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Dr. Chris Alexander, Stress Engineering Services, Inc., Houston, Texas A significant amount of work has transpired over the past several years in generating consensus- based standards that include ASME PCC-2 and ISO 24817 for developing composite repair systems. The intent in developing these standards has been to provide industry with guidelines for designing composite repair systems to ensure that damaged pipelines are safely and properly reinforced. With the numerous composite repair systems currently available to pipeline operators, the importance of evaluating the capabilities of each system cannot be over- stated. The fundamental design variables available to manufacturers are stiffness, strength, and thickness of the composite. A properly-designed repair system ensures that strains in the reinforced steel and reinforcing composite material do not reach unacceptable levels. This article provides a basic overview of the design philosophy embedded into the current design codes, as well as present- ing results associated with several spe- cific studies that were conducted to eval- uate composite repair performance. Repair codes In order for composite systems to repair damaged pipelines, it is critically important that they be designed to ensure that adequate reinforcement is present. The ASME PCC-2 and ISO 24817 composite repair codes (here- after referred to as Codes) provide the required details to design a system that has sufficient stiffness, strength, and thickness. Provided below are the Scope and Applicability sections from Part 4 (Nonmetallic and Bonded Repairs) of the 2006 edition of ASME PCC-2. 1.1 Scope. This Article provides the requirements for the repair of pipework and pipelines using a qualified Repair System. The Repair System is defined as the combination of the following ele- ments for which qualification testing has been completed. (a) substrate (pipe) (b) surface preparation (c) composite material (repair laminate) (d) filler material (e) adhesive (f) application method. The composite materials allowed for the Repair System include, but are not limited to, glass, aramid, or carbon fiber reinforcement in a thermoset resin (e.g. polyester, polyurethane, phenolic, vinyl ester, or epoxy) matrix. Fibers shall be continuous. 1.2 Applicability. This Article addresses the repair of pipework and pipelines origi- nally designed in accordance with ASME B31.1/B31.3/B31.4/B31.8, and ISO 15649 and 13623. The Applicability section goes on to state that the Code covers situations involving damage that include internal and external corrosion, external damage such as dents, gouges, and cracks, as well as manufacturing defects. The repair of leaks is also permitted, although for high pressure transmission pipelines, this repair option is unacceptable at the pres- ent time based on the author’s opinion. Because the focus of this article is repair- ing high pressure gas and liquid trans- mission pipelines, there is no discussion Recently developed ASME PCC-2 and ISO 24817 standards offer guidance on the design of composite repair technologies. Developing standards for composite repair systems Figure 1. Strains measured in composite reinforced corroded pipe sample. October 2009 www.pipelineandgastechnology.com As seen in the Ocotober 2009 issue of
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Page 1: Developingstandardsfor compositerepairsystemscompositerepairstudy.com/downloads/Developing_standards...nally designed in accordance with ASME B31.1/B31.3/B31.4/B31.8, and ISO 15649

Dr. Chris Alexander, Stress EngineeringServices, Inc., Houston, Texas

Asignificant amount of work hastranspired over the past severalyears in generating consensus-

based standards that include ASMEPCC-2 and ISO 24817 for developingcomposite repair systems. The intent indeveloping these standards has been toprovide industry with guidelines fordesigning composite repair systems toensure that damaged pipelines are safelyand properly reinforced.

With the numerous composite repairsystems currently available to pipelineoperators, the importance of evaluating thecapabilities of each system cannot be over-stated. The fundamental design variablesavailable to manufacturers are stiffness,strength, and thickness of the composite.

A properly-designed repair systemensures that strains in the reinforcedsteel and reinforcing composite materialdo not reach unacceptable levels. Thisarticle provides a basic overview of thedesign philosophy embedded into thecurrent design codes, as well as present-ing results associated with several spe-cific studies that were conducted to eval-uate composite repair performance.

Repair codesIn order for composite systems to repairdamaged pipelines, it is critically importantthat they be designed to ensure that adequatereinforcement is present. The ASME PCC-2and ISO 24817 composite repair codes (here-after referred to as Codes) provide therequired details to design a system that hassufficient stiffness, strength, and thickness.

Provided below are the Scope and

Applicability sections from Part 4(Nonmetallic and Bonded Repairs) of the2006 edition of ASME PCC-2.

1.1 Scope. This Article provides therequirements for the repair of pipeworkand pipelines using a qualified RepairSystem. The Repair System is defined asthe combination of the following ele-ments for which qualification testing hasbeen completed.

