U.S. patent application number 11/593338 was filed with the patent office on 2007-05-24 for containers and methods for containing pressurized fluids using reinforced fibers and methods for making such containers.
Invention is credited to Ronald R. Bowen, Moses Minta.
Application Number | 20070113959 11/593338 |
Document ID | / |
Family ID | 28457177 |
Filed Date | 2007-05-24 |
United States Patent
Application |
20070113959 |
Kind Code |
A1 |
Minta; Moses ; et
al. |
May 24, 2007 |
Containers and methods for containing pressurized fluids using
reinforced fibers and methods for making such containers
Abstract
Containers suitable for storing pressurized fluids at cryogenic
temperatures of -62.degree. C. (-80.degree. F.) and colder are
provided and comprise a self-supporting liner and load-bearing
composite overwrap, whereby means are provided for substantially
preventing failure of the container during temperature changes.
Inventors: |
Minta; Moses; (Sugar Land,
TX) ; Bowen; Ronald R.; (Magnolia, TX) |
Correspondence
Address: |
Brent R. Knight;ExxonMobil Upstream Research Company
P. O. Box 2189
CORP-URC-SW348
Houston
TX
77252-2189
US
|
Family ID: |
28457177 |
Appl. No.: |
11/593338 |
Filed: |
November 6, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10396895 |
Mar 25, 2003 |
7147124 |
|
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11593338 |
Nov 6, 2006 |
|
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60367824 |
Mar 27, 2002 |
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Current U.S.
Class: |
156/184 |
Current CPC
Class: |
F17C 2201/0128 20130101;
F17C 2260/013 20130101; F17C 1/002 20130101; F17C 2203/0621
20130101; F17C 2205/0305 20130101; F17C 2203/0651 20130101; F17C
2223/033 20130101; F17C 2201/054 20130101; F17C 2223/035 20130101;
F17C 2203/0673 20130101; F17C 2201/0123 20130101; F17C 2201/035
20130101; F17C 2203/0607 20130101; F17C 2260/012 20130101; F17C
2203/0663 20130101; F17C 2205/018 20130101; F17C 2203/0619
20130101; F17C 2201/0109 20130101; F17C 2209/221 20130101; F17C
2209/232 20130101; F17C 2270/0105 20130101; F17C 2201/052 20130101;
F17C 2223/0161 20130101; F17C 2221/033 20130101; F17C 2203/0604
20130101; F17C 2203/0646 20130101; F17C 2203/066 20130101; F17C
2203/0639 20130101; F17C 2203/0648 20130101 |
Class at
Publication: |
156/184 |
International
Class: |
B31C 1/00 20060101
B31C001/00 |
Claims
7. A method of making a container suitable for storing a
pressurized fluid at a pressure of about 1035 kPa (150 psia) to
about 7590 kPa (1100 psia) and at a temperature of about
-123.degree. C. (-190.degree. F.) to about -62.degree. C.
(-80.degree. F.), said method comprising the steps of: (a)
constructing a self-supporting liner, said self-supporting liner
being suitable for providing a substantially impermeable barrier to
said pressurized fluid; and (b) overwrapping said self-supporting
liner with adequate composite materials to form a load-bearing
vessel in contact with said self-supporting liner, said
load-bearing vessel being suitable for withstanding pressures of
about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and
temperatures of about -123.degree. C. (-190.degree. F.) to about
-62.degree. C. (-80.degree. F.), and said composite materials
having a coefficient of thermal expansion (i) that is substantially
the same as the coefficient of thermal expansion of said
self-supporting liner at the interface with said self-supporting
liner, and (ii) that gradually decreases through the thickness of
said load-bearing vessel as the distance from said interface
increases.
8. The method according to claim 7 wherein said composite materials
comprise an intermediate material at the interface with said
self-supporting liner, wherein said intermediate material has
adequate shear strength or strain to substantially prevent failure
of said container during changes in temperature between ambient and
about -123.degree. C. (-190.degree. F.).
9. The method according to claim 7 wherein said self-supporting
liner of step (a) is made of a material consisting essentially of
aluminum and said composite materials comprise fibers selected from
the group consisting of (i) carbon, (ii) glass, (iii) aramid, and
(iv) Ultra-High Molecular Weight Polyethylene.
10. The method according to claim 7 wherein said self-supporting
liner of step (a) is made of a material consisting essentially of a
steel having a yield strength of at least about 690 MPa (100 ksi)
and a ductile to brittle transition temperature lower than about
-62.degree. C. (-80.degree. F.) in the base material and in its
heat-affected-zone after welding and said composite materials
comprise fibers selected from the group consisting of (i) carbon,
(ii) glass, (iii) aramid, and (iv) Ultra-High Molecular Weight
Polyethylene.
11. A method of making a container suitable for storing a
pressurized liquefied natural gas at a pressure of about 1035 kPa
(150 psia) to about 7590 kPa (1100 psia) and at a temperature of
about -123.degree. C. (-190.degree. F.) to about -62.degree. C.
(-80.degree. F.), said method comprising the steps of: (a)
constructing a self-supporting liner, said self-supporting liner
being suitable for providing a substantially impermeable barrier to
said pressurized liquefied natural gas; and (b) over-wrapping said
self-supporting liner with adequate composite materials to form a
load-bearing vessel in contact with said self-supporting liner,
said load-bearing vessel being suitable for withstanding pressures
of about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and
temperatures of about -123.degree. C. (-190.degree. F.) to about
-62.degree. C. (-80.degree. F.), and said composite materials
having a coefficient of thermal expansion that is substantially the
same as the coefficient of thermal expansion of said
self-supporting liner at the interface with said self-supporting
liner.
12. A method of storing a pressurized fluid at a pressure of about
1035 kPa (150 psia) to about 7590 kPa (1100 psia) and at a
temperature of about -123.degree. C. (-190.degree. F.) to about
-62.degree. C. (-80.degree. F.), said method comprising the steps
of containing said pressurized fluid in at least one container,
said at least one container comprising (a) a self-supporting liner,
said self-supporting liner providing a substantially impermeable
barrier to said pressurized fluid; and (b) a load-bearing vessel in
contact with said self-supporting liner, said load-bearing vessel
having been made from composite materials and being suitable for
withstanding pressures of about 1035 kPa (150 psia) to about 7590
kPa (1100 psia) and temperatures of about -123.degree. C.
(-190.degree. F.) to about -62.degree. C. (-80.degree. F.), and
said composite materials having a coefficient of thermal expansion
that is substantially the same as the coefficient of thermal
expansion of said self-supporting liner at the interface with said
self-supporting liner and that gradually decreases through the
thickness of said load-bearing vessel as the distance from said
interface increases.
13. A method according to claim 12 wherein said at least one
container comprises (a) a self-supporting liner, said
self-supporting liner providing a substantially impermeable barrier
to said pressurized fluid; and (b) a load-bearing vessel in contact
with said self-supporting liner, said load-bearing vessel having
been made from composite materials and being suitable for
withstanding pressures of about 1035 kPa (150 psia) to about 7590
kPa (1100 psia) and temperatures of about -123.degree. C.
(-190.degree. F.) to about -62.degree. C. (-80.degree. F.), and
said composite materials having a coefficient of thermal expansion
(i) that is substantially the same as the coefficient of thermal
expansion of said self-supporting liner at the interface with said
self-supporting liner, and (ii) that gradually decreases through
the thickness of said load-bearing vessel as the distance from said
interface increase.
14. The method according to claim 12 wherein said composite
materials comprising an intermediate material at the interface with
said self-supporting liner, wherein said intermediate material has
adequate shear strength or strain to substantially prevent failure
of said container during changes in temperature between ambient and
about -123.degree. C. (-190.degree. F.).
15. The method according to claim 12 wherein said self-supporting
liner is made of a material consisting essentially of aluminum said
composite materials comprise fibers selected from the group
consisting of (i) carbon, (ii) glass, (iii) aramid, and (iv)
Ultra-High Molecular Weight Polyethylene.
16. The method according to claim 12 wherein said self-supporting
liner is made of a material consisting essentially of a steel
having a yield strength of at least about 690 MPa (100 ksi) and a
ductile to brittle transition temperature lower than about
-62.degree. C. (-80.degree. F.) in the base material and in its
heat-affected-zone after welding and said composite materials
comprise fibers selected from the group consisting of (i) carbon,
(ii) glass, (iii) aramid, and (iv) Ultra-High Molecular Weight
Polyethylene.
17. The method according to claim 12 wherein said pressurized fluid
is pressurized liquefied natural gas.
18. The method according to claim 7 wherein said pressurized fluid
is pressurized liquefied natural gas.
19. A method for importing pressurized fluid comprising: providing
at least one container having a pressurized fluid, wherein the at
least one container comprises (a) a self-supporting liner providing
a substantially impermeable barrier to pressurized fluid; and (b) a
load-bearing vessel in contact with the self-supporting liner and
made from composite materials, the composite materials having a
coefficient of thermal expansion that is substantially the same as
the coefficient of thermal expansion of the self-supporting liner
at the interface with the self-supporting liner and that gradually
decreases through the thickness of the load-bearing vessel as the
distance from the interface increases; and unloading the
pressurized fluid from the at least one container.
20. The method of claim 19 wherein the at least one container is
disposed in a transportation vessel.
21. The method of claim 20 wherein the transportation vessel is a
marine transport vessel.
22. The method of claim 21 wherein the at least one container is
secured to the marine transportation vessel by a two-point support
system.
23. The method of claim 19 wherein the pressurized fluid is
pressurized liquefied natural gas.
24. The method of claim 19 wherein the load-bearing vessel is
suitable for withstanding temperatures of about -123.degree. C.
(-190.degree. F.) to about -62.degree. C. (-80.degree. F.).
25. The method of claim 19 wherein the load-bearing vessel is
suitable for withstanding pressures of about 1035 kPa (150 psia) to
about 7590 kPa (1100 psia).
26. The method of claim 19 wherein the at least one container
comprising an overwrap layer having one of carbon fibers, a
material that provides similar creep performance as carbon fibers,
and any combination thereof.
27. The method of claim 19 wherein the composite material
comprising an intermediate material at the interface with the
self-supporting liner, wherein the intermediate material is
configured to provide shear strength that prevents failure of the
composite material in a temperature range from ambient to about
-123.degree. C. (-190.degree. F.).
