Note: Descriptions are shown in the official language in which they were submitted.
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CA 02286956 1999-10-18
llBm vanlSYSICLIENTS11P00110075 calspec final 990603.wpd
STEEL-COMPOSITE PIPE AND METHOD OF MANUFACTURING SAME
s Field of the Invention
The present patent application relates generally to
pipelines and specifically to a pipe for a high pressure gas
pipeline and a method of manufacturing the pipe, the materials
io and dimensions of which are optimally selected.
Background Of The Invention~
Pipelines for transmitting natural gas and other fluids
i5 typically span many hundreds of kilometres of terrain. These
pipelines tend to be pressurized at high pressures; some known
pipelines are designed to operate around 1750 psi. Pipes that
are connected together to form such pipelines are typically
made from a high-strength steel so that the pipeline can
2o withstand stresses caused by high internal pressures and
resist adverse external environmental conditions.
To protect the exterior of the pipeline and to provide
additional circumferential strength, high-pressure pipelines
2s have been known to be wrapped with a covering such as a wire
winding or a flexible non-metallic sheet. Such coverings also
serve to arrest propagation of cracks that originate from
1
CA 02286956 1999-10-18
..
failures in the pipe wall. For example, U.S. patent no.
4,559,974 (Fawley) discloses a plurality of circumferential
bands of unidirectional non-metallic fibres that wrap around
a conventional steel pipe. The patent states that the
plurality of bands are intended in part to equalize the
circumferential strength of the pipe with its longitudinal
strength so that the pipe can bear an increased stress before
failing. However, the patent does not appear to disclose any
particulars about the amount of increased stress that can be
io safely borne for a particular amount of band material and for
a particular selection of pipe parameters; without such
particulars, increasing the stress level beyond specified
maximum operating levels would be hazardous. Further,
covering-wrapped pipes such as those disclosed in Fawley tend
i5 to be costlier to manufacture than non-wrapped pipes having a
comparable steel layer thickness and operating at a comparable
pressure. Further, the covering adds undesirable weight to
the underlying pipe, making pipeline construction more
difficult and more costly.
It is apparent that a covering that provides some
circumferential strength to the pipe structure will permit a
covering-wrapped pipeline to withstand some increase in its
internal operating pressure. Increasing the internal
2s operating pressure will increase the rate of gas transmittable
by the pipeline, thereby increasing the revenue that can be
generated. Therefore, it is a desirable objective to
2
CA 02286956 1999-10-18
..
construct a covering-wrapped pipeline that enjoys the benefits
of a covering and whose increased construction costs are
sufficiently offset by an increased rate of fluid transmission
so that the covering-wrapped pipeline is as or more profitable
than a comparable non-covered pipeline.
Alternatively, the cost of construction of the covering-
wrapped pipeline may be offset by utilizing the
circumferential strength of the covering in place of some of
io the circumferential strength provided by the steel, so that
the amount of steel used in the covering-wrapped pipeline can
be reduced relative to an all-steel pipeline operating at a
comparable pressure. This reduction can in some cases reduce
the weight and material cost of the covering-wrapped pipe.
No known method has been devised to accomplish optimally
the above objectives. That is, there is no known method to
select the materials, dimensions, and operating parameters of
the covering and pipe portions of the covering-wrapped pipe so
2o that the strength and other properties of the materials are
fully utilized, thereby optimizing the fluid transmission
capacity of the covered pipeline relative to its cost of
construction and operation.
2s The present invention overcomes some of the shortcomings
of the prior technology and achieves further advantages that
will be apparent after reviewing the following summary and
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CA 02286956 1999-10-18
detailed description of the invention.
Summary Of The Invention:
s The present invention in one aspect is a pipe for use in
high pressure fluid-bearing pipelines, wherein the pipe has a
wall with an inner steel layer and an outer composite layer
continuously wrapped around the steel layer. The outer
composite layer provides exterior protection for the pipe and
io provides circumferential strength to the pipe. The dimensions
and other significant parameters of the layers are selected in
accordance with criteria to be discussed further below.
In another aspect, the invention is a method of
i5 manufacturing a pipe with optimal selection of materials,
dimensions, and other significant parameters of the pipe.