(a) substrate (pipe)(b) surface preparation(c) composite material (repair laminate)(d) filler material(e) adhesive(f) application method.The composite materials allowed for

the Repair System include, but are notlimited to, glass, aramid, or carbon fiberreinforcement in a thermoset resin (e.g.polyester, polyurethane, phenolic, vinylester, or epoxy) matrix. Fibers shall becontinuous.

1.2 Applicability. This Article addressesthe repair of pipework and pipelines origi-nally designed in accordance with ASMEB31.1/B31.3/B31.4/B31.8, and ISO 15649and 13623.

The Applicability section goes on tostate that the Code covers situationsinvolving damage that include internaland external corrosion, external damagesuch as dents, gouges, and cracks, aswell as manufacturing defects. The repairof leaks is also permitted, although forhigh pressure transmission pipelines, thisrepair option is unacceptable at the pres-ent time based on the author’s opinion.Because the focus of this article is repair-ing high pressure gas and liquid trans-mission pipelines, there is no discussion

Recently developed ASME PCC-2 and ISO 24817 standardsoffer guidance on the design of composite repair technologies.

Developing standards forcomposite repair systems

Figure 1. Strains measured in composite reinforced corroded pipe sample.

October 2009 www.pipelineandgastechnology.com

As seen in the Ocotober 2009 issue of

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on the repair of leaking pipes.The function of the Codes is design

and within ASME PCC-2 there are threebasic approaches for determining theminimum required thickness for a par-ticular composite material in repairingcorrosion and are listed below.

• Section 3.4.3 − Pipe Allowable Stress• Section 3.4.4 − Repair Laminate

Allowable Strains• Section 3.4.5 − Repair Laminate

Allowable Stresses Determined byPerformance Testing.

The contents of this article should notbe used as a substitute for actually con-sulting and utilizing the compositerepair design codes (i.e. ASME PCC-2and ISO 24817). These design codesprovide details that deal with specificissues when using composite materialsin repairing and reinforcing damagedpipelines that should not be ignored.

Determining repair thicknessThe sections that follow provide specificdetails on the above referenced ASMEPCC-2 sections and their unique designapproaches. An example problem is alsoprovided to demonstrate the level ofconservatism associated with each calcu-lation method and the benefits indesigning a performance-based system asdetailed in Section 3.4.5, even thoughadditional efforts and costs are associ-ated in qualifying a given composite sys-tem to this level. Due to limited space inthis article, all calculations assumestructural contribution of the remaining

corroded steel, although the option fornot including this contribution is analternative provided by the Codes thatresults in a greater required minimumcomposite thickness.

The following ASME PCC-2 nomen-clature (i.e. variable descriptions) isused in the calculations that follow.

D External pipe diameter (inches)Ec Tensile modulus for the compos-

ite laminate in the circumferentialdirection (psi)

Es Tensile modulus for the pipe steel(psi)

f Service factor (inverse of safetyfactor, provided in ASME PCC-2Table 5)

P Internal design pressure (psi)Ps Maximum Allowable Working

Pressure (MAWP) for corrodedpipe using B31G, etc.

s Specified Minimum Yield Strength(SMYS) for pipe (psi)

slt 95% lower confidence limit of thelong-term composite strength viatesting (psi)

t Nominal wall thickness of pipe(inches)

tmin Minimum repair thickness ofcomposite (inches)

ts Minimum remaining wall thick-ness of pipe (inches)

εc Allowable circumferential strainThe first design option that is pro-

vided in ASME PCC-2 is the most con-servative of the three options presentedin this article. Equation 1 is used to cal-culate the minimum required thickness

considering hoop stresses based oninternal pressure. Note that by includingthe Ps term credit is taken for strengthsassociated with the remaining steel.

(ASME PCC-2 Equation 1)

In reviewing Equation 1, is it clear thatthe relative stiffness values of the steel(Es) and the composite (Ec) are inte-grated to calculate the minimum requiredthickness. The use of this equationassumes that the substrate (e.g. remainingreinforced pipe material) does not yieldand remains elastic throughout operation.

The second design option that isavailable in PCC-2 is calculating theminimum composite thickness based onhoop strain due to internal pressureusing Equation 4. Also included in PCC-2 is Equation 3 that integrates the effectsof internal pressure in the pipe at thetime of the composite installation,although this equation is not presentedin this article.