28. The method of claim 19 wherein the self-supporting liner
comprises one of aluminum and steel having a yield strength of at
least about 690 MPa (100 ksi) and a ductile to brittle transition
temperature lower than about -62.degree. C. (-80.degree. F.).
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/367824, filed 27 Mar. 2002 and is a divisional
of U.S. patent application Ser. No. 10/396,895 filed Mar. 25,
2003.
FIELD OF THE INVENTION
[0002] The present invention relates to improved containers and
methods for containing pressurized fluids and to methods for making
such containers. More particularly, the present invention relates
to containers comprising a self-supporting liner and a load-bearing
composite overwrap, whereby means are provided for substantially
preventing container failure during changes in temperature between
ambient and about -123.degree. C. (-190.degree. F.), and to methods
for containing pressurized fluids using such containers and to
methods of making such containers. In some embodiments, the present
invention relates to improved containers and methods for storing
pressurized liquefied natural gas (PLNG).
BACKGROUND OF THE INVENTION
[0003] Various terms are defined in the following specification.
For convenience, a Glossary of terms is provided herein,
immediately preceding the claims.
[0004] U.S. Pat. No. 6,085,528 (the "PLNG Patent") entitled
"Improved System for Processing, Storing, and Transporting
Liquefied Natural Gas", describes containers and transportation
vessels for storage and marine transportation of pressurized
liquefied natural gas (PLNG) at a pressure in the broad range of
about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and at a
temperature in the broad range of about -123.degree. C.
(-190.degree. F.) to about -62.degree. C. (-80.degree. F.).
Containers described in the PLNG Patent are constructed from
ultra-high strength, low alloy steels containing less than 9 wt %
nickel and having tensile strengths greater than 830 MPa (120 ksi)
and DBTTs (a measure of toughness, as defined in the Glossary)
lower than about -73.degree. C. (-100.degree. F.). As discussed in
the PLNG Patent, at the preferred operating pressures and
temperatures of the invention described therein, about 31/2 wt %
nickel steel can be used in the coldest operating areas of a PLNG
plant for the process piping and facilities, whereas more expensive
9 wt % nickel steel or aluminum is generally required for the same
equipment in a conventional LNG plant (i.e., a plant for producing
LNG at atmospheric pressure and about -162.degree. C. (-260.degree.
F.)). Preferably, high strength, low alloy steels with adequate
strength and fracture toughness at the operating conditions of the
PLNG plant, are used to construct the piping and associated
components (e.g., flanges, valves, and fittings), pressure vessels,
and other equipment of the PLNG plant in order to provide economic
advantage over a conventional LNG plant. U.S. Pat. No. 6,212,891
the ("Process Component Patent") entitled "Process Components,
Containers, and Pipes Suitable For Containing and Transporting
Cryogenic Temperature Fluids", describes process components,
containers, and pipes suitable for containing and transporting
cryogenic temperature fluids. More particularly, the Process
Component Patent describes process components, containers, and
pipes that are constructed from ultra-high strength, low alloy
steels containing less than 9 wt % nickel and having tensile
strengths greater than 830 MPa (120 ksi) and DBTTs lower than about
-73.degree. C. (-100.degree. F.). U.S. Pat. No. 6,460,721 (the
"Non-load-bearing Liner Container Patent"), entitled "Systems And
Methods For Producing And Storing Pressurized Liquefied Natural
Gas", describes containers and transportation vessels for storage
and marine transportation of pressurized liquefied natural gas
(PLNG) at a pressure in the broad range of about 1035 kPa (150
psia) to about 7590 kPa (1100 psia) and at a temperature in the
broad range of about -123.degree. C. (-190.degree. F.) to about
-62.degree. C. (-80.degree. F.). Containers described in the
Non-load-bearing Liner Container Patent are constructed from (a) a
load-bearing vessel made from a composite material, said vessel
being suitable for withstanding pressures of about 1035 kPa (150
psia) to about 7590 kPa (1100 psia) and temperatures of about
-123.degree. C (-190.degree. F.) to about -62.degree. C.
(-80.degree. F.); and (b) a substantially non-load-bearing liner in
contact with said vessel, said liner providing a substantially
impermeable barrier to said pressurized liquefied natural gas. The
PLNG Patent, the Process Component Patent, and the Non-load bearing
liner Container Patent are hereby incorporated herein by
reference.
[0005] The PLNG Patent and the Process Component Patent utilize
ultra-high strength, low alloy steels as the connecting theme
between the PLNG plant and the containers used for storing and
transporting the PLNG. If use of the steels for constructing the
containers did not provide a commercially viable means for storing
and transporting the PLNG on marine vessels, then any use of the
steels in the plant would be meaningless since there would be no
mechanism for commercially transporting the PLNG produced by the
plant. Conversely, while use of the steels in the PLNG plant
generates some economic savings over conventional LNG operations,
the most substantial economic benefit is derived from the enormous
simplification (and consequent cost reductions) in the plant.
Because of its relatively simple design, the PLNG plant is
substantially cheaper than a conventional LNG plant of similar
capacity. Additionally, while use of the steels in the PLNG
transportation system is commercially viable and does generate some
economic savings over conventional LNG operations, the weight of
the steel containers is high compared to that of its PLNG cargo,
resulting in a relatively low cargo-carrying capacity performance
factor (PF). The PF for compressed fluid storage containers relates
the pressure exerted by the cargo (P) to the volume (V) of the
container and the weight (W) of the container by the equation
PF=PV/W. What is currently missing from the all-steel PLNG system
(i.e., plant plus transportation) is a combination of the PLNG
plant with a low cost, higher PF, container-based transportation
system that is capable of handling PLNG.
[0006] High-performance fibers, which offer high strength-to-weight
ratios, are used to construct lightweight composite-overwrapped
pressure vessels. Such lightweight pressure vessels have been used
extensively in the aerospace industry and for life-support systems
such as emergency breathing apparatus for professional
firefighters, miners, and rescue workers. These pressure vessels
are also used for portable oxygen for medical applications and for
flight crew and passengers. Seal et al. (U.S. Pat. No. 5,822,838)
describe the two primary technologies used in the design of such
high-pressure gas containment systems. The first approach, the most
prevalent, uses thin metallic liners (e.g. aluminum) that yield
during the service cycle because each pressure cycle results in
fiber/composite strain higher than the yield strain (or elastic
capability) of the liner. This generally limits the cycle life of
the liner and hence of the pressure vessel. In this approach, the
liner is non-load bearing; it provides essentially no contribution
to carrying the structural load, but only serves as a
gas-permeation barrier for the pressure vessel. Such liners are
typically bonded to the composite. In the second approach, a
material with a higher elastic range relative to the fiber strain
during the pressure service is selected for the liner. This
increases the liner life since the liner remains elastic during the
operating pressure cycles. The liner is also required to share the
structural load and is therefore characterized as load-bearing.
Typically, the composite is applied only in the hoop direction
since the liner must be thick enough to operate in the elastic
range. Seal et al. prefer a titanium liner. Both U.S. Pat. No.
5,577,630 (Blair et al.) and U.S. Pat. No. 5,798,156 (Mitlitsky et
al.) describe lined, composite pressure vessels for storing and
transporting compressed natural gas.
[0007] Use of such composite-overwrapped pressure vessels in
cryogenic service introduces another problem inherent in the design
due to the difference in the CTE, or coefficient of thermal
expansion or contraction, of the liner material and the composite.
Typical values of CTE are about -5.6.times.10.sup.-7 m/m/K
(-1.times.10.sup.-6 in/in/.degree. F.) for carbon fiber composite,
about 3.3.times.10.sup.-6 m/m/K (6.times.10.sup.-6 in/in/.degree.
F.) for glass fiber composite, and about 7.2.times.10.sup.-6 m/m/K
(13.times.10.sup.-6 in/in/.degree. F.) for aluminum. As a typical
composite pressure vessel is cooled to cryogenic temperatures, the
liner, which is typically aluminum, tends to contract more than the
composite material causing the liner to separate from the windings
and subsequently causing pre-mature failure. Innovative approaches
to address the CTE problem are the subject of several patents,
e.g., U.S. Pat. No. 4,835,975 (Windecker), U.S. Pat. No. 3,830,180
(Bolton), and U.S. Pat. No. 4,073,400 (Brook et al). For example,
Windecker (U.S. Pat. No. 4,835,975) proposes using a low-carbon
steel liner (having a CTE of about 3.1.times.10.sup.-6 m/m/K
(5.5.times.10.sup.-6 in/in/.degree. F.)) and fiberglass composite
which have comparable CTE's to avert the problem.
[0008] U.S. Pat. No. 3,830,180 ("Bolton") discusses use of a
double-walled, composite cylindrical vessel configuration for
transport of regular LNG, i.e., LNG at atmospheric pressure and at
temperatures of about -162.degree. C. (-260.degree. F.). However,
the load-bearing, inner wall of Bolton's vessel is designed for a
maximum pressure of approximately 0.34 to 0.41 MPa (50 to 60 psi)
and, thus, Bolton's vessel is not suitable for transport and
storage of PLNG. Further, Bolton does not discuss liner material
but proposes the use of a plastic material, such as FRP pipe (fiber
reinforced plastic pipe), or other suitable material "capable of
enduring exposure and stress at cryogenic temperatures" for
construction of the inner and outer walls of the vessel; however
use of FRP necessitates use of a liner since the resin for the FRP
will micro-crack at cryogenic temperatures and will not be
impermeable to the product, as will be familiar to those skilled in
the art.
[0009] S. G. Ladkany, in "Composite Aluminum-Fiberglass Epoxy
Pressure Vessels for Transportation of LNG at Intermediate
Temperature", published in Advances in Crvogenic Engineering,
Materials, volume 28 (Proceedings of the 4th International
Cryogenic Materials Conference), San Diego, Calif., USA, 10 Aug.
1981-14 Aug. 1981, discusses the design of pressure vessels for the
transportation of liquefied natural gas (LNG) at temperature and
pressure conditions between the critical conditions, 191 K, 4.69
MPa (-116.degree. F., 680 psi) and atmospheric conditions 106 K,
0.1 MPa (-268.degree. F., 14.7 psi). Ladkany's design consists of a
47 mm (1.85 inch) thick aluminum vessel circumferentially
reinforced with 17 mm (0.67 in) thick layers of high strength
fiberglass epoxy or 51 mm (2 in) thick layers of pultruded glass
polyester overwrap and stiffened against buckling by
circumferential frames that are placed at 2.16 m (7.1 ft)
intervals. The stiffening frames are also used for structurally
supporting and fastening the free-standing vessel during
transportation and operation. The metal liners for the hoop-wound
pressure vessel are load-sharing and are not bonded to the
composite overwrap. Stiffening frames are therefore required for
buckling resistance, which adds to the complexity of the design and
limits the size of the pressure vessel. Ladkany opts for a welded
aluminum pressure vessel for containing the intermediate
temperature LNG.