The parameters to be selected for the layers forming the
pipe of the invention depend upon the internal cross-sectional
2o area and maximum operating pressure of the pipe. These
selected area and pressure values relate to the transmission
capability of the pipeline and are usually specified by the
customer.
2s Then, a suitable grade of steel is selected for the steel
layer and a suitable composite is selected for the composite
layer, in accordance with steps of the invention to be
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CA 02286956 1999-10-18
discussed below. High-strength steels typically used in
conventional all-steel pipelines are suitable choices for the
steel layer. A composite outer layer that provides
circumferential strength, protection from impacts, and some
s ability to arrest crack propagation in the steel layer is a
suitable choice for the composite layer.
Then, suitable steel and composite design operating
stresses are selected for the steel and composite layers,
io respectively. Preferably, the steel design operating stress
should not exceed a value that would cause fatigue failure of
the steel layer under normal pressure fluctuations.
Preferably, the composite design operating stress is selected
to be a safe operating percentage of the composite's ultimate
i5 tensile strength that is consistent with industry practice.
While it is possible to choose design operating stress values
that are greater than the foregoing preferred values, there is
an obvious risk of failure if such values are chosen. While
it is possible to choose design operating stress values that
2o are significantly lower than the preferred values, such choice
will not optimally use the strength of the materials, thus
entailing higher manufacturing cost than may be acceptable to
the customer.
2s Then, a preferred range of steel and composite layer
thickness combinations are determined as those thickness
combinations wherein the steel and composite layers are
CA 02286956 1999-10-18
stressed at their respective design operating stresses when
the steel-composite pipe is at the maximum operating pressure.
The minimum preferred thickness of the steel layer in this
range is determined to be the minimum thickness at which the
s steel layer only is able to withstand pressurization at the
maximum operating pressure. However, in certain
circumstances, a thinner minimum thickness is suitable, for
example, in situations where there is little or no risk that
the composite layer will suffer complete failure. The minimum
to preferred thickness of the composite layer in this range is
the minimum thickness that is able to provide suitable
circumferential strength, toughness against impacts, and
resistance against crack propagation in the steel layer.
i5 Other acceptable layer thickness combinations other than
the preferred combinations may be selected in accordance with
the invention that will provide some advantages of the
invention; however, these other acceptable layer thickness
combinations do not optimally utilize the strength and other
2o properties of the steel and composite and are therefore not
preferred. The acceptable-but-not-preferred thickness
combinations may be any combination that has steel and
composite layers that are above the respective minimum
thickness values, above the respective thicknesses in each of
2s the preferred thickness combinations, and whose combined per
unit length mass is less than the per unit length mass of a
comparable all-steel pipe, i.e. a steel pipe made of the same
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CA 02286956 1999-10-18
grade of steel, having the same internal diameter and
operating at the same maximum operating pressure.
Because the composite layer is continuously wrapped
s around the steel layer, the two layers will behave as one from
a strain standpoint; the layers will strain together at the
same rate and degree . As the steel has an elastic modulus
that is substantially higher than the composite, the steel
will bear a substantially higher stress than the composite at
io a particular strain. In order for the steel and composite
layers having a selected thickness combination to be at their
respective design operating stresses when the pipe is at the
maximum operating pressure, the stress distribution must be
appropriately distributed between the steel and composite.
15 The stress distribution between the steel and composite layers
may be selectively changed by plastically deforming the steel
by a selected amount.
The steel layer is plastically deformed by autofrettaging
2o the steel-composite pipe at a selected autofrettage pressure.
Autofrettage testing is the application of hydrostatic
pressure wherein the steel layer is stressed beyond its
specified minimum yield stress. An autofrettage pressure is
selected that will effect the appropriate amount of plastic
2s deformation; this selected autofrettage pressure is determined
by mathematically modelling the stress-strain curves of the
steel and composite layers respectively, and the pressure-
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CA 02286956 1999-10-18
strain curve of the steel-composite pipe.
Then, the pipe is manufactured for the most part in
accordance with conventional composite/steel pipe
s manufacturing practice by manufacturing a circular tubular
steel inner layer whose strength and dimensions are as
determined by the applicable constraints, and then applying to
that steel layer an outer composite layer whose strength and
dimensions are as determined by the applicable constraints.
io The general nature of the constraints has been discussed above
and will be elaborated below. Then, the composite/steel pipe
is autofrettaged at the selected autofrettage pressure so that
an appropriate amount of plastic strain is effected to
distribute the stresses between the steel and composite layers
i5 so that the steel layer and the composite layer are at their
respective design operating stresses when the pipe is at the
selected operating pressure.
