Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR EXPANDING A STEEL TUBING AND WELL WITH SUCH A
TUBING
The invention relates to expansion of tubings. More
particularly the invention relates to a method of
expanding a steel tubing by moving an expansion mandrel
through the tubing.
Numerous methods and devices are known for expansion
of tubings.
European patent specification 643794 discloses a
method of expanding a casing against the wall of an
underground borehole wherein the casing is made of a
malleable material which preferably is capable of plastic
deformation of at least 25% uniaxial strain and the
casing may be expanded by an expansion mandrel which is
pumped, pulled or pushed through the casing.
Other expansion methods and devices are disclosed in
German patent specification No. 1583992 and in US patent
specification Nos. 3,203,483; 3,162,245; 3,167,122;
3,326,293; 3,785,193; 3,489,220; 5,014,779; 5,031,699;
5,083,608 and 5,366,012.
Many of the known expansion methods employ an
initially corrugated tube and the latter prior art
reference employs a slotted tube which is expanded
downhole by an expansion mandrel.
The use of corrugated or slotted pipes in the known
methods serves to reduce the expansion forces that need
to be exerted to the tube to create the desired
expansion.
A method is known from US patent specification No. 5,366,012. In
this known method a slotted tube is expanded by an
expansion mandrel having a tapering expansion section.
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It is an object of the present invention to
provide a method for expanding an at least partly solid,
i.e. unslotted, tubing which requires exertion of a low
force to expand the tubing and which provides a tubing
having a larger diameter and higher strength than the
unexpanded tubing and which can be carried out with a tubing
which already may have a tubular shape before expansion.
The method according to the invention thereto
comprises the step of moving an expansion mandrel of which
the tapering expansion section has a tapering ceramic outer
surface through an at least partly solid tubing which is
made of a formable steel grade which is subject to strain
hardening without incurring any necking and ductile
fracturing as a result of the expansion process.
A broad aspect of the invention provides a method
of expanding an at least partly solid steel tubing which is
made of a formable steel grade subject to strain hardening,
the method comprising the step of moving an expansion
mandrel having a tapering expansion section which has a
tapering ceramic outer surface, through the tubing thereby
plastically expanding the tubing.
As a result of strain hardening the tubing becomes
stronger during the expansion process since for any further
increment of expansion always a higher stress is required
than for the preceding expansion.
It has been found that the use of a formable steel
grade for the tubing in combination with a ceramic tapering
outer surface of the expansion mandrel has a synergetic
effect since the resulting expanded tubing will have an
adequately increased strength while the expansion forces
remain low. The low yield strength and high ductility of
the tubing before expansion enables, if the tubing is to be
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used in an underground borehole, the use of a tubing which
is reeled from a reeling drum into the borehole.
It is observed that in the art of metallurgy the
terms strain-hardening and work-hardening are synonyms and
are both used to denote an increase of strength caused by
plastic deformation.
The term formable steel grade as used in this
specification means that the tubing is able to maintain
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its structural integrity while being plastically deformed
into various shapes.
Ways of determining forming characteristics of a
steel are set out in the Metals Handbook, 9th edition,
volume 14, Forming and Forging, issued by ASM
Internationa.L, Metals Park, Ohio (USA).
The term necking refers to a geometrical effect
leading to non-uniform plastic deformations at some
location by occurrence of a local constriction. From the
point of nec:{ing on, the continual work hardening in the
necked region no longer compensates for the continual
reduction of the smallest cross-section in the neck, and
therefore, the load carrying capacity of the steel
decreases. W:Lth continuing loading, practically all
further plasi--ic deformation is restricted to the region
of the neck, so that a highly non-uniform deformation
occurs to develop in the necked region until fracture
occurs.
The term ductile fracturing means that a failure
occurs if pliistic deformation of a component that
exhibits duc,--ile behaviour is carried to the extreme so
that the component separates locally into two pieces.
