiesranging from 1 part hardener per 100 parts resin (1 phr) to greater than 100 phr [5].The most common types are either of the amine or acid anhydride type. Somehardeners promote curing by catalytic action and other participates directly into thereaction and become chemically bound into the resin. The choice of hardener greatlyinfluences the material properties of the final product such as glass transitiontemperature Tg, viscosity, chemical resistance and pot life (amount of time the mix


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stays workable). It also influence time and thermal conditions required for propercuring.

Diluents are reactive or non-reactive materials added sometimes to lower the viscosityof the resin system.

2.3.1 Temperature dependence of polymer resins

As glassy materials the thermal response of polymers plays a large role in theirproperties and behaviour. Close to the glass transition temperature (Tg) the polymerresin experiences drastic changes in physical (heat capacity, density) and mechanical(stiffness, damping) properties. FRP strengthening system dependent on polymer resinfor transfer of forces between fibres and the adhered surface are often restricted totemperatures well below Tg. In [6] it is recommended that Tg is at least 20C higherthan the design temperature. Values for Tg and/or melting temperature is given inTable 2.4 for the most commonly used polymer resins.

In order to use adhesive bonded FRP strengthening in high temperature environmentsthe use of high temperature resins is necessary. High temperature resins are a class ofthermally stable thermosets with outstanding high temperature mechanical properties[5]. Today there are high temperature resins with glass temperature (Tg) over 300C[5] [7] [4]. Some epoxy polymers exhibit thermal stability at temperatures up to 250C[5]. If better low-temperature properties or lower stiffness is desirable then epoxiescan be mixed with plasticizers or flexibilizers. These additions do however have anegative impact on other properties such as high-temperature stability and [5].

Table 2.4 Comparison of properties between thermosets and thermoplastics [1]and [3]

Thermosets Thermoplastics

Epoxyresins

Polyester resins

Nylon 6.6 Polypropylene

PEEK

Melting temperature [ C]

Tg [ C]

Shrinkage on curing [%]

-

150-200

1-2

-

140-200

4-8

265

50

-

164

100

-

334

143

-

2.3.2 Curing of polymer resins

The time it takes for a resin-hardener system to cure is strongly affected by the heatapplied during the curing process. Higher curing temperature generally reduces thecuring time; the exact correlation is dependent upon the choice of constituents in the


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resin-hardener system. But for thermosetting resins the final material properties arealso highly dependent upon thermal conditions during curing.

Most high temperature resins require high curing temperature during application inorder to obtain their properties. As a rule, the higher the glass transition temperatureof the resin the higher curing temperature it requires. The same is true about high

stiffness thermosetting resins or other resins with especially high material properties.

Elevated temperature curing can be applied either as a pure high temperature cure orin the form of temperature cycling.

2.4 Fibre architecture

Many FRP properties are strongly dependent on the arrangement and distribution offibres. Choice of fibre architecture depends on what type of structure that is to bestrengthened and against what type of load. The reinforcement materials used in the

pipeline industry are normally in the form of a fabric [4].

2.4.1 FRP Laminates

Laminates are thin plates made up of several, even thinner, layers called laminaes thatare stacked on top of each other. The number of laminaes used to construct a laminatevaries, see Figure 2.1. What marks the laminaes is that they have a homogenus fiberarrangement, may it be in one or more directions. A unidirectional laminae is oftencalled a ply.

Figure 2.1 Simple laminate made by unidirectional plies [8]

2.4.2 FRP fabrics

Although the maximum mechanical properties are achieved by using fibres in theirunidirectional orientation, this architecture is not always practical to use in the field.


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FRP fabrics can be divided into three broad categories, woven, unidirectional andstitched.

Woven carbon fibres are made of fibres oriented along two perpendicular directions:one is called warp and the other one fill, see Figure 2.2. Some woven fabric, calledbalanced fabrics, contains an equal number of fibres in the warp direction as in the fill

direction.

Figure 2.2 Notation for a woven fabric [8]

In all woven fabrics all the fibres pass over and under either one or multiple fibres. Asevery point where the fibres go under and over another is a weak point a woven fabrichas a lower tensile strength and stiffness than would be expected from the number offibres in the loading direction. To counter the problem with crossing fibres designerstry have suggested various patterns, see Figure 2.3. However, the fibre crossing alsorestricts local buckling to a degree. This gives a carbon fibre weave a highercompressive strength and stiffness than a comparable unidirectional ply.

Figure 2.3 Left: plain weave Middle: satin weave Right: twill weave [8]


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Unidirectional fabrics are fabrics where all of the reinforcing fibres are in the warpdirection. An example of an undirected weave can be seen in Figure 2.4. Such fibrearchitecture allows the reinforcing fibres to be flat in the warp direction. The fillfibres, often nylon or glass [4], are only present to hold the weave together andprovides no mechanical properties in that direction.

2.5 Prepegs

Prepegs are pre-engineered, ready-to-mould combinations of resin and reinforcementfibres. Available forms, among others, include resin impregnated woven fabrics, fibremats and laminates. Prepegs are manufactured by impregnating continuous webs offabric or fibre with resin and then advancing the resin impregnated material to thepartly polymerized B-stage [5]. After the prepeg have been installed the curing of theresin is completed and the final, permanent, product takes form. As prepeg materialscontain resins that have been partially cured they can seldom be stored more than 90days without loosing drape properties [5].


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3 Filament winding

This chapter deals with filament wound structures, their applications, modeling ofbehavior and technology of production.

3.1 Filament wound structures.

A winding operation is the basic fabrication technique for forming load-bearingstructural elements made of polymer matrix-based fibrous composites, which have theshapes of bodies of revolution.