Summary of the Drawings:
A detailed description of the preferred embodiments is
provided herein below with reference to the accompanying
drawings, in which:
2s Figure 1 is a schematic three-dimensional view of a pipe
according to an embodiment of the invention;
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Figure 2 is a stress/strain graph for E-Glass type composite;
Figure 3 is a stress/strain graph for X-80 steel;
Figure 4 is a graph of acceptable composite and steel layer
thickness combinations for a 48~~ inner diameter pipe having an
X-80 steel layer and an E-glass composite layer and operating
at a pressure of 2200 psi.
io Figure 5 is a stress/strain curve of X-80 steel and E-glass
composite, superimposed with a pressure-strain curve of the
same, the three curves being for a 48 inch diameter pipe
operating at 2200 psi, 0.80 steel design stress percentage,
0.661 inch thick steel layer, and 0.160 inch thick composite
i5 layer;
Figure 6 is a stress/strain curve of X-80 steel and E-glass
composite, superimposed with a pressure-strain curve, the
three curves being for a 48 inch diameter pipe operating at
20 2200 psi, 0.80 steel design stress percentage, 0.723 inch
thick steel layer, and 0.100 inch thick composite layer.
Detailed Description
25 A pipe 10 in accordance with the invention is shown
generally in Figure 1. The pipe 10 is a circular cylindrical
pipe section of a pipeline used for transmitting gas or other
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CA 02286956 1999-10-18
fluids under high pressures. The following discussion relates
to gas pipelines, but it is understood that the invention also
applies to pipelines bearing other fluids, such as oil. The
wall of the pipe 10 comprises an inner steel layer 12 and an
s outer composite layer 14 that is attached to and covers the
steel layer 12. The steel layer 12 primarily provides
circumferential, radial and axial strength to the pipe 10; the
composite layer 14 primarily provides circumferential and
radial strength, fracture propagation arrest capability, and
io protection against external environmental conditions. The
materials and dimensions of the wall layers of the pipe 10 are
selected in accordance with a method of the invention,
described in detail below, so that the steel and composite
portions are optimally utilized to achieve the above described
i5 purposes and the steel-composite pipe is more cost effective
and lighter than a comparable all-steel pipe. Optimal
utilization means that for given operating conditions and
selected materials, the dimensions of the pipe, including the
steel and composite layer thicknesses, are selected so that
2o the strength of the steel and composite are fully utilized,
i.e. at maximum operating conditions, the steel and composite
layers are at their respective maximum design operating
stresses.
2s According to the preferred method of the invention, the
materials and dimensions of the pipe 10 are optimally selected
in accordance with a given internal cross-sectional area and
CA 02286956 1999-10-18
a maximum internal operating pressure of the pipe 10. These
values are usually customer-specified, as they relate to the
transmission capacity of the pipeline. Typical pipeline
diameters in conventional mainline transmission high pressure
s steel pipelines vary, the most common diameters being less
than or equal to 48 inches. A typical operating pressure of
such pipelines is 1400 psi, although some high-strength
pipelines have been known to transmit gas at operating
pressures in excess of 1700 psi. To illustrate the method of
io the invention, the pipe materials and dimensions selected
below are based on a maximum operating pressure of 2200 psi
and an inner diameter of 48". However, the method of the
invention may be applied to optimally select the materials and
dimensions of a pipe 10 having any pipe diameter and operating
i5 pressure typically selected for conventional high pressure
pipes and still achieve the advantages of the invention.
Further, operating pressures beyond typical operating
pressures in conventional all-steel pipelines can be achieved
by the pipe 10, as will be described in further detail below.