Nucleation, growth and coalescence of internal voids
propagate to failure, leaving a dull fibrous rupture
surface. A detailed description of the terms necking and
ductile frac-:uring is given in the handbook "Failure of
Materials in Mechanical Design" by J.A. Collins second
edition, issl.ied by John Wiley and Sons, New York (USA) in
1993.
Preferably the tubing is made of a high-strength
steel grade with formability and having a yield strength-
tensile strength ratio which is lower than 0.8 and a
yield strength of at least 275 MPa. When used in this
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specification, the term high-strength steel denotes a
steel with a yield strength of at least 275 MPa.
It is also preferred that the tubing is made of a
formable steel grade having a yield stress/tensile stress
ratio which is between 0.6 and 0.7.
Dual phase (DP) high-strength, low-alloy (HSLA)
steels lack a definite yield point which eliminates
Luders band formation during the tubular expansion
process which ensures good surface finish of the expanded
tubular.
Suitable HSLA dual phase (DP) steels for use in the
method according to the invention are grades DP55'"' and
DP60TM developed by Sollac having a tensile strength of at
least 550 MPa and grades SAFH 540 DTM and SAFH 590 DT'
developed by Nippon Steel Corporation having a tensile
strength of at least 540 MPa.
It is observed that US patent specification
No. 4,938,266 discloses a method for producing dual phase
steels.
Other suitable steels are the following formable
high-strength steel grades
- an ASTM A106 high-strength low alloy (HSLA) seamless
pipe;
- an ASTM A312 austenitic stainless steel pipe, grade
TP 304 L;
- an ASTM A312 austenitic stainless steel pipe, grade
TP 316 L; and
- a high-retained austenite high-strength hot-rolled
steel (low-alloy TRIP steel) such as grades
SAFH 590 ETM, SAFH 690 E'"" and SAFH 780 ETM developed by
Nippon Steel Corporation.
The above-mentioned DP and other suitable steels each
have a strain hardening exponent n of at least 0.16 which
allows an expansion of the tubing such that the external
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diameter of the expanded tubing is at least 20% larger
than the external diameter of the unexpanded tubing.
Detailed explanations of the terms strain hardening,
work hardening and the strain hardening exponent n are
given in chapters 3 and 17 of the handbook "Metal
Forming-Mechanics and Metallurgy", 2nd edition, issued by
Prentice Hall, New Jersey (USA), 1993.
Suitably, the expansion mandrel contains an expansion
section that has a conical ceramic outer surface. It is
observed that US patent specification No. 3,901,063
discloses a plug having a conical ceramic outer surface
for use in tube-drawing operations. If the expansion
mandrel is pumped through the tubing then the mandrel
preferably comprises a sealing section which is located
at such a distance from the tapering expansion section
that when the expansion mandrel is moved through the
tubing by means of exerting a hydraulic pressure behind
the mandrel tr.e sealing section engages a plastically
expanded part of the tubing. This will generally be
achieved if said distance is at least three times the
wall thickness of the expanded tubing.
The use of' a ceramic conical surface reduces friction
forces during the expansion process and by having a
sealing section which engages the expanded tube it is
avoided that hydraulic forces would result in an
excessive expansion of the tubing.
In such caLse it is preferred that the expansion
mandrel contains a vent line for venting to the surface
any fluids that are present in the borehole and tubing
ahead of the expansion mandrel.
Alternatively the tubing is expanded such that the
outer diameter of the expanded tubing is slightly smaller
than the internal diameter of the borehole or of any
casing that is present in the borehole and any fluids
that are present in the borehole and tubing ahead of the
;aN1ENDED SHEEi'
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expansion mandrel are vented to surface via the annular
space that remains open around the tubing after the
expansion process.
MD03/TS6025PC7.'
