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A CONCEPTUAL LNG CONTAINMENT DESIGN
UTILIZING EXPLOSION WELDED TRANSITION
JOINTS
BY CHAD N. FUHRMANN
2
1. ABSTRACT
2. INTRODUCTION
a. Short History
b. What is LNG?
c. Future Growth in LNG market
3. DESCRIPTION OF LNG CONTAINMENT AND TRANSPORT
a. Basic Types and Designs of LNG Vessel Containment Systems
i. Containment Design Considerations
ii. Kvaerner-Moss
iii. GTT No 96 Membrane Containment System
iv. GTT Mark III Membrane Containment System
b. Advantages/Disadvantages of Membrane vs. Moss Containment Designs
4. STAINLESS STEEL AND INVAR® VS. ALUMINUM IN LNG
CONTAINMENT
a. Thermal Properties of Each Metal
i. Invar®
1. Coefficient of Thermal Expansion
2. Coefficient of Thermal Conductivity
3. Effect of Thermal Properties on Containment Design
ii. Stainless Steel
1. Coefficient of Thermal Expansion
2. Coefficient of Thermal Conductivity
3. Effect of Thermal Properties on Containment Design
iii. Aluminum
1. Coefficient of Thermal Expansion
2. Coefficient of Thermal Conductivity
3. Effect of Thermal Properties on Containment Design
b. Kvaerner-Moss Design Considerations and Explosion Welded Transition
Joint
3
5. INTRODUCTION TO EXPLOSION WELDING OF ALUMINUM AND
STEEL
a. Process and Method
b. Basic Properties of Explosion Welded Aluminum and Steel Transition
Pieces
6. DESCRIPTION OF PROPOSED DESIGN FOR LNG TANKER
CONTAINMENT SYSTEM
a. Containment Cell Design
i. Aluminum Interior Similar to Moss Design
ii. Advantages of Design
1. Aluminum Interior with Cryogenic Advantages
2. Reduced Material Usage
3. Reduced Weight
4. Simplicity/Relative Ease of Installation
b. Explosion Welded Transition Joints/“Floating” Design
i. Description
1. Basic Design
2. Materials Used
ii. Advantages
1. Reliable Structural Support in Lieu of Wood Insulation-
Filled Boxes
2. Collision Safety
c. Piping Considerations
7. FURTHER CALCULATIONS
a. Coefficients of Thermal Expansion and Thermal Conductivity
i. Containment
ii. Transition Joints
iii. Vessel Support Structure
iv. Insulating Foam
b. Shear/Compressive Strength
i. Normal Loading
ii. Collision
4
8. LIMITATIONS OF PROPOSED DESIGN
a. Lack of Information Available
i. Current Designs Used in Naval Applications
ii. Design Details are Closely Guarded
b. Possible Design Flaws
i. Rough Estimates For Calculations
ii. Unproven Design
9. CONCLUSIONS
10. WORKS CITED
11. WORKS CONSULTED
5
Abstract
This paper introduces a new conceptual design for a liquefied natural gas (LNG)
vessel containment design. This “Floating Containment System,” or FCS, takes
advantage of recent advances in explosion welding, particularly between aluminum alloy
5083 and carbon steel for use as transition joints between the aluminum LNG
containments and the vessel’s hull. The new design will be studied using the coefficients
of thermal expansion for each of the metals used and the advantages and disadvantages of
the design will be described as applicable to cryogenic fluids in general and liquefied
natural gas in particular.
Introduction
Up until the 1970’s, most natural gas was burned off at wells in the oil field.
When it was recognized that this gas could be used for energy production, it was stored
for future use or re-injected back into oil deposits to assist in maintaining pressure within
the well. Today, natural gas is recognized as an extremely efficient, cost effective and
clean-burning fuel. It is being utilized in gas and steam turbines for electrical power
generation, used as an alternative fuel in transportation, heating homes and office
buildings and finding uses in countless other areas of life.
Natural gas is a fossil fuel found, as its name would suggest, in gaseous form and
is the byproduct of the anaerobic decay of organic material. The natural gas is trapped
and then processed for transport and used for energy production is normally found in oil
and natural gas fields but is also produced as biogas in places such as landfills and
swamps. Natural gas is made up of 70-90% methane (CH4), 5-15% ethane (C2H6),
6
propane (C3H8) and butane (C4H10), as well as small amounts of carbon dioxide, nitrogen
and other hydrocarbons and sulfur-containing gases.
Before it is transported any great distance, natural gas undergoes extensive
processing. This processing removes acids, moisture, mercury, nitrogen and other
contaminants producing a gas that is roughly 95% methane.
Demand for the fuel is growing, primarily for residential and commercial heating,
power generation and for industrial heat processes. As concerns for the environment
grow, demand for cleaner burning personal and public transportation will also increase
the demand for natural gas.
