Floating LNG/CNG Processing & Storage Offshore Platforms Utilizing a New Tank Containment System - Technical Paper

Floating LNG/CNG Processing and Storage
Offshore Platforms Utilizing a New Tank
Containment System

Regu Ramoo
Director of Engineering, Altair ProductDesign, Inc.
1820 E. Big Beaver Road, Troy, MI 48083, USA

Prof. Thomas Lamb
University of Michigan
Ann Arbor, MI 48109 USA

www.altairproductdesign.com
copyright Altair Engineering, Inc. 2011


Abstract
Current interest in Natural Gas offshore systems is focused on the Floating Oil/LNG
Processing and Storage Offshore Platforms (FOLNGPSO) Floating LNG Processing and
Storage Offshore Platforms (FLNGPSO, Floating Oil/CNG Processing and Storage Offshore
Platforms (FOCNGPSO) Floating CNG Processing and Storage Offshore Platforms
(FCNGPSO). A number have been built and many more are in design. A new tank
containment system which has shown significant operation and acquisition cost benefits is
even more beneficial to the FOGNGPSOs and FCNGPSOs especially its ability to withstand
sloshing loads in partially filled LNG tanks.

The paper reports on the benefits of the Cubic Dough-nut tank containment system on the
supporting platform design especially its ability to operate with liquid levels in the tank from
empty to full.

Keywords: LNG and CNG Containment Tank, FOLNGPSO, FOCNGPSO

1.0 Introduction
In the past few years interest in Floating Oil/LNG Processing and Storage Offshore Platforms
(FOLNGPSO) Floating LNG Processing and Storage Offshore Platforms (FLNGPSO) has
increased and a number have been built, are under construction, and many more are in
design. More recently interest in Floating CNG Platforms has developed and a number of
designs completed. It has been concluded that the best way to collect and transport gas from
a small field is by compressing it. Compressed natural gas (CNG) requires a storage volume
approximately twice that of LNG but does not need the expensive refrigeration plant at the
source or the gasifying equipment at the receiving end.

Even though the capacity of natural gas is small, a CNG platform would still be relatively large
in length. The problem is that the weight of the gas storage tanks is great, about 50,000 t for a
cargo deadweight of only 15,000 t. This results in a very low deadweight ratio.

A solution that has superior volumetric and weight usage is proposed that utilizes a new
containment tank system that can be applied to both LNG and CNG, namely the Cubic
Doughnut Tank System (CDTS). Previous papers (LAMB 2009OTC, RAMOO 2009, LAMB
2009) have described the development of the CDTS and its applications to both LNG and
CNG Car-riers with a brief mention of its application to floating production and storage
platforms for both LNG and CNG. The first two of these papers also presented detailed
structural analysis for LNG and the last one for CNG applications and these will not be
repeated in this paper. However, updates to the analysis will be presented.

2.0 CDTS Description
The CDTS was developed over 30 years ago but nothing was done with it as interest in
importing LNG disappeared along with the cessation of diplomatic relations with Algeria. The
basis for its design was constructing a self-standing tank surface composed of 12 identical, in

Copyright Altair Engineering, Inc., 2011 2


form, intersecting cylinders that formed the twelve edges of a cube that would have a
significantly better volumetric efficiency than a spherical tank. Where the intersecting cylinders
met in the center of each face a closing cap was provided. Figure 1 (from the original patent)
shows the form of the tank.

Since 2005 ALTAIR Engineering joined Lamb in developing the CDTS using their advanced
structural analysis and simulation systems. A detailed description of the tank development can
be found in the first three references to this paper. The most recent tank configuration is
shown in Figure 2.

Figure 1. Cubic Doughnut Tank System (CDTS)

Figure 2. Latest Configuration of CDTS



3.0 Comparison of CDTS with other Containment Systems
Figure 3 shows the outlines in two views of membrane, spherical and CDTS tanks of equal
volume.

Figure 3. Comparison of Outlines for Different Containment

It can be seen that the spherical tank is larger in all dimensions whereas the membrane tank
is only larger than the CDTS in length and breadth. The CDTS has a volumetric efficiency
between the current membrane tanks system and the proposed PRISM membrane sys-tem
(Noble 2005). The volumetric efficiency of different types of tanks is compared in Table 2. It
can be seen from the table that the CDTS is 60% better that spherical tanks.

