Floating LNG/CNG Processing and Storage                     Offshore Platforms Utilizing a New Tank                       ...
www.altairproductdesign.comAbstractCurrent interest in Natural Gas offshore systems is focused on the Floating Oil/LNGProc...
www.altairproductdesign.comform, intersecting cylinders that formed the twelve edges of a cube that would have asignifican...
www.altairproductdesign.com3.0     Comparison of CDTS with other Containment SystemsFigure 3 shows the outlines in two vie...
www.altairproductdesign.comThe major operating problem is the sloshing of the liquefied natural gas especially in partiall...
www.altairproductdesign.com                          Figure 4. Current Tank Arrangement Design                            ...
www.altairproductdesign.com         Table 3. Platform Characteristics for 160,000 m3 /1.4 M Bbls FOLNGPSO                 ...
www.altairproductdesign.comSystem requires a significant time afloat to install the insulation and membrane lining, often ...
www.altairproductdesign.com                                     Figure 8. Midship SectionBaseline CDTS vs. Membrane TANKIn...
www.altairproductdesign.com                                   Figure 10. Enforced RotationThe period of the sloshing motio...
www.altairproductdesign.com         Figure 12: Sloshing Loads at 50% Tank Capacity (Roll Motion Bow Seas)CDTS Sloshing Loa...
www.altairproductdesign.comThe simulation was composed of two steps. In the first step a constant gravitational load (9.81...
www.altairproductdesign.com              Figure 14. Fluid Motion and Impingement Loads at 2.9 secondsImpact on CNG Platfor...
www.altairproductdesign.comA range of CDTS size was explored in the preliminary structural analysis to determine tankvolum...
www.altairproductdesign.comFigure 16 shows the midship structural arrangement. A smaller and A larger combinations aregive...
www.altairproductdesign.comIntroductionALTAIR Engineering Hyperworks was used to analyse the tank structure for CNG. Altai...
www.altairproductdesign.comThe goal of any topology optimization is to achieve a value of either 0 or 1 for the densityvar...
www.altairproductdesign.com                             Figure 17. Baseline Design of the CDTSAny size parameter in the fi...
www.altairproductdesign.comwere added as shown in Figures 22 and 23. Since topology optimization is a design tool usedto p...
www.altairproductdesign.com   Figure 23. Load Path from Topology         Figure 24. Load pat from Topology               O...
www.altairproductdesign.com   Figure 27. Thickness from Free Size          Figure 28. Thickness from               Optimiz...
www.altairproductdesign.com       Figure 31. Trimmed Bulkhead          Figure 32. Contour Plot of Von Mises               ...
www.altairproductdesign.com  Figure 35. Contour Plot of Von Mises            Figure 36. Contour Plot of Von Mises  Stress ...
www.altairproductdesign.com6.0     ConclusionsThe paper has shown the benefits of a new tank containment system, namely th...
www.altairproductdesign.comInternational Conference on the Design & Operations of Gas Carriers, September 2004,LondonRADIO...
