Design-Construction-Marketing Highlights of

World's LargestPrestressed LPGFloating Vessel

Arthur R. Anderson, ScDChairman of the BoardABAM Engineers IncorporatedConsulting EngineersTacoma, Washington

SynopsisThe 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.

12

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

T he world's first large prestressedconcrete floating, totally offshore

faci!ity for liquefied petroleum gas(LPG), was recently installed at a per-manent mooring in Indonesia.

Shown in Fig. 1, the 65,000-tonvessel was designed by ABAM En-gineers, Inc., and constructed byConcrete Technology Corporation, inTacoma, Washington. It was thentowed 10,000 miles (16,000 km)across the Pacific Ocean to the Ard-juna oil and gas field in the Java Sea(Fig. 2). This field is in an earthquakebelt, where the water depths rangefrom 100 to 135 ft (30 to 40 m).

LPG at +45 C (113 F) and pressur-ized in underwater pipelines is trans-mitted to the facility for cooling to—45 C (-49 F) and stored in 12 insu-lated steel tanks whose capacity is375,000 barrels (30,000 metric tons).

The nearest coast, some 20 miles(approximately 32 km) to the south,is marsh and mud terrain, unsuitablefor heavy industrial construction. Al-though the Java Sea is subjected togales up to 75 knots (138.9 km/hr),with sea waves reaching 27 ft (8 m),the following factors favored a float-ing structure:• Seismic loads on the structure and

its complex equipment were avoid-ed.

• Time and cost demands were lessthan those for land-based con-struction.The total facility, including the hull,tanks, machinery, piping and elec-trical systems, was constructed ata location near sources of techni-cally-skilled manpower and sup-port industries, making it possibleto deliver a complete package.

PCI JOURNAL/January-February 1977 13

LPGOil Storage Barge I I iL__–J

SBM +

Junction Platform Quarters PlatformCentral Plant

ter------ --- - _ v

[E 'structurei

Q Compression Platform'L structure ^ B'structure Q Production Platform

I ® Flow StationK' structure Q Service Platform

--- Gas Pipeline

'U' structure _ Oil PipelineDIAGRAM OF ARDJUNA OPERATIONS

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

The following factors favored pre-stressed concrete for the hull con-struction:

• Lower initial cost.• Less maintenance cost, since con-

crete is corrosion-free in salt wa-te r.

• Periodic drydocking for inspection,repair and painting is unnecessary.

• Superior fire resistance.• No problems associated with cy-

clic loading and fatigue.

Marketing Aspects

For more than a century, the ma-rine industry has been iron-and-steeloriented. Only during times of plateshortage (the two world wars) hasconcrete been seriously consideredfor large ships' hulls. Although anumber of concrete ships performedwell on the high seas, steel regainedits exclusive status when it again be-came available.

14

In gaining acceptance for a pre-stressed concrete floating facility, anearly obstacle was removed whenthe owners received assurance fromthe American Bureau of Shippingthat it would classify the vessel forinsurance purposes.

Ship classification societies haveserved a unique role in the designand construction of sea-going ves-sels for more than 150 years. Institu-tions such as Lloyds Registry (Brit-ish), Bureau Veritas (French), Detnorske Veritas (Norwegian) and theAmerican Bureau of Shipping haveformulated "rules" for ship designand construction.

These societies also critically re-view vessel design for rules compli-ance as they relate to structuraladequacy and seaworthiness. Their"surveyors" carry out detailed in-spections, not only during construc-tion but also periodically during theservice life of each vessel.

Before a vessel is deemed insur-able, the owner must obtain a docu-ment from the ship classification so-ciety, whose dry-land counterpartwould be a combination of institu-tions like the ACI and AISC forcodes, structural, electrical andmechanical engineering firms for de-sign review, building officials for ap-provals and permits, and inspectionagencies for compliance.

Obviously, the ship classifica-tion societies have tremendous au-thority and responsibility in their roleof passing judgment on the insurancerisks for ships. Over the years, theirrules have been largely based onpast experience (hindsight), andgradually extrapolated into the fu-ture.

Thus, the introduction of pre-stressed concrete as a hull materialposed some unusual problems re-garding classification, mainly be-

cause it lacked sufficient seagoingexperience.

When comparing sea structures tobuildings and other land structures,one is immediately impressed withthe complexity of loadings enduredby ships and other sea structures;not only are the stress magnitudesmore difficult to quantify, but alsothey are further complicated by mil-lions of cycles of stress reversal en-countered during service life.

Failures in steel ship structuresdue to fatigue and stress concentra-tions have caused the maritime au-thorities many traumatic experiences.It is only natural that they would takea cautious approach to a brittle, low-tensile-strength material like con-crete which is susceptible to crack-ing. But unlike steel plating, thecracks in prestressed concrete closewith the passing of peak loads.

