Composite Structures 92 (2010) 499–507

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Composite Structures

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Extending onshore pipeline repair to offshore steel risers with carbon–fiberreinforced composites

Chris Alexander a, Ozden O. Ochoa b,*

a Stress Engineering Services, Inc., Houston, TX, United Statesb Department of Mechanical Engineering, Engineering Physics Building, Room 100, Texas A&M University, College Station, TX 77843-3123, United States

a r t i c l e i n f o

Article history:Available online 21 August 2009

Keywords:RepairDesignRisersCombined loadsCarbon–epoxy composites

0263-8223/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.compstruct.2009.08.034

* Corresponding author. Tel.: +1 979 845 2022; faxE-mail address: OOCHOA@TAMU.EDU (O.O. Ochoa

a b s t r a c t

In the last 15 years, glass fiber hoop reinforced composite systems emerged as an acceptable, successfulmethod for repairing corroded and mechanically-damaged onshore pipelines where the primary load isinternal pressure. The feasibility of extending these repairs to offshore pipes such as risers require a thor-ough understanding of the complex combined load profiles; overlay of significant tension, bending, inter-nal and external pressure. Herein an innovative design based on integrated computational models andfull-scale tests is presented to address the viability of reinstating capacity to offshore pipelines and risers.The experimental program was based on a collaborative test matrix developed with the participation ofcomposite repair manufacturers. The outcome guided and led to an easily deployable carbon–fiber com-posite repair system which was based on limit analysis methods and strain-based design techniques. It isanticipated that the results of this program will foster future investigations by integrating operator’sinsight and in situ data gathering to extend composite repair for offshore needs.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Risers are critical components in offshore operations as they ex-tend the wellhead at the mud line to the surface as shown in Fig. 1.Risers are subject to external corrosion and mechanical damageduring their life cycle and undergo repairs to assure safe opera-tions. Conventional repair techniques incorporate external steelclamps that are either welded or bolted to the outside surface ofthe riser. There are numerous challenges during the installationof steel clamps such as mobilizing the heavy clamp, welding toan operating riser pipe (including safety issues), and installationexpenses. Alternative solutions such as load bearing compositerepair sleeves provide an attractive option as they are relativelyinexpensive, lightweight, do not require welding, and are simpleto install. The transition from onshore to offshore can be demon-strated by undertaking case studies for sites located at top-sideplatform and above the splash zone.

Significant body of work has been conducted to assess the use ofcomposite materials in offshore applications [1–14], primarilyaddressing risers, choke and kill lines and spoolable tubulars. Inparallel for more than a decade, composites were also consideredand adopted to repair damaged pipelines. The majority of thisremediation work focused on the repair of onshore pipelines to re-store hoop strength due to localized wall thickness loss in the steel.

ll rights reserved.

: +1 979 845 3081.).

At present, both the ASME B31.4, Liquid Transportation Systems forHydrocarbons, Liquid Petroleum Gas, Anhydrous Ammonia, and Alco-hols [15] and ASME B31.8, Gas Transmission and Distribution PipingSystem [16] pipeline codes approved their use to re-rate corrodedpipelines to restore operating pressure partially or fully. Addition-ally, mechanical damage (e.g. dents with gouges) has been repairedin situ using composite materials and validated experimentallyusing both burst and cyclic pressure fatigue testing. The historyof onshore pipeline repairs using composite materials is presentedby Alexander and Francini [17]. Even though the long-term perfor-mance of composites incorporating physical and chemical degra-dation is of great importance in pipeline applications, yet thedata reflecting relevant environmental conditions is still sparse[18,19].