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Floating LNG/CNG Processing & Storage Offshore Platforms Utilizing a New Tank Containment System - Technical Paper

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

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

  1. 1. 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 USAwww.altairproductdesign.comcopyright Altair Engineering, Inc. 2011
  2. 2. www.altairproductdesign.comAbstractCurrent 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, FOCNGPSO1.0 IntroductionIn the past few years interest in Floating Oil/LNG Processing and Storage Offshore Platforms(FOLNGPSO) Floating LNG Processing and Storage Offshore Platforms (FLNGPSO) hasincreased and a number have been built, are under construction, and many more are indesign. More recently interest in Floating CNG Platforms has developed and a number ofdesigns completed. It has been concluded that the best way to collect and transport gas froma small field is by compressing it. Compressed natural gas (CNG) requires a storage volumeapproximately twice that of LNG but does not need the expensive refrigeration plant at thesource or the gasifying equipment at the receiving end.Even though the capacity of natural gas is small, a CNG platform would still be relatively largein length. The problem is that the weight of the gas storage tanks is great, about 50,000 t for acargo 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 storageplatforms 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 DescriptionThe CDTS was developed over 30 years ago but nothing was done with it as interest inimporting LNG disappeared along with the cessation of diplomatic relations with Algeria. Thebasis for its design was constructing a self-standing tank surface composed of 12 identical, inCopyright Altair Engineering, Inc., 2011 2
  3. 3. www.altairproductdesign.comform, intersecting cylinders that formed the twelve edges of a cube that would have asignificantly better volumetric efficiency than a spherical tank. Where the intersecting cylindersmet 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 advancedstructural analysis and simulation systems. A detailed description of the tank development canbe 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 CDTSCopyright Altair Engineering, Inc., 2011 3
  4. 4. www.altairproductdesign.com3.0 Comparison of CDTS with other Containment SystemsFigure 3 shows the outlines in two views of membrane, spherical and CDTS tanks of equalvolume. Figure 3. Comparison of Outlines for Different ContainmentIt can be seen that the spherical tank is larger in all dimensions whereas the membrane tankis only larger than the CDTS in length and breadth. The CDTS has a volumetric efficiencybetween 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. Itcan be seen from the table that the CDTS is 60% better that spherical tanks. Table 2. Comparison of Tank Volumetric EfficiencyNext the use of ship space was compared. Figure 6 shows the “hold” space required by eachof the systems being compared for a 300,000 m3 LNG Tank Capacity. It can be seen that theLength usage for the CDTS is better than the other systems.Copyright Altair Engineering, Inc., 2011 4
  5. 5. www.altairproductdesign.comThe major operating problem is the sloshing of the liquefied natural gas especially in partiallyfilled large membrane tanks. Liquid sloshing limits the carriage of LNG in large side to sidemembrane tanks to be either over 80% or less than 10% full to avoid damage to the tanklining and insulation. This is impractical for a FOLNGPSO and FNGPSO where the tanks willbe filled and emptied continuously. Spherical containment tanks do not suffer from thisproblem 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(dimensions). Even the tanks in the largest tankers were less than 50 m in length and 30 m inbreadth whereas LNG tanks can be over 50 m in length and over 40 m in breadth. Also thetanks in tankers are integral structural tanks and thus more able to withstand the sloshingloads and usually have a transverse SWASH Bulkhead at mid-length of the tanks whichreduces the fore and aft sloshing loads on the tight transverse bulkheads, whereas the currenttrend in LNG carriers is the membrane lined and insulation box supported tanks, which hasbeen shown to have sloshing problems (damage to lining and insulation) as size increases.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 tanks emptythe hogging moment is enormous.To overcome this bending moment problem a unique approach for arranging the tanks, whichwas developed by a team of students in 2006 for their final Capstone Design Course at theUniversity of Michigan, is shown in Figure 5. This arrangement reduced the maximumBending Moment by 30%. By using the CDTS for LNG Tanks there are even further benefitsin 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 asFigures 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 design FOLNGPSOwith one of equal capacity using the CDTS. The reduction in length has immediate impact onthe structural design in that the Wave Bending Moments are reduced to half those of thelonger membrane FOLNGPSO. The maximum Bending Moment will also be reduced by thetank arrangement compared to the arrangement shown in Figure 8 by 50%. Both thesebending moment reductions will result in a smaller required Sectional Modulus and Moment ofInertia which in turn will be met with less longitudinal section-al area thus reducing structuralweight.Copyright Altair Engineering, Inc., 2011 5