The proposal for the prestressedconcrete vessel was made to theowners, a group of American com-panies and Pertamina, the Indones-ian State-owned oil company, withthe Atlantic-Richfield Company asthe operator.

Design Considerations

The following functional require-ments governed the design of theprestressed concrete hull:

(a) Liquefaction by refrigeration of18,000 barrels of petroleum gasper day.

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

(c) A complete installation of elec-tric power plant, refrigeration,piping and instrumentation.

(d) Complete accommodations fora 50-man crew.

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

PCI JOURNAL/January-February 1977 15

Fig. 3. Cross section of the concrete hull.

(g) Berthing and mooring for tank-er ships.

(h) Cargo transfer system.The optimum geometry for LPG

storage was determined to be 12 cy-lindrical tanks with hemisphericalends, and with a 38-ft (11.6-m) diame-ter and an overall length of 168 ft(51.2 m). With these dimensions, anarrangement with six tanks belowand six tanks above deck was adopt-ed.

Allowing for the electric power andrefrigeration plant abaft the tanks,and the crew's accommodations for-ward, the overall dimensions wereestablished:

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

ed to satisfy functional requirements,along with strength and vessel stabil-ity. For strength, the hull was requir-

ed to satisfy two principal loadingconditions:

1. Local loads against the shellfrom hydrostatic pressure headsup 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) high and 461 ft(140.5 m) long (equal to thelength of the hull).

The hull bottom naturally evolvedinto three cylindrical barrel shells(Fig. 3) whose shape most efficientlyfunctions to resist a 3000 psf (14.6T1/m 2 ) hydrostatic pressure, and alsoreduces the vertical span for sideshell pressure.

To minimize the transverse bend-ing moments in the deck, Y-shapedsides and Y-shaped longitudinal bulk-heads were introduced. This config-uration eliminated the necessity fortransverse ribs and longitudinal stiff-

16

eners of the type used with steel hullconstruction.

To resist the longitudinal bendingcaused by the various combinationsof weights acting downward, andbuoyancy forces acting upward, thehull functions as a multicell box gir-der, in which the fore and aft globalbending stresses and shears are ac-commodated unencumbered by thelocal hydrostatic pressures againstthe bottom shell. Moreover, trans-verse local bending stresses in theside shell and deck are not directlyadditive to the global load stresses.

The development of design criteriafor the hull structure took into ac-count the loadings for ships appli-cable to steel vessels of comparabledimensions, as required by the rulesof the American Bureau of Shipping.Longitudinal hull girder bendingstress limits were established for (1)Delivery voyage; (2) Normal servicein the Java Sea; 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 regardingallowable tension in the concrete un-

der longitudinal hull bending. This re-sulted in the ultimate load factorsgiven in Table 1.

In the case of local bending dueto hydrostatic pressure, a tensionof 5V/f'- was permitted, in recogni-tion of the fact that, in plate bend-ing, the cracks would not penetrateinto the tendons, which were en-cased in steel tubes filled with grout.

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

Also, a special analysis was madeto determine the rate of roll of thevessel, and its effect on the tankfoundations and certain items ofequipment. Moreover, a special caseof damaged stability was investigatedfor the case of a collision and flood-ing 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 buildingways and graving docks, ConcreteTechnology Corporation had onlyone thing in common, a site locatedon deep industrial waterway. Conse-

Table 1. Assumed Wave Heights and Stress Limits,ARCO Facility.

DeliverySea Conditions Voyage

NormalService

100-YearStorm

Wave height 23 ft 11 ft 27 ftMax. allowed Zero tension Zero tension -

stress 0.45 f' compr. 0.45 F. compr. -Cracking load

factor 1.65 2.00 -Ultimate load factors:

Required 2.0 2.6 1.3Actual 2.1 3.5 2.0

PCI JOURNAL/January-February 1977 17

Fig. 4. Graving dock and delivery of40-ton bottom hull segment.

quently, upon receipt of notice toproceed, a graving dock 500 ft (152.5m) in length and 160 ft (47.8 m) widehad to be constructed.

Providing for a launching draft of13 ft (3.96 m) concurrently with thegraving dock construction, detail de-sign of the concrete hull and produc-tion tooling was underway (Fig. 4).

Construction

The hull construction scheme isshown in Fig. 5. The hull bottom ismade of precast concrete shell seg-ments, (a), which are delivered tothe graving dock, (b). Vertical sideshell and longitudinal bulkheads arecast in place, (c). The partially com-pleted hull is launched, (d). Whenafloat, the lower six tanks are in-stalled, (e), and remaining hull con-crete is cast in place, (f).