A number of grades of steel are typically used for
conventional high pressure steel pipelines, including X-70 and
X-80 steel. The next step of the preferred method of the
invention is to select an appropriate grade of steel for the
2s steel layer 12 of the pipe 10. For illustration, X-80 grade
steel having a specified minimum yield strength (SMYS) of 80
ksi and a specified minimum ultimate tensile strength (SMUTS)
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CA 02286956 1999-10-18
of 90 ksi is selected, it being understood that any grade of
steel typically used for high pressure pipelines is a suitable
selection for the steel layer of the pipe.
s Grades of steel with higher yield strengths than X-80 are
typically not used in conventional all steel high-pressure
pipelines because the increased yield strength provided by
these steels aver X-80, is not accompanied by a proportionate
increase in fracture toughness. However, as the composite
io layer of the pipe 10 provides some fracture toughness in
conjunction with that provided by the steel layer, it is
possible to use such higher grades of steel in addition to
grades conventionally used in high-pressure pipelines for the
pipe 10, provided that the dimensions of the composite and
i5 steel wall layers are appropriately selected to provided
adequate strength and fracture toughness.
The maximum desired stress in the pipe wall when the pipe
is pressurized at the maximum operating pressure is known
2o as the design operating stress. For conventional all-steel
pipes, the pipe is typically designed so that the
circumferential and axial stresses in the pipe wall when the
pipe 10 is pressurized at the maximum operating pressure are
at a level that is less than or equal to a specified design
2s stress percentage (SDSP) of the yield strength of the steel.
The SDSP provides a °safety factor" for the safe operation of
the pipe at the design operating conditions. For high
12
CA 02286956 1999-10-18
pressure pipelines, the Canadian Standards Association (CSA)
requires that steel pipes be designed with a SDSP that
typically does not exceed 80% of the steel's SMYS. However,
this value depends on a number of factors including the
s proximity of the pipeline to inhabited areas.
As there does not appear to be any applicable regulation
for composite wrapped steel pipes, the steel design operating
stress (circumferential) of pipe 10 is selected to be 80% of
io the steel's SMYS in accordance with current CSA standards for
all-steel pipes, so that for X-80 steel having a SMYS of
80,000 psi, the steel design operating (circumferential)
stress is 64,000 psi. However, the current North American
pipe standards do not take into consideration the additional
i5 strength provided by the composite layer, and therefore, such
a safety factor is probably overly conservative. Preferably,
the steel design operating stress should be defined as the
maximum stress that would not cause a fatigue induced failure
when the pipe is pressurized at the maximum operating pressure
2o and subjected to typical pressure fluctuations in gas
transmission lines.
In accordance with typical industry standards, the pipe
should not be pressurized beyond its maximum operating
2s pressure for a prolonged period of time, and should not at any
time exceed a pressure that would stress the steel layer
beyond 110% of its specified minimum yield stress ("maximum
13
CA 02286956 1999-10-18
allowable overpressure").
Preferably, E-glass fibre composite is selected as the
material for the composite layer 14 of the pipe 10 wall. The
s composite comprises a plurality of lightweight high-strength
unidirectional E-glass fibres encased in a polyester or epoxy
resin matrix. The composite is wrapped around the steel layer
12 so that the unidirectional fibres are aligned
circumferentially around the pipe 10. Depending on the
l.o selection of the type of resin, the composite may provide the
pipe 10 with some corrosion resistance. Alternatively,
corrosion resistance may be provided by applying a
conventional layer of fusion-bond epoxy onto the pipe 10.
Although E-glass is selected as the preferred material for the
i5 composite layer 14 of the pipe 10, other materials may be
suitably substituted if they are cost-effective, can bear
substantial circumferential stress, and can provide some
protection against crack formation and propagation. Such
alternative materials includes a composite manufactured by
2o Owens Corning and sold under the commercial trade-name
Advantex, which offers superior resistance to environmental
degradation than standard E-glass.
As shown in Figure 2, the E-Glass composite is a brittle
2s material that has an elastic modulus of 6x106psi. It has an
ultimate strength of 164 ksi. The ultimate strength
represents the highest short term stress that the composite
14
CA 02286956 1999-10-18
can withstand before failing. Typical industry standards
recommend that the maximum design operating stress of a
composite should not exceed 40% of the composite's ultimate
strength. It has been found that the composite maximum design
operating stress also represents the highest repeated cyclic
stresses that the composite can withstand without suffering
fatigue failure. In this connection, to fully utilize the
strength of the composite, the magnitude of circumferential
stress in the composite wall layer of the pipe 10 should be
to close to the composite design operating stress when the pipe
is at the maximum operating pressure, and should be at a
stress below the composite's ultimate tensile strength when
the pipe 10 is temporarily pressurized beyond the maximum
operating pressure.
