joining composite pipes using hybrid prepreg welding and adhesive bonding

Joining Composite Pipes Using Hybrid Prepreg Welding and Adhesive Bonding

GUOQIANG LI*, DISHILI DAVIS, CARLOS STEWAFU'. JERRY PECK, and SU-SENG PANG

Department of Mechanical Engineering Louisiana State University Baton Rouge, LA 70803

A critical technology for composite piping systems in offshore platforms is the joining technique. This paper discusses the development of a hybrid joining approach by using heat-activated prepreg welding and adhesive bonding. The joining procedure was demonstrated via specimens' fabrication. Four adhesives, with vary- ing mechanical properties, were used to seal the gap between the two pipes. A glass fiber reinforced prepreg was used to wrap the pipes. A total of forty-five specimens were prepared and evaluated using standardized internal pressure tests. A finite element analysis was conducted to aid in the understanding of the mechanisms of the hybrid joining method. Recommendations for further studies were made based on the test and finite element analysis results.

1. INTRODUCTION

eepwater activities are the future for the offshore D oil and gas industries. Large reserves have been discovered in the Gulf of Mexico and off the coasts of West Africa and Brazil. The development of floating production plafforms and vessels challenges the facilities engineer who must consider new materials to meet the stringent topside weight limitations. With the advent of deepwater activity comes a new expanding opportunity for fiber reinforced plastic (FW) composite materials. FRP composite materials have been used as tethers or tendons, production risers, pressure vessels, caissons, and of course, piping systems in the offshore oil and gas industries (1-9). Among these, composite piping systems have been well developed and are generally accepted. It is not difficult to show the corrosion resistance of composite piping, in flow- ing and stagnant seawater, to be in excess of 30 years. Composite piping is approximately one-fourth the weight of carbon steel piping and, other than carbon steel piping, is the least expensive piping material to install (9). The United States Coast Guard has granted "Subchapter Q Type Approval" for the use of composite piping for various services. These include both wet and dry firewall applications in hazardous areas on deepwater offshore oil and gas platforms and floating production storage offloading vessels (FPSOs) operat- ing in the Gulf of Mexico (9). International standards

l o whom correspondence should be addressed.

POLYMER COMPOSITES, DECEMBER 2003, Vol. 24, No. 6

are being developed and more companies are relying on composite-materials specialists to assist in meet- ing the critical topside weight limitations that exist on floating facilities. The growth of the composite piping industry will inevitably be in direct proportion to the growth of deepwater oil and gas activity. The chal- lenge to the composite-pipe manufacturer will be to recognize the special needs of marine piping and to develop innovative new approaches to solve the unique problems presented in a timely manner.

As the composite-pipe manufacturing technique using filament winding has been well developed, joining composite pipes to form piping systems in offshore applications becomes a critical technology. Ideally, a piping system would be designed without joints, since any joints could be a source of weakness and/or excess weight. However, limitations on component size imposed by manufacturing processes, and the require- ments for inspection, accessibility, repair and trans- portation/assembly. mean that some load-canying joints are inevitable in piping systems. Actually, it is estimated that one joint is installed for every four feet of composite piping in offshore plafforms (8).

Currently, there are several approaches for joining composite pipes, including bolted joints, adhesive bonded joints, and butt-weld joints. Bolted joints may be a source of leakage at the gasket. In addition, owing to space limitation, bolted joints may not be the most desirable choice. Adhesive bonded socket joints provide ease of installation and low cost. However, their associated strength is comparatively low and can only be used as secondary bonding. The butt-weld

697

Guoqiang Li, Dishili Davis, Carlos Stewart, Jeny Peck, and Su-Seng Pang

________________________________________--------__------ Flg. 1. Schematic of a heat-activated coupling joint.

technique using the wet lay-up approach results in high strength joints and is widely used in composite piping industries. However, the wet lay-up technique requires more time for the curing of epoxy adhesive and thus lowers joining efficiency.

In order to increase the joining efficiency, prepreg butt-weld composite pipe joints have been investigated recently by Stubblefield et aL (lo) and Mensah et aZ. (1 1). Fgwe 1 shows a schematic of a heat-activated coupling joint. Compared to the wet lay-up process, prepreg weldmg reduces the coupling time from about 24 hours to within 3 hours. In this approach, the prepreg laminate, made of thermosetting resins impregnated with fiberglass reinforcements, is placed over the pipe joint. If heat is applied around the laminate at a temperature of approximately 155"C, the thermoset resins will cure and the laminate shrinks to seal the joints. Although prepreg butt-weld composite pipe joints have the potential to provide the composite market with a user-friendly, portable and cost-effective process for on-site joining service, it is found that the internal pressure rating is not high enough to satisfy the industry requirement of at least 2.07 MPa internal pressure for 200 psig composite pipes. The prepreg butt-weld technique has to be reevaluated to achieve a higher internal pressure rating.