Description of LNG Containment and Transport
There are five basic sets of conditions for storage and/or transport of liquids or
gases:
1. Liquid at atmospheric pressure and temperature (atmospheric storage);
2. Liquefied gas under pressure and at atmospheric temperature (pressure
storage);
3. Liquefied gas under pressure and at low temperature (refrigerated pressure
storage, semi-refrigerated storage);
4. Liquefied gas at atmospheric pressure and at low temperature (fully
refrigerated storage);
5. Gas under pressure (Mannan pp.4-5).
7
For ease of pumping and transport via ship, natural gas is contained as per
conditions 3 or 4, depending on the containment method. For these conditions, the
natural gas must be liquefied. The liquefaction process occurs when the temperature of
the gas is reduced to approximately ( )FC °−°− 260160 . This extensive cooling reduces
the gas to a liquid that is one six hundredth of its initial volume making containment and
transport much more economically feasible. The extremely low temperature, however,
creates a unique set of difficulties that must be addressed.
Any fluid with a boiling temperature of ( )FC °−°− 1309.89 or lower at
atmospheric pressure ( psia7.14 or kPa3.101 absolute) is by definition a cryogenic fluid
(2006 International Fire Code p.279) with inherent dangers and handling/containment
precautions over and above those normally associated with a flammable liquid. At a
temperature of C°−160 , liquefied natural gas falls well within these parameters.
In addition to the precautions that must be taken in regard to safety of personnel
when handling LNG, there are also rules and guidelines that must be followed regarding
its containment and transport on board LNG vessels. Dry cargo or liquid petroleum
vessels are constructed of various types of steel depending on design criteria and the
owner’s preference and budget. However, LNG, due to its cryogenic state must be
treated differently. LNG is transported in vessels specifically designed to carry it. The
fluid is stored on board these vessels in specialized containments that can only be
constructed of specific materials.
There are two basic designs used for LNG vessels today. The general shape and
specific structure of these two containment systems is very different. The first design is
the Kvaerner-Moss spherical containment design. This design was introduced in 1965
8
and is the property of the Norwegian Kvaerner Group (www.akerkvaerner.com). The
design consists of a spherical aluminum tank interior covered in steel with a layer of
insulation in between. This design relies on an explosively welded transition joint as a
means of securing the aluminum sphere to the steel structure of the hull. It is this
transition joint that allows the combined advantage of the excellent cryogenic properties
of aluminum and the economic and strength advantage of the vessel’s structural steel.
The second type of LNG vessel containment is the membrane design. Much like
the Kvaerner-Moss design, this design consists of the membrane inner surface that is in
contact with the liquefied natural gas, surrounded by thick layers of insulation. The
entire containment is encased within the protective structural steel shell of the vessel’s
hull. While the spherical Kvaerner-Moss design is a one piece (relatively simple) design
9
that can be simply dropped into the hull of the vessel during construction, the membrane
type design is more complicated in its design and construction.
Within the membrane design group are two unique designs designated as the Gaz
Transport & Technigaz No. 96 and Gaz Transport & Technigaz Mark III Membrane
Containment Systems. In both of these containment systems, the liquid cargo membranes
are independent of the vessel’s structure, supported instead by a base of insulation and
plywood that surrounds the containment. Neither of the systems is anchored directly to
the steel hull of the vessel.
Generally, the No. 96 and Mark III Membrane Containment Systems are very
similar in their basic designs. However, each system utilizes different insulation and
membrane materials. The GTT No. 96 Membrane Containment System is built upon a
base of stacked plywood boxes filled with perlite insulation. On top of the stacked boxes
Figure 2--General Membrane Type LNG Vessel Containment Design
10
is layered an Invar membrane. Invar® is an iron and nickel alloy known for its uniquely
low coefficient of thermal expansion. The Invar® membrane is anchored in the stacked
layers of plywood boxes.
Secondary Box Level
Primary Box Level
Invar® Membrane
Steel Tank Top
Plywood Base Layers
Foam Insulation
Stainless Steel Membrane Triplex® Liner Sheet
Figure 3--GTT No. 96 Membrane Containment
System Construction (Curt p. 19)
Figure 4--GTT Mark III Membrane Containment System Construction (Curt p. 21)
11
The GTT Mark III Membrane Containment System is layered similarly to the No.
96 System. However, the Mark III System consists of layers of insulating foam divided
by layers of plywood and a Triplex®
plastic liner. The Triplex® plastic liner serves to
strengthen the overall membrane and also acts as an additional layer of insulation and as
a moisture guard. On top of the insulation, a baffled stainless steel membrane is installed.
Unlike the Invar® membrane of the GTT No. 96 System, expansion and contraction of
the stainless steel membrane, with its coefficient of thermal expansion of Kmm
mm17 ,
must be considered in its design and construction. The baffles of the stainless steel
membrane thus serve to absorb the thermal movement of the material.