The composite is able to provide some protection against
propagation of fractures occurring in the steel layer in a
longitudinal direction of the pipe 10. In both long-seam weld
and spiral weld pipes subjected to high circumferential
2o stresses, failure is typically observed in the form of a small
longitudinal fracture that if left unchecked, quickly
propagates into long ductile longitudinal fractures. The
ability of the composite to impede propagation of a fracture
in the steel layer is related to the amount of energy that can
be absorbed by the composite layer before failing. This
energy can be estimated by determining the amount of energy
required to cause fracture of a given cross-sectional area of
CA 02286956 1999-10-18
the composite fibres.
The E-glass composite does not offer much strength in the
axial direction of the pipe 10. The unidirectional fibres
provide only tensile strength around the circumference of the
pipe wall, and the resin matrix does not offer significant
strength in any direction. Therefore, the steel layer 12 must
be thick enough to withstand axial stresses caused by
prolonged pressurization at the maximum operating pressure,
l.o and temporary pressurization at the maximum allowable
overpresssure. In this connection, the axial stress in the
steel layer of the pipe 10 should not exceed the maximum
limits as set by the Canadian Standards Association (or some
other appropriate regulatory authority) for a steel pipe
15 pressurized at the maximum design operating conditions; for
the purposes of illustrating the method of the invention, a
steel design operating axial stress is selected to be 80°s of
the steel s SMYS, which is 64,000 psi for X-80 steel.
2o One of the objectives of pipeline design is to minimize
the mass of a unit length of a pipe with respect to its other
properties. This will tend to reduce the cost of manufacturing
and make the pipe easier to handle. In this connection, the
mass of a unit length of the composite-steel pipe 10 should
25 not exceed the mass of a unit length of a comparable all-steel
pipe designed to operate under the same conditions.
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CA 02286956 1999-10-18
The above discussed design parameters constrain the
suitable selection of materials and dimensions of the
invention and are summarized below:
(1) the internal pressure does not exceed a level that
would cause the circumferential and axial stresses in the
steel layer 12 to exceed 110% of the steel's SMYS, i.e.
the internal pressure does not exceed the maximum
allowable overpressure during autofrettage;
to
(2) the internal pressure does not exceed a level that
would cause the circumferential stress in the composite
layer 14 to exceed the composite's ultimate tensile
strength during autofrettage;
(3) the circumferential stress in the steel layer 12 does
not exceed the steel design operating (circumferential)
stress when the pipe 10 transmits gas at the maximum
operating pressure; and
(4) the axial stress in the steel layer 12 does not
exceed the steel design operating (axial) stress when
the pipe 10 transmits gas at the maximum operating
pressure; and
(5) the circumferential stress in the composite layer 14
is at or below a selected composite design operating
17
CA 02286956 1999-10-18
stress when the pipe 10 transmits gas at the maximum
operating pressure.
(6) the mass of a unit length of the composite-steel
pipe 10 does not exceed the mass of a unit length of a
comparable all-steel pipe operating under comparable
conditions.
Having defined these design parameters, the next step of
io the method of the invention is to select suitable steel and
composite layer thickness combinations. This step is
described in the following paragraphs:
Let an internal pressure p be exerted on the wall of pipe
i5 10, the pipe 10 having an inside diameter di, a composite
layer of thickness tromp and a steel layer 12 of thickness
tateel ~ The force tending to separate two halves of a unit
length of the pipe 10 is p~di . This force is resisted by the
circumferential stress, acting uniformly over the stressed
2o area. Assuming that, under pressure, the radial stress in the
pipe 10 is relatively small compared to the circumferential
stress, the relationship between the circumferential stresses
in the wall layers and the internal pressure is:
25 p ~ di/ 2 - ~eteel ~ tateel + comp ~ tromp
(equation 1)
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CA 02286956 1999-10-18
wherein 68tee1 is the circumferential stress in the steel layer
12 and 6°°mp is the circumferential stress in the composite
layer 14 at pressure p. At the maximum operating pressure p°p,
6steel 1S eCjual t0 the steel design operating StreSS 6gteel°p and
s a°°mp is equal to the composite design operating stress
a°°mp°p:
p°p ~ dil 2 - ~eteel°p ~ tsteel + comp p tcomp
(equation 2)
io As discussed above, the values for p°p, di, 6eteel°pi
comp°p have
been selected. Therefore equation 2 provides an optimal range
of steel and composite layer thickness combinations that fully
utilize the strength of the steel and composite; that is,
layer thickness combinations in accordance with equation 2
15 will provide a pipe having steel and composite layers that are
at their respective maximum design operating stresses when the
pipe is pressurized at the maximum operating pressure.
