Large Floating Structure with Free-Floating, Self-Stabilizing Tanks for Hydrocarbon Storage

energies

Article Large Floating Structure with Free-Floating, Self-Stabilizing Tanks for Hydrocarbon Storage

Jian Dai 1,* , Kok Keng Ang 2,*, Jingzhe Jin 3, Chien Ming Wang 4 , Øyvind Hellan 3 and Arnstein Watn 5 1 Department of Marine Technology, Norwegian University of Science and Technology, 7491 Trondheim, Norway 2 Department of Civil and Environmental Engineering, National University of Singapore, Singapore 117576, Singapore 3 SINTEF Ocean, 7052 Trondheim, Norway 4 School of Civil Engineering, University of Queensland, St Lucia, QLD 4072, Australia 5 SINTEF, 7465 Trondheim, Norway * Correspondence: [email protected] (J.D.); [email protected] (K.K.A.)

Received: 30 July 2019; Accepted: 6 September 2019; Published: 10 September 2019

Abstract: Hydrocarbon is a major source of energy for sustainable development. Storage of hydrocarbon products, however, requires a significant amount of land space to land-scarce countries like Singapore. This paper presents an alternative way of storing hydrocarbon in Singapore coastal waters through the innovative design of a floating hydrocarbon storage facility. The design comprises free-floating and self-stabilizing tanks enclosed by barges that form a floating hydrocarbon storage facility. The tanks are made of prestressed concrete and they are designed to be self-stabilized when floating in the sea water. Owing to the lack of available design guidelines, design requirements on the stability and motion criteria for floating storage tanks are developed based on a review of existing codes of practice and design specifications for both onshore tanks and offshore vessels. A comprehensive study on the hydrostatic performance of various proposed floating tank design concepts with different storage capacities is carried out. This paper aims to give design recommendations on the tank’s storage capacity and dimensional aspect ratios that fulfill the recommended stability requirements and motion criteria.

Keywords: hydrocarbon storage; ﬂoating storage tank; self-stabilizing; ﬂoaters

1. Introduction Singapore relies heavily on the import of fuels to ensure a secure, reliable, and diversified supply of competitively-priced energy. Among various energies sources, petroleum products and crude oil occupied 94% of the total imported energy in 2015. Thus, a vast amount of space is required to store and trade hydrocarbon products. In land-scarce countries like Singapore, there is a need to create additional space through innovative engineering solutions. For example, Singapore created an artificial island, called Jurong Island, in 2009 by amalgaming seven offshore islands through land reclamation to grow the petrochemical industry. Subsequently in 2014, the Jurong Rock Caverns, excavated at some 120 m beneath the ground and was put into operational use for massive oil storage. These approaches of creating space have their own limitations. Storing oil in the sea may be an appealing alternative solution, particularly for coastal countries. This idea appeals to the oil & gas industry for decades. In 1969, a submerged fuel storage facility named Khazzan No. 1 was built in Dubai (see Figure1). This bottomless inverted funnel-shaped storage tank was designed to store crude oil based on the water displacement principle [1]. The same principle was later adopted by Mo [2] in the design of the gravity-based Condeep oil storage platforms (see Figure2).

Energies 2019, 12, 3487; doi:10.3390/en12183487 www.mdpi.com/journal/energies Energies 2019, 12, x FOR PEER REVIEW 2 of 30 Energies 2019, 12, x FOR PEER REVIEW 2 of 30

principleprinciple waswas laterlater adoptedadopted byby MoMo [2][2] inin thethe dedesignsign ofof thethe gravity-basedgravity-based CondeepCondeep oiloil storagestorage Energies 2019, 12, 3487 2 of 30 platformsplatforms (see (see Figure Figure 2). 2).

FigureFigureFigure 1.1. 1. Sectional Sectional view view of of KhazzanKhazzan Khazzan No.No. No. 1 1 [[1]. 1[1].].

FigureFigure 2. 2. An An example example of of Condeep Condeep oil oil storagest storageorage platform: platform: Troll Troll A A [[3]. 3[3].]. Although gravity-based offshore oil storage facilities stand steadily under harsh environmental AlthoughAlthough gravity-based gravity-based offshore offshore oil oil storage storage facilities facilities stand stand steadily steadily under under harsh harsh environmental environmental conditions, their applications are largely limited by the seabed condition. To promote the storage of conditions,conditions, their their applications applications are are largely largely limited limited by by the the seabed seabed condition. condition. To To promote promote the the storage storage of of fuel in relatively calm waters, Hirata [4] proposed the concept of floating bottomless tanks entitled fuelfuel in in relatively relatively calm calm waters, waters, Hirata Hirata [4] [4] proposed proposed the the concept concept of of floating floating bottomless bottomless tanks tanks entitled entitled “Floating Oil Storage Installation” (see Figure3). The floatability is also achieved by water displacement “Floating“Floating OilOil StorageStorage Installation”Installation” (see(see FigureFigure 3).3). TheThe floatabilityfloatability isis alsoalso achievedachieved byby waterwater principle, and the roof can be used to provide buoyancy when oil level is low. A key concern for displacementdisplacement principle, principle, and and the the roof roof can can be be used used to to provide provide buoyancy buoyancy when when oil oil level level is is low. low. A A key key adopting the water displacement principle, however, is the contamination of both the petrochemical concernconcern for for adopting adopting the the water water displacement displacement princi principle,ple, however, however, is is the the contamination contamination of of both both the the product and the marine environment, and is certainly not suitable for the storage of clean petrochemical products (CPP).

Energies 2019, 12, x FOR PEER REVIEW 3 of 30 Energies 2019,, 12,, xx FORFOR PEERPEER REVIEWREVIEW 3 of 30 petrochemical product and the marine environment, and is certainly not suitable for the storage of Energiespetrochemical2019, 12, 3487 product and the marine environment, and is certainly not suitable for the storage3 of 30of cleanclean petrochemicalpetrochemical productsproducts (CPP).(CPP).

FigureFigure 3.3. FloatingFloating Oil Oil Storage Storage Installation Installation involving involving fl ﬂoatingfloatingoating tank tank moored moored toto ﬁxedfixedfixed anchorsanchors [[4].[4].4].

VLFSVLFS (very(very largelarge floatingfloatingfloating structures) structures) technologytechnology has has also also been been adopted adopted in in thethe oiloil && gasgas industry.industry. industry. InInIn Japan,Japan,Japan, theretherethere areareare twotwotwo floatingfloatingfloatingfloating fuelfuelfuel storagestoragestoragestorage facilities:facilifacilifacilities:ties:ties: Kamigoto KamigotoKamigoto Oil OilOil Storage StorageStorage Base BaseBase and andand the thethe Shirashima ShirashimaShirashima OilOil StorageStorage BaseBase (see(see FigureFigure4 4).).4). Both BothBoth storage storagestorage facilities facilifacilitiestiesties comprise comprisecomprisecomprise several severalseveralseveral very veryveryvery large largelargelarge box-shaped box-shapedbox-shapedbox-shaped steel steelsteelsteel storagestorage modulesmodulesmodules that thatthat are areare compartmentalized compartmentalizedcompartmentalized into intointo a number aa nunumbermber of oil ofof tanksoiloil tankstanks [5]. There[5].[5]. ThereThere is also isis aalsoalso similar aa similarsimilar large prestressedlargelargelarge prestressedprestressedprestressed concrete concreteconcreteconcrete floating floatingfloatingfloating production productionproductionproduction unit (FPU) unitunitunit (FPU)(FPU)(FPU) for crude forforfor crudecrudecrude oil storage oiloiloil storagestoragestorage stationed stationedstationedstationed on N’Kossa ononon N’KossaN’KossaN’Kossa field, Congofield,field,field, CongoCongoCongo (see Figure (see(see(see FigureFigureFigure5)[6]. 5)5)5) [6].[6].[6].

(a) ((a)) (((bb)))

FigureFigure 4.4. FloatingFloating oil oil storagestorage facilities:facilities: ( (aa))) Kamigoto KamigotoKamigoto Oil OilOil Storage StorageStorage Base BaseBase and andand ( ((bb))) Shirashima ShirashimaShirashima Oil OilOil StorageStorageStorage BaseBase [[7].[7].7].

Figure 5. FigureFigure 5.5. N’KossaN’Kossa floating ﬂoatingfloating prestressed prestressed concreteconcrete bargebarge [[8].[8].8]. The aforementioned existing VLFS storage facilities provided solid solution to long-term and/or TheThe aforementionedaforementioned existingexisting VLFSVLFS storagestorage facilitiesfacilities providedprovided solidsolid solutionsolution toto long-termlong-term and/orand/or emergency oil stockpiling in the sea. These facilities, however, are designed to be a subsidiary system emergencyemergency oiloil stockpilingstockpiling inin thethe sea.sea. TheseTheseThese facilities,facilities,facilities, however,however,however, areareare designeddesigneddesigned tototo bebebe aaa subsidiarysubsidiarysubsidiary systemsystemsystem to the onshore oil farms and they also lack the capability for frequent processing and trading operations

Energies 2019, 12, x FOR PEER REVIEW 4 of 30 Energies 2019, 12, 3487 4 of 30 to the onshore oil farms and they also lack the capability for frequent processing and trading operations of the petrochemical products. Therefore, a stand-alone floating storage facility that has of the petrochemical products. Therefore, a stand-alone floating storage facility that has the full the full functionality as a land-based oil terminal is highly desirable. To this end, the authors have functionality as a land-based oil terminal is highly desirable. To this end, the authors have proposed proposed an innovative design entitled “Floating Hydrocarbon Storage and Bunker Facility” which an innovative design entitled “Floating Hydrocarbon Storage and Bunker Facility” which has been has been granted a provisional patent [9]. Figure 6 shows the plan and side views of the FHSBF. granted a provisional patent [9]. Figure6 shows the plan and side views of the FHSBF. Figure7 shows a Figure 7 shows a 3D architectural rendering view. This concept is self-contained with all essential 3D architectural rendering view. This concept is self-contained with all essential facilities such as power facilities such as power generation plant, desalination plant, slop and wastewater treatment plant, generation plant, desalination plant, slop and wastewater treatment plant, control room, warehouse, control room, warehouse, pump rooms, offices and accommodation quarters for workers on the pump rooms, offices and accommodation quarters for workers on the floating offshore base. It also has floating offshore base. It also has floating berths on the sides for oil tankers to load/offload floating berths on the sides for oil tankers to load/offload hydrocarbon products. The floating berths hydrocarbon products. The floating berths and barges also serve as a protection of the fuel storage and barges also serve as a protection of the fuel storage modules against waves and ship berthing modules against waves and ship berthing forces. Note that the dimensions are indicative only and forces. Note that the dimensions are indicative only and they are subject to changes depending on the they are subject to changes depending on the local design requirements, tank size, environmental local design requirements, tank size, environmental conditions, etc. conditions, etc. Enclosed within the floating berths and barges are free-floating self-stabilizing hydrocarbon Enclosed within the floating berths and barges are free-floating self-stabilizing hydrocarbon storage tanks. These tanks can be easily constructed in a standard dockyard. During operation, storage tanks. These tanks can be easily constructed in a standard dockyard. During operation, the the tanks are loaded/unloaded independently without affecting each other. The cross-section of the tanks are loaded/unloaded independently without affecting each other. The cross-section of the tanks tanks is cylindrical that enjoys the benefit of optimal resistance to hydrostatic pressure (see Figure8). is cylindrical that enjoys the benefit of optimal resistance to hydrostatic pressure (see Figure 8). The The tanks are made of prestressed concrete and they are designed to be self-stabilized when floating in tanks are made of prestressed concrete and they are designed to be self-stabilized when floating in the water. As the tanks are freely floating, its lateral position is held in place through contact between the water. As the tanks are freely floating, its lateral position is held in place through contact between the floater walls, which are relatively stronger than the tank hull, with the rubber fenders installed the floater walls, which are relatively stronger than the tank hull, with the rubber fenders installed alongalong the the surrounding surrounding barges. barges. The The space space within within the the floaters floaters can can be be used used for for piping piping routing, routing, personnel personnel accessaccess and and water water ballasting ballasting when when necessary. necessary.

280 30 30 15 15 208 208

40 30 30 6 12 12 6 18~21 18~21

FigureFigure 6. 6. PlanPlan and and sectional sectional views views of of FHSBF. FHSBF.

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Figure 7. 3D architectural rendering view of FHSBF. Figure 7.7. 3D architectural rendering viewview ofof FHSBF.FHSBF.

(a) (b) (a) (b) Figure 8. Isometric view of hydrocarbon storage tank: ( a) small tank and ( b) big tank. Figure 8. Isometric view of hydrocarbon storage tank: (a) small tank and (b) big tank.

As thethe floating floating tanks tanks are are surrounded surrounded by floatingby floating barges barges serving serving as breakwaters as breakwaters [10], the [10], wave-induced the wave- As the floating tanks are surrounded by floating barges serving as breakwaters [10], the wave- motionsinduced ofmotions the tanks of the are tanks expected are expected to be small to [be11 ].small However, [11]. However, other environment other environment loads such loads as water such induced motions of the tanks are expected to be small [11]. However, other environment loads such currentas water and current wind, and may wind, be significantmay be significant and may and lead may to lead undesirable to undesirable levels oflevels motion of motion of the of tanks the as water current and wind, may be significant and may lead to undesirable levels of motion of the thattanks could that leadcould to lead instability. to instability. This paper This is paper concerned is concerned with the with stability the andstability motion and of motion the proposed of the tanks that could lead to instability. This paper is concerned with the stability and motion of the self-stabilizingproposed self-stabilizing tanks under tanks environmental under environmenta loads. Thel loads. tanks mustThe tanks satisfy must the followingsatisfy the design following and proposed self-stabilizing tanks under environmental loads. The tanks must satisfy the following functionaldesign and requirements: functional requirements: design and functional requirements: 1. The tank shall be self-stabilizing with or without internal free surface effect; 1.1. The tank shall be self-stabilizing with or without internal free surface eeffect;ﬀect; 2. The tank shall have a flexible range of storage capacity to cater for operators’ commercial needs; 2.2. The tank shall have a flexible ﬂexible range of storage capacity to cater for operators’ commercial needs; 3. Motions of the tanks shall be controlled within an acceptable range so that it would not pose a 3.3. Motions of the tanks shall be controlled within an acceptable range so that it wouldwould notnot posepose aa problem during operation under working condition and survivability is not compromised under problem during operation under working condition and survivability is not compromised under extreme environmental condition; extreme environmental condition; 4. The tank sizes shall be designed with the desire to use minimum sea space; 4. The tank sizes shall be designed with the desire to use minimum sea space;

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4. The tank sizes shall be designed with the desire to use minimum sea space; 5. The clearance between the seabed and tank bottom must be greater than a minimum speciﬁed value so as not to hinder the ﬂow of water.

