Note: Descriptions are shown in the official language in which they were submitted.
CA 02299644 2001-11-26
1 FIELD OF THE INVENTION
2 The present invention relates to a hybrid structure comprising an inner
3 metal liner (selected from the group consisting of pipes and pressure
vessels)
4 wrapped with and bonded to an outer layer of fiber-reinforced, cured polymer
resin matrix composite. In another aspect the invention relates to a method
6 for producing such a structure.
7
8 BACKGROUND OF THE INVENTION
9 It is known to reinforce a steel liner, such as a pipe or vessel, by
wrapping it with a layer of fiber-reinforced, cured polymer resin matrix
11 composite. The product can be referred to as a 'hybrid structure'.
12 The fibers can be selected from the group consisting of glass, carbon,
13 graphite or aramid. The polymer resin can be selected from the group
14 consisting of epoxy, vinylester, polyester, peek, nylon and polyethlene.
The
preferred combination is glass or carbon fibers in an epoxy resin matrix.
16 The hybrid structure can be formed in either of the following ways.
17 Layers of partially cured, pre-impregnated, fiber-reinforced tape or
18 sheet can be sequentially applied to the liner. The innermost layer is
bonded
19 to the steel with structural adhesive. The layers are bonded to each other
by
a curing resin, positioned between them, which interacts chemically with the
21 layer resin. This technique is disclosed in my published Canadian patent
22 application No. 2,181,497.
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CA 02299644 2001-11-26
1 An alternative technique involves drawing rovings of fiber through a
2 liquid bath of resin and winding the resin-coated fibers onto the liner to
form a
3 wrapping. The wrapping is then cured in place. This technique is described,
4 for example, in U.S. Patent 4,559,974, issued to Fawley.
SUMMARY OF THE INVENTION
6 The present invention is based on the following discovery:
7 ~ if a hybrid structure, comprising an inner metal liner, preferably
8 steel, and an outer wrapping of layers of fiber-reinforced, cured
9 polymer resin matrix composite (non-metallic), bonded to the liner
and arranged in an angle-ply pattern, is internally pressurized, to
11 cause the steel of the liner to yield while the composite remains
12 elastic, and then is de-pressurized;
13 ~ it will be found that, in the de-pressurized state, the steel will have a
14 compressive residual stress while the composite wrapping will have
a tensile residual stress;
16 ~ with the result that, when subsequently re-pressurized, the elastic
17 regime (that is, the linear part of the pressure versus strain curve) of
18 the steel liner is extended and the liner will not yield at the previous
19 yield pressure. Thus, it takes greater pressure to burst the steel
liner.
21 This means that a thinner walled pipe or vessel, when wrapped and
22 treated as described, can operate as safely in pressure service as a
thicker
23 walled pipe or vessel formed of the same steel. Otherwise stated, a lighter
24 pipe or vessel can be modified to achieve the same pressure rating as a
heavier pipe or vessel.
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1 By way of further explanation, if one takes a hybrid pipe or pressure
2 vessel prepared as described and subjects it to increasing internal
3 pressurization, and plots a pressure versus strain curve, the following will
be
4 noted:
~ The curve will ascend linearly and with a steep slope, characteristic
6 of the stiffness of the steel alone, until the point at which the steel
7 yields. Over this interval, both the steel and composite undergo
8 elastic deformation;
9 ~ After the yield point of the steel, the curve continues ascending
generally linearly, but with a lesser slope, characteristic of the
11 composite. Over this interval, there is plastic deformation of the
12 liner and elastic deformation of the composite. The steel now
13 deforms more rapidly;
14 ~ When the pressure is released, the composite wants to shrink back
to its original diameter. However, the liner is now permanently
16 deformed and can only shrink back to a diameter greater than its
17 original diameter. As a result, the composite now is in tension and
18 the steel is in compression.
19 When the so treated hybrid pipe or vessel is again internally pressurized
and
the pressure versus strain is plotted, a linear (elastic) response curve is
21 produced up to the previously applied pressure.
4
CA 02299644 2001-11-26
1 By applying the technique of the invention, the fiber reinforced
2 composite shares a substantial part of the applied load. In addition, the
3 fatigue resistance of the hybrid pipe has been found to be improved because
4 for a part of the operating pressure cycle the stress in the steel liner
remains
compressive. Because of these features, a lighter hybrid pipe treated in
6 accordance with the invention has better mechanical performance than a
7 hybrid pipe that has not been so treated or the liner alone.