Filament winding technique is used in production of such structures as different tanks,reservoirs, wind turbines, risers, and other types of structures. The Figures 3.1 and 3.2illustrate some filament wound structures. Filament winding is most commonly usedin strengthening against pressure. However axial and flexural load bearing capacity

can also be achieved in the structure by varying the winding angle (see Figures 3.1and 3.4).

Figure 3.1 Riser tubes [9]


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.

Figure 3.2 Filament winding for wind turbine [9]

Among the number of existing prefabricated filament wound structures the most

common ones are high-pressure vessels and filament wound composite pipes. Thesetypes of filament wound structures are widely used and well investigated.

3.2 High pressure vessels

High pressure vessels were the first field of application of filament winding technique.A metal-lined composite pressure vessel was originally designed and developed bythe National Aeronautics and Space Administration (NASA) in 1970 for spaceapplications. [9]. During the early stages fiberglass were used as reinforcing material.

Since then, two different liner approaches have been developed. In the first, filamentsare used to reinforce a high strength metal shell with a thickness about one-quarter toone-half the thickness of an unreinforced homogenous metal vessel. This concept withthe load-bearing metal liner is referred to as a filament-reinforced metal pressurevessel. In the second design approach, filament windings are used to reinforce a verythin metal liner having the minimum possible thickness required for impermeabilityand fabrication. This concept, with the liner carrying only a small share of the load, isreferred to as a metal-lined filament-wound pressure vessel. [9]

Filament-wound products are produced by using one of the three basic types ofwinding patterns: polar, helical, and hoop. [9] According to [5] in polar or planar typeof winding, the mandrel remains stationary while the fiber-feed arm rotates about it in

a longitudinal axis. In helical and hoop winding methods mandrel rotates continuouslywhile the feed carriage shuttles back and forth at a speed regulated to generate thedesired helical angle.

Hoop winding is a special form of helical winding and is used to deposit fiber close to90 to the longitudinal axis. Hoop windings are generally applied only to thecylindrical or straight portion of a mandrel and results in a single layer ofreinforcement material. Figures 3.3 to 3.5 illustrate different winding patterns.


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Figure 3.3 Polar winding pattern [9]

Figure 3.4 Helical-winding pattern [9]

Figure 3.5 Hoop-winding pattern [9]


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3.3 Prefabricated pipes

Prefabricated composite pipes are produced using the same winding techniques asused for pressure vessels. We can distinguish two types of filament wound pipes:filament wound composite pipes with or without metal liner. Filament woundcomposite pipes without metal liner are usually treated in literature as composite

pipes. Filament wound composite pipes with metal liner are some times calledhybrid composite pipes.

Prefabricated hybrid pipes are not a wide used product and few papers such as [10]are available in this field. Pure composite pipes achieved much wider field ofapplication due to their high corrosion resistance in comparison with steel and otherpositive properties. The behavior of these structures is subjected to broad research.

Reinforced pipe consist of wrapped composite (composite shell) and old pipe (metalliner). As soon as behavior of both parts their self is known the problem of modelingof behavior of entire structure is limited by finding the way to model interaction. Itshould be noticed that researchers use similar computational techniques for hybrid and

pure composite pipes.

Figure 3.6 Hybrid pipe specimen after burst [10]

3.4 Analysis methods

There are several types of computational techniques for filament wound structures. Ifcomputational methods are going to be used in design a number of requirements needto be met. The first requirement is that the method provides a good approximation ofbehaviour of the structure. The second requirement is that computational time shouldbe minimized. These two requirements are in contradiction. Precise modelling ofbehaviour requires a lot of computational time and vice versa.


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This study deals with preliminary design and quite limited in time, it makes thesecond criteria to be governing. This makes the use of simplified and traditionalcomputational methods more attractive. Methods which are not well established yet orrequire significant amount of computational time should be used on later stages ofresearch.

3.4.1 Netting Analysis

The basic method used in design and analysis of high pressure vessels with thincomposite layer is Netting Analysis. This method described in [5] came into wide useduring early filament winding development and still used for high pressure vesselsdesign. For example: a computer program using the netting analysis method wasdeveloped by Aerojet Corporation to perform calculations for the design andstructural analysis of pressure vessels fabricated from filament-reinforced metalshells. Netting analysis is quite limited in its applicability but it gives opportunity todetermine hoop and meridian stresses in quite good approximation. Netting analysis is

also conservative approach due to its assumptions. These properties in combinationwith its simplicity induce designers to continue use of this method.

The principles of Netting Analysis are the following. It assumes that only reinforcingfibers have load carrying capability and that all fibers are uniformly stressed intension. The only function of the resin matrix is to hold the fibers in position. Theanalysis applies primary to a determination of membrane stress in shells in whichoriented filaments are the load bearing members. It can not be used to determinebending, shear or discontinuity stresses or resistance to buckling. The elastic constantsand Poissons ratio have no physical meaning in netting analysis theory, nor can theybe determined by it. [5]

Disadvantage of Netting Analysis is that it can be used only to determine membranestress. At the same time it does not rely on adhesive or resin matrix mechanicalproperties, i.e. only properties of fibers considered. At the special conditions of powerplant cooling systems where the temperature of pipe can exceed 200-300C behaviorof matrix resin is uncertain. The temperature affects polymers mechanical properties.Especially if it approaches Tg (glass transition temperature of the polymer) while theproperties of fibers are generally less temperature dependable.

3.4.2 Other computational techniques

There are different analysis techniques for shell and membrane composite structuresdescribed in polymers handbooks.

