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13Permanently Floating Structures

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13.1 General

These are structures that are intended to remain floating during their service life. Theymay be moored. They may be relocated during service, or they may be self-propelled.

This is a concept of great potential for the future as coastal land becomes increasinglyunavailable, forcing us to utilize ocean space, despite the difficulties imposed bythe environment.

Since many of the structures are oil storage vessels, which float with varying freeboard,even when partial ballasting is carried out, their mooring must give thorough consider-ation to transverse wind forces. This is especially true of steel vessels because of theirinherent small draft when empty. Concrete vessels, on the other hand, have greaterinherent draft but more limited capacity, especially in shallow water.

Typically, these permanently moored vessels employ single-point moorings, but insome areas of shallow water, fixed moorings are used. Storage vessels for oil are usuallyequipped with both a ballasting system and a transfer system.

Although the classification societies require dry docking for bottom inspection everyyear, they will usually waive that requirement for five years if an annual underwaterinspection reveals no defects, especially for concrete vessels. This means that cleaningof the hull also has to be carried out, by high-pressure water jet, supplemented bywire brush.

The great majority of these oil storage vessels are steel ships whose construction are inaccordance with International Rules of IMPCO and Ship Classification Societies such asABS, DNV, Bureau Veritas, and Lloyds. Many of these steel vessels now are required tohave double hulls. The design, construction, and maintenance is well covered by the rulesof the classification agencies, so will not be further covered here.

Concrete floating structures have a history of equal longevity, the first use of reinforcedconcrete being a small boat. However, they have had only sporadic development since,due to their inherent weight. However, during World Wars I and II, a number of smallreinforced concrete tankers and oil storage barges were built, due to the shortage of steel.

Floating bridges of concrete, floating piers, ferry slip docks, floating guide walls fornavigation locks, and large floating storage and production vessels have been constructedin recent years and, utilizing the improved technologies of prestressing and high-per-formance lightweight concrete, appear increasingly attractive. Prestressed concrete isproving its value by the successful performance of such structures as the ARCO FloatingProduction Storage and Offloading (FPSO) for LPG, in service in the Java Sea since 1975,the N’Kossa FPSO, moored off West Africa; the floating breakwater and pier in Monaco;the Heidron TLP (Tension Leg Platform) and the Troll West Semi-Submersible, both in the

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7 by Taylor & Francis Group, LLC

Construction of Marine and Offshore Structures534

North Sea. While all of the above are permanently moored, several in-depth studies haveshown concrete’s viability for self-powered vessels. See Figure 13.1 through Figure 13.4.

There are many potential uses for permanently floating structures of prestressedconcrete, including floating FPSOs (spars, TLPs and semisubmersibles, and ship-shapehulls), floating Liquefied Natural Gas (LNG) production and gasification terminals,floating heliports and airports and ice-resistant vessels. Permanently floating bridgepiers and marine terminals, anchored by driven piles have been employed (e.g., TheTappan Zee Bridge across the Hudson River in New York.)

The properties of significance for these uses are the ability of prestressed and reinforcedconcrete to resist high local import forces, durability, resistance to fatigue, fire resistance,and overall safety. Unit weight, the density of the concrete, is a generally limiting propertythat affects draft and inertia, the latter requiring greater power to achieve the same speed.In some structures, the greater draft is an asset, reducing the requirement for ballasting.

The bold concept by the U.S. Navy of a Mobile Offshore Basing System has beentemporarily shelved. However, a great deal has been learnt about the almost insurmoun-table problems of mating two massive floating structures in the open sea, each reacting todifferent phases of the six degrees of freedom. The most difficult appears to be differential

FIGURE 13.1Sakti Ardjuna floating FPSO vessel for refriger-ation and storage of Liquefied Petroleum Gas(LPG). Shown on station in Java Sea, Indonesia.Prestressed concrete hull. (Courtesy of Berger-ABAM.)

q 2007 by Taylor & Francis Group, LLC

FIGURE 13.2Mating of steel hulls for floating offshore base. Test of prototype.