AMENDED SHEET
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The invention also relates to a well provided with a
tubing which is expanded using the method according to
the inventior-. In such case the tubing may serve as
production tubing through which hydrocarbon fluid is
transported to the surface and a reelable service and/or
kill line passes through at least a substantial part of
the length of: the tubing, through which line fluid can be
pumped towarcis the bottom of the borehole while
hydrocarbon fluid is produced via the surrounding
production tubing. The use of such an expanded production
tubing allows the use of almost the full wellbore for the
transport of hydrocarbon fluids so that a relatively slim
borehole may be utilized to attain the desired production
rate.
Alternatively the tubing may be expanded against the
inner surface of a casing which is present in the bore-
hole. In such case the tubing may either be used as a
production ttibing and/or as a protective cladding for
protecting the well casing against corrosive well fluids
and damage from tools that may be lowered into the well
during maintenance and workover operations.
These anci other objects, features and advantages of
the method and well system according to the present
invention will be apparent from the accompanying claims,
abstract and the following detailed description with
reference to the accompanying drawing, in which
Fig. 1 is schematic longitudinal sectional view of an
underground borehole in which a tubing is expanded in
accordance w:-th the method according to the invention.
Now referring to Fig. 1, there is shown a borehole
traversing arl underground formation 1 and a casing 2 that
is fixed within the borehole by means of an annular body
of cement 3.
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A production tubing 4 which is made of a dual phase,
high-strength low-alloy (HSLA) steel or other formable
high-strength steel is suspended within the casing 2.
An expansion mandrel 5 is moved in longitudinal
direction through the tubing 4 thereby expanding the
tubing 4 such that the outer diameter of the expanded
tubing is slightly smaller than or is about equal to the
internal diarreter of the casing 2.
The expansion mandrel 5 is equipped with a series of
ceramic surfaces 6 which restrict frictional forces
between the pig and tubing 4 during the expansion
process. In the example shown the semi top angle A of the
conical cerair.ic surface that actually expands the tubing
is about 25 . It has been found that zirconium oxide is a
suitable ceramic material which can be formed as a smooth
conical ring. Experiments and simulations have shown that
if the semi cone top angle A is between 20 and 30 the
pipe deforms such that it obtains an S-shape and touches
the tapering part of the ceramic surface 6 essentially at
the outer tip or rim of said conical part and optionally
also about halfway the conical part.
The experiments also showed that it is beneficial
that the expanding tubing 4 obtains an S-shape since this
reduces the length of the contact surface between the
tapering part of the ceramic surface 6 and the tubing 4
and thereby also reduces the amount of friction between
the expansior.~. mandrel 5 and the tubing 4.
Experiments have also shown that if said semi top
angle A is smaller than 15 this results in relatively
high frictior.al forces between the tube and pig, whereas
is said top angle is larger than 30 this will involve
redundant plastic work due to plastic bending of the
tubing 4 which also leads to higher heat dissipation and
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to disruptions of the forward movement of the pig 5
through the tubing 4. Hence said semi top angle A is
preferably selected between 15 and 30 and should always
be between 5 and 45 .
ExperimentS have also shown that the tapering part of
the expansion mandrel 5 should have a non-metallic outer
surface to avoid galling of the tubing during the
expansion process. The use of a ceramic surface for the
tapering part of the expansion mandrel furthermore caused
the average roughness of the inner surface of the
tubing 4 to decrease as a result of the expansion
process. The experiments have also shown that the
expansion mandrel 5 provided with a ceramic tapering
surface 6 could expand a tubing 5 made of a forinable
steel such that the outer tubing diameter D2 after
expansion was at least 20% larger than the outer diameter
Dl of the unexpended tubing and that suitable formable
steels are dual phase (DP) high-strength low.alloy (HSLA)
steels known as DP55TM and DP60TM; ASTM A106 HSLA seamless
pipe, ASTM A312 austenitic stainless steel pipes, grades
TP 304 L and TP 316 L and a high-retained austenite high-
strength hot rolled steel, known as TRIPTM steel manu-
factured by the Nippon Steel Corporation.