8 In a preferred feature, the angle-ply wrapping is provided in the form of
9 a stack of sheets or layers of fiber-reinforced polymer resin matrix. The
sheets are wrapped about the liner in what is referred to as an angle ply
11 pattern. That is, one sheet or ply is wrapped with its fibers at an angle
of +x°
12 relative to the axis of the pipe and the next ply is wrapped with its
fibers at an
13 angle of -~f' relative to the axis. Most preferably the fibers of the
sheets of the
14 stack are arranged on the basis of +8°, -8°, +A°, -
0°... The phrase "angle ply
pattern" is intended to cover these variants. It is to be understood that the
16 word "wrapping" is to be given a broad connotation. It denotes the cured
17 covering, which may have involved winding sheets, tapes or fibers onto the
18 liner.
19 The stack is bonded to the liner and the sheets are bonded one to
another by curing the stack after it has been wrapped on the liner.
21 In one aspect the invention is directed to a fiber-reinforced hybrid
22 structure comprising: an inner metal liner selected from the group
consisting
23 of pipes and vessels; and an outer wrapping of fiber-reinforced, cured
polymer
24 resin matrix composite, said wrapping being wrapped around the liner in an
angle-ply pattern and bonded thereto; said structure having been internally
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CA 02299644 2001-11-26
1 pressurized, to cause the metal of the liner to yield while the composite
layer
2 remained elastic and then de-pressurized so that the liner has compressive
3 residual stress and the composite layer has tensile residual stress after de-
4 pressurization.
In another aspect the invention is directed to a method for making a
6 reinforced hybrid structure comprising: providing a metal liner selected
from
7 the group consisting of pipes and vessels; wrapping the liner with a
plurality of
8 plies of uncured or partly-cured fiber-reinforced polymer resin composite
9 arranged in an angle-ply pattern; curing the plies and bonding them to the
liner and to one another to produce a wrapped liner; internally pressurizing
the
11 wrapped liner to cause the steel of the liner to yield while the composite
12 remains elastic; and then de-pressurizing the wrapped liner to produce a
13 reinforced hybrid structure in which the liner has compressive residual
stress
14 and the composite has tensile residual stress.
DESCRIPTION OF THE DRAWINGS
16 Figure 1 is a set of stress-strain curves based on the results of
17 Example 1;
18 Figure 2 is a sectional side view of the test assembly used to carry out
19 the test of Example 1;
Figure 3 is a set of stress-strain curves based on the results of test run
21 #1 of Example 2;
22 Figure 4 is a set of stress-strain curves based on the results of test run
23 #2 of Example 2;
24 Figure 5 is a curve showing the cyclic pressure spectrum of test run #3
of Example 2;
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CA 02299644 2001-11-26
1 Figure 6 is a set of curves showing the slope of ~P/~s for three runs
2 reported in Example 2;
3 Figure 7 is a set of stress-strain curves based on the results of
4 Example 3; and
Figure 8 is a perspective, partly cut-away view of a liner wrapped with a
6 plurality of layers or plies in an angle-ply pattern.
7 As shown in Figure 8, the hybrid structure 1, shown as a wrapped pipe,
8 comprises a steel liner 2 wrapped in a stack 3 of sheets 4 of cured epoxy
9 resin containing reinforcing fibers 5 of glass, the sheets 4 being arranged
in
an angle-ply pattern.
11
12 DESCRIPTION OF THE PREFERRED EMBODIMENT
13 The invention is illustrated by the following examples.
14 EXAMPLE I
A longitudinal seamed steel pipe having an inside diameter of 305 mm
16 (12 inch) and 1.21 mm (0.0476 inch) wall thickness was used in this test.
The
17 yield stress of the pipe steel was known to be 180 MPa (26,000 psi).
18 According to accepted design practice, the maximum operation pressure
19 (MOP) of this pipe was 0.714 MPa (104 psi). The MOP equates with half of
the expected yield pressure.
21 The pipe was prepared by sand blasting its surface and washing it with
22 acetone, to clean it.
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1 Multiple (14) layers of partly cured ("pre-preg") fiber glass - reinforced
2 epoxy composite were cut from a pre-preg sheet roll, available from 3M under
3 the designation 3M-type 1003 E-glass fiber/epoxy resin. The layers were cut
4 using a template and sharp scalpel blade. The templates were made from a
high quality metal sheet to length and shape which depend on the pipe
6 dimensions (external diameter and length) and fiber direction wrap angle.
7 The procedure was as follows.
8 The pre-preg roll was laid on a flat surface working table, which was
9 overlaid with a plastic sheet on which two longitudinal guides were fixed.
All
surfaces were thoroughly cleaned by acetone and wiped out. The pre-preg
11 sheet roll was laid on the table and cut using the template and scalpel
blade.