In the design of a composite repair system for offshore pipes,fatigue loads, impact and the potential for galvanic corrosion formthe basis for additional consideration. Factors such as wave motionand contact with other structures, such as ships and other risersare realistic sources for impact damage. Galvanic corrosion whichmay result due to electrochemical coupling of carbon fibers withsteel alloys is another mechanism where design of interfaces, treat-ment technology, and environmental conditions are carefully char-acterized. The effect of cyclic pressure loads on the performance ofa composite repair system is quite important in tailoring its stiff-ness and strengths especially for hoop and off-axis fiber reinforce-ments. Numerous studies have been performed that addressdamage initiation and propagation during fatigue of composite

MUDLINE

BOP

W.L.

Flex Joint

Bare Riser Joints

Riser Joints w/ Buoyancy

Telescopic Joint

Diverter & Flex/Ball JointTensioner & Tensioner Ring

LMRP

Fig. 1. Layout for a semi submersible rig showing position of the riser.

500 C. Alexander, O.O. Ochoa / Composite Structures 92 (2010) 499–507

laminates leading to the observation of a gradual decrease in thestatic strength (and modulus of elasticity) as it is subjected to anincreasing number of cycles at a given stress level [9]. One of thegeneral concerns across industry regarding the use of compositematerials is their long-term performance and the potential for deg-radation in strength. In the absence of long-term data, designsusing composite materials rely on large safety factors.

In order to gain field experience, the State of California Depart-ment of Transportation (CALTRANS) conducted tests to assess thelong-term performance of composite materials for infrastructureapplications such as highways bridge columns [20]. This reportpresents data on the effects of environmental exposure on themechanical and physical properties of carbon/epoxy and glass/epoxy systems subjected to 10,000 h in salt water, dry heat at60 �C and hot–wet conditions at 38 �C. Another practical resourceis the ASME STP/PT-005, Design Factor Guidelines for High-PressureComposite Hydrogen Tanks [21]. This report provides recommendeddesign factors relative to short-term burst pressure and interimmargins for long-term stress rupture based on a fixed 15-year de-sign life for fully wrapped and hoop wrapped composite tanks withmetal liners. Similarly, ASTM D2992, Standard Practice for FiberglassPipe and Fittings, recommends the design to be based on one-half(i.e. 0.5) the minimum expected fiber stress to rupture in100,000 h (95% confidence level), or the 50-year strength, which-ever is less [22].

2. Strain-based design methods

It is most desirable to enforce stress/strain limits on both thereinforced steel and reinforcing composite material in any pro-posed composite repair design methods. In this study, the limitstate design is engaged to evaluate the plastic collapse load ofthe reinforced structure. Once the plastic collapse load is deter-mined, a design load can be calculated with an appropriate designmargin. Both analysis and testing are used to predict the maximumstrain in the reinforced steel at both the design and plastic collapse

loads. It is prudent to limit strain in the steel, although it is recog-nized that the contribution of the composite material will alter themaximum strains that would be permitted if no reinforcementwere present. Since limit analysis is based on the use of elastic–plastic material properties for the steel, the strain in the reinforcingcomposite material can be obtained after load has been transferredfrom the steel carrier structure. This is an important point as apurely elastic analysis will fail to account for the mechanics ofthe load transfer and underestimate the amount of load actuallycarried by the composite. Both Division 2 and Division 3 of SectionVIII of the ASME Boiler and Pressure Vessel Codes describe andspecify the use of limit state methods for demonstrating the ade-quacy of design [23]. The plastic collapse load using pressuredeflection is obtained from either an analytical or experimentalsource as described by Twice-Elastic Slope Pressure in WRC Bulletin254 [24,25].

3. Composite repair system design

The principal aim of this study was to design a composite sys-tem to repair offshore risers incorporating design requirements,material selection, and installation techniques. The design require-ments for this effort were to ensure that the stresses due to com-bined pressure, tension, and bending loads remain below anacceptable level. The present design approach is summarized inFig. 2, highlighting the essential interaction between computa-tional and experimental simulations from preliminary to final de-sign stages. Note that since no document exists that candesignate the design requirements for a composite repair in a pre-scriptive manner, both the design itself, as well as the identifica-tion of design limits, are sought after simultaneously.

