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1. Introduction

In November 2004, IHI Marine United Inc. (hereinaftercalled IHIMU) delivered the world’s first LPG FloatingProduction, Storage and Offloading (LPG-FPSO) systemto the owner, Single Buoy Moorings Inc. (hereinaftercalled SBM) in Monaco (Fig. 1). This LPG-FPSO isinstalled in Sanha Field, about 50 km off the Republic ofAngola, about 60 m deep, by means of single-pointmooring and receives through pipelines the associatedgasses of propane and butane that are produced togetherwith crude oil from the sea bottom. Then, LPG-FPSOseparates the associated gas into pure propane and butane,with the process plant on the deck and cools them with therefrigeration unit, then stores them in tanks in the hull, andoffloads the LPG into LPG carriers assigned from time totime.

The LPG FPSO was ordered on the basis of the hightechnology of IHI-SPB (Self-supporting Prismatic shapeIMO type B) tanks, our record in the construction of manycrude oil FPSO and FSO (Floating Storage and Offloadingsystems) and the high evaluation of LPG-FSO (Escravos),which has been operating continuously off Nigeria since1997(1)-(3) without shutdown since its delivery by us. (IMOstands for International Maritime Organization.)

This LPG-FPSO was delivered to SONASING, a jointventure between SBM and a public oil corporation that is

run by the Angolan government (SONANGOL), and as ofMay 2005, LPG-FPSO has been under trial operationusing actual associated gasses in the aforementioned seaarea where it is installed.

2. Background of development/construction

The associated gasses (gasses such as propane and butane)produced together with the crude oil have beentraditionally burnt on the vent tower or returned into theoriginal oil field to keep the natural flow from the oil/gas

AWASHIMA Yuji : Manager, Engineering, IHI Marine United Inc. AOKI Eiji : Technical Executive, IHI Marine United Inc.

ISHIKAWA Hiroki : Manager, Production Administration Department,Kure Shipyard, IHI Marine United Inc.

WATANABE Kazuo : Manager, Ship & Offshore Design Department, Kure Shipyard, IHI Marine United Inc.

MORITA Takashi : Manager, Ship & Offshore Design Department, Kure Shipyard, IHI Marine United Inc.

Development and Construction of LPG-FPSO

IHI Marine United Inc. has developed, designed and constructed the world’s first LPG-FPSO, with the largeststorage capacity as a marine storage facility, by applying IHI’s excellent technology, design capability and highlyqualified construction skills. To meet the unique requirements of the LPG-FPSO, e.g. long operability at a site farfrom dry docks, reduced vessel motions, cargo transfer between floaters, easiness of maintenance, high safety,etc., IHI has applied serveral unique technologies such as the IHI-SPB tank system. Continuous operation of theLPG-FPSO to commercialize the associated petroleum gas field will contribute toward reduction of CO2 emissionsto moderate climate change. The outline of the LPG-FPSO is described.

Fig. 1 Overall view of LPG-FPSO

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field. Recently, however, from the viewpoints ofenvironmental issue, CO2 reduction, and effectiveutilization of energy resources, attention has been paid tothe movement of commercializing the associated gasses asliquefied gas and transporting it to consumption areas. Tothis end, gas separating equipment, reliquefaction andrefrigerating units and storage tank facilities are required.

Various systems are available for this purpose, such asa system utilizing offshore platforms with land tanks and asystem in which all facilities are installed on land only.Among them, the LPG-FPSO having storage tanks in thehull structure and plant and reliquefaction/refrigeratingunits on the deck allows maximum cost reduction. Weexpect that its concept will increase in the future.Especially, this LPG-FPSO system can be utilized inmarginal sea areas such as deep sea areas far away fromthe land.