Permanently Floating Structures 535

heave combined with pitch. Currently, the navy is testing a floating modular pier ofprestressed concrete for the mooring of combatant ships.

To attain the desirable qualities listed above and the best performance of concretestructures, prestressing is essential, as is well-distributed reinforcement. Concretestrength and quality must be as high as possible in order to reduce draft and to obtainthe impermeability essential to long-term durability. Because of their intended long-termservice in a dynamic environment, the levels of prestressing and the quantity of passivereinforcement must be adequate to ensure that after cracking of concrete, the steel is

FIGURE 13.3Conceptual design of mobile offshore basing system (MOBS).


FIGURE 13.4Heidrun concrete semisubmersible being fabricated while afloat in Bergen Fjord. (Courtesy of NorwegianContractor.)


below yield. The sections are typically thin as compared with offshore concrete platforms.Therefore, tolerances become of increased importance, both for the concrete sections andfor the placement of reinforcing steel and prestressing ducts.

Large floating structures are usually fabricated in a construction basin or graving dockbut some have been fabricated on ways. Smaller vessels may be fabricated on a largebarge. In all cases, draft at launching may well be critical, so very accurate control ofas-built weight and external dimensions must be maintained. Weight is affected by theconcrete density, and the weight of reinforcing steel actually placed, including splices.

Hull forms for concrete vessels are typically rectangular with a flared bow and taperedstern, since the towing and service wave responses are primarily inertial. Being deep indraft, due to their mass, they typically respond less to waves and swells than their steelcounterparts. For TLPs and semisubmersible hulls, spars, Ocean Thermal Energy Conver-sion (OTEC), and other deep draft structures, the cylindrical or oval cross sections aremuch more efficient, due to the ability to use the entire cross section in compression toresist hydrostatic loads. Concrete, being readily molded to any desired shape, is alsoexcellent for such complex shapes as the elbow of TLPs and the torus. This suggeststhat bow and stern segments can be separately precast and joined by cast-in-place concretewith the hull. By keeping the prestress concentric, it is technically feasible to prestresscurved sections, but shear reinforcement is required to resist radial shear on curves. Use ofsteel fibers in the concrete or ultra high strength concrete is beneficial.

Both standard-weight concrete and lightweight concrete are employed. The lightweightconcrete, while saving 25% or more in unit weight and hence draft, may require additionalreinforcement in both in-plane and out-of-plane shear. However, where the plate thicknessis determined by cover and the need to properly space the inner and outer reinforcements,as well as to accommodate ducts for prestressing, the lighter unit weight usually results ina more efficient and practicable vessel. Lightweight concrete further possesses better



thermal properties and, when combined with concrete containing silica fume, increasesfatigue endurance. It behaves especially well at cryogenic temperatures.

13.2 Fabrication of Concrete Floating Structures

Maximum utilization should be made of prefabrication using precast slabs and shells.Typically, joints are cast-in-place, to ensure full continuity of reinforcing steel and topermit splicing of ducts. Precasting permits the attainment of dimensional accuracy,while dispersing construction activities and increasing production (see Figure 13.5 andFigure 13.6). Segmental construction methods, similar to bridges, can be utilized.

A base slab is laid or cast on a carefully screeded crushed rock underbase, to permit laterflooding to penetrate entirely under the rock. This also permits attachment of any fittingsthat must protrude below the hull proper. In the case of a cast-in-place bottom slab, thebase is covered with polyethylene or similar. Drainpipe is placed in the rock underbase.

Base slabs have also been successfully cast on a finished concrete slab, which has beencoated with bond breaker. The base slab is typically haunched upward, 100–150 mm or so atthe wall panel intersections, so that the eccentricity of the prestressing tendons will eliminatethe negative moment tension cracking. Haunches also provide increased shear resistance.