The focus of this paper is placed on establishing recommended stability and motion criteria of ﬂoating storage tanks as well as evaluating the hydrostatic performance of various proposed tank designs. Owing to Singapore’s benign sea conditions, it is justiﬁable to evaluate the tank’s performance based on a hydrostatic analysis. Nevertheless, comprehensive studies on hydrodynamic and structural performances of the proposed storage tanks as well as hydroelastic response of the surrounding barges were also carried out and interested readers may refer to results reported in references [12–16] for more details.

2. Review of Existing Codes of Practice and Design Specifications As mentioned earlier, the stability and motion of the self-stabilizing floating tanks under environmental loads are critical. The magnitudes of tank motion must be constrained within appropriate limits so that the loading/offloading process is not disrupted during operation under working weather condition. Under extreme weather conditions, the tank motion must not be excessive as to cause damage to surrounding structures and facilities. Obviously, the stability of the floating tank must be ensured throughout its service life. As there is a lack of available design codes and specifications for floating storage tanks, recommendations on intact stability and motion criteria of floating tanks will first be established based on existing design standards for both onshore tanks and offshore vessels.

2.1. Existing Design Standards for Land-Based Storage Tanks Onshore storage tanks are bottom founded and thus they are generally free of lateral movements. However, uneven settlement at the bottom may lead to rigid body tilting of a tank, which is often addressed as the planar tilt. According to API standard 653, excessive planar tilt will induce a number of undesirable consequences as listed below [17]:

1. Malfunction of ﬂoating roof seals; 2. Binding of a ﬂoating roof; 3. Problems with connecting pipes; 4. Problems with surface water drainage from the tank pad; 5. Tank shell buckling; 6. Overstress of shell or bottom plates.

While API 653 does not specify any explicit limit on the planar tilt of storage tanks, EEMUA 159 emphasizes that the out-of-verticality at top of the tank shell shall not exceed 1% of the tank height (see Figure9)[ 18]. In terms of angular tilt, this is about 0.6◦. Similar to the clauses listed in API 653, this stringent limit is imposed to onshore storage tanks in order to avoid malfunctioning of floating roof, damage to the tank structure as well as pipeline connections. It is worth mentioning that there will be no floating roof in the proposed floating tanks in view of the structural difficulty to effectively resist liquid movements under tank angular motions. As it can be seen from Figure6, loading arms and /or flexible hoses are utilized in the floating storage base for loading/offloading operations, which allow a certain degree of differential movements between the tanks and pipelines. Furthermore, as the proposed floating tanks are made of prestressed concrete instead of conventional thin-walled steel plates that are often used for land-based tanks, shell buckling is not a concern. For these reasons, the limit on tank’s tilt as specified by EEMUA 159 may not be applicable to the proposed free floating self-stabilizing storage tanks. Energies 2019, 12, 3487 7 of 30 Energies 2019, 12, x FOR PEER REVIEW 7 of 30

Figure 9. Planar tilt of storage tanks. Adapted from [18]. Figure 9. Planar tilt of storage tanks. Adapted from [18]. 2.2. Existing Design Standards for Offshore Vessels 2.2. Existing Design Standards for Offshore Vessels The degrees of ship motions have a pivotal impact on their survivability under environmental loads,The and degrees are normally of ship associated motions withhave the a pivotal stability impa of vessels.ct on their There survivability are well-established under environmental classification societiesloads, and issuing are codesnormally of practice associated for vessels’ with the stability. stability For example,of vessels. DNV There rules are for well-established classification of shipsclassification list the following societies criteriaissuing forcodes intact of stabilitypractice forfor allvessels’ ships [stability.19]: For example, DNV rules for classification of ships list the following criteria for intact stability for all ships [19]: 1. The area under the righting lever curve (GZ curve) is not to be less than 0.055 m-radians up to 1. The area under the righting lever curve (GZ curve) is not to be less than 0.055 m-radians up to θ θ = 30◦ angle of heel and not less than 0.09 m-radians up to θ = 40◦ or the angle of flooding θf = 30° angle of heel and not less than 0.09 m-radians up to θ = 40° or the angle of flooding θf if if this angle is less than 40◦. Additionally, the area under the righting lever curve between the this angle is less than 40°. Additionally, the area under the righting lever curve between the angles of heel of 30◦ and 40◦ or between 30◦ and θf, if this angle is less than 40◦, is not to be less angles of heel of 30° and 40° or between 30° and θf, if this angle is less than 40°, is not to be less than 0.03 m-radians; than 0.03 m-radians; 2. The righting lever (GZ) is to be at least 0.20 m at an angle of heel equal to or greater than 30 ; 2. The righting lever (GZ) is to be at least 0.20 m at an angle of heel equal to or greater than 30°;◦ 3. The maximum righting lever should occur at an angle of heel preferably exceeding 30 but not 3. The maximum righting lever should occur at an angle of heel preferably exceeding 30°◦ but not less than 25 ; less than 25°;◦ 4. The initial metacentric height, GM is not to be less than 0.15 m. 4. The initial metacentric height, GM0 is not to be less than 0.15 m. ItIt shouldshould be be noted noted that that ships ships are designedare designed to cruise to cr inuise the openin the sea. open Thus, sea. the Thus, capability the capability of restoring of torestoring its original to its upright original position upright when position the ship when undergoes the ship a largeundergoes tilt under a large a severe tilt under environmental a severe conditionenvironmental is crucial. condition Furthermore, is crucial. large Furthermore, heel angles maylarge not heel induce angles detrimental may not consequencesinduce detrimental to the structureconsequences when to it the is far structure away from when other it is shipsfar away or structures. from other ships or structures. TheThe proposedproposed floatingfloating tanks,tanks, however,however, areare enclosedenclosed byby bargesbarges//platformsplatforms withwith rubberrubber fendersfenders (see(see FigureFigure6 6).). Large Large tilting tilting angles angles could could disrupt disrupt working working processprocess oror even even damage damage the the tank tank and and the the surroundingsurrounding barges. barges. For For this this reason, reason, the the first first three three requirements requirements on on ship’s ship’s stability stability by by DNV DNV may may not not be applicablebe applicable to floating to floating tanks tanks that arethat kept are in kept position in position by floating by bargesfloating via barges rubber via fenders. rubber However, fenders. theHowever, requirement the requirement on minimum on minimum initial metacentric initial metace heightntric not height smaller not than smaller 0.15 than m shall 0.15 be m retained. shall be Additionally,retained. Additionally, the tilting the angle tilting shall angle notexceed shall no 5◦t underexceed extreme 5° under weather extreme conditions weather conditions to ensure noto damageensure no to damage the tank to itself the andtank the itself surrounding and the surrounding structures asstructures well. as well. InIn additionaddition toto stabilitystability andand motionmotion ofof ships,ships, thethe InternationalInternational ConventionConvention ofof thethe PreventionPrevention ofof PollutionPollution fromfrom ShipsShips (MARPOL)(MARPOL) mademade it mandatory that new oil tanks must have double hulls [20]. [20]. ThisThis isis toto preventprevent oiloil pollutionpollution fromfrom operationaloperational measuresmeasures andand accidentalaccidental discharges.discharges. TheThe proposedproposed floatingfloating hydrocarbonhydrocarbon storagestorage facility,facility, however,however, isis designeddesigned toto bebe aa permanentlypermanently mooredmoored floatingfloating structure instead of a marine vessel. Therefore, the design does not necessarily comply with

Energies 2019, 12, 3487 8 of 30 structure instead of a marine vessel. Therefore, the design does not necessarily comply with MARPOL regulations. Nevertheless, the relevant prevention of pollution shall be taken into account as the facility is to be used for hydrocarbon storage. It should be noted that all the storage tanks are protected by external barge structures from environmental loads and direct contact with ships. As such, the design of the entire floating hydrocarbon storage facility may be regarded as a “double hull” concept. In addition, the storage tanks have appropriate separation distance from each other with the aid of structural spacers and floating barges [21]. The enclosing barges are also designed to be able to contain leaked oil from a fully loaded tank. Furthermore, prestressed concrete structures have much thicker walls than double hull oil tankers. A separate study by the research team showed that the proposed design has a better resistance to ship impact than conventional oil tankers [22]. As a result, it is justifiable to consider either single hull or double hull for the design of the floating tanks.

2.3. Existing Recommendation on Motion Criteria of Vessels during Berthing In order to satisfy operational requirements, the motion of floating tanks under normal working environmental conditions shall be limited to ensure smooth and efficient loading/offloading operations. According to PIANC Working Group 24, the acceptable movements of both oil tankers and gas tankers are determined by the allowable reach of loading arms [23]. Table1 summarizes the recommended motion criteria for various types of vessels during berthing. For example, the horizontal translations of oil tankers and gas tankers shall not exceed 3 m and 2 m, respectively to ensure a safe loading operation. There are no limits imposed on the allowable yaw, pitch and roll degrees of freedom of an oil tanker as the angular motions are generally well within the design motion envelopes of the loading arms [23]. However, as the proposed storage tanks are significantly smaller than huge oil tankers, they are thus more vulnerable to the environmental conditions. Their angular motions must therefore be controlled within acceptable limits. It is proposed that the 2◦ limit on rotational motions for a lesser massive gas tanker (see Table1) be adopted for the floating tanks. A joint Nordic group conducted a similar research on the acceptable movements of moored ships in a harbor under working conditions. The results are somewhat agreeable with those published by PIANC 24. Furthermore, the Nordic group has suggested that the occurrence frequency of critical ship movements should be less than 1 week in a year for most of the cargo ships [24]. In view of this, it is reasonable to apply the 1-year characteristic environmental values to evaluate the motion of floating tanks under working conditions.

Table 1. Recommended motion criteria for safe working conditions [23].

Cargo Handling Ship Type Surge (m) Sway (m) Heave (m) Yaw ( ) Pitch ( ) Roll ( ) Equipment ◦ ◦ ◦ Elevator crane 0.15 0.15 Fishing Lift-on-Lift-off 1 1 0.4 3 3 3 vessels Suction pump 2 1 Freighters, Ship’s gear 1 1.2 0.6 1 1 2 Coasters, Quay cranes 1 1.2 0.8 2 1 3 Side ramp 0.6 0.6 0.6 1 1 2 Ferries, Bow/stern ramp 0.8 0.6 0.8 1 1 4 RO-RO Linkspan 0.4 0.6 0.8 3 2 4 Rail ramp 0.1 0.1 0.4 1 1 General – 2 1.5 1 3 2 5 cargo Container 100% efficiency 1 0.6 0.8 1 1 3 vessels 50% efficiency 2 1.2 1.2 1.5 2 6 Cranes 2 1 1 2 2 6 Bulk carriers Elevator/bucket-wheel 1 0.5 1 2 2 2 Conveyor belt 5 2.5 3 Oil tankers Loading arms 3 3 Gas tankers Loading arms 2 2 2 2 2 Energies 2019, 12, x FOR PEER REVIEW 9 of 30 Energies 2019, 12, 3487 9 of 30 2.4. Recommended Stability and Motion Design Criteria for Floating Tank 2.4. RecommendedBased on the Stability review and of Motionexisting Design codes Criteriaof practi force Floating and design Tank specifications on both onshore storageBased tanks onthe and review offshore of existing cruising/berthing codes of practice vessels, and designtogether specifications with the consideration on both onshore of storagedesign tanksrequirements and offshore of the cruising proposed/berthing floating vessels, tanks, together the following with the design consideration guidelines of designon tanks’ requirements stability and of themotion proposed criteria floating are proposed: tanks, the following design guidelines on tanks’ stability and motion criteria are1. proposed:The initial metacentric height GM0 of moored tanks shall not be smaller than 0.15 m; 2. Additional to 1 above, floating tanks that are fully exposed to the open sea need to fulfill all 1. The initial metacentric height GM0 of moored tanks shall not be smaller than 0.15 m; stability requirements as specified by DNV rules for classification of ships [19]; 2. Additional to 1 above, floating tanks that are fully exposed to the open sea need to fulfill all 3. Sway and surge movements of moored tanks shall not exceed 3 m, and rotational stability requirements as specified by DNV rules for classification of ships [19]; (roll/pitch/yaw) motions of moored tanks shall not exceed 2° under working environmental 3. Sway and surge movements of moored tanks shall not exceed 3 m, and rotational (roll/pitch/yaw) condition; motions of moored tanks shall not exceed 2 under working environmental condition; 4. Rotational (roll/pitch/yaw) movements of ◦moored floating tanks shall not exceed 5° under 4. Rotationalextreme environmental (roll/pitch/yaw) condition. movements of moored floating tanks shall not exceed 5◦ under extreme environmental condition. The acceptable limits on the movement of tanks under extreme condition need to be further reviewedThe acceptable and revised limits after on detailed the movement hydrostatic of tanksand structural under extreme analyses. condition The principle need to is bethat further these reviewedcritical movements and revised should after detailednot cause hydrostatic damage or and loss structural of integrity analyses. and stability The principle to the tank is that itself these and criticalsurrounding movements structures should and not faciliti causees. damage Furthermore, or loss additional of integrity motion and stability criteria to for the the tank construction itself and surroundingand installation structures stages andneed facilities. to be fulfilled. Furthermore, However, additional these will motion not be criteria addressed for the herein, construction as they and are installationnot within the stages scope need of this to be paper. fulfilled. However, these will not be addressed herein, as they are not within the scope of this paper. 3. Stability and Motion Analysis of Floating Storage Tanks under Wind and Current Loads 3. Stability and Motion Analysis of Floating Storage Tanks under Wind and Current Loads In this section, the hydrostatic response of the floating storage tanks under wind and current In this section, the hydrostatic response of the floating storage tanks under wind and current will will be presented. The stability and motions of the tanks will be investigated against the design be presented. The stability and motions of the tanks will be investigated against the design criteria criteria established in the previous section. Several types of tanks of various shapes, configurations established in the previous section. Several types of tanks of various shapes, configurations and aspect and aspect ratios are examined with the aim of designing tanks that are self-stabilizing and fulfill the ratios are examined with the aim of designing tanks that are self-stabilizing and fulfill the prescribed prescribed motion criteria under given environmental conditions. motion criteria under given environmental conditions. 3.1. Methodology 3.1. Methodology Consider a floating hydrocarbon storage tank enclosed by floating barges on the sides. As the Consider a floating hydrocarbon storage tank enclosed by floating barges on the sides. As the barges protect the tanks from incoming waves, the tank is mainly subject to wind and water current barges protect the tanks from incoming waves, the tank is mainly subject to wind and water current loads, as illustrated in Figure 10. The shaded region in the figure represents the stored hydrocarbon loads, as illustrated in Figure 10. The shaded region in the figure represents the stored hydrocarbon product. The wind and water current speeds are assumed to be sustained in the model so that a static product. The wind and water current speeds are assumed to be sustained in the model so that a static analysis is possible [24]. Also note, that the fenders are designed to restrain sway and surface motions analysis is possible [24]. Also note, that the fenders are designed to restrain sway and surface motions within prescribed limits. Thus, only angular movements, i.e., roll and pitch, will be studied. within prescribed limits. Thus, only angular movements, i.e., roll and pitch, will be studied.