Table 2. Comparison of Tank Volumetric Efficiency

Next the use of ship space was compared. Figure 6 shows the “hold” space required by each
of the systems being compared for a 300,000 m3 LNG Tank Capacity. It can be seen that the
Length usage for the CDTS is better than the other systems.



The major operating problem is the sloshing of the liquefied natural gas especially in partially
filled large membrane tanks. Liquid sloshing limits the carriage of LNG in large side to side
membrane tanks to be either over 80% or less than 10% full to avoid damage to the tank
lining and insulation. This is impractical for a FOLNGPSO and FNGPSO where the tanks will
be filled and emptied continuously. Spherical containment tanks do not suffer from this
problem but they are un-suitable for floating processing and storage platforms as their
arrangement restricts the available deck space for the processing equipment.

Tank sloshing has been around with ship designers and operators since liquids were first
carried in ships. However the liquids were carried in tanks with much smaller capacities
(dimensions). Even the tanks in the largest tankers were less than 50 m in length and 30 m in
breadth whereas LNG tanks can be over 50 m in length and over 40 m in breadth. Also the
tanks in tankers are integral structural tanks and thus more able to withstand the sloshing
loads and usually have a transverse SWASH Bulkhead at mid-length of the tanks which
reduces the fore and aft sloshing loads on the tight transverse bulkheads, whereas the current
trend in LNG carriers is the membrane lined and insulation box supported tanks, which has
been shown to have sloshing problems (damage to lining and insulation) as size increases.

3.1 Impact on LNG Platform Design
LNG has a specific gravity slightly more than half that of oil. Thus the LNG tank space
dominates the design. To date the existing FONG and those in design follow a tank
arrangement as shown in Figure 4. Due to the heavier oil in the extreme forward and aft
tanks, this tank arrangement results in a large still water and wave at midship hogging
moment that increases the required section modulus in the longitudinal structure of the hull,
even when the LNG tanks are fully loaded. If the oil tanks were full and the LNG tanks empty
the hogging moment is enormous.

To overcome this bending moment problem a unique approach for arranging the tanks, which
was developed by a team of students in 2006 for their final Capstone Design Course at the
University of Michigan, is shown in Figure 5. This arrangement reduced the maximum
Bending Moment by 30%. By using the CDTS for LNG Tanks there are even further benefits
in that the tank length is reduced by 80 m or 25% and the Length Overall by 100 m or 29%
and the bending moment by a further 40%. This is shown in Figure 6 to the same scale as
Figures 4 and 5.

The reduced hold length for the CDTS is the clear ad-vantage. Coupled with the proposed
unique tank arrangement it results in a significantly smaller platform length as can be seen
from Table 3 that compares plat-form characteristics an existing and one design FOLNGPSO
with one of equal capacity using the CDTS. The reduction in length has immediate impact on
the structural design in that the Wave Bending Moments are reduced to half those of the
longer membrane FOLNGPSO. The maximum Bending Moment will also be reduced by the
tank arrangement compared to the arrangement shown in Figure 8 by 50%. Both these
bending moment reductions will result in a smaller required Sectional Modulus and Moment of
Inertia which in turn will be met with less longitudinal section-al area thus reducing structural
weight.



Figure 4. Current Tank Arrangement Design

Figure 5. Parallel Tank Arrangement

Figure 6: Tank Arrangement with CDTS

Table 3. Platform Characteristics for 160,000 m3 /1.4 M Bbls FOLNGPSO

Table 4 shows the difference in characteristics for a hypothetical 300,000 m3 FOLNGPSO
Figure 7 shows the General Arrangement of the proposed FOLNGPSO using the CDTS.



Table 3. Platform Characteristics for 160,000 m3 /1.4 M Bbls FOLNGPSO

Figure 7. CDTS FOLNGPSO Profiles

3.2 Impact of LNG Platform Cost
Building Cost Estimates were made for the FOLNGPSO with Membrane and CDTS LNG
containment systems in Table 3 which shows that the CDTS design would cost 7% less than
the membrane design. It also shows that the Gross Tonnage would be 5% less which would
result in operating cost savings.