Interior wall segments are usually precast concrete slabs, tilted up using tilt-up buildingtechnology, and joined, usually at the juncture of four wall slabs. While this is a verycongested area, it may be enlarged by 458 fillets. The external side walls should behaunched over the frames.

What appears to be the optimum system for joining the precast wall slab to the base is toleave a space in the base slabs and design the joint in the base slabs at the same location asthe joint with the wall. The wall slab now is constructed with its reinforcing steel projectinginto the joint, as are the bars from the two slabs. Thus the bars need to be set in exact

FIGURE 13.5Hood canal floating replacement bridge is fabricated of precast concrete segments. These will be post-tensioned toform hull.


FIGURE 13.6Assembling precast concrete segments for Hood Canal Bridge.


location so that the mating bars can pass without interference. The wall slab is supportedby struts at its sides which rest on the precast slabs. Prestressing ducts are spliced, and thenthe concrete joint is placed, integrating the haunches previously described.

Cast-in-place walls and slip forms minimize the number of joints. Both slip forms andjump forms have been used. Flowing concrete should be used, due to the thin,congested sections.

Mechanically headed reinforcing bars are especially useful in regions of high shear,since they can be placed more readily and effectively in the congested areas. Mechanicalrebar splices are also not only better from the point of view of congestion butreduce weight.

The upper deck may similarly be constructed of precast slabs, resting on ledges bolted tothe walls. The joints then are made over the walls.

The prestressed concrete modular floating pier now being tested at half-scale by the U.S.Navy and the designer, ABAM, uses all precast slabs of high performance lightweightconcrete, for both exterior and interior of the hulls. Since the pier is designed for ahundred-year life, several types of corrosion-resistant steel reinforcement were tested,including stainless steel, fusion-bonded epoxy-coated and corrosion-resistant steelMMFX-2, with minimum concrete cover. Based on these tests and a consideration ofcosts, MMFX-2 was selected for the test module. Cover was 44 mm.

Since the post-tensioning profile was straight, plastic ducts, with plastic anchorageencapsulation, were selected for the post-tensioning tendons. Self-consolidating high per-formance lightweight concrete was employed, using lightweight coarse aggregate andnatural sand. The unit weight was 1954 Kg/m3 (122 pcf) and the design strength was48 MPa at fifty-six days. Haunched precast concrete panels were joined by cast-in-placefine concrete, a scheme initially employed on the Hay Point Terminals in Queenslandin 1975.

In many cases, prefabrication is used only for a portion of the hull (Figure 13.7). Forexample, on the N’Kossa FPSO the interior walls were precast but all exterior walls cast in


FIGURE 13.7The curved bottom of the Sakti Ardjuna floating vessel for LPG is formed by match-cast segments of precastconcrete. (Courtesy of Berger-ABAM.)


place. Precasting is especially useful with double-curved portions of the hull, such as thosefor the bow. Slip forms have been used for cast-in-place walls. Stay-in-place forms havebeen used for horizontal members such as the deck.

Construction joints should be properly prepared by water jet blasting to expose thecoarse aggregate. Typically, this will cut back the joint surface about 6 mm. Then, justbefore the concrete is placed, bonding epoxy may be sprayed on the joint surface. Itmust still be wet as the concrete covers it. Some experienced fabricators prefer to brushon a heavy coat of latex mortar.

Decks should be sloped to drain so that saltwater spray does not pond. Decks should betreated with silane at the time of construction and at two- to five-year intervals thereafter.In vessels scheduled for service in cold regions, air entrainment should be used in theconcrete to prevent freeze-thaw attack.

Thermal and setting shrinkage may produce cracks, especially at construction joints.Proper concrete mix and curing procedures can minimize these. Epoxy coating of thejoint, just in advance of concreting, and early partial prestressing will minimize cracking.

Anchorage zones need to be well reinforced to distribute and confine the transverse,longitudinal, and radial strains that occur. Although this is a matter of detailed design,these details need to be determined in cooperation with the contractor or the prestressingsubcontractor. Further, since many of these large floating structures are constructed on adesign-build basis, the constructor is vitally involved. The highest stresses around theanchorage typically occur at the time of prestressing, not under service or extremeloads, and it is then when cracking occurs if proper details are not employed.