There is a preferred minimum steel layer thickness for
2o the optimal range of selectable thicknesses; this minimum
thickness and the corresponding composite thickness
(calculated from equation 2) define one boundary of the above
range. Preferably, the minimum steel layer thickness is the
minimum thickness of steel that the steel layer of the pipe 10
2s can by itself withstand the maximum internal operating
pressure without yielding. Thus, where the composite layer
cannot provide any circumferential strength, e.g. if it is
19
CA 02286956 1999-10-18
damaged, the steel layer of the pipe 10 will bear the
pressurized gas long enough to effect the necessary repairs.
This steel layer thickness is determined by solving for teteel
in the following equation:
~]~P ' [~y~2 = SMYS ' tHteel
(equation 3)
A composite thickness corresponding to the minimum steel layer
io thickness is determined by solving for t~omP in equation 2 to
obtain a steel and composite layer thickness combination
(°first boundary combination~~).
A thinner minimum steel layer thickness may be acceptably
i5 selected in certain circumstances. For example, the
conditions may be such that there is a low risk that the
composite layer will suffer damage. Or, a user may not desire
the added safety feature provided by a thicker steel layer.
In this case, the minimum steel layer thickness is the minimum
2o thickness that will provide sufficient strength to withstand
axial stresses when the pipe 10 is pressurized at prolonged
periods at the maximum operating pressure, and temporary
periods at the maximum allowable overpressure. To determine
this thickness, assume again that the radial stresses are
25 minor relative to the circumferential and axial stresses;
then, the axial stress in the steel layer 12 of the pipe 10 is
approximately one-half of the circumferential stress in both
CA 02286956 1999-10-18
the steel and composite layers. Making the appropriate
substitutions, the following equation may be derived:
/ 0
pip ~ dil 4 - ~eteel, axial P ~ tateel
(equation 4)
wherein 6eteel, axial~p 1S the steel axial stress and is equal to
the SDSP multiplied by the steel SMYS. This equation may then
be solved for tHteel
io The other boundary of the above optimal range is defined
by the minimum composite layer thickness t~omp'"ln that is able
to provide suitable circumferential strength, crack arrest
capability, and toughness against impacts; this composite
layer thickness will depend on the external environment and
i5 other circumstances, but for the sake of illustration in this
example has been selected to be 0.100". The corresponding
steel layer thickness can be determined by solving for t9teel in
equation 2 to obtain a composite and steel layer thickness
combination ("second boundary combination").
The following table illustrates the optimal range of
steel and composite layer thickness combinations for a 48"
inner diameter pipe having X-80 steel and E-glass composite
layers and operating at a pressure of 2200 psi, based on the
2s above calculations. The bolded combinations represent the
preferred thickness layer combinations wherein the steel layer
can bear by itself the stress associated with pressurization
21
CA 02286956 1999-10-18
at the maximum operating pressure.
Table 1: optimal steel and composite layer thickness combinations for 48"
diameter
X-80 and E-glass composite pipe operating at 2200 psi.
Wall Thickness Axial Autofrettage
inches Stress Pressure
psi psi
Composite Steel Steel
11.100 (1.723 365411 2879
(1.115 (1.711737334 2868
(1.1311 11.692 38164 2856
11.145 (1.676 39032 2845
(1.1611 (1.661 39939 2834
0.175 0.646 40891 2822
0.190 0.63() 41888 2811
0.205 0.615 42936 2800
0.220 0.60() 44037 2788
0.235 0.584 45196 2777
0.250 0.569 46418 2766
0.265 0.553 47707 2754
0.28(1 (1.538 49071 2743
().295 0.523 50514 2732
0.311) 0.507 52045 2720
0.325 0.492 53672 2709
0.340 0.477 55404 2698
0.355 0.461 57251 2686
0.370 0.446 59226 2675
().385 0.43() 61342 2664
().40(1 0.415 63614 2653
Other layer thickness combinations other than the above
optimal combinations may be acceptably selected in accordance
22
CA 02286956 1999-10-18
with the invention that will provide some advantages of the
invention, including a steel-composite pipe that is lighter
than a comparable all-steel pipe, having a composite layer
that provides some circumferential strength, fracture arrest
s capability, and protection from the external environment.