The Kvaerner-Moss spherical containment design and the GTT membrane
containment designs each have advantages and disadvantages in comparison to one
another. The spherical containment has two distinct advantages over the membrane
design. First of all, the spherical tanks are simple in design and construction. They can
be completely fabricated before and during the construction of the vessel and then
installed in one piece on the ship. The aluminum and steel transition joint offers a degree
of structural stability that is not found in the membrane design. However, the spherical
shape of the containment does not fit the generally rectangular design of the vessel.
While the unused cargo space on this type of design serves as an additional safety zone in
the event of collision, it is also serves as dead space within the vessel that can be more
economically used in the better fitting membrane design.
The economic advantage of the membrane design is the primary reason that it is
the preferred design for LNG transport. Because of the better use of space in the
membrane containment system, it can carry more product on a vessel of similar size in
12
comparison to the Kvaerner-Moss design. To account for this, vessels using the
Kvaerner-Moss design must be on average 10% longer than the LNG vessels using the
membrane design. Thus, the construction cost is inherently lower for a LNG vessel using
a membrane containment system due to its comparatively smaller size. The size
difference between vessels also accounts for increased shipyard capacities, less difficulty
with canal passage, etc.
The “collision zone” created by the additional space in the Kvaerner-Moss
spherical containment is a safety advantage of this particular design. The same dead
space that makes the design less efficient also serves as an impact zone, protecting the
spherical containments from damage in the event of a collision or grounding.
The membrane containment systems create their own collision safety zone by
virtue of the independent design of the membranes. Again, the membranes are not
directly supported by or connected to the structural members of the vessel. In the event
of a marine casualty, the layers of insulation surrounding the containments creates a
cushioned safety zone around the cargo holds and the unattached design of the membrane
resists the damage from being transferred from structural members to the containment.
Stainless Steel and Invar® vs. Aluminum in LNG Containment
Members of the Workforce should verify that material and equipment that are
used in cryogenic applications are constructed only of materials that do not become
brittle and hazardous at low temperatures (Shrouf, p.7).
13
Material Properties Aluminum
Alloy (6061-T6)
Aluminum Alloy (6061-Untempered)
Mild Steel 1010 CREW
304 Stainless Annealed ASTM A269
321 Stainless Annealed
Titanium Cp2
Grade 2 ASTM B
338
Invar®
Tensile Strength MPa (20 C) 310 124 380 590 620 50,000 345 Yield Strength MPa (20 C) 275 55 275 240 245 275 360 Elongation Percent 12 25 20 55 55 20 57.5 Density Kg/m3 2700 2700 7800 8000 8000 4500 8100
Modulus of Elasticity x 103 MPa 0.07 0.07 0.20 0.19 0.19 0.10 0.15 Coefficient of Thermal
Expansion /K (or C) (x 10-6)
(20 C) 23 23 11 17 17 20 1.2
Coefficient of Thermal Conductivity
W/m K (20 C)
225 225 45 20 20-25 15.6 125.7
In addition to construction and design, the major difference between the
Kvaerner-Moss and GTT containment designs is the type of material used that is in
contact with the liquefied natural gas. The above table illustrates the different
characteristics of a selection of different materials. Aluminum, Invar® and stainless steel
are all materials used in the different containment types. The design of the containment
determines the type of the material used in its construction because each of the three
materials reacts differently to temperature extremes. A lower or higher coefficient of
thermal expansion may be incompatible or uneconomical with a given design.
Invar®, as used in the GTT Mark III Membrane Containment system is an alloy
consisting of 64% iron and 36% nickel with additional carbon and chromium. The
coefficient of thermal conductivity of Invar® is approximately Km
W6.125 at C°20 .
This value falls between stainless steel on the low end and aluminum on the high end of
the three materials in question.
14
Invar®, however, is especially known for its low coefficient of thermal
expansion. While the average value at low temperatures is Kmm
mm6101 −× , some
formulations of the material have a negative value for the coefficient of thermal
expansion (Incropera, p.537). The lower the value of this coefficient, the less a material
will expand or contract as its temperature varies. As the lining of the Mark III system,
Invar® is therefore an excellent material in that it is largely unaffected by the temperature
of the cryogenic LNG. The minimal contraction of the material at low temperatures
minimizes the risk of fractures due to tension stress in the material. Likewise, when the
membrane is subject to atmospheric conditions when empty, the material will not expand
to the point of buckling with the increase in temperature.
A disadvantage of Invar® is its tendency to creep. As the material is thermally
stressed over a period of time it has a tendency to deform to relieve that stress. This
could lead to weakened areas in the membrane that could eventually lead to leakage or
rupture of the tanks, particularly in the event of a collision.
The coefficient of thermal conductivity of the membrane material will have an
effect the insulation used in the membrane construction. The perlite-filled plywood boxes
used in the construction of the GTT No. 96 Membrane construction help to counteract the
higher thermal conductivity of the Invar®. The perlite insulation has a lower coefficient
of thermal conductivity (approximately Km
W029.0025.0 − at an average temperature
of C°−126 ) as compared to other forms of insulation. Perlite by itself, however, has
little or no compressive strength due to its loose form. Thus, the insulation is used inside
the plywood boxes which assist in adding compressive strength to the insulation layer
along with the Invar® anchors that support the unattached membrane.