12 Individual layers thus cut were properly positioned relative to the guide
with
13 the pre-preg backing paper facing the top, and the lower surface lightly
14 sticking on the plastic overlay; and using a flat heavy block, trapped air
was
squeezed out by moving the block on the backing paper. Following this, the
16 backing paper was gently peeled off from the end closer to the pipe on to
17 which it was to be wrapped.
18 The steel pipe liner was then rolled along a guide as the peeling
19 proceeded. The pre-preg stuck on the pipe surface and the first layer was
thus wrapped on the steel liner. The hybrid pipe was then rolled several times
21 on the table while applying small pressure to ensure that no air remained
22 trapped in between the layer and the steel liner. Prior to applying the
next
23 layer, the table surface and the guide were thoroughly cleaned with
acetone,
24 and the layer was applied in an alternating angle ply pattern. In this
example,
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1 the ply angles were ~ 70° relative to the axis of the pipe. Fourteen
layers
2 were wrapped in the above described manner.
3 A nylon release peel ply-fabric (E4760 Non-perforated, available from
4 Northern Fiber Glass Sales Inc., Alberta) was then wrapped on the exterior
surface of the pipe. Finally, a shrink tape (Oriented Polyester Tape) was
6 wrapped on the fabric. The two extremities of the pipe were then sealed by
7 using a high temperature tape.
8 The pipe thus wrapped was then placed in an oven and heated to
9 149°C (300°F) for 12 hours to cure the composite and
consolidate the pre-
preg layers. Upon cooling, the shrink tape and release peel ply fabric were
11 removed. The final thickness of the wrapped glass fiber epoxy resin layers
12 was 3.5 mm (0.138 inch).
13 The cured wrapped pipe was subsequently placed in a device to apply
14 an internal pressure. This device consisted of an inner thick-walled
cylinder,
two end flanges and a rubber bladder. The space between the thick-walled
16 cylinder and the rubber bladder was filled with oil and then pressurized.
The
17 thick-cylinder was attached to the end flanges and sealed by o-rings so no
oil
18 could penetrate inside.
19 The pressurized rubber bladder transmitted pressure to the inner wall
of the hybrid pipe. The axial pressure force on the two flanges was
21 transferred directly to the inner thick-walled cylinder. In this manner,
the
22 hybrid pipe was subjected to a pure hoop stress with negligible axial
stress.
23 The pipe was instrumented by placing a strain gauge in the hoop direction
at
24 its mid length.
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1 The hybrid pipe was tested by gradually increasing the internal
2 pressure. As shown in Figure 1, curve a, all three wrapped pipe samples
3 yielded at a pressure of about 2.1 MPa (300 psi). At this pressure the slope
of
4 the curve changed as the fiber glass wrap carried a larger proportion of the
pressure load. The pressure was increased to about 6.2 MPa (900 psi) after
6 which the pipe was de-pressurized to about 0.35 MPA (50 psi). Note that at
7 this pressure there exists a residual strain of about 0.2 percent, that is,
the
8 inner steel liner is plastically deformed (permanent deformation).
9 The pipe was then again pressurized and as shown by curve b, no
yielding was observed up to 4.1 MPa (600 psi) and no failure (burst) occurred
11 when the pressure was increased as high as 14.5 MPa (2100 psi). Note that
12 Figure 1 shows the data points for three samples with negligible deviations
13 from one test to another. Table 1 summarizes the improved performance of
14 the hybrid pipe, relative to a non-reinforced steel pipe. Due to glass
fiber
epoxy resin reinforcement and the described procedure of inducing residual
16 compressive stresses in the steel pipe, the yield pressure of the hybrid
pipe
17 was increased by about three folds and the burst pressure by more than six
18 times.
19 TABLE 1
a) THIN-WALLED STEEL PIPE
Internal Diameter 305 mm (12 inches)
Longitudinal Seamed Steel Pipe Thickness1.21 mm (0.0476 inch)
Weight per Unit Length 9.08 kg/m (6.101b/foot)
Yield Pressure 1.45 MPa (210 psi)
21
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1 TABLE 2
2 HYBRID STEELIGLASS FIBER REINFORCED PIPE
Internal Diameter 305 mm (12 inches)
Seamed Steel Pipe Thickness 1.21 mm (0.0476 inch)
Composite Wrap Thickness 3.5 mm (0.138 inch)
Weight per Unit Length 15.3 kg/m (10.3 Ib/foot)
Yield Pressure 4.1 MPa (600 psi)
Failure Pressure >14.5 MPa (2,100 psi)
3
4 EXAMPLE 2
A standard 4 inch gas pipeline (NPS-4) designed according to present
6 codes for a maximum operating pressure (MOP) of 8,450 kPa (1,225 psi) will
7 result in a pipe having the following dimensions: the nominal inside
diameter
8 with be 102 mm (4 in.) and the wall thickness will be 6 mm (0.24 in.) when
the
9 pipe is made of grade 241 steel (ASTM A333 grade G seamless).
