[0001] This invention relates, in general, to the prestressing of tubular apparatus, for
example elongated conduits for conveying hot or cold fluid, and in particular to prestressed
tubular apparatus and methods of manufacturing and prestressing tubular apparatus
made of two or more nested (e.g. coaxial) tubular members or tubes.
[0002] Heavy oil and tar sands represent huge untapped resources of liquid hydrocarbons
which will be produced in increasing quantities to help supplement declining production
of conventional crude oil. These deposits must, however, be heated to reduce the oil
viscosity before it will flow to the producing wells in economical quantities. A dominant
method of heating is by injection of surface generated steam in either a continuous
(steam flood) or intermittent (steam stimulation or "huff and puff") mode.
[0003] When steam is injected down long injection pipes or "strings", a significant amount
of thermal energy is lost to the rock overburden (150 to 2130 m or 500 to 7,000 feet)
which covers the oil deposit. In the initial steam injection projects, the price of
oil did not justify the prevention of this heat loss, but now with the price of oil
at $30 or more a barrel, insulation systems for the well injection pipe have become
economically justified.
[0004] Thermally insulated double wall piping structures are known and used, for example,
as insulated steam injection tubing in oil wells, or in pipelines for carrying fluids
at elevated temperatures. Such piping is disclosed, for example, in US Patent No.
3 574 357 to Alexandru et al and US Patent No. 3 397 745 to Owens et al.
[0005] It is common practice for such tubes to be prestressed in order to compensate for
differential expansion of the inner and outer coaxial walls or tubes. Such prestressing
is done, for example, by elongating the inner tube through such means as heating or
mechanically stretching and attaching the outer tube while the inner tube is in such
an elongated state. While still held in the elongated state, any heat treatment required
for the attachment is completed. However, it is difficult to heat treat welds while
the tubes are under stress. For this reason, it is believed that such heat treatment
of the welds is not normally done in the industry, resulting in welds which are more
brittle, more damage prone, and more corrosion prone.
[0006] After cool down of the heat treatment, if any, the heating or mechanical stretching
is then removed and the tubes assume a state of tensile prestress on the inner tube
and. compressive prestress on the outer tube. While in service, carrying a hot fluid,
the inner tube becomes hot and expands. This relaxes the tensile prestress before
the inner tube goes into compression. In this manner, the inner tube is prevented
from buckling.
[0007] An an analogous fashion, where the inner tube is to be used to convey cold fluids,
the outer tube is heated or mechanically stretched before the inner tube is connected
thereto.
[0008] A disadvantage of these prior approaches to prestressing double walled tubes or conduits
is that the inner, outer, or both tubes (also referred to hereinafter as "tubulars")
must be held in their compressed or stretched state while other manufacturing steps,
such as the connection of the tubes, the heat treatment thereof and the cool-down
therefrom, are accomplished.
[0009] According to the present invention there is provided a method of prestressing tubular
apparatus having at least one inner tubular and an outer tubular connected to the
inner tubular at at least two spaced locations along the length thereof, the method
being characterised by:
heating at least a portion of one of the inner and outer tubulars to a temperature
sufficient for reducing the yield strength of said portion of said one of the inner
and outer tubulars to a yield strength which is less than the yield strength of the
other of the inner and outer tubulars;
stretching the inner and outer tubulars by a selected amount which is beyond the yield
point of said one tubular and which is not beyond the yield point of said other tubular;
and
permitting said one of the inner and outer tubulars to cool while said tubulars are
stretched whereby the tubular apparatus is prestressed.
[0010] The invention also provides a method of prestressing a tubular apparatus having at
least one inner tubular and an outer tubular connected to the inner tubular at two
spaced locations along the length thereof, the method being characterised in that
the inner and outer tubulars are of materials having different yield strengths and
the inner and outer tubulars are mechanically stretched so that the tubular having
the lower yield strength is stretched beyond its lower yield strength.
[0011] Further, the invention provides a prestressed tubular apparatus characterised by:
at least one inner tubular made of material having a first yield strength;
an outer tubular positioned around the inner tubular and made of a material having
a second yield strength; and
at least two joints mechanically connecting the inner and outer tubulars at spaced
locations along the length thereof;
the first and second tubulars being in a stretched state sufficient to have plastically
deformed the one of said inner and outer tubulars having a lower yield strength but
not to have plastically deformed the other of said inner and outer tubulars having
a higher yield strength.