(ASME PCC-2 Equation 4)

In solving Equation 4 the designationof an allowable long-term strain, εc, isrequired. Table 4 from ASME PCC-2specifies that for continuous (sustained)loading conditions the allowable long-term strain for the repair laminate is lim-ited to 0.25%, while for rarely occurringloads it is 0.40%.

The minimum required thickness usingthe ASME PCC-2 Section 3.4.5 method isbased on performance testing of the com-posite material itself and is the third ASMEPCC-2 design option. This approachrequires additional testing of the compositematerial beyond what is required for theother calculation methods, such as the1,000 hour survival test as presented inSection V-2.1 in Appendix V of ASMEPCC-2 based on ASTM D 1598. In this

www.pipelineandgastechnology.com October 2009

Figure 2. Schematic diagram of composite repair pipe test sample.

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particular test internal pressure is appliedto a test sample having a minimum diame-ter of 4 inches and a minimum thickness of0.120 inches. The sample’s internal pres-sure and composite laminate thickness areselected to maximize the long-term com-posite stress, slt, using the equation below.

(ASME PCC-2 Equation V-1)

In this qualification test, three identi-cal test samples must be repaired andsurvive 1,000 hours of testing with nodeterioration of the laminate in the formof cracking, delamination, or leaking.

Once the long-term composite designstrength is established based on the 95%lower confidence limit, the minimum com-posite repair thickness is calculated usingEquation 9 from ASME PCC-2. In review-ing this equation, the use of a service fac-tor, f, is required. The service factors arebasically the reciprocals of safety factorsand are listed in Table 5 of ASME PCC-2. Ifone opts to establish long-term laminatestrength using the 1,000 hour data, theservice factor is 0.5 (i.e. safety factor of 2.0for the composite material’s strength).

(ASME PCC-2 Equation 9)

Calculating repair thicknessTable 1 provides calculations associatedwith the reinforcement of a 12.75-in. x0.375-in., Grade X42 pipeline having50% corrosion where the MAOP is de-rated from 1,778 psi to a MAWP of 1,000psi due to the corrosion. Presented areresults for all three calculation methodsdiscussed previously (i.e. pipe allowablestress, repair laminate allowable strains,and repair laminate allowable stressesdetermined by performance testing). Itshould be noted that the contribution ofthe remaining steel is considered in allprovided calculations.

An extremely important observation in

reviewing the calculated results provided inTable 1 is the reduction in the minimumrequired laminate repair thickness associ-ated with the three calculation options. It isclear from this presentation that with theinclusion of the long-term data as requiredfor using Equation (9), a less conservativecomposite thickness results due to thegreater effort undertaken in determiningthe actual long-term strength.

Case studiesOne of the consistent elements associatedwith the development and qualification ofcomposite repair systems has been experi-mental evaluation. This evaluation hasinvolved assessments at both the couponand full-scale levels. Evaluating materialperformance at the coupon level is aneffective means for determining thestrength of the composite, while at thesame time being less expensive than full-scale testing. The primary emphasis in theCodes up to this point in time has been indesigning composite repair systems toreinforce corrosion; however, there is alsoan abundance of data demonstrating thatcomposite materials can be used to rein-force wrinkle bends, elbows, field branchconnections, dents, and others anomalies.Results from several prior studies havebeen presented in the previous articlesassociated with this series.

Two case studies are presented belowthat deal specifically with the reinforce-ment of corrosion using composite materi-als. The first case study involves the repairof an 8-in. nominal diameter pipeline with

50% corrosion that was reinforced using acarbon-epoxy system. During testing straingages monitored strain in the reinforcedsteel region and were used to demonstratethe level of reinforcement provided by thecomposite material. The second case studydiscusses results associated with a testingprogram to evaluate the capacity for a car-bon-epoxy system to reinforce 75% corro-sion in a 12-in. nominal diameter pipelinesubjected to cyclic pressures.

Case study No. 1. In 2006, a programwas conducted for the U.S. MineralsManagement Service to evaluate the useof composite materials in repairing off-shore risers. Part of this study involvedrepairing a burst test sample having 50%corrosion using a 0.60-in. thick carbon-epoxy system that included two pre-cured half-shells. Strain gages wereinstalled in the corroded region of the8.625-in. x 0.406-in., Grade X46 pipesample and monitored during pressuriza-tion to failure. Results from this test areprovided in Figure 1. Included in thisplot are a few annotations that designatethe lower bound collapse load (5,975 psi)from which the design pressure (2,988psi) is calculated. This design pressureexceeds the maximum allowable operat-ing pressure of 2,887 psi of a non-cor-roded pipe. The results of this programdemonstrated that the carbon repair waseffective in reinforcing the corroded pipeand ensured that strains in the reinforcedsteel did not reach an unacceptable level.This study is classified as one based onstrain-based design limits.