3. Required specifications and applicabletechnologies

The main specifications and applicable technologiesrequired for realizing this LPG-FPSO are as follows.3.1 High reliability to realize continuous operation

without docking for 20 yearsWe adopted cargo tanks of the IHI-SPB system to assurethe reliability over the life of the LPG-FPSO and alsoadopted materials and paints superior in durability forvarious units of equipment. As to the structural strengthincluding hull, tanks, and plant parts, we confirmedadequate strength by conducting analyses based on the seaconditions of the North Atlantic Ocean, which is severerthan those of the sea area of installation.3.2 Technologies to avoid plant shutdownIn addition to the high specifications and high redundancyof the plant itself, we were requested to prevent the plantperformance from deteriorating even in large waves thatare experienced once in 10 years. For this reason, wepursued a type of hull shape to minimize hull motionbased on motion analysis and water tank testing. As aresult, we developed the optimum type of hull shape withthe rolling angle within 5 degrees. To reduce the hullmotion even in a combination of winds and wavesexpected, we installed a 3 000 kW-class thruster formaintaining the LPG FPSO direction. The generators andother utilities were provided with sufficient redundancy. 3.3 Easy maintenance on the oceanWith the floating facility, many units of equipment arearranged on a limited area. To facilitate maintenance onthe ocean, we fully investigated the arrangement elementssuch as securing carry-out routes and repair areas ofvarious units of equipment. As a result, we installed 5deck cranes and realized smooth maintenanceperformance through optimum arrangement. 3.4 Securing safetyAs to the plant part, cargo part, machinery part and related

equipment, piping, and apparatuses, we studied safetysystems called HAZID (Hazard Identification) andHAZOP (Hazard and Operability) together with theowners. We took various measures including pipearrangement with less accumulated damage, which willallow emergency venting to discharge LPG. And weprovided heat-resistance for pressure vessels, and installedblast-reducing panels on refrigeration/reliquefactionrooms and designed strong accommodation structureassuming an explosion at the plant part.3.5 Challenge toward larger typeThis LPG-FPSO has the largest cargo capacity ofany exist ing LPG ships in the world (maximumabout 85 000 m3). We paid detailed attention in thedesign analysis and construction.

4. Specifications of LPG-FPSO

4. 1 OutlineThe principal particulars are shown below and the generalarrangement is shown in Fig. 2.

Length overall (Loa) 262.65 mLength between perpendicular (Lbp)

230.00 mBreadth (Mould) 49.00 mDepth (Mould) 29.30 mDesign draft (Mould) 13.20 mScantling draft (Mould) 13.20 mCargo storage capacity (-50 deg. C)

135 000 m3

Cargo production capacity 5 940 m3/dCargo offloading capacity 5 400 m3/hDeadweight Approx. 93 000 tInternational gross tonnage 111 246International net tonnage 33 374

The cargo tank consists of 6 tanks having a capacity of135 000 m3 in total at -50 deg. C, and all the tanks can beloaded with both propane and butane. The fore endportion of the hull is provided with single point mooring(turret) and accommodations for 60 persons. On the deck,from the fore portion of the hull, propane refrigeration unitroom, propane/butane separating plant (depropanizer), andbutane refrigeration unit/reliquefaction unit room arearranged. At the stern, the LPG discharging process venttower is installed.

The flag is Bahamas and the classification is ABS(American Bureau of Shipping).4.2 Plant & cargo partThe propane/butane mixture of room temperature/highpressure (18 deg. C, 22 bar) is introduced into this LPG-FPSO through undersea pipelines and risers and separatedinto propane and butane by the depropanizer. Figure 3shows the process flow.

Within the plant, the mixture is adjusted to 6.2 bar, 18deg. C by the LPG surge vessel, and the sulfur content isremoved by the H2S scrubber, and then it is heated to 30

1 1

deg. C and separated into propane and butane of 17 bar,100 deg. C by the fractionating column. The separatedpropane and butane are cooled to -40 deg. C and -10 deg.C, respectively, in the refrigeration unit room and storedas liquefied gas (LPG) in the cargo tanks. Therefrigeration units for propane and butane have thecapacity of 50% of necessary capacity × 3 units and 100%× 2 units, respectively.

The LPG in the tanks is vaporized due to heat ingressfrom outside, but liquefied by the reliquefaction units andreturned into the tanks. The reliquefaction units have a

capacity of 100% of necessary capacity × 2 units forpropane, 100% × 1 unit for propane/butane, and 100% × 1unit for butane. As the refrigerant, propane is used. Thedepropanizer was made by NIKKI K.K. (JGC) in Japanand the refrigeration/reliquefaction equipment by HGS(Hamworthy Gas Systems AS) in Norway.

The LPG in the tanks is offloaded into LPG carriersassigned from time to time by a maximum of 10 cargopumps. The pressurized butane can also be offloaded at arate of 550 m3/h.