All cost estimates were made using a Preliminary De-sign Cost Estimating Model. This
approach (or methodology) has been found over time to predict shipbuilding cost within plus
or minus 10% with very few outliers.

4.0 Construction Benefit
A major construction benefit results for the CDTS by uncoupling the tank building and
installation schedule from the ship construction schedule, whereas the Membrane Tank



System requires a significant time afloat to install the insulation and membrane lining, often as
long as the hull erection time. Like other independent tank systems the CDTS would
significantly reduce the tank installation time afloat to almost zero compared to the membrane
tank system.

The CDTS offers all the benefits of the independent tank systems such as the spherical and
prismatic self-standing tank systems, but with a simpler hull construction and tank/hull
integration such as:

no need to stage the hold to apply insulation and lining to the structure,
tanks can be installed in one piece at the best time in the ship construction build
sequence,
tanks can be constructed from aluminum or special steel,
tanks can be structurally and leak tested before installation in the ship,
eliminates the significant welding of the insulation and lining securing strips and the
lining onboard the ship,
is not subject to the same damage from dropped items as the membrane tank
containment system,
a smaller skirt system compared to the spherical tank containment system,
the service/maintenance benefit in that the internal ship’s structure and the tank
insulation can be inspected, and
tank insulation is shaped only in two dimensions not three as in spherical tanks.

Further, the CDTS can be constructed using typical shipyard rolling and forming equipment. It
is made up of 12 identical partial cylindrical tubes (made from identical or mirror image plates)
and 8 identical spherical corners. One design option even deletes the spherical corners to
simplify the construction and increase capacity, but at an additional material cost and design
complexity. While the CDTS offers benefits just from the tank design, construction and
installation in the ship, it offers unique benefits in the design of the ship including significant
reduction in length from 370m to 264m, which has construction benefits in reduced steel
weight and less work content for the same capacity ship com-pared with any other system.

The Impact of the CDTS on the platform’s structural arrangement can be seen from Figure 8,
the Midship Section and Figure 14, the Centerline Profile.



Figure 8. Midship Section

Baseline CDTS vs. Membrane TANK
In the previous study (Lamb and Ramoo, 2009), sloshing simulations of a rigid CDTS
(baseline) and a rigid membrane tank of nearly equal capacity were per-formed using the SPH
(Smooth Particle Hydrodynamics) approach available in RADIOSS. The finite element model
of the membrane tank used is shown in Figure 12a. The volume of both the tanks was
40,000m3. The tanks were subjected to an oscillatory motion (Figure 12b) about their
longitudinal, transverse, and mid off-axis to simulate the motion of the ship in beam, bow and
bow-quartering seas.

Figure 9. Rigid Membrane Tank



Figure 10. Enforced Rotation

The period of the sloshing motion was 8 seconds. This was based on the anticipated roll
period of an LNG ship carrying six CDTS and a peak roll amplitude of 30o.

The sloshing simulations were performed with two different tank capacities. One with 80% and
another with 50% tank capacity. The total sloshing loads on the sides of the tank at 50% and
80% tank capacities for roll motion (bow seas) are shown in Figure 11 and 12 respectively.

The higher sloshing loads in the case of the membrane tank could be attributed to the waves
impinging directly on the flat walls of the membrane tank unlike in the case of CDTS where the
waves could decelerate along the curved walls. Also, the free surface was larger in the case
of the membrane tank whereas in the case of CDTS the cross braces appeared to break the
waves and there-by reduced the velocity of the fluid before impacting the walls of the tank.

Figure 11: Sloshing Loads at 80% Tank Capacity (Roll Motion/Bow Seas)



Figure 12: Sloshing Loads at 50% Tank Capacity (Roll Motion Bow Seas)

CDTS Sloshing Loads and Wall Stress
In this study, the sloshing simulation was performed using the current design of the CDTS,
employing the ALE (Arbitrary Lagrangian Eulerian) approach available in RADIOSS. The ALE
approach was opted since it gives a smooth variation of the sloshing load compared to the
SPH approach for the same level of discretization. The CDTS was considered deformable
with a uniform thickness of 100 mm. The skirt is considered rigid. The tank was filled to 80%
capacity. The fluid (LNG, specific gravity 0.5) was modeled using hexahedral elements and 2-
phase liquid-gas mixture material model with a Mïe Grüneisen equation of state (material
law37). The rest of the tank was filled with air (hexahedral elements and material law37).The
finite element half model of the CDTS used for the sloshing simulation is shown in Figure 18.
Symmetry boundary conditions were imposed on both the structural and fluid nodes.