Post-tensioning ducts should be of plastic, fused at their splice joints, or galvanized steelwith heat shrink joints. Plastic ducts should not be used where the tendons are sharplycurved, since the strands may cut deep grooves in them. In any event, the sharp curvesshould be watertight steel tubes, pre-curved to the proper profile. Grout should contain anantiwashout and thixotropic admixture to prevent the formation of bleed pockets. Plasticcaps should be used to seal over the anchorages. Prestressing anchorages should be



protected by being located in a pocket, into which reinforcing bars protrude from thesurrounding concrete structure. This is filled with concrete after all prestressing activitiesare finished. The joint surfaces should be coated with bonding epoxy just before placementof the concrete. In regions subjected to prolonged freezing, latex concrete should be usedinstead and the epoxy coating eliminated.

13.3 Concrete Properties of Special Importance to Floating Structures

Although somewhat repetitive of the prior presentations, such as Chapter 4, a discussionof those properties of concrete of principal concern to floating structures is presented inmore detail in this section.

1. Fatigue. The vessels are exposed to a large number of cycles of loading fromwaves through their service life. Being prestressed prevents the hog and sagmoments from exceeding more than half the concrete tensile strength, even incases of severe storm. Under such high-cycle low-amplitude cycles, thereappears to be no endurance limit. If the stress were to be allowed to exceedthe full tensile strength of the concrete so that cracks develop, then waterwould be sucked in and create a hydraulic ram effect as the crack closes underhigh compression. Provided there is enough total steel, both mild andprestressed, across the crack so that the steel stays below yield stress at cracking,then many tests demonstrate that fatigue endurance is still highly satisfactory.

Prestressed lightweight concrete has been shown in tests to have both goodinsulating and excellent fatigue resistance even when intentionally cracked. Forthe concrete ships of WWI and II, which had simple passive reinforcementalthough no prestressing, no fatigue failures have ever been reported. Thefully satisfactory endurance of prestressed concrete has been demonstrated bythe shafts of the offshore platforms in the North Sea, several of which have beenin service for over thirty years. Research has shown that the combination oflightweight coarse aggregate and microsilica is especially resistant to fatigue.

2. Fire Resistance of concrete is dependent on the cover thickness over the steel andthe insulating and spalling characteristics of the concrete.

In an intense fire, spalling of the cover could occur in the very impermeableconcrete expected in seagoing vessels. The phenomenon is caused by steamgeneration in the pores of the concrete. To relieve the excessive pore pressures,polypropylene fibers are incorporated. In a fire, these melt, leaving a tiny void forthe expulsion of steam. This could be considered for interior bulkheads inespecially vulnerable areas such as the engine room.

3. Durability. The concrete envisioned for seagoing structures has been developedand improved over the last fifteen years by the addition of various admixtures tothe mix. The principle admixtures now available and utilized on importantstructures such as the Channel Tunnel, offshore concrete platforms, and floatingstructures in the seawater environment are:

a. High-range water reducing admixture—to reduce the water required for work-ability during construction and thus enhance the strength and impermeability.

b. Pozzolanic replacements such as fly ash and blast furnace slag, to increase thesulfate resistance, the resistance to alkali-aggregate reactivity, and enhance theimpermeability. It replaces an approximately equal amount of cement.



c. Air entrainment—to prevent freeze-thaw attack.

d. Corrosion inhibitors, such as DCI.

e. Viscosity admixtures such as anti-washout admixture (AWA) or

f. Microsilica (silica fume) to give higher strengths and greater impermeability, aswell as viscosity and fatigue resistance.

g. Drainage of decks—to prevent ponding of spray.

The proper use of the above admixtures, together with small size aggregate (10–16 mmmax) will produce “flowing concrete” of high compressive and tensile strength andexcellent durability.

