Note: Descriptions are shown in the official language in which they were submitted.
CA 02203643 1997-04-24
WO 96112911 PCTlUS95/13698
1
HIGH-PRESSURE FIBER REINFORCED COMPOSITE PIPE JOINT
Background
This invention concerns a joint between adjacent pieces of fiber reinforced
composite
pipe which is suitable for use with conventional fiberglass pipe, high
strength carbon fiber
' composite pipe or with high strength fiber reinforced pipe containing
embedded steel strips.
Fiber reinforced composite pipe finds appreciable utility where corrosive
materials are
carried in a pipeline or where the pipeline is buried or laid on the sea floor
or is otherwise
subjected to an external corrosive environment. Techniques have been developed
for
producing fiber reinforced pipe for carrying high internal pressures. For
example, until
recently a typical high pressure pipe might have a 10 cm nominal diameter and
an internal
burst pressure of about 600 bar. More recently, fiber reinforced high pressure
pipes with
a 20 cm nominal diameter have been rated at about 1200 bar burst pressure.
Such fiber reinforced composite pipe, when reinforced with glass fibers, may
have a
wall thickness on the order of 5 cm, which clearly makes it costly and heavy.
There is
currently development of another variety of high pressure pipe which includes
helically
wound steel strips embedded in fiber reinforced resin. Such an embodiment has
such good
strength that the wall thickness may be as little as 7 mm for a 25 cm nominal
diameter pipe.
Such a pipe is described and illustrated in U.S. Patent No. 4,351,364, for
example.
A substantial concern in such high strength pipe, either fiber wound or with
steel
reinforcement, is the coupling or joint between adjacent pipes. The pipe joint
needs to have
a circumferential burst strength at least as great as, and preferably more
than, the principal
length of pipe. More significantly, the joint must have sufficient
longitudinal shear strength
to prevent the pipes from separating under internal pressure or other axial
loads. Preferably
the joints are designed to have sufficient longitudinal shear strength that
they will not fail
before rupture of the pipe itself.
Design of a suitable joint for fiber reinforced composite pipe differs
appreciably from
metal since the fiber reinforced composite pipe, as contrasted with steel, for
example, has
very little ductility. This places significant limitations on what can be done
in pipe joints.
In a conventional bell and spigot joint secured by filling the joint with
adhesive, the high
stiffness of the adherent places a high shear stress on adhesive in the joint.
The distribution
of stress along the joint is not uniform. The shear stress is quite high at
the ends of the
adhesive, as much as three times the average stress, and decreases rapidly
from the ends
toward the middle. In a long adhesive joint, the shear stress in the middle of
the joint may
be near zero.
The high stress at the ends of adhesive in such a lap shear joint can result
in failure
of the adhesive in shear adjacent to an end of the joint. This simply
transfers the shear stress
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CA 02203643 1999-06-24
further along the joint and there is progressive failure at average stresses
that
would appear to be well within the capability of the adhesive.
Other joints for fiber reinforced pipe are also difficult because of the
stiffness of the fiber reinforced composite. It is desirable to provide a pipe
joint
that redistributes stress along the length of the joint to avoid such
progressive
failure of the joint. Preferably the joint has a higher strength than the wall
of the
pipe remote from the joint. The pipe joint should have a high safety margin,
i.e.
a failure stress greater than the rated capability of the joint. The joint
should be
easily and economically assembled in field conditions.
Brief Summary of the Invention
There is therefore provided in practice of this invention according to the
presently preferred embodiment, a pipe joint having an external taper on one
end
of a fiber reinforced composite pipe and a matching internal taper in a
coupling or
the like. For example, the tapers have an included half angle in the range of
from
one to five degrees. A half groove extends helically along the length of each
of
the internal and external tapers. A ductile key member extends helically along
both grooves for locking the internal and external tapers together.
Accordingly, the present invention provides a pipe joint for fiber reinforced
composite pipe comprising:
a fiber reinforced composite pipe having an external taper on one end with
an included half angle in the range of from one to five degrees;
a groove extending helically along the length of the external taper;
a coupling having an internal taper having an included half angle the same
as the half angle on the external taper;
a groove extending helically along the length of the internal taper having
the same pitch as the groove in the external taper; and
a ductile key member extending helically a plurality of turns along both
grooves for locking the external and internal tapers together.
In a further aspect, the present invention provides a pipe joint comprising:
a glass fiver reinforced composite pipe having an external taper on one end
with an included half angle in the range of from two to four degrees;
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CA 02203643 1999-06-24
a half round groove extending helically a plurality of turns along the length
of the external taper;
a fiver reinforced composite coupling having an internal taper having an
included half angle the same as the half angle on the external taper, at least
a
portion of the coupling being formed of a sufficient amount of material having
a
higher modulus of elasticity than glass to provide a hoop stiffness greater
than the
hoop stiffness of the pipe adjacent to the external taper;
a half round groove extending helically a plurality of turns along the length
of the internal taper having the same pitch as the groove in the external
taper;
a round key member extending helically a plurality of turns along both
grooves for locking the external and internal pipe joint moieties together,
the key
member having sufficient ductility for plastically deforming in shear and
redistributing longitudinal loads along the length of the pipe joint;
a first sealing surface outside the pipe between the end of the key member
and the end of the pipe;
a second sealing surface inside the coupling facing the first sealing surface;
a pair of spaced apart circumferential elastomeric seals between the first
and second sealing surfaces;
a sealant between the elastomeric seals and adhered to both surfaces, the
sealant being sufficiently elastic of accommodating the maximum longitudinal
shear strain of the pipe joint;
a sealant injection passage through a wall of the coupling between the
elastomeric seals for injecting sealant between the elastomeric seals after
the pipe
joint is assembled; and
a bleed passage through a wall of the coupling between the elastomeric
seals for releasing air from between the elastomeric seals as sealant is
injected.
In a still further aspect, the present invention provides a pipe joint for
fiber
reinforced composite pipe comprising:
a fiber reinforced composite pipe having an external pipe joint moiety on
one end;
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CA 02203643 1999-06-24
a coupling having an internal pipe joint moiety matching the external pipe
joint moiety;
means for locking the external and internal pipe joint moieties together in a
longitudinal direction; and
means for sealing the joint spaced apart from the means for locking the
pipe joint moieties together comprising:
a first surface outside the pipe,
a second surface inside the coupling facing the first surface,
a pair of spaced apart circumferential elastomeric seals between the first
and second surfaces, and
a sealant between the elastomeric seals and adhered to both surfaces, the
sealant being sufficiently elastic for accommodating the maximum longitudinal
shear strain of the pipe joint.
Preferably the joint is sealed by having a pair of surfaces facing each other
adjacent an end of the key member with an elastic sealant between and adhered
to
both facing surfaces. An elastomeric seal ring adjacent each longitudinal end
of
the sealant retains the sealant when it is injected as a liquid and before it
cures.
Preferably the hoop stiffness of the external coupling adjacent to the
internal taper is greater than the hoop stiffness of the pipe adjacent tot he
external
taper so that a substantial fraction of the longitudinal shear strength of the
joint is
obtained by friction between the tapers. The higher hoop stiffness may be
obtained by forming the external portion of the pipe joint with a fiver
reinforcement that has a larger modulus of elasticity than the modulus of
elasticity
of material use in l:he inner pipe.
Such a pipe joint is particularly useful in a fiber reinforced composite pipe
including helically wound steel strips embedded in the resin. In such a joint,
the
steel strips end within the pipe joint and at different distances from the end
of the
pipe joint.
The present invention also provides a composite pipe comprising a fiber
reinforced composite pipe including a plurality of helically wound steel
strips each
having its end completely embedded in fiber reinforced resin spaced apart from
the
end of the pipe, and having an external joint moiety on at least one end of
the pipe
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CA 02203643 1999-06-24
tapering on its outside diameter to a smaller diameter adjacent to the end of
the
pipe, each of the steel stxops ending at a different distance from the end of
the
pipe joint and within the tapered portion of the pipe joint.
Drawings
Fig. 1 is an exemplary fiver reinforced composite pipe joint with the outer
coupling being in longitudinal cross section;
Fig. 2 is a side view of the inner member of a joint partially in longitudinal
cross section for a second embodiment of fiber reinforced composite pipe with
embedded steel strips;
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WO 96J12911 PCT/US95/13698
1 FIG. 3 is a fragmentary longitudinal cross section of the pipe of FIG. 2 at
a location
away from the pipe joint;
FIG. 4 is a side view of an end portion of the pipe of FIG. 2 in an
intermediate stage
of manufacture;
FIG. 5 is a transverse cross section of an embodiment of key member for a pipe
joint;
and
FIG. 6 is a transverse cross section of another embodiment of key member.
Description
A typical pipe joint has an inner member of fiber reinforced composite pipe 10
such
as is conventionally made of epoxy resin reinforced with helically wound glass
fibers. These
are conventional pipes, albeit with a thick wall for withstanding high
internal pressures. A
high pressure pipe having a nominal inside diameter of about 20 cm may have a
wall
thickness of about 5 cm. The pipe has an external taper 11 adjacent its end. A
half round
groove 12 extends helically along the length of the external taper. (It will
be recognized that
the "helical" groove in the tapered surface is not a cylindrical helix but
instead has the same
taper as the tapered surface 11 and a uniform depth throughout the length of
the groove.)
The external taper on the pipe fits into a coupling 13 having an internal
taper 14
matching the external taper on the pipe. The internal taper also has a half
round groove 16
with the same pitch as the groove on the pipe. In this description, the outer
member of the
pipe joint combination is referred to as a "coupling" since that is a usual
embodiment for a
pipe joint. Alternatively, one may make a pipe with an external taper on one
end and an
internal taper on the other end and for purposes of this description, the end
with an internal
taper would be referred to as a "coupling". The "coupling" may be in any of a
broad variety
of pipe fittings such as valves, flange transition fittings, unions, etc. In
one type of pipe for
which this invention is useful, there are external tapers at both ends of each
length of pipe
and adjacent pieces of pipe are interconnected by a short coupling having two
internal tapers.
The internal and external tapers are interconnected by a round ductile key
member 17,
half of which lies in each of the half round grooves on the internal and
external tapers
respectively. A suitable material for a key comprises nylon or similar
ductile, relatively
strong thermoplastic material. A preferred key for many applications is formed
of high
purity aluminum, which has relatively high strength, is quite ductile and is
corrosion
resistant. Aluminum has about twice the shear strength of nylon. Aluminum is
desirable for
applications where the pipe may be immersed in water. Nylon tends to swell
during
exposure to water.
Another suitable key, as illustrated in FIG. 5, may be formed by embedding a
flexible
steel core 18 in a layer of ductile plastic sheath 19 such as nylon. The steel
core may be a
single wire or is preferably a cable twisted from multiple strands of wire for
greater
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WO 96112911 PCT/US95/13698
1 flexibility. In such an embodiment, the outer plastic provides the ductility
important in the
key and the steel provides shear strength. It is believed to be important to
closely embed the
metal core in the plastic sheath without bubbles or voids.
Another embodiment of key, as illustrated in FIG. 6, may be a hollow tube of
metal
stronger than high purity aluminum, such as stainless steel. Shear strength is
provided by
the metal and ductility is provided by denting of the tube wall 21. The
desired ductility and
shear strength of such a key is obtained by selection of the material and the
wall thickness
of the tube.
It is preferred that the shear strength of the key be up to one half of the
shear strength
of the fiber reinforced pipe joint moiety so that deformation certainly occurs
in the key and
loads that could cause the fiber reinforced composite to fail are avoided.
A round key is preferred so that the grooves in the pipe joint are rounded.
This
minimizes stress concentrations in the essentially non-ductile fiber
reinforced composite.
Furthermore, a round key can be wound smoothly in a helical groove extending
along a
tapered surface. A key with a non-round cross section twists when wound in a
tapered helix
and would need to be deformed to lie smoothly in a groove.
Some dimensions of such a pipe joint may be given to provide an idea of scale.
For
a pipe with a nominal inside diameter of about 20 cm, the total length of the
pipe joint is
about 48 cm. The pitch of the grooves in the tapered surfaces is about 2.6 cm.
The groove
is made long enough to accommodate 13 full turns of a key around the taper.
The key has
a diameter of about 7.5 mm.
An important dimension of the pipe joint is the angle of the taper in the
joint. The
included half angle of the taper is in the range of from one to five degrees
from the axis of
the taper. As explained hereinafter, the friction in the joint provides a
significant fraction
of the total shear strength of the joint. If the included half angle is more
than about five
degrees, the joint may become too short to accommodate enough turns of the key
and to
provide enough friction contribution. If the included half angle is less than
about one degree,
a joint may become unreasonably long and there is difficulty in maintaining
appropriate
diametral tolerances on the internal and external tapers. A relatively small
diametral
tolerance is required for minimizing axial length d:~screpancies in an
assembled joint.
The internal taper and groove are formed in the course of winding a coupling
on a
mandrel and there is good control of dimensions. The external taper and groove
are
machined and a diametral tolerance between the two parts within less than t
250 ~cm can
readily be maintained. The taper is more forgiving than a cylindrical surface
to discrepancies
in diameter. A close fit between matching tapers can be obtained by axial
shifting, whereas
a cylindrical joint must be made with closer tolerances to get as tight a fit.
It is preferred that the included half angle of the taper be in the range of
from two to
four degrees. A four degree taper angle is found to be quite suitable for .
glass fiber
CA 02203643 1997-04-24
WO 96J12911 PCTIUS95/13698
1 reinforced composite pipe using a nylon key. A typical joint length might be
from about one
to 1'h times the diameter of the pipe. A high strength fiber reinforced pipe
may have a
sufficient wall thickness that a taper angle as high as four or five degrees
is needed to extend
a suitable distance through the thickness of the pipe wall.
' S Although a four degree half angle is suitable for an all fiber reinforced
pipe, a fiber
reinforced pipe with embedded steel strips (described in greater detail
hereinafter) may
advantageously employ a lower taper angle of as little as two degrees. Such
steel reinforced
pipe may employ a thinner pipe wall than a pipe made solely with fiberglass
and a low taper
angle may be sufficient. It might be noted that a higher taper angle is often
preferred to
make it easier to remove internal tooling used for winding fittings such as
elbows with an
internal taper on each end. The diametral tolerances required for
manufacturing the pipe
joint are also relaxed for larger taper angles.
The pipe joint is assembled by first winding the ductile key into the half
round groove
in the internal taper. A straight nylon key springs outwardly within the taper
as it is put in
place and readily fits into the groove. An aluminum key may be wound onto a
tapered
mandrel to have a diameter somewhat larger than the internal taper. When the
key is
threaded into the taper, the key fits into the groove.
The external taper is then inserted in the internal taper and one or the other
of the
members is rotated to essentially thread the joint together. The pipe and
coupling are
threaded together until the tapers engage tightly.
The key in the tapered pipe joint does not provide a fluid tight seal. A seal
is
provided by an adhesive elastic sealant (too thin to be illustrated in FIG. 1)
between an
external sealing surface 22 on the pipe and a facing internal sealing surface
23 in the
coupling. In the illustrated embodiment, the sealing surfaces have the same
taper at a four
degree half angle as provided on the tapered surfaces forming the mechanical
pipe joint.
A pair of circumferential O-ring grooves 24 straddling the sealing surface
near the end
of the pipe accommodate elastomeric O-rings 26 which seal against the facing
sealing surface
within the coupling. After the joint is assembled, a liquid sealant may be
injected through
one of a pair of passages 27 through the wall of the coupling. The second
passage serves
as an air vent and indicates when the sealant has filled the space between the
sealing
surfaces. The elastomeric O-rings retain the sealant within that space while
it is liquid before
curing. The O-rings also serve as a buffer at each end of the sealant within
the annular
sealing space for minimizing shear strain and keeping the sealant from
shearing from the
facing surfaces.
A preferred sealant comprises a two part thermosetting polysulfide resin. This
is a
relatively low cost, temperature and solvent resistant resin that adheres well
to surfaces, even
if wet. By selection of activators, etc., the pot life of the resin can be
adjusted to provide
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WO 96J12911 PCT/US95/13698
1 adequate time for inspection and rework of the seal before it cures. Another
suitable sealant
is a fluorinated silicone resin. Elastomeric polyurethane resins may also be
used.
Typical thickness of the sealant between the facing surfaces is about 0.75 mm.
The
length of the sealing surfaces between the O-rings in this embodiment is about
3.9 cm,
although the taper on the two moieties of the joint is appreciably longer to
assure that the
passages 27 are between the O-rings when the joint is assembled.
It is preferred that the sealing surfaces are at the smaller diameter
(internal) end of the
pipe joint and beyond the end of the key. This places the O-ring grooves in a
portion of the
pipe which has very low longitudinal stress.
A second embodiment of pipe joint illustrated in FIGS. 2 to 4 is described
before
describing functioning of the pipe joint. In this embodiment, the fiber
reinforced composite
pipe also comprises a plurality of helically wound steel strips embedded in
the wound fiber
reinforcement. The end of such a pipe with a pipe joint is illustrated in FIG.
2 with a
portion illustrated in longitudinal cross section. FIG. 3 is a fragmentary
longitudinal cross
section of the wall of the pipe significantly enlarged to show detail. FIG. 4
illustrates the
end of the pipe in an intermediate stage in its manufacture. The drawing is as
if some of the
outer layers of the pipe were peeled away.
This moiety of the pipe joint mates with a coupling (not shown) having an
internal
taper generally similar to the pipe coupling illustrated in FIG. 1, except
that the dimensions
and geometry match the extennal dimensions of the pipe joint moiety
illustrated in FIG. 2.
The principal portion of the length of the pipe, i.e., away from the pipe
joint, includes
four steel strips 31. The steel strips are too thin to illustrate in cross
section in FIG. 2 but
are illustrated in the fragmentary cross section of FIG. 3. In an exemplary
embodiment of
pipe having a nominal 25 cm diameter, there are four steel strips helically
wound within the
fiber reinforced composite. Each strip is from 10 to 15 cm wide and has a
thickness of about
0.5 mm. The strips are helically wound with the edges in close proximity,
typically 2 mm
or less. Successive strips are staggered so that the gaps 30 between the edges
of the strips
are not aligned. A thin layer 32 of epoxy resin (about 50 ~,m) is between each
adjacent pair
of steel strips. Inwardly from the innermost steel strip, there is a layer 33
of glass fiber
reinforced epoxy with a thickness of about 2.5 mm. On the outer wall of the
pipe, outwardly
from the steel strips, there is another layer 34 of glass fiber reinforced
epoxy having a
thickness of about 1.5 mm. Thus, the steel strips are completely embedded in
the fiber
reinforced composite.
FIG. 4 illustrates an end of the pipe with outer layers peeled away showing
essentially
the innermost layer 33 of fiber reinforced composite and the innermost layer
of helically
wound steel strip 31. The end of the steel strip is cut off at the helix angle
of the strip
winding so that the cut edge is parallel to the end 36 of the pipe. The
otherwise sharp point
on the end of the strip is likewise cut off. There is a hole 37 near the
centerline of the strip
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WO 96J12911 PCT/US95/13698
1 near the end for receiving a tooling pin (not shown) to hold the end of the
strip during
winding. After the layer of steel strip is wound, a circumferential winding of
glass rovings
38 is wrapped over the end portion of the strip to secure it in place while
subsequent steel
strips and the outer layer of fiber reinforced resin are added.
' S The next overlying steel strip is helically wound with the same helix
angle and the
same direction of winding. The end of the overlying steel strip, however, is
cut off at a
longer distance from the end of the pipe than the innermost layer. Each
succeeding layer is
similarly cut off at successively greater distances from the end of the pipe.
This is illustrated
by the widening black line in FIG. 2 which increases in width at successively
greater
distances from the end of the pipe. The scale of the drawing is too small to
show the strips
individually and in cross section. By staggering the ends of the successive
steel strips, load
is distributed to the joint at the end of the pipe over an appreciable
distance instead of at a
single location where stress concentrations could damage the pipe when loaded
by pressure
or otherwise.
It will also be noted that all of the steel strips end at a distance from the
end of the
pipe so that the steel strips are completely embedded in the fiber reinforced
resin. The
surrounding epoxy shields the steel from corrosive media that may be present
inside or
outside of the pipe.
In an exemplary embodiment where the steel strips are each about 10 cm wide,
the end
of the innermost strip is about 2.5 cm from the end of the pipe. Each
successive steel strip
ends about 5 cm, or half the width of the strip, away from the end of the
pipe.
After the inner and outer layers of fiber reinforced resin and the embedded
steel strips
are wound, the pipe joint moiety is added over these layers at the end of the
pipe. Additional
layers of glass fiber rovings wetted with epoxy are wound over the outside of
the pipe to
build up sufficient thickness to machine the finished geometry of the pipe
joint. Typically
the fibers are wound at a helix angle of about 70 ° to 80 ° with
some outer wraps being
substantially circumferential. In an exemplary embodiment with a nominal 25 cm
diameter
pipe, the diameter of the thickest portion of the built up windings from which
the joint is
made is as much as 34 cm. The end 41 of the added fiber reinforced composite
is gradually
feathered to the smaller diameter of the principal length of the pipe either
in the process of
winding or by machining after winding is completed. Feathering of the end
minimizes stress
concentrations adjacent to the joint.
The pipe joint has an external tapered surface 42 with a half round groove 43
. extending helically along its length. The taper has an included half angle
of four degrees.
In an exemplary embodiment of a nominal 25 centimeter diameter pipe, the
length of the
taper which could engage an internal taper in a coupling is at least 25
centimeters and
preferably 26 or 27 centimeters. The pitch of the groove is about 3
centimeters, yielding a
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CA 02203643 1997-04-24
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1 sufficient length of groove to accommodate six full turns of a ductile key.
A typical key
diameter is about 9.5 millimeters.
As previously described in the embodiment of fiber reinforced composite pipe
without
the embedded steel strips, there is a sealing surface 46 near the end of the
pipe beyond the
end of the key groove. The sealing surface is between O-ring grooves 47 for
retaining
sealant as it is pumped into the space adjacent the sealing surfaces.
