High Pressure Composite Pipeline and Joints for GasDistribution Network

J. F. McNamara a, A. Connolly b and J. Steen c

aDepartment of Mechanical Engineering, National University of Ireland, Galway,Ireland.

bMCS International, Lismoyle House, Merchant’s Road, Galway, Ireland.

cAmeron B.V., J.F. Kennedy laan 7, P.O. Box 6, 4190 CA Geldermalsen, TheNetherlands.

ABSTRACT

The problem addressed in this paper is the development of high pressurecomposite pipelines and joints suitable for a broad spectrum of applicationsincluding water and gas distribution and the transport of corrosive chemicals.Two types of pipe wall are considered including traditional filament wound glassreinforced epoxy (GRE) and a hybrid composite with GRE layers reinforced bysteel strip laminates (SSL). Both types are joined by a patented system of helicalgrooves on a tapered lap section with an integral ductile key known as a coil-lockjoint. Typical specifications are 70 bar operating pressure with a factor of safetyof 4.5 for a 200mm diameter pipe and corresponding values of 150 bar and 2.5 forthe stronger SSL type. Details on the pipeline designs, coil-lock joint, testprogrammes and supporting finite element calculations are outlined and indicatethat the required performance criteria are achieved.

1. INTRODUCTION

There is an increasing demand from both the hydrocarbon and industrialmarkets for relatively high pressure composite material pipelines that arecapable of transporting a broad spectrum of fluids, including corrosive chemicals,and that are easy to install and maintain. The operating pressures underconsideration are in the range from 70 bar to 150 bar such that the joint designbecomes important. In addressing this problem, two composite pipelines areconsidered, one is a traditional filament wound glass reinforced epoxy (GRE) pipewhile the other is an innovative hybrid design with GRE inner and outer layers

reinforced by a steel strip laminate (SSL) core. The GRE pipe wall is targeted atthe lower internal pressure of 70 bar and a factor of safety 4.5 whereas the SSLpipe may sustain a higher pressure level of 150 bar at a factor of safety of 2.5.The pipelines for both GRE and SSL constructions are joined using a patentedcoil-lock threaded connection which combines a tapered lap joint with an integralductile helical key.

Development of the GRE pipeline was partially supported under an EC Brite-Euram programme and test results with supporting finite element calculationsare presented. The modelling of the coil-lock joint including the details of thelocking key and contact between the male and female sections of the joint areoutlined. Ameron performed an extensive series of tests on the SSL pipe andthese are discussed also. The tests and computations indicate that the requiredperformance levels for the respective pipelines and coil-lock joint are attainedand exceeded in many cases.

2. PIPELINE DESCRIPTIONS

The two types of pipewall design are now described in more detail. Theconfiguration of the coil-lock joint is the same for both and details are also given.

2.1 Glass reinforced epoxy pipe (GRE)The epoxy pipe is made by means of pulling continuous tows through a resin

bath and then applying them via a transverse feed eye to a rotating mandrel. Thewinding angle is mostly set to ±54o and the matrix consists of a hardener and anepoxy resin, which is cured at elevated temperatures. The glass used is E-glassand the pipe can be used up to 110oC and is highly corrosion resistant. The pipewall design is based on API-15LR with ASTM 2992 being used for testing andregression analysis. Prototype pipe sections, ranging in internal diameters from100 mm to 300 mm, were manufactured by Ameron B.V. for testing purposes.

2.2 Steel strip laminate pipe (SSL)A cross-section of the steel strip laminate (SSL) pipe wall construction is

shown in Figure 1. The pipe combines conventional GRE composite constructionon the inside and outside of the pipewall with high strength steel strip layerssandwiched in the middle. The GRE inner and outer jackets provide the corrosionresistance of conventional GRE and the steel layers provide low costreinforcement for the GRE composite. The joining system further encapsulatesthe steel strips at the ends of each pipe section to provide complete corrosionprotection of the reinforcing steel strips. The laminated steel strip structure isbased on proven technology developed over 30 years in the manufacture of highperformance rocket motor casings [1]. The GRE component of the SSL pipe isdesigned in compliance with API-15HR, while the steel component is in

Fig. 1. Steel strip laminate pipe cross section (US patent 5,579,890)

compliance with design stresses specified in API-5L.