During the graving dock construc-tion, bottom shell segments werematch-cast in steel forms (Fig. 6) andstockpiled. Each element was rein-forced and provided with ducts forboth longitudinal and transverse ten-dons. The 40-ton bottom shell mem-bers were placed on precast con-

SIZE OF HULL ANDNUMBER AND DIMENSIONS

OF PRECASTCOMPONENTS

Size of StructureHull dimensions: 461 ft long x

136 ft wide x 56.4 ft diameterConcrete volume: 12,000 cu ydsPrestressing: 600 miles of ch-

in. diameter strandWeight: 65,000 tons displace-

ment (fully loaded)

Number and dimensions ofprecast components

® Quantity of precast curvedshells: 120

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

• Midship bulkheadUniform thickness: 1 ft 4 in.Twelve precast plates, ap-

proximately 40 ft wide and8 ft high

Three crescent-shapedplates at keel

• Bow plateThree beams: 3 ft thick by

40 ft long, haunched from6 ft at midspan to 9 ft atends.

Twelve plates 40 ft long by8 ft high with taperedcross section from 18 to24 in.

Three crescent-shapedplates at keel.

Three plates 6 ft high by 40ft long, pentagon shapedcross section.

18

a. Match Castings of Shell Segments

e: Placing Hull Tanks

b. Setting of Shells in Graving Dock

f. In-place Casting of "s" Platesand Deck

c. In-place Casting of Longitudinal Bulkheads,Saddles and Transverse Bulkheads

d. Launching Stage g. Placing Deck Tanks and FinalOutfitting

Fig. 5. Construction sequence.

PCI JOURNAL/January-February 1977 19

rig. b. Match-casting, precast concrete bottom shell segments.

Fig. 7. Erection of the bottom shell.

Fig. 8. Formwork forcast-in-place longitudinalbulkhead, and delivery pipe-line for pumped concrete.

Fig. 9. Reinforcing steeland ductwork for post-tensioned tendons.

Fig. 10. Bottom shell, longitudinal bulkheads and prestressed concrete sad-dle tank supports.

PCI JOURNAL/January-February 1977 21

Fig. 11. Erection of precast midships bulkheads and construction p1u9Ic**on vertical front.

Fig. 12. Bottom rake construction at rorwaru e,nu.

22

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

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

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

PCI JOURNAL/January-February 1977 23

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

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

Fig. 18. Lower tier LPG tanks in place.

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

Fig. 20 Stressing of U-shapedvertical tendons.

Fig. 21. Erection of precast saddlesfor above-deck tanks.

Fig. 22. Completion of upper hull construction, and erection of precast ver-tical shell closure plates.

4-

0

z

N/mm2

55 62 69 76 8^50

X = 9.8 KSIO'= 0.6 KSIV2 6%

10-= 66 N/mm

O' 8 4.1 N/mmV 2 6%

30

20

1

ILIIII Iii8 9 10 11 1.

28-day Strength – KSI

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

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

Fig. 25. Departure of the 65,000-ton "Ardjuna Sakti" from Concrete Tech-nology Corporation, 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.

crete supports which, in turn, weresupported by prestressed concretepiles (Fig. 7).

During assembly of the shell seg-ments, each joint was coated withepoxy adhesive, after which the seg-ment was then promptly stressed toits neighbor by means of Dywidaghigh-tensile thread bars.

Following immediately after erec-tion of the bottom shell came theconstruction of the vertical sides andlongitudinal bulkhead with cast-in-place concrete (Fig. 8). The trans-verse tendon ducts projecting fromthe bottom segments were coupledwith those in the vertical sections(Fig. 9). Prior to closing the form, acoating of epoxy adhesive was ap-plied to the hardened concrete jointsurface, and shortly thereafter, thenew concrete was cast against it.

Erection of bottom shell segments,followed by cast-in-place verticalshells and longitudinal bulkheads,proceeded on a vertical front in aroutine manner. The only exceptionwas for a change in detail at the tanksaddles (Fig. 10) and at the midshipsection, where vertical precast ele-ments were introduced for the amid-ship bulkheads (Fig. 11).

A 45-deg rake to the bottom shellat the forward end was provided toreduce towing resistance on the 10,-000-mile delivery voyage (Fig. 12).

Concreting of the lower 40 ft (12 m)of the hull and erection of the endshell members (Fig. 13) made thehull ready for launching (Fig. 14).

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

Concurrently with hull construc-tion work on the tanks, refrigerationand electrical plant and crew's hous-ing structure was underway nearby.Tho fabrication and insulation of the

12 tanks was an operation of thesame order of magnitude as the hullconstruction. This work was done byAmerican Bridge Company at theConcrete Technology Corporationsite (Fig. 16).

The 400-ton tanks were loweredinto the hull by a pair of stiff-leg der-ricks installed especially for this pur-pose (Fig. 17). Each tank was seatedon end-grain cedar pads built into theconcrete saddles, and then securelystrapped down (Fig. 18).