  6. 6. www.altairproductdesign.com 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 FOLNGPSOTable 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.Copyright Altair Engineering, Inc., 2011 6
  7. 7. www.altairproductdesign.com Table 3. Platform Characteristics for 160,000 m3 /1.4 M Bbls FOLNGPSO Figure 7. CDTS FOLNGPSO Profiles3.2 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% less thanthe membrane design. It also shows that the Gross Tonnage would be 5% less which wouldresult 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 BenefitA major construction benefit results for the CDTS by uncoupling the tank building andinstallation schedule from the ship construction schedule, whereas the Membrane TankCopyright Altair Engineering, Inc., 2011 7
  8. 8. www.altairproductdesign.comSystem requires a significant time afloat to install the insulation and membrane lining, often aslong as the hull erection time. Like other independent tank systems the CDTS wouldsignificantly reduce the tank installation time afloat to almost zero compared to the membranetank 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 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. Itis 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 tosimplify the construction and increase capacity, but at an additional material cost and designcomplexity. While the CDTS offers benefits just from the tank design, construction andinstallation in the ship, it offers unique benefits in the design of the ship including significantreduction in length from 370m to 264m, which has construction benefits in reduced steelweight 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.Copyright Altair Engineering, Inc., 2011 8
  9. 9. www.altairproductdesign.com Figure 8. Midship SectionBaseline 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 element modelof the membrane tank used is shown in Figure 12a. The volume of both the tanks was40,000m3. 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, bow andbow-quartering seas. Figure 9. Rigid Membrane TankCopyright Altair Engineering, Inc., 2011 9
  10. 10. www.altairproductdesign.com Figure 10. Enforced RotationThe 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% andanother with 50% tank capacity. The total sloshing loads on the sides of the tank at 50% and80% 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 wavesimpinging directly on the flat walls of the membrane tank unlike in the case of CDTS where thewaves could decelerate along the curved walls. Also, the free surface was larger in the caseof the membrane tank whereas in the case of CDTS the cross braces appeared to break thewaves 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)Copyright Altair Engineering, Inc., 2011 10
  11. 11. www.altairproductdesign.com 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. The ALEapproach was opted since it gives a smooth variation of the sloshing load compared to theSPH approach for the same level of discretization. The CDTS was considered deformablewith 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 (materiallaw37). The rest of the tank was filled with air (hexahedral elements and material law37).Thefinite 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 SimulationCopyright Altair Engineering, Inc., 2011 11
  12. 12. www.altairproductdesign.comThe simulation was composed of two steps. In the first step a constant gravitational load (9.81m/s2) was applied to the tank and the fluid for 0.5secs. In the next step a roll motion wasenforced on the tank for 7.5 seconds. The gravitational load was held constant for the entireduration 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) and consi-dered asstatic load cases for further optimization of the tank design. The contour plots of von Misesstress in MPa at these instances of time from the sloshing simulation are shown 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 CNGIt 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.Copyright Altair Engineering, Inc., 2011 12
  13. 13. www.altairproductdesign.com Figure 14. Fluid Motion and Impingement Loads at 2.9 secondsImpact 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 CNG Marinecontainment systems. It has a hold volumetric efficiency of 0.33 (VOTRANS 0.18 and SEA NG0.20) a ship volumetric efficiency of 0.14 (VOTRANS 0.09 and SEA NG 0.09) and theplatforms utilizing CDTS would have a cargo deadweight coefficient of 0.133The 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 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 manifoldsThe result of its many benefits is significant acquisition and life cycle cost savings comparedto the other pro-posed designs.Copyright Altair Engineering, Inc., 2011 13