Fig. 2 General arrangement

Fig. 3 Process flow of plant part

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Fig. 5 Structural model of plant part and hull part for FEM analysis

4.3 Hull structure & tank partAs to the structure including hull, tanks, and plant deck,the maximum stress and fatigue analyses (design life 30years) were conducted assuming the marine conditions ofNorth Atlantic (return period 20 years), marine conditionsat the site (return period 100 years), and marine conditionswhen towing, and sufficient strength was provided.

As aforementioned, the tanks are based on the IHI-SPBsystem, a product of our design and constructionphilosophy, and are completely different from theconventional self-supporting prismatic shape IMO type ALPG ships. Figure 4 shows the typical cross section ofthe SPB tank. There has been so far no reports of damageamong tanks of the IHI-SPB system of LPG/LNG carriers,or LPG-FSO constructed by IHIMU. Owners consider anoptimum system for FPSO required of continuousoperation over a long period of time like this LPG-FPSO.

With this LPG-FPSO, the depropanizer plant,refrigeration unit room and reliquefaction unit room areinstalled about 8 m or more above the deck. Since they arevery large structures, it is not proper to design themseparately from the hull. So, the structural analysis ofthese large structures was made integral with the hull deckportion. Figure 5 shows the entire analysis model.Especially for the depropanizer plant installed on the hullthat has motion in waves, unlike land plants, IHIMUconducted detailed strength checking, including fatiguestrength.4.4 Hull partWhen LPG is offloaded, an LPG carrier (maximum85 000 m3) is moored alongside the LPG-FPSO, and thefenders, winches and quick release hooks necessary forthe mooring are installed on the deck. When thespecifications of this equipment were decided, weestimated the relative motion of this LPG-FPSO and LPGcarriers based on sea conditions for LPG offloading using

an analysis program developed by IHIMU to design forsecuring safe operation.(4) In preparation for futureincrease in transporting volume, the hull aft portion isprovided with tandem type mooring equipment and cargohandling equipment to cope with LPG carriers of amaximum 125 000 m3 class.

The piping design was made optimum using the three-dimensional CAD system “Ajisai” that has beendeveloped independently by IHIMU, as in the case of thehull design. Figure 6 shows the typical three-dimensionalpiping arrangement on the deck.

The 60-person accommodations with luxurious interiorsassure comfortable living even for a long stay. The safetyis strengthened by adopting walls, windows and doors ofH-class structural fire protection (H-60). In designing thehelicopter deck installed at the top of the accommodationspace, tunnel tests were conducted by Research &Development Division (Yokohama) of Ishikawajima-Harima Heavy Industries Co., Ltd. (hereinafter called IHI)to investigate the effects of the gust and exhaust gas from

Deck Deck girderAnti-rolling chock

Anti-flotation chock

Water ballast tank

Horizontal girder

Insulation

Access space

SupportCL

Fig. 4 Cross section of IHI-SPB tank

Depropanizer Plant

No.4 Deck Crane

Ref./ Reliq. Unit Room

No.6 Hold

No.5 Hold

No.4 Hold

No.3 Hold

Fig. 6 Three dimensional piping arrangement on deck

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the funnel, thereby realizing the optimum shape/arrangement.Figure 7 shows the tunnel testing.4.5 Machinery partTo cover all the necessary power supplies, boilersfiring marine gas oil/fuel gas (90 t/h ¥ 3 units),turbine generators (9 000 kW ¥ 3 units), a dieselgenerator (3 000 kW ¥ 1) and an emergency generator(850 kW ¥ 1 unit) are equipped. As other utilities, aninert gas generator, cooling water pumps (50% ¥ 3 units),fire pumps (50% ¥ 2 units), deluge pumps (50% ¥ 2units), and emergency fire pumps (50% ¥ 2 units) areequipped.4.6 Electrical & instrumentation partThis LPG-FPSO is equipped with a huge computer systemfor centralized control of the entire LPG-FPSO (Monitorand Control System: M&CS), which is located at theFPSO control center (FCC) (Fig. 8). The M&CS consistsof a process control system to automatically control andmonitor units of equipment and safety shutdown systemfor alarming and emergency shutdown.

Figure 9 shows the M&CS system diagram. With thisM&CS, continuous operation is realized by a minimumnumber of persons.