FigureFigure 10.10. FloatingFloating tanktank enclosedenclosed byby bargesbarges andand subjectsubject toto environmentalenvironmental loads.loads.

Figure 11a,b show the floating tank in the upright and tilted positions, respectively. Given the information on structural configuration and geometry of tank components as well as fuel loading condition, the locations of the tank’s center of gravity (CG) and center of buoyancy (CB) can be determined as shown in Figure 11. The initial metacentric height, GM0, may be calculated as follows

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Figure 11a,b show the ﬂoating tank in the upright and tilted positions, respectively. Given the information on structural conﬁguration and geometry of tank components as well as fuel loading condition, the locations of the tank’s center of gravity (CG) and center of buoyancy (CB) can be determined as shown in Figure 11. The initial metacentric height, GM0, may be calculated as follows

Energies 2019, 12, x FOR PEER REVIEW 10 of 30 GM0 = KB + BM KG, (1) Energies 2019, 12, x FOR PEER REVIEW − 10 of 30

where KB is the height of the center of buoyancyGM0 = KB above + BM the − keep,KG, KG the height of the center of gravity (1) above the keel; BM the metacentric radiusGM and0 = canKB be+ BM written− KG, as (1) where KB is the height of the center of buoyancy above the keep, KG the height of the center of gravity abovewhere theKB keel;is the BM height the ofmetacentric the center ofradius buoyancy andBM can ab= ovebeIo/ Vwritten the, keep, as KG the height of the center of gravity (2) above the keel; BM the metacentric radius and can be written as BM = Io/V, (2) where Io is the second moment of area of the waterplane, and V the displaced water volume. When the BM = Io/V, (2) wheretank is I ﬁlledo is the with second hydrocarbon moment of products, area of the freewaterplane, surface ofand the V stored the displaced liquid leads water to volume. a virtual When rise in thewherethe tank vertical Io is filledthe center second with of gravity, hydrocarbonmoment which of area reducesproducts, of the the waterplane,the stability free surface of theand tank ofV thethe as displaced shownstored inliquid Figurewater leads volume.11b. to The a virtual When rise in risetheCG tankin due the tois vertical freefilled surface with center hydrocarbon eﬀ ofect gravity, may be products,which expressed reduce the as sfree the surface stability of of the the stored tank asliquid shown leads in Figureto a virtual 11b. Therise risein the in verticalCG due centerto free ofsurface gravity, effect which may reduces be expressed the stability as of the tank as shown in Figure 11b. The rise in CG due to free surface effectGG’ may= be(ρ fuelexpressed/ρwater) I ias/V , (3) GG’ = (ρfuel⁄ρwater) Ii/V, (3) GG’ = (ρfuel⁄ρwater) Ii/V, (3) wherewhere ρfuelfuel andand ρρwaterwater denotedenote the the densities densities of of fuel fuel and and water, water, respectively; respectively; G’ G’ is is the the location location of of the the new new centerwherecenter of ρfuel gravity; and ρwater and denote IIii the secondsecondthe densities momentmoment of fuelof of area area and of ofwater, the the liquid liquid respectively; waterplane. waterplane. G’ is The Thethe metacentric locationmetacentric of heightthe height new of ofcenterthe the tilted tiltedof gravity; tank tank is is givenand given Ii bythe byG’M second G’M0.0. Note Notemoment that that of the the area location location of the of ofliquid thethe metacentermetacenter waterplane. MM The00 remains metacentric virtually height the the sameofsame the for fortilted small small tank tilting tilting is given angle angle by of of G’M the the0 tank. tank Note (< (< that 5°).5◦). the location of the metacenter M0 remains virtually the same for small tilting angle of the tank (< 5°).

(a) (b) (a) (b) FigureFigure 11. 11. FloatingFloating tank tank at at ( (aa)) upright upright and and ( (bb)) tilted tilted positions. positions. Figure 11. Floating tank at (a) upright and (b) tilted positions. Next,Next, we maymay estimateestimate the the e ﬀeffectect of of wind wind and and water water current current loads loads on the on tilting the tilting angle ofangle the storageof the storagetank.Next, According tank. we Accordingmay to estimate DNV to standards theDNV effect standards [25 of,26 wind], the[25,26 and wind ],water the and windcurrent water and current loads water on pressures thecurrent tilting arepressures angle reasonably of arethe reasonablystorageassumed tank. to assumed be According uniformly to be distributed uniformlyto DNV standards distributed on the tank [25,26 on wall, the], as tankthe shown windwall, in asand Figure shown water 12 .in Also Figurecurrent shown 12. pressures Also in Figure shown are 12 inreasonablyare Figure two load 12 assumed are combinations, two load to be combinations, uniformly where the distributed wind where and the currenton wind the tank and are inwall,current the as (a) areshown same in thedirection in (a)Figure same and 12. direction Also (b) opposite shown and (b)indirections. Figure opposite 12 Withoutdirections.are two load further Without combinations, detailed further environmental detailedwhere the enwindvironmental data and on current the data directions areon thein the directions and (a) same likelihood and direction likelihood of theirand of(b)maxima their opposite maxima occurring directions. occurring simultaneously, Without simultan furthereously, the twodetailed the load two en combinations vironmentalload combinations will data be on consideredwill the bedirections considered herein and andherein likelihood the and one theofresulting their one resultingmaxima in the maximumoccurring in the maximum simultan tilting momenttiltingeously, moment willthe two be will used load be to combinationsused evaluate to evaluate the will maximum the be maximum considered tilting tilting angle herein angle of and the oftheﬂoating the one floating resulting tank. tank. in the maximum tilting moment will be used to evaluate the maximum tilting angle

of the floating ank.

(a) (b) (a) (b) Figure 12. Tank subject to wind and current forces acting in the (a) same and (b) opposite directions. FigureFigure 12. 12. TankTank subject subject to to wind wind and and curre currentnt forces forces acting in the ( a)) same same and and ( b) opposite directions. Figure 13 shows the forces acting on the floating storage tank. Point A on the figure denotes the intersectionFigure 13between shows thethe resultantforces acting rubber on fenderthe floating reaction storage and tank.the metacentric Point A on radius the figure (BM). denotes By taking the momentintersection about between A, the themoment resultant equili rubberbrium fender equation reaction can be and written the metacentric as radius (BM). By taking moment about A, the moment equilibrium equation can be written as W × GM tan α =|Fw dw| + |Fc dc|, (4) W × GM tan α =|Fw dw| + |Fc dc|, (4) where W denotes the weight of the tank; Fw and Fc are the resultant wind load and current load action onwhere the Wtank; denotes dw and the d cweight refer to of the the lever tank; armsFw and fromFc are wind the resultantload center wind and load current and current load center load actionto the pointon the A, tank; respectively; dw and d cαrefer the tankto the tilting lever angle arms due from to windsustained load windcenter and and current current loads. load center to the pointAccording A, respectively; to DNVα codesthe tank of practice,tilting angle the winddue to and sustained water current wind and forces current exerting loads. on the tank may be writtenAccording as [25,26] to DNV codes of practice, the wind and water current forces exerting on the tank may be written as [25,26]

Energies 2019, 12, 3487 11 of 30

Figure 13 shows the forces acting on the ﬂoating storage tank. Point A on the ﬁgure denotes the intersection between the resultant rubber fender reaction and the metacentric radius (BM). By taking moment about A, the moment equilibrium equation can be written as

W GM tan α =|Fw dw| + |Fc dc|, (4) × where W denotes the weight of the tank; Fw and Fc are the resultant wind load and current load action on the tank; dw and dc refer to the lever arms from wind load center and current load center to the Energiespoint A,2019 respectively;, 12, x FOR PEERα theREVIEW tank tilting angle due to sustained wind and current loads. 11 of 30 According to DNV codes of practice, the wind and water current forces exerting on the tank may be written as [25,26] 𝐹 =1 𝜌𝑣𝐴𝐶𝐶, (5a) F = ρ v2 A C C , (5a) w 2 air w w s h 𝐹 =1 𝜌𝑣 𝐴𝐶, (5b) F = ρ v2A C , (5b) c 2 water c c DS where vw and vc denote the characteristic wind and water current speeds, respectively; Aw and Ac refer where vw and vc denote the characteristic wind and water current speeds, respectively; Aw and Ac to the projected areas normal to the wind and current actions, respectively; Cs is the shape coefficient refer to the projected areas normal to the wind and current actions, respectively; Cs is the shape and Ch the height coefficient, according to DNV-OS-C301 [25]; and CDS is the steady drag coefficient coefficient and C the height coefficient, according to DNV-OS-C301 [25]; and C is the steady drag and may be selectedh according to DNV-RP-H103 [26]. DS coefficient and may be selected according to DNV-RP-H103 [26].

FigureFigure 13. 13. ForcesForces acting acting on on the the floating ﬂoating storage storage tank. tank.

3.2.3.2. Hydrostatic Hydrostatic Stability Stability and and Motions Motions of of Floating Floating Tanks Tanks ThisThis section shallshall examineexamine the the hydrostatic hydrostatic stability stability of theof the various various designs designs of ﬂoating of floating storage storage tanks tankswith diwithﬀerent different storage storage capacities. capacities. This sectionThis section shall alsoshall investigate also investigate the tilting the tilting angle angle of storage of storage tanks tanksunder under combined combined wind andwind current and current loads. loads. The proposed The proposed hydrocarbon hydrocarbon storage storage and bunkering and bunkering facility facilityhas two has sizes two of sizes tanks, of namelytanks, namely small andsmall big and tanks. big tanks. The small The tankssmall aretanks designed are designed for storing for storing Clean 3 3 CleanPetrochemical Petrochemical Products Products (CPP) (CPP) with with a practical a practical storage storage capacity capacity ranging ranging from from 5000 5000 m tom3 15,000 to 15,000 m . mThe3. The big big tanks tanks are are designed designed for for crude crude and and fuel fuel oil oil storage storage and and theirtheir storagestorage capacities range from from 3 3 17,50017,500 m m3 toto 35,000 35,000 m m3. .In In this this study, study, only only small small tanks tanks are are analyzed analyzed as as they they are are inherently inherently less less stable stable thanthan larger larger and and stockier stockier tanks. tanks. The The big big tanks, tanks, wh whichich can can be be designed designed by by adopting adopting the the same same design design principlesprinciples applied applied to to the the small small tanks, tanks, will will not not be be covered herein. FiguresFigures 14 andand 1515 show show the the initial initial design design of theof the tank tank structure. structure. The The tanks tanks are made are made of light-weight of light- weightprestressed prestressed concrete concrete and they and have they a doublehave a hulldouble structure. hull structure. Note that Note L and that D L denote and D the denote length the of lengththe outer of the tank outer wall andtank diameterwall and of diameter the inner of wall, the respectively.inner wall, respectively. Hr,Hw and H Hrd, Hare,w and respectively, Hd are, respectively,the heights/depths the heights/depths of the tank roof, of th walle tank and roof, bottom wall deck.and bottomH is the deck. total H height is the of total the height tank. Tableof the2 tank.summarizes Table 2 summarizes the key geometric the key information geometric information and material and properties material of properties the storage of tank.the storage In addition, tank. In addition, a 1/8 roof height to span ratio (Hr/L) is adopted. The roof is assumed to be stiffened by beams that contribute another 15% to the roof slab weight. Furthermore, a uniformly distributed load of 0.5 kPa due to roof maintenance operation and another equipment weight of 3% of the total tank’s self-weight are assumed to be acting on top of the roof. The equivalent mass of these loads is used to compute the center of gravity (CG) location of the entire tank and mass radius of gyration. The tank bottom is assumed to be a solid concrete slab of constant thickness. When the tank is filled with hydrocarbon product to the designed storage capacity, there will be a minimum 1 m gap between the product top surface and the roof bottom. This gap is necessary to prevent spillover of the product as well as it creates space for inert gas. Unless otherwise noted, all the values listed in Table 2 will be used throughout this paper. Note that the structural properties employed here are based on the results of detailed structural analysis of the floating tanks [15].

Energies 2019, 12, 3487 12 of 30

a 1/8 roof height to span ratio (Hr/L) is adopted. The roof is assumed to be stiffened by beams that contribute another 15% to the roof slab weight. Furthermore, a uniformly distributed load of 0.5 kPa due to roof maintenance operation and another equipment weight of 3% of the total tank’s self-weight are assumed to be acting on top of the roof. The equivalent mass of these loads is used to compute the center of gravity (CG) location of the entire tank and mass radius of gyration. The tank bottom is assumed to be a solid concrete slab of constant thickness. When the tank is filled with hydrocarbon product to the designed storage capacity, there will be a minimum 1 m gap between the product top surface and the roof bottom. This gap is necessary to prevent spillover of the product as well as it creates space for inert gas. Unless otherwise noted, all the values listed in Table2 will be used Energiesthroughout 2019, 12 this, x FOR paper. PEER Note REVIEW that the structural properties employed here are based on the results12 of of30 detailedEnergies 2019 structural, 12, x FOR analysis PEER REVIEW of the floating tanks [15]. 12 of 30 Table 2. Geometric and material properties for storage tanks. TableTable 2.2. GeometricGeometric andand materialmaterial propertiesproperties forfor storagestorage tanks.tanks. Parameter Value Parameter Value ParameterConcreteParameter density Value 1800Value kg/m 3 Outer Parameter Parameter wall thickness 450 Value mm Value ConcreteWater density density 1025 1800 kg/mkg/m33 Inner Outer wall wall thickness thickness 300 450 mmmm Concrete density 1800 kg/m3 Outer wall thickness 450 mm Fuel density 870 kg/m3 3 Roof slab thickness 200 mm Water densityWater density 1025 kg 1025/m3 kg/m Inner Inner wallwall thicknessthickness 300 mm 300 mm Air density 1.25 kg/m33 Diagonal stiffener thickness 350 mm Fuel densityFuel density 870 kg 870/m3 kg/m Roof Roof slabslab thicknessthickness 200 mm 200 mm Bottom slab thickness 750 mm 3 Air densityAir density 1.25 kg 1.25/m3 kg/m Diagonal Diagonal stistiffenerffener thicknessthickness 350 mm 350 mm Bottom slabBottom thickness slab thickness 750 mm 750 mm

Figure 14. InitialInitial design design of of floating ﬂoating storage tank (planar cross-sectional view). Figure 14. Initial design of floating storage tank (planar cross-sectional view).