15
The GTT Mark III Containment System uses a membrane constructed of stainless
steel. Described briefly earlier, this membrane design utilizes a baffle-type design to
prevent stress in the membrane from leading to fractures or buckling over the wide
temperature differential experienced between loading operations. Stainless steel has a
coefficient of thermal expansion of approximately ( )Kmm
mm6100.17 −× at C°20 . This
value is higher than that of Invar®, indicating that there will be more movement in the
material over the expected temperature differential, hence the need for the baffled design.
In comparison to Invar®, the coefficient of thermal conductivity of stainless steel
is low, approximately Km
W45 . In this design a foam insulation product is used. Foam
insulation can have a thermal conductivity in the area of Km
W039.0 at a temperature of
C°0 and lower at cryogenic temperatures (Pittsburgh Corning). This is higher than that
of perlite insulation, but in combination with the lower thermal conductivity of the
stainless steel, is suitable for the application.
The foam has the added benefit of a relatively high compressive strength, about
kPa620 , thus eliminating the need for the plywood boxes and simplifying the overall
construction of the membrane. Plywood sheets are still incorporated as layers of
separation between the foam layers as is a Triplex® sheet which further improves the
thermal conductivity and strength of the insulation layer.
The Kvaerner-Moss spherical containment design uses aluminum as the
construction material for its inner shell. The aluminum sphere is surrounded by foam
insulation. Perlite is not practical in this design for a number of reasons. First, the
insulation-in-box type of structure is not necessary by design as the tank is supported
16
directly by the vessel’s steel structure. Secondly, the loose nature of the perlite makes it
impractical for installation around the exterior of the aluminum shell. Foam, though not
necessary for support of the containment, is a solid material that can be pre-formed and
will remain in place around the spherical structure once installed.
Aluminum and its alloys have a thermal expansion coefficient of approximately
( )Kmm
mm61023 −× . In comparison to the other materials, aluminum is therefore subject
to much more expansion and contraction over the anticipated temperature range. For this
reason, the Kvaerner-Moss containment design can use aluminum as its interior material
while it may not be practical for the membrane design. The spherical design of this
containment system allows for the tension and compression forces experienced over the
temperature range to be evenly distributed over the surface of the hold. If aluminum
were to be used in a membrane-type containment system, the forces experienced in the
corners (stress concentrators) of the membrane containment could be excessive and may
very well lead to failure.
The cryogenic nature of liquefied natural gas and the thermal characteristics of
aluminum require that the Kvaerner-Moss containment be designed as it is. Meaning that
the spherical shape of the containment is required to withstand the stresses involved in
the containment. However, this shape requires that the Kvaerner-Moss vessel be larger
than membrane type vessels to be able to carry a comparable amount of LNG.
There are distinct advantages of aluminum in LNG containment and transport.
Despite the higher coefficient of thermal expansion, aluminum has excellent cryogenic
qualities. Unlike other materials, aluminum becomes stronger at cryogenic temperatures
and is not prone to brittle fractures at these low temperatures.
17
Unlike the membrane containments described above, the aluminum sphere in the
Kvaerner-Moss design is directly attached to the structural steel portion of the hull. This
is accomplished by the aluminum and steel transition joint located around the circular
base of the spherical tank.
This is the critical area in this type of containment design. Again, because of the
spherical shape of the containment, the forces applied to the structure of the tank are
evenly distributed. Placing the transition joint around the base circumference of the
containment allows the stress of thermal expansion and contraction to be evenly
distributed around the ring’s perimeter, supporting the structure without the risk of
fracture or deformation.
However, having the containment system directly connected with the hull of the
vessel has a disadvantage. In the even of a collision, the joint between the containment
and the structure of the vessel presents a potential danger area. If the support is damaged,
that damage could be transferred to the containment itself.
Introduction to Explosion Welding of Aluminum and Steel
In the Kvaerner-Moss containment design, the aluminum/steel transition joint is
created by a process known as explosion welding. This is a method by which two
dissimilar metals are welded together using tremendous force. The resultant bond is as
strong as any normal welding process but leaves the individual characteristics of the
dissimilar metals intact. In this instance, the aluminum retains its excellent cryogenic
properties, while the strength and toughness of the steel is also preserved.