Vasiliev V. V. in [11] p736-757 offers general equations for composite structure andoffers solution for the case of cylindrical shells subjected to axisymmetricdeformation. He also offers to use semimembrane theory for the case ofnonsymmetric deformation. One of the assumptions for general equations is followingthe radii of curvatures are assumed to be the same for each lamina and the laminate


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thickness is much smaller than the other dimensions. These two assumptions makemethod inappropriate for pipes with thick laminate layer.

Paper [12] deals with analysis of the behavior of multi-laired composite pipe underinternal pressure. Both these papers work with alternative theory for thick laminatelayers. At paper [13] Xia M. and other authors analyze behavior of filament-wound

fiber-reinforced sandwich pipes under internal pressure combined withthermomechanical loading. At [14] they deal with bending behavior of the samestructure.

Experimental study of bending behavior of large diameter pipes (caissons) has beencarried out at [15]. Commercial software has been used for prediction of behavior oftubes. No information about theoretical assumptions for modeling included in report.

The most part of presented above techniques are under development and requiredquite advanced calculations. Most of them can not be applied directly to the case ofreinforced pipe since problem of interaction between steel and composite componentsshould be solved.

The methods above and their applicability for prediction of behavior of reinforcedpipe instead of prefabricated pipe is a subject of future research. They can be used onlater stages of design.


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4 Existing FRP based strengthening techniques

This Chapter presents a review of existing commercial strengthening and repairtechniques. Presented methods are developed by different companies. Only generalinformation available from public sources is presented. The purpose of this chapter is

to give a review only. Sometimes the same or similar names are used for the differentproducts of different companies; names of the companies and their structure are alsothe subject to change. Only those methods most commonly mentioned in the literatureare listed in this chapter. Translations from Fahrenheit to Centigrade and inches tomm are expressed in round numbers for comfort of use.

The producers don not always provide detailed information about behaviour of theirsystems. Scientific reports often do not reveal names of the commercial products thatare investigated. Due to these reasons the information about behaviour of reinforcedpipes is presented in separate subchapter.

4.1 General information

Application of fiber composites for strengthening and repair of pipes is relatively newtopic. There are possible two main alternatives. The first alternative is to adopt thefilament winding technique to use it on the field conditions i.e. to develop mobilewinding equipment. The second option is to use hand layup of prefabricated wovenfabric for reinforcement of existing pipe. The most companies preferred to go for thesecond method, but there is also an example of adopting the first technique.

4.2 Diamond WrapDiamondWrap is the main product offered by Citadel Technologies. TheDiamondWrap system includes a bidirectional weave of carbon fibre and a 100%solids epoxy, forming a composite system. The structural system forms a pipe arounda pipe; see Figure 4.1. Company claims that the bidirectional weave allows forstrength in both the hoop and axial directions. Because of its low profile,DiamondWrap can be installed on tees, elbows, straight runs of pipe, confinedspaces and irregular surfaces that require structural reinforcement or leakcontainment. [7].


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Figure 4.1 DiamondWrap system [7]

The company recently issued a new line of products, high temperature kits:DiamondWrap High Temperature Ambient, DiamondWrap High Temperature701* and DiamondWrap Ultra-High Temperature*

Information about products is from Citadel Technologies web-site [7]. Temperaturevalues were given in Fahrenheit. The company claims that values are not exact andcan be modified due to different exploitation conditions.

DiamondWrap High Temperature Ambient is designed to be used onpipes that operate at temperatures up to 265 F (130C). The High

Temperature Ambient kit is formulated to cure at ambient temperatures withno additional heat required. It is great for pipes that operate at elevatedtemperatures, but will be wrapped while the line is out of service.

DiamondWrap High Temperature 701kit has been designed to be appliedon pipes that operate at temperatures up to 350 F (175C). These kits havebeen formulated to cure with additional heat, which allows them to be appliedto pipes at elevated temperatures without the threat of reduced workability. Toproperly cure, these kits must be heated to at least 150 Fahrenheit (65C) afterinstallation unless the pipe is operating above this temperature when theproduct is applied.

DiamondWrap Ultra-High Temperature* system is pre-impregnated withCitadels Ultra-High Temperature epoxy. This epoxy has been formulated tobe applied to pipes operating at temperatures up to 690 F (365C). Curing ofthis epoxy requires a minimum temperature of 300 Fahrenheit (150C) eitherfrom the piping system itself, or with the addition of external heat.

Companys web-site [7] also contains some technical information about materialproperties and operating pressures. No information about structure of reinforcementsystem has been found.


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4.3 Clock Spring

Clock Spring is a permanent repair alternative for corrosion, mechanical damage orother defects on high-pressure pipelines [16]. According to the companys web-site[16] Clock Spring system is designed to repair/reinforce damaged or corrodedsections of pipe with up to 80% loss of wall thickness. The Clock Spring System

consists of a patented, unidirectional composite structure; a patented, two-partadhesive system and high compressive strength load transferring component.

The composite is E-glass and polyester resin. All the glass fibres are continuousstrands aligned in the hoop direction providing high tensile strength. The system isavailable in different performance. The main size is a system consisting of 8 wraplayers with a width of 292mm, see figure 4.3. The thickness and the number of layerscan vary. Clock Spring applied manually by wrapping the fabric over the pipe (seeFigure 4.2). Regular service temperature is between -29C and 54C. Clock SpringHT based on vinyl ester resins instead of polyester resin and has service temperaturebetween -29C and 120C (82C, variation in text in [16]).