[0012] According to a preferred embodiment of the present invention described hereinbelow,
a desired state of prestress is established in a double wall tubing structure, while
difficulties and disadvantages of the prior art methods are avoided or at least alleviated.
According to the preferred embodiment, tubes or pipes (also referred to herein as
"tubular members" or "tubulars") of the double wall tubing structure are assembled
and fixedly joined to each other without prestressing. Any required heat treatment
of the structure or the joint is then performed, again without any prestress condition.
To achieve a prestress, the outer tube is locally heated to reduce its yield strength
and then is mechanically stressed beyond its yield strength. The heat source is removed
so that the mechanical stretching is rendered permanent. The outer tube is thus plastically
deformed while the inner tube remains elastic. After cooling, the load establishing
the mechanical stretching can be removed. Upon complete cooling, the desired prestress
condition is present with a tensile force on the inner tube and a compressive force
on the outer tube.
[0013] This structure is useful in conveying hot fluids such as steam in the inner tube
portion.
[0014] Where cold fluids, such as liquefied natural gas, are to be conveyed, it is desirable
to have a tensile prestressing on the outer tube and a compressive prestressing on
the inner tube. This can be achieved by heating at least a portion of the inner tube
to reduce its yield strength and mechanically stressing the inner tube beyond its
yield strength. The heat source is then removed. The inner tube is thus plastically
deformed while the outer tube remains elastic.
[0015] The preferred embodiment of the present invention eliminates the need to maintain
the elongation of one tube relative to the other tube while joining them together
or the need to maintain such elongation while perfoming heat treatment operations.
This simplifies these operations and reduces their cost, especially since heat treatment
of the members connecting the tubulars is very difficult to perform while the tubulars
are in a prestressed condition. The present method permits the prestressing to be
performed at a convenient time in the production sequence and after any operations
which may produce rejectable parts. Thus, the prestressing steps are achieved only
after all previous steps have been accomplished satisfactorily. This results in a
faster and less expensive production sequence and decreases the production investment
in rejectable parts.
[0016] Accordingly, the invention provides a method of prestressing a double wall tube having
an inner tubular and an outer tubular connected to the inner tubular at at least two
spaced locations along their length, the method comprising heating at least a portion
of one of the inner and outer tubulars sufficiently to reduce the yield strength thereof,
mechanically stretching said one of the inner and outer tubulars to elongate said
one of the inner and outer tubulars by a selected amount, and permitting said one
of the inner and outer tubulars to cool.
[0017] The invention also provides a method of manufacting a prestressed double wall tube
having an inner tubular connected to an outer tubular at at least two spaced locations
along their length, wherein the inner tubular is of a material having a different
yield strength than the outer tubular and the tubular which has a lower yield is stretched
past its yield point but the tubular which has the higher yield strength is not stretched
past its yield point to prestress the double wall tube.
[0018] The invention will now be further described by way of illustrative and non-limiting
example, with reference to the accompanying drawings, in which:
Figure 1 is a side sectional view of a double wall tube embodying the invention, showing
at the top half an unstressed condition and at the bottom half a prestressed condition;
Figure 2 is a graph showing the relationship between stresses in outer and inner tubular
members of the double wall tube after prestressing due to an externally applied force;
Figure 3 is a graph showing the yield strength of a typical carbon steel versus temperature;
Figure 4 is a graph showing the stress in the inner tubular member as it relates to
the stress in the heated outer tubular member during the prestressing process;
Figure 5 is a graph showing the relationship between stress and strain for a typical
carbon steel at 5930C (1100°F); and
Figure 6 is a graph relating the plastic (heated) length of the outer tubular member
to the plastic strain needed for a given total elongation.
[0019] A description will now be given, with reference to the drawings, of a method of prestressing
a double wall tube, generally designated at 10 in Figure 1, which comprises an outer
tube or tubular member (hereinafter abbreviated to "tubular") 12 and an inner tube
or tubular 14 which are connected to each other at axially spaced joints 16 and 18,
which are preferably at or near the ends of the tubulars 12, 14.
[0020] The upper half of Figure 1 shows the double wall tube 10 before it is prestressed.