  14. 14. www.altairproductdesign.comA 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 tank wasselected to demonstrate its appli-cation to CNG platforms, as it was the best compromisebetween shell thickness, weight and other construction limits. Table 5. CDTS Tank CharacteristicsFOCNGPSO 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 10.5 MMscm/200,000 BblFOCNGPSO. 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 ArrangementsCopyright Altair Engineering, Inc., 2011 14
  15. 15. www.altairproductdesign.comFigure 16 shows the midship structural arrangement. A smaller and A larger combinations aregiven in Table 6.Ongoing WorkThe initial class certification and a detailed manufacturing/facility plan are all underway. Also adetailed cost estimate for the CDTS tanks is being performed along with the manufacturing/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 Figure 16. FOCNGPSO Midship SectionCopyright Altair Engineering, Inc., 2011 15
  16. 16. www.altairproductdesign.comIntroductionALTAIR Engineering Hyperworks was used to analyse the tank structure for CNG. AltairEngineering’s Hyperworks is a computer-aided engineering (CAE) simulation softwareplatform that allows businesses to create superior, market-leading products efficiently andcost 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, theHyperworks suite of advanced structural de-sign, analysis and optimization tools were used toimprove the design to meet the structural objectives which could not otherwise be attained bythe proposed original design. This involved connecting the center of all faces by an internalcross brace. The finite element analysis and optimization was performed using Altair OPTISTRUCT, 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 theCDTS 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 design isdriven 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 the cylinderswere directly connected.A brief overview of the different optimization techniques that are available in Altair Optistruct ispresented in the next section. Results of the analyses and optimization of the CNG tank underinternal pressure are discussed in the subsequent sections.Optimization TechniquesThe mathematical statement of any structural optimization problem can be posed asMinimize f(X) = f(X1,X2,…Xn) Subject to gj (X) ≤ 0 j = 1,2,…mWhere 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 = Σ wiCi , where Ci and wi are the compliance and weight associatedwith each load case respectively.Topology OptimizationTopology 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-sizeoptimization, the objective function is typically the weighted compliance of the structure for thegiven load cases. The design variable is the material density of each element in the finiteelement 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 between thisdensity and the physical mass density of the material of the structure.Copyright Altair Engineering, Inc., 2011 16
  17. 17. www.altairproductdesign.comThe 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 arepossible though not desirable. In order to avoid intermediate values for the density variable, apenalization technique is use and is given by K (ρ) = ρp KWhere K is the actual element stiffness matrix (the real density of the material is used tocompute the actual element stiffness matrix), K is the penalized element stiffness matrix, ρ isthe material density or the design variable and p is the penalization constant which variesbetween 2 and 4. Using a value of p 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 User’s Guide, AltairEngineering 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 between free-size and conventional size/gage optimization. Unlike conventional size optimization, free-sizeoptimization results in continuously variable shell thickness in the design space, between thegiven lower and upper bounds of the thickness. A part with variable thickness is typically farmore expensive to manufacture and may not be a viable choice at first glance. It should beemphasized that the results of free-size optimization should not be considered as a finaldesign. Based on this result, the design space should be subdivided into smaller zones and aconventional size optimization could then be performed to fine tune the thickness of thedifferent zones. The design variables for this size optimization would be the thickness ofvarious zones.Size/Gage OptimizationConventional finite-element based size optimization techniques require the use of engineeringjudgment or intuition to make a priori decisions as to how the design space should bediscretized using different design variables. Based on how the design variables are defined,the optimization algorithm then iteratively explores the combination of design variable levelsthat minimizes the objective function subject to the constraints that were imposed. Thenumber of design variables is typically limited to about 50 to 300 due to computational costand effectiveness of computational search algorithms.Copyright Altair Engineering, Inc., 2011 17