Figure 4.2 Wrapping the Clock Spring over a pipe [16]


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Figure 4.3 Clock Spring System [16]

1. composite structure, 2- adhesive system, 3- load transferring

component

4.4 Armor Plate

Armor Plate Pipe Wrap (APPW) is the first and main product developed byArmor Plate, Inc. APPW was originally developed for the structural repair of high-pressure pipelines. The system is composed of a patented fibreglass cloth combinedwith one of the several types of resins. The material is usually applied by sections12.5 inches (320mm) wide, sections 24 (610mm) inches wide are available, seeFigures 4.4 and 4.5.

Figure 4.4 APPW straight pipe repair [17]


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Figure 4.5 APPW straight saddle repair [17]

No information about systems behaviour under the high temperature conditions isavailable on the official web-site.

4.5 FACS Group

FACS Group (Field-Applied Composite Systems Group) offers two mainproduct groups - Aquawrap and PowerSleeve. [18]

Aquawrap is a low cost composite system for use in repair andreinforcement of existing mechanical systems, structures and piping. Thecompany offers both uni-axial and bi-axial woven glass fabric for this system.The adhesive cures with the application of water. Resin system is patented, nodata available. Among general characteristics mentioned Tg 288 F (about 142C).


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Figure 4.6 Aquawrap [18]

PowerSleeve is the high strength system offered by the company. It uses

special hybrid fabric made of combination of glass and aramid (Kevlar)fibres. The system has been successfully used for repair of acid vessel andcooling tower. Tg for standard matrix system is 185 F (85 C) but there arealternative systems for high temperature applications. X-TEMP-2 is one ofthem providing maximum temperature resistance. A two-component, heatresistant matrix it requires treating at 450 F(about 232 C) during applicationand has maximum service temperature of 525 F (about 270 C) according to[18]

Figure 4.7 Use of PowerSleeve on cooling tower [18]


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4.6 SNAMs method

Strengthening method developed by SNAM partially adopted filament wound methodfor in-field application. The method involves wrapping a thermoplastic polymercomposite tape around the pipe. The use of a winding machine allows the creation ofsome precompression on the pipe wall over the length concerned. As a result, induced

hoop stress in the pipe wall is reduced and crack growth prevented. The technique canbe applied to both straight pipes and pipes with bends. [19]

Materials used are aramid fibre tape and adhesive tape made of poly-isobutyl rubber(glue) and low-density polyethylene (support). The adhesive and the composite tapeare assembled, loaded into special storage bobbin, and then mounted on the windingmachine. The outer side of the adhesive layer is protected by a release paper, whichhas to be removed before the tape is laid on the pipe.

In order to be able to apply filament winding technique SNAM has developed arelatively simple orbiting machine, which can easily be transported by a light truckand mounted on the pipe to be repaired. The machine winds the composite tape with

pretensions of up to about 500 N and delivery speeds of up to 60 m/min. Thecircumferential pretension applied on the composite tape during the winding convertsinto radial stress, which counterbalances the internal pressure of the fluid within. [19]Figure 4.8 represents scheme of repair (a) system and section of wound pipe (b), thewhite layers are the composite tape and the black layers are the adhesive tape.

Figure 4.8 Composite repair system [19]

Paper [19] developed an analytical two-dimensional model to compute the hoopstresses in repaired system. In the model it is assumed that only the composite tapebears the circumferential stress in the winding, and that adhesive layers simply


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transfer the radial pressure between the neighbouring composite tapes, see Figure 4.9.Stress in the composite tape is expressed like.

( ) ( )

( )

cE

xol

xc

Rxol

c

=

2(4.1)

Figure 4.9 The unit cell in the theoretical model [19]

Radial compressive stress in adhesive is expressed like:

( )( ) ( )

( )xRxhx

xc

ccr

= (4.2)

Where r(x) is radial compressive stress in adhesive, c(x) is the circumferentialstress in the composite tape in the xth layer, hcis the thickness of the tape,Rc(x) is the

radius ofxth layer, cE is the Youngs modulus of the composite in the fibre

direction, l0(x) is the length of the tape originally laid on the winding.

An iterative procedure is required in order to obtain values of stresses and strains.

The important feature of this particular repair system is the reduction of elastic stresswithin the pipe. Applied precompression on the pipe wall over the length concernedmakes the composite repair to bear considerable part of the load before the linerplasticizes unlike the other repair systems. No information about failure mode ofrepaired pipe was found. But behaviour of the system on early stages of loadingdiffers significantly from repairs with wrapped fabric.

4.7 Behaviour of repaired structure

Several papers have been found about general behaviour of pipes with compositerepairs. Investigated repairs are mostly used for repair of corroded or damaged pipesagainst internal pressure. Different tests have been carried out on pipes with up to80% section thickness loss repaired with the composite wrap. Investigated repairs


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presented in the literature are mostly glass fibre based due to its low price. Repairsutilize fibres strength after plasticity in the liner occurs.

At [20] authors carried out burst tests on some anonymous composite repairs based onglass fibre composite. Pipe specimens had artificial defects of 70% wall thicknessloss. Rupture test were combined with FE modelling of the structure, authors also

used strain gages in the tests. Their results indicated that up to the start of yielding ofpipe defect region, practically entire load is carried by steel liner, due to the steel highYoung modulus. After yielding, the composite material started working efficiently.Some repair systems allowed the pipes to reach the original design pressure.

Paper [21] is a report indicating test work on repair system DiamondWrap. Thestatic pressure burst tests have been carried out on specimens with 50% and 80% lossof wall thickness. Burst strength was significantly improved in comparison withunstrengthen control pipe. Tests were not combined with FE analysis and no straingauges have been used. As a result no information about behaviour up to failure isavailable.