In the early days of offshore oil storage, considerable concern was expressed oversulphate attack as a result of the anaerobic bacteria in the crude oil. However, this hasturned out to be of low probability for the high quality concretes used in modern seastructures, especially when fly ash and/or silica fume has been included in the mix.

† Corrosion of the reinforcing and prestressing steel is prevented by an adequateconcrete cover and the impermeability of the concrete. A corrosion-inhibitorsuch as calcium nitrite will delay the onset of corrosion as will the use ofcorrosion-resistant steel reinforcement such as MMFX-2. Stainless steel reinforce-ment is expensive but non-corrosive.

† Leak tightness, both for seawater in leakage and stored hydrocarbon leakageoutward, is provided by the impermeability of the concrete and adequatereinforcement and prestressing to ensure that any cracks that do form areclosed tightly after the high load has passed.

† Certain refined petroleum products require coatings or liners of the concretetank surfaces.

† Impact resistance. Concrete slabs or shells, reinforced with closely spacedreinforcement and prestressing tendons, are very ductile and can thus absorba large amount of energy. If more is required in certain locations, steel fibers canbe added or three-dimensional mesh. T-headed bars are effective.

† Abrasion resistance, especially around mooring attachments, and on decks.

13.4 Construction and Launching

The most common method of construction of large and heavy prefabricated structures isby construction in a basin, excavated on the bank of a harbor or river. In effect, thisconstruction basin is a temporary dry dock. Actual fabrication takes place in the bottomof the basin. Such a basin was employed in which to build and launch the Arco SaktiArdjuna. This vessel was towed from Takoma, Washington, to the Jara Sea where it hasserved as an LPG terminal for over thirty years (Figure 13.8).

Alternatively, a structural shipyard dry dock may be leased. This was the case with theN’Kossa barge, the largest concrete vessel yet constructed.

For structures built of concrete, the base of the structure is separated from the floor of thebasin by such means as a layer of pervious material (e.g., sand), covered by a layer ofplywood and polyethylene plastic. When it comes time to launch, the dock is flooded andwater injected in the pervious underlayer. By waiting a few hours with pressure on, the


FIGURE 13.8Arco ”Sakti Ardjuna“ enroute to Java Sea. (Courtesy of Berger-ABAM.)


vessel will float up. Some of the plywood and polyethylene may stick to the bottom andhave to be removed by diver.

Two basins, side by side, one deep, one shallow, may be beneficial, especially wheremultiple elements are to be built. Then, construction of the lower hull may take place in theshallow basin, floating it sidewise into the deeper basin for completion and finallaunching, while construction of another vessel is started in the shallow basin.

Exit into the river or harbor must be controlled by tugs and shore lines, especially wherethere is a wind or current in the channel.

If the structure is constructed at grade level, transfer of the heavy prefabricated vessel,up to 20,000 tn. or more, is carried out by skidding on finely leveled and firmlysupported grade beams, using Teflon bearing pads and horizontal jacks. On otherprojects such as the Øresund Bridge, Hillman Rollers were employed. Steel jackets foroffshore platforms have been slid on heavy timber or steel beams using Teflon-stainlesslaunch pads. Coefficients of friction as low as 0.03 have been experienced. The huge100 m long prefabricated concrete girders for the Great Belt Bridge were slid on concretebeams, using grease. Regular ratchet recesses in the beams allowed jacks to grip andmove the girders progressively. For the Confederation Bridge in eastern Canada, lowprofile rubber-tired carts lifted the 8000 tn. units by hydraulic jacks and rolled themforward on concrete rails.

Use of gaskets on the sides and injection underneath large flat-bottom steel orconcrete vessels with pressurized water reduces friction to near zero, allowing easyre-positioning.

Side-launching is a second method used to launch barges and similar vessels. Side-launching reduces the bending moment in the vessel during launching. It also distributesthe concentrated bearing loads which travel down the ways as the vