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Design-Construction-Marketing Highlights of

World’s LargestPrestressed LPGFloating VesselArthur R. Anderson, ScDChairman of the BoardABAM Engineers IncorporatedConsulting EngineersTacoma, Washington

S y n o p s i sS y n o p s i sS y n o p s i sS y n o p s i sS y n o p s i s

The world’s largest precast prestressedconcrete floating liquefied petroleum gas(LPG) facility is described.Criteria for design and construction of the375,000-barrel floating facility are given.Construction procedures and prestressingmethodology are discussed.Problems related to the development andmarketing of prestressed concrete for marineconstruction, and prospects for futureapplications of prestressed concrete seastructures are presented.

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The world’s first large prestressedconcrete floating, totally offshore fa-

cility for liquefied petroleum gas (LPG),was recently installed at a permanentmooring in Indonesia.

Shown in Fig. 1, the 65,000-ton ves-sel was designed by ABAM Engineers,Inc. and constructed by Concrete Tech-nology Corporation, in Tacoma,Washington. It was then towed 10,000miles (16,000 km) across the PacificOcean to the Ardjuna oil and gas field inthe Java Sea (Fig. 2). This field is in anearthquake belt, where the water depthsrange from 100 to 135 ft (30 to 40 m).

LPG at +45 C (113 F) and pressurizedin underwater pipelines is transmitted to thefacility for cooling to -45 C (-49 F) and storedin 12 insulated steel tanks whose capacityis 375,000 barrels (30,000 metric tons).

The nearest coast, some 20 miles(approximately 32 km) to the south, ismarsh and mud terrain, unsuitable forheavy industrial construction. Althoughthe Java Sea is subjected to gales up to75 knots (138.9 km/hr), with sea wavesreaching 27 ft (8 m), the following fac-tors favored a floating structure:

Seismic loads on the structure andits complex equipment were avoided.Time and cost demands were lessthan those for land-based construc-tion.The total facility, including the hull,tanks, machinery, piping and electri-cal systems, was constructed at alocation near sources of technically-skilled manpower and support indus-tries, making it possible to deliver acomplete package.

Fig. 1. Conceptual model of LPG processing and storage facility onsingle-buoy mooring in the Java Sea.

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The following factors favored pre-stressed concrete for the hull constru-ction:

Lower initial cost.Less maintenance cost, since con-crete is corrosion-free in salt water.Periodic drydocking for inspection,repair and painting is unnecessary.Superior fire resistance.No problems associated with cyclicloading and fatigue.

Fig. 2. Location of the Ardjuna Field and operations diagram.

Marketing Aspects

For more than a century, the marineindustry has been iron-and-steel ori-ented. Only during times of plate short-age (the two world wars) has concretebeen seriously considered for largeships’ hulls. Although a number of con-crete ships performed well on the highseas, steel regained its exclusive statuswhen it again became available.

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In gaining acceptance for a pre-stressed concrete floating facility, anearly obstacle was removed when theowners received assurance from theAmerican Bureau of Shipping that itwould classify the vessel for insurancepurposes.

Ship classification societies haveserved a unique role in the design andconstruction of sea-going vessels formore than 150 years. Institutionssuch as Lloyds Registry (British), Bu-reau Veritas (French), Det NorskeVeritas (Norwegian) and the AmericanBureau of Shipping have formulated“rules” for ship design and construc-tion.

These societies also critically re-view vessel design for rules compli-ance as they relate to structural ad-equacy and seaworthiness. Their “sur-veyors” carry out detailed inspections,not only during construction but alsoperiodically during the service life ofeach vessel.

Before a vessel is deemed insur-able, the owner must obtain a docu-ment from the ship classification so-ciety, whose dr y - land counterpar twould be a combination of institutionslike the ACI and AISC for codes, struc-tural, electrical and mechanical engi-neering firms for design review, build-ing officials for approvals and permits,and inspection agencies for compli-ance.

Obviously, the ship classificationsocieties have tremendous authorityand responsibility in their role of pass-ing judgment on the insurance risksfor ships. Over the years, their ruleshave been largely based on past ex-perience (hindsight), and graduallyextrapolated into the future.

Thus , the in t roduct ion o f p re -stressed concrete as a hull materialposed some unusual problems re-garding classification, mainly because

it lacked sufficient seagoing experi-ence.

When comparing sea structures tobuildings and other land structures,one is immediately impressed with thecomplexity of loadings endured byships and other sea structures; notonly are the stress magnitudes moredifficult to quantify, but also they arefur ther complicated by mil l ions ofcycles of stress reversal encounteredduring service life.

Failures in steel ship structures dueto fatigue and stress concentrationshave caused the maritime authoritiesmany traumatic experiences. It is onlynatural that they would take a cau-tious approach to a brittle, low-tensile-strength material like concrete whichis susceptible to cracking. But unlikesteel plating, the cracks in prestressedconcrete close with the passing ofpeak loads.

The proposal for the prestressedconcrete vessel was made to the own-ers, a group of American companiesand Pertamina, the Indonesia State-owned oil company, with the Atlantic-Richfield Company as the operator.

Design Considerations

The following functional requirementsgoverned the design of the prestressedconcrete hull:

(a) Liquefaction by refrigeration of18,000 barrels of petroleumgas per day.

(b) Storage capacity of 375,000barrels of LPG.

(c) A complete installation of electricpower plant, refrigeration, piping,and instrumentation.

(d) Complete accommodations for a50-man crew.

(e) Crane service for maintenance.(f) Life boats and heliport.

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Fig. 3. Cross section of the concrete hull.

(g) Ber th ing and moor ing fortanker ships.

(h) Cargo transfer system.

The optimum geometry for LPG stor-age was determined to be 12 cylindri-cal tanks with hemispherical ends,and with a 38-ft (11.6-m) diameterand an overall length of 168 ft (51.2m). With these dimensions, an ar-rangement with six tanks below andsix tanks above deck was adopted.

Allowing for the electric power andrefrigeration plant abaft the tanks, thecrew’s accommodations forward, theoverall dimensions were established:

Length: 461 ft (140.5 m)Beam: 136 ft (41.5m)Depth 56.5 ft (17.2 m)The hull section was then devel-

oped to satisfy functional require-ments, along with strength and vesselstability. For strength, the hull was re-

quired to satisfy two principal loadingconditions:

1. Local loads against the shellf rom hydrostat ic pressureheads up to 50 ft (16 m).

2. Global loading on the total hullfunctioning as a box girderpoised on waves up to 27 ft(8 .25 m) h igh and 461 f t(140.5 m) long (equal to thelength of the hull).

The hull bottom naturally evolvedinto three cylindrical barrel shells (Fig.3) whose shape efficiently functionsto resist a 3,000 psf (14.6 T/m2) hy-drostatic pressure, and also reducesthe vertical span for side shell pres-sure.

To minimize the transverse bendingmoments in the deck, Y-shaped sidesand Y-shaped longitudinal bulkheadswere introduced. This configurationeliminated the necessity for trans-

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verse ribs and longitudinal stiffenersof the type used with steel hull con-struction.

To resist the longitudinal bendingcaused by the various combinations ofweights acting downward, and buoy-ancy forces acting upward, the hullfunctions as a multicell box girder, inwhich the fore and aft global bendingstresses and shears are accommo-dated unencumbered by the local hy-drostatic pressures against the bottomshel l . Moreover, transverse localbending stresses in the side shell anddeck are not directly additive to theglobal load stresses.

The development of design criteriafor the hull structure took into accountthe loadings for ships applicable tosteel vessels of comparable dimen-sions, as required by the rules of theAmerican Bureau of Shipping. Longi-tudinal hull girder bending stress lim-its were established for (1) Deliveryvoyage; (2) Normal service in the JavaSea; and (3) the 100-year storm.

The assumed wave heights andstress limits are given in Table 1:

Note that zero tension in the con-crete was stipulated for the deliveryvoyage and for normal service at themooring in the Java Sea. A conserva-tive policy was adopted regarding al-

lowable tension in the concrete underlongitudinal hull bending. This re-sulted in the ultimate load factorsgiven in Table 1.

In the case of local bending due tohydrostatic pressure, a tension of 5√f’cwas permitted, in recognition of thefact that, in plate bending, the crackswould not penetrate into the tendons,which were encased in steel tubesfilled with grout.

In addition to the stresses underservice conditions, stresses duringconstruction were analyzed to insurethe hull integrity during launching andsubsequent construction afloat.

Also, a special analysis was madeto determine the rate of roll of the ves-sel, and its effect on the tank founda-tions and certain items of equipment.Moreover, a special case of damagedstability was investigated for the caseof a collision and flooding of a below-deck compartment.

It was found that the vessel cansurvive a collision with one compart-ment fully flooded. In this case, how-ever, the vessel would heel to a 25-degree angle.

When compared with establishedshipyards, equipped with bui ldingways and graving docks, ConcreteTechnology Corporation had only one

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thing in common, a site located ondeep industrial waterway. Conse-quently, upon receipt of notice to pro-ceed, a graving dock 500 ft (152.5 m)in length and 160 ft (47.8 m) wide hadto be constructed.

Providing for a launching draft of 13ft (3.96 m) concurrently with the grav-ing dock construction, detail design ofthe concrete hull and production tool-ing was underway (Fig. 4).

Fig. 4. Graving dock and delivery of 40-ton bottom hull segment.

C o n s t r u c t i o n

The hull construction scheme isshown in Fig. 5. The hull bottom ismade of precast concrete she l lsegments, (a), which are delivered tothe graving dock, (b). Vertical sideshell and longitudinal bulkheads arecast in p lace , (c ) . The par t ia l l ycompleted hull is launched, (d). Whenaf loat , the lower s i x tanks areinstal led, (e) , and remaining hul lconcrete is cast in place, (f).

Dur ing the grav ing dockconstruction, bottom shell segmentswere match-cast in steel forms (Fig. 6)and stockpiled. Each element wasreinforced and provided with ducts forboth long i tud ina l and t ransversetendons. The 40-ton bottom shell

SIZE OF HULL ANDSIZE OF HULL ANDSIZE OF HULL ANDSIZE OF HULL ANDSIZE OF HULL ANDNUMBER AND DIMENSIONSNUMBER AND DIMENSIONSNUMBER AND DIMENSIONSNUMBER AND DIMENSIONSNUMBER AND DIMENSIONS

OF PRECOF PRECOF PRECOF PRECOF PRECASTASTASTASTASTCOMPONENTCOMPONENTCOMPONENTCOMPONENTCOMPONENTSSSSS

Size of StructureSize of StructureSize of StructureSize of StructureSize of Structure

Hull dimensions: 461 ft long x136 ft wide x 56.4 ft diameter

Concrete volume: 12,000 cu ydsPrestressing: 600 miles of 1/2-

in. diameter strandWeight: 65,000 tons displace-

ment (fully loaded)

NNNNNumber of dimensions of precastumber of dimensions of precastumber of dimensions of precastumber of dimensions of precastumber of dimensions of precastcomcomcomcomcomponentsponentsponentsponentsponents

Quant i ty of precast curvedshells: 120

Length: 11 ft 2 in.Overall width: 45 ftRise: 13 ft 2 in.

Midship bulkhead

Uniform thickness: 1 ft 4 in.Twelve precast plates, approxi-

mately 40 ft wide and 8 fthigh

Three crescent-shaped platesat keel

Bow plate

Three beams: 3 ft thick by 40ft long, haunched from 6 ftat midspan to 9 ft at ends.

Twelve plates 40 ft long by 8f t high with tapered crosssection from 18 to 24 in.

Three crescent-shaped platesat keel.

Three plates 6 ft high by 40 ftlong, pentagon-shaped crosssection.

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Fig. 5. Construction sequence.

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Fig. 6. Match-casting, precast concrete bottom shell segments.

Fig. 7. Erection of the bottom shell.

Fig. 8. Formwork for cast-in-place longitudinal bulkhead,and delivery pipeline forpumped concrete.

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Fig. 9. Reinforcingsteel and ductwork forpost-tensioned ten-dons.

Fig. 10. Bottom shell, longitudinal bulkheads and prestressedconcrete saddle tank supports.

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Fig. 11. Erection of precast midships bulkheads and constructionprogress on vertical front.

Fig. 12. Bottom rake construction at forward end.

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Fig. 13. Erection ofvertical shell elementat after end of hull.

Fig. 14. Aerial view of hull readying for launching.

Fig. 15. Float-out of the hull lower section.

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Fig. 16. 400-ton steel tank for LPG storage.

Fig. 17. Erection of steel tank into concrete hull.

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Fig. 18. Lower tier LPG tanks in place.

Fig. 19. Concreting of upper hull and deck with movable steel forms.

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Fig. 20. Stressing of U-shaped verticaltendons.

Fig. 21. Erection of precast saddles forabove-deck tanks.

Fig. 22. Completion of upper hull construction, and erection of precastvertical shell closure plates.

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Fig. 23. Histogram of 28-day cylinder compression strength tests, hullconcrete.

Fig. 24. Electric power plant and refrigeration module in place.

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Fig. 25. Departure of the 65,000-ton “Ardjuna Sakti” from Concrete TechnologyCorporation, on 10,000-mile delivery voyage to Indonesia.

Fig 26. Aerial view of completed vessel being towed to its destination inIndonesia for installation at a permanent mooring.

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members were placed on precast concretesupports which, in turn, were supportedby prestressed concrete piles (Fig. 7).

During assembly of the shellsegments, each joint was coated withepoxy adhesive, after which the segmentwas then promptly stressed to itsneighbor by means of Dywidag high-tensile thread bars.

Following immediately after erectionof the bottom shell came theconstruction of the vertical sides andlongitudinal bulkhead with cast-in-placeconcrete (Fig. 8). The transverse tendonducts projecting from the bottomsegments were coupled with those in thevertical sections (Fig. 9). Prior to closingthe form, a coating of epoxy adhesivewas applied to the hardened concretejoint surface, and shortly thereafter, thenew concrete was cast against it.

Erection of bottom shell segments,followed by cast-in-place vertical shellsand longitudinal bulkheads, proceededon a vertical front in a routine manner.The only exception was for a change indetail at the tank saddles (Fig. 10) andat the midship section, where verticalprecast elements were introduced forthe amidship bulkheads (Fig. 11).

A 45-deg rake to the bottom shell atthe forward end was provided to reducetowing resistance on the 10,000-miledelivery voyage (Fig. 12).

Concreting of the lower 40 ft (12 m)of the hull and erection of the end shellmembers (Fig. 13) made the hull readyfor launching (Fig. 14).

The dock was then flooded andopened, and with the aid of tugs, the hullwas moved to the outfitting pier (Fig. 15).

Concurrently with hull constructionwork on the tanks, refrigeration andelectrical plant and crew’s housingstructure was underway nearby. Thefabrication and insulation of the 12tanks was an operation of the sameorder of magnitude as the hull

construction. This work was done byAmerican Bridge Company at the ConcreteTechnology Corporation site (Fig. 16).

The 400-ton tanks were lowered intothe hull by a pair of stiff-leg derricksinstalled especially for this purpose (Fig.17). Each tank was seated on end-graincedar pads built into the concretesaddles, and then securely strappeddown (Fig. 18).

The tanks at -45 C (-49 F) were thusinsulated from the concrete. In addition,the tanks were insulated with a heavylayer of butyl-covered polyurethanematerial.

Af ter tank placement, the upperportion of the hull was cast in placeutilizing movable steel forms (Fig. 19).Starting at the after end the concretingprogressed forward, using the sametechniques for tendon alignment andepoxy bonding at the construction joints.

When the concreting of the upper hulland deck structure had reachedmidship, erection of the upper-tier tanksaddles commenced. These heavilyreinforced elements were precast ontheir side and post-tensioned with U-shaped tendons. The 1,200-kip (545metric ton) final force in the circular pathprovided the desired safeguard againstcracking during tilt-up and erection ofthe saddle element (Fig. 20). Afterplacement on deck, additional tendonswere installed and post-tensioned for theconnection to the hull.

Upper hull post-tensioning followedclosely behind the concreting.Horizontal tendons stressed the deckand bulkheads, both fore and aft andathwar tships. U-shaped tendonsanchored in the deck were stressedsimultaneously at both ends, providingtransverse compression to thelongitudinal bulkheads, side shells andbottom shell elements (Fig. 21).