John Wolodko at [22] presents simplified method of predicting of stress-strainresponse of hoop wound composite pipe. Both simplified analysis and FE modelingindicate that up to steel yielding point the major part of the load is carried by the linerdue to low stiffness of analyzed composite. Figure 4.10 from [22] represents the hoopstress distribution for the pipe with composite repair.

Figure 4.10 Comparison of the Hoop stress response predicted by analytical solution

and FE model [22]

4.8 Summary

The properties of the presented strengthening techniques are summarized in Table 4.1.Most producers use manual lay up. There are two products available at the marketadapted to high temperature applications. All producers offer strengthening againstpressure, some of the systems even have axial strength.


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Table 4.1 Strengthening methods.

Materials

Strength-ening

against

Y-yes

N- no

MethodMan

uallay-upofcloth(ML),or

machinewinding(W)

Fibre

composite

Matrix

resin

Adhesive

Max

servicetemperatureC

Pressure

Axial

DiamondWrapML

Carbon,fabric

epoxy epoxy 365 Y Y

Clock Spring MLE-Glass,

fabric

vinylester/polyes

ter

vinyl ester/polyester

120 Y N

APPW MLGlass,fabric

? ? ? Y ?

Aquawrap

MLGlass,fabric

? ?About120

Y YFACS PowerSleev

eML

HybridGlass-Aramidfabric

? ? 270 Y ?

SNAM WAramid

band- polyisobutyl ? Y N

Neither the information about the behaviour of the composite nor the way to model itis supplied by the manufacturers. A few reports about behaviour were found. Thesescientific papers treat the composite repairs anonymously. The composite repairs thatwere analyzed rely on plastic stage. Before plasticity occurred these composites donot work efficiently.


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There are composite repair systems with high temperature application available on themarket but the question about their acceptability can not be answered. The desiredsystem should provide a significant reduction of the stresses in the steel. In order toensure this the design procedure and analysis of the systems behaviour is required.The information necessary for design of presented systems is not available for public.The commercial composite repairs are designed by their producers only.

A special composite repair system therefore needs to be developed from scratch.


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5 Design of composite repair

5.1 Design procedure

Design criterion for the composite method

The composite repair method should be designed with the aim of replacing the steelliner as the load bearing part of the pipe. More precisely, a well designed compositestrengthening system should replace the tensile stresses in the steel liner with a smallamount of compression.

Necessary choices to be made:

The composite repair method should, when presented, contain recommendations onthe following:

Type of fibre and fibre properties

Type of fibre architecture (e.g. weave or strand)

Type of resin and resin properties Curing temperature and curing time for the resin

Number of wraps and thickness of all constituents in each layer

Method of application

When selecting appropriate material properties no consideration is taken to whether ornot these specific properties exist in the form of a commercially available material.Instead recommendations are given for desirable material properties.

Approach:

In order to simplify the design procedure a simple analytical model was created. Some

initial assumptions were combined with data from this model in order to find acomposite repair method that meets the previously formulated requirements. Thedesign procedure used in chapter 5 is shown in Figure 5.1. All choices, calibrated ordetermined by other factors, are presented in Chapter 5. 7.


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Preliminaty choice ofconfiguration

Analytical model:

Choice of promisingalternatives

Analytical model:Control and comparison of

choosen alternatives

Preliminary model

Some limitations have to be

implemented in order forthe method to be useful in

practice

Determining suitablecomposite properties

regarding stiffness andthermal expansion

coefficient

Selecting a poper type ofcomposite

Check if the selectedcomposite material meets

the requirement

Detremine a suitablecomposite thickness

Figure 5.1 Design procedure for the choice of a preliminary reinforcement model

5.2 Description of the pipe and loads

A corroded pipe of Class 3 was chosen in this study as example for application of thecomposite repair. The design temperature for the pipe is 135 C. Design internalpressure is 30 bar (3 MPa). Maximum allowable stress in the steel pipe, max, is 130MPa.

The chosen pipe has nominal diameter of 168mm and a nominal wall thickness of7.1mm. Due to heavy pitting corrosion the wall thickness has been reduced form theoriginal 7.1mm to the present 2.88mm. Geometric variables are presented in Table5.1. The pipe is made out of steeland is assumed to be very long and straight withoutany attached details that can act as stress raisers. The pipe supports are designed toallow some movement along the length of the pipe, in order to minimize the influence

of temperature induced stresses.


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Table 5.1 Geometric variables for examined pipe

Diameter

D [mm]

Nominalthicknesst1[mm]

Currentthicknesstred[mm]

Maximumstress

max[MPa]

168,3 7,1 2,88 130

Two load cases are treated. The first one represent the conditions during normaloperation, the second represent the conditions during shut down. The reason forhaving the shut down as a design load case will be explained later in Chapter 5.3. Thedesign pressures and temperatures are presented in the Table 5.2. The correct way toobtain temperature difference Tfrom the values of temperatures will be described inChapter 5.3

Table 5.2 Design temperatures and pressure

Load case Pressure [MPa] Temperature [C]

Operating

Shut down

3

0

135

20

5.3 Preliminary choice of configuration

The choice of the system configuration needs to be done. As was shown in Chapter 4composite repairs can have both axial and hoop strength and stiffness. The hoopstrength is utilized against internal pressure. The axial strength can be utilized ifreinforcement is done against bending and axial forces. Reinforcement can be appliedboth locally and globally. In the first case only the damaged area of the pipe isreinforced. In the second case the entire pipe is reinforced.