  18. 18. www.altairproductdesign.com Figure 17. Baseline Design of the CDTSAny size parameter in the finite element model of the design space like the thickness of a shellcomponent, the moment of inertia of a beam component etc. could be used as a designvariable.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 Poisson’s ratio of 0.3. The massof the baseline design is 873 t.An internal pressure of 2000 psi was applied on the walls of the tank. Due to symmetry, aquarter 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 is abovethe 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 optimizationwas minimization of the compliance with a constraint on the volume fraction of the material as30%. The design space was filled with first order tetrahedral elements. The load path or theoptimal material distribution obtained from the topology optimization is shown in Figures 20and 21. Using the load path of the topology optimization as a guideline, internal bulkheadsCopyright Altair Engineering, Inc., 2011 18
  19. 19. www.altairproductdesign.comwere added as shown in Figures 22 and 23. Since topology optimization is a design tool usedto provide critical insight to the structural load path, manufactura bility and fabricationconsiderations must be taken into account when interpreting these results. Figure 18. Von Misses stress in Map – BaselineA 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 OptimizationCopyright Altair Engineering, Inc., 2011 19
  20. 20. www.altairproductdesign.com Figure 23. Load Path from Topology Figure 24. Load pat from Topology Optimization OptimizationFigure 25. Bulkheads Incorporated based Figure 26. Bulkheads Incorporated based on the Load on the Load Path from Topology from TopologyCopyright Altair Engineering, Inc., 2011 20
  21. 21. www.altairproductdesign.com 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)Copyright Altair Engineering, Inc., 2011 21
  22. 22. www.altairproductdesign.com 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 ModelCopyright Altair Engineering, Inc., 2011 22
  23. 23. www.altairproductdesign.com 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 ModelOptimizationUsing the load path of the topology optimization as a guideline, internal bulkheads were addedas shown inBased on these results, discrete thickness values were assigned to different parts of the tankso as to minimize the number of regions with disparate thicknesses. The mass of the tank isabout 594 t. This discrete thickness map is shown in Figures 29. The resulting von Misesstress distribution is shown in Figure 30. With this thickness distribution the maximum stressis 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 theearlier topology runs also indicated a sparser material distribution on the bulkheads adjacentto the spherical caps. Additionally, limiting the welding of the bulkheads to the seams of theintersecting cylinders and cap instead of the center of the cap will significantly reduceconstruction complexities and the need to weld the bulkheads to the spherical caps. Highstress concentration seen at the corners of the trimmed bulk-heads (Figure 32) could beaddressed by designing in generous fillets in these regions.In order to determine the accuracy of the results of the shell model it was deemed necessaryto compare the results of the shell model with that of an equivalent solid model. Hence a solidmodel with the same thickness as the shell model was created using hexahedral andpentahedral elements and the analysis was Figures 33 to 36 compare the results obtainedusing the shell and solid models. The results obtained are in good agreement. This addscredibility to the design and analysis approach using the shell model. For future work only theshell model will be used as it is easier to implement design changes to a shell model than asolid model.Copyright Altair Engineering, Inc., 2011 23
  24. 24. www.altairproductdesign.com6.0 ConclusionsThe 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, 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 andFOCNGPSO /FCNGPSO.It also presented the results of the preliminary structural analysis showing the adequacy of thedesign while de-monstrating the use of ALTAIR Engineerings Hyper-works suite of software.Structural simulation studies evaluating trade-offs between material and fabricating cost withcontainment pressures and temperatures are currently ongoing.7.0 AcknowledgementsThe authors would like to acknowledge with thanks the support of ALTAIR Engineering andtheir vision of a future for the CDT system.8.0 ReferencesLAMB, T, and RAMOO, R, "The Application of a New Tank Containment System to ULTRA-Large LNG Carriers," Paper, OTC 2009LAMB, T, and RAMOO, R., "A New Concept for CNG Carriers and Floating CNG/OilProcessing and Storage Offshore Platforms," CNG Forum, London 2009RAMOO, R., PARTHASARATHY, M., SANTANI, J., and LAMB, T., "The use of AdvancedStructural Analysis and Simulation Tools to Validate a New Independent LNG TankContainment System," ICCAS 2009NOBLE, P., LEVINE, R., and COLTON, T., “Planning the Design, Construction and Operationof a New LNG Transpor-tation System – Ships, Terminals and Operations,” (2004) RINACopyright Altair Engineering, Inc., 2011 24
  25. 25. www.altairproductdesign.comInternational Conference on the Design & Operations of Gas Carriers, September 2004,LondonRADIOSS/OPTISTRUCT 9.0 User’s Guide, Altair Engineer-ing Inc., 2008.Copyright Altair Engineering, Inc., 2011 25

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