3.1. Design requirements

Herein the Primary Requirements are considered as those thatgovern the structural design of the composite repair effectively

Design Development Process

Preliminary sizing based on classical mechanics.

Refined evaluation using finite element analysis.

Acceptablestresses?

DESIGN DEVELOPMENT

YES

NO(refine analysis)

Fabricate prototypes and perform testing to failure.

YESNO(refine analysis)

FINAL DESIGNIntroduce composite repair to industry as basis for additional research to prove viability of technology and approval from regulators.

Additional InvestigationsIn addition to the analyses used to asses performance of the repair system relative to pressure, tension, and bending loads, additional investigations were completed including:•Compressive radial stress generated by the composite on the steel pipe•Effects of composite end taper on stresses in the steel•Effects of thermal cooling during the fabrication process and the “free”residual stress state in the composite•Effects of disbonding on the adhesive shear stress and the stress/strain in the steel.

Identify critical elements associated with design requirements. Sub-divide into Primary and Secondary design requirements.

Identify or establish a design basis to which calculated stresses and strains can be compared (i.e. allowable stress/strain values). For the steel-composite interaction, the only real option is a strain-based design approach.

Develop preliminary design concepts.

Acceptableresults?

Fig. 2. Design steps to create CRA repair system.

C. Alexander, O.O. Ochoa / Composite Structures 92 (2010) 499–507 501

guiding the composite architecture and geometric options. In sum-mary they are listed as:

� Prevent bulging of the corroded pipe section duringpressurization.

� Provide sufficient reinforcement so that bending strains remainbelow allowable.

� Maintain integrity of the interface bond in the repair zone.

On the other hand, the Secondary Requirements encompass easeof installation, economic viability, quality control as well as struc-tural integrity during installation, impact resistance and absence ofgalvanic corrosion.

The composite repair system (hereafter referred to as the CRAsystem) was developed and evaluated relative to a pre-establishedset of design criteria. For the problem at hand, this fundamentallyinvolved determining the appropriate fiber orientation and thick-ness to resist internal pressure, tension, and bending loads associ-ated with the operation of an offshore riser. Both prototype

fabrication and full-scale testing of CRA system are reported andcompared to analytical predictions,

3.2. Basic CRA configuration

Provided below are elements finally incorporated in the CRAsystem consistent with the specific pipe geometry. Further detailson the preliminary candidates are articulated in [25]. Inner andouter layers of E-glass were used in the reinforcement. The innerlayer acts to protect the pipe from potential corrosion due to car-bon interaction with steel (i.e. formation of a galvanic cell), whilethe outer layers protect the carbon fibers against potential impactand wear. Circumferentially-oriented carbon fibers are placed inthe region of corrosion. The majority of the carbon fibers are ori-ented axially to provide rigidity in bending and tension. The lengthof the repair is identified to be at least 41 cm (16 in.) on each sideof the corroded region with a total repair zone length of 152 cm(60 in.) The final CRA repair zone configuration is composed ofthe following features:

502 C. Alexander, O.O. Ochoa / Composite Structures 92 (2010) 499–507

Inner layer of 50–50 E-glass, spiral wrap, 0.076 cm (0.030 in.)thick.Circumferential carbon stitched fabric, 0.508 cm (0.200 in.)thick.Axial carbon (pre-cured half shells), 1.106 cm (0.400 in.) thick.Circumferential-spiral carbon (stitched fabric), 0.254 cm(0.100 in.) thick.Outer layer of 50–50 E-glass, spiral wrap, 0.076 cm (0.030 in.)thick.