From previous analysis (12), it is found that for butt- weld joints, there is a gap at the bond line between two sections of pipe when subjected to an internal pressure. This gap enlarges as the internal pressure increases and pressurized water is forced into the gap. According to Pascal's Law, the water within this gap will apply the same pressure in axial and radial direction to the pipes and welding layers, resulting in additional interfacial shear and peel stresses. These additional interfacial stresses lead to premature leakage and reduce the capacity to cany internal pressure. Thus, the gap has to be reduced or removed. This can

be done by sealing the gap with an adhesive, i.e., ap- plying a layer of adhesive around the pipe's rim. The layer of adhesive will not only act as a sealant, but also provide a secondary structural bonding, increasing the load-carrying capacity of the joint. This new technique is a hybrid joining approach using prepreg welding and adhesive bonding. This hybrid approach is distinguished from prepreg welding or adhesive bonding in the mode with which it canies the applied load. In the prepreg welding approach, the prepreg layers carry the applied load. In the adhesive bonding approach, the adhesive sealant layer carries the applied load. However, the applied load will be carried by both the prepreg layers and the adhesive sealant layer in the proposed hybrid joining method.

The purpose of this study was to experimentally in- vestigate the proposed hybrid joining technique. Four adhesives and one prepreg were used. Specimens were prepared and standard internal pressure tests were conducted to evaluate the effectiveness of the proposed hybrid joining technique. A finite element analysis was also conducted to aid in the understanding of the joining mechanism of the hybrid joining technique. The feasibility of this new joining technique in offshore piping systems was discussed based on the internal pressure test and !bite element analysis results.

2. TEST PROCEDURES 2.1 Raw Materials

After considering the mechanical properties, materials availability, chemical compatibility, cost, and curing temperature, a glass fabric reinforced crossply phenolic prepreg was used. I t s mechanical properties and curing cycles recommended by the manufacturer are listed in Table 1 . Four types of adhesives were used in this study. The first was a milled glass fiber-

Table 1. PhysicallMechanical Properties of Prepregs.

Curing temperature ( O F ) Curing time (h) Tensile strength (MPa) Elastic modulus (GPa)

275 1.5 330 24.8

698 POLYMER COMPOSITES, DECEMBER 2003, Vol. 24, No. 6

Joining Composite Pipes

filled polyester putty; the second was a two-part epoxy polyamide adhesive; the third was a modified two-part methacrylate adhesive; and the last was a two-part methacrylate adhesive. HereaRer, they are designated A l , A2, A3, and A4, respectively. The physical and mechanical properties for each adhesive are listed in Table 2. The composite pipes were provided and sur- faces prepared by ED0 Specialty Plastics. They were FIBEFBOND@ 2OFW-HV 200 psig pipes. The inner di- ameter was 101.6 mm and the wall thickness was 6.35 mm. They were manufactured using the filament-wound technique by winding E-glass fibers onto epoxy vinyl ester resins with an angle of 54" to the longitudinal axis. The composite pipe was cut into 304.8-mm-long sections and the paint on the surface was removed using a side grinder. The mechanical properties of the pipe are shown in Table 3.

2.2 Preparation of Specimens All pipe joints were prepared using the hybrid join-

ing procedure except for the control joints, which were prepared using prepreg butt-weld only. The hybrid joining procedure was as follows:

1) A thin layer of adhesive or putty was uniformly applied to the rim of the two pieces of pipe. The two sections of pipe were then aligned and put together to form a coupled pipe. Pressure was applied to the coupled pipe along the axial direction to squeeze out the excess adhesive. Each sample was cured at a room temperature for a specified period of time based on the adhesive used. When the adhesive was cured, the coupled pipes were ready to be wrapped with prepreg.

2) A piece of prepreg 152.4 mm wide was cut from a roll. Depending on the number of layers to be used to wrap the pipes, the length of the piece of prepreg was 1.435 m for four wraps, 2.153 m for six wraps, and 2.871 m for eight wraps.

3) The cut pieces of prepreg were wrapped onto the pipes tightly. Care was taken to avoid wrinkles in the prepreg layers.