However, these other layer thickness combinations do not
optimally utilize the strength and other properties of the
steel and composite and are therefore not preferred. The
acceptable non-preferred thickness combinations may be any
io combination that falls within an area on a steel and composite
thickness graph illustrated in Figure 4, that is, within the
area defined by the following lines: (1) the minimum preferred
steel layer thickness (line A), (2) the minimum preferred
composite layer thickness (line B), (3) the range of optimal
i5 thickness combinations (line C), and (4) the range of
thickness combinations of a unit length of steel-composite
pipe that equals the mass of a unit length of a comparable
all-steel pipe (Line D). Line D may be readily plotted given
the respective densities and dimensions of the steel and
2o composite layers; for the exemplary pipe illustrated in
Figure 6, the steel and composite densities are 0.2829 lb/in3
and 0.0779 lb/in3, respectively.
Once the foregoing parameters are determined for the
2s steel and composite layers and acceptable dimensions for the
pipe 10 have been selected, the pipe 10 is manufactured for
the most part in accordance with conventional composite/steel
23
CA 02286956 1999-10-18
pipe manufacturing practice by manufacturing a circular
tubular steel inner layer, and then applying to that steel
layer an outer composite layer by known means. In an
exemplary known means, the fibres of the composite are drawn
s from a plurality of spools contained in a creel, are fed
through gathering and aligning devices, through a bath of
polyester resin, and onto the outside surface of the metal
layer. While the above is a suitable means of attaching the
composite to the steel layer, any suitable known means for
to attaching the composite may be alternatively used, so long as
continuous contact is maintained between the composite and
steel layers when the pipe 10 is subjected to axial,
compressive and radial stresses, and so long as that under
such stresses, the steel and composite layers 12, 14 will
i5 strain at the same rate and by the same amount.
Referring to Figure 5, it can be seen that when the
manufactured pipe is first pressurized to the maximum
operating pressure of 2200 psi, the stress in the composite
20 layer is well below the composite design operating stress of
64 ksi, and the stress in the steel layer exceeds the steel
design operating stress of 64 ksi. This is because the steel
and composite layers are in continuous contact and will have
the same strain for a given internal pressure. As the elastic
2s modulus of the steel is significantly higher than that of the
composite, the steel will bear a higher amount of stress than
the composite for a given strain. For the steel and composite
24
CA 02286956 1999-10-18
layers to be at their respective design operating stresses
when the pipe is at the maximum operating pressure, the stress
distribution between the steel and composite layers at the
maximum operating pressure must be changed. This is achieved
s by stressing the steel beyond its yield stress so that the
steel is plastically deformed. This overstressing of the
steel may be effected by autofrettaging the pipe during a
field hydrostatic pressure test, i.e. by pressurizing the pipe
beyond the maximum operating pressure to a selected
io autofrettage pressure.
In a conventional field hydrostatic pressure test, a
portion of a field-installed pipeline is pressurized with
water to a selected pressure. Evidence of leakage is then
is monitored; leakage is particularly prevalent around the weld
seams between pipe sections but may occur anywhere on the pipe
wall that experiences a failure. If no leakage occurs, then
the pipeline portion has passed the pressure test.
2o Autofrettage is the application of hydrostatic pressure
wherein the steel layer is stressed beyond its specified
minimum yield stress. Generally accepted industry practice
recommends that the autofrettage pressure not exceed a level
that would cause the wall of the pipe to be stressed beyond
2s 110% of the steel's SMYS. An effect of pressure testing
conventional all-steel pipes between 100-110% of the steel's
yield stress is that the steel wall will experience some
CA 02286956 1999-10-18
plastic deformation. As illustrated in Figure 3, when
plastically deformed, the elastic range of the steel is
increased. Referring to Figures 5 and 6, plastically
deforming the steel in the steel-composite pipe will impart a
s compressive stress in the steel layer and a tensile stress in
the composite layer, thereby changing the stress distribution
between the respective layers when the pipe is re-pressurized.