4. Computational assessment of CRA system

Finite element models were developed to determine (i) stressand strain in the composite material and the steel for the designload conditions, and (ii) axial and hoop reinforcement capacityfor the design loads. The finite element model was constructedusing the PATRAN modeling package and analyzed and post-pro-cessed using the general-purpose ABAQUS� Standard general-pur-pose finite element code (version 6.4). The S4R shell element wasused in the analysis to conveniently model layers having differentthicknesses, orientations, and materials. The carbon layers weremodeled using lamina properties with elastic moduli of 70 GPaand 7 GPa parallel and transverse to the direction of the uni-axialstitched fibers, respectively. For the steel, a simple isotropic elas-tic–plastic model was used with yield and ultimate strength of427 MPa (61 ksi) and 522 MPa (74.6 ksi), respectively. The CRA sys-tem was subjected simultaneously to internal pressure of 20 MPa(2887 psi), axial tension 645 kN (145 kips), and a range of bendingmoments (four-point bending configuration) to asses its perfor-mance. The axial strains in the steel pipe beneath the compositelayers as a function of applied bending load are presented inFig. 3. Note that a steel pipe without any corrosion is also includedto illustrate the full capacity of the original pipe. Furthermore, theunrepaired case with corrosion was modeled without any internalpressure to capture the benchmark full-scale test of the experi-mental program. If pressure had been applied, an excessively lowbending capacity would have resulted for the corroded unrepairedcase due to gross plastic yielding in the steel. The primary source of

Bending Strain versus Results from FEA model of pipe with elasticreinforcement using carbon fibers. Data als

0 0.5

Axial Strain in Steel Ben

Acceptable strain regionfor design conditions

Ben

ding

Mom

ent (

kN-m

)

220

176

132

88

44

0

Fig. 3. Bending force versu

the design limits is based on the virgin pipe response (green curve).The following points are determined:

� Plastic analysis collapse load of 149.5 kN (33.6 kips).� Design load (bending force) of 74.8 kN (16.8 kips) with a design

margin of 2.0 on the collapse load, which corresponds to a bend-ing moment of 66.6 kN-m (49.1 kip-ft).

� At the design condition, the maximum permissible axial strainin the steel beneath the repair is 0.214% (corresponds to theintersection of the horizontal line designating the design loadand the double elastic curve).

The maximum principal strain contours in the steel at designand plastic collapse loads from the finite element model are pre-sented in Fig. 4. Note that at the design condition, the maximumstrain in the steel beneath the composite repair is 0.166%. If thecomposite reinforcement had not been installed, the deformationin this region would have undergone extensive yielding. Once theplastic collapse load is reached, the maximum strain occurs outsidethe corroded and reinforced region, signaling that the compositereinforcement starts carrying a significant portion of the bendingload. Thus, the maximum bending strain in the pipe actually occursoutside the composite reinforced region.

In summary, the following design limits are imposed on the CRAsystem design:

� Carbon/epoxy material stress limit of 275.8 MPa (40 ksi) inaccordance with the methods outlined in ASME STP/PT-005Design Factor Guidelines for High-Pressure Composite HydrogenTanks, which corresponds to a strain limit of 0.40%.

� Strain limit on corroded steel beneath the reinforcement of 0.214%� The maximum permissible bending load (based on design condi-

tions with a design margin of 2.0 on the collapse load) is 74.8 kN(16.8 kips).

5. Fabrication and installation of the CRA prototype repair

Six carbon/epoxy half shells, each 152 cm (60 in.) long, werefabricated at Comptek Structural Composites, Inc. The stacking

Applied Bending Load-plastic material properties with and without o for conditions with and without corrosion.

1 1.5 2

eath the Repair (percent)

Reinforced corrosion (0.40-inch axial carbon)

No composite (corroded, no pressure)

No composite (no corrosion)

Double elastic curve

Plastic analysis collapse load

Design load with margin of 2.0

s axial strain in pipe.

Fig. 5. a. Installation of the carbon half shells b. Final CRA composite repair.

Fig. 4. Axial strains in steel at design (left) and plastic collapse (right) load conditions.