4) Two layers of polyester shrink tape were wrapped in a spiral pattern onto the prepreg surface. This

was to apply the required pressure during curing and to squeeze out the excess resin.

5) The specimen was moved to a programmable oven. The curing cycle was programmed according to the recommendations by the manufacturer of the prepreg. At the end of the programmed cycle, the specimens were cured and ready for the internal pressure testing.

This procedure is illustrated in Figs. 2-4. Using this procedure, a total of 15 groups of samples were prepared. Each group contained 3 effective samples: thus 45 effective samples were prepared. The details of each group of samples are shown in Table 4. Among them, groups 1-3 were control samples without adhesive. Groups 4-6 were prepared using adhesive Al. Groups 7-9 were prepared using adhesive A2. Groups 10-12 were prepared using adhesive A3. Groups 13- 15 were prepared using adhesive A4. Some of the cured samples with shrink tape layers are shown in Q. 5.

2.3 Internal Pressure Test

The internal pressure tests on the joints were conducted according to ASTM D2992 (Procedure B- Steady pressure). The test setup is shown in Fig. 6. The test data, including internal pressure and time, were recorded by the assembled computer. In this test, the pressure was increased to 2.07 MPa at a rate of 0.14 MPa/sec. The constant pressure of 2.07 MPa was used because this is the pressure required by this study's industrial partner. When the pressure of 2.07 MPa was achieved, it was held constant for 40 minutes. If no leakage or fracture occurred during the 40-minute period, the pressure was increased again at the same rate of 0.14 MPa/sec until a bursting failure of the joint. It is noted that, for some samples, leakage was detected before the internal pressure reached 2.07 MPa. For some other samples, leakage was detected during the 40-minute constant pressure period. All of the test details are summarized into Table 5. In Table 5, the test results are averaged results from three samples.

Table 2. PhysicaVMechanical Properties of Adhesives.

Adhesive A1 A2 A3 A4

Curing temperature ("C) 25 25 25 25 Curing time (min.) 10 2,880 20 15 Tensile strength (MPa) 68 30 22 16 Elastic modulus (MPa) 3,450 2,370 1,000 4,850 Elongation ("/.) 5 2 18 2

Table 3. Mechanical Properties of the Composite Pipes.

Axial tensile strength Hoop tensile strength Axial modulus of elasticity Hoop modulus of elasticity

55 207 9.65 15.17

( M W (MPa) ( G W (GPa)

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Guoqiang Li, Dishili Davis, Carlos Stewart, Jerry Peck, and Su-Seng Pang

internal pressure rating. It is thus inferred that further increasing the wrapping layers would achieve the requirement for the internal pressure rating. However, this would also increase the cost considerably. There- fore, further increasing the welding layers is not a cost-effective choice. Other joining approaches such as the proposed hybrid joining method are worth in- vestigating.

Using the hybrid joining approach, samples with adhesive A2 passed the test. Samples using adhesive A3 also passed the test except for those with four prepreg wraps. Samples with adhesive A4 failed to pass the test except for those with eight prepreg wraps. Samples with adhesive A1 failed to pass the test. This means the adhesive layer plays an important role in achieving the desired internal pressure rating. The adhesive layer not only acts as a sealant to seal the gap, but also provides additional structural bonding. Con- sidering the purpose of using prepreg is to reduce the fabrication time, adhesive A3 is more suitable for the hybrid joining technique than adhesive A2. The reason is that A2 needs a whole day to cure, while A3 needs only 20 minutes. I t is noted that A2 can also be cured in two hours if an elevated temperature of 9OoC

Fig. 2. Adhesiw coupled pipes.

3. RESULTS AND DISCUSSION

From Table 5, the joints using prepreg alone (samples without adhesive) cannot satisfy the requirement of an internal pressure rating of 2.07 MPa. It is also seen that increasing the wrapping layers increases the

Fig. 3. Wrap glass_Fber prepreg layers.

700

Fig. 4. Wrap shrink tapes.

POLYMER COMPOSTES, DECEMBER 2003, Vol. 24, No. 6


Table 4. Details of Each GrouD of Samdes. ~~ ~~

Group No. 1 2 3 Number of prepreg wraps 4 6 8 Adhesive - - - Group No. 4 5 6 Number of prepreg wraps 4 6 8 Adhesive A1 A1 A1

Group No. 7 8 9 Number of prepreg wraps 4 6 8 Adhesive A2 A2 A2



Ffg. 5. Some of the fablicated composite pipe joints.

is used. In such a case, A2 is also a good candidate for the hybrid joining technique because it produces composite pipe joints with a higher internal pressure rating. Therefore, for the four adhesives considered, A2 and A3 are recommended for joining vinyl ester composite pipes using the developed hybrid joining technique.