[0010] U.S. Pat. No. 5,499,739 (Greist, III et al.) discusses a
thermoplastic liner made of a modified nylon 6 or nylon 11 material
for use in a pressure vessel to control gas permeation and allow
operation at low temperatures, the low end of which is stated to be
-40.degree. C. (-40.degree. F.). U.S. Pat. No. 5,658,013 (Bees et
al.) discusses a fuel tank for vehicles for holding and dispensing
both a liquid and gaseous fuel, and suggests that fully-composite
or fiberglass reinforced materials could be used in construction
thereof. The liquid fuels discussed in the patent are conventional
liquid fuels at ambient temperature and pressure. Both Bees et al.
and Mitlitsky et al., previously discussed, propose metal-coated,
polymer-based liners that provide further enhancements in
performance factors of their tanks/vessels. However, the complexity
and hence high cost of the metal deposition process and the liner
fabrication process make the tanks/vessels of Bees et al. and
Mitlitsky et al. suitable primarily for applications where
maximized payload-carrying capacity is the primary objective and,
thus, low tank/vessel weight is of very high premium. U.S. Pat. No.
5,695,839 (Yamada et al.) discusses a composite container that has
a gas barrier property, wherein the packaging material for
constituting such a container is caused to have a laminate
structure, and a layer of a material such as an aluminum foil is
disposed or interposed in the laminate structure. However, none of
the containers discussed in these publications are designed for
containing fluids that are at both temperatures less than
-40.degree. C. (-40.degree. F.) and high pressures, such as the
temperatures and pressures of PLNG.
[0011] Conventional liquefied natural gas ("LNG") is typically
transported by sea at temperatures of about -162.degree. C.
(-260.degree. F.) and at atmospheric pressure using spherical or
close-to-spherical tanks (often called Moss Spheres) made of
aluminum or steel capable of cryogenic service. The service
pressure for these spherical tanks is too low for PLNG application.
Designing very large tanks for the PLNG service pressure using
conventional materials presents fabrication challenges due to the
unusually large material thicknesses required. Containers for
storing and transporting PLNG as described in the PLNG Patent are
constructed from ultra-high strength, low alloy steels. However, in
spite of the high strength of the steels used in the construction
of the PLNG containers described in the PLNG Patent, the weight of
a containment system using these containers will be high relative
to the cargo and will constrain the ship design through parameters
such as draft and stability. Further, these containers will likely
be cylindrical in shape and have small diameters, relative to a
typical Moss Sphere LNG container, and thus will likely require
interconnection with cryogenic-grade materials into a smaller
number of containers to simplify loading and unloading.
Furthermore, the arrangement of the cylindrical containers will
likely affect the geometric design of the ship affecting the ship
block coefficient and hence increasing the power requirement, and
obstructing the line-of-sight from the engine room. As used herein,
the ship block coefficient is defined as V/(L)(B)(T) where V is the
volume of fluid displaced by the ship, L is the length between the
ship's perpendiculars, B is the ship's beam and T is the ship's
draft.
[0012] The Non-load-bearing Liner Patent proposes an alternative
containment system design based on lightweight, high-performance
composite containers with non-load-bearing liners. The reduced
weight enhances the ship design by removing weight-related
constraints. However, the fabrication complexity of thin-lined
composite containers limits the size and geometry of the containers
and thereby increases the complexity of piping requirements and
impact on geometric design of the ship.
[0013] In spite of the aforementioned advances in technology, even
those providing systems and methods for producing and storing
pressurized liquefied natural gas (PLNG), it would be advantageous
to have improved containers and methods for storing and
transporting PLNG.
[0014] Therefore, an object of this invention is to provide such
improved containers and methods. Other objects of this invention
will be made apparent by the following description of the
invention.
SUMMARY OF THE INVENTION
[0015] In one embodiment ofthis invention, a container suitable for
storing a pressurized fluid at a pressure of about 1035 kPa (150
psia) to about 7590 kPa (1100 psia) and at a temperature of about
-123.degree. C. (-190.degree. F.) to about -62.degree. C.
(-80.degree. F.) is provided, said container comprising: (a) a
self-supporting liner, said self-supporting liner providing a
substantially impermeable barrier to said pressurized fluid; and
(b) a load-bearing vessel in contact with said self-supporting
liner, said load-bearing vessel having been made from composite
materials and being suitable for withstanding pressures of about
1035 kPa (150 psia) to about 7590 kPa (1100 psia) and temperatures
of about -123.degree. C. (-190.degree. F.) to about -62.degree. C.
(-80.degree. F.), and said composite materials having a coefficient
of thermal expansion (i) that is substantially the same as the
coefficient of thermal expansion of said self-supporting liner at
the interface with said self-supporting liner, and (ii) that
gradually decreases through the thickness of said load-bearing
vessel as the distance from said interface increases. In one
embodiment, said load-bearing vessel of said container has an
outermost overwrap layer consisting essentially of carbon fibers or
of a material that provides similar creep performance as carbon
fibers would provide. In another embodiment, element (b) of said
container is replaced with the following: (b) a load-bearing vessel
in contact with said self-supporting liner, said load-bearing
vessel having been made from composite materials and being suitable
for withstanding pressures of about 1035 kPa (150 psia) to about
7590 kPa (1100 psia) and temperatures of about -123.degree. C.
(-190.degree. F.) to about -62.degree. C. (-80.degree. F.), and
said composite materials comprising an intermediate material at the
interface with said self-supporting liner, wherein said
intermediate material has adequate shear strength or strain to
substantially prevent failure of said container during changes in
temperature between ambient and about -123.degree. C. (-190.degree.
F.). In another embodiment, said self-supporting liner of said
container is made of a material consisting essentially of aluminum
and element (b) is replaced with the following: (b) a load-bearing
vessel in contact with said self-supporting liner, said
load-bearing vessel having been made from composite materials and
being suitable for withstanding pressures of about 1035 kPa (150
psia) to about 7590 kPa (1100 psia) and temperatures of about
-123.degree. C. (-190.degree. F.) to about -62.degree. C.
(-80.degree. F.), said composite materials comprising fibers
selected from the group consisting of (i) carbon, (ii) glass, (iii)
kevlar.RTM., (iv) aramid, and (v) Ultra-High Molecular Weight
Polyethylene. In another embodiment, said self-supporting liner of
said container is made of a material consisting essentially of a
steel having a yield strength of at least about 690 MPa (100 ksi)
and a ductile to brittle transition temperature lower than about
-62.degree. C. (-80.degree. F.) in the base material and in its
heat-affected-zone after welding and element (b) is replaced with
the following: (b) a load-bearing vessel in contact with said
self-supporting liner, said load-bearing vessel having been made
from composite materials and being suitable for withstanding
pressures of about 1035 kPa (150 psia) to about 7590 kPa (1100
psia) and temperatures of about -123.degree. C. (-190.degree. F.)
to about -62.degree. C. (-80.degree. F.), said composite materials
comprising fibers selected from the group consisting of (i) carbon,
(ii) glass, (iii) kevlar.RTM., (iv) aramid, and (v) Ultra-High
Molecular Weight Polyethylene.
[0016] In another embodiment of this invention, a container
suitable for storing a pressurized liquefied natural gas at a
pressure of about 1035 kPa (150 psia) to about 7590 kPa (1100 psia)
and at a temperature of about -123.degree. C. (-190.degree. F.) to
about -62.degree. C. (-80.degree. F.) is provided, said container
comprising: (a) a self-supporting liner, said self-supporting liner
providing a substantially impermeable barrier to said pressurized
liquefied natural gas; and (b) a load-bearing vessel in contact
with said self-supporting liner, said load-bearing vessel having
been made from composite materials and being suitable for
withstanding pressures of about 1035 kPa (150 psia) to about 7590
kPa (1100 psia) and temperatures of about -123.degree. C.
(-190.degree. F.) to about -62.degree. C. (-80.degree. F.), and
said composite materials having a coefficient of thermal expansion
that is substantially the same as the coefficient of thermal
expansion of said self-supporting liner at the interface with said
self-supporting liner.
[0017] Also provided is a method of making a container suitable for
storing a pressurized fluid at a pressure of about 1035 kPa (150
psia) to about 7590 kPa (1100 psia) and at a temperature of about
-123.degree. C. (-190.degree. F.) to about -62.degree. C.
(-80.degree. F.), said method comprising the steps of: (a)
constructing a self-supporting liner, said self-supporting liner
being suitable for providing a substantially impermeable barrier to
said pressurized fluid; and (b) overwrapping said self-supporting
liner with adequate composite materials to form a load-bearing
vessel in contact with said self-supporting liner, said
load-bearing vessel being suitable for withstanding pressures of
about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and
temperatures of about -123.degree. C. (-190.degree. F.) to about
-62.degree. C. (-80.degree. F.), and said composite materials
having a coefficient of thermal expansion (i) that is substantially
the same as the coefficient of thermal expansion of said
self-supporting liner at the interface with said self-supporting
liner, and (ii) that gradually decreases through the thickness of
said load-bearing vessel as the distance from said interface
increases. In another embodiment, step (b) of said method is
replaced with the following: (b) overwrapping said self-supporting
liner with adequate composite materials to form a load-bearing
vessel in contact with said self-supporting liner, said
load-bearing vessel being suitable for withstanding pressures of
about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and
temperatures of about -123.degree. C. (-190.degree. F.) to about
-62.degree. C. (-80.degree. F.), and said composite materials
comprising an intermediate material at the interface with said
self-supporting liner, wherein said intermediate material has
adequate shear strength or strain to substantially prevent failure
of said container during changes in temperature between ambient and
about -123.degree. C. (-190.degree. F.). In another embodiment of
said method, said self-supporting liner of step (a) is made of a
material consisting essentially of aluminum and step (b) is
replaced with the following: (b) overwrapping said self-supporting
liner with adequate composite materials to form a load-bearing
vessel in contact with said self-supporting liner, said
load-bearing vessel being suitable for withstanding pressures of
about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and
temperatures of about -123.degree. C. (-190.degree. F.) to about
-62.degree. C. (-80.degree. F.), said composite materials
comprising fibers selected from the group consisting of (i) carbon,
(ii) glass, (iii) kevlar.RTM., (iv) aramid, and (v) Ultra-High
Molecular Weight Polyethylene. In another embodiment of said
method, said self-supporting liner of step (a) is made of a
material consisting essentially of a steel having a yield strength
of at least about 690 MPa (100 ksi) and a ductile to brittle
transition temperature lower than about -62.degree. C. (-80.degree.