18
Figure 5--Basic Explosion Welding Process (DMC Clad Metal Groupe SNPE p. 1)
As seen in Fig. 5, the explosion welding process begins with preparation of the
two materials to be joined (specifically aluminum and steel in this instance). After
preparation is complete, the two sheets are placed a specified distance apart with the
cladder material (aluminum) over the backer, or base material (steel). A precise amount
of explosive powder is evenly spread across the aluminum and is detonated from a
predetermined starting point. The explosion front travels across the face of the aluminum
at a speed of s
m40002000 − forcing it down upon the steel base metal at pressures
ranging from MPa000,4700 − (Murr p. 186, and High Energy Metals, Inc. p. 1). The
force of collision across the plate surfaces forces out oxides and other impurities
promoting a clean metal-to-metal bond. The metals are forced together at such great
19
velocity and with such force as to briefly become viscous fluids. The metals “splash”
together and bond with each other via interlocking waveforms.
Figure 6--Magnified View of Explosion Welded Joint
(Murr p. 188)
Figure 7--X-Ray Photograph of Aluminum/Steel Transition Piece
(Photo Courtesy of Joe Kass Photography, LLC, 2007)
20
The end result of the process is a single piece of metal consisting of steel and
aluminum completely fused together across their surface areas each with their original
characteristics still intact. These metal pieces are commonly produced in long strips
approximately cm3801 − wide by cm45.39.1 − thick and can be used in any number of
applications, including transition pieces in LNG containment systems.
Aluminum Plate
Aluminum/Steel Transition Joint
Steel Support Structure
Figure 9--Typical Explosion Welded Transition Joint (Merrem &
laPorte p. 11)
Figure 8--Explosion Welded Stock (Young p. 5)
21
The aluminum and steel transition joints manufactured by this process provide
many advantages over traditional fastening methods. The bond between the two different
materials provides a gap-free transition that greatly reduces galvanic corrosion. In
interior compartments, where there is no exposure to seawater or sea air, corrosion is
virtually eliminated. In areas where the transition point is subject to corrosion-inducing
environments, regular painting or coating can inhibit and even prevent corrosion.
Furthermore, the bond created between the metals is permanent and requires no
further maintenance. This is in contrast to other fastening methods between dissimilar
metals, such as nuts and bolts that can loosen or wear over time and exposure to
vibration, thermal expansion, loading, etc.
The explosion bonding process does not affect the original characteristics of the
materials. Contraction and expansion due to temperature changes still affect the
materials according to their thermal characteristics but the continuous joint, as mentioned
earlier, evenly distributes any stress that may occur due to these thermal effects on the
materials as well as any loading on the members they are a part of.
MATERIAL Steel Chemically Pure
Aluminum Aluminum Alloy 5083
Explosively Bonded Metal (Triclad®)
Tensile Strength
( )MPa 380-515
65-95 275-350 80 (min)
181 (typical)
Yield Strength
( )minMPa 205 20 125 ---
% Elongation
min 27 35 17 ---
Shear Strength
( )MPa --- --- ---
70 (min) 94 (typical)
Figure 10--Material Characteristics (Merrem & laPorte, pp. 6-7)
22
Proposed Design for LNG Tanker Containment System
The new LNG containment design proposed here attempts to take advantage of
the technological advances made in explosion welding in a new way, offering better
support of the containment cell and higher cargo capacity with less danger of
containment damage in the event of collision or other catastrophic failure.
Like the Kvaerner-Moss LNG containment design, the Floating Containment
System utilizes the excellent cryogenic properties of aluminum for the main component
of the LNG hold. Not only is aluminum excellent for cryogenic applications, it is also
very light (approximately 32700m
kg). This compared to the stainless steel and Invar®
used in the membrane designs, both of which have densities of approximately
38000m
kg. The decreased weight of the construction material allows for more cargo to
be carried on a vessel of a given size, making its use more economical in comparison to
other materials.
Figure 11--Proposed Containment Design Utilizing
"Floating" Supports
23
The FCS also utilizes a rounded shape to better withstand the contraction and
expansion of the aluminum with its high coefficient of thermal expansion. The
ellipsoidal containment cell lies within a similar shaped ellipsoidal shell (Fig. 10). While
this creates more dead space within the vessel as compared to the membrane design, the
modified design of the FCS ellipsoidal containment allows for more cargo capacity in
comparison with the Kvaerner-Moss design and, potentially, a slightly lower profile. A
vessel using the ellipsoidal containment allows for a capacity equivalent to a vessel of
equal size using the Kvaerner-Moss design, however, the FCS would only require two
ellipsoidal containments as opposed to the four required by the spherical containments on
an identical vessel.
Figure 12--Ellipsoid
As an example, a large LNG vessel using the spherical containment design can
carry approximately 3000,155 m of liquefied natural gas with four spherical tanks each
measuring approximately 42 meters in diameter. Utilizing two ellipsoid tanks with the
same measurement for width and height and double the length of one sphere, the
volumetric comparison between the two designs is as follows:
24
cbaVolume ⋅⋅⋅⋅= π3
4 , where ma 42= , mb 21= and, mc 21=
≈⋅⋅⋅⋅=∴ 2121423
4 πV3600,77 m
=× 2600,76 3m 3200,155 m
While the FCS design will not result in a straightforward increase in cargo
capacity, it will result in cost savings by reducing the amount of materials required to
build two containment cells as opposed to four containments of the traditional Kvaerner-
Moss design.