The tanks at —45 C (-49 F) werethus insulated from the concrete. Inaddition, the tanks were insulatedwith a heavy layer of butyl-coveredpolyurethane material.

After tank placement, the upperportion of the hull was cast in placeutilizing movable steel forms (Fig.19). Starting at the after end theconcreting progressed forward, usingthe same techniques for tendonalignment and epoxy bonding at theconstruction joints.

When the concreting of the upperhull and deck structure had reachedmidship, erection of the upper-tiertank saddles commenced. Theseheavily reinforced elements wereprecast on their side and post-ten-sioned with U-shaped tendons. The1200-kip (545 metric ton) final forcein the circular path provided the de-sired safeguard against cracking dur-ing tilt-up and erection of the saddleelement (Fig. 20). After placement ondeck, additional tendons were in-stalled and post-tensioned for theconnection to the hull.

Upper hull post-tensioning fol-lowed closely behind the concreting.Horizontal tendons stressed the deckand bulkheads, both fore and aft andathwartships. U-shaped tendons an-chored in the deck were stressed si-multaneously at both ends, providingtransverse compression to the longi-

PCI JOURNAL/January-February 1977 29

tudinal bulkheads, side shells andbottom shell elements (Fig. 21).

Most tendons consisting of 6, 8and 12-in. 270-kip seven-wire strandswere stressed and anchored, utilizingthe Anderson system. Approximately600 miles (about 1000 km) of 1h -in.270-kip strand wire was used in theconcrete hull structure.

When the concrete deck was com-pleted, the ends were closed in byerection of the precast vertical pan-els (Fig. 22).

Quality control of the constructionwas rigorous and under constant en-gineering supervision. Special atten-tion was given to the concrete mix, inwhich a water-cement ratio was heldto 0.4 or less.

As can be seen from Fig. 23, an av-erage 28-day cylinder compressionstrength of 9800 psi (66 MPa) with a

coefficient of variation of 6 percentwas achieved.

The mechanical, piping and electri-cal installations were subcontractedto firms specializing in services to themarine and petroleum industry. A500-ton machinery module was pre-assembled and erected on the afterend as a package unit (Fig. 24).

Concurrently, a multistory housingmodule for a 50-man crew was con-structed. It was moved to the edge ofa 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 vesselbeing towed to its destination in In-donesia are shown in Figs. 26 and 27.

Concluding Remarks

The potential for utilization of pre-stressed concrete in marine con-struction is cause for optimism.

Ship classification societies andgovernment regulatory agenciesmust be persuaded that, when prop-erly designed and constructed underappropriate quality control, pre-stressed concrete should become apreferred (or at least equally accept-able) material for hull construction.

However, prestressed concretemight not be competitive for hullscarrying cargo on long voyages be-cause the payload/total weight ordead weight/displacement, in nauti-cal language ratio is substantiallyless with concrete hulls compared tosteel construction. This was clearlydemonstrated with the self-propelledconcrete ships built and operatedduring the periods 1917-1920 and1942-1945.

When used as barges or floatingstructures, mainly on stationary ser-vice afloat, the advantages for pre-stressed concrete versus steel be-come obvious. Among these advan-tages, the following must be recog-nized:

• Lower initial construction cost.• Superior durability in sea water

environment.• Ductile behavior when severely

overloaded.• Freedom from damage under fa-

tigue-type loads.• Excellent properties at extreme-

ly low (cryogenic) temperatures.• Superior behavior when exposed

to fire.• Easy to repair when damaged by

collision, etc.• Drydocking at regular intervals

for inspection, repair and main-tenance not necessary.

30

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

The concrete industry lacks a long,continuous track record for sea-go-ing vessels. Nevertheless, a few ex-amples, some old and some recent,are impressive:

1. A breakwater at Powell River,Canada, consists of ten concreteships, one built in 1918 and ninebuilt in 1942-1944. None have beendrydocked, nor has any money beenspent on maintenance.

2. About 12 years ago Alfred Yeedesigned and constructed severaldry and liquid cargo barges made ofpretensioned concrete. These 200-ft (approx. 60m) barges, with a 200-ton DWT capacity, have performedexcellently in the Southeast Asiaarea during the last 10 years.

3. Several large North Sea oilstructures are now in place. Despitesevere storms, they have performedwell.

Credits

Structural Engineer: ABAM Engi-neers, Inc., Tacoma, Washington.

General Contractor-Precaster-Pre-stresser: Concrete TechnologyCorporation, Tacoma, Washing-ton.

Seller: Concrete Energy Systems,Inc., a joint company consisting ofConcrete Technology Corporationand Trans-Energy InternationalInc., Hamilton, Bermuda.

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

Discussion of this paper is invited.Please forward your discussion toPCI Headquarters by July 1, 1977.

PCI JOURNAL/January-February 1977 31