Figure 13: Finite Element Model of CDTS used for Sloshing Simulation



The simulation was composed of two steps. In the first step a constant gravitational load (9.81
m/s2) was applied to the tank and the fluid for 0.5secs. In the next step a roll motion was
enforced on the tank for 7.5 seconds. The gravitational load was held constant for the entire
duration of the roll period.

Figure 14 depicts the fluid motion during the event as well as the distribution of the sloshing
loads or the fluid impingement loads at different instances of time (2.6 seconds, 2.9 seconds).
These loads were extracted from the sloshing simulation (ALE/RADIOSS) and consi-dered as
static load cases for further optimization of the tank design. The contour plots of von Mises
stress in MPa at these instances of time from the sloshing simulation are shown in Figure 15.

Structural Analysis
An earlier paper (LAMB OTC2009) presented details of the structural analysis for the CDTS
containing LNG and it will not be presented in this paper.

5.0 Application to CNG
It was always the intention to explore the use of the tank for pressures above atmospheric,
and recently the application of the CDTS to CNG Carriers and FOCNGPSO and PCNGPSO
was examined. Whereas the CDTS size for LNG application had no limitation up to that
required for the largest LNG Carriers under development, the CDTS tank for the carriage of
CNG will be much smaller due to its thicker shell and thus weight and will be a compromise
between shell thickness, weight and manufacturability.



Figure 14. Fluid Motion and Impingement Loads at 2.9 seconds

Impact on CNG Platforms
The CDTS has been applied to CNG Carrier design (Lamb 2009) and offers significant
acquisition and life cycle cost savings compared to other tank containment systems. It has
been found to offer similar cost savings for CNG offshore platform applications. The CDTS
has superior volumetric efficiency and weight com-pared to any other proposed CNG Marine
containment systems. It has a hold volumetric efficiency of 0.33 (VOTRANS 0.18 and SEA NG
0.20) a ship volumetric efficiency of 0.14 (VOTRANS 0.09 and SEA NG 0.09) and the
platforms utilizing CDTS would have a cargo deadweight coefficient of 0.133

The CDTS offers many of the benefits to LNG also to CNG and in addition the following
benefits compared to other proposed systems for CNG Carriers and offshore platforms:

1. Building cost reduction of 12 % for platform and 10 to 20% for the containment system,
2. significant reduction in platform length,
3. significant reduction in Gross Tonnage
4. significant reduction in tank surface area and thus CNG gain of heat. This is important
as it impacts the heat transfer into or out of the contained CNG and this in turn
increases the CNG pressure due to increasing in gas temperature.
5. the in service maintenance benefit in that the tank structure can be inspected, and
6. significantly reduced number of tank manifolds

The result of its many benefits is significant acquisition and life cycle cost savings compared
to the other pro-posed designs.



A range of CDTS size was explored in the preliminary structural analysis to determine tank
volume and aver-age shell thickness, and is presented in Table 5. The 10 m CDTS tank was
selected to demonstrate its appli-cation to CNG platforms, as it was the best compromise
between shell thickness, weight and other construction limits.

Table 5. CDTS Tank Characteristics

FOCNGPSO Tank Arrangement
Before the natural gas can be transported by ships it must first be collected. Unfortunately
many of the gas fields are small compared to the large oil fields. Thus it has not been
economically viable to recover the gas from them up until now. However with increasing
demand, and a decreasing supply of easily recovered energy it is becoming necessary to
investigate how to change the situation. The first Floating Liquefied Natural Gas (FLNGPSO)
platform is in operation. Figure 15 shows a concept design for a 10.5 MMscm/200,000 Bbl
FOCNGPSO.