October 2009 www.pipelineandgastechnology.com

Composite Repair Focus Series

Figure 3. Schematic showing location of strain gages of photo of machined region.

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Case Study No. 2. Most of the experi-mental research associated with the com-posite repair of corroded pipelines hasfocused on burst tests. The general philos-ophy has been that in the absence ofcyclic pressures during actual operation,there are few reasons to be concernedwith qualifying composite repairs forcyclic conditions. One could argue thatonly liquid transmission pipelines need tobe concerned about cyclic pressures.However, recent studies have indicatedthat for severe corrosion levels (on theorder of 75%) there is a need to take acloser look at the ability of the compositeto provide reinforcement. The case studypresented herein was actually preceded bya series of tests using E-glass materialsthat evaluated the number of pressurecycles to failure in reinforcing 75% corro-

sion in a 12.75-in. x 0.375-in., Grade X42pipeline. Figure 2 is a schematic showingthe geometry of the test sample used inthis study, while Figure 3 shows the posi-tioning of strain gages beneath each repairin the corroded region. The test sampleswere pressure cycled at a pressure range of36% SMYS (i.e. 894 psi for this pipe).

Tests were performed on five differentcomposite systems that included the fol-lowing cycles to failure:

• E-glass system: 19,411 cycles to failure• E-glass system: 32,848 cycles to failure• E-glass system: 140,164 cycles to failure• E-glass system: 165,127 cycles to failure• Carbon system (Pipe #1): 212,888

cycles to failure• Carbon system (Pipe #2): 256,344

cycles to failure• Carbon system (Pipe #3): 202,903

cycles to failure• Carbon system (Pipe #4): 532,776

cycles to failure.Minimal information is provided

with the above data (e.g. no informa-tion provided on thickness, compositemodulus, filler materials, fiber orienta-tion, etc.). However, one can definitelyconclude that all composite repair sys-tems are not equal. The study on thecarbon composite system having fourdifferent pipe samples was specificallyconducted by a manufacturer to deter-mine the optimum design conditionsfor reinforcing the severely corrodedpipe. Figure 4 shows the strainsrecorded in the four carbon-reinforcedtest samples. What is noted in this plotis that the lowest recorded mean strainsoccur in Pipe #4, which also corre-sponds to the test sample that had thelargest number of cycles to failure.

ConclusionComposite materials continue to play animportant role in repairing damagedpipelines. When properly designed andinstalled, they are able to restore theintegrity of damaged pipelines back totheir original integrity. The relativelyrecent development and application ofcomposite repair standards such as ASMEPCC-2 and ISO 24817 are contributingsignificantly to the proper design of thecomposite repair technologies. Thesestandards will continue to develop as thepipeline industry requires that compositematerials provide repair solutions forpipeline anomalies as part of theirintegrity management programs. �

Table 1. ASME PCC-2 Calculated Thickness Values

Figure 4. Measured strain range in 75% corroded test sample.

ASME PCC-2Equation Number

ASME PCC-2Equation

Calculated Values(see Note below forvariable values)

(1) 0.787 inches

(4) 0.306 inches

(9) 0.138 inches

Notes (input variables used in above equations)Es 30 x 106 psi (steel pipe modulus)Ec 4.5 x 106 psi (composite laminate modulus)s 42,000 psi (pipe Minimum Specified Yield

Strength, or SMYS)P 1,778 psi (MAOP)

Ps 1,000 psi (de-rated operating pressure due topresence of corrosion)

t 0.375 inches (pipe nominal wall thickness)εc 0.25% (allowable long-term composite strain

from ASME PCC-2 Table 4)f 0.5 (Service Factor from ASME PCC-2 Table 5)

slt 50,000 psi (long-term composite strength basedon ASME PCC-2 Appendix V directives)

ts 0.188 inches (remaining pipe wall thickness dueto corrosion)

Copyright, Hart Energy Publishing, 1616 S. Voss, Ste. 1000, Houston, TX 77057 USA (713)260-6400, Fax (713) 840-8585