1 For interpretation of color in Figs. 3–9, the reader is referred to the web version ofthis article.

C. Alexander, O.O. Ochoa / Composite Structures 92 (2010) 499–507 503

sequence was used and they were cured under a vacuum seal. Priorto testing and installation of the carbon/epoxy half shells, three (3)steel pipe test samples were fabricated using 8.625-in � 0.406-in(219 mm � 10.3 mm), grade X46 pipe. A 50% simulated corrosioncircumferential groove spanning 60.9 cm (24 in.) in length was ma-chined in each sample. The sample lengths were as follows: burst –244 cm (8 ft), tension – 244 cm (8 ft) and bending – 457 cm (15 ft).The surface of the pipe where the composite repair is to be in-stalled is sandblasted first. The uni-axial stitched carbon clothmaterial was cut to length to repair the 60.9 cm (24 in.) long cor-roded section of pipe. The cloth was saturated with two part epoxyand wrapped around the pipe in the hoop direction in the damagedregion and cured overnight. Subsequently two part epoxy was ap-plied to the surface to bond the carbon half shells on the outsidesurface of the pipe. The half shells were centered and placed axiallyon the corroded region as shown in Fig. 5a. Once the carbon halfshells were locked in place with the steel banding clamps, the out-er hoop-wrapped carbon layers (uni-axial stitched carbon/epoxy).The samples were cured overnight. Fig. 5b shows the final repairedpipe.

6. Full-scale tests

Three full-scale tests that included burst (pressure only), ten-sion (tension with constant pressure), and bending (bend loads

with both pressure and tension held constant) are undertaken withthe repaired pipes. Biaxial (i.e. hoop and axial) strain gage rosetteswere used to determine the level of strain in the steel and CRA sys-tem. Strain gages were installed at different stations, A, B and C asdepicted in Fig. 6, which also displays the basic geometric dimen-sions; (1) on the steel prior to the installation of the repair both atthe center and at the end of repair zone, (2) on the top surface ofcarbon half shells, and (3) on the surface of the hoop-wrapped car-bon cloth layers installed over the half shells.

6.1. Burst pressure

The hoop strains as measured in the steel on various sections inthe CRA repair system during the burst pressure test are reportedin Fig. 7.1 A careful study of the graph leads to the following obser-vations; (i) the ideal level of reinforcement is one that parallels theinitial response of the uncorroded bare pipe (RED), (ii) the strain inthe corroded region (BLUE) is a direct indication of the level of rein-forcement that is provided by the repair system, (iii) the strain gageplaced on the carbon half shell (GREEN) registers lower strains sincethe axial fibers are not being loaded as much due to the internalpressure, and (iv) the strain experienced by the outermost carbon

198-cm

15-cm

Bi-axial strain gage location(install gages at 0°, 90°, and 180°)

Gages located on outside of carbon composite half shell

Gages located on outside of outer carbon hoop-oriented layers

Gages @ A and B are beneath composite repair

Center of groove

61-cm(corroded region)

Three (3) additional gages installed on outside of repair aligned with Station A.

106-cm

A

B

C

152-cm longcompositerepair

Gages reported in bending data plot

457-cm

198-cm

15-cm

Bi-axial strain gage location(install gages at 0°, 90°, and 180°)

Gages located on outside of carbon composite half shell

Gages located on outside of outer carbon hoop-oriented layers

Gages @ A and B are beneath composite repair

Center of groove

61-cm(corroded region)

Three (3) additional gages installed on outside of repair aligned with Station A.

106-cm

A

B

C

152-cm longcompositerepair

Gages reported in bending data plot

457-cm

Fig. 6. Basic geometry and locations for strain gages of interest on CRA system samples.

Hoop Strain as a Function of Internal Pressure

0

6894

13788

20682

27576

34470

41364

48258

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Hoop Miscrostrain (10,000 me = 1 percent strain)

Inte

rnal

Pre

ssur

e (k

Pa)

On pipe in corroded regionBare pipe (uncorroded)Outside surface of repair (on hoop layer)Carbon half shell

Lower bound collapse load, LBCL (41,192 kPa)

Design load, 0.5 * LBCL = 20,601 kPa (19,902 kPa actual design)

Region with permissible design strain levels.