It can be seen that when the pipe is re-pressurized to
to the maximum operating pressure, the stress distribution
between the steel and composite layers is different than that
before autofrettaging. To achieve the desired stress
distribution, namely, that distribution wherein the steel
layer and composite layers are at their respective design
15 operating stresses, the steel is plastically deformed by an
amount that will locate line II-III in Figure 6 so that it
passes through a point corresponding to the steel design
operating circumferential stress at the operating strain.
2o Once the appropriate amount of plastic deformation has
been determined, an appropriate autofrettage pressure that
will effect the appropriate plastic deformation must be
determined. This is determined by mathematically modelling
the stress-strain curves of steel and composite, and the
2s pressure-strain curve of the pipe 10. Using Figure 6 to
illustrate, the stress-strain curves of the steel and
composite may be modelled using a combination of the Ramberg-
26
CA 02286956 1999-10-18
Osgood equation and linear equations:
For line I-II:
(equation 5)
steel f~ ( steel' n
steel L.' pY F
steel Y
Where Eateei is the steel' strain
s
~ateel 1.S the steel' stress
s
EHceei is the steel' Elastic Modulus
s
F
a =0.05-- y
py E
steel
to FY is the steel's effective specified minimum yield
strength according to CAN/CSA 2662-96.
18.5 for X-80 - X-100 grade steel and 17.5 for
X-70 grade steel
for line II-III
+b (equation 6)
steel steel steel
At line B, the steel is at its operating stress and strain.
cs =c3°p =F ~s (elation 7)
steel steel y steel
2o Where SHteel 1S the steel design stress percentage. The strain
of the steel layer and composite layer at the operating strain
27
CA 02286956 1999-10-18
are the same:
s UTS (equation 8)
fop -fop comp comp
steel comp $
comp
Where S~omp UTScomp and Eoomp are the composite design stress
s percentage, ultimate tensile strength and Elastic Modulus,
respectively.
Then, at operating conditions,
(equation 9)
b=S F - steel ~ S . UTS )
steel y F comp comp
comp
to At the autofrettage strain (Line A)
(equation 10)
1 c c~'' -b >
steel E steel
steel
and
(equation 11)
1 ( ~5p __b' - «steel +~ ( (3steel' n
steel E' PY F
steel steel y
i5 Solving for 'SHteei at line A provides the maximum allowable
stress of the steel, i.e. the steel autofrettage stress:
(equation 12)
-b
c5A =F ~ ) 1 / n
steel y g $
py steel
2a
CA 02286956 1999-10-18
With the steel autofrettage stress, the autofrettage strain
may be readily determined. Then, the appropriate autotrettage
pressure can be readily determined from equation 1. Figures
and 6 illustrate the steel and composite stress-strain
s curves and pressure-strain for the first and second boundary
thickness combinations having an appropriate degree of plastic
strain.
The last step of the method of the invention is to
io autofrettage the pipe 10 at the above determined autof rettage
pressure. The resultant steel-composite pipe 10 will be able
to operate at a higher internal pressure than a conventional
all-steel pipe having a wall thickness equal to the thickness
of the steel layer of the steel-composite pipe. The composite
layer bears a proportion of the load caused by the internal
pressure; in the all-steel pipe, this load must be borne
entirely by the steel wall. Therefore, maximum operating
pressures of a steel-composite pipeline can be increased
beyond those presently specified for conventional all-steel
2o pipelines, thereby increasing the flow rate of gas and
profitability of the steel-composite pipeline.
Other alternatives and variants of the above described
methods and apparatus suitable for practising the methods will
2s occur to those skilled in the technology. For example,
aluminum may be substituted for the material for the inner
layer if a lighter overall weight is desired. Other
29
CA 02286956 1999-10-18
alternative materials for the inner layer include stainless
steel or titanimum. Carbon fibre or other materials may be
substituted for the E-glass fibres to enhance the modulus of
the composite, increase the composite's operating stress, or
s provide enhanced corrosion resistance. The scope of the
invention is as defined in the following claims.