There are a number of features of the pipe joint which are of importance for
carrying
a large longitudinal load in the joint. These include use of a ductile key for
redistribution
of load along the length of the joint in the brittle fiber reinforced
composite; location of the
key along a tapered surface so that there are individual shear layers rather
than transferring
all of the shear load in a single layer in the brittle material; a round key
and key way groove
for minimizing stress concentrations in the brittle material; friction
actuation of the pipe joint
which significantly amplifies the load carrying capability provided by the
key; and means for
sealing the joint independent of the mechanical load carrying structure.
Fiber reinforced epoxy pipe has very little ductility. If one makes up a bell
and spigot
joint between pieces of fiberglass pipe bonded with adhesive, shear loading is
transferred
from the stiff adherent to the ductile adhesive nonuniformly. It has been
shown that there
is a very high shear loading at the end of the adhesive which decreases
significantly with
distance. The high shear load can cause progressive failure of the adhesive
and failure of
the joint. The tapered joint with a ductile key avoids this by redistributing
the load along the
length of the pipe joint.
The proportion of shear stress at the end of the joint depends on the length
of the joint.
A long joint shows an end stress much higher than the average stress along the
joint length.
A short joint has more uniform shear stress. It may be considered that the
ductile helical key
divides the long joint into a number of short joints, each of which has a
smaller stress
concentration at the end. Furthermore, plastic shear failure of a part of the
key does not
propagate to subsequent turns of the key.
When an increasing longitudinal load is applied to the pipe joint it affects
the first turn
of the key a major proportion compared with subseauent turns of the key. When
the load
exceeds the shear strength of the key in the first turn it deforms in shear
and additional load
is applied to the next turn of the key. There is progressive deformation of
the ductile key
along the length of the joint, thereby redistributing the stress along the
length of the joint.
In effect, one obtains the benefit of a ductile thread for a non-ductile pipe.
In one test a pipe joint was pressurized until incipient failure was detected.
The first
turns of the key showed plastic deformation in shear with progressively
decreasing
deformation along the length of the key. Such deformation redistributes stress
to achieve
reasonably uniform stress on all or most of the length of the key. Thus, the
ductile key in
the non-ductile fiber reinforced composite redistributes the longitudinal load
along the full
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1 length of the joint. High stress at the first turn of the key is avoided as
the shear area of the
successive turns of the key are added for carrying the load. In a lap shear
joint, the shear
load at the end of a joint may be more than three times the average shear
load. A nylon
helical key as described herein has a shear load on the first turn only about
30 % greater than
' S the average shear load on the key. The fiber reinforced composite is much
stiffer than the
ductile key. Thus, a portion of the longitudinal load is carried on each turn,
or at least most
of the turns, of the helical key.
The very high stresses at the loaded end have been minimized in threaded
joints by
employing what is called a variable pitch thread. Each turn of the thread has
a slightly
different pitch from the preceding turn so that when a predetermined
longitudinal load
stretches the threaded member, the threads are uniformly engaged and the
stress is distributed
along a major portion of the thread instead of being concentrated at the end.
Such an
arrangement is used, for example, in the breech blocks of cannons.
The ductile "thread" in the fiber reinforced composite pipe joint effects a
load
redistribution somewhat analogous to a variable pitch thread, without the high
cost associated
with precision machining of a variable pitch thread. The pitch of the grooves
in the pipe
joint is uniform along the length of the joint, but the ductile key deforms
nonuniformly,
thereby redistributing the load along a major portion of the length of the
joint. Because of
this, essentially all of the shear area of the key (the cross-sectional area
of the key at the
interface of the conical tapered portions) is bearing load. The pressure
carrying capability
of the joint can therefore be determined by knowing the strength of the key
material and the
shear area available.
The longitudinal strain of a pipe joint can be measured as pressure is
applied. It is
found that there is a linear increase in joint length with increasing pressure
up to a certain
limit and above that limit there is non-linear increase. If, however, pressure
is released
within the linear region, the joint decreases in length only part way to its
original length.
It may be that the required "ductility" is a combination of softness or low
modulus of
elasticity relative to the rigid fiber reinforced composite to accommodate
appreciable elastic
deformation, as well as plastic deformation usually associated with the term
"ductile" . Thus,
as used herein the term "ductile" encompasses soft, flexible materials like
nylon, high purity
aluminum and their equivalents.