Fig. 7. Annotated pressure test plot showing limit state design parameters.

504 C. Alexander, O.O. Ochoa / Composite Structures 92 (2010) 499–507

cloth hoop wrap (GOLD) clearly demonstrates that they are beingloaded and that the outer layers provide restraint to the carbon halfshells.

The outcome of limit state design method is also presented inFig. 7, where the lower bound collapse load is identified as41.2 MPa (5.98 ksi) and the corresponding design pressure as20.6 MPa (2.99 ksi) with a design margin of 0.6 based on API RP1111. Note that the design pressure for the base benchmark pipeis 19.9 MPa (2.89 ksi), which is 97% of the limit state design pres-sure. The highlighted region embodies the acceptable design pres-

sure and strain levels. Note that the maximum strain in the outsidesurface of repair (hoop-wrapped carbon cloth layers), approxi-mately 0.10%, is significantly less than 0.40% pointing to a long-term performance.

6.2. Tension test

Fig. 8 displays axial strains measured during the tension testswhere the final failure occurred at 2643.3 kN (594 kips). The limitstate design LBCL is reflected in as 2118.2 (476 kips) corresponding

C. Alexander, O.O. Ochoa / Composite Structures 92 (2010) 499–507 505

to a design load of 1059.1 kN (238 kips). This value is 64% greaterthan the specified design load of 645 kN (145 kips).

As expected, the maximum strain occurs in the corroded steelregion of the sample beneath the repair. From the beginning ofloading this region carries a greater percentage of load than ob-served in the CRA composite repair; however, it should be notedthat if it was not repaired, it would have failed at approximately1424 kN (320 kips), about 50% of the test load recorded for this

Axial Strain as a Fun

0

445

890

1335

1780

2225

2670

0 2000 4000Miscrostrain (10,00

Axia

l Loa

d (k

N)

Design l

Region with permissible design strain levels.

0

445

890

1335

1780

2225

2670

0 2000 4000

Design l

Region with permissible design strain levels.

Fig. 8. Annotated tension test plot show

Axial Strain as a Fun

10

30

40

50

60

70

80

90

0 2000 4000

Miscrostrain (10,000

Region with permissibldesign strain levels.

O

B

C

O

Ben

ding

Mom

ent (

kN-m

)

44

0

88

132

176

220

264

308

352

396

440

Design load, 0.5 * LB

Fig. 9. Annotated bending test plot show

particular sample based on the remaining area and ultimatestrength. As yielding occurs in both the corroded region and thebase pipe, a greater percentage of the load is distributed to thecomposite material. That is, as the base pipe (RED) started yieldingat approximately 2003 kN (450 kips), the axial strains in the carbonhalf shell (GREEN) increased. Axial strains measured in the outerhoop-wrapped carbon are less than those measured in both thepipe (corroded and uncorroded) and the carbon half shells as ex-

ction of Tension Loading

6000 8000 100000 = 1 percent strain)

On pipe in corroded region

Bare pipe (uncorroded)

Outside of repair (on hoop layer)

Carbon half shell

oad, 0.5 * LBCL = 1059 kN (645 kN actual design)

6000 8000 10000me

On pipe in corroded region

Bare pipe (uncorroded)

Outside of repair (on hoop layer)

Carbon half shell

Lower bound collapse load, LBCL (2118 kN)

oad, 0.5 * LBCL = 1059 kN (645 kN actual design)

ing limit state design parameters.

ction of Bending Load

6000 8000 10000

me = 1 percent strain

e

n pipe in corroded region

are pipe (uncorroded)

arbon half shell

utside of repair (on hoop layer)

Lower bound collapse load, LBCL (194 kN-m)

CL = 97 kN-m (96 kN-m actual elastic design)

ing limit state design parameters.

Table 1Comparison of axial strains in CRA composite repair system.

Configuration Design strain limita (%) Calculated strain (analysis) (%) Experimental measured strain (testing) (%)b

Loading at design conditionsPressure loading (at 19.9 MPa) 0.169 0.116 0.106Bending loading (at 72.8 kN-m bending load) 0.214 0.057 0.055

Loading at lower bound collapse load conditionsPressure loading (at 39.3 MPa) N/A 0.370 0.458Bending loading (at 150 kN-m bending load) N/A 0.138 0.152

a Design strain limit based on finite element results for undamaged pipe subject to specified loading.b Experimental measured strains were extracted from strain gage positioned on steel beneath composite repair in center of corrosion region.

506 C. Alexander, O.O. Ochoa / Composite Structures 92 (2010) 499–507

pected since this region is the last to be loaded in tension. It isimportant to acknowledge that the strain in the reinforcedcorroded section (BLUE) generally exists within the acceptable de-sign region demonstrating that adequate reinforcement is pro-vided by the composite repair system.

6.3. Four-point bend test

The bend test results are displayed in Fig. 9. Note that duringtesting internal pressure of 19.9 MPa (2.89 ksi) and axial tensionof 645.3 kN (145 kips) were included in addition to the bendingload. At a bending load of approximately 89 kN (20 kips), allstrain gages demonstrate deviation from the proportional limit(i.e. response is no longer elastic). As expected, the maximumstrain occurred in the corroded region of the test sample be-neath the repair (BLUE). At a bending load of 178 kN (40 kips),the axial strain is 0.20%. The strain in the carbon half shell(GREEN); although less than the strain in the reinforced steel,demonstrates that it is engaged with increasing bending loads.Another important observation is that as the bending load is in-creased, the axial strains in the region of the reinforcement (i.e.everything except the RED) do not increase proportionally. Therationale is that once a plastic hinge forms in the pipe (1.5 timesthe yield load, or approximately 289.3 kN (65 kips)), deformationinitiates in the base pipe away from the composite repair. Addi-tional loading only plastically deforms the pipe at the points ofcontact with the hydraulic cylinders and does not transfer loadinto the reinforced region, indicating that the actual plastic col-lapse of the pipe will not occur in the repaired region, but ratheroutside the repair zone where local bending stresses are thegreatest.

Within the range of acceptable highlighted strain levels, thereinforcement provided by the CRA system is once again satisfac-tory. Due to the relatively low lower bound collapse load observedexperimentally, all strains in the reinforced region of the sampleare below the strains observed in the base pipe away from thereinforcement.

7. Integration of computational models and test data

The analysis served as the foundation for the design of experi-ments as well as for the final design for CRA repair system to estab-lish the required thickness, and stacking sequence. Table 1provides a comparison of strain values from both the analysisand testing efforts. The results are provided at the reinforced re-gion of the steel for two cases: pressure only and combined pres-sure-tension-bending loads. In general, all measured strains areless than those calculated with finite element methods, includingthe results at both the design and the limit load conditions. Theexception to this observation is the strains recorded for the pres-sure sample near the limit load of 39.3 MPa (5.7 ksi), where the ac-tual burst occurred at 42.3 MPa (6.2 ksi).

8. Conclusions

Extension of onshore composite repair techniques to offshorerisers by developing integrated analytical and experimental meth-ods is accomplished by designing a carbon/epoxy based compositerepair system incorporating computational simulation, prototypefabrication and experimental verification. The unique approach ofthe current effort is its embodiment of limit analysis and strain-based design methods for steel pipes with composite reinforce-ment. Furthermore the fabrication, installation and combined load-ing prototype tests validated both the safety of the selected designmargins as well as the CRA design methodology.

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[21] American Society of Mechanical Engineers. Design factor guidelines for high-pressure composite hydrogen tanks. New York: American Society ofMechanical Engineers; 2006.

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