Length BP = 230m Beam = 60m Depth at Side = 21m Operating Draft = 8.84m
Displacement = 111,284t Light Ship = 70,962t CNG = 8,000t Oil=25,000t

Figure 15. 10.5 MMscm/200,000 Bbls CDTS FOCNGPSO Arrangements



Figure 16 shows the midship structural arrangement. A smaller and A larger combinations are
given in Table 6.

Ongoing Work
The initial class certification and a detailed manufacturing/facility plan are all underway. Also a
detailed cost estimate for the CDTS tanks is being performed along with the manufacturing
/facility Plan.

5.1 Structural Analysis
Again the structural analysis of the CDTS for CNG was presented in an earlier paper (LAMB
2009) and only updates to those findings will be presented.

Table 6. CDTS FOCNGPSO Series

Figure 16. FOCNGPSO Midship Section



Introduction
ALTAIR Engineering Hyperworks was used to analyse the tank structure for CNG. Altair
Engineering’s Hyperworks is a computer-aided engineering (CAE) simulation software
platform that allows businesses to create superior, market-leading products efficiently and
cost effectively. The Hyperworks platform offers modeling & visualization as well as analysis &
optimization solutions. The CDTS is a complex shape and as such does not lend itself to
simple analysis. An advanced structural analysis approach is required. Starting from 2005, the
Hyperworks suite of advanced structural de-sign, analysis and optimization tools were used to
improve the design to meet the structural objectives which could not otherwise be attained by
the proposed original design. This involved connecting the center of all faces by an internal
cross brace. The finite element analysis and optimization was performed using Altair OPTI
STRUCT, which is a linear finite element solver available in Altair Engineering’s Hyperworks.
An earlier paper (RAMOO 2009) describes the finite element analysis and optimization of the
CDTS as applied for LNG applications.

The CDTS was originally intended for LNG applications and was designed to withstand the
hydrostatic and sloshing loads. A CNG tank will see none of those loads. Instead the design is
driven by internal pressure and must meet ASTM and Classification Society Rules for
pressure vessels. In this study a modified version of the CDTS is considered for CNG
applications. The central cross brace was eliminated as shown in Figure 2 and the cylinders
were directly connected.

A brief overview of the different optimization techniques that are available in Altair Optistruct is
presented in the next section. Results of the analyses and optimization of the CNG tank under
internal pressure are discussed in the subsequent sections.

Optimization Techniques
The mathematical statement of any structural optimization problem can be posed as
Minimize f(X) = f(X1,X2,…Xn) Subject to gj (X) ≤ 0 j = 1,2,…m

Where f(x) is the objective function, X1,X2,…Xn are the design variables and gj(X) are the
constraints. Typically the objective function is the compliance of the structure for the given
loading and boundary conditions and the constraint is on the mass, volume fraction of the
material in the design space or any response like displacement, stress, etc. When there are
multiple load cases, a weighted compliance is used as the objective. The weighted
compliance is given by Cw = Σ wiCi , where Ci and wi are the compliance and weight associated
with each load case respectively.

Topology Optimization
Topology Optimization is a mathematical technique that produces an optimized material
distribution/shape of the structure within a given package space. As in the size and free-size
optimization, the objective function is typically the weighted compliance of the structure for the
given load cases. The design variable is the material density of each element in the finite
element model of the design space and it varies continuously between 0 and 1 which
represent the states of void and solid respectively. A distinction should be made between this
density and the physical mass density of the material of the structure.



The goal of any topology optimization is to achieve a value of either 0 or 1 for the density
variable. Since the density variable is continuously varying, many intermediate values are
possible though not desirable. In order to avoid intermediate values for the density variable, a
penalization technique is use and is given by

K (ρ) = ρp K

Where K is the actual element stiffness matrix (the real density of the material is used to
compute the actual element stiffness matrix), K is the penalized element stiffness matrix, ρ is
the material density or the design variable and p is the penalization constant which varies
between 2 and 4. Using a value of p greater than 1 gives a small value for the stiffness and
thus penalization is achieved when the optimization problem is posed as minimization of
compliance (or maximization of stiffness). For details of the different optimization techniques
mentioned above the reader is directed to RA-DIOSS/OPTISTRUCT 9.0 User’s Guide, Altair
Engineering Inc., 2008.

Free-Size Optimization
In free-size optimization, the thickness of each element in the finite element model of the
design space is treated as a design variable. This is the fundamental difference between free-
size and conventional size/gage optimization. Unlike conventional size optimization, free-size
optimization results in continuously variable shell thickness in the design space, between the
given lower and upper bounds of the thickness. A part with variable thickness is typically far
more expensive to manufacture and may not be a viable choice at first glance. It should be
emphasized that the results of free-size optimization should not be considered as a final
design. Based on this result, the design space should be subdivided into smaller zones and a
conventional size optimization could then be performed to fine tune the thickness of the
different zones. The design variables for this size optimization would be the thickness of
various zones.

Size/Gage Optimization
Conventional finite-element based size optimization techniques require the use of engineering
judgment or intuition to make a priori decisions as to how the design space should be
discretized using different design variables. Based on how the design variables are defined,
the optimization algorithm then iteratively explores the combination of design variable levels
that minimizes the objective function subject to the constraints that were imposed. The
number of design variables is typically limited to about 50 to 300 due to computational cost
and effectiveness of computational search algorithms.



Figure 17. Baseline Design of the CDTS

Any size parameter in the finite element model of the design space like the thickness of a shell
component, the moment of inertia of a beam component etc. could be used as a design
variable.

Results: CNG Tank Baseline Design
The baseline design of the CNG tank is shown in Figure 17. The tank made of 12 identical
cylinders of diameter about 4.7m which intersect at the four corners to spherical caps. The
size of the cube circumscribing the CDTS (excluding the base) is 10m. A uniform shell
thickness of 100mm was initially assumed. The material used for the tank is manganese-
molybdenum steel alloy with a modulus of 210,000 MPa and Poisson’s ratio of 0.3. The mass
of the baseline design is 873 t.

An internal pressure of 2000 psi was applied on the walls of the tank. Due to symmetry, a
quarter model of the tank was considered for the finite element analysis. The base was
constrained in vertical displacement and symmetry boundary conditions were applied to the
two planes of symmetry. The contour plot of von Mises stress is shown in Figure 18. The
average value of the ultimate strength of manganese-molybdenum steel alloy is about 800
MPa. The desired stress level was set as 400 MPa which is about 50% of the average value
of the ultimate strength. As can be seen in Figure 18, a significant portion of the tank is above
the desired stress level of 400 MPa.

Topology optimization was then performed on the base-line design in order to determine the
optimal material distribution that would result in a lower stress level. The design space used
for the topology optimization is shown in Figure 19. The objective of the topology optimization
was minimization of the compliance with a constraint on the volume fraction of the material as
30%. The design space was filled with first order tetrahedral elements. The load path or the
optimal material distribution obtained from the topology optimization is shown in Figures 20
and 21. Using the load path of the topology optimization as a guideline, internal bulkheads



were added as shown in Figures 22 and 23. Since topology optimization is a design tool used
to provide critical insight to the structural load path, manufactura bility and fabrication
considerations must be taken into account when interpreting these results.

Figure 18. Von Misses stress in Map – Baseline

A free size optimization was then performed on the modified design in order to determine an
optimal thickness distribution that reduces the mass and yet maintains a lower stress level.
The free-size optimization was posed as minimization of compliance due to the 2000 psi
internal pressure with a stress constraint of 400 MPa and mass constraint of 500 t. The
thickness of the various components of the tank was allowed to vary from15mm to 120mm.
The continuously variable thickness distribution obtained from the free-size optimization is
shown in Figures 27 and 28.

Figure 19. Design Space used for Topology Optimization



Figure 23. Load Path from Topology Figure 24. Load pat from Topology
Optimization Optimization

Figure 25. Bulkheads Incorporated based Figure 26. Bulkheads Incorporated based
on the Load on the Load Path from Topology from
Topology



Figure 27. Thickness from Free Size Figure 28. Thickness from
Optimization Free Size Optimization

Figure 29. Discrete Thickness Map Figure 30. Contour Map of Von Mises
Stress (MPa)



Figure 31. Trimmed Bulkhead Figure 32. Contour Plot of Von Mises
Stress (MPa)

Figure 33. Contour Plot of Von Mises
Figure 34. Contour Plot of Von Mises
Stress (MPa) at the outer surface from
Stress (MPa) at the outer Surface (skin)
Shell Model
from Solid Model



Figure 35. Contour Plot of Von Mises Figure 36. Contour Plot of Von Mises
Stress (MPa) at the inner surface from Stress (MPa) at the inner surface (skin)
Shell Model from Solid Model

Optimization
Using the load path of the topology optimization as a guideline, internal bulkheads were added
as shown in

Based on these results, discrete thickness values were assigned to different parts of the tank
so as to minimize the number of regions with disparate thicknesses. The mass of the tank is
about 594 t. This discrete thickness map is shown in Figures 29. The resulting von Mises
stress distribution is shown in Figure 30. With this thickness distribution the maximum stress
is just above the desired level of 400 MPa.

Considering the stress contours of Figure 30 and factoring manufacturability considerations,
the internal bulk-heads were trimmed (Figure 31). The critical load path contours from the
earlier topology runs also indicated a sparser material distribution on the bulkheads adjacent
to the spherical caps. Additionally, limiting the welding of the bulkheads to the seams of the
intersecting cylinders and cap instead of the center of the cap will significantly reduce
construction complexities and the need to weld the bulkheads to the spherical caps. High
stress concentration seen at the corners of the trimmed bulk-heads (Figure 32) could be
addressed by designing in generous fillets in these regions.

In order to determine the accuracy of the results of the shell model it was deemed necessary
to compare the results of the shell model with that of an equivalent solid model. Hence a solid
model with the same thickness as the shell model was created using hexahedral and
pentahedral elements and the analysis was Figures 33 to 36 compare the results obtained
using the shell and solid models. The results obtained are in good agreement. This adds
credibility to the design and analysis approach using the shell model. For future work only the
shell model will be used as it is easier to implement design changes to a shell model than a
solid model.



6.0 Conclusions
The paper has shown the benefits of a new tank containment system, namely the CDTS, for
the storage of LNG and/or CNG in floating offshore production and storage platforms, which
compared to other existing designs:

eliminated the sloshing problem for LNG platforms,
improved volumetric efficiency for CNG storage,
significantly reduced the size (length and displacement) compared to the other LNG
and CNG systems currently being developed,
reduced the estimated acquisition cost of platform (excluding containment system and
processing plant cost) by 7%,
reduced the Gross Tonnage and therefore many operating costs by 5% to 10%,
reduced surface area for CNG containment sys-tems, and thus heat transfer, by a
factor of 8 com-pared to VOTRANS and 50 compared to SEA NG,

all combining to offer a technical cost effective solution for both FDLNGPSO/FLNGPSO and
FOCNGPSO /FCNGPSO.

It also presented the results of the preliminary structural analysis showing the adequacy of the
design while de-monstrating the use of ALTAIR Engineering's Hyper-works suite of software.
Structural simulation studies evaluating trade-offs between material and fabricating cost with
containment pressures and temperatures are currently ongoing.

7.0 Acknowledgements
The authors would like to acknowledge with thanks the support of ALTAIR Engineering and
their vision of a future for the CDT system.

8.0 References
LAMB, T, and RAMOO, R, "The Application of a New Tank Containment System to ULTRA-
Large LNG Carriers," Paper, OTC 2009

LAMB, T, and RAMOO, R., "A New Concept for CNG Carriers and Floating CNG/Oil
Processing and Storage Offshore Platforms," CNG Forum, London 2009

RAMOO, R., PARTHASARATHY, M., SANTANI, J., and LAMB, T., "The use of Advanced
Structural Analysis and Simulation Tools to Validate a New Independent LNG Tank
Containment System," ICCAS 2009

NOBLE, P., LEVINE, R., and COLTON, T., “Planning the Design, Construction and Operation
of a New LNG Transpor-tation System – Ships, Terminals and Operations,” (2004) RINA



International Conference on the Design & Operations of Gas Carriers, September 2004,
London

RADIOSS/OPTISTRUCT 9.0 User’s Guide, Altair Engineer-ing Inc., 2008.


Floating LNG/CNG Processing & Storage Offshore Platforms Utilizing a New Tank Containment System - Technical Paper

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