Figure 15. Side and isometric views of tank with dome roof. Figure 15. Side and isometric views of tank with dome roof. Figure 15. Side and isometric views of tank with dome roof. The environmental conditions for wind and water current at the selected site are assumed as shownThe in environmentalTable 3. As me conditionsntioned earlier, for wind the windand water and currentcurrent speedsat the selectedtaking on site the are characteristic assumed as maximumshown in Tablevalues 3. at As their me 1-yearntioned return earlier, period the wind will beand adopted current for speeds stability taking analysis on the under characteristic working environmentalmaximum values condition. at their For1-year analysis return under period the will extr beeme adopted environmental for stability condition, analysis it under is essential working to considerenvironmental the values condition. at their For 100-year analysis return under period the extr [27,28].eme Inenvironmental the following condition, study, Equations it is essential 5a,b are to usedconsider to calculate the values the at wind their and100-year water return current period forces [27,28]. exerted In onthe the following tanks. Equationsstudy, Equations (1) and 5a,b (4) areare thenused employed to calculate to theassess wind the and stability water and current roll motion forces ofexerted the tanks on the in question.tanks. Equations (1) and (4) are thenBesides employed wind to assessand water the stability current, and floating roll motion tanks are of thealso tanks subjected in question. to wave loads. The dynamic effectBesides of wave wind loads and may water lead current, to sloshing floating of th tankse stored are hydrocarbonalso subjected product. to wave However,loads. The the dynamic wave conditioneffect of wave in Singapore loads may coastal lead toregions sloshing is ofrather the storedbenign hydrocarbon due to the product.shielding However, from islands the waveand neighboringcondition in lands.Singapore According coastal to regionsthe available is rather hindcast benign data, due the to meanthe shielding significant from wave islands height and is neighboring lands. According to the available hindcast data, the mean significant wave height is

Energies 2019, 12, 3487 13 of 30

The environmental conditions for wind and water current at the selected site are assumed as shown in Table3. As mentioned earlier, the wind and current speeds taking on the characteristic maximum values at their 1-year return period will be adopted for stability analysis under working environmental condition. For analysis under the extreme environmental condition, it is essential to consider the values at their 100-year return period [27,28]. In the following study, Equations (5a) and (5b) are used to calculate the wind and water current forces exerted on the tanks. Equations (1) and (4) are then employed to assess the stability and roll motion of the tanks in question. Besides wind and water current, floating tanks are also subjected to wave loads. The dynamic effect of wave loads may lead to sloshing of the stored hydrocarbon product. However, the wave condition in Singapore coastal regions is rather benign due to the shielding from islands and neighboring lands. According to the available hindcast data, the mean significant wave height is generally below 0.25 m [29], although it may exceed 1 m in some exposed areas. Nevertheless, the present hydrocarbon storage facility is proposed to be located in shielded regions and the surrounding floating barges serve as breakwaters that reduce wave loads exerted on the floating tanks. Therefore, waves are expected to have a limited effect on the floating tanks and thus not considered in this study. Furthermore, the water depth of the coastal regions around Singapore is shallow and a daily tidal variation of 2 m to 3 m is expected [29,30]. In this study, a water depth of 18 m under low tide condition is taken into account [16]. Considering the motion of tanks and a clearance between tank bottom and seabed, the allowable draft of tanks is set to 16.5 m.

Table 3. Characteristic wind and water current speeds at selected site.

Return Period (years) Extreme Hourly Mean Wind Speed (m/s) Current Speed (m/s) 1 15.9 1.46 5 18.4 1.51 10 19.6 1.52 25 21.3 1.74 50 22.6 1.77 100 24.0 1.90

Figure 16 shows the maximum tilting angles of various tanks with L/D = 1.05. This ratio represents the case where the inner face of the square external wall is in contact with the outer face of the circular inner wall. For a given capacity, the maximum tilting motion is found to decrease with increasing D/H ratio. Likewise, for a given D/H ratio, the tilting motion decreases with increasing storage capacity. Note that the acceptable limits of tank’s rotational motion are 2◦ and 5◦ under working and extreme environmental conditions, respectively, as recommended in Section 2.4. All tanks with D/H > 1.3 fulfill both recommended design requirements on stability and motion as specified in Section 2.3. Tanks with D/H = 1.3 are noted to be operationally stable only if the storage capacity is greater than 7500 m3 under working environmental condition. For a smaller D/H ratio of 1.2, tanks for the entire range of storage capacities considered for small tanks are found to be unstable. The ratio L/D is a parameter that controls the tilting motion of the tank. Larger L/D ratios would lead to a smaller tilting motion as the water plane moment of inertia would be increased. However, for the same capacity, larger L/D ratios would also result in more costly use of materials. Figures 17 and 18 show the maximum tilting angles of various floating storage tanks with L/D = 1.1 and 1.2, respectively. As stated earlier, the results reveal that larger L/D ratios reduce the tilting motion of storage tanks. For example, all storage tank capacities considered with D/H 1.3 satisfy the prescribed ≥ stability requirements when L/D is increased from 1.05 to 1.1. As L/D ratio increases further to 1.2, even more slender tanks meet the prescribed stability requirements. As it can be seen from Figure 18, all storage tank capacities considered with D/H 1.2 now satisfy the prescribed stability requirements. ≥ In addition, highly slender tanks with D/H = 1.1 are now able to fulfill the stability requirements provided the storage capacity is greater than 6500 m3. Energies 2019, 12, x FOR PEER REVIEW 13 of 30

generally below 0.25 m [29], although it may exceed 1 m in some exposed areas. Nevertheless, the present hydrocarbon storage facility is proposed to be located in shielded regions and the surrounding floating barges serve as breakwaters that reduce wave loads exerted on the floating tanks. Therefore, waves are expected to have a limited effect on the floating tanks and thus not considered in this study. Furthermore, the water depth of the coastal regions around Singapore is shallow and a daily tidal variation of 2 m to 3 m is expected [29,30]. In this study, a water depth of 18 m under low tide condition is taken into account [16]. Considering the motion of tanks and a clearance between tank bottom and seabed, the allowable draft of tanks is set to 16.5 m.

Table 3. Characteristic wind and water current speeds at selected site.

Current Speed Return Period (years) Extreme Hourly Mean Wind Speed (m/s) (m/s) 1 15.9 1.46 5 18.4 1.51 10 19.6 1.52 25 21.3 1.74 50 22.6 1.77 100 24.0 1.90

EnergiesEnergies 20192019,, 1212,, xx FORFOR PEERPEER REVIEWREVIEW 1414 ofof 3030

andand 1818 showshow thethe maximummaximum tiltingtilting anglesangles ofof variouvariouss floatingfloating storagestorage tankstanks withwith L/DL/D == 1.11.1 andand 1.2,1.2, respectively.respectively. AsAs statedstated earlier,earlier, thethe resultsresults revealreveal ththatat largerlarger L/DL/D ratiosratios reducereduce thethe tiltingtilting motionmotion ofof storagestorage tanks.tanks. ForFor example,example, allall storagstoragee tanktank capacitiescapacities consideredconsidered withwith D/HD/H ≥≥ 1.31.3 satisfysatisfy thethe prescribedprescribed stabilitystability requirementsrequirements whenwhen L/DL/D isis increasedincreased fromfrom 1.051.05 toto 1.1.1.1. AsAs L/DL/D ratioratio increasesincreases furtherfurther toto 1.2,1.2, eveneven moremore slenderslender tankstanks meetmeet thethe prprescribedescribed stabilitystability requirements.requirements. AsAs itit cancan bebe seenseen fromfrom FigureFigure 18,18, allall storagestorage tanktank capacitiescapacities consideredconsidered withwith D/HD/H ≥≥ 1.21.2 nownow satisfysatisfy thethe prescribedprescribed stabilitystability requirements.requirements. InIn additionaddition,, highlyhighly slenderslender tankstanks withwith D/HD/H == 1.11.1 areare nownow ableable toto fulfillfulfill the the 3 stabilitystability requirementsrequirementsFigureFigure 16. provided16.providedMaximum Maximum thethe tilting tilting storstorageage angleangle capacitycapacity of double-hull isis greatergreater storage storage thanthan tanks65006500 tanks m(L/Dm3.(L. =/D 1.05).= 1.05).

The ratio L/D is a parameter that controls the tilting motion of the tank. Larger L/D ratios would lead to a smaller tilting motionWorkingWorking as the water environmentalenvironmental plane moment conditioncondition of inertia would be increased. However, for the same capacity, larger L/DExtremeExtreme ratios would environmentalenvironmental also result conditio conditioin more nncostly use of materials. Figures 17

FigureFigureFigure 17. 17.17.Maximum MaximumMaximum tilting tiltingtilting angle angleangle of of double-hulldouble-hull storagestorage storage tankstanks tanks (L/D(L/D (L ==/ D 1.1).1.1).= 1.1).

WorkingWorking environmentalenvironmental conditioncondition ExtremeExtreme environmentalenvironmental conditioconditionn

FigureFigureFigure 18. 18.18.Maximum MaximumMaximum tilting tiltingtilting angle angleangle of of double-hulldouble-hull storagestorage storage tankstanks tanks (L/D(L/D (L ==/ D 1.2).1.2).= 1.2).

However,However, thethe resultsresults presentedpresented aboveabove areare basedbased onon aa uniqueunique setset ofof tanktank wallwall thicknesses.thicknesses. NoteNote thatthat thethe diameterdiameter ofof thethe tanktank cancan rangerange fromfrom 2020 mm forfor aa 50005000 mm33 tanktank withwith D/HD/H == 1.11.1 toto aboutabout 4040 mm forfor aa 15,00015,000 mm33 tanktank withwith D/HD/H == 2.0.2.0. Thus,Thus, thethe tanktank wallwall thicknessthickness isis unlikelyunlikely toto remainremain thethe samesame asas D/HD/H ratioratio andand storagestorage capacitycapacity vary.vary. TheThe thicknessthickness ofof thethe tanktank wallwall needsneeds toto bebe designeddesigned toto satisfysatisfy strengthstrength andand serviceabilityserviceability requirements,requirements, andand thisthis dimensiodimensionn affectsaffects thethe overalloverall massmass andand hencehence thethe tiltingtilting

Energies 2019, 12, 3487 15 of 30

However, the results presented above are based on a unique set of tank wall thicknesses. Note that the diameter of the tank can range from 20 m for a 5000 m3 tank with D/H = 1.1 to about 40 m for a 15,000 m3 tank with D/H = 2.0. Thus, the tank wall thickness is unlikely to remain the same as

D/HEnergies ratio and2019, 12 storage, x FOR PEER capacity REVIEW vary. The thickness of the tank wall needs to be designed15 of to 30 satisfy strength and serviceability requirements, and this dimension affects the overall mass and hence the Energies 2019, 12, x FOR PEER REVIEW 15 of 30 tiltingmotion motion and and stability stability of the of thetank. tank. In view In view of this, of this,it is ituseful is useful to study to study the effect the e ffofect different of diff erentwall wall thicknessesthicknessesmotion and on theon stability the tilting tilting of motion themotion tank. of of tanks Intanks view under of this, both both it working workingis useful and to and extreme study extreme the environmental effect environmental of different conditions. conditions. wall thicknessesFiguresFigures 19 onand 19 the and 20 tilting 20present present motion the the of results results tanks under showingshowing both the the working effect effect of and of wall wall extreme thickness thickness environmental on the on maximum the maximum conditions. tilting tilting 3 anglesangles ofFigures 5000 of 5000 m 193 m tankand tank 20 of presentof various various the D D/Hresults/H ratiosratios showing under the work working effecting of and wall and extreme thickness extreme enviro on environmental thenmental maximum conditions, tilting conditions, respectively. This3 tank capacity is chosen as smaller capacity has more stability concerns. Results respectively.angles of 5000 This m tank tank capacity of various is D/H chosen ratios as under smaller work capacitying and extreme has more enviro stabilitynmental concerns. conditions, Results showrespectively. that when This D/H tank > 1.3, capacity the maximum is chosen tilting as sm anglealler iscapacity virtually has independent more stability of tank concerns. wall thickness. Results show that when D/H > 1.3, the maximum tilting angle is virtually independent of tank wall thickness. However,show that whenfor smaller D/H > D/H 1.3, theratios, maximum the maximum tilting angletilting is angle virtua tendslly independent to increase rapidlyof tank wallwhen thickness. the tank However, for smaller D/H ratios, the maximum tilting angle tends to increase rapidly when the wallHowever, thickness for smaller reaches D/H a certain ratios, critical the maximum value. Under tilting aangle safe workingtends to increasecondition, rapidly it can when be seen the fromtank tankFigurewall wall thickness thickness19 that reachesthe reaches stability a certain a certainrequirement critical critical value. is viol value. Underated Underfora safe D/H working a < safe 1.35. working condition,However, condition, itunder can be an seen itextreme can from be seen fromenvironmentalFigure Figure 19 19 that that condition,the the stability stability it isrequirement noted requirement from Figureis viol is violatedated 20 that for tanks forD/H D with 1.25. in ForFigures smaller 21 andD/H 22, ratios the maximum tilting angle is not affected by the wall thickness when D/H > 1.25. For smaller D/H ratios

Energies 2019, 12, 3487 16 of 30

Energies 2019, 12, x FOR PEER REVIEW 16 of 30

EnergiesA 2019 similar, 12, x FOR study PEER was REVIEW carried out for 15,000 m3 tanks. As it can be seen in Figures 21 16and of 2230 , such as 1.2 and 1.25, the tilting angle is observed to increase sharply as the wall reaches a critical the maximum tilting angle is not aﬀected by the wall thickness when D/H > 1.25. For smaller D/H thickness, similar to the case of the smaller tank capacity considered earlier. It is found that tanks suchratios as such 1.2 asand 1.2 1.25, and the 1.25, tilting the tilting angle angle is observed is observed to increase to increase sharply sharply as asthe the wall wall reaches reaches a a critical critical with D/H < 1.3 do not satisfy stability requirements. In view of this, it may be concluded that the thickness,thickness, similar toto thethe casecase of of the the smaller smaller tank tank capacity capacity considered considered earlier. earlier. It is It found is found that that tanks tanks with stability and tilt motion of the tanks are not affected by the wall thickness provided that the withD/H