As this study do not deal with reinforcement against bending, little is gained by usinga composite with high axial stiffness. Furthermore, the thermal effects will cause highshear stresses in the axial direction on steel-to-composite interface if the compositehave a high axial stiffness. The behaviour of the composites with axial stiffness ismore complicated in comparison with the ones with hoop stiffness only. Because ofthe factors described above the axial stiffness of the composite repair is not onlyuseless for the local repairs but also unfavourable. For this reason a composite repairwith a low axial stiffness was chosen. This allows the exclusion of the axial directionform the calculations.


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Figure 5.2 Composite repair systems with and without axial stiffness

The composite repair system with stiffness only in the hoop direction can be producedby the following ways:

Machine winding. This method also allows prestressing fibres in thecomposite. (see Chapter 4.6)

Manual application of unidirectional fabric i.e. the fabric with load bearingfibres in one direction only (see Chapter 2.4.2). The fabric can be appliedeither as a plain, fibre only, fabric or as a prepeg. A prepeg is a fabric that ispre-impregnated with resin and partially cured before application, see Chapter2.5.

Design temperature is one of the most important factors. The composite repair systemshould be able to hold a design temperature of 135 C. The general recommendations

state that the exploration temperature must be at least 20 C below Tg [6]. The choiceof matrix resin and adhesive is critical for the composite repair system. The fibresthemselves usually have high temperature resistance. However, most high temperatureresins require high curing temperature during application. This temperature can vary.As a rule the higher the design temperature of the resin the higher curing temperatureit requires. For the example case studied in this thesis, the curing temperature of theresin was assumed to be 65 C, analogous with the system available on the market(see Chapter 4.2).


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5.3.1 Zero stress temperature

The rate of the temperature difference depends on adhesive properties. As mentionedin Chapter 2 and Chapter 4 most high temperature resins require curing on elevated

temperatures. Assume that the composite is applied at ambient temperature and thencured. The temperature on which adhesive is applied is treated in the text asapplication temperature. A change in temperature leads to thermally induced stressesif the adhesive have stiffness to transfer the forces between the liner and the fibres.Before the adhesive is hardened no thermal stresses can appear.

The term zero stress temperature indicates the temperature at which thermallyinduced stresses are zero. This temperature should be used in order to determinetemperature ranges for calculations, i.e. the different values of T. Based on thedefinition of zeros stress temperature above, Tcan be calculated as the differencebetween the temperature of the pipe and the zero stress temperature. As the pipe canhave a temperature of either 20 C or 135 C, T can either be a positive value,

Tpositive, or a negative value, Tnegative.

Figure 5.3 Illustration of relations between curing temperature, application

temperature and zero stress temperature.


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The value of zero stress temperature is generally unknown but the upper and lowerboundaries of this value are known. There are two so called edge scenario when thecuring process takes place. The first scenario indicates the situation when the adhesivein not hardened before curing and cant transfer stresses, the hardening only takesplace under the curing temperature. In this case the zero stress temperature will beequal to the curing temperature. The case is illustrated in Figure 5.3. The second

scenario is the case when the adhesive is hardened at the ambient temperature and thecure needed only in order to obtain the high temperature resistance. In this case theZero Stress Temperature is equal to the ambient temperature. The reality issomewhere in between these two edge scenarios. As a result zero stress temperatureis somewhere in between application temperature and cure temperature.

The case when Tis negative indicates the negative thermal effect. This effect causestension in the steel and compression in the composite shell in the hoop direction andtension in the adhesive layers in the radial direction. In case of high tensile stressesthe tensile failure of adhesive or debonding can occur. The compressive failure of thecomposite shell also can occur since the compressive strength of fibre composites ismuch lower than their tensile strength. The value of compressive stresses in the

composite shell can be determined analytically and should be taken into accountduring design.

The safe design approach is to use the curing temperature as the zero stresstemperature in the calculations regarding the steel stress, Case 1 Figure 5.3. Thisassumption will indicate the highest possible negative temperature effect (T=-45 C)and the lowest positive temperature effect (T=70 C), see Figure 5.3. In case whenthe highest possible tensile stress in the composite part is investigated the safeapproach is to use ambient temperature as the zero stress temperature, Case 3 (T=-115 C) Figure 5.3.

PositiveT Tis favourable (F) orunfavourable (U)

Case number from the

Figure 5.2

Stress in the steel F Case 1

Stress in the composite U Case 3

Negative T - -

Stress in the steel U Case 1

Stress in the composite U Case 1

The algorithm to calculate the design value ofT:


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It can be possible in case of temporary repairs that the piping system operatescontinuously and under no circumstances its temperature goes bellow the certain limitunder the service time of the composite repair. In this case this temperature limit canbe used instead of ambient temperature in calculations of negative thermal stress but aspecial investigation is required in order to ensure that the temperature will not gobellow this limit during the service time.

5.3.2 Application of the composite repair

The composite repair is aiming at reducing the hoop stress in the steel part. Thereduction of hoop stress can be efficiently used to treat different problems such ascorrosion, cracks and weld defects. Before the composite repair is applied, the surfaceof the pipe should be prepared. There is a number of standard surface preparationtechniques e.g. sand blasting. Standard surface preparation regularly includescleaning, creation of required roughness and application of primer. Cleaning andcreation of rough surface are often done simultaneously by sand blasting. When the

circular shape of the pipe is not affected the standard surface preparation is sufficient.This includes cracks, weld defects and internal corrosion (both pitting and uniform).These cases are schematically illustrated on the Figure 5.4.