In addition, to the strength of the key, the longitudinal strength of the
joint is enhanced
by friction between the tapers due to load transferred across the pipe-
coupling interface. This
can be a substantial fraction of the total load carrying capability of the
joint. In fact, the
friction actuation of the joint can contribute as much or more of the load
carrying capability
as the ductile key. To enhance the friction actuation, the circumferential or
hoop stiffness
of the outer coupling is made at least as large as the stiffness of the joint
moiety on the pipe
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CA 02203643 1997-04-24
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1 inserted into the coupling. Preferably the hoop stiffness of the coupling is
at least twice as
large as the hoop stiffness of the pipe.
The hoop stiffness of the coupling may be made large by making it of a
material such
as steel which has a much higher modulus of elasticity than glass reinforced
epoxy. The
modulus of steel is about 20 x 105 kg/cm2, whereas the modulus of glass fiber
reinforced
epoxy is about 2.5 x 105 kg/cm2. Generally, however, it is preferable to avoid
steel
couplings and employ a coupling that is fiber reinforced composite.
The stiffness of the coupling can be enhanced by increasing the wall
thickness. The
stiffness is a function of wall thickness and hoop or circumferential modulus
of elasticity of
the material used to fabricate the coupling. It is preferable, however, to
keep the couplings
small by using a material with a higher modulus of elasticity than the glass
reinforced epoxy
typically used in fiber reinforced composite pipe.
A high stiffness coupling can be obtained by winding the coupling with carbon
or
graphite fibers. Such a composite of carbon fiber in epoxy has a modulus in
the order of 7
x 105. In such an embodiment, a coupling is wound with glass or carbon fibers
nearer the
internal part of the coupling with customary helix angles for obtaining a
desired proportion
of longitudinal and circumferential strength. Once sufficient longitudinal
strength is obtained,
carbon or glass fibers in epoxy resin may be wrapped at a helix angle of
70° to 90° to build
up additional hoop stiffness.
Furthermore, the hoop stiffness of the coupling may be increased by winding
high
tensile strength steel wire instead of carbon fibers. Although the modulus of
steel is
somewhat below the modulus of the least expensive carbon fibers, the total
stiffness of the
coupling can be enhanced at lower cost with steel windings. Such windings may
include
steel strips (analogous to those in the pipe wall) as well as steel "fibers".
In such an
embodiment the steel is completely embedded within the fiber reinforced resin
and therefore
protected from corrosion.
Friction actuation of the pipe joint can be visualized by imagining that the
coupling
is infinitely stiff. In that case, the entire internal pressure in the pipe is
carried by the
coupling and all of the pressure is transmitted across the tapered pipe-
coupling interface.
This would provide 100 % of the available friction to add to the strength of
the key to prevent
longitudinal extension of the joint.
On the other hand, assume that the stiffness of the coupling is exactly equal
to the
stiffness of the pipe. In this embodiment half of the pressure load is carried
by the coupling
and half is carried by the pipe. Thus, half of the internal pressure is
carried through the
pipe-coupling interface. The addition to the longitudinal load carrying
capability of the pipe
joint is thus 50% of the total friction available.
If the coupling stiffness is twice of the pipe, two-thirds of the load is
carried by the
coupling and one-third by the pipe. Since two-thirds of the pressure load is
conveyed
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CA 02203643 1997-04-24
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1 through the pipe-coupling interface the addition of friction to the strength
of the key is 66
of the total theoretical for an infinitely stiff coupling.
In addition to increasing the stiffness of the coupling one may increase the
proportion
of the longitudinal load carried by friction by increasing the length of the
joint. Ordinarily
' S one does not consider that friction increases by increasing the area for a
given load. That
principle is not applicable, however, in an increasing length of this pipe
joint since the total
radial load is not constant. In the pipe joint the radial load applied at the
friction interface
is directly proportional to the area and the pressure within the pipe joint
(as well as stiffness).
Thus, as a pipe joint is made longer the friction component of the load
carrying capability
goes up exponentially.
For example, if one doubles the length of the joint and the length of the key
in the
joint, the friction area (and load) are doubled, as is the shear area of the
key. This means
that it takes twice the longitudinal load to shear the key and the pressure
rating of the pipe
joint (based on key shear) can also be doubled. The doubling of the pressure
rating, of
course, increases the radial load and the contribution of friction. Thus,
doubling of the
friction area and pressure rating increases the friction component four times.
The seal between the pipe and coupling has a distinct advantage in assembling
a
pipeline, for example, since the seal can be inspected to assure that there is
good sealing.
After the pipe joint is assembled with the tapers in engagement, the space
between the
O-rings is pressurized with gas to determine whether the O-rings have formed a
seal. This
can be quickly verified. A liquid thermosetting sealant is then pumped into
one of the
passages, with the opposite passage serving as a bleed hole to verify that the
sealed space
between the O-rings has been filled. A radiopaque material such as a barium
compound is
included in the sealant. The seal can then be X-rayed for detecting any
bubbles or voids in
the sealant. A sealant with a relatively slow curing cycle can be employed so
that the
inspection can be performed before the sealant is cured and any defective seal
can be readily
reworked while the sealant is still liquid.