8.07.0 D/H 1.2 7.06.0 D/H 1.21.25 6.05.0 D/H 1.251.3 5.04.0 D/H 1.31.35 4.03.0 D/H 1.351.4 Max. Tilt AngleTilt Max. 3.02.0 D/H 1.41.45 Max. Tilt AngleTilt Max. 2.01.0 D/H 1.451.5 D/H 1.52.0 1.00.0 300 350 400 450 500 550 D/H 2.0 0.0 300 350Wall 400 Thickness 450(mm) 500 550 Wall Thickness (mm) Figure 21. Effect of wall thickness on tilting angle for 15,000 m3 tank (L/D = 1.05) under working Figure 21. Eﬀect of wall thickness on tilting angle for 15,000 m3 tank (L/D = 1.05) under working environmental condition. Figureenvironmental 21. Effect condition. of wall thickness on tilting angle for 15,000 m3 tank (L/D = 1.05) under working environmental condition. 16.0

16.014.0

14.012.0 D/H 1.2 D/H 1.25 12.010.0 D/H 1.2 D/H 1.251.3 10.08.0 D/H 1.31.35 8.0 6.0 D/H 1.351.4 Max Tilt Angle (°) Tilt Angle Max 6.04.0 D/H 1.41.45 Max Tilt Angle (°) Tilt Angle Max 4.02.0 D/H 1.451.5 D/H 1.52.0 2.00.0 300 350 400 450 500 550 D/H 2.0 0.0 300 350Wall 400 Thickness 450(mm) 500 550

Figure 22. Effect of wall thicknessWall on Thickness tilting angle (mm) for 15,000 m3 tank (L/D = 1.05) under extreme Figureenvironmental 22. Effect condition. of wall thickness on tilting angle for 15,000 m3 tank (L/D = 1.05) under extreme environmental condition. Figure 22. Effect of wall thickness on tilting angle for 15,000 m3 tank (L/D = 1.05) under extreme Similar to ships, tanks tend to have low hydrostatic stability and thus large tilt angles under environmental condition. environmentalSimilar to ships, loads tanks when tend the storageto have levellow hydros is eithertatic close stability to empty and thus or fully large loaded. tilt angles When under the environmentalfillingSimilar level to is loads low,ships, thewhen tanks free the tend surface storag to havee fflevelect low isis predominanteitherhydros closetatic tostability as empty the displaced andor fully thus loaded. waterlarge tiltWhen volume angles the is underfilling small environmentallevel(see Equationis low, the (3)loads freefor surface reference).when theeffect storag Significant is predominante level free is either surface as the close edisplacedffect to reducesempty water or the fullyvolume metacentric loaded. is small heightWhen (see the andEquation filling hence level(3)the for stability is reference). low, the of thefree Significant tank. surface On effect free the othersurface is predominant hand, effect as redu the asces filling the the displaced levelmetacentric increases, water height volume the and CG is ishence small raised the (see and stability Equation reaches of (3)thethe for tank. highest reference). On levelthe other Significant when hand, the tankfreeas the surface is filling fully effect loaded.level reduincrea Thus,cesses, the thethe metacentric stabilityCG is raised of height the and tank andreaches reduceshence the the highest as stability the filling level of thewhen tank. the On tank the is otherfully loaded.hand, as Thus, the filling the stability level increa of theses, tank the reduces CG is raised as the andfilling reaches level increases.the highest Unlike level whenthe fully the filled tank condition,is fully loaded. a filling Thus, level the that stability is close of to th thee tank fully reduces filled condition as the filling would level have increases. an additional Unlike the fully filled condition, a filling level that is close to the fully filled condition would have an additional

Energies 2019, 12, 3487 17 of 30

Energies 2019, 12, x FOR PEER REVIEW 17 of 30 level increases. Unlike the fully filled condition, a filling level that is close to the fully filled condition negativewould have effect an of additional the free surface negative and eff ectit would of the ther freeefore surface have and lesser it would stability. therefore Thus, have it is lesserimportant stability. to investigateThus, it is importantthe stability to of investigate tanks under the stabilitynear empt ofy tanks and fully under loaded near empty conditio andns fullyand propose loaded conditions methods toand improve propose the methods stability toof improvethe tanks the under stability these oftwo the critical tanks underfill conditions. these two critical fill conditions. FigureFigure 2323 shows shows the the shape shape of ofthe the inner inner tank tank wall wall,, which which is segmented is segmented into three into threeportions. portions. The heightsThe heights of the oftop, the middle top, middleand bottom and portions bottom portionsare denoted are as denoted ht, hm, and as hhtb,, hrespectively.m, and hb, respectively.The middle segmentThe middle has segmenta constant has radius a constant throughout radius throughout its height, itswhile height, the whiletop and the bottom top and segments bottom segments have a truncatedhave a truncated conical conical shape. shape.This shap Thise shapeaims aimsto improve to improve the thestability stability of tank of tank by by reducing reducing the the second second momentmoment of of area area of of the the internal internal liquid liquid waterplane waterplane at at both both tank tank top top and and the the bottom bottom segments. segments.

FigureFigure 23. 23. InnerInner tank tank wall wall with with tapering tapering at at top top and and bottom. bottom.

TwoTwo tanks tanks with with a a capacity capacity of of 5000 5000 m m33 andand 15,000 15,000 m m33 areare considered considered to to investigate investigate the the effect eﬀect of of thethe tapered tapered inner inner wall wall on on the the stability stability performance. performance. The The dimensions dimensions of of the the tanks tanks are are summarized summarized in in TableTable 44.. Note that the diameters of the tanks with tapered inner walls are slightly greater than those withwith cylindrical cylindrical inner inner walls. walls. This This is is to to compensate compensate the the reduced reduced storage storage capacity capacity at at low low and and high high filling ﬁlling zoneszones so so that that the the overall overall height height of of the the tank tank remains remains the the same. same.

TableTable 4. 4. TankTank dimensions. dimensions.

50005000 m 3m3 15000 m3 15000 m3 Case Case OriginalOriginal Tapered Tapered Original OriginalTapered Tapered H (m) 16.9 16.9 23.9 23.9 H (m) 16.9 16.9 23.9 23.9 L (m) 26.0 26.0 36.8 36.8 L (m)D (m) 26.0 23.6 24.8 26.0 33.5 36.8 35.0 36.8 D (m)ht (m) 23.6 - 4.0 24.8 - 33.5 5.5 35.0 ht (m) - 4.0 - 5.5 hm (m) - 4.4 - 7.0 hm (m) - 4.4 - 7.0 hb (m) - 4.0 - 5.5 hb (m) - 4.0 - 5.5 θ (°) - 26.6 - 26.6 θ (◦) - 26.6 - 26.6 Figure 24 presents the tilting angle of tanks at different fuel filling levels under extreme environmentalFigure 24 conditions. presents the As tiltingit can be angle seen, oftilting tanks motion at di fftendserent to fuel be higher filling when levels nearly under empty extreme or fullenvironmental for the case conditions.of a non-tapered As it cantank. be In seen, general, tilting it motionis found tends that there to be higherare two when local nearlypeaks emptyin the tiltingor full angle for the corresponding case of a non-tapered to these two tank. critical In general, fillingit levels. is found One that of these there two are twolevels, local depending peaks in theon thetilting geometry angle correspondingof the tank, would to these result two in critical the absolute filling levels.maximum One tilting of these motion. two levels, By introducing depending taperingon the geometry at the top of and the bottom tank, would segments result for in the the ta absolutenks, stability maximum is found tilting to improve motion. for By the introducing low and hightapering filling at levels. the top There and bottom is a slight segments reduction for thein the tanks, stability stability when is foundthe fuel to level improve is between for the lowthe two and criticalhigh filling filling levels. levels. There This isis adue slight to the reduction small in increase the stability in the diameter when the at fuel the level middle is between segment the of twothe tank.critical However, filling levels. this Thisdrawback is due is to more the small than increase compensated in the diameterby the appreciable at the middle decrease segment in of the the maximumtank. However, tilting this angle. drawback It is therefore is more thanrecommended compensated that by tapering the appreciable of the inner decrease tank in wall the profile maximum be adopted to achieve an improved stability performance.

Energies 2019, 12, 3487 18 of 30 tilting angle. It is therefore recommended that tapering of the inner tank wall proﬁle be adopted to achieveEnergiesEnergies 2019 an 2019 improved, 12, 12, x, xFOR FOR PEER stabilityPEER REVIEW REVIEW performance. 1818 of of 30 30

2.52.5

2.02.0

1.51.5

1.01.0 Tilting angle (°) angle Tilting Tilting angle (°) angle Tilting 50005000 m3 m3 tank tank 50005000 m3 m3 tank tank with with tapered tapered inner inner wall wall 0.50.5 1500015000 m3 m3 tank tank 1500015000 m3 m3 tank tank with with tapered tapered inner inner wall wall 0.00.0 00 20406080100120 20406080100120 Fuel filling level (%) Fuel filling level (%) FigureFigureFigure 24. 24. 24.E Effectﬀ Effectect ofof of taperedtapered tapered inner inner inner wall wall wall conﬁguration configuration configuration on on on tilting tilting tilting angle angle angle of of of tanks. tanks. tanks.

ItIt isIt is is important important important to to to recognize recognize recognize that that that a a single asingle single hull hull hull tank tank tank design design design does does does not not not provide provide provide satisfactory satisfactory satisfactory stability stability stability underunderunder environmental environmental environmental loads. loads. loads. AA A doubledouble double hullhull hull wallwall wallwould would woulddefinitely definitely definitelylead lead leadto to tobetter better betterstability stability stabilityfor for for the the the same same same storagestoragestorage capacity.capacity. capacity. However,However, However, thethe the voidvoid void betweenbetween between thethe the tanktank tank wallswalls walls meansmeans means thatthat that thethe the outerouter outer wallwall wall isis is subject subject subject toto to strongstrongstrong sea sea sea water water water pressure pressure pressure acting acting acting on on the on the outer the outer outer side si andsidede theand and inner the the inner wallinner fromwall wall storedfrom from stored liquidstored pressureliquid liquid pressure pressure acting onactingacting the inner on on the the side. inner inner Consequently, side. side. Consequently, Consequently, these walls these these would walls walls be would thicker would be than be thicker thicker the case than than of athe the single case case hullof of a wall asingle single which hull hull enjoyswallwall which thewhich benefit enjoys enjoys of the negatingthe benefit benefit pressure of of negating negating on both pressure pressure sides on ofon theboth both wall. sides sides To of overcomeof the the wall. wall. thisTo To disadvantage,overcome overcome this this itdisadvantage, woulddisadvantage, desirable it it would would to fill desirable the desirable void betweento to fill fill the the thevoid void tank between between walls the with the tank tank a suitable walls walls with with material, a asuitable suitable such material, asmaterial, sea water such such orasas asea syntacticsea water water foam.or or a asyntactic syntactic Figure 25foam. foam. shows Figure Figure a proposed 25 25 sh showsows design a aproposed proposed where thedesign design space where where between the the space the space double between between hull the isthe segmenteddoubledouble hull hull into is is segmented 8segmented compartments, into into 8 8 allco compartments, filledmpartments, with sea all all water filled filled to with with a desired sea sea water water level. to to The a adesired desired effect oflevel. level. the seaThe The water effect effect levelofof the the in sea thesea water voidwater spacelevel level in on in the thethe void stabilityvoid space space performance on on the the stability stability of the performance performance tank will be of investigated.of the the tank tank will will be be investigated. investigated.

FigureFigureFigure 25. 25. 25.Plan Plan Plan andand and sideside side viewview view ofof of tanktank tank with with with sea sea sea water water water between between between double double double hulls. hulls. hulls.

NoteNoteNote that that that the the the sea sea watersea water water in the in in voidthe the spacevoid void alsospace space contributes al alsoso contributes contributes to the free to tosurface the the free free eff ect.surface surface The calculationeffect. effect. The The ofcalculationcalculation the section of of momentthe the section section of areamoment moment of the of of waterplanesarea area of of the the waterplanes ofwaterplanes the sea water of of the the may sea sea bewater water reasonably may may be be simplifiedreasonably reasonably bysimplifiedsimplified assuming by by zero assuming assuming thickness zero zero of thickness stithicknessffeners of andof stiffeners stiffeners sharp cornersand and sharp sharp of thecorners corners tank of wall, of the the astank tank shown wall, wall, in as as Figure shown shown 26 in .in Consequently,FigureFigure 26. 26. Consequently, Consequently, the eight water the the compartmentseight eight water water compartments compartments have one typical have have planar one one typical typical shape planar withplanar di shapeff shapeerent with orientations.with different different Theorientations.orientations. resulting secondThe The resulting resulting moments second second of area moments moments corresponding of of ar areaea to corresponding onecorresponding typical ballast to to one waterone typical typical compartment ballast ballast water water are givencompartmentcompartment by are are given given by by Ix = Ix,tri Ix,cir, (6a) 𝐼𝐼=𝐼=𝐼,,− 𝐼𝐼,,, , (6a)(6a) Iy = I I , (6b) y,tri − y,cir 𝐼𝐼=𝐼=𝐼,,𝐼𝐼,,, , (6b)(6b) wherewhere I xI xand and I yI ydenote, denote, respectively, respectively, the the second second moments moments of of area area of of the the ballast ballast water water compartment compartment aboutabout x -x -and and y -axes,y-axes, I x,triIx,tri, I, y,triIy,tri and and I x,cirIx,cir, I, y,cirIy,cir are are the the second second moments moments of of area area of of the the triangle triangle and and 1/8 1/8 circle circle illustratedillustrated in in Figure Figure 26b, 26b, respectively. respectively.