SSL pipeline specimens were exposed to a range of corrosive chemicalsincluding crude oil and, in all cases, the steel strips were completely protectedfrom corrosion by the GRE laminates. Additional tests on the permeation rates ofH2S and CO2 through the hybrid wall of GRE liner, steel straps and GRE outerjacket were executed at rated pressure and results show that SSL pipe isresistant to sour environments.

2.3 Joining system (coil-lock)The patented [2] joint for both GRE and SSL pipelines is similar and is a

tapered and mechanically locked joint with a ductile key as shown in Figures 2and 3 for the SSL case. The joint can be sealed utilising dual mechanical seals orcan incorporate dual mechanical seals with injection of an adhesive sealantbetween the seals for maximum leak integrity as shown in Figure 4. The joint forthe GRE pipe is formed by machining the spiral grooves on the male end of thepipe and matching those to grooves on the female end, manufactured by the useof a key former mounted in machined grooves on the mandrel.

In the SSL design, the steel strip laminates are progressively terminated in astaircase pattern as shown in Figures 3 and 4. This allows for a gradual loadtransfer to the GRE and termination of the steel within the joint buildup. This

Glass Reinforced EpoxyOuter Jacket

Helically WoundSteel Strip Layers

Glass ReinforcedEpoxy Inner Liner

Fig. 2. 3D view of coil-lock joint (US patent 5,579,890)

protects the steel by completely encasing it within the GRE at both ends of eachpipe section.

Injection Hole for Sealant StaggeredSteel Strips

Stagger Pattern

Leading Edge of FirstSteel Strip

Fig. 3. SSL pipe coupled joint detail

Fig. 4. Seal detail showing injection of adhesive sealant between dual O-ringseals

The coil-lock joint construction provides ductile redistribution of axial loadsacross the full length of the joint. As shown in Figure 5, the coil-lock jointincreases the allowable shear stress in the GRE by utilising a ductile key tobreak up the total shear path into multiple short shear paths. Additionally, thetaper of the joint places shear planes in separate GRE layers. Only a fewrotations of the pipe are required in order to make the joint, due to the taper andlarge pitch of the thread.

3. TEST PROGRAMMES

Both GRE and SSL type pipeline sections including coil-lock joints weresubjected to pressure testing. The tests on GRE pipes were carried out as part ofan EC Brite-Euram research programme and are fully reported in [3].Independent testing of both GRE and SSL sections were also executed by

O-Ring seals

Spigot end of pipe

Integral bell

Pressure point forchecking O-Ringintegrity and injectingsealant

Polysulfide adhesivesealant

Table 1Ameron GRE burst tests

Diameter Number of tests Average burstpressure (bar)

Failure type

100mm 2 340 Leaking200mm 5 350 Weeping300mm 4 297.5 Weeping

Ameron, and Ameron provided the samples for all tests. The main results of thetesting programmes are summarised above.

3.1 GRE pipeline test resultsBurst tests carried out by Ameron on a variety of GRE pipes and joints are

summarised in Table 1. The mode of failure of the pipe sections was in axialtension in the female section in a region near the O-ring seal. An additional 200mm diameter prototype with axial reinforcement in this failure region achievedan increase in pressure from the original 350 bar average burst pressure to 550bar and was held at this level for 2 minutes without leakage. Thus, the objectiveof designing a high pressure joint with 70 bar operating pressure and a factor ofsafety of 4.5 is readily achievable for all diameters tested.

Tests on two 100 mm diameter GRE pipes and joints reported in [3] indicatean average failure pressure of 300 bar with a mostly linear elastic response up to200 bar. Failure due to weeping occurred in the same area as for the Amerontests in the female section at the junction between the uniform pipe section andthe joint taper. A major difficulty in all of these burst pressure tests is in decidingat what level failure occurs. For example, one of the tests sections from [3] isrecorded as failing due to a slight leakage at the endcap fixture at 320 bar butthis did not prevent the pipe from attaining a maximum burst pressure of 450bar. If the value of 450 bar is accepted the average failure pressure rises to 365bar, which is close to the Ameron value of 340 bar for 100mm diameter pipe.