The example studied in this thesis is a corroded pipe. Due to this reason corrosioncases are going to be studied in more details. The repair has limited length and isgenerally expected to deal with pitting corrosion. Uniform corrosion can be alsotreated by the repair if the corroded part is short enough.


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Figure 5.4 Application of the repair in case of crack, weld defect and internal

corrosion.

In case of large surface deviations, caused by e.g. external corrosion, the standardsurface preparation may be not enough. The external shape of the pipe sectionchanges due to external corrosion. The shape of the composite shell is determined bythe external shape of the pipe. In case of large corroded area the shape of thecomposite shell will deviate from the circular form significantly. Any shapedeviations act as stress raisers. There is also possibility of appearance of air pocketsduring application, which also will lead to local deviations in the stress field. Theinfluence of these factors is hard to analyse. The matter of influence is strongly

depends on the defects geometry which is unique in every certain case. One canguess that in most cases these local stress raisers can be neglected but in order toestablish the boundary between ignorable case and the case where local defect can notbe ignored more research is needed. Due to these facts it is better to minimize theinfluence of local shape deviations. A possible way to do this is to fill the cavitieswith a polymer load transfer agent before the composite wrap is applied, see Figure5.5. The polymer load transfer agent must be compatible with the adhesive and primerused in repair.


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Figure 5.5 Schematic view of externally corroded pipe section with composite

repair. Without or with polymer agent applied.

In case of uniform corrosion when the circular shape of the pipe is unaffected the

standard surface preparation should be sufficient. If the uniform corrosion iscombined with pitting one, polymer agent should be used, see Figure 5.6.

Figure 5.6 Uniform external corrosion


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5.3.3 Risk of galvanic corrosion

There is a risk of galvanic corrosion when non-impregnated carbon fibre based fabricsare used and come in contact with the steel pipe.

The steel pipe expands with temperature while the composite shell does not. This

phenomena cause temperature induced pressure. The change of pipe diameter due totemperature changes can be expressed as following.

TDDl

= (5.1)

Assuming the range of pipe diameters from 40mm up to 1000mm, Tabout 150 C,and l=1110

-6/C we obtain the range of diameter changes D from 0.066 mm up to1.65 mm. For example D=168.3 mm, T=65-20=45C and T=135-20=115C (65Cis curing temperature, 135 C is design temperature), and l=1110

-6/C, D=0.083mm and D=0.21 mm.

Carbon fibres have high electrical conductivity. In case when the steel structure is

reinforced with carbon fibre the adhesive layer isolates the steel from fibres andprevents galvanic corrosion. In case when the steel expands while the adhesive is nothardened yet one can suggest that there is a risk that the filament can penetrateadhesive layer. This will cause galvanic corrosion. In order for the filament to be ableto penetrate the adhesive the increase of pipe radius should be bigger than thethickness of the first adhesive layer.

For the small diameter pipe there is low risk that the carbon filament will penetrate theadhesive layer due to thermal expansion of the steel. (The risk of galvanic corrosion isminimized if only the innermost adhesive layer is thicker than 0.0415mm in thestudied example). For the bigger pipe diameters the thickness of the first adhesivelayer can be artificially increased in order to ensure that the filament will not be able

to penetrate it.

Galvanic corrosion can also be minimized by using a primer that has been hardenedbefore the application of the composite repair. But the used primer should be able towithstand high temperatures and be compatible with the adhesive used. The mostcommon way to solve this problem is to use the same polymers for adhesive andprimer. Due to this reason a full hardening of the primer can require elevatedtemperature treatment.

As discussed above, there are several ways to avoid the risk of galvanic corrosion andthe choice of the best way depends on the properties of the materials used.

5.4 Analytical model

5.4.1 Basic model

For the preliminary design a modified version of the analytical method used in [22]has been adopted. This method was developed for systems with hoop reinforcement.The critical point is the fibre arrangement in the final system. The method is suitable


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for hoop reinforcement only. The original method presented in [22] does not take intoaccount the thermal effects or prestressing.

The nuclear code does not allow any plasticity in steel under primary loads [23]. Dueto this reason only the elastic stage is investigated. Stress-strain equations used in theoriginal method were resolved by implementing new boundary conditions and with

the addition of thermal strains. The effect of prestressing was also taken into account.Detailed derivations are presented in Appendix A.

The final equations for the hoop stress when no prestressing is included are:

For the hoop stress in the steel:

( )12

cclc

TE

lt

ct

lt

PD

l

=

(5.2)

For the hoop stress in the composite:

( )22

ccll

TE

ct

lt

ct

PD

c

+=

(5.3)

Where:

lElt

cE

ct

B = (5.4)

Bc

+=

1

1

1(5.5)

B

Bc

+=

12(5.6)

The derivation for the case in which prestressing is included is presented in AppendixA.

c

E represents the E-modulus of the composite. This value is assumed to be the same

in tension and compression.D,ct , and

lt are described in Figure 5.7. Compression is

generally not expected but can take place due to thermal effects. Further discussion ispresented in Section 5.5.3.

The value ofc

represent the thermal expansion coefficient of the composite layer.

For simplicity this value is set to zero in order to represent a median value for allcomposite fibres involved.


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Figure 5.7 Nominal diameter (D), total composite thickness (ct ) and steel wall

thickness (lt )

5.4.2 Obtaining the hoop stress in corroded partThe hoop stresses in the steel pipe are expressed by the following expression.

l

lt

DP

=

2 (5.7)

Where l is the hoop stress, P is the internal pressure,D is the nominal diameter of

the pipe and tl is the wall thickness.