For samples with adhesive A4, only those with eight layers of prepreg wraps passed the test. The poor performance of A4 is understandable because it has the highest rigidity and the least tensile strength. The high rigidity translates into a hgh stress in the adhesive layer. The combination of high stress and low strength makes A4 perform poorly.

For samples with adhesive A l , none passed the test. The poor performance of A1 cannot be explained Ffg. 6. Internal pressure test set-up.

by stress-&-ength relations because it has the highest tensile strength and a moderate rigidity. The poor performance of A1 comes from its degradation when subjected to high temperature. In this study, 135°C was used to m e the prepreg wraps for 90 minutes. Sub- jected to such a high temperature for such a consider- able period of time, the A1 adhesive was degraded significantly. In fact, it was found that the adhesive layer

was debonded from the pipe ends. The debonding makes the adhesive layer lose its ability to seal the gap between the two sections of pipe. Consequently, the in- temal pressure rating was reduced. In addition, it was found that, when adhesive A1 was used, the prepreg layers were not well cured, as shown in FQ. 7. This

Table 5. Averaged Internal Pressure Test Results.

Number of prepreg Duration to hold 2.07 Bursting pressure Adhesive wraps MPa pressure (min.) (MPa)

- 4 - 1.23 6 - 1.65 8 - 1.86

A1 4 - 0.21 6 - 0.48 8 - 1.03

A2 4 40 2.24 6 40 4.14 8 40 5.17

6 40 2.76 8 40 3.31

A4 4 - 1.72 6 - 1.93 8 40 2.96

- A3 4 5

~~~



Rg. 7.

may be why the internal pressure rating with A1 adhesive was even lower than that without any adhesive (control samples). The reason for the incomplete curing is still unclear at this time. Further investigation is desired.

In order to better understand the joining mechanism using the hybrid joining approach, stress-strain distribution analysis is essential. To this end, a finite

element analysis was conducted. In this analysis, the software package COSMOS/M (version 2.7) was used. Eight-node composite shell element SHELLSL was used to model the composite components, while eight- node shell element SHELL9 was used to model the adhesive. A total of 8,256 elements were used to mesh the joint. The mechanical properties shown in Tab2e-s 1 - 3 were used. The Poisson's ratios were assumed based on the magnitudes of similar materials. An internal pressure of 300 psi (2.07 MPa) was applied throughout the analysis. It is noted that the internal pressure was applied not only to the pipe inner surface in the radial direction, but also to the two end caps in the axial direction, simulating the joint under internal pressure test. Only the cases with four prepreg welding layers were considered.

mure 8 shows the axial stress (psi) distributions in the deformed mode. From &. 8, the axial stress is con',entrated at the adhesive layer. This means the ad ive layer is critical in the whole joint. Once the te. stress surpasses the tensile strength of the adhe-: layer, a cohesive failure mode will occur. An- other possibility is that the interfacial stresses at the pipe/adhesive interface surpass the interfacial bonding strength, resulting in an adhesive failure mode. Generally speaking, the interfacial bonding strength

702

Fig. 8. Axial stress distributions in deformed shape in the joint.



between dissimilar materials is much smaller than the tensile strength of each bulk material. Hence, it is very possible that an adhesive failure mode will occur before a cohesive failure mode. No matter what hap- pens, the adhesive layer will lose its ability to seal the gap and will not provide the required structural bonding. This will inevitably reduce the internal pressure rating of the joint.

From the test results, it is found that the adhesive layer plays an important role in the hybrid joining technique. Therefore, a careful adhesive design should decrease the stress in the adhesive layer while main- taining a reasonable stress level in the prepreg layers. Two major parameters can be considered in designing the adhesive layer: its stiffiess and its thickness.

Rgure 9 shows the effect of the stiffness of the adhesive layer on the maximum stresses in the adhesive layer and the prepreg layers. From Rg. 9, when the modulus of elasticity of the adhesive layer is decreased from about 2,500 MPa to around 1.000 MPa, the stress in the adhesive layer is decreased considerably, particularly the axial stress. In addition, within this range, the stress level in the prepreg layers is still much lower than their corresponding tensile strength. Therefore, the stiffness within this range is ideal. I t is noticed that the stiffnesses of the adhesives A2 and A3 are within this range. This may be why A2 and A3 perform well in the test. For adhesives with hgher stiffnesses, the stress (particular the hoop stress) in the

