Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02808531 2013-02-13
WO 2012/019275
PCT/CA2011/000216
CLAMP SUITABLE FOR INCREASING THE FATIGUE LIFE OF THE BUTT
WELDS OF A PIPE PRESSURE VESSEL WHICH IS SUBSEQUENTLY BENT
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the priority of U.S. Patent Application No. 12/855,970
entitled "CLAMP SUITABLE FOR INCREASING THE FATIGUE LIFE OF THE
BUTT WELDS OF A PIPE PRESSURE VESSEL WHICH IS SUBSEQUENTLY
BENT," filed August 13, 2010, the contents of which are hereby incorporated by
reference.
FIELD OF THE INVENTION
The invention relates to method of minimizing bending strain for pipe welds
and for a pipe clamp to facilitate the same. More particularly, the invention
relates to a
pipe clamp for locating over a butt weld in a length of pipe to be subjected
to bending
forces to protect against excessive strain in the weld area.
BACKGROUND OF THE INVENTION
The invention relates to the storage of natural gas (CNG) or other fluids or
gases
under pressure in pressure vessels comprised of very long lengths (generally
over 1 km
and under 30 km) of pipe, e.g., steel pipe. For practical management, these
lengths of
pipe are coiled on a steel spool by a simple winding process. The result is
termed a
"Coselle ". Many Coselles may be installed in the hold of a ship. A Coselle
carrying ship loads gas in a first port and discharges in a second port. A
Coselle
carrying ship may make ¨60 cycles per year. The pressurizing and de-
pressurizing
associated with the loading and unloading of gas induces major stress changes
in the
steel of the Coselles , which could possibly result in fatigue cracking. The
long lengths
of pipe are necessarily constructed by welding shorter lengths of pipe
together. The
welds and pipe material adjacent the welds are more sensitive to fatigue
problems than
the parent metal of the pipe.It has become apparent during prototype Coselle
testing where 6" pipe was
wound about a hub of ¨9 meters diameter, that higher than average strains are
experienced in the outside wall of the pipe at the heat affected zone (HAZ)
interface of
the circumferential joining welds, i.e., butt welds, contained in the pipe.
The highly
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localized strains, (approximately 6% as opposed to approximately 1.8% average)
shorten the fatigue life of the weld and are responsible for limiting the
ultimate fatigue
life of the Coselle . As a result, it was necessary to operate at a lower
working
pressure than would be allowed if the fatigue life were extended, which
reduced the gas
volume and raised the overall cost of gas transport.
In addition to exceptional strain, the weld properties may also be adversely
affected by ovalization of the pipe during the bending process. At the four
points of
high and low curvature of the oval, stress is increased during the
pressurization process
and this will reduce fatigue life, particularly for the butt welds.
SUMMARY OF THE INVENTION
It is economically important to ensure a long fatigue life for the butt welds
in the
Coselle pipe pressure vessel by protecting against both excessive strain of
the heat
affected zone and ovalization, which are both induced by the coiling process.
This is
achieved by the clamp apparatus of the invention.
The solution chosen to the excessive strain problem is to severely limit the
bending of the pipe and to prevent ovalling in the region of butt welds and
simultaneously to largely prevent strain in the outside of the pipe. This may
be
accomplished by applying a bend restraining member, such as a clamp, to a pipe
over a
butt weld. The following describes the clamp that was developed and tested
successfully. However, other embodiments of bend restraining members or clamps
could be derived to accomplish the desired results.
An important concern when bending the pipe in the vicinity of a butt weld is
the
straining of the heat affected zone adjacent to the weld, e.g. in the heat
affected zone on
the outside or "stretched" portion of bent pipe. The clamp of the invention
does two
things. First, the clamp largely eliminates bending of the pipe. Second, the
clamp also
prevents the upper half of the pipe from being strained. In one embodiment,
the latter is
accomplished by means of scoring on the inside of the clamp. The narrow ridges
of the
scoring, when compressed onto the pipe by the tightening of bolts, produce a
high
friction force that prevents the outside of the pipe from moving significantly
and so
nearly eliminates the strain of the weld region.