F.) in the base material and in its heat-affected-zone after
welding and step (b) is replaced with the following: (b)
overwrapping said self-supporting liner with adequate composite
materials to form a load-bearing vessel in contact with said
self-supporting liner, said load-bearing vessel being suitable for
withstanding pressures of about 1035 kPa (150 psia) to about 7590
kPa (1100 psia) and temperatures of about -123.degree. C.
(-190.degree. F.) to about -62.degree. C. (-80.degree. F.), said
composite materials comprising fibers selected from the group
consisting of (i) carbon, (ii) glass, (iii) kevlar.RTM., (iv)
aramid, and (v) Ultra-High Molecular Weight Polyethylene.
[0018] Also provided is a method of making a container suitable for
storing a pressurized liquefied natural gas at a pressure of about
1035 kPa (150 psia) to about 7590 kPa (1100 psia) and at a
temperature of about -123.degree. C. (-190.degree. F.) to about
-62.degree. C. (-80.degree. F.), said method comprising the steps
of: (a) constructing a self-supporting liner, said self-supporting
liner being suitable for providing a substantially impermeable
barrier to said pressurized liquefied natural gas; and (b)
overwrapping said self-supporting liner with adequate composite
materials to form a load-bearing vessel in contact with said
self-supporting liner, said load-bearing vessel being suitable for
withstanding pressures of about 1035 kPa (150 psia) to about 7590
kPa (1100 psia) and temperatures of about -123.degree. C.
(-190.degree. F.) to about -62.degree. C. (-80.degree. F.), and
said composite materials having a coefficient of thermal expansion
that is substantially the same as the coefficient of thermal
expansion of said self-supporting liner at the interface with said
self-supporting liner.
[0019] In yet another embodiment of this invention, a method of
storing a pressurized liquefied natural gas at a pressure of about
1035 kPa (150 psia) to about 7590 kPa (1100 psia) and at a
temperature of about -123.degree. C. (-190.degree. F.) to about
-62.degree. C. (-80.degree. F.) is provided, said method comprising
the steps of containing said pressurized liquefied natural gas in
at least one container, said at least one container comprising (a)
a self-supporting liner, said self-supporting liner providing a
substantially impermeable barrier to said pressurized liquefied
natural gas; and (b) a load-bearing vessel in contact with said
self-supporting liner, said load-bearing vessel having been made
from composite materials and being suitable for withstanding
pressures of about 1035 kPa (150 psia) to about 7590 kPa (1100
psia) and temperatures of about -123.degree. C. (-190.degree. F.)
to about -62.degree. C. (-80.degree. F.), and said composite
materials having a coefficient of thermal expansion that is
substantially the same as the coefficient of thermal expansion of
said self-supporting liner at the interface with said
self-supporting liner. In another embodiment of said method, said
at least one container comprises (a) a self-supporting liner, said
self-supporting liner providing a substantially impermeable barrier
to said pressurized fluid; and (b) a load-bearing vessel in contact
with said self-supporting liner, said load-bearing vessel having
been made from composite materials and being suitable for
withstanding pressures of about 1035 kPa (150 psia) to about 7590
kPa (1100 psia) and temperatures of about -123.degree. C.
(-190.degree. F.) to about -62.degree. C. (-80.degree. F.), and
said composite materials having a coefficient of thermal expansion
(i) that is substantially the same as the coefficient of thermal
expansion of said self-supporting liner at the interface with said
self-supporting liner, and (ii) that gradually decreases through
the thickness of said load-bearing vessel as the distance from said
interface increases. In another embodiment of said method, said at
least one container comprises (a) a self-supporting liner, said
self-supporting liner providing a substantially impermeable barrier
to said pressurized fluid; and (b) a load-bearing vessel in contact
with said self-supporting liner, said load-bearing vessel having
been made from composite materials and being suitable for
withstanding pressures of about 1035 kPa (150 psia) to about 7590
kPa (1100 psia) and temperatures of about -123.degree. C.
(-190.degree. F.) to about -62.degree. C. (-80.degree. F.), and
said composite materials comprising an intermediate material at the
interface with said self-supporting liner, wherein said
intermediate material has adequate shear strength or strain to
substantially prevent failure of said container during changes in
temperature between ambient and about -123.degree. C. (-190.degree.
F.). In another embodiment of said method, said at least one
container comprises (a) a self-supporting liner made of a material
consisting essentially of aluminum and that provides a
substantially impermeable barrier to said pressurized fluid; and
(b) a load-bearing vessel in contact with said self-supporting
liner, said load-bearing vessel having been made from composite
materials and being suitable for withstanding pressures of about
1035 kPa (150 psia) to about 7590 kPa (1100 psia) and temperatures
of about -123.degree. C. (-190.degree. F.) to about -62.degree. C.
(-80.degree. F.), said composite materials comprising fibers
selected from the group consisting of (i) carbon, (ii) glass, (iii)
kevlar.RTM., (iv) aramid, and (v) Ultra-High Molecular Weight
Polyethylene. In another embodiment of said method, said at least
one container comprises (a) a self-supporting liner made of a
material consisting essentially of a steel having a yield strength
of at least about 690 MPa (100 ksi) and a ductile to brittle
transition temperature lower than about -62.degree. C. (-80.degree.
F.) in the base material and in its heat-affected-zone after
welding and that provides a substantially impermeable barrier to
said pressurized fluid; and (b) a load-bearing vessel in contact
with said self-supporting liner, said load-bearing vessel having
been made from composite materials and being suitable for
withstanding pressures of about 1035 kPa (150 psia) to about 7590
kPa (1100 psia) and temperatures of about -123.degree. C.
(-190.degree. F.) to about -62.degree. C. (-80.degree. F.), said
composite materials comprising fibers selected from the group
consisting of (i) carbon, (ii) glass, (iii) kevlar.RTM., (iv)
aramid, and (v) Ultra-High Molecular Weight Polyethylene.
[0020] Unlike the conventional approach using a non-load bearing
liner, the container design of this invention uses a
self-supporting metallic liner overwrapped with high-performance
composite fibers with a cryogenic resin. As used herein, the term
"self-supporting" in regard to a liner means capable of maintaining
its structural integrity while supporting its own weight. Once the
overwrap is applied, the composite provides additional buckling
resistance for the container. For example, referring to FIG. 6,
which is a graph having an abscissa 60 representing liner thickness
in millimeters and an ordinate 61 representing collapse pressure in
pounds per square inch, line 62 shows the critical pressure line,
line 63 shows the collapse pressure for a liner having a diameter
of 10 meters (32.8 feet), line 64 shows the collapse pressure for a
liner having a diameter of 20 meters (65.6 feet), and line 65 shows
the collapse pressure for a liner having a diameter of 40 meters
(131.2 feet). Other criteria, besides collapse pressure, may be
used to determine whether a container liner is self-supporting, as
is familiar to those skilled in the art. As used herein, the term
"high-performance" in regard to composites or fibers means having a
tensile strength greater than about 3401 MPa (500 ksi) and a
modulus greater than about 136054 MPa (20 million pounds per square
inch (msi)). One embodiment comprises a basic LNG spherical tank
over-wrapped with a high-performance composite to provide the
structural integrity requirements for PLNG containment. The
advantages and characteristics of this invention are more clearly
described in the following.
[0021] The liner itself provides the primary structural support for
the forces imposed by the tension in the fibers as they are wound
over the liner. Fibers that are wound over the liner contribute to
the support. The liner may bear some of the loads exerted by the
pressurized, cryogenic temperature fluids contained in the
container. As is familiar to those skilled in the art, design
details such as the thickness of the liner, or the percentage of
load that the liner will bear, is determined by one skilled in the
art based on the materials that comprise the liner and the
composite overwrap and on other factors that are familiar to those
skilled in the art.
[0022] Second, several innovative design approaches to address the
CTE differences between the metallic liner and the composite are
provided. In one embodiment, the differences in CTE are graduated
by use of intermediate matrix-fiber materials that have a CTE
substantially the same as the CTE of the liner at the interface
with the liner, and have gradually decreasing CTE's as the distance
from the liner increases. In one embodiment, the outermost overwrap
layer consists essentially of carbon fibers for improved creep
performance, or of a material that provides similar creep
performance as carbon fibers would provide. The matrix-fiber
material design comprises an intra-ply hybrid fiber mixture in
which carbon and glass fibers are mixed within the ply (or tow).
This has the added beneficial effect of attaining good compaction
for the composite. A variation of this hybrid approach is inter-ply
mixing whereby alternating layers of different fibers are used. A
third variation involves differing resin fraction for the
laminates: the laminates adjacent to the liner have a higher resin
fraction than laminates further away from the liner, and the resin
fraction in laminates extending further from the liner is gradually
reduced as the distance from the liner increases. The resin can be
specially formulated with customized CTE properties to enhance the
performance of each layer of composite or laminate. In the case
where an aluminum liner is used, this approach recognizes the
relatively large difference in CTE between aluminum and carbon
fiber, a preferred fiber for this design due to its better creep
performance, and the relatively small difference in CTE between
aluminum and glass fiber. As used herein, the term "creep" means
time-dependent strain caused by stress.
[0023] In another embodiment, the self-supporting liner is designed
to withstand the critical buckling loads for the application.
Consequently, the interface between the metallic liner and the
composite overwrap is left unbonded. This differs from conventional
lined composite container designs in which the non-load bearing
liner is bonded to the composite overwrap with an adhesive that can
withstand the interface shear associated with the application; this
is done to mitigate against liner failure by preventing liner
separation from the composite overwrap.