5. Construction

The main hull and cargo tanks were constructed at theIHIMU Kure Shipyard, depropanizer at IHI Kure-ShinguWorks, accommodation quarters at IHI AMTEC Co., Ltd.,and pipe racks on the deck at IHI Aioi Works, and finally,they were installed on this LPG-FPSO at IHIMU KureShipyard. Especially the turret, depropanizer plant and

Fig. 7 Tunnel test

Fig. 8 FPSO control center (FCC)

FPSO Control Center ( FCC )

Plant Infomation Network

Reliquefaction Refrigeration

CS3000 node

PLC for #1 propane Ref. Compressorcontrol

PLC for #1 propane Reliq. compressorcont.

PLC f or # 1boiler c ont.

Protec. relay forMVSW G001

ProsafeCP U/node

Prosaferemotenode

R1

R4

C4

C5

C1

C2

C3

Depropanizer

FIRE DETECTOR

HIS 8 HIS 7

*1

MulcomMulcom

YNT

176

177

PC PC

ExaquantumHIS 2 HIS 1

CS3000node

FOR

FOR

FOR

GAS DETECTOR

CS3000 node

FOR

ProsaefRemotenode

ProsaefRemotenode

CS3000node

：Process Control System

：Safety Shutdown System

EXT Alarm

Machinery Room

Mach. RemoteCont. Center ( MRCC )

Switch GearRoom ( SWGR )

PA/GA

ProsafeCPU/node

ProsafeCPU/node

ProsafeCPU/node

Opt

ical

Fibe

rCab

le

PLC for #2 propane Ref. Compressorcontrol

PLC for #3 propane Ref. Compressorcontrol

PLC for #4 butane Reliq. compressorcont.

PLC for #4 butane Ref. compressorcontrol

PLC for #5 butane Ref. compressorcontrol

…

……

…

…

…

…

…

…

…

Fig. 9 System diagram of monitoring and control system (M&CS)

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accommodation quarters were installed integrally usingthe floating crane. Figure 10 and 11 show the tankconstruction and plant installation, respectively.

The construction was started in November 2002. Theturret, accommodation quarters, and depropanizer plantwere installed from November 2003, and variousperformances were adjusted and checked, and then at thenaming ceremony in June 2004, the ship was named“SANHA LPG-FPSO.” In October, the gas trial wascompleted, and delivery was made in November 2004.Subsequently it was towed to the site, and since May2005, has been under final adjustment and performance

testing at the sea area of installation off Angola.Concerning not only the design of FPSOs but also

construction and quality control, the applicable rules andowner’s requirements were different between the hull partand plant part. Therefore, adjustment and observationwere part of our efforts. Since this LPG-FPSO wasdesigned and constructed incorporating specifications andsafety check items of an international consortiumincluding Chevron Corporation (U.S.A.), the field projectimplementing body, we also endeavored to control theschedules of the construction.

6. Conclusion

This FPSO is the world’s largest LPG floating system inthe field including LPG ships as well as the world firstLPG-FPSO. We carried out the whole project from thedevelopment to design and construction in a short periodof 33 months from the contract to the delivery, utilizingthe total technology and construction capability ofIHIMU. We sincerely hope that this LPG-FPSO willcontinue its stable operation and contribute to theprogress of the owners and Republic of Angola andprevention of global warming. And we expect thatprojects based on the similar concept (LPG-FPSO, LNG-FPSO, etc.) that can utilize our experience of this LPG-FPSO will increase in the future.

REFERENCES

(1) M. Ximense, G. J. Adia and A. Abe : Design andConstruction of a Floating Storage and OffloadingVessel Escravos LPG FSO, SNAME TransactionsVol.105 (1997) pp.455-489

(2) E. Aoki, Y. Okumura, A. Abe, S. Yamashita and T.Ishiguro : Technological Innovation of SPB FloatingTerminal GASTECH’98 Proceeding (1998)

(3) Y. Okumura, A. Ando and O. Ushirokawa : Outlineand Features of SPB LNG Carrier Technology,Ishikawajima-Harima Engineering Review Vol.34No.4 July 1994 pp.235-240

(4) H. Yokozawa, E. Aoki, S. Yamashita and R. Ishii :Experimental Study on Relative Motions betweenLNG FPSO and LNG Carrier in Waves, 15th OceanEngineering Symposium in 2000, Japan Society ofNaval Architects and Ocean Engineers

Fig. 10 Tank construction

Fig. 11 Installation of topsides