To further accommodate the expansion and contraction of the aluminum
containment cell and assist in its support, expandable foam insulation would be used in
the void spaces surrounding the aluminum containment. Again, this is similar to the
Kvaerner-Moss design in that the foam insulation can be pre-cut to conform to the
curvature of the containment cell. This foam will be compressed upon installation to
allow for expansion as the containment contracts.
Finally, the FCS also utilizes explosion welded transition joints between the
aluminum of the containment and the steel structure of the vessel. Like the explosion
welded joint of the Kvaerner-Moss design, these joints are integral to the structure of the
containment system. However, unlike the transition joint in the Kvaerner-Moss design,
the FCS design uses “floating” joints (Figs. 12&13) to compensate for the expansion and
contraction of the aluminum containment. These joints surround the containment cell in
broken bands and allow the aluminum to expand and contract freely in all directions
while still securing the containment cell within the vessel’s structure.
25
Figure 13--Floating Joint Utilizing Explosion Welded Transition Piece (Cross Sectional View)
Figure 14--Floating Joint (Side View)
Aluminum/Steel Transition
Aluminum Support (To Containment)
Steel Support (To Vessel Structure)
Floating Joint
Explosion Welded Transition
Floating Joint
Structural Steel
Aluminum
To Containment
To Vessel
Hardened Steel
Air Gap
26
The joints are constructed of three different materials—the aluminum portion
which is welded to the containment cell, the structural steel member welded to the
vessel’s hull and the hardened steel floating joint. The lower half of the floating joint
(closest to the containment) is bonded to the aluminum via the explosion welded
transition piece.
The hardened steel portion of the joint is designed to allow movement within the
joint without being subject to excessive wear, galling, etc. The surfaces of the joint are
highly polished to have a low coefficient of friction between the surfaces and machined
to be press-fit together at ambient conditions ( )C°20 .
Together, the foam insulation and floating joint system provide a better support
structure for the containment than the Gaz Transport & Technigaz membrane designs.
There is no requirement for plywood structures to assist in the support of the
containment. The foam and floating joint combination provide a lasting, reliable,
maintenance free support structure. The plywood support structures found in the GTT
No. 96 and Mark III membrane containment systems, on the other hand, may fall victim
to problems caused by the effects of moisture and time.
With the dead space surrounding the outer shell of the containment, a safety zone
is built into the design in the event of a collision or grounding. The floating joint support
system adds an additional element of safety. As seen in Figs. 12&13, there are several
stress concentrators present in the floating joint design. Also, the floating joints are not
welded at 90 degree angles to the containment. These two characteristics are safety
features of the FCS design. In normal conditions, the foam insulation and individual
floating joints act as a common system supporting the containment and its contents. In
27
the event of a collision, if the vessel’s support structure is damaged to the point of
endangering the containment, these supports are designed to fail at the welded seams
rather than puncture the containment cell.
Further Calculations
Please note that all calculations are rough estimates only. The alloys,
insulating materials, etc. were chosen for the purpose of analyzing this conceptual
design. More accurate analyses must be made for the specific materials chosen.
Before it is filled, the containment is at an ambient temperature of C°20 . As the
cell fills with cryogenic LNG the temperature of the containment and the immediate
surrounding area is reduced to a temperature of approximately C°−160 . Using the
dimensions of the containment cell from the previous example, it can be determined
approximately how much of a decrease in volume will be experienced over the
temperature differential.
Using the equation for the change in volume of a geometric solid, and the
containment dimensions from the previous example, the change in volume of the
ellipsoid containment is as follows:
TVV oellipsoid ∆⋅⋅=∆ α
Where, 3500,66 mV
o≈
Cmm
linearumalu °×=
−6
)(min 1023α
( ) CT °=−−=∆ 18016020
( )( )( ) ≅×=∆∴− 1801023600,77 6V 3320m
28
This equates to approximately 3.3 centimeters of joint travel required in the
floating joints around the surface of the ellipsoidal containment cell to compensate for its
contraction over the expected temperature range. This amount will vary, however, as the
contraction around the cell will not be equal in all directions. Specifically, the
contraction will have the greatest effect at the top of the cell with minimal effect at the
bottom.