[0024] In yet another embodiment, the outermost overwrap layer
consists essentially of carbon fibers for improved creep
performance, or a material that provides similar creep performance
as carbon fibers would provide. An intermediate layer of
predominantly glass fiber is placed between the outermost carbon
fiber overwrap and liner made of aluminum capable of cryogenic
service. The autofrettage process is used to provide a residual
compressive pre-stress in the liner to a degree that offsets the
differential thermal contraction of the system. Without the
intermediate layer of glass fiber, the residual compressive
pre-stress would be inadequate to offset the much higher
differential contraction between the aluminum and the carbon. The
following data for a spherical pressure vessel design using
aluminum 5083-0, illustrates the point. An interface bearing
pressure of 34 kPa (5 psi) in tension is developed at the girth
between the aluminum and the carbon fiber composite when the
pressure vessel is cooled to -95.degree. C. (-140.degree. F.). This
results after an autofrettage pressure of 5.78 MPa (850 psig),
followed by a proof pressure of 5.1 MPa (750 psig) both at room
temperature. The corresponding bearing pressure after rebound from
proof pressure is 340 kPa (50 psig) in compression. The glass
ensures positive bearing pressure at the interface thereby
preventing bondline fissuring. The low yield strength of aluminum
limits the residual compressive pre-stress induced in the liner
after the autofrettage process.
[0025] In another embodiment of this invention, the outermost
overwrap layer consists essentially of carbon fibers for improved
creep performance, or a material that provides similar creep
performance as carbon fibers would provide. The liner uses a
material of high yield strength thereby enabling a much higher
residual compressive pre-stress to be induced. This higher
pre-stress essentially offsets the differential contraction between
the liner and the carbon, and essentially no intermediate material
such as glass fiber or adhesive is required at the interface
between the liner and the carbon composite. In addition to the high
yield strength, the material must have adequate low temperature
toughness. Preferably, such high yield strength material has a
yield strength of at least about 690 MPa (100 ksi) and a Ductile to
Brittle Transition Temperature ("DBTT") lower than about
-62.degree. C. (-80.degree. F.) in the base material and in its
heat-affected-zone ("HAZ") after welding, if any. Example materials
that meet the yield strength and DBTT requirements are discussed in
International Publication Nos. WO 99/32672, WO 00/39352, WO
99/32670, WO 00/40764, WO 99/32671, WO 00/37689, and WO 99/05335,
and in U.S. Pat. Nos. 6,251,198, 6,254,698, 6,066,212, 6,159,312,
and 6,264,760 (all of which U.S. patents are hereby incorporated
herein by reference). Useful welding techniques for joining such
steels are discussed in International Publication Nos. WO 01/63974,
WO 99/05335, and WO 00/56498, and in U.S. Pat. Nos. 6,114,656 and
6,336,583 (both of which U.S. patents are hereby incorporated
herein by reference). Other suitable steels and welding techniques
may exist or be developed hereafter. All such steels and welding
techniques are within the scope of the present invention. A
non-limiting liner steel and welding example is provided at the end
of the Detailed Description of the Invention.
[0026] The proposed design. has several advantages over the
conventional PLNG containment system based on steel including the
following: (i) The fabrication process is simplified; (ii) The
containment system weight is reduced which favorably impacts the
transport ship design; (iii) The product piping requirements are
tremendously simplified; (iv) The off-loading scheme is also
simplified; and (v) The insulation requirement is reduced.
DESCRIPTION OF THE DRAWINGS
[0027] The advantages of the present invention will be better
understood by referring to the following detailed description and
the attached drawings in which:
[0028] FIG. 1 is a cross section of a container according to this
invention having a spherical geometry;
[0029] FIG. 2A is a front, cross sectional view of a container
according to this invention having a spherical geometry and located
in a PLNG transportation vessel hull;
[0030] FIG. 2B is a side, cross sectional view of an arrangement in
a PLNG transportation vessel hull of several containers according
to this invention having spherical geometries;
[0031] FIG. 2C is a top, cross sectional view of an arrangement in
a PLNG transportation vessel hull of several containers according
to this invention having spherical geometries;
[0032] FIG. 3 is a cross section of a container according to this
invention having an oblate spheroidal geometry;
[0033] FIG. 4 is a cross section of a container according to this
invention having geodesic-isotensoid half domes attached to a
relatively short cylindrical section;
[0034] FIG. 5 illustrates with cut-away views, one embodiment of a
container according to this invention having a cylindrical geometry
and geodesic-isotensoid half domes;
[0035] FIG. 6 is a graph showing the relation between container
liner collapse pressure, container liner thickness, and container
liner diameter;
[0036] FIG. 7A is a front, cross sectional view of an arrangement
in a PLNG transportation vessel hull of horizontally-situated
containers according to this invention having cylindrical
geometries;
[0037] FIG. 7B is a side, cross sectional view of an arrangement in
a PLNG transportation vessel hull of horizontally-situated
containers according to this invention having cylindrical
geometries;
[0038] FIG. 7C is a top, cross sectional view of an arrangement in
a PLNG transportation vessel hull of horizontally-situated
containers according to this invention having cylindrical
geometries;
[0039] FIG. 8A illustrates a plot of critical flaw depth for a
given flaw length, as a function of CTOD fracture toughness and of
residual stress; and
[0040] FIG. 8B illustrates the geometry (length and depth) of a
flaw.
[0041] While the invention will be described in connection with its
preferred embodiments, it will be understood that the invention is
not limited thereto. On the contrary, the invention is intended to
cover all alternatives, modifications, and equivalents which may be
included within the spirit and scope of the present disclosure, as
defined by the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
Composite Overwrap
[0042] The composite overwrap in a container according to this
invention preferably provides the primary structural support for
the operating loads. The composite overwrap is preferably a
material system comprising high-performance fibers in a resin
matrix capable of cryogenic temperature service. As used herein
"cryogenic temperature" means any temperature of about -62.degree.
C. (-80.degree. F.) and colder. An example of such a resin is the
CTD 525 epoxy cryogenic resin. Two classes of material systems have
been designed for this invention.
[0043] The first class of material systems comprises: (i) a high
performance fiber, preferably selected from the following fibers
(carbon, glass, kevlar.RTM., aramid, UHMWP); and (ii) a
thermo-setting resin (such as the CTD-525 epoxy cryogenic resin).
One embodiment uses high-performance carbon fiber such as TORAY
T-700, GRAFIL 34-600 or ZOLTEK PANEX 35 for better creep
performance. This first class of material system is characterized
by a constant CTE value. For example, the average CTE value
measured for carbon/epoxy resin laminate between room temperature
and -73.degree. C. (-100.degree. F.) is 1.1.times.10.sup.-7 m/m/K
(0.19.times.10.sup.-6 in/in/.degree. F.). This value typically
results in a mismatch with several metallic liners and particularly
with aluminum and the high yield strength steels discussed herein,
i.e., steels having a yield strength of at least about 690 MPa (100
ksi).
[0044] The second class of material systems is characterized by
tunable CTE values and comprises combinations of fibers (mixing
different fibers such as glass and carbon) with various resin
combinations. The resin combinations may comprise substantially
pure resins as well as resins with additives designed to affect the
CTE of the resin. By judicious optimization of parameters such as
the ratio of fibers, resin fraction, and additive content, a
preferred CTE value is obtained. Representative measured CTE values
are, for example without hereby limiting this invention,
7.2.times.10.sup.-6 m/m/K (13.times.10.sup.-6 in/in/.degree. F.)
for aluminum, 18.8.times.10.sup.-6 m/m/K (33.9.times.10.sup.-6
in/in/.degree. F.) for neat resin, and 1.1.times.10.sup.-7 m/m/K
(0.19.times.10.sup.-6 in/in/.degree. F.) for carbon. The
optimization process required is familiar to those skilled in the
art based on the desired performance parameters of the container
being constructed. Further, different laminates of the overwrap are
tuned to different CTE values resulting in a gradation of CTE from
the liner interface to the exterior surface of the overwrap. This
gradation is designed to achieve acceptable inter-laminate stress.
This acceptable value can be determined from analytical techniques
such as a detailed finite element analysis (FEA analysis), as is
familiar to those skilled in the art.
[0045] The use of the second class of materials enables use of
liner materials with any CTE characteristic. Conversely, the fixed
CTE value characteristic of the first class of materials constrains
its use to liners of closely matching CTE such as liners made of
INVAR. The average CTE value for INVAR is of the same order of
magnitude as carbon 5.0.times.10.sup.-7 m/m/K compared to
1.1.times.10.sup.-7 m/m/K (0.9.times.10.sup.-6 in/in/.degree. F.
compared to 0.19.times.10.sup.-6 in/in/.degree. F.). Another aspect
of this invention is the use of an intermediate material, with
high-strain capability, i.e., a strain energy absorption capability
greater than about 34 Joules per square meter (3.0.times.10.sup.-3
Btu per square foot), at the interface between the liner and the
material system of the first class.
Metallic Liner
[0046] The metallic liner of a container according to this
invention preferably serves three primary functions: (i) It
provides an impermeable barrier for the fluid contained; (ii) It
provides the primary structural support required during the winding
process due to tension in the fibers; and (iii) It provides at
least partial structural support for the operating load. Further,
the liner provides at least partial structural support for the
operating loads imposed by the internal pressure due to the PLNG as
well as external loads such as due to ship motions.
[0047] Referring to FIG. 5, one embodiment of a container 5
according to this invention comprises a composite vessel 12 and a
liner 10 made of a substantially impermeable material such as
aluminum or the high yield strength steels discussed herein, i.e.,
steels having a yield strength of at least about 690 MPa (100 ksi),
which provides a barrier for PLNG contained in container 5. In this
embodiment, composite vessel 12 bears the structural load,
including the internal pressure load, of container 5. Liner 10 is
completely surrounded by composite vessel 12, and is therefore a
fully-wrapped pressure vessel. A hoop-wrapped alternative may be
designed in which case the liner 10 is sized to take the full load
in the half dome sections. Container 5 is preferably protected by
an outer-coating 14 made of a material capable of protecting
composite vessel 12 from moisture, acids, ultra-violet rays and
other environmental hazards as necessary. For example, without
limiting this invention, outer-coating 14 could be made from
polyurethane. Container 5 may also include provision for a support
system. For example, a reinforcement boss (not shown in FIG. 5) may
be provided at the lower end of container 5 for interfacing with a
support skirt (not shown in FIG. 5). The design of the support
skirt may be of any typical design, as will be familiar to those
skilled in the art. Preferably, any added reinforcement boss is
integrally wound with composite vessel 12. This provides for
significant economic benefit and also for improved structural
strength and integrity of the interface between a support system
and container 5. A nozzle 20 is provided, preferably at an upper
end of container 5, to allow for penetrations into container 5,
e.g. for the loading or unloading of PLNG. In one embodiment,
nozzle 20 is derived from a metallic boss (not shown in FIG. 5)
installed prior to winding the composite that makes up composite
vessel 12. The metallic boss is over-wrapped with the composite
providing a leak-tight and high-strength interface for access into
container 5.