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An example clamp has a length of 300 mm. Preferably, the clamp has a length
of 1.75 external diameters of pipe, but larger or smaller clamps could also be
used. The
clamp functions as a reinforcing brace comprised of two semi-circular,
straight half-
cylinders of steel bolted together on both sides of the pipe. The clamp is
made up of a
first half-cylinder located on the outside of the bend of the pipe, and is
made up of a
second half-cylinder located on the inside of a bend of the pipe. The inside
radius of
curvature of both clamp half-cylinders is preferably equal to the outside
radius of the
pipe. The thickness of the clamp is such that the clamp is not plastically
bent by the
bending forces.
The rigidity of the clamp limits bending of the pipe in the region of the weld
and, with its special features described below, ensures that the localized
axial strains in
the outside wall of the pipe remain low during and after winding.
Simultaneously, the
clamp ensures that ovalization is minimized.
In the preferred embodiment, the outer half of the clamp contains scoring on
its
inside to ensure bonding with the outside surface of the pipe. The primary
mechanism
by which the clamp works to limit strain in the outside of the pipe is inter-
particle static
friction on the surface of the pipe, and this is the reason for providing
scoring. Scoring
(which preferably resembles threading) has a pitch such that the ridges will
bite slightly
into the pipe and an overall contact area so as not to damage the pipe by
compression
during the bending process. For purposes of this application, when Applicant
refers to
"threads" or "threading", what is meant is "having the characteristics of a
helical ridge
of a screw or bolt". The inside half of the clamp is not scored and in fact,
slippage on
the inside of the pipe is not undesirable.
As discussed in the background, localized strains are induced by coiling a 6"
pipe on a hub of approximately 9 m in diameter. The localized strains without
the use
of a clamp were found to be approximately 6% in the area of the weld and 1.8%
average for the pipe. With the use of the clamp of the invention, the
localized strains
were found to be approximately 0.5%.
In the vicinity of the pipe weld, the pipe is almost completely surrounded by
the
rigid clamp. Therefore, ovalization of the pipe is almost completely
prevented.
However, the presence of the clamp can induce serious ovalling, even kinking,
of the
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pipe at both ends of the clamp as the pipe is wound around a spool. To address
this
problem, two shape modifications to the inner half of the clamp are made.
First, the
two ends of the inside half of the clamp are ground away in a specific pattern
to ease the
concentration of force on the pipe at the ends of the clamp. The preferred
shape of the
metal removed, i.e. of the voids, is a semicircle of radius equal to the
radius of the pipe,
but approximations to this semi-circular shape may be used. Secondly,
ovalization in
the vicinity of the clamp may be further reduced by grinding away some of the
thickness of the inside half of the clamp so that it is significantly thinner
at its outside
edges than at its center (roughly half the thickness is preferred). The reason
is to further
minimize the transverse force on the pipe when the clamp first hits the
underlying pipes
beyond the relief provided by the removed semicircles.
While both halves of the clamp have their radii of curvature identical to the
outside of the pipe, the two halves of the clamp do not comprise a complete
3600 turn.
When bolts are fully tightened, the two halves of the clamp preferably do not
touch
each other so that the full force of the clamp halves is impressed on the
pipe.
When the bending of the pipe is complete, the clamp is removed as follows: the
inward force of the pipe is reduced by a very slight reverse turn of the
spooling hub.
The clamp is then disassembled. The outside half of the clamp is removed and
then the
inside half of the clamp is rotated around the pipe and lifted off. To
facilitate the
rotation, the bottom half of the clamp preferably has a reasonably uniform or
reduced
thickness profile in the rotary sense.
In use, the clamp halves are very strongly tightened on the pipe by a series
of
bolts. The scoring and the tightening threads in the bolt holes wear down with
use so
that the clamp will have to be replaced after a number of uses.
Both halves of the clamp have a small channel of metal removed from the inside
centers of the halves in the hoop direction to accommodate the weld bead of
the butt
weld. This obviates the need to grind the weld.
The clamp is preferably made of steel with a Young's Modulus closely equal to
that of the steel of the pipe. The yield stress of the steel used in the clamp
is preferably
at least that of the pipe and up to 40% greater.
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A first embodiment of the invention is rigid and heavy (about 200 lbs weight).
This embodiment totally protects the weld and even with 1,000,000 cycles no
fatigue
damage was found to have occurred to the weld. The first embodiment is,
however,
cumbersome to manipulate. A second embodiment is a lighter version (about 40
lbs).