The materials in the joints themselves will contract as well. Assuming a void
space thickness of 2 meters, the actual floating joint will be approximately 1 meter from
the containment. The explosion welded transition piece in turn will be located 50
centimeters from the containment, half way in between the aluminum containment and
the floating joint. The thermal conductivity of aluminum is very high (approximately
CmW
°225 ) and, since the length of the aluminum portion of the support is relatively
short, the entire segment will be assumed to experience the same temperature differential
as the containment. The approximate contraction of the aluminum portion of the support
is therefore,
( )( )( ) =×=∆⋅⋅=∆− 18010235.0 6TLL
oα m002.0
Likewise, the hardened steel portion of the support, with a coefficient of thermal
expansion of approximately Cm
m°
×−61012 , will be assumed to experience the same
temperature differential as the aluminum portion due to its own thermal conductivity
29
(approximately Cm
W°
35 ), the thermal conductivity of the aluminum and its short
distance from the containment. Thus,
( )( )( ) =×=∆⋅⋅=∆− 18010125.0 6TLL
oα m001.0
Finally, the 1 meter section of hardened and carbon structural steel that makes up
the stationary section of the floating joint and the structural member of the support
system will have a change in length that can be assumed to be negligible in comparison to
the first two sections analyzed. The total joint movement is thus,
=++ 001.0002.0033.0 m036.0
Surrounding the floating joints and the containment cell is the foam insulation.
Again, this insulation is compressed prior to installation to compensate for the increasing
volume seen in the void space as the containment contracts. The material chosen in for
this analysis is Pittsburgh Corning FOAMGLAS® insulation. This insulation provides a
thermal conductivity of only Cm
W°
10.0 , while providing extremely low water
permeability and having a density of 3120m
kg. It also provides a compressive force of
kPa620 . FOAMGLAS® is comparable to the best characteristic of the foam and perlite
insulations used in the GTT No. 96 and Mark III membrane systems (Pittsburgh
Corning).
Insulation Material Perlite Foam FOAMGLAS®
Thermal Conductivity
°CmW 0.14 0.10 0.10
30
Permeability (% absorption by volume) 2-90 0.7 0.2
Density
3mkg
80-208 32 120
Compressive Force ( )kPa 620 310 620
Perhaps the most important characteristic is the compressive strength of the
FOAMGLAS® insulation. In the FCS containment design, the insulation serves an
integral role in the support of the containment. In fact, much like the membrane
containment designs, the insulation is the main source of support for this design. For the
containment cell of the dimensions described above, the surface area is approximately
2300,10 m , the lower half of which ( 2150,5 m ) is supported by the foam. The foam
provides a total vertical supporting force exceeding 300,000 metric tons. Considering
that a full containment of LNG, at a mass of about 3420m
kg, weighs approximately
30,000 tons, the FOAMGLAS® will provide excellent support for the containment.
Perhaps the only drawback to this choice of insulation would be the weight. If the
outside shell of the FCS were 2 meters wider all the way around the containment, the
total volume of space filled by the insulation would be approximately 3500,18 m .
( ) ( ) ( )[ ] ( ) ≈+−−⋅−⋅−⋅⋅=∴ 275500,662182212423
4 πV3500,18 m
This amount of FOAMGLAS® insulation would add about 2,200 metric tons to
each individual containment system. However, if necessary, the top portion of the
containment, which does not require the compressive force of the lower half, could be
insulated using a much lighter foam (polystyrene, for example) at the cost of a small
additional loss of product due to evaporation.
31
( ) ≅
+
32
2
500,18
2
2200 1400 metric tons
The foam and floating joint system together also provide added protection in the
event of a collision or grounding. The floating joints have additional room for movement
due to the contraction of the containment. In addition to its insulating qualities, the
density of the foam provides a cushioning effect that acts against the forces experienced
during a collision or grounding and serves as an additional buoyant layer should the hull
be compromised. The layer of FOAMGLAS® is also another barrier of defense against
puncture of the containment cell.
The supports themselves are installed at angles greater than 90 degrees to the
perpendicular to the containment. Particularly in the areas of the containment where
there would be the greatest likelihood of damage due to collision or grounding. The weld
seams provide stress concentrators in addition to those outlined in the previous
description of the floating joints. Additionally, the welds are weaker than the rest of the
containment cell. The floating joint support structure is purposely designed to break at
the weld seems and/or other stress concentrators rather than puncture the containment
cell.
A number of design considerations and construction materials may be
incorporated in the piping systems required with the Floating Containment System.
Piping may be made of an aluminum alloy similar to the containment cell material. An
explosion welded transition joint can again be used to bond this pipe to a steel or stainless
steel pipe as it penetrates the outer shell of the containment. Another design may utilize
an expanding/contracting bellows of suitable material attaching the containment to the
32
outer shell via a standpipe. Accommodations must also be made for the recovery of gas
vapors for possible use in the vessel’s propulsion engines.
Limitations of Proposed Design
While the FCS design promises advantages over previous design, it has
limitations. For example, while an LNG vessel utilizing the FCS design has greater
capacity than a comparable Kvaerner-Moss vessel, a vessel utilizing either of the GTT
membrane designs still has greater capacity and thus, greater short term economic
benefits.
The FCS design is a new, unproven concept. No design like it has been tested so
its economic and operational feasibility cannot be proven at this time. Likewise, the FCS
floating joint system is a new application of explosion welded transition joints that has
not been used before.
Although LNG containment systems are not new designs, details surrounding
their construction are considered proprietary. The Kvaerner-Moss spherical containment
and the GTT membrane containment systems are exclusive to the respective companies
that designed them. Their construction, particularly the explosion welded transition joint
of the Kvaerner-Moss design is not completely known and the details are kept fairly
guarded.
Likewise, the process of explosion welding is by no means a new technology. It
has been in existence since soldiers first noticed the copper jackets from bullets bonding
to the steel armor of tanks and other armored vehicles. However, the process is still
33
untested in many applications. Many traditional fastening methods on the other hand, are
tried and true in countless industries and functions.
Where explosion welding technology has been tested is in many military
applications. Naval vessels have been constructed with steel hulls and aluminum
interiors that utilize explosion welded joints as transition pieces. For reasons of security,
the details of materials used and results of construction testing such as sea trials are not
public knowledge and are available to very few.
Protected proprietary information and industry secrets make it difficult to
determine whether or not a conceptual design is feasible. As construction details and
results of real world industrial and military applications become a matter of public
knowledge, designs can be determined to be practical or not earlier on in the design
process. Until then designs can be conceived and tested only as far as the technologies
and designs are known to architects, engineers, grad students, etc.
All of the calculations made throughout the design process have been rough
estimates based on certain assumptions. Not all of the assumptions made may be the
correct. Values were used for coefficients of thermal expansion and conductivity based
on general alloy versions of the materials used. They do not take into account certain
factors. For some materials, the coefficient of thermal expansion changes with the
temperature. As a result, further research may show that alternate materials would fare
better in this application. Additionally, the details and characteristics of materials such as
insulation were based off of specifications from manufacturer’s data sheets and given on
their websites. These numbers are more than likely ideal values and may not prove true
in application.
34
Conclusions
The proposed design attempts to use the best aspects of already proven LNG
vessel containment designs. Taking a characteristic from the Kvaerner-Moss
containment design, aluminum is the material of choice in the Floating Containment
System. It is the lightest material of those analyzed allowing for a lighter and therefore
more economical vessel design. Aluminum is also the material with the best cryogenic
properties in comparison to the GTT membrane materials stainless steel and Invar®.
The ellipsoidal containment cell design allows more product to be carried on a
vessel of similar size to one with the Kvaerner-Moss design. The compressed foam
insulation in the support system mimic the support structure of the membrane designs by
holding the containment cell securely, while still protecting it from damage by avoiding a
direct transition joint, unlike the explosion welded transition joint found in the spherical
containment design. The floating joint structure adds an element of support not seen in
the membrane design that increases the strength and safety of the containment system.
While aspects of the Floating Containment System may prove to be impractical or
not economically feasible, it is a new approach that attempts to increase the efficiency of
LNG containment and transport. However, there are many more aspects of the design
and the materials used that must be analyzed.
As demand for cleaner, cheaper fuel sources grows the demand on the LNG
market will grow. The Floating Containment System may prove to be a legitimate
direction for the future of LNG vessel design and construction.
35
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2006 International Fire Code. “Cryogenic Fluids”. Chapter 32, pp.279-284. 2006
Aker Kvaerner. History.
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Avallone, Eugene A. and Baumeister, Theodore III. Editors. Mark’s Standard Handbook
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Conference. March 29, 2004
High Energy Metals, Inc. Explosion Bonding Engineering and Design Basics. March 8,
2000
DMC Clad Metal Groupe SNPE. “Explosion Clad Plate Manufacturing”. URL
http://www.dynamicmaterials.com/data/brochures/EXWprocess2.pdf
Incropera, Frank P.; DeWitt, David P. Fundamentals of Heat and Mass Transfer, 5th
Edition, Wiley. ISBN 0-471-38650-2. August 9 2001
Mannan, Sam. Editor. Lees' Loss Prevention in the Process Industries (3rd Edition).
Chapter 22, pp.1-78. Elsevier Publications. 2005
38
Merrem & laPorte. “Triclad®: Welding Aluminum to Steel”. URL
http://www.triclad.com/pictures/triclad.pdf
Murr, Lawrence E. Shock Waves for Industrial Applications. Chapter 5, pp. 170-215.
William Andrew Publishing/Noyes. 1988. URL
http://www.knovel.com/knovel2/Toc.jsp?BookID=775&VerticalID=0
Pittsburgh Corning. “About FOAMGLAS® Insulation”. URL
http://www.foamglasinsulation.com/mechanicalspecs.asp
Robin Materials Inc. “Invar®”. URL http://www.rmat.com/invar.html
Shrouf, Robert. Safe Handling of Cryogenic Fluids. GN470100, Issue B. October 31,
2006. URL http://www-irn.sandia.gov/corpdata/esh-manuals/gn470100/g100.htm
Triplex GmbH. Basic Information: Triplex Plastic Panels: Extremely Rigid and
Lightweight. 2006. URL http://www.triplex-kunststoffplatten.de/english/basic-
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Proceedings of the 13th Offshore Symposium. February 24, 2004