[0048] In an alternative embodiment, a horizontal orientation for
containers according to this invention on a marine transportation
vessel 90 maximizes the cargo volume and results in a finer hull in
transportation vessel 9, as illustrated in FIG. 7A-FIG. 7C.
Referring now to FIG. 7B, the lengths of horizontally oriented
containers 92 are preferably defined such that each container 92
can be supported at two points, e.g., points 93 and 94. In view of
the complex motion of a PLNG transportation vessel 90, a simple
two-point support system is preferred for horizontally oriented
containers 92, as will be familiar to those skilled in the art. As
will also be familiar to those skilled in the art, the two-point
support system imposes limitations on the lengths of containers 92.
When a project requires greater storage capacity than is provided
by containers of the lengths allowed by a two-point support system,
a moderate increase in support system complexity enables the use of
containers of longer lengths.
[0049] As illustrated in FIG. 1, a container 1 according to this
invention comprising a self-supporting liner 3 and a
composite-overwrap 2 can have a spherical shape. Container 1 can
include a nozzle 4 to allow for penetrations into container 1.
Referring now to FIG. 2A-FIG. 2C, for an embodiment of four
spherical containers 24 according to this invention onboard a ship
22 and carrying about 200,000 cubic meters of PLNG product, the
following geometric parameters for the liner (e.g., self-supporting
liner 3 as illustrated in FIG. 1) have been found to meet the ship
and cargo requirements as well as the above-stated functional
requirements of the liner: [0050] Diameter--about 46 meters (150.9
feet) [0051] Liner material--Aluminum alloy 5083-0 [0052] Yield
Strength--about 190 MPa (28000 psi) [0053] Average thickness--about
45 mm (1.77 inch) [0054] Unit Thermal Contraction (UTC) at
27.degree. C. to -95.degree. C. (80.degree. F. to -140.degree.
F.)--about 0.256% or 2.56 mm/m (2.56.times.10.sup.-3 in/in) The
choice of aluminum in this embodiment results in a substantial
mismatch of the coefficient of thermal expansion with a carbon
overwrap (UTC <0.02%). Therefore the preferred composite
material system is chosen from the set in which different laminates
of the overwrap are tuned to different CTE values resulting in a
gradation of CTE from the liner interface to the exterior surface
of the overwrap
[0055] Referring again to FIG. 2A-FIG. 2C, for an alternative
embodiment of this invention for the four spherical containers 24
on a ship 22 carrying about 200,000 cubic meters (7062891 cubic
feet) of product, the following geometric parameters for the liner
have been found to meet the ship and cargo requirements as well as
the above-stated functional requirements of the liner: [0056]
Diameter--about 46 meters (150.9 feet) [0057] Liner
material--INVAR-36 (alloy of iron with 36% nickel) [0058] Yield
Strength--about 236.7 MPa (34.8 ksi) [0059] Ultimate
strength--about 432.7 MPa (63.6 ksi) [0060] Average
thickness--about 35 mm (1.38 inch) [0061] Unit Thermal Contraction
(UTC) at about 27.degree. C. to about -162.degree. C. (80.degree.
F. to about -260.degree. F.)--about 0.03% or 0.3 mm/m
(3.0.times.10.sup.-4 in/in) This alternative embodiment is designed
for minimal CTE mismatch with the basic material system, such as
carbon-fiber-epoxy system as defined above.
[0062] In another such alternative embodiment, the following
geometric parameters for the liner have been found to meet the ship
and cargo requirements as well as the above-stated functional
requirements of the liner: [0063] Diameter--about 46 meters (150.9
feet) [0064] Liner material--a high yield strength steel as
discussed herein [0065] Yield Strength--about 120000 psi (816 MPa)
[0066] Average thickness--about 25.4 mm (1 inch) [0067] Unit
Thermal Contraction (UTC) at 27.degree. C. to -95.degree. C.
(80.degree. F. to -140.degree. F.)--about 0.128% or 1.28 mm/m
(1.28.times.10-3 in/in) This alternative embodiment allows a higher
residual compressive pre-stress in the liner to offset the
difference in thermal contraction between the liner and the carbon
overwrap.
[0068] An alternative geometry, over the spherical geometry, for
this embodiment is a cylinder with geodesic-isotensoid half domes.
A geodesic-isotensoid contour is a dome contour in which the
filaments are placed on geodesic paths so that the filaments will
exhibit uniform tensions throughout their length under pressure
loading. The geodesic path is the shortest distance between two
points on a surface. Consequently, this geometry results in reduced
fiber requirement (about 30% less) relative to the spherical
configuration. Further the geodesic-isotensoid domed cylinder is a
more efficient shape for space utilization than a sphere. Referring
now to FIG. 4, a container 40 having geodesic-isotensoid half domes
41 attached to a relatively short cylindrical section 45 comprises
a self-supporting liner 43 and a composite overwrap 42. Container
40 may have a nozzle 44. Referring now to FIG. 3, a container 30
having an oblate spheroidal geometry comprises a self-supporting
liner 33 and a composite overwrap 32. Container 30 may have a
nozzle 34.
[0069] Benefits of the composite containment system of this
invention for PLNG include the following. Ship design can be
optimized for the geometry and large dimensions of the PLNG
containers. The composite containment system of this invention can
be fabricated for the uniquely large dimensions required for PLNG
transportation, i.e., by providing a self-supporting structure for
a filament-winding fabrication process. Also, the system performs
structurally at cryogenic conditions because differences in CTE
between the liner and the composite overwrap material are
adequately matched.
Liner Steel and Welding Example
[0070] As described in U.S. Pat. No. 6,066,212 (and in
corresponding International Publication No. WO 99/32671), a method
is provided for preparing an ultra-high strength, dual phase steel
plate having a microstructure comprising about 10 vol % to about 40
vol % of a first phase of substantially 100 vol % (i.e.,
substantially pure or "essentially") ferrite and about 60 vol % to
about 90 vol % of a second phase of predominantly fine-grained lath
martensite, fine-grained lower bainite, or mixtures thereof,
wherein the method comprises the steps of (a) heating a steel slab
to a reheating temperature sufficiently high to (i) substantially
homogenize the steel slab, (ii) dissolve substantially all carbides
and carbonitrides of niobium and vanadium in the steel slab, and
(iii) establish fine initial austenite grains in the steel slab;
(b) reducing the steel slab to form steel plate in one or more hot
rolling passes in a first temperature range in which austenite
recrystallizes; (c) further reducing the steel plate in one or more
hot rolling passes in a second temperature range below about the
T.sub.nr temperature and above about the Ar.sub.3 transformation
temperature; (d) further reducing said steel plate in one or more
hot rolling passes in a third temperature range below about the
Ar.sub.3 transformation temperature and above about the Ar.sub.1
transformation temperature (i.e., the intercritical temperature
range); (e) quenching said steel plate at a cooling rate of about
10.degree. C. per second to about 40.degree. C. per second
(18.degree. F./sec-72.degree. F./sec) to a Quench Stop Temperature
(QST) preferably below about the M.sub.s transformation temperature
plus 200.degree. C. (360.degree. F.); and (f) stopping said
quenching. In another embodiment of this steel example, the QST is
preferably below about the M.sub.s transformation temperature plus
100.degree. C. (180.degree. F.), and is more preferably below about
350.degree. C. (662.degree. F.). In one embodiment of this steel
example, the steel plate is allowed to air cool to ambient
temperature after step (f). This processing facilitates
transformation of the microstructure of the steel plate to about 10
vol % to about 40 vol % of a first phase of ferrite and about 60
vol % to about 90 vol % of a second phase of predominantly
fine-grained lath martensite, fine-grained lower bainite, or
mixtures thereof. (See Glossary for definitions of T.sub.nr
temperature, and of Ar.sub.3, A.sub.1, and M.sub.s transformation
temperatures.)
[0071] To ensure ambient and cryogenic temperature toughness, the
microstructure of the second phase in steels of this steel example
comprises predominantly fine-grained lower bainite, fine-grained
lath martensite, or mixtures thereof. It is preferable to
substantially minimize the formation of embrittling constituents
such as upper bainite, twinned martensite and MA in the second
phase. As used in this steel example, and in the claims,
"predominantly" means at least about 50 volume percent. The
remainder of the second phase microstructure can comprise
additional fine-grained lower bainite, additional fine-grained lath
martensite, or ferrite. More preferably, the microstructure of the
second phase comprises at least about 60 volume percent to about 80
volume percent fine-grained lower bainite, fine-grained lath
martensite, or mixtures thereof. Even more preferably, the
microstructure of the second phase comprises at least about 90
volume percent fine-grained lower bainite, fine-grained lath
martensite, or mixtures thereof.
[0072] To make a steel according to this example, a steel slab is
manufactured in a customary fashion and comprises iron and the
following alloying elements, preferably in the weight percentage
ranges indicated in the following: 0.04-0.12 carbon (C), more
preferably 0.04-0.07 C; 0.5-2.5 manganese (Mn), more preferably
1.0-1.8 Mn; 1.0-3.0 nickel (Ni), more preferably 1.5-2.5 N;
0.02-0.1 niobium (Nb), more preferably 0.02-0.05 Nb; 0.008-0.03
titanium (Ti), more preferably 0.01-0.02 Ti; 0.001-0.05 aluminum
(Al), more preferably 0.005-0.03 Al; and 0.002-0.005 nitrogen (N),
more preferably 0.002-0.003 N. Chromium (Cr) is sometimes added to
the steel, preferably up to about 1.0 wt %, and more preferably
about 0.2 wt % to about 0.6 wt %. Molybdenum (Mo) is sometimes
added to the steel, preferably up to about 0.8 wt %, and more
preferably about 0.1 wt % to about 0.3 wt %. Silicon (Si) is
sometimes added to the steel, preferably up to about 0.5 wt %, more
preferably about 0.01 wt % to about 0.5 wt %, and even more
preferably about 0.05 wt % to about 0.1 wt %. Copper (Cu),
preferably in the range of about 0.1 wt % to about 1.0 wt %, more
preferably in the range of about 0.2 wt % to about 0.4 wt %, is
sometimes added to the steel. Boron (B) is sometimes added to the
steel, preferably up to about 0.0020 wt %, and more preferably
about 0.0006 wt % to about 0.0010 wt %. The steel preferably
contains at least about 1 wt % nickel. Nickel content of the steel
can be increased above about 3 wt % if desired to enhance
performance after welding. Each 1 wt % addition of nickel is
expected to lower the DBTT of the steel by about 10.degree. C.