adhesive layer is in increase almost linearly. Because it is the key to protect the adhesive layer from premature failure, it is not recommended to use adhesives with higher stifhesses, as they increase the stress levels in the adhesive layer. This is validated by the test. From the test, A1 and A4 have higher stiffuesses than A2 and A3; thus A1 and A4 bonded samples failed in lower internal pressure ratings.

From Rg. 10, increasing the adhesive layer thickness can reduce the stresses in the adhesive layer. Al- though this increases the stress in the prepreg layers, the stress level is still acceptable. Therefore, using a slightly thicker adhesive has a positive effect. It is noted that the effect of increasing the adhesive layer thickness is not very significant, particularly for the hoop stress. In fact, it is found that the adhesive layer thickness cannot be increased without limit. The reason is that in order to obtain a uniform adhesive distribution, some axial force needs to be applied. The applied force will squeeze out the excess adhesive, re- ducing the adhesive thickness. From the test results, it is found that the averaged adhesive thickness is

In order for the developed technique to be used in offshore piping systems, further studies are required. These studies may include, but are not limited to, long-term environmental attacks (seawater immer- sion, ultraviolet radiation), creep, fatigue, and other mechanical property tests.

around 0.5 IIUI- 1.0 111111.

18

16 -

14 - h (D g 12 - v

Hoop stress in the adhesive layer Axial stress in the adhesive layer Hoop stress in the prepreg layer \

\ \

0 1 I I I I

0 1000 2000 3000 4000 5000

Modulus of elasticity (MPa)

Q. 9. Effmt of the modulus of elasticity of the adhesive layer on the maximum stresses in the adhesive layer and the prepreg welding layers.


12

10

n m n z 8 Y

.- z z

cn to. Q)

z 6 E

x 4

2

0


/

Hoop stress in adhesive layer Axial stress in adhesive layer Hoop stress in prepreg layers

-\

--- -- -z

0.0 0.5 1 .o 1.5 2.0 2.5

Adhesive layer thickness (mm) FYg. 10. Effkct of the adhesiue layer thickness on the maximum stresses in the adhesive layer and the prepreg welding layers.

4. CONCLUSION adhesives A2 and A3 are recommended for con-

A hybrid joining technique using prepreg welding and adhesive bonding was investigated in this study. Forty-five composite pipe joints were prepared using the proposed hybrid joining technique. Four adhesives were used to evaluate the effect of the adhesive on the internal pressure rating of the joint. A finite element analysis was conducted to understand the joining mechanism of the hybrid joining method. Parameters affecting the internal pressure rating were evaluated using the finite element model. Based on the test results and the finite element analysis results, the fol- lowing preliminary conclusions were obtained:

1) The adhesive layer plays an important role in the hybrid joining technique. Without the adhesive layer, none of the samples satisfies the internal pressure rating requirement. With the adhesive layer, all the samples coupled by adhesive A2 pass the test. With six layers of prepreg wraps, samples coupled by adhesive A3 pass the test. The number of prepreg wraps becomes eight in order for the A4 coupled samples to pass the test. A1 coupled samples fail to pass the test.

2) Considering the internal pressure rating, the cost, and the time required to prepare the samples,

sideration in industry. Adhesive A4 is not recommended because it is too stif€ and its strength is too low. Adhesive A1 is not recommended because it degrades significantly ' at elevated tem- peratures.

3) Using flexible adhesives with moderate strength is recommended for the proposed hybrid joining technique. Increasing the adhesive layer thickness also has a positive effect, but this effect is insignifkant.

4) Further studies are required in order for the proposed joining technique to be used in offshore piping systems.

ACKNOWLEDGMENTS

This study was partially sponsored by the Louisiana Board of Regents, BoRSF, under agreement NASA/ LEQSF (1996-200 1)-LaSPACE-0 1, NASA/LEQSF (200 1-2005)-LaSPACE, NASA/LaSPACE under grant

(2001-04)-RD-B-03, and ED0 Specialty Plastics. Mr. Kirk Wells, Mr. Mark Hale, Mr. Ben McCurry, Mr. Kevin Schmit. and Mr. Randy Jones helped prepare the samples and conduct the tests. Their assistance is greatly appreciated.

NGT5-40 1 15, LEQSF(2000-03)-RD-B-05, LEQSF

704 POLYMER COMPOSITES, DECEMBER 2003, Vol. 24, No. 6


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