The clamp described above in detail is a preferred embodiment of a device to
minimize the strain in the outer edge of a pipe and to minimize ovalling in
the region of
a butt weld while being bent around a coil (FIG. 13). Other embodiments may
also be
used.
For example, the clamping force may be provided not by bolts, but by external
hydraulic pressure. The halves of the clamp may be held together by welding or
binding with wire. Alternatively the clamp halves may be significantly heated
before
being applied and welded. The clamping force would be due to metallic
contraction as
the clamp cools. Alternatively, the two clamp halves could be drawn together
by a
buckling mechanism, with or without the assistance of a hydraulic press.
Alternative means of creating friction may be developed to eliminate the need
for the threading, such as metal to metal adhesives, low temperature metal
alloy solders,
and a layer of sharp particles of a substance harder than steel, such as
silicon carbide,
etc. Additionally, strips of steel, centered on the butt weld may be welded
axially on
the pipe. The butt weld and adjacent areas may be wound with a strong material
that is
bonded to the pipe. The material may be steel wire, e-glass, Kevlar or other
material
that could be permanently bonded to the pipe, which would reduce the induced
winding
strains and/or reduce ovalization.
In a further embodiment, a section of pipe whose ID is the same as the OD of
the main pipe may be slipped onto the main pipe before the welding operation.
The
section of pipe may then be centered over the butt weld. This pipe section
resembles
the clamp but may remain in place after the main pipe is bent.
Alternatively, a small pipe section or other steel structure may be inserted
inside
the main pipe and restrained so as to be centered on the butt weld. The small
section of
pipe remains in place after welding and bending.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded perspective view of the pipe weld clamp of the invention
including an inside clamp segment and an outside clamp segment;
FIG. 2 is a partial cross-sectional view of the pipe weld clamp of FIG. I
shown
installed within a pipe;
FIG. 3 is a perspective view of the inside pipe clamp segment of FIG. 1;
FIG. 4 is a schematic view of an outside surface of the inside pipe clamp
segment of FIG. 1;
FIG. 5 is a schematic view of an inside surface of the inside pipe clamp
segment
of FIG. 1;
FIG. 6 is a perspective view of the outside pipe clamp segment of FIG. 1;
FIG. 7 is a schematic view of an outside surface of the inside pipe clamp
segment of FIG_ 1;
FIG. 8 is a schematic view of an inside surface of the inside pipe clamp
segment
of FIG. 1;
FIG. 9 is an elevation view of a modified pipe having scoring formed on ends
adjacent to a circular weld;
FIG. 10 is an elevation view of a modified pipe having small particles affixed
to
ends of pipe segments adjacent to a circular weld;
FIG. 11A is an elevation view of a modified pipe clamp secured to pipe
segments, wherein the modified pipe clamp has Nelson studs affixed within
drilled
holes on an interior surface of the clamp;
FIG. 11B is an enlarged view of the clamp of FIG. 11A showing a Nelson stud
within a hole in a clamp member;
FIG. 12A is an elevation view of a modified pipe having a reinforcing strap or
fin affixed to an exterior thereof, adjacent to a circular weld;
FIG. 12B is a plan view of the modified pipe of FIG. 12A; and
FIG. 13 is a perspective view of the pipe of FIG. 2 wound on a core.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to Figures 1 and 2, shown is pipe clamp designated generally 10.
Pipe clamp 10 is for affixing to pipe 12 (FIGS 2, 13). Pipe 12 consists of a
plurality of
pipe segments joined together end to end via butt welds, e.g., first pipe
segment 14 and
second pipe segment 16 are joined together via butt weld 18. Pipe clamp 10 is
provided
to protect butt weld 18 and its heat affected zones from excessive strain and
ovallization, both induced by a coiling process of pipe 12 to form a Coselle ,
e.g., as
shown in FIG. 13.
Pipe clamp 10 consists of inside clamp segment 20 (FIGS. 1, 3-5). Inside clamp
segment 20 has a semi-cylindrical shape and has a first end 22 and a second
end 24.
Inside clamp segment 20 additionally has an inside surface 26, an upper
lengthwise
mating surface 28, and a lower lengthwise mating surface 30. First end 22
defines first
stress relief area 32. Second end 24 defines second stress relief area 34
(FIGS. 3-5).