The mandrel 5 is provided with a pair of sealing
rings 7 which are located at such a distance from the
conical ceramic surface 6 that the rings 7 face the
plastically expanded section of the tubing 4. The sealing
rings serve to avoid that fluid at high hydraulic
pressure would be present between the conical ceramic
surface 6 of the mandrel 5 and the expanding tubing 4
which might lead to an irregularly large expansion of the
tubing 4.
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The expansion mandrel 5 is provided with a central
vent passage 7 which is in communication with a coiled
vent line 8 through which fluid may be vented to the
surface. After completion of the expansion process the
pig 5 may be pulled up to surface by the vent line and a
coiled kill and/or service line (not shown) may be
lowered into the expanded tubing 4 to facilitate
injection of kill and/or treatment fluids towards the
hydrocarbon f'luid inflow zone which is normally be done
via the annulus between the production tubing and the
well casing. However, if the tubing 4 is expanded to a
smaller diameter then the residual annular space between
the casing 2 and expanded tubing 4 can be used for
venting of fluids during the expansion process and for
injection of fluids during the production process, in
which case there is no need for using a vent line 8 and
kill and/or s.ervice lines.
In conventional wells it is often necessary to use a
production tubing having an outer diameter which is less
than 50% of the inner diameter of the well casing to
enable a smooth insertion of the tubing even if the well
is deviated and the casing has an irregular inner
surface. Therefore it is apparent that the in-situ tubing
expansion method according to the present invention
enhances an efficient use of the wellbore.
It will be understood that instead of moving the
expansion mar.Ldrel through the tubing by means of
hydraulic pressure, the mandrel can also be pulled
through the tubing by means of a cable or pushed through
the tubing by means of pipe string or rod.
The method according to the invention can also be
used to expand tubings that are used outside a wellbore,
for example to expand oilfield tubulars at surface
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facilities or to expand a tubing inside an existing
tubing which has been damaged or corroded.
The invention will now be further described on the
basis of the following comparative experiments.
Experiment 1
An expansion mandrel having a conical ceramic surface
(semi top angle A of cone = 20 ) was moved through a
conventional oil field tubular, known as casing grade L80
13% Cr, which is a widely used casing type, having an
initial outer diameter of 101.6 mm (4"), an initial wall
thickness of 5.75 mm, a burst pressure of 850 bar and a
strain hardening exponent n = 0.075. The expansion
mandrel was designed such that the outer diameter of the
expanded tubular would be 127 mm, so that the increase in
diameter would be 20%. The tubular burst during the
expansion process. Analysis showed that the ductility
limit of the material had been exceeded so that ductile
fracturing occurred.
Experiment 2
An experiment was carried out with a coiled tubing of
the type QT-800 which is increasingly used as a pro-
duction tubing in oil or gas wells. The tubing had an
initial outer diameter of 60.3 mm, a wall thickness of
5.15 mm, a burst pressure of 800 bar and a strain
hardening exponent n= 0.14. An expansion mandrel was
moved through the tubing which mandrel comprised a
conical ceramic surface such that the semi top angle A of
a cone enveloping the conical surface was 5 and which
was designed such that the outer diameter of the expanded
tubing would be 73 mm (increase of about 210). This
tubing burst during the expansion process. Analysis
revealed that due to high friction forces the expansion
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pressure had exceeded the burst pressure of the pipe
during the expansion process.
Experiment 3
An experiment was carried out with a seemless pipe
made of a formable steel grade known as ASTM
A 106 Grade B. The pipe had an initial outer diameter of
101.6 mm (4"), an initial wall thickness of 5.75 mm and a
strain hardening exponent n = 0.175.
An expansion mandrel was pumped through the pipe,
which mandrel comprised a ceramic conical surface such
that the semi top angle A of a cone enveloping the
conical surface was 20 and such that the outer diameter
of the expanded pipe was 127 mm (5") and the outer
diameter incileased by 21%.
The pipe was expanded successfully and the hydraulic
pressure exerted to the mandrel to move the mandrel
through the pipe was between 275 and 300 bar. The burst
pressure of the expanded pipe was between 520 and
530 bar.