3.2 SSL pipeline test resultsA total of 19 joints in three different configurations were tested during the

initial phase of the development programme. In twelve cases the joint key failedin shear, the O-ring seals failed in six cases and the fibreglass threads failed inone case. These data were utilised to develop an analytical computational modelfor predicting the structural performance of the joint. The actual measuredpressures at structural failure of the joint are compared in Figure 6 with thepredictions of the computational model for the twelve specimens that failed dueto shearing of the key. Frictional forces between the key and the grooves areincluded with bearing forces in the computational model. These results are being

Fig. 5. Shear distribution comparison of a long bonded joint to coil-lock

used to optimise the joint design such that it should be at least as strong as theSSL pipewall under all loading conditions.

As noted from Figure 6, the test pressures at failure range from 4000 psi(27.2MPa) to 9000 psi (61.2MPa), or from 272 bar to 612 bar with an averagevalue equal to 450 bar. Further improvements in joint key design indicate thateven higher ultimate pressures are possible leading to operating pressuresapproaching 150 bar, with a safety factor of 2.5.

4. FINITE ELEMENT ANALYSIS OF GRE PIPE AND COIL-LOCK JOINT

In the interests of further detailing the performance of the pipeline and coil-lock joint a finite element analysis of the GRE section with the dimensions shownin Figure 7 is presented.

4.1 Finite element modelSince the GRE pipeline tests indicate failure occurring externally to the joint

threads, it is felt that an axisymmetric structural model is sufficient.Additionally, the overall finite element model includes the female, male and

Short Shear Length Average Shear StressElastic Shear Stress

Bonded Joint

Coil – Lock Joint

Elastic Shear Stress

Average Shear Stress

Long Bondline

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

3 4 5 6 7 8 9 10 11 17 18 19

Specimen number

Fai

lure

Pre

ssur

e (p

si)

ACTUAL

PREDICTED

Fig. 6. Actual structural failure pressure of the joint versus predicted failure

locking key as separate components joined by contact elements as shown inFigure 8 for the region of the coil-lock joint. The polyamide key is approximatedas seven separate rings around the joint, rather than a helical spiral, which is notpossible to model in two dimensions.

The elements used are four-noded axisymmetric quadrilaterals and contactingsurfaces include elastic deformations of the surfaces and sliding contact [4].Orthotropic material properties consistent with a constant winding angle of ±54°are taken from [5] and are listed in Table 2; properties of the composite pipe inthe radial direction are approximated as those of the unreinforced matrix. Theproperties for the polyamide key are assumed isotropic and are listed in Table 2also.

Table 2Material properties for the pipe and key

Pipe/Joint KeyRadial Axial Circumferential

Young’s Modulus (MPa) 2,000 10,000 25,300 350Ultimate Strength (MPa) 65 65 300 48Poisson’s Ratio 0.35 0.4 0.49 0.4Shear Modulus (MPa) 741 3471 8490 1250

Fig. 7(a). Female section of pipe

Fig. 7(b). Male section of pipe

Fig. 8. Axisymmetric FE model of coil-lock pipe/joint system

The mesh is continued into the uniform section of the pipe extending to alength of 2m for the pipe and joint assembly. One end is fully restrained whilethe other end is free but subjected to an axial tensile load generated by theendcap effect due to the applied internal pressure. A series of elastic staticanalyses are carried out on the pipeline over a range of internal pressures from70 to 310 bar in order to determine the location and magnitude of the criticalstresses.