In the embodiment shown, the length L
0 is chosen to be 12.2 m (40 ft) and the material, at last of the outer tubular, is
chosen to be carbon steel.
[0021] The lower half of Figure 1 shows the stretched and prestressed state of the double
wall tube 10. The length has been increased by an amount A L.
[0022] For this example, suppose that the tubulars are chosen to be:
Outer tubular: 12.2 m (40 ft) long 114.3 mm (4.5 in) outside diameter (OD) 6.88 mm
(0.271 in) wall carbon steel 379 MPa (55 klbf/in2 or "KSI") room temperature yield strength Area of cross section = 23.23 cm 2 (3.600 in2 );
Inner tubular: 12.2 m (40 ft) long 73.0 mm (2.875 in) outside diameter (OD) 5.51 mm
(0.217 in) wall carbon steel 552 MPa (80 KSI) room temperature yield strength Area
of cross section = 11.69 cm2 (1.812 in2);
and that the desired level of prestress in the inner tubular is 172 MPa (25 KSI) tension.
At isothermal conditions (same temperature on both tubulars), the corresponding stress
in the outer tubular is 86.9 MPa (12.6 KSI) compression.
[0023] The inner tubular 14 is inserted into the outer tubular 12, the tubulars are welded
together at each end with no prestress, and the welds are heat- treated as required.
[0024] To produce the desired condition of prestress, the outer tubular 12 is first heated
to 593°C (1100
0F) over a length of 305 mm (12 in). A typical stress-strain curve for a carbon steel
at this temperature is shown in Figure 5. Both tubulars 12, 14 are then subjected
to a load of 1.209 MN (271.8 Kips (thousand pounds force)). This load produces a stress
in the inner tubular 14 of 517 MPa (75 KSI) tension (elastic) and in the outer tube
of 260 MPa (37.75 KSI) tension. In the heated portion of the outer tubular 12, this
stress produces 5% plastic strain, while in the cooler portion, the stress is still
elastic. The 5% plastic strain over a 305 mm (12 in) length results in a total overall
length increase of 15.2 mm (0.6 in). When the outer tubular 12 cools to about 427°C
(800°F), the load is removed. When the outer tubular 12 has cooled to room temperature,
the 15.2 mm (0.6 in) length increase results in the desired stress state: 172 MPa
(25 KSI) tension in the inner tubular and 86.9 MPa (12.6 KSI) compression in the outer
tubular 12.
[0025] In its prestressed condition, the inner tubular 14 thus is exposed to an incremental
stress
Cï of 172 MPa (25 KSI). Factoring in the difference in area of the inner and outer tubulars,
this corresponds to a compressive stress on the outer tubular of σ = 86.9 MPa (12.6
KSI).
[0026] Figure 2 shows the relationship between the incremental stresses on the inner and
outer tubulars with a maximum on the outer tubular being 259 MPa (37.5 KSI). This
maximum level is established since above this level the yield strength for the inner
tubular is approached.
[0027] Figure 3 shows the relationship between temperature in degrees Fahrenheit and yield
strength for a typical carbon steel used for the outer tubular (e.g. 8260 annealed
steel). In order to reduce the yield strength to less than 259 MPa (37.5 KSI), a temperature
of at least about 538°C (1000 F) is required. In fact, the yield strength must be
somewhat lower since the outer tubular 12 must not only yield but it must also undergo
some strain.
[0028] Figure 4 illustrates how the force applied to the outer tubular 12 initially effects
a linear increase in length. Once the yield point is reached for the outer tubular
12, however, the increase becomes non-linear and corresponds to plastic deformation
of the outer tubular. With a release of the load, the prestress on the inner tubular
14 decreases until it reaches the desired level of 172 MPa (25 KSI). This is a condition
which is in equilibrium with the 86.9 MPa (12.6 KSI) compressive prestress on the
outer tubular 12.
[0029] By selecting the temperature and the heated length for the outer tubular 12, the
prestress on the inner tubular 14 can be controlled. The stress (strain state) at
the completion of yielding must fall on the curve shown in Figure 2. Once the stress-strain
curve for the outer tubular 12 is known, the heated length can be determined, as can
the temperature of the operation.