Test Pipe
11 A test pipe, formed of grade 241 steel and having 102 mm (4 in.) inside
12 diameter and wall thickness of 3 mm (0.12 in.) was welded to a flange in
both
13 ends as shown in drawing Figure 2. The pipe was then wrapped with 12
14 layers of 3M-type 1003 E-glass/epoxy resin and cured in accordance with the
procedure of Example 1, to provide a glass-reinforced composite wrap having
16 a thickness of 3 mm (0.12 in.).
17 The resulting hybrid pipe was then internally hydraulically pressurized.
18 (Note that an internal thick-walled pipe is inserted inside the hybrid
pipe,
19 similar to Example 1, to take the axial pressure load.)
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1 Test Run #1
2 The hybrid pipe was tested by gradually increasing pressure, as shown
3 in Figure 3. The steel liner yielded at about 9,522 kPa (1,380 psi). The
4 average slope of ~P/~s in the elastic range was 16,700 MPa (2.42x106 psi)
(average value from two strain gauge readings). The pressure was then
6 increased to 19,320 kPa (2,800 psi) and held for 15 minutes with no creep
7 being observed. The pressure was finally increased to 22,080 kPa (3,200
8 psi), that is by a factor of more than 2.3 times the yield pressure of the
steel
9 liner.
The pressure was then bled off to zero. Note that there was a residual
11 strain of about 0.14%.
12 Test Run #2
13 The hybrid pipe was then again subjected to increasing internal
14 pressure. As seen in Figure 4, the yield pressure this time was about
22,000
kPa (3,190 psi) close to the previous maximum pressure. This increased
16 yield pressure was due to the compressive residual stresses induced in the
17 first loading, Figure 3. The average slope of OP/DE in the elastic range
was
18 17,250 MPa (2.5x106 psi). At a pressure of 30,950 kPa, that is, 3.66 times
its
19 maximum operating pressure, the hybrid pipe still did not burst. A steel
pipe
of the same thickness, that is, NPS-4 pipe would have already burst.
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1 Test Run #3
2 Furthermore, the hybrid pipe was subjected to a block of cyclic
3 pressure spectrum as shown in Figure 5. This block containing 31 cycles with
4 a maximum pressure of 6,200 kPa (900psi) was extracted from the recorded
pipeline pressure fluctuation spectrum. The test was run for 13, 200 blocks,
6 equivalent to 200 years of operation. The slope of OP/DE was measured at
7 several time intervals; it remained almost constant and was about 16,600 kPa
8 (2.4x106 psi).
9 Figure 6 shows the slope of OP/Os for the three different test histories,
showing a constant value indicative of no damage being sustained by the
11 hybrid pipe.
12 EXAMPLE 3
13 A hybrid pipe prepared as in EXAMPLE 1, was subjected to different
14 pressure histories as follows:
Test Run #1
16 The hydraulic fluid in the pipe was pressurized gradually up to 11,850
17 kPa (1,720 psi) held for 15 minutes and de-pressurized. Neither creep
strain
18 at the hold pressure nor residual strain at the zero pressure was observed.
19 The pipe was again pressurized to 16,500 kPa (2,390 psi), and de-
pressurized. Yielding of steel liner was observed at about 15,000 kPa (2,175
21 psi) and the residual strain was less than 0.05% as shown in Figure 7. The
22 final step was to pressurized the hybrid pipe to a maximum pressure of
23 19,500 kPa (2,830 psi), that is, 2.3 times its maximum operating pressure.
24 Figure 7 shows the pressure versus hop strain for the above pressurization
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CA 02299644 2001-11-26
1 history. The average slope of OP/DE in the elastic range was 17,800 MPa
2 (2.58x106 psi).
3 Test Run #2
4 The hybrid pipe of test run #1 (Example 3) was connected to a main
gas line loop for 42 days. The pipe was thus subjected to the actual pressure
6 loading in a gas transport main line, as well as pulsation tests. The latter
7 consisted of a week (4-5 hours/day) at a line pressure of about 5,000 kPa
8 (725 psi) with a frequency of 10-50 Hz and a peak pulsation increment of 100
9 kPa (15 psi). After 42 days of testing program described above, the hybrid
pipe stiffness was measured, OP/Os=17,150MPa (2.49x106 psi).
11 It is seen from this example that the hybrid pipe performance is quite
12 remarkable under both laboratory tests Run #1 and actual field tests, Run
#2.
13 The stiffness measurement also indicates that the pipe did not sustain any
14 damage.
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