4.2 Results of finite element analysisThe contours of axial stresses in the assembly for an applied internal pressure

of the 31 MPa (310 bar) as computed by the finite element method are shown inFigure 9. It is noted that the pipe walls of the uniform sections are fully stressed

Fig. 9. Axial stress (MPa) due to applied pressure of 31 MPa

at 65 MPa while the stresses in the coil-lock tapered sections attain moremoderate values of 25 MPa. However, very highly localised axial stresses arepredicted at the two transition areas between the uniform pipe and tapered jointon the male and female sides, respectively. The stresses attain maximum valuesof 119 MPa for the applied pressure of 31 MPa and an enlarged view of the stressconcentration in the transition on the male side is shown in Figure 10. Theseresults are in general agreement with experiments where failure in axial tensionoccurred near the female transition location under internal pressures of 28 MPaand 32 MPa in separate tests.

For the applied pressure of 31 MPa, the circumferential stresses reach amaximum value of 190 MPa which is well below the ultimate stress of 300 MPagiven in Table 2. Typical values vary from 180 MPa to 115 MPa in the uniformpipe, and from 120 MPa to 10 MPa in the joint region. Thus, the circumferentialstresses are less critical that the axial stresses.

The locking keys are in compression circumferentially and a typical value ofhoop stress is –15 MPa.

4.3 Comparison of computed axial stresses with measured dataA sample of the results from experiments performed on the pipe and coil-lock

joint section described above, and reported in [3], are compared with

Fig. 10. Enlarged view of stress concentration (MPa)

corresponding finite element results. Stresses are computed using strain gaugedata for three locations on the joint as follows:

(i) over the polyamide key at 100mm from the female opening end ofthe joint;

(ii) over the O-ring seal at 175mm from the female opening end of thejoint;

(iii) near the transition between the pipe and joint at 200 mm from thesealed end of the joint.

Test results for the critical axial stresses are compared with the finite elementpredictions in Figure 11. Agreement is excellent for the two gauges in the area ofthe tapered joint but poor for the gauge at the transition area. Other comparisonsfor hoop stresses not shown here are excellent for all three locations so the gaugeresults for the axial stresses at the transition location must be suspect.Additionally, the value of axial stress for the transition zone area should exceedthe values in the tapered section and this is not the case for the measurements.Therefore, this limited comparison with test data allows some confidence in thepredictions of the finite element computation.

0

10

20

30

40

50

60

70

80

90

0 100 200 300 400 500

Axial Stress (MPa)

Pre

ssu

re (

bar

) Female (FE)

Female (Test) [3]

Key (FE)Key (Test) [3]

O-Ring (FE)

O-Ring (Test) [3]

Fig. 11. Comparison of experimental and computed axial stresses

5. CONCLUSIONS

Tests on GRE pipeline sections for diameters in the range from 100mm to300mm indicate that the target operating pressure of 70 bar with a safety factorof 4.5 is satisfactorily achieved. The patented coil-lock joint system, developed byAmeron, is shown to be an excellent solution for relatively high pressure GREpipelines. Similarly, the SSL design achieved its desired specification of 150 baroperating pressure and a factor of safety of 2.5.

Extensive finite element calculations on the GRE pipeline section includingthe joint are presented and show good correlation with measured data. Efforts onimproving the performance of the coil-lock joint are continuing such that higherpressures will be possible with future designs.

REFERENCES

1. Rocket Motors Division, Royal Ordnance, British Aerospace.2. Ameron, Coil-Lock Joint, US Patent No. 5,520,4223. Thornhill, K., Curley, L. & Gilchrist, M.D., “Ultimate pressure testing of

joints between sections of filament wound glass fibre reinforced epoxy (GRE)gas pipelines”, 5th International Conference on Deformation and Fracture ofComposites, 18th - 9th March 1999, London, UK.

4. ANSYS Version 5.4, ANSYS Inc. Southpointe, 275 Technology Drive,Canonsburg, PA 15317, USA.

5. Ameron Bondstrand Reference Guide FP 452 C, Ameron Fibreglass PipeDivision, Geldermalsen, The Netherlands, 1990.

ACKNOWLEDGEMENTS

The authors are grateful to the EC Brite-Euram programme for fundingreceived under Contract Number BRE2-CT94-1012 for part of the researchreported here. The support of MCS International and Ameron and theirpermission to publish these results is gratefully acknowledged.