Most tendons consisting of 6-, 8-, and12-in. 270-kip, seven-wire strands were

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stressed and anchored, utilizing theAnderson system. Approximately 600miles (about 1,000 km) of 1/2-in. 270-kip strand wire was used in the concretehull structure.

When the concrete deck wascompleted, the ends were closed in byerection of the precast vertical panels.(Fig. 22).

Quality control of the constructionwas r igorous and under constantengineering supervision. Specialattention was given to the concrete mix,in which a water-cement ratio was heldto 0.4 or less.

As can be seen from Fig. 23, anaverage 28-day cylinder compressionstrength of 9,800 psi (66 MPa) with acoefficient of variation of 6 percent wasachieved.

The mechanical, piping, and electricalinstallations were subcontracted to firmsspecializing in services to the marineand petroleum industry. A 500-tonmachinery module was preassembledand erected on the after end as apackage unit (Fig. 24).

Concurrently, a multistory housingmodule for a 50-man crew wasconstructed. It was moved to the edgeof a nearby wharf. The hull was nosedinto the wharf with tugs, and the 600-ton module was rolled on board andbolted securely to its foundation.

Fig. 25 shows the vessel completedand ready for departure on the 100-day,10,000-mile (16,000 km) deliveryvoyage. Aerial shots of the vessel beingtowed to its destination in Indonesia areshown in Figs. 26 and 27.

Concluding Remarks

The potential for utilizing prestressedconcrete in marine construction is causefor optimism.

Ship classification societies and gov-ernment regulatory agencies must bepersuaded that, when properly designedand constructed under appropriate qual-ity control, prestressed concrete shouldbecome a preferred (or at least equallyacceptable) material for hull construc-tion.

However, prestressed concrete mightnot be competitive for hulls carryingcargo on long voyages because the pay-load/total weight or dead weight/dis-placement, in nautical language ratio issubstantially less with concrete hullscompared to steel construction. Thiswas clearly demonstrated with the self-propelled concrete ships built and oper-ated during the periods of 1917-1920and 1942-1945.

When used as barges or floatingstructures, mainly on stationary serviceafloat, the advantages for prestressedconcrete versus steel become obvious.Among these advantages, the followingmust be recognized:

Lower initial construction cost.Superior durability in sea water en-vironment.Ductile behavior when severely over-loaded.Freedom from damage under fa-tigue-type loads.Excellent properties at extremely low(cryogenic) temperatures.Superior behavior when exposed tofire.Easy to repair when damaged by col-lision, etc.Drydocking at regular intervals forinspection, repair, and maintenancenot necessary.

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The concrete industry lacks a long,continuous track record for sea-goingvessels . Never theless, a few ex -amples, some old and some recent,are impressive:

11111..... A breakwater at Powell River,Canada, consists of 10 concrete ships,one built in 1918 and nine built in1942-1944. None have beendrydocked, nor has any money beenspent on maintenance.

2.2.2.2.2. About 12 years ago Alfred Yeedesigned and constructed several dryand l iqu id cargo barges made ofpretensioned concrete. These 200-ft(approximately 60 m) barges, with a200- ton DW T capac i ty, have per-formed excellently in the SoutheastAsia area during the last 10 years.

3.3.3.3.3. Several large North Sea oil struc-tures are now in place. Despite severestorms, they have performed well.

C r e d i t s

Structural Engineer: ABAM Engineers Inc.,Tacoma Washington.

General Contractor-Precaster-Prestresser:Concrete Technology Corporation,Tacoma, Washington.

Seller: Concrete Energy Systems, Inc., ajoint company consisting of ConcreteTechnology Corporation and Trans-Energy International Inc., Hamilton,Bermuda.

Owner: Atlantic Richfield Indonesia, Inc.,and the Per tamina Group, agovernment-owned Indonesian oilCompany, Djakarta, Indonesia.

Fig. 27. Aerial view of completed vessel being towed to Indonesia.

PCI JOURNAL

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This paper is reprinted from the copyrightedJOURNAL of the Prestressed Concrete Institute

V. 22, No. 1, Jan.-Feb. 1977.

BERGER/ABAM Engineers Inc.33301 Ninth Avenue South, Suite 300Federal Way, Washington 98003-2600

206/431-2300

JR183