The seal arrangement is also particularly well suited for laying sub-sea
pipelines or
installing the pipes in other locations under water. A joint may be assembled,
sealed and
tested on the deck of a ship and then passed overboard before the sealant has
cured. The
O-rings protect the sealant from the water while the sealant cures.
An exemplary seal has a length between the O-rings of about 2.5 cm. The
thickness
of the sealant between the facing surfaces is about 0.75 mm. The thickness of
the space
between the O-rings should be sufficient to permit the liquid sealant to be
pumped in without
too much back pressure. It should also be sufficient that the cured sealant
can accommodate
the strain of longitudinal deformation of the pipe joint as the joint is
pressurized. In effect,
a "rectangle" of sealant deforms into a parallelogram and the thickness must
be sufficient to
accommodate this strain without exceeding the shear strength of the sealant or
adhesive bond
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1 between the sealant and adherent. The presence of the elastomeric O-rings at
each end of
the sealant minimize the high stresses at the ends and enhance the ability of
the sealant to
accommodate shear strain. The relative radial stiffness of the pipe and
coupling also
contribute to adhesive sealant strength since the radial stress in the
adhesive helps prevent
peeling failure due to internal pressure.
On the other hand the sealant cannot be too thick or it may be extruded from
the seal
by the internal pressure of the fluid in the pipe. It turns out that a major
portion of the load
on the sealant is due to the pressure on the projected area of the end of the
sealant. Thus,
the sealant should have sufficient length between the O-rings to resist
internal pressure in the
pipe. For a sealant with a lap shear strength of about 20 kg/cm2 and radial
thickness of
about 0.75 millimeters, a length of adhesive of only about 15 mm is sufficient
to
accommodate an internal pressure of 630 bar (9,000 psi). A short seal length
is preferred
so that the joint can be readily disassembled.
For field assembly of pipelines it is preferred to assemble a short coupling
onto one
end of each length of pipe at the manufacturing site. This enables the seal to
be tested and
the pipe provided in lengths (typically about 12 meters) with what amounts to
a male thread
at one end and a female thread at the other end. Thus, in field assembly only
one joint needs
to be made up between adjacent lengths of pipe.
The new pipe joint makes glass fiber reinforced epoxy pipe suitable for oil
well
casings. It has good longitudinal load carrying capability for hanging casing
in a well. The
joint can be made up quickly using the standard tools present on drill rigs
and the like. In
such usage sealing with a sealant between the O-rings may not be required and
the O-ring
seals alone should be su~cient in most cases.
Although limited embodiments of pipe joint constructed according to principles
of this
invention have been described and illustrated herein, it will be understood
that many
modifications and variations may be made by those skilled in the art. For
example, in a
relatively thin wall pipe having steel strips embedded in the fiber reinforced
resin, an added
band of glass fiber reinforced resin may be wrapped around the pipe beyond the
larger
diameter end of the taper. This enlarged area may be engaged by tooling for
assembling the
joint without hazard of damaging the thin walled portion of the pipe.
It will also be recognized that a ductile key with a plurality of helical
turns in such a
pipe joint may be employed where the pipe joint does not have the described
taper. A ductile
key in such an embodiment may have sufficient accommodation for deforming in
shear and
redistributing the longitudinal load along the length of the pipe. A taper in
such a joint is,
however, preferred so that the shear load at each turn of the helix is at a
different radial
distance and the shear layers do not line up. This enhances the shear strength
of the pipe
joint.
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1 Although the structure of the pipe joint has been described for a pipe and
coupling,
it will be apparent that a similar structure may be provided on a pipe with an
external taper
on one end and an internal taper on the other end. The joint is also
particularly well suited
for a high pressure transition between a fiber reinforced composite pipe and a
steel coupling,
' S valve or other fitting. It may be useful, for example, to include a steel
coupling in a fiber
reinforced composite pipeline where it may be desired to tap into the
pipeline.
- Steel has a significantly different modulus of elasticity than fiberglass
reinforced resin.
When such materials are threaded together and undergo elongation under load,
the stretching
is rather different and the resulting thread pitch is different in the two
materials. The ductile
key in the joint accommodates the difference in modulus without degrading the
strength of
the joint. The sealing arrangement for the joint is also advantageous when the
pipe and
coupling materials are different.
20
30
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