(18.degree. F.). Nickel content is preferably less than 9 wt %,
more preferably less than about 6 wt %. Nickel content is
preferably minimized in order to minimize cost of the steel. If
nickel content is increased above about 3 wt %, manganese content
can be decreased below about 0.5 wt % down to 0.0 wt %. Therefore,
in a broad sense, up to about 2.5 wt % manganese is preferred.
[0073] Additionally, residuals are preferably substantially
minimized in the steel. Phosphorous (P) content is preferably less
than about 0.01 wt %. Sulfur (S) content is preferably less than
about 0.004 wt %. Oxygen (O) content is preferably less than about
0.002 wt %.
[0074] In somewhat greater detail, a steel according to this steel
example is prepared by forming a slab of the desired composition;
heating the slab to a temperature of from about 955.degree. C. to
about 1065.degree. C. (1750.degree. F.-1950.degree. F.); hot
rolling the slab to form steel plate in one or more passes
providing about 30 percent to about 70 percent reduction in a first
temperature range in which austenite recrystallizes, i.e., above
about the T.sub.nr temperature, further hot rolling the steel plate
in one or more passes providing about 40 percent to about 80
percent reduction in a second temperature range below about the
T.sub.nr temperature and above about the Ar.sub.3 transformation
temperature, and finish rolling the steel plate in one or more
passes to provide about 15 percent to about 50 percent reduction in
the intercritical temperature range below about the Ar.sub.3
transformation temperature and above about the Ar.sub.1
transformation temperature. The hot rolled steel plate is then
quenched at a cooling rate of about 10.degree. C. per second to
about 40.degree. C. per second (18.degree. F./sec-72.degree.
F./sec) to a suitable Quench Stop Temperature (QST) preferably
below about the M.sub.s transformation temperature plus 200.degree.
C. (360.degree. F.), at which time the quenching is terminated. In
another embodiment of this example, the QST is preferably below
about the M.sub.s transformation temperature plus 100.degree. C.
(180.degree. F.), and is more preferably below about 350.degree. C.
(662.degree. F.). In one embodiment of this steel example, the
steel plate is allowed to air cool to ambient temperature after
quenching is terminated.
[0075] As is understood by those skilled in the art, as used herein
"percent reduction in thickness" refers to percent reduction in the
thickness of the steel slab or plate prior to the reduction
referenced. For purposes of explanation only, without thereby
limiting this example, a steel slab of about 25.4 cm (10 inches)
thickness may be reduced about 50% (a 50 percent reduction), in a
first temperature range, to a thickness of about 12.7 cm (5 inches)
then reduced about 80% (an 80 percent reduction), in a second
temperature range, to a thickness of about 2.5 cm (1 inch). Again,
for purposes of explanation only, without thereby limiting this
example, a steel slab of about 25.4 cm (10 inches) may be reduced
about 30% (a 30 percent reduction), in a first temperature range,
to a thickness of about 17.8 cm (7 inches) then reduced about 80%
(an 80 percent reduction), in a second temperature range, to a
thickness of about 3.6 cm (1.4 inch), and then reduced about 30% (a
30 percent reduction), in a third temperature range, to a thickness
of about 2.5 cm (1 inch). As used herein, "slab" means a piece of
steel having any dimensions.
[0076] For this example steel, as is understood by those skilled in
the art, the steel slab is preferably reheated by a suitable means
for raising the temperature of substantially the entire slab,
preferably the entire slab, to the desired reheating temperature,
e.g., by placing the slab in a furnace for a period of time. The
specific reheating temperature that should be used may be readily
determined by a person skilled in the art, either by experiment or
by calculation using suitable models. Additionally, the furnace
temperature and reheating time necessary to raise the temperature
of substantially the entire slab, preferably the entire slab, to
the desired reheating temperature may be readily determined by a
person skilled in the art by reference to standard industry
publications.
[0077] For this example steel, as is understood by those skilled in
the art, the temperature that defines the boundary between the
recrystallization range and non-recrystallization range, the
T.sub.nr temperature, depends on the chemistry of the steel, and
more particularly, on the reheating temperature before rolling, the
carbon concentration, the niobium concentration and the amount of
reduction given in the rolling passes. Persons skilled in the art
may determine this temperature for each steel composition either by
experiment or by model calculation. Likewise, the Ar.sub.1,
Ar.sub.3, and M.sub.s transformation temperatures referenced herein
may be determined by persons skilled in the art either by
experiment or by model calculation.
[0078] For this steel example, as is understood by those skilled in
the art, except for the reheating temperature, which applies to
substantially the entire slab, subsequent temperatures referenced
in describing the processing methods of this example are
temperatures measured at the surface of the steel. The surface
temperature of steel can be measured by use of an optical
pyrometer, for example, or by any other device suitable for
measuring the surface temperature of steel. The cooling rates
referred to herein are those at the center, or substantially at the
center, of the plate thickness; and the Quench Stop Temperature
(QST) is the highest, or substantially the highest, temperature
reached at the surface of the plate, after quenching is stopped,
because of heat transmitted from the mid-thickness of the plate.
For example, during processing of experimental heats of a steel
composition according to the examples provided herein, a
thermocouple is placed at the center, or substantially at the
center, of the steel plate thickness for center temperature
measurement, while the surface temperature is measured by use of an
optical pyrometer. A correlation between center temperature and
surface temperature is developed for use during subsequent
processing of the same, or substantially the same, steel
composition, such that center temperature may be determined via
direct measurement of surface temperature. Also, the required
temperature and flow rate of the quenching fluid to accomplish the
desired accelerated cooling rate may be determined by one skilled
in the art by reference to standard industry publications.
[0079] A person of skill in the art has the requisite knowledge and
skill to use the information provided herein to produce ultra-high
strength, low alloy steel plates having suitable high strength and
toughness for use in constructing liners in accordance with the
present invention.
[0080] A person of skill in the art has the requisite knowledge and
skill to use the information provided herein to produce ultra-high
strength, low alloy steel plates having modified thicknesses,
compared to the thicknesses of the steel plates produced according
to the examples provided herein, while still producing steel plates
having suitable high strength and suitable cryogenic temperature
toughness for use in the liners of the present invention. Other
suitable steels may exist or be developed hereafter. All such
steels are within the scope of the present invention.
[0081] When a dual phase steel is used in the construction of
composite-overwrapped container liners according to this invention,
the dual phase steel is preferably processed in such a manner that
the time period during which the steel is maintained in the
intercritical temperature range for the purpose of creating the
dual phase structure occurs before the accelerated cooling or
quenching step. Preferably the processing is such that the dual
phase structure is formed during cooling of the steel between the
Ar.sub.3 transformation temperature to about the Ar.sub.1
transformation temperature. An additional preference for steels
used in the construction of liners according to this invention is
that the steel has a yield strength greater than 690 MPa (100 ksi)
and a DBTT lower than about -73.degree. C. (-100.degree. F.) upon
completion of the accelerated cooling or quenching step, i.e.,
without any additional processing that requires reheating of the
steel such as tempering. More preferably the yield strength of the
steel upon completion of the quenching or cooling step is greater
than about 690 MPa (100 ksi).
[0082] In order to join a steel to construct a liner according to
the present invention, a suitable method of joining the steel
plates is required. Any joining method that will provide joints
with adequate strength and toughness for the present invention, is
considered to be suitable. Preferably, a welding method suitable
for providing adequate strength and fracture toughness to contain
the fluid being contained or transported is used to construct the
liners of the present invention. Such a welding method preferably
includes a suitable consumable wire, a suitable consumable gas, a
suitable welding process, and a suitable welding procedure. For
example, both gas metal arc welding (GMAW) and tungsten inert gas
(TIG) welding, which are both well known in the steel fabrication
industry, can be used to join the steel plates, provided that a
suitable consumable wire-gas combination is used.
[0083] In a first example welding method, the gas metal arc welding
(GMAW) process is used to produce a weld metal chemistry comprising
iron and about 0.07 wt % carbon, about 2.05 wt % manganese, about
0.32 wt % silicon, about 2.20 wt % nickel, about 0.45 wt %
chromium, about 0.56 wt % molybdenum, less than about 110 ppm
phosphorous, and less than about 50 ppm sulfur. The weld is made on
a steel, such as any of the above-described steels, using an
argon-based shielding gas with less than about 1 wt % oxygen. The
welding heat input is in the range of about 0.3 kJ/mm to about 1.5
kJ/mm (7.6 kJ/inch to 38 kJ/inch). Welding by this method provides
a weldment (see Glossary) having a tensile strength greater than
about 900 MPa (130 ksi), preferably greater than about 930 MPa (135
ksi), more preferably greater than about 965 MPa (140 ksi), and
even more preferably at least about 1000 MPa (145 ksi). Further,
welding by this method provides a weld metal with a DBTT below
about -73.degree. C. (-100.degree. F.), preferably below about
-96.degree. C. (-140.degree. F.), more preferably below about
-106.degree. C. (-160.degree. F.), and even more preferably below
about -115.degree. C. (-175.degree. F.).
[0084] In another example welding method, the GMAW process is used
to produce a weld metal chemistry comprising iron and about 0.10 wt
% carbon (preferably less than about 0.10 wt % carbon, more
preferably from about 0.07 to about 0.08 wt % carbon), about 1.60
wt % manganese, about 0.25 wt % silicon, about 1.87 wt % nickel,
about 0.87 wt % chromium, about 0.51 wt % molybdenum, less than
about 75 ppm phosphorous, and less than about 100 ppm sulfur. The
welding heat input is in the range of about 0.3 kJ/mm to about 1.5
kJ/mm (7.6 kJ/inch to 38 kJ/inch) and a preheat of about
100.degree. C. (212.degree. F.) is used. The weld is made on a
steel, such as any of the above-described steels, using an
argon-based shielding gas with less than about 1 wt % oxygen.
