High-pressure fiber reinforced composite pipe joint

A pipe joint for fiber reinforced composite pipe has internal and external matching tapers with a low taper angle. A half round helical groove is formed in each of the internal and external tapers for receiving several turns of a ductile key. The pipe joint is usable on high strength lightweight fiber reinforced composite pipe having helically wound steel strips embedded in the fiber reinforced resin. The pipe joint on such pipe is on an enlarged portion of glass fiber reinforced composite built up on an end of the pipe. The steel strips are terminated at differing distances from the end of the pipe joint and the ends of all of the steel strips are completely embedded in the fiber reinforced resin.

BACKGROUND 
This invention concerns a 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. Pat. 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 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 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.

DESCRIPTION 
This application is a division of U.S. patent application Ser. No. 
08/327,616, which concerns a pipe joint. The subject matter of the parent 
application is hereby incorporated by reference. 
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 plastic. 
It is preferred to place the key inside the internal taper for ease of 
assembling and disassembling the joint. The external taper threading into 
the internal taper tends to force the key outwardly and permit free 
insertion of the pipe into the coupling. If a key firmly on the pipe were 
used, the joint could be self-locking and it could be difficult to fully 
tighten the joint. Likewise, the key in the coupling tends to be self 
releasing from the pipe when the joint is unthreaded. 
The key in the tapered pipe joint does not provide a fluid tight seal. A 
seal is provided by an adhesive elastic sealant 25 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 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 external 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 gm) 
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 
51 edge is parallel to the end 36 of the pipe. The otherwise sharp point 
on the end of the strip is likewise cut off along an edge 52. There is a 
hole 37 near the centerline of the strip 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. 
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 700 to 800 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. 
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. 
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. 
It should therefore be understood that within the scope of the appended 
claims, this invention may be practiced otherwise than as specifically 
described.