Inside clamp segment 20 preferably defines radial groove 36 for receiving butt
weld 18 when inside clamp segment 20 is positioned on pipe 12. Inside surface
26 of
inside clamp segment 20 is preferably substantially smooth for permitting
slippage of
an outside surface of pipe 12 with respect to inside surface 26 of inside
clamp segment
20.
In one embodiment, first stress relief area 32 defines a first semi-circle and
second stress relief area 34 defines a second semi-circle. The first semi-
circle defined
by first stress relief area 32 and a second semi-circle defined by the second
stress relief
area 34 preferably each have a radius approximately equal to the radius of
pipe 12.
Approximations of semi-circular shapes are also contemplated for use as stress
relief
areas.In one embodiment, as best seen .in FIG. 4, a thickness of inside clamp
segment
20 proximate first end 22 and second end 24 is less than a thickness of inside
clamp
segment 20 at a center of inside clamp segment 20.
Pipe clamp 10 additionally consists of outside clamp segment 40 (FIGS 1, 2, 6-
8), having a semi-cylindrical shape. Outside clamp segment 40 has a first end
42 and a
second end 44. Outside clamp segment 40 additionally defines an upper
lengthwise
mating surface 46 and a lower lengthwise mating surface 48. Outside clamp
segment
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40 additionally has an inside surface 50. Preferably, inside surface 50
defines a friction
element 52. Friction element 52 is for gripping an outside surface of pipe 12.
Outside clamp segment 40 additionally preferably defines radial groove 54
(FIGS. 6, 8) for receiving butt weld 18 when outside clamp segment 40 is
located on
pipe 12. In a preferred embodiment, friction element 52 is made up of scoring
on inside
surface 50. More preferably, scoring is made up of threading. An example
threading
has a depth of 0.77" and a peak to peak measurement of 0.166". Friction
element 52
may also be created through use of metal adhesives, low temperature metal
alloy
solders, or by a layer of hard particles located on inside surface 50.
Preferably, first lengthwise surface 28 and second lengthwise surface 30 of
inside clamp segment 20 define a plurality of orifices 60. Additionally, in a
preferred
embodiment, first lengthwise surface 46 and second lengthwise surface 48 of
outside
clamp segment 40 defines a plurality of orifices 62. Plurality of bolts 64
(FIGS. 1, 2)
are provided for locating in plurality of orifices 60 defined by first
lengthwise surface
28 and second lengthwise surface 30 of inside clamp segment 20. Bolts 64
additionally
locate within plurality of orifices 62 defmed by first lengthwise surface 46
and second
lengthwise surface 48 of outside clamp segment 40. Bolts 64 secure inside
clamp
segment 20 to outside clamp segment 40. In a preferred embodiment, bolts 64
have a
head 64a and a shank 64b of smaller diameter. Preferably, inside clamp segment
20
defines orifices 60 that are threaded for receiving threaded shank 64b of bolt
64.
Outside clamp segment 40 preferably defines orifices 62 that are smooth walled
and
define bolt head engaging surface 62a. Therefore, when bolt 64 is located
within holes
60, 62, bolt head engaging surface 62a is drawn towards threads formed in
holes 60,
thereby drawing first lengthwise surface 28 of inside clamp segment 20 towards
first
lengthwise surface 46 of outside clamp segment 40.
In a preferred embodiment, inside clamp segment 20 and outside clamp segment
40 have a thickness such that pipe clamp 10, i.e., inside clamp segment 20 and
outside
clamp segment 40, is not plastically deformed by bending forces induced by the
coiling
process when pipe 12 is bent after pipe clamp 10 is secured to pipe 12.
In a preferred embodiment, inside clamp segment 20 and outside clamp segment
have a radius of curvature substantially equal to a radius of curvature of
pipe 12.
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Preferably, inside clamp segment 20 and outside clamp segment 40 are sized
such that
when inside clamp segment 20 and outside clamp segment 40 are fully tightened
about
pipe 12, first lengthwise surface 28 of inside clamp segment 20 and first
lengthwise
surface 46 of outside clamp segment 40 do not touch one another. Similarly, it
is
preferred that second lengthwise surface 30 of inside clamp segment 20 and
second
lengthwise surface 48 of outside clamp segment 40 do not touch one another so
that the
full compressive force of inside clamp segment 20 and outside clamp segment 40
is
impressed upon pipe 10 when bolts 64 are tightened.
Other methods of compressing pipe 12 within pipe clamp 10 are also
contemplated, including forcing inside clamp segment 20 and outside clamp
segment
40 together with hydraulic pressure. Inside clamp segment 20 and outside clamp
segment 40 may also be forced together with tightened wire. Further, inside
clamp
segment 20 and outside clamp segment 40 may be forced together by metallic
cooling
of inside clamp segment 20 and outside clamp segment 40.
Referring now to FIG. 9, an embodiment of the invention is shown wherein
scoring 90 is formed on the outside of pipe 12 on both sides of butt weld 18
by an
abrasive tool or other suitable method such as by roughening the outside of
pipe 12 by
sanding, etc. The process is the reverse of scoring clamp 10, as described
above, but
has a similar effect. Outside clamp segment 40 of clamp 10 could then have a
smooth
interior.
Referring to FIG. 10, sharp, small particles 100 of a very hard agent (harder
than
the steel, corundum, Aluminum oxide, Carborundum) are inserted (e.g., painted
on in a
viscous liquid or sprinkled onto a still liquid viscous adhesive on an outside
half of pipe
12 or on inside surface 26 of outside clamp segment 40) between a smooth
inside
surface 26 of outside clamp segment 40 and an outside half of pipe 12. Clamp
10 is
then tightened so that particles 100 will penetrate the surfaces of outside
clamp segment
40 and pipe 12, creating a desired friction force.
Referring to FIGS. 11A and 118, an additional alternative pipe bend
restraining
device is shown. Many small holes 110, e.g., 15 mm in diameter, are drilled in
the
upper clamp. Nelson studs 112, e.g., 10 mm diameter x 10 mm high, are affixed,
e.g.,
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spot welded, onto pipe 12. After bending of pipe 12 is complete, clamp 10 is
removed
and studs 112 may be ground down.
Referring to FIGS. 12A and 12B, an alternative pipe bend restraining device is
shown. At least one strap or fin 120 is welded on the outside of pipe 12, on
the outside
of the bend. No clamp is required. Fin 120 may be, for example, 20" long, 4
inches
high and 0.4" thick. Fin 120 preferably has a small circle cut out 122 (FIG.
9B) in the
middle to accommodate the weld bead of butt weld 18. Fin 120 will prevent the
outside
of pipe 12 from being strained while pipe 12 is being bent. Once pipe 12 is on
the
Coselle, fm 120 should be cut off and the surface ground smooth.
It is further contemplated that a pipe segment may be centered over butt weld
18. It is additionally contemplated that a pipe segment may be Centered under
butt weld
18 inside of pipe 12.
When the bending of pipe 12 is complete, clamp 10 is removed as follows: the
inward force of pipe 12 is reduced by a very slight reverse turn of the
spooling hub 66
(FIG. 13). Clamp 10 is then disassembled. Outside clamp segment 40 is removed
and
then inside clamp segment 20 of the clamp 10 is rotated around pipe 12 and
lifted off.
To facilitate the rotation, the bottom half of the clamp 10 preferably has a
reasonably
uniform or reduced thickness profile in the rotary sense.
In use, clamp segments 20, 40 are very strongly tightened up on pipe 12 by a
series of bolts 64. Other clamp mechanisms may also be used. Friction element
52,
e.g., scoring, small particles or studs, and the tightening threads in the
bolt holes 60
wear down with use so that clamp 10 will have to be replaced after a number of
uses.
Both segments 20, 40 of clamp 10 have a small channel or radial groove 36 of
metal removed from the inside surface 26 of clamp segment 20 and inside
surface 50 of
clamp segment 40 in the hoop direction to accommodate the weld bead of weld
18. This
obviates the need to grind the weld bead of weld 18.
The clamp 10 is preferably made of steel with a Young's Modulus closely equal
to that of the steel of pipe 12. The yield stress of the steel used in the
clamp 10 should
be at least that of pipe 12 and up to 40% greater.
* * * *
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Thus, the present invention is well adapted to carry out the objectives and
attain
the ends and advantages mentioned above as well as those inherent therein.
While
presently preferred embodiments have been described for purposes of this
disclosure,
numerous changes and modifications will be apparent to those of ordinary skill
in the
art. Such changes and modifications are encompassed within the spirit of this
invention
as defined by the specification.
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