Welding by this method provides a weldment having a tensile
strength greater than about 900 MPa (130 ksi), preferably greater
than about 930 MPa (135 ksi), more preferably greater than about
965 MPa (140 ksi), and even more preferably at least about 1000 MPa
(145 ksi). Further, welding by this method provides a weld metal
with a DBTT below about -73.degree. C. (-100.degree. F.),
preferably below about -96.degree. C. (-140.degree. F.), more
preferably below about -106.degree. C. (-160.degree. F.), and even
more preferably below about -115.degree. C. (-175.degree. F.).
[0085] In another example welding method, the tungsten inert gas
welding (TIG) process is used to produce a weld metal chemistry
containing iron and about 0.07 wt % carbon (preferably less than
about 0.07 wt % carbon), about 1.80 wt % manganese, about 0.20 wt %
silicon, about 4.00 wt % nickel, about 0.5 wt % chromium, about
0.40 wt % molybdenum, about 0.02 wt % copper, about 0.02 wt %
aluminum, about 0.010 wt % titanium, about 0.015 wt % zirconium
(Zr), less than about 50 ppm phosphorous, and less than about 30
ppm sulfur. The welding heat input is in the range of about 0.3
kJ/mm to about 1.5 kJ/mm (7.6 kJ/inch to 38 kJ/inch) and a preheat
of about 100.degree. C. (212.degree. F.) is used. The weld is made
on a steel, such as any of the above-described steels, using an
argon-based shielding gas with less than about 1 wt % oxygen.
Welding by this method provides a weldment having a tensile
strength greater than about 900 MPa (130 ksi), preferably greater
than about 930 MPa (135 ksi), more preferably greater than about
965 MPa (140 ksi), and even more preferably at least about 1000 MPa
(145 ksi). Further, welding by this method provides a weld metal
with a DBTT below about -73.degree. C. (-100.degree. F.),
preferably below about -96.degree. C. (-140.degree. F.), more
preferably below about -106.degree. C. (-160.degree. F.), and even
more preferably below about -115.degree. C. (-175.degree. F.).
[0086] Similar weld metal chemistries to those mentioned in the
examples can be made using either the GMAW or the TIG welding
processes. However, the TIG welds are anticipated to have lower
impurity content and a more highly refined microstructure than the
GMAW welds, and thus improved low temperature toughness.
[0087] A person of skill in the art has the requisite knowledge and
skill to use the information provided herein to weld ultra-high
strength, low alloy steel plates to produce joints having suitable
high strength and fracture toughness for use in constructing the
liners of the present invention. Other suitable joining or welding
methods may exist or be developed hereafter. All such joining or
welding methods are within the scope of the present invention.
[0088] As will be familiar to those skilled in the art, the
operating conditions taken into consideration in the design of
composite-overwrapped container liners constructed from a welded
steel for storing and transporting pressurized, cryogenic fluids,
such as PLNG, include among other things, the operating pressure
and temperature, as well as additional stresses that are likely to
be imposed on the steel and the weldments (see Glossary). Standard
fracture mechanics measurements, such as (i) critical stress
intensity factor (K.sub.IC), which is a measurement of plane-strain
fracture toughness, and (ii) crack tip opening displacement (CTOD),
which can be used to measure elastic-plastic fracture toughness,
both of which are familiar to those skilled in the art, may be used
to determine the fracture toughness of the steel and the weldments.
Industry codes generally acceptable for steel structure design, for
example, as presented in the BSI publication "Guidance on methods
for assessing the acceptability of flaws in fusion welded
structures", often referred to as "PD 6493: 1991", may be used to
determine the maximum allowable flaw sizes for the liner based on
the fracture toughness of the steel and weldment (including HAZ)
and the imposed stresses on the liner. A person skilled in the art
can develop a fracture control program to mitigate fracture
initiation through (i) appropriate liner design to minimize imposed
stresses, (ii) appropriate manufacturing quality control to
minimize defects, (iii) appropriate control of life cycle loads and
pressures applied to the liner, and (iv) an appropriate inspection
program to reliably detect flaws and defects in the liner. A
preferred design philosophy for the system of the present invention
is "leak before failure", as is familiar to those skilled in the
art. These considerations are generally referred to herein as
"known principles of fracture mechanics."
[0089] The following is a non-limiting example of application of
these known principles of fracture mechanics in a procedure for
calculating critical flaw depth for a given flaw length for use in
a fracture control plan to prevent fracture initiation in a liner
according to this invention.
[0090] FIG. 8B illustrates a flaw of flaw length 315 and flaw depth
310. PD6493 is used to calculate values for the critical flaw size
plot 300 shown in FIG. 8A (having abscissa 302 representing CTOD
fracture toughness in mm and ordinate 301 representing critical
flaw depth in mm) based on the following design conditions for a
pressure vessel or liner according to this invention:
TABLE-US-00001 Vessel Diameter: 4.57 m (15 ft) Vessel Wall
Thickness: 25.4 mm (1.00 in.) Design Pressure: 3445 kPa (500 psi)
Allowable Hoop Stress: 333 MPa (48.3 ksi).
[0091] For the purpose of this example, a surface flaw length of
100 mm (4 inches), e.g., an axial flaw located in a seam weld, is
assumed. Referring now to FIG. 8A, plot 300 shows the value for
critical flaw depth as a function of CTOD fracture toughness and of
residual stress, for residual stress levels of 15 percent of yield
stress (line 303), 50 percent of yield stress (line 304), and 100
percent of yield stress (line 305). Residual stresses can be
generated due to fabrication and welding; and PD6493 recommends the
use of a residual stress value of 100 percent of yield stress in
welds (including the weld HAZ) unless the welds are stress relieved
using techniques such as post weld heat treatment (PWHT) or
mechanical stress relief.
[0092] Based on the CTOD fracture toughness of the steel at the
minimum service temperature, the liner fabrication can be adjusted
to reduce the residual stresses and an inspection program can be
implemented (for both initial inspection and in-service inspection)
to detect and measure flaws for comparison against critical flaw
size. In this example, if the steel has a CTOD toughness of 0.025
mm at the minimum service temperature (as measured using laboratory
specimens) and the residual stresses are reduced to 15 percent of
the steel yield strength, then the value for critical flaw depth is
approximately 4 mm (see point 320 on FIG. 8A). Following similar
calculation procedures, as are well known to those skilled in the
art, critical flaw depths can be determined for various flaw
lengths as well as various flaw geometries. Using this information,
a quality control program and inspection program (techniques,
detectable flaw dimensions, frequency) can be developed to ensure
that flaws are detected and remedied prior to reaching the critical
flaw depth or prior to the application of the design loads. Based
on published empirical correlations between CVN, K.sub.IC and CTOD
fracture toughness, the 0.025 mm CTOD toughness generally
correlates to a CVN value of about 37 J. This example is not
intended to limit this invention in any way.
[0093] For liners that require bending of the steel, e.g., into a
cylindrical shape, the steel is preferably bent into the desired
shape at ambient temperature in order to avoid detrimentally
affecting the excellent cryogenic temperature toughness of the
steel. If the steel must be heated to achieve the desired shape
after bending, the steel is preferably heated to a temperature no
higher than about 600.degree. C. (1112.degree. F.) in order to
preserve the beneficial effects of the steel microstructure as
described above.
[0094] Although this invention is well suited for storage and
transport of PLNG, it is not limited thereto; rather, this
invention is suitable for storage and transport of any fluid,
including cryogenic fluids, pressurized fluids, and cryogenic,
pressurized fluids. Additionally, while the present invention has
been described in terms of one or more preferred embodiments, it is
to be understood that other modifications may be made without
departing from the scope of the invention, which is set forth in
the claims below.
Glossary of Terms
[0095] Ar.sub.1 transformation temperature: the temperature at
which transformation of austenite to ferrite or to ferrite plus
cementite is completed during cooling;
[0096] Ar.sub.3 transformation temperature: the temperature at
which austenite begins to transform to ferrite during cooling;
[0097] CNG: compressed natural gas;
[0098] coefficient of thermal expansion or contraction: the
increment in volume of a unit volume of a solid for a rise of
temperature of 1.degree. at constant pressure;
[0099] creep: time-dependent strain caused by stress;
[0100] cryogenic temperature: any temperature of about -62.degree.
C. (-80.degree. F.) and colder;
[0101] CTE: coefficient of thermal expansion or contraction;
[0102] DBTT (Ductile to Brittle Transition Temperature): delineates
the two fracture regimes in structural steels; at temperatures
below the DBTT, failure tends to occur by low energy cleavage
(brittle) fracture, while at temperatures above the DBTT, failure
tends to occur by high energy ductile fracture.
[0103] high-performance: in regard to composites or fibers means
having a tensile strength greater than about 3410 MPa (500 ksi) and
a modulus greater than about 136054 MPa (20 msi);
[0104] INVAR: a material consisting essentially of iron and
nickel;
[0105] ksi: thousand pounds per square inch;
[0106] LNG: liquefied natural gas at atmospheric pressure and about
-162.degree. C. (-260.degree. F.);
[0107] M.sub.s transformation temperature: the temperature at which
transformation of austenite to martensite starts during
cooling;
[0108] msi: million pounds per square inch;
[0109] Non-load-bearing Liner Container Patent: U.S. Pat. No.
6,460,721;
[0110] PLNG: pressurized, liquefied natural gas at a pressure in
the broad range of about 1035 kPa (150 psia) to about 7590 kPa
(1100 psia) and at a temperature in the broad range of about
-123.degree. C. (-190.degree. F.) to about -62.degree. C.
(-80.degree. F.);
[0111] PLNG Patent: U.S. Pat. No. 6,085,528;
[0112] Process Component Patent: U.S. Pat. No. 6,212,891;
[0113] psi: pounds per square inch;
[0114] self-supporting: in regard to a liner means capable of
maintaining its structural integrity while supporting its own
weight;
[0115] ship block coefficient: V/(L)(B)(T) where V is the volume of
fluid displaced by the ship, L is the length between the ship's
perpendiculars, B is the ship's beam and T is the ship's draft;
[0116] T.sub.nr temperature: the temperature below which austenite
does not recrystallize;
[0117] weldment: a welded joint, including: (i) the weld metal,
(ii) the heat-affected zone (HAZ), and (iii) the base metal in the
"near vicinity" of the HAZ. The portion of the base metal that is
considered within the "near vicinity" of the HAZ, and therefore, a
part of the weldment, varies depending on factors known to those
skilled in the art, for example, without limitation, the width of
the weldment, the size of the item that was welded, the number of
weldments required to fabricate the item, and the distance between
weldments.
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