The code suggest that the stresses in corroded pipes can be calculated assuming a pipethickness to be equal to the reduced thickness [23].

lred

newt

DP

=

2 (5.8)

Here tred represents the reduced thickness due to corrosion. This formula wasdeveloped for pipe to be made out of one material only. It allows one to calculate thehighest primary stress and ignores the secondary stresses. In case of the uniformcorrosion formula 5.8 just represents the new hoop stress value. In case of pittingcorrosion only the hoop stress at the defect area will be equal to the calculated one.The hoop stresses at the uncorroded part will remain the same as for the pipe withnominal thickness in this case. The alternative way to express the hoop stress in thecorroded part is the following following.

lred

lnew

t

t= (5.9)

Formula 5.9 indicates that the hoop force is the same over the entire pipe section. Atthe corroded part it should be carried by smaller amount of material and the stressincreased in the defect area proportionally to the thickness ratio.


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In case of two material systems such as a pipe with composite repair the hoop forcewill be distributed between the parts. One part of the load will be carried by the steelcomponent and another part by the FRP component. The ratio of distribution can becalculated, for example using the analytical model introduced above. The load isdistributed among the systems parts according to their stiffness. The strains of thecomponents are the same in case of full interaction. Note that the stiffer parts of the

system carry a bigger part of the load.

The analytical model allows one to calculate the hoop stress in the steel for originalnot corroded pipe but do not give the hoop stress in corroded zone directly. One canthink about different ways to determine this stress.

The first way is to use the reduced thickness in the calculations directly i.e. to use theapproach described in the code for homogeneous systems. Assuming the entirethickness of the pipe to be equal to the reduced one is equal to reducing the stiffnessof the steel component. The component with reduced thickness will take a smallerpart of the load. This means that the real hoop stress will be higher than the calculatedone. This approach is not conservative.

The second way is to determine the hoop stress in the uncorroded part of the pipesection assuming the wall thickness to be equal to the nominal one. The relationsbetween stresses in uncorroded part of the pipe and reduced part still relates to eachother as in expression (5.9). This approach will provide more realistic stressestimation in comparison with the first one.

In case of uniform corrosion the stiffness of the steel component can be reduced. Thesteel part will actually carry smaller part of the load. The real stresses will be lowerthan the calculated ones. This stress calculation approach is conservative regardingthe hoop stress in the steel.

5.5 Estimation of behaviour of repaired structure

5.5.1 Stresses in the steel and in the FRP layer for pressure only

The stress reduction in the FRP reinforced steel pipe is shown in Figure 5.8-5.12. Thestress in the FRP layer and the steel liner is plotted as functions of composite stiffnessand the composite thickness. E-modulus of the composite repair can vary from about40 GPa for glass-fibre composite repair up to 200-300 GPa for composite repairsmade of high-modulus carbon fibre. The thickness of the FRP layer was assumed tobe between 0.1mm and 9mm.


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0

20

40

60

80

100

120

0,

1

0,

9

1,

7

2,

5

3,

3

4,

1

4,

9

5,

7

6,

5

7,

3

8,

1

8,

9

9,

7

Thickness of the composite [mm]

Stressinthesteelliner(%)

FRP E=50GPa

FRP E=100GPa

FRP E=150GPa

FRP E=200GPa

FRP E=250GPa

FRP E=300GPa

Figure 5.8 Hoop stresses in the liner; expressed in percent of the value for the un-

strengthened pipe

Figure 5.8 illustrates the reduction of the stress in the steel part in percentage tooriginal value. This graph is the most convenient one since it is valid for any pressure

value. (Note that the temperature effects are not included here)

One can observe that in order to achieve a reduction of the hoop stresses in the steelpart by 50% the composite shall be of the same thickness and of the same stiffness asliner. If the stiffness of the composite is less than stiffness of the liner a thicker shell isrequires in order for one to obtain the same reduction factor and vice versa. Thethickness of the composite shell should remain relatively small in order for the thin-wall approach to be applicable. Otherwise the thin-wall model is not relevant anymore.

Figures 5.9 and 5.10 represent the stresses in the steel and in the compositerespectively. The high strength of the composite materials is not utilized under elastic

stage. One can observe on Figure 5.10 that the stress in the composite part is reducedwith the increase of the composite thickness. This means that the use of thickcomposite shell leads to inefficient way of using expensive FRP.


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0

5

10

15

20

25

30

35

40

0,

1

0,

9

1,

7

2,

5

3,

3

4,

1

4,

9

5,

7

6,

5

7,

3

8,

1

8,

9

9,

7

Thickness of the composite [mm]

Stressinthesteelliner[MPa]

FRP E=50GPa

FRP E=100GPa

FRP E=150GPa

FRP E=200GPa

FRP E=250GPa

FRP E=300GPa

Figure 5.9 Hop stresses in the liner [MPa]

0

10

20

30

40

50

60

0,

1

0,

9

1,

7

2,

5

3,

3

4,

1

4,

9

5,

7

6,

5

7,

3

8,

1

8,

9

9,

7

Composite thickness [mm]

Hoopstresinthecomposite[M

Pa]

FRP E=50GPa

FRP E=100GPa

FRP E=150GPa

FRP E=200GPa

FRP E=250GPa

FRP E=300GPa

Figure 5.10 Hoop stresses in the composite (FRP) layer [MPa]


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Figure 5.11 shows the relation between the hoop stress in the steel and the hoop stressin the composite shell with varying shell thickness. The steel stress is reduced as thethickness of the composite increase but the rate of reduction is not constant. Thegradient of change is gradually diminishing. It

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