Energies 2019, 12, 3487 19 of 30 Energies 2019, 12, x FOR PEER REVIEW 19 of 30

Table 5 lists the second moments of area of two typical sea water compartments about the center Energies 2019where, 12I,x xand FORI yPEERdenote, REVIEW respectively, the second moments of area of the ballast water compartment 19 of 30 of the entire tank, Ix and Iy, as well as their centers of gravity, xc and yc. Note that in the calculation of about x- and y-axes, Ix,tri, Iy,tri and Ix,cir, Iy,cir are the second moments of area of the triangle and 1/8 circle freeEnergies surface 2019 ,effect, 12, x FOR the PEER second REVIEW moment of inertia about its own centroid is needed, which can be easily19 of 30 Tableillustrated 5 lists inthe Figure second 26b, moments respectively. of area of two typical sea water compartments about the center obtainedTable5 listsby using the second parallel moments axis theorem. of area ofSubsequent two typically, sea the water stability compartments and rotational about motion the center of the of the entirefloating tank,Table tanks Ix 5 and lists can Ithebey, assecondevaluated well momentsas by their including ofcenters area theof of two fr gravity,ee typical surface seax effectc and water duey ccompartments. Note to the that water in aboutcompartments the calculationthe center of of the entire tank, Ix and Iy, as well as their centers of gravity, xc and yc. Note that in the calculation free surfaceof effect,the entire the tank, second Ix and moment Iy, as well of as inertia their centers about of its gravity, own centroidxc and yc. Noteis needed, that in thewhich calculation can be of easily ofby free using surface Equation effect, (3). the second moment of inertia about its own centroid is needed, which can be free surface effect, the second moment of inertia about its own centroid is needed, which can be easily obtainedeasily by obtainedusing parallel by using parallelaxis theorem. axis theorem. Subsequent Subsequently,ly, the the stability and and rotational rotational motion motion of the of the obtained by using parallel axis theorem. Subsequently, the stability and rotational motion of the floatingfloating tanks tankscan be can evaluated be evaluated by by including includingthe the free free surface surface effect effect due to due the waterto the compartments water compartments by floating tanks can be evaluated by including the free surface effect due to the water compartments by usingusing Equation Equation (3). (3). by using Equation (3).

(a) (b)

Figure 26. Simplified model showing tank walls and sea water: (a) planar view and (b) typical compartment.

(a) (b) Table 5. Second moment of area and center of gravity of sea water compartment. Figure 26. Simplified(a) model showing tank walls and sea water:(b ()a ) planar view and (b) typical compartment. L D 𝐼 = 𝜋 2 Figure Figure26. Simplified 26. Simplified model model showingshowing tank tank walls walls and sea and water: sea (a) planarwater:192 view (a512 and) planar (b) typical view compartment. and (b) typical compartment. Table 5. Second moment of area and center of gravity Lof seaD water compartment. 𝐼 = 𝜋 2 Table 5. Second moment of area and center of gravity of sea64 water512 compartment. 𝜋 Orientation 1 1LL DD sin Table 5. Second moment of area and center of gravity𝐼 = of sea water𝜋 compartment.4 2 𝑥 =192 512 L4 D4 3 Ix =𝜋D (π 2) L 192 − 512 − L D 4L4 D4 Iy = (π + 2) 𝐼 =L D 𝜋64 𝜋 2512 1 L644D512sin −3 3 π 𝐼 = 𝜋1 8L 4D 2 sin( 4 ) Orientation 1 𝑦 = xc = 𝜋− 6192 L D512𝜋Dsin3 2 πD2 Orientation 1 1 ± 4 L 4 𝑥 = L − 2 4 L3 4D3 sin π L 3 D 𝜋D1 ( ( 8 )) ycL= − 2 𝐼 = L D ±𝜋64 L 22 πD 64 512 − 4 𝐼 = 𝜋 2𝜋 L 4D sin4 𝜋4 641 512 L 8D Orientation 1 1 L DIx = sin (π + 2) 𝑦 = 64 5124 𝑥 =L6 D 𝜋D4 − 4 L = L D ( ) 𝐼 = 3 Iy 𝜋192𝜋D4 2 512 π 2 192 512L 3− 3 −π 2 Orientation 2 1 L4 4D(sin( 8 )) L x D= 𝜋− Orientation 2 1 L 4Dc sin6 𝜋2 πD2 𝐼 = ±𝜋8 2 L 4 𝑥 =1 L64 4D512 sin 3 − 3 π 𝜋D L 8D sin( ) 𝑦 =6 1 4 Lyc= − 2 6 L D 4𝜋D3 L2 πD ± 𝜋 4 𝐼 = LDL sin𝜋 2 − 1921 512 44 𝑦 = 𝜋 Orientation 2 3L 4D𝜋Dsin 1L LD 8 There are several schemes of filling the compartments𝑥 between= the double4 hull with sea water. 𝐼 = 6 𝜋𝜋D 2 It would be interesting to investigate the effect of the scheme on64 the performance512L of the tank with 4 𝜋 regardsThere to stability are several and floatability. schemes of Figure filling 27 the shows compartm five possibleents between schemes. L theD Thedouble sin sea hull water with level sea in water. the L 1 D 4 𝐼 𝑦== 𝜋 2 compartmentsIt would be interesting is variable to for investigate the first four the schemes. effect of the The scheme level depends on3 the performance on𝜋D either the of hydrocarbon the tank with 192 L512 productregards level to stability only, the and average floatability. of the draft Figure and 27 hydrocarbon shows five possible product schemes. level, or the The4 draft sea𝜋 only.water An level active in the Orientation 2 L 4D sin 1 8 controlcompartments system is is therefore variable neededfor the forfirst these four fourschemes. schemes. The𝑥 =level For thedepends fifth scheme,on either the the level hydrocarbon in the There are several schemes of filling the compartments between6 the double𝜋D hull with sea water. compartmentsproduct level is only, fixed the at halfaverage the heightof the draft of the and tank hydrocarbonwall. This schemeproductL haslevel, the or advantage the draft only. of not An 4 requiringactiveIt would control a control be interestingsystem system. is therefore to investigate needed the for effect these of fo theur schemes. scheme onFor the the performance fifth scheme,𝜋 of the the level tank in withthe L D sin compartmentsregardsTables6 to–9 stability present is fixed and the at resultsfloatability. half the of storageheight Figure tanksof 27 the shows with tank five various wall. possible This aspect1 scheme schemes. ratios has and The the storage4 sea advantage water capacities level of in not the 𝑦 = compartments is variable for the first four schemes. The level 3depends on𝜋D either the hydrocarbon underrequiring working a control environmental system. conditions. Since the infilled sea water reducesL the hydrostatic pressure on bothproductTables tank level walls, 6–9 only,present the wallthe the average thickness results of of is the storage assumed draft tanks and to be hydrocarbonwith reduced various from productaspect 450 mm ratios level, to4 300and or mm. storagethe draft The capacities results only. An underactive workingcontrol system environmental is therefore conditions. needed for Since these fothure infilledschemes. sea For water the fifth reduces scheme, the the hydrostatic level in the Therepressurecompartments are several on both schemesis tank fixed walls, at of half thefilling thewall height the thickness compartm of the is assumedtankents wall. betweento This be reduced scheme the double fromhas the450 hull advantagemm withto 300 seaofmm. not water. It would berequiring interesting a control to system.investigate the effect of the scheme on the performance of the tank with regards to stabilityTables and 6–9 floatability.present the results Figure of storage27 shows tanks five with possible various schemes.aspect ratios The and sea storage water capacities level in the under working environmental conditions. Since the infilled sea water reduces the hydrostatic compartments is variable for the first four schemes. The level depends on either the hydrocarbon pressure on both tank walls, the wall thickness is assumed to be reduced from 450 mm to 300 mm. product level only, the average of the draft and hydrocarbon product level, or the draft only. An active control system is therefore needed for these four schemes. For the fifth scheme, the level in the compartments is fixed at half the height of the tank wall. This scheme has the advantage of not requiring a control system. Tables 6–9 present the results of storage tanks with various aspect ratios and storage capacities under working environmental conditions. Since the infilled sea water reduces the hydrostatic pressure on both tank walls, the wall thickness is assumed to be reduced from 450 mm to 300 mm.

Energies 2019, 12, 3487 20 of 30 Energies 2019, 12, x FOR PEER REVIEW 20 of 30 showThe results the floatability show the offloatability the tank, of i.e., the whether tank, i.e., it floatswhether or sink,it floats the or maximum sink, the draft,maximum the maximum draft, the tiltingmaximum angle tilting and the angle possibility and the of possibility the tank bottom of the hittingtank bottom the seabed. hitting The the maximum seabed. allowableThe maximum draft isallowable taken to draft be 16.5 is taken m. Any to be draft 16.5 exceedingm. Any draft the exceeding allowable the limit allowable may result limit in may potential result detrimentalin potential environmentaldetrimental environmental effects due to effects restricted due naturalto restricted flow natural of sea water flow withinof sea water the narrowed within the gap narrowed between tankgap between bottom and tank seabed. bottom Also, and thereseabed. is a Also, possible there danger is a possible that the tankdanger bottom that the may tank hit thebottom seabed may when hit the tankseabed heaves when or the rolls tank under heaves wave or action.rolls under The allowablewave action. limit The depends allowable on severallimit depends factors, on including several thefactors, tank diameter,including thethe tidal, tank wave diameter, and wind the conditions tidal, wave and and the degreewind conditions of environmental and the concern degree at theof site.environmental As previously concern stated, at thethe maximumsite. As previously limit for the stated, tilting the angle maximum for the tanklimit isfor set the to tilting 2 degrees angle under for workingthe tank is environmental set to 2 degrees condition. under working environmental condition. When the draft draft of of the the tank tank exceeds exceeds the the allowable allowable limit, limit, the the tank tank may may or ormay may not not be befloating. floating. In Inthe the latter latter case, case, the the result result would would show show a a“sink” “sink” value value indicating indicating that that the the tank tank will will sink sink and and rest rest on the seabed. ForFor convenience, convenience, the the tank tank will bewill referred be referred to as Scheme to as NSchemetank, where N tank,N = 1where to 5, corresponding N = 1 to 5, tocorresponding the five different to the schemes five different for filling schemes the compartments for filling the compartments with sea water with (see sea Figure water 27 ).(see It canFigure be seen27). It from can be Tables seen6 from–9 that Tables Schemes 6–9 that 1 and Schemes 5 tanks 1 satisfyand 5 tanks the design satisfy requirementsthe design requirements when D/H when1.6. ≥ ForD/H the ≥ 1.6. other For schemes, the other the schemes, tanks will the eventuallytanks will eventual sink underly sink high under fuel load high even fuel withload aeven large withD/H a =large2.0. ItD/H is also= 2.0. observed It is also observed that when thatD/H when= 1.3, D/Hany = scheme1.3, any chosenscheme wouldchosen resultwould in result lower in maximum lower maximum tilting anglestilting angles whencompared when compared to the same to the tank same without tank with anyout filling any in filling the compartments. in the compartments. This implies This thatimplies the tankthat the stability tank stability is improved is improved by filling by the filling compartments the compartments with sea with water. sea water. As D/H As ratio D/H increases ratio increases to 1.6, onlyto 1.6, Schemes only Schemes 1 and 51 tanksand 5 havetanks maximum have maximum tilting t anglesilting angles smaller smaller than tanks than withouttanks without any filling any filling in the compartments.in the compartments. When When D/H reaches D/H reaches 2.0, the 2.0, maximum the maximum tilting angletilting of angle tanks of using tanks any using filling any scheme filling willscheme be larger will be than larger tanks than without tanks wi anythout filling. anyIn filling. view In of view this, itof is this, recommended it is recommended that Scheme that Scheme 5 tanks with5 tanks D/ Hwith= 1.6 D/H be = adopted 1.6 be adopted as it gives as it the gives best the performance best performance in terms in terms of stability of stability and it and is also it is most also economicalmost economical since itsince does it notdoes require not require any active any active control control system. system.

Figure 27. Sea water ﬁlling schemes. Figure 27. Sea water filling schemes.

Energies 2019, 12, 3487 21 of 30

Table 6. Results of storage tank with D/H = 1.3, L/D = 1.05.

Volume 5000 m3 7000 m3 9000 m3 11,000 m3 13,000 m3 15,000 m3 Roll Roll Roll Roll Roll Roll Scheme Draft Draft Draft Draft Draft Draft (◦) (◦) (◦) (◦) (◦) (◦) No ﬁll Pass 2.23 Pass 2.15 Pass 2.10 Pass 2.05 Hit 2.01 Hit 1.98 1 Pass 1.86 Pass 1.71 Hit 1.64 Hit 1.58 Hit 1.53 Hit 1.49 2 Sink 1.86 Hit&Sink 1.76 Hit&Sink 1.68 Hit&Sink 1.62 Hit&Sink 1.57 Hit&Sink 1.53 3 Sink 1.93 Hit&Sink 1.82 Hit&Sink 1.74 Hit&Sink 1.67 Hit&Sink 1.62 Hit&Sink 1.58 4 Sink 2.06 Hit&Sink 1.93 Hit&Sink 1.83 Hit&Sink 1.75 Hit&Sink 1.69 Hit&Sink 1.64 5 Pass 1.81 Pass 1.71 Hit 1.63 Hit 1.57 Hit 1.52 Hit 1.48

Table 7. Results of storage tank with D/H = 1.5, L/D = 1.05.

Volume 5000 m3 7000 m3 9000 m3 11,000 m3 13,000 m3 15,000 m3 Roll Roll Roll Roll Roll Roll Scheme Draft Draft Draft Draft Draft Draft (◦) (◦) (◦) (◦) (◦) (◦) No ﬁll Pass 1.32 Pass 1.24 Pass 1.18 Pass 1.14 Pass 1.10 Pass 1.07 1 Pass 1.24 Pass 1.16 Pass 1.09 Pass 1.05 Pass 1.01 Hit 0.98 2 Sink 1.32 Sink 1.24 Sink 1.17 Hit&Sink 1.13 Hit&Sink 1.09 Hit&Sink 1.06 3 Sink 1.38 Sink 1.29 Hit&Sink 1.22 Hit&Sink 1.17 Hit&Sink 1.12 Hit&Sink 1.09 4 Sink 1.48 Sink 1.37 Hit&Sink 1.29 Hit&Sink 1.23 Hit&Sink 1.18 Hit&Sink 1.14 5 Pass 1.24 Pass 1.16 Pass 1.09 Pass 1.05 Pass 1.01 Hit 0.98

Table 8. Results of storage tank with D/H = 1.6, L/D = 1.05

Volume 5000 m3 7000 m3 9000 m3 11,000 m3 13,000 m3 15,000 m3 Roll Roll Roll Roll Roll Roll Scheme Draft Draft Draft Draft Draft Draft (◦) (◦) (◦) (◦) (◦) (◦) No ﬁll Pass 1.04 Pass 0.97 Pass 0.92 Pass 0.88 Pass 0.85 Pass 0.82 1 Pass 1.03 Pass 0.95 Pass 0.90 Pass 0.86 Pass 0.83 Pass 0.80 2 Sink 1.11 Sink 1.04 Sink 0.99 Sink 0.94 Hit&Sink 0.91 Hit&Sink 0.88 3 Sink 1.16 Sink 1.08 Sink 1.02 Hit&Sink 0.98 Hit&Sink 0.94 Hit&Sink 0.91 4 Sink 1.25 Sink 1.15 Sink 1.08 Hit&Sink 1.03 Hit&Sink 0.99 Hit&Sink 0.95 5 Pass 1.03 Pass 0.96 Pass 0.90 Pass 0.86 Pass 0.83 Pass 0.80

Table 9. Results of storage tank with D/H = 2.0, L/D = 1.05.

Volume 5000 m3 7000 m3 9000 m3 11,000 m3 13,000 m3 15,000 m3 Roll Roll Roll Roll Roll Roll Scheme Draft Draft Draft Draft Draft Draft (◦) (◦) (◦) (◦) (◦) (◦) No ﬁll Pass 0.45 Pass 0.41 Pass 0.37 Pass 0.37 Pass 0.36 Pass 0.34 1 Pass 0.50 Pass 0.46 Pass 0.44 Pass 0.42 Pass 0.40 Pass 0.38 2 Sink 0.57 Sink 0.53 Sink 0.50 Sink 0.48 Sink 0.46 Sink 0.44 3 Sink 0.60 Sink 0.55 Sink 0.52 Sink 0.50 Sink 0.48 Sink 0.46 4 Sink 0.65 Sink 0.60 Sink 0.56 Sink 0.53 Sink 0.50 Sink 0.48 5 Pass 0.51 Pass 0.47 Pass 0.44 Pass 0.42 Pass 0.40 Pass 0.39 Energies 2019, 12, 3487 22 of 30

Figures 28 and 29 show, respectively, the maximum hydrostatic pressure head on the tank outer and inner walls for D/H = 1.6. As it is to be expected, the filling of the compartments with sea water reduces the hydrostatic pressure on the tank’s walls considerably. For example, the half-fuel scheme (Scheme 1) reduces the hydrostatic pressure on outer and inner walls by 42% and 50%, respectively. The corresponding reductions in the constant half-wall scheme (Scheme 5) are 45% and 37%. Both the average-level and full-fuel schemes (Schemes 3 and 2) are able to give even more significant reductions. It is obvious that the draft-level scheme results in no hydrostatic pressure on the outer wall. However, there is less benefit to the inner wall with consequently lesser reduction in the hydrostatic Energiespressure. 2019,As 12, ax FOR significant PEER REVIEW volume of sea water is used to fill the compartments in the last three schemes,22 of 30 the additional heavy weight of the contained sea water will lead to reduced storage capacity and hence Energies 2019, 12, x FOR PEER REVIEW 22 of 30 an uneconomical16 tank design. 16 14 14 12 12 10 No Ballast 10 HalfNo FuelBallast 8 FuelHalf Fuel 8 6 AverageFuel Draft 6 Average 4 HalfDraft Wall 4 2 Half Wall Max. hydrostatic pressure head (m) head pressure hydrostatic Max. 2

Max. hydrostatic pressure head (m) head pressure hydrostatic Max. 0 5000 7000 9000 11000 13000 15000 0 Storage capacity (m3) 5000 7000 9000 11000 13000 15000

Storage capacity (m3) Figure 28. Maximum hydrostatic pressure head on outer tank wall. Figure 28. Maximum hydrostatic pressure head on outer tank wall. Figure 28. Maximum hydrostatic pressure head on outer tank wall. 16

1416

1214

1012 No Ballast HalfNo FuelBallast 810 FuelHalf Fuel 6 8 AverageFuel DraftAverage 4 6 HalfDraft Wall 2 4 Half Wall Max. hydrostatic pressure head (m) head pressure hydrostatic Max. 0 2 Max. hydrostatic pressure head (m) head pressure hydrostatic Max. 5000 7000 9000 11000 13000 15000 0 5000 7000Storage 9000 capacity 11000 (m3) 13000 15000

Figure 29. MaximumStorage hydrostatic capacity pressure (m3) head on inner tank wall. Figure 29. Maximum hydrostatic pressure head on inner tank wall. Figure 29. Maximum hydrostatic pressure head on inner tank wall. In an attempt to obtain a significant reduction in hydrostatic pressure without compromising the designed storage capacity, it is worthwhile to explore filling the compartments with durable and In an attempt to obtain a significant reduction in hydrostatic pressure without compromising light-weight (about 332 kg/m3) composite syntactic foam. The foam comprises composite hollow the designed storage capacity, it is worthwhile to explore filling the compartments with durable and microspheres that are bonded together with syntactic foam. The material can have a minimum life of light-weight (about 332 kg/m3) composite syntactic foam. The foam comprises composite hollow 40 years with a maximum 5% loss of buoyancy throughout its lifespan [31]. Figure 30 illustrates the microspheres that are bonded together with syntactic foam. The material can have a minimum life of use of syntactic foam to fill the compartments. The slab beneath the compartments has holes or slits 40 years with a maximum 5% loss of buoyancy throughout its lifespan [31]. Figure 30 illustrates the to allow seawater to flow through to serve the purpose of balancing the hydrostatic pressure acting use of syntactic foam to fill the compartments. The slab beneath the compartments has holes or slits on both sides of each wall. Gaps between the tank walls and the foam are provided in order to allow to allow seawater to flow through to serve the purpose of balancing the hydrostatic pressure acting the flow of seawater. on both sides of each wall. Gaps between the tank walls and the foam are provided in order to allow the flow of seawater.

Energies 2019, 12, 3487 23 of 30

In an attempt to obtain a significant reduction in hydrostatic pressure without compromising the designed storage capacity, it is worthwhile to explore filling the compartments with durable and light-weight (about 332 kg/m3) composite syntactic foam. The foam comprises composite hollow microspheres that are bonded together with syntactic foam. The material can have a minimum life of 40 years with a maximum 5% loss of buoyancy throughout its lifespan [31]. Figure 30 illustrates the use of syntactic foam to fill the compartments. The slab beneath the compartments has holes or slits to allow seawater to flow through to serve the purpose of balancing the hydrostatic pressure acting on both sides of each wall. Gaps between the tank walls and the foam are provided in order to allow the

ﬂow ofEnergies seawater. 2019, 12, x FOR PEER REVIEW 23 of 30

Energies 2019, 12, x FOR PEER REVIEW 23 of 30 Syntactic foam Holes on bottom plate

Syntactic foam Holes on bottom plate

(a) (b)

FigureFigure 30. Use 30. ofUse syntactic of syntactic foam foam as as ﬂoaters: floaters: ( (aa) )configuration conﬁguration andand (b) bottom (b) bottom plate. plate.

(a) (b) Figure Figure31 presents 31 presents the maximum the maximum tilting tilting angles angles of storage of storage tanks tanks with with syntactic syntactic foam foam under under working working environmentalFigure 30. Use conditions. of syntactic Allfoam tank as floaters: walls (a )are configuration assumed and to ( bbe) bottom 300 plate.mm thick. Curves environmental conditions. All tank walls are assumed to be 300 mm thick. Curves highlighted in red highlighted in red denote that the maximum draft of the tank has exceeded the allowable limit of 16.5 denotem. that Conversely, theFigure maximum 31 other presents curves draft the denote ofmaximum the that tank thetilting has maxi exceededanglesmum of draft storage the meets allowable tanks the withallowable limitsyntactic limit of 16.5foam requirement. m.under Conversely, other curvesAs workingit can denote be seen,environmental that the the results maximum conditions. show that draft slenderAll tank meets tanks wa thells (smaller are allowable assumed D/H ratio) limitto be will requirement. 300 result mm in thick. largerAs Curves draft it can for be seen, the resultsa givenhighlighted show storage that in capacity, slenderred denote tanksas that to thebe (smaller maximumexpected. D draftFor/H ratio)aof given the tank will tank has result D/H exceeded ratio, in larger the larger allowable draft storage limit for tanks aof given 16.5 may storage m. Conversely, other curves denote that the maximum draft meets the allowable limit requirement. violate the draft limit requirement. On the other hand, smaller storage tanks may not meet the capacity, asAs toit can be be expected. seen, the results For show a given that tankslender D tanks/H ratio, (smaller larger D/H ratio) storage will result tanks in larger may draft violate for the draft stability or tilting limit requirement. Figure 31 shows that for the entire range of storage capacities limit requirement.a given storage On the capacity, other as hand, to be smallerexpected. storageFor a given tanks tank may D/H notratio, meet larger the storage stability tanks ormay tilting limit considered, it is necessary that the tank D/H ≥ 1.6 to ensure that the tank is stable and the limits on requirement.violate Figure the draft 31 shows limit requirement. that for the On entire the other range hand, of storagesmaller storage capacities tanks considered,may not meet it the is necessary draft and tilting angle are not exceeded. that the tankstability D/H or tilting1.6 to limit ensure requirement. that the Figure tank 31 is shows stable that and for the entire limits range on of draft storage and capacities tilting angle are considered,≥ it is necessary that the tank D/H ≥ 1.6 to ensure that the tank is stable and the limits on not exceeded.draft and tilting angle are not exceeded.

Figure 31. Maximum tilting angle of floating tanks with syntactic foam. Figure 31. Maximum tilting angle of floating tanks with syntactic foam. The syntacticFigure foam 31. Maximum filling scheme, tilting just angle like the of ﬂoatingsea water tanks filling with schemes syntactic discussed foam. earlier, will result inThe reduction syntactic offoam the filling maximum scheme, hydrostatic just like the pressuressea water fillingat the schemes tank walls. discussed The earlier,reduction will in pressuresresult in are reduction presented of inthe Figure maximum 32. Figure hydrostatic 32a shows pressures the variations at the tank of thewalls. maximum The reduction hydrostatic in pressurepressures head are ( ΔpresentedH) on tank in Figure inner 32. wall Figure with 32a respect shows tothe storage variations capacity. of the maximum Figure 32b hydrostatic shows the percentagepressure reduction,head (ΔH) as on compared tank inner to wallthe tank with with respect no fillingto storage scheme. capacity. It can Figurebe seen 32b from shows Figure the 32a percentage reduction, as compared to the tank with no filling scheme. It can be seen from Figure 32a

Energies 2019, 12, 3487 24 of 30

The syntactic foam filling scheme, just like the sea water filling schemes discussed earlier, will result in reduction of the maximum hydrostatic pressures at the tank walls. The reduction in pressures are presented in Figure 32. Figure 32a shows the variations of the maximum hydrostatic pressure head (∆H) on tank inner wall with respect to storage capacity. Figure 32b shows the percentage reduction, Energies 2019, 12, x FOR PEER REVIEW 24 of 30 as compared to the tank with no filling scheme. It can be seen from Figure 32a that the maximum thatpressure the maximum head increases pressure with head storage increases capacity with but reducesstorage withcapacity increasing but reduces D/H. Whenwith increasing compared toD/H the. Whentank with compared no filling to the scheme, tank Figurewith no 32 fillingb shows scheme, there isFigure at least 32b 43% shows reduction there inis theat least maximum 43% reduction∆H for a 3 3 in5000 the m maximumtank. When ΔH thefor storage a 5000 capacitym3 tank. increases When the to 15,000storage m capacity, the reduction increases increases to 15,000 to as m3, high the as reduction55%. Furthermore, increases itto can as behigh observed as 55%. from Furthermore, Figure 32b thatit can this be percentage observed reductionfrom Figure in ∆ 32bH is that found this to percentagebe virtually reduction independent in Δ ofH is D /foundH. The to e ffbeect virtually of syntactic independent foam filling of D/H. scheme The is effect more of pronounced syntactic foam with fillingrespect scheme to the is outer more wall. pronounced Owing to with the respect free flow to ofthe sea outer water wall. to Owing within to the the gap free between flow of tanksea water walls toand within syntactic the gap foam, between the level tank attained walls withinand syntacti the compartmentc foam, the level will beattained at the draftwithin level. the compartment Consequently, willthere be is at no the net draft pressure level. eConsequently,ffect on the outer there wall. is no net pressure effect on the outer wall.

(a) (b)

FigureFigure 32. 32. HydrostaticHydrostatic pressure pressure on on tank tank inner inner wall: wall: (a (a) )maximum maximum pressure pressure head head and and ( (bb)) reduction reduction in in maximummaximum pressure pressure head. head.

AnAn alternative alternative to usingusing ballastballast water water or or syntactic syntactic foam foam for for reducing reducing hydrostatic hydrostatic pressures pressures on tank on tankwalls walls is to is consider to consider using using a single a single hull cylindricalhull cylindrical storage storage tank. tank. It is obviousIt is obvious that that a single a single hull hull tank tankwould would enjoy enjoy the benefit the benefit of nearly of nearly balanced balanced pressure pres onsure both onsides both ofsides the of wall. the However,wall. However, thereis there a need is ato need examine to examine carefully carefully the stability the stability and floatability and floatabili of thety tankof the of tank the sameof the storage same storage capacity capacity as a double as a doublehull tank. hull Figure tank. 33Figure presents 33 presents a single hulla single cylindrical hull cylindrical tank held intank lateral held position in lateral by surroundingposition by surroundingbarges and spacers. barges and Rubber spacers. fenders Rubber are providedfenders are to provided absorb the to kineticabsorb energythe kinetic to prevent energy to damage prevent to damagetank wall to andtank barge. wall and barge. FigureFigure 3434 showsshows the the maximum maximum tilting tilting angle angle of of single single wall wall cylindrical cylindrical tanks tanks under under wind wind and and currentcurrent loads loads at at their their 1-year 1-year return return period. period. As As in in earlier earlier cases, cases, a a relatively relatively thinner thinner 300 300 mm mm wall wall is is assumedassumed which which is is deemed deemed feasible feasible in in view view of of the the balanced balanced hydrosta hydrostatictic pressures pressures on on the the tank tank wall. wall. ResultsResults show show that that tanks tanks with with D/H D/H == 1.71.7 fulfill fulfill the the stability stability and and operational operational requirements requirements on on tilting tilting motionmotion when when the the storage storage capacity capacity is is above above 10,500 10,500 m m33. .Tanks Tanks of of all all storage storage capacities capacities considered considered in in the the studystudy satisfy satisfy the the aforementioned aforementioned design design checks checks provided provided D/H D/H ≥ 1.8.1.8. ≥ TheThe maximum maximum hydrostatic pressurepressure on on the the tank tank wall wall is shownis shown in Figurein Figure 35. 35. It is It interesting is interesting to note to notethat thethat maximum the maximum hydrostatic hydrostatic pressure pressure head ishead below is below 2 m when 2 m the when tank the D/ Htank> 1.8. D/H These > 1.8. pressure These pressurehead values head are values found toare be found much smallerto be much than thosesmaller for doublethan those hull tanksfor double with compartment hull tanks filledwith compartmentwith water or filled syntactic with foam. water Results or syntactic also reveal foam. that Results the maximum also reveal∆H that occurs the whenmaximum the tank ΔH is occurs empty. whenAs the the single tank hull is empty. tank generally As the single would hull result tank in generally a lighter would weight result design, in asa lighter compared weight to double design, hull as comparedtank, the draft to double experience hull tank, would the be draft smaller. experien Consequently,ce would thebe smaller. maximum Consequently, hydrostatic pressure the maximum exerted hydrostaticon the wall pressure would be exerted smaller. on the wall would be smaller.

Energies 2019, 12, x FOR PEER REVIEW 25 of 30 Energies 2019, 12, 3487 25 of 30 Energies 2019, 12, x FOR PEER REVIEW 25 of 30

Energies 2019, 12, x FOR PEER REVIEW 25 of 30

Figure 33. Single hull tank enclosed by floating barges.

Figure 33. SingleSingle hull tank enclosed by floating ﬂoating barges. FigureTilt 33. Single Angle hull tank(for enclosed CPP by Tank) floating barges. 2.5 Tilt Angle (for CPP Tank) D/H=1.7

) Tilt Angle (for CPP Tank) o 2.5 2.5 D/H=1.8 2.0 D/H=1.7D/H=1.7 ) ) o o D/H=1.8D/H=1.9D/H=1.8 2.0 2.0 1.5 D/H=1.9 D/H=2D/H=1.9 1.5 1.5 D/H=2 1.0 D/H=2.2D/H=2 1.0 D/H=2.2

Maximum Maximum Tilt ( Angle D/H=2.4D/H=2.2

1.0 Maximum Tilt ( Angle D/H=2.4 0.5 0.5 D/H=2.6 Maximum Maximum Tilt ( Angle D/H=2.6D/H=2.4 0.5 0.0 0.0 D/H=3D/H=3D/H=2.6 55 7.5 7.5 10 10 12.5 12.5 15 15 0.0 Storage Capcity(m33) 3 D/H=3 Storage Capcity(m ) x10x103 5 7.5 10 12.5 15

3 FigureFigure 34. Maximum 34. Maximum tilting Storagetilting angle angle Capcity(m of of singlesingle hull hull) cylindrical cylindrical tank tankx10 (5000–17,5003 (5000–17,500 m3). m3). Figure 34. Maximum tilting angle of single hull cylindrical tank (5000–17,500 m3).

Figure 34. Maximum tilting angle of single hull cylindrical tank (5000–17,500 m3).

(a) (b)

Figure 35. Hydrostatic pressure on cylindrical tank wall: (a) maximum pressure head and (b) reduction in maximum pressure head. (a) (b)

Figure 35.35. HydrostaticHydrostatic(a pressure ) pressure on on cylindrical cylindrical tank tank wall: wall: (a) maximum (a) maximum pressure pressure(b head) and head (b) reduction and (b) reductionin maximum in maximum pressure head. pressure head. Figure 35. Hydrostatic pressure on cylindrical tank wall: (a) maximum pressure head and (b) reduction in maximum pressure head.

Energies 2019, 12, 3487 26 of 30 Energies 2019, 12, x FOR PEER REVIEW 26 of 30

TheThe singlesingle hullhull tanktank hashas thethe advantagesadvantages ofof easyeasy construction,construction, lesserlesser constructionconstruction materialmaterial andand smallersmaller hydrostatichydrostatic pressurepressure didifferencefference onon thethe tanktank wall.wall. However,However, thisthis conceptconcept susuffersffers fromfrom lesserlesser Energies 2019, 12, x FOR PEER REVIEW 26 of 30 stabilitystability andand buoyancybuoyancy asas comparedcompared toto aa doubledouble hullhull tanktank ofof samesame storagestorage capacity.capacity. ForFor thisthis reason,reason, aa largerlarger DD/HThe/H singleratioratio ofofhull aboutabout tank 1.8has1.8 istheis necessarynecessary advantages inin of orderorder easy toconstruction,to fulfillfulfill thethe lesser prescribedprescribed construction stabilitystability material requirements.requirements. and ThisThis wouldsmallerwould resulthydrostaticresult inin aa greater greaterpressure useuse difference ofof seasea spacespace on the asas tank comparedcompared wall. However, toto thethe otherother this conc alternativealternativeept suffers concepts.concepts. from lesser InIn orderorder toto improveimprovestability thethe and singlesingle buoyancy hullhull tankastank compared designdesign to concept,concept, a double itit hull isis proposedproposed tank of same toto attachatta storagech fourfour capacity. smallersmaller For hollowhollow this reason, cylinderscylinders toto thethea exteriorlargerexterior D/H ofof theratiothe tank tankof about wall,wall, 1.8 asas is shownshown necessary inin Figure Figuin orderre 36 36. to. TheseThesefulfill the hollowhollow prescribed cylinderscylinders stability areare maderequirements.made ofof thethe same same lightweightlightweightThis would concreteconcrete result asinas a thethe greater storagestorage use tankoftank sea and andspace areare as intendedintecomparednded toto acttheact asotheras floaters,floaters, alternative i.e.,i.e., toconcepts.to provideprovide In additionalorderadditional buoyancy.buoyancy.to improve InIn addition,addition, the single thesethese hull tank floatersfloaters design areare concept, alsoalso designeddesi it isgned proposed toto interactinteract to attach withwith four the thesmaller rubberrubber hollow fendersfenders cylinders installedinstalled to the exterior of the tank wall, as shown in Figure 36. These hollow cylinders are made of the same onon thethe sidessides ofof thethe floatingfloating barges.barges. TheThe meritsmerits ofof thisthis designdesign includeinclude feasiblefeasible andand easyeasy construction,construction, lightweight concrete as the storage tank and are intended to act as floaters, i.e., to provide additional balancedbalanced hydrostatichydrostatic pressurepressure andand relativelyrelatively lowlow constructionconstruction cost. cost. ResultsResults plottedplotted inin FigureFigure 3737 showshow buoyancy. In addition, these floaters are also designed to interact with the rubber fenders installed that a D/H ratio of 1.6 satisfies the prescribed stability requirement and motion criteria. that aon D/H the sidesratio ofof the1.6 floatingsatisfies barges. the prescrib The meritsed stability of this design requirement include feasibleand motion and easy criteria. construction, balanced hydrostatic pressure and relatively low construction cost. Results plotted in Figure 37 show that a D/H ratio of 1.6 satisfies the prescribed stability requirement and motion criteria.

(a()a ) (b) (b)

FigureFigureFigure 36. 36.Single Single36. Single hull hull hull tank tank tank with with with four four four hollow hollow hollow cylindrical cylindrical ﬂoaters:floaters: floaters: (a ( )a( aplan)) plan plan view viewview and andand (b) isometric ((b)) isometric view. view. view.

3 FigureFigure 37. Maximum37. Maximum tilting tilting angle angle of of single single hullhull cylindricalcylindrical tank tank (5000–17,500 (5000–17,500 m ) m under3) under working working environmental conditions. environmental conditions. Figure 37. Maximum tilting angle of single hull cylindrical tank (5000–17,500 m3) under working

environmental conditions.

Energies 2019, 12, 3487 27 of 30

4. Discussion A study on the hydrostatic stability analyses of floating hydrocarbon storage tanks with various storage capacities ranging from 5000 m3 to 17,500 m3 has been carried out. Various tank designs were investigated. Static analysis of tank tilting angle under sustained wind and current loads was carried out. Note that both 1-year and 100-year environmental conditions listed in Table3 are employed to define the working and extreme environmental conditions. In the evaluation of hydrostatic performance of the proposed tank designs, the acceptable tank rotational motion is limited to 2◦ and 5◦, respectively. The maximum draft of tanks is set to 16.5 m to avoid undesirable contact between the tank bottom and seabed in view of the shallow water depth in Singapore coastal regions. The local wave conditions are generally benign and thus their effect on the tanks are not considered in the study. Wind loads also contain dynamic components that may give rise to sloshing of hydrocarbon product inside the storage tanks. In order to evaluate the effect of dynamic wind loads, the fundamental tank sloshing period was first identified to be in the range of 8–10 s [32]. The peak energy of short-term oscillatory wind is, however, normally located at a period above 1 min [33]. As a result, the dynamic wind energy at tank sloshing period is expected to be small. Thus, it is reasonable to neglect the dynamic effect of wind loads. In addition to wind, the flow of water current passing through a floating tank forms vortex shedding in the downstream, which may also give rise to excitation of sloshing. The frequency of vortex shedding may be evaluated by using

St = fv Do/vc, (7) where St is the Strouhal number and can be reasonably taken as 0.2 [34], fv the frequency of vortex shedding, Do the overall diameter of the tank, and vc the current speed. Based on the tank dimension considered and the current speed listed in Table3, the period of vortex shedding is expected to be ranging from 75 s to 140 s. Again, this is much greater than the aforementioned sloshing periods. Therefore, it is justiﬁable to neglect the dynamic eﬀect of water current loads in the study. The following conclusions may be drawn from the study:

1. Larger storage tanks have better stability than smaller tanks due to the larger restoring moment of inertia. 2. Based on the environmental data at the selected site, the double hull tank with L/D =1.05 shall have a D/H 1.3 to ensure a stable and economical design. ≥ 3. For a given storage capacity, tank stability may be improved by choosing tank dimensions with as large as practically possible L/D and D/H ratios. However, large L/D ratio results in high cost due to additional construction materials whereas large D/H ratio leads to use of more sea space. 4. Once the prescribed stability requirements have been fulﬁlled, the thickness of tank walls has negligible eﬀect on the stability of the tank.

Tanks tend to have the lowest stability when the fuel level is either very low or very high. Stability may be improved by adopting tapered inner wall profile. The double hull arrangement requires thick walls to resist high hydrostatic pressures due to pressure acting on one face of each wall. In order to achieve an economical design, it is necessary to reduce the effect of unbalanced hydrostatic pressures. Four design proposals are considered. The first two proposes to fill the tank compartments with (1) sea water or (2) composite syntactic foam. The third requires redesigning the cylindrical tank, as a single wall and the fourth is a modification by attaching hollow cylindrical floaters to the exterior of the tank wall. In the first proposal, five different sea water filling schemes were investigated. All the five schemes are found to significantly reduce the effect of hydrostatic pressure acting on the tank walls. The “half wall height” filling scheme is recommended as it does not require any control system. A minimum D/H ratio of 1.6 is needed to ensure the stability of the tank. The disadvantage of this Energies 2019, 12, 3487 28 of 30 proposal is that the addition of sea water in the tank compartments increases the weight of the storage tanks substantially. Thus, the floatability of the storage tanks is significantly decreased, which reduces the tank storage capacity. The second proposal employs light-weight composite syntactic foam to fill the tank compartments, which alleviates the problem due to the extra weight of filling sea water suffered in the first proposal. A minimum D/H ratio of 1.6 is suggested for tanks with durable and light-weight syntactic foam. Though the use of syntactic foam appears superior, it is significantly more expensive than the use of sea water as filling material. Single wall tanks were also examined as the third proposal. This proposal enjoys the advantage of nearly balanced hydrostatic pressure acting on both sides of the tank wall. However, it has a low stability. Consequently, a minimum D/H ratio of 1.8 is required, which leads to the need for a larger sea space. The fourth proposal improved the single wall tank design by attaching four concrete floaters to the tank wall. Results show that a D/H ratio of 1.6 satisfies the prescribed stability requirement and motion criteria. In addition to increasing the buoyancy of the tank, the floaters are also designed to interact with the rubber fenders installed on the sides of the floating barges. They are relatively small in size when compared to the single wall tank. Thus, they do not introduce significant increase in the construction cost. The maximum allowable rotation of the tank is limited to 5◦ under extreme environmental conditions. This matches with the comprehensive hydrodynamic analysis results presented in references [13,14]. Within such a small rotation range, the investigation on the initial stability is therefore deemed sufficient in an initial feasibility study phase. However, unexpected large tank rotation may occur accidentally. Undesirable damage to the floating tanks may also affect the stability. More detailed studies are thus needed to examine the stability curve and damaged stability of the proposed storage tanks.

5. Conclusions This paper presents a novel stand-alone floating hydrocarbon storage and bunkering facility to cope with the growth in the demand for hydrocarbon storage and the shortage of available land for such storage facilities. The facility can be easily scaled up and down to suit any sea space and required storage capacity. It also has the advantage of mobility, as it can be relocated to another location if necessary. Inside the facility, there are self-stabilizing floating storage tanks of various storage capacities to cater for the operators’ commercial needs. These tanks have many advantages such as modular design for easy construction, and easy maintenance, as well as replacement. Owing to the lack of design specifications for floating fuel storage tanks, codes of practice for both onshore tanks and offshore tankers were reviewed. Based on the review, design guidelines for stability and motion criteria of floating tanks under both working and extreme environmental conditions were established. Different tank structural configurations were proposed and a comprehensive study on the hydrostatic performance of the tank designs was carried out. The most appealing design is a single wall cylindrical tank with floaters attached to the tank wall. The merits of this design include feasible and easy construction, balanced hydrostatic pressure and relatively low construction cost. Results from a hydrostatic analysis on a practical range of storage capacities show that these self-stabilizing tanks are very doable in calm water zones. Results also suggest that the tank shall have a D/H ratio of at least 1.6 to furnish a stable and economical design in Singapore coastal waters. Finally, it is worth mentioning that although the present study is based on Singapore environmental conditions, the proposed concept is deemed to be applicable to coastal waters in other parts of the world with appropriate considerations of local conditions. Energies 2019, 12, 3487 29 of 30

6. Patent Wang, C.M.; Ang, K.K.; Dai, J.; Lim, B.K.; Magee, A.R.; Watn, A.; Hellan, Ø.; Lie, H.; Justnes, H.; Tan, T.L.; Heah, S.P.; Lee, E.Y. Floating hydrocarbon storage and bunker facility. Singapore provisional patent, No. 10201702691X.

Author Contributions: Conceptualization, J.D., K.K.A., C.M.W., Ø.H. and A.W.; methodology, J.D. and J.J.; formal analysis, J.D. and J.J.; writing—original draft preparation, J.D.; writing—review and editing, K.K.A., C.M.W., Ø.H. and A.W.; visualization, J.D.; funding acquisition, C.M.W. Funding: This research is supported in part by the Singapore Ministry of National Development and the National Research Foundation, Prime Minister’s Office under the Land and Liveability National Innovation Challenge (L2 NIC) Research Programme (L2 NIC Award No L2 NICTDF1-2015-2.). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not reflect the views of the Singapore Ministry of National Development and National Research Foundation, Prime Minister’s Office, Singapore. Acknowledgments: The support from JTC Corporation is deeply appreciated by the authors. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript. The funders have given approval to publish the results.

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