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

7/30/2019 Floating LNG-CNG Processing and Storage Offshore Platforms Utilizing a New Tank Containment System

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Altair Engineering 2011 X-1

Floating LNG/CNG Processing and Storage Offshore

Platforms Utilizing a New Tank Containment SystemRegu RamooAltair ProductDesignTroy, MichiganUSAand

Thomas LambUniversity of MichiganMichiganUSA

Abstrac t

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

The paper reports on the benefits of the Cubic Dough-nut tank containment system on thesupporting platform design especially its ability to operate with liquid levels in the tank fromempty 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 OffshorePlatforms (FOLNGPSO) Floating LNG Processing and Storage Offshore Platforms(FLNGPSO) has increased and a number have been built, are under construction, andmany 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 andtransport 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 theexpensive 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 relativelylarge in length. The problem is that the weight of the gas storage tanks is great, about50,000 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 newcontainment tank system that can be applied to both LNG and CNG, namely the CubicDoughnut Tank System (CDTS). Previous papers (LAMB 2009OTC, RAMOO 2009, LAMB2009) have described the development of the CDTS and its applications to both LNG andCNG Car-riers with a brief mention of its application to floating production and storage


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Altair Engineering 2011 Floating LNG/CNG Processing and Storage Offshore 5-2Platforms Utilizing a New Tank Containment System

platforms for both LNG and CNG. The first two of these papers also presented detailedstructural analysis for LNG and the last one for CNG applications and these will not berepeated in this paper. However, updates to the analysis will be presented.

2.0 CDTS Descr ipt ion

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 12identical, in form, intersecting cylinders that formed the twelve edges of a cube that wouldhave a significantly better volumetric efficiency than a spherical tank. Where the intersectingcylinders met in the center of each face a closing cap was provided. Figure 1 (from theoriginal patent) shows the form of the tank.

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

Figure 1. Cubic Doughnut Tank System (CDTS)

Figure 2. Latest Configuration of CDTS


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3.0 Comparison of CDTS with other Containment Systems

Figure 3 shows the outlines in two views of membrane, spherical and CDTS tanks of equalvolume.

Figure 3. Comparison of Outlines for Different Containment

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

Table 2. Comparison of Tank Volumetric Efficiency

Next the use of ship space was compared. Figure 6 shows the hold space required byeach of the systems being compared for a 300,000 m3 LNG Tank Capacity. It can be seenthat 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 inpartially filled large membrane tanks. Liquid sloshing limits the carriage of LNG in large sideto side membrane tanks to be either over 80% or less than 10% full to avoid damage to thetank lining and insulation. This is impractical for a FOLNGPSO and FNGPSO where thetanks will be filled and emptied continuously. Spherical containment tanks do not suffer fromthis problem but they are un-suitable for floating processing and storage platforms as theirarrangement restricts the available deck space for the processing equipment.

Tank sloshing has been around with ship designers and operators since liquids were firstcarried in ships. However the liquids were carried in tanks with much smaller capacities


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(dimensions). Even the tanks in the largest tankers were less than 50 m in length and 30 min breadth whereas LNG tanks can be over 50 m in length and over 40 m in breadth. Alsothe tanks in tankers are integral structural tanks and thus more able to withstand thesloshing loads and usually have a transverse SWASH Bulkhead at mid-length of the tankswhich reduces the fore and aft sloshing loads on the tight transverse bulkheads, whereasthe 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 sizeincreases.

3.1 Impact on LNG Platform DesignLNG has a specific gravity slightly more than half that of oil. Thus the LNG tank spacedominates the design. To date the existing FONG and those in design follow a tankarrangement as shown in Figure 4. Due to the heavier oil in the extreme forward and afttanks, this tank arrangement results in a large still water and wave at midship hoggingmoment 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 tanksempty 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 Courseat the University of Michigan, is shown in Figure 5. This arrangement reduced themaximum Bending Moment by 30%. By using the CDTS for LNG Tanks there are evenfurther benefits in that the tank length is reduced by 80 m or 25% and the Length Overall by100 m or 29% and the bending moment by a further 40%. This is shown in Figure 6 to thesame scale as Figures 4 and 5.

The reduced hold length for the CDTS is the clear ad-vantage. Coupled with the proposedunique tank arrangement it results in a significantly smaller platform length as can be seenfrom Table 3 that compares plat-form characteristics an existing and one designFOLNGPSO with one of equal capacity using the CDTS. The reduction in length hasimmediate 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 alsobe reduced by the tank arrangement compared to the arrangement shown in Figure 8 by50%. Both these bending moment reductions will result in a smaller required SectionalModulus and Moment of Inertia which in turn will be met with less longitudinal section-alarea thus reducing structural weight.

Figure 4. Current Tank Arrangement Design

Figure 5. Parallel Tank Arrangement


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Figure 6: Tank Arrangement with CDTS

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

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

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


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Figure 7. CDTS FOLNGPSO Profiles

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

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

4.0 Construction Benefit

A major construction benefit results for the CDTS by uncoupling the tank building andinstallation schedule from the ship construction schedule, whereas the Membrane TankSystem requires a significant time afloat to install the insulation and membrane lining, oftenas 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 themembrane tank system.

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

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 tankcontainment system,

a smaller skirt system compared to the spherical tank containment system,

the service/maintenance benefit in that the internal ships structure and the tankinsulation 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 sphericalcorners to simplify the construction and increase capacity, but at an additional material costand 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 shipincluding significant reduction in length from 370m to 264m, which has construction benefitsin reduced steel weight and less work content for the same capacity ship com-pared withany other system.

The Impact of the CDTS on the platforms structural arrangement can be seen from Figure8, the Midship Section and Figure 14, the Centerline Profile.


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Figure 8. Midship Section

Baseline CDTS vs. Membrane TANKIn 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 elementmodel of the membrane tank used is shown in Figure 12a. The volume of both the tankswas 40,000m3. The tanks were subjected to an oscillatory motion (Figure 12b) about theirlongitudinal, transverse, and mid off-axis to simulate the motion of the ship in beam, bowand bow-quartering seas.

Figure 9. Rigid Membrane Tank


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Figure 10. Enforced Rotation

The period of the sloshing motion was 8 seconds. This was based on the anticipated rollperiod 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 at50% and 80% tank capacities for roll motion (bow seas) are shown in Figure 11 and 12respectively.

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

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


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

CDTS Sloshing Loads and Wall StressIn this study, the sloshing simulation was performed using the current design of the CDTS,employing the ALE (Arbitrary Lagrangian Eulerian) approach available in RADIOSS. TheALE approach was opted since it gives a smooth variation of the sloshing load compared tothe SPH approach for the same level of discretization. The CDTS was considereddeformable with a uniform thickness of 100 mm. The skirt is considered rigid. The tank wasfilled to 80% capacity. The fluid (LNG, specific gravity 0.5) was modeled using hexahedralelements and 2-phase liquid-gas mixture material model with a Me Grneisen equation ofstate (material law37). The rest of the tank was filled with air (hexahedral elements andmaterial law37).The finite element half model of the CDTS used for the sloshing simulationis shown in Figure 18. Symmetry boundary conditions were imposed on both the structuraland 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 motionwas enforced on the tank for 7.5 seconds. The gravitational load was held constant for theentire duration of the roll period.

Figure 14 depicts the fluid motion during the event as well as the distribution of the sloshingloads 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) andconsi-dered as static load cases for further optimization of the tank design. The contour


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plots of von Mises stress in MPa at these instances of time from the sloshing simulation areshown in Figure 15.

Structural AnalysisAn earlier paper (LAMB OTC2009) presented details of the structural analysis for the CDTScontaining 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 PCNGPSOwas examined. Whereas the CDTS size for LNG application had no limitation up to thatrequired for the largest LNG Carriers under development, the CDTS tank for the carriage ofCNG will be much smaller due to its thicker shell and thus weight and will be a compromisebetween shell thickness, weight and manufacturability.

Figure 14. Fluid Motion and Impingement Loads at 2.9 seconds


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Impact on CNG PlatformsThe CDTS has been applied to CNG Carrier design (Lamb 2009) and offers significantacquisition and life cycle cost savings compared to other tank containment systems. It hasbeen found to offer similar cost savings for CNG offshore platform applications. The CDTShas superior volumetric efficiency and weight com-pared to any other proposed CNGMarine containment systems. It has a hold volumetric efficiency of 0.33 (VOTRANS 0.18and 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 followingbenefits 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 containmentsystem,

2. significant reduction in platform length,3. significant reduction in Gross Tonnage4. 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 inturn increases the CNG pressure due to increasing in gas temperature.

5. the in service maintenance benefit in that the tank structure can be inspected, and6. significantly reduced number of tank manifolds

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

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

Table 5. CDTS Tank Characterist ics

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


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Length BP = 230m Beam = 60m Depth at Side = 21m Operating Draft = 8.84mDisplacement = 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 combinationsare given in Table 6.

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

5.1 Structural AnalysisAgain the structural analysis of the CDTS for CNG was presented in an earlier paper (LAMB2009) and only updates to those findings will be presented.

Table 6. CDTS FOCNGPSO Series


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Figure 16. FOCNGPSO Midship Section

IntroductionALTAIR Engineering Hyperworks was used to analyse the tank structure for CNG. AltairEngineerings Hyperworks is a computer-aided engineering (CAE) simulation softwareplatform 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 tosimple analysis. An advanced structural analysis approach is required. Starting from 2005,the Hyperworks suite of advanced structural de-sign, analysis and optimization tools wereused to improve the design to meet the structural objectives which could not otherwise beattained by the proposed original design. This involved connecting the center of all faces byan internal cross brace. The finite element analysis and optimization was performed usingAltair OPTISTRUCT, which is a linear finite element solver available in Altair EngineeringsHyperworks. An earlier paper (RAMOO 2009) describes the finite element analysis andoptimization of the CDTS as applied for LNG applications.

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

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

Optimization Techniques

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

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

Topology Optimization

Topology Optimization is a mathematical technique that produces an optimized materialdistribution/shape of the structure within a given package space. As in the size and free-size


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optimization, the objective function is typically the weighted compliance of the structure forthe given load cases. The design variable is the material density of each element in thefinite element model of the design space and it varies continuously between 0 and 1 whichrepresent the states of void and solid respectively. A distinction should be made betweenthis 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 densityvariable. 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 () = p K

Where K is the actual element stiffness matrix (the realdensity of the material is used tocompute the actual element stiffness matrix), K is the penalized element stiffness matrix, is the material density or the design variable and p is the penalization constant which variesbetween 2 and 4. Using a value ofp greater than 1 gives a small value for the stiffness andthus penalization is achieved when the optimization problem is posed as minimization ofcompliance (or maximization of stiffness). For details of the different optimization techniquesmentioned above the reader is directed to RA-DIOSS/OPTISTRUCT 9.0 Users Guide,Altair Engineering Inc., 2008.

Free-Size OptimizationIn free-size optimization, the thickness of each element in the finite element model of thedesign space is treated as a design variable. This is the fundamental difference betweenfree-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 thicknessis typically far more expensive to manufacture and may not be a viable choice at firstglance. It should be emphasized that the results of free-size optimization should not beconsidered 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 finetune the thickness of the different zones. The design variables for this size optimizationwould be the thickness of various zones.

Size/Gage OptimizationConventional finite-element based size optimization techniques require the use ofengineering judgment or intuition to make a priori decisions as to how the design spaceshould be discretized using different design variables. Based on how the design variablesare defined, the optimization algorithm then iteratively explores the combination of designvariable levels that minimizes the objective function subject to the constraints that wereimposed. The number of design variables is typically limited to about 50 to 300 due tocomputational cost and effectiveness of computational search algorithms.


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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 adesign variable.

Results: CNG Tank Baseline DesignThe baseline design of the CNG tank is shown in Figure 17. The tank made of 12 identicalcylinders of diameter about 4.7m which intersect at the four corners to spherical caps. Thesize of the cube circumscribing the CDTS (excluding the base) is 10m. A uniform shellthickness of 100mm was initially assumed. The material used for the tank is manganese-molybdenum steel alloy with a modulus of 210,000 MPa and Poissons ratio of 0.3. Themass 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 wasconstrained in vertical displacement and symmetry boundary conditions were applied to thetwo planes of symmetry. The contour plot of von Mises stress is shown in Figure 18. Theaverage value of the ultimate strength of manganese-molybdenum steel alloy is about 800MPa. The desired stress level was set as 400 MPa which is about 50% of the average valueof the ultimate strength. As can be seen in Figure 18, a significant portion of the tank isabove the desired stress level of 400 MPa.

Topology optimization was then performed on the base-line design in order to determine theoptimal material distribution that would result in a lower stress level. The design space usedfor 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 ofthe material as 30%. The design space was filled with first order tetrahedral elements. Theload path or the optimal material distribution obtained from the topology optimization isshown in Figures 20 and 21. Using the load path of the topology optimization as aguideline, internal bulkheads were added as shown in Figures 22 and 23. Since topologyoptimization is a design tool used to provide critical insight to the structural load path,manufactura bility and fabrication considerations must be taken into account wheninterpreting these results.


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Figure 18. Von Misses stress in Map Baseline

A free size optimization was then performed on the modified design in order to determine anoptimal 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 psiinternal pressure with a stress constraint of 400 MPa and mass constraint of 500 t. Thethickness 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 isshown in Figures 27 and 28.

Figure 19. Design Space used for Topology Optimization


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Figure 23. Load Path f rom TopologyOptimization

Figure 24. Load pat from TopologyOptimization

Figure 25. Bulkheads Incorporated basedon the Load

Figure 26. Bulkheads Incorporated basedon the Load Path from Topology from

Topology

Figure 27. Thickness from Free SizeOptimization

Figure 28. Thickness fromFree Size Optimization


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

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

Figure 33. Contour Plot of Von MisesStress (MPa) at the outer surface from

Shell Model

Figure 34. Contour Plot of Von MisesStress (MPa) at the outer Surface (skin)

from Solid Model


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Figure 35. Contour Plot of Von MisesStress (MPa) at the inner surface from

Shell Model

Figure 36. Contour Plot of Von MisesStress (MPa) at the inner surface (skin)

from Solid Model

Optimization

Using the load path of the topology optimization as a guideline, internal bulkheads wereadded as shown in

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

Considering the stress contours of Figure 30 and factoring manufacturabilityconsiderations, the internal bulk-heads were trimmed (Figure 31). The critical load pathcontours 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 bulkheadsto the seams of the intersecting cylinders and cap instead of the center of the cap willsignificantly reduce construction complexities and the need to weld the bulkheads to thespherical 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 deemednecessary 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 usinghexahedral and pentahedral elements and the analysis was Figures 33 to 36 compare theresults 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 designchanges 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, forthe storage of LNG and/or CNG in floating offshore production and storage platforms, whichcompared to other existing designs:

eliminated the sloshing problem for LNG platforms,

improved volumetric efficiency for CNG storage,


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significantly reduced the size (length and displacement) compared to the other LNGand CNG systems currently being developed,

reduced the estimated acquisition cost of platform (excluding containment systemand 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 afactor 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 andFOCNGPSO /FCNGPSO.

It also presented the results of the preliminary structural analysis showing the adequacy ofthe design while de-monstrating the use of ALTAIR Engineering's Hyper-works suite ofsoftware. Structural simulation studies evaluating trade-offs between material andfabricating 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/OilProcessing 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 TankContainment System," ICCAS 2009

NOBLE, P., LEVINE, R., and COLTON, T., Planning the Design, Construction andOperation 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 Users Guide, Altair Engineer-ing Inc., 2008.

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