[0030] As long as the temperature is such that the minimum yield of the outer tube is greater
than 86.9 MPa (12.6 KSI), it is probably not necessary to hold the prestress once
the yielding has occurred. This is assuming that the heated length is short enough
as not to buckle.
[0031] The required plastic deformation (L L) is about 15.2 mm (0.6 in) with the plastic
strain needed as a function of the heated length being shown in Figure 6.
[0032] The double wall tube described above is useful where the inner tubular 14 is intended
to convey heated substances such as steam. Where the inner tubular 14 is intended
to convey cold substances such as liquefied natural gas, the inner tubular 14 rather
than the outer tubular 12 can be heated and stretched.
[0033] As an alternative measure, the material making up the inner and outer tubulars can
be chosen to have different yield strengths, with the tubular to be plastically deformed
having the lower yield strength.
[0034] It is noted that two or more inner tubulars or tubes may be provided within the outer
tubular or tube and may be prestressed to different levels. This is possible by providing
the tubulars with different yield strengths. The inner tubulars may be axially spaced
and aligned, disposed one next to the other or one within the other.
[0035] It is also advantageous to insulate the annular space formed between the inner and
outer tubulars. This can be done by providing fibres or layered insulation which is
preferably wrapped around the inner tubular. A thermal barrier can also be established
by evacuating the annular space. The evacuated space may be used in conjunction with
the fibrous or layered insulation, or alone. To maintain the vacuum over a prolonged
period of use for the tubing, a getter material is provided, preferably at a high
temperature location within the annular space, that absorbs such gases. Such a getter
material is preferably adjacent the inner tube and activatable at a temperature between
204°C and 371°C (400°F and 700°F). Gases which may leak into the vacuum include hydrogen
formed by corrosion on the outer tubular migrating through the outer tubular and such
gases as nitrogen and carbon monoxide outgassed from the material of the inner tubular.
[0036] In an alternative embodiment of this invention, the inner tubular 14 is composed
of a material which has a higher yield strength than the material of the outer tubular
12, and the stress in the inner tubular 14 is allowed to exceed its yield strength
while the outer tubular 12 is stretched such that its yield strength is exceeded.
This results in a prestressed condition which is limited by the difference in the
yield strengths of the tubulars.
1. A method of prestressing tubular apparatus having at least one inner tubular (14)
and an outer tubular (12) connected to the inner tubular at at least two spaced locations
(16, 18) along the length thereof, the method being characterised by:
heating at least a portion of one of the inner and outer tubulars (14, 12) to a temperature
sufficient for reducing the yield strength of said portion of said one of the inner
and outer tubulars to a yield strength which is less than the yield strength of the
other of the inner and outer tubulars;
stretching the inner and outer tubulars (14, 12) by a selected amount which is beyond
the yield point of said one tubular and which is not beyond the yield point of said
other tubular; and
permitting said one of the inner and outer tubulars to cool while said tubulars are
stretched whereby the tubular apparatus is prestressed.
2. A method according to claim 1, including heating and mechanically stretching the
outer tubular (12) so as to apply a compressive prestressing thereto and so as to
apply a tensile prestressing to the inner tubular (14).
3. A method according to claim 1, including heating and stretching the inner tubular
(14) so as to apply a compressive prestressing thereto and so as to apply tensile
prestressing to the outer tubular (12).
4. A method of prestressing a tubular apparatus having at least one inner tubular
(14) and an outer tubular (12) connected to the inner tubular at two spaced locations
(16, 18) along the length thereof, the method being characterised in that the inner
and outer tubulars (14, 12) are of materials having different yield strengths and
the inner and outer tubulars are mechanically stretched so that the tubular having
the lower yield strength is stretched beyond its lower yield strength.
5. A prestressed tubular apparatus characterised by:
at least one inner tubular (14) made of material having a first yield strength;
an outer tubular (12) positioned around the inner tubular (14) and made of a material
having a second yield strength; and
at least two joints (16, 18) mechanically connecting the inner and outer tubulars
(14, 12) at spaced locations along the length thereof;
the first and second tubulars (14, 12) being in a stretched state sufficient to have
plastically deformed the one of said inner and outer tubulars having a lower yield
strength but not to have plastically deformed the other of said inner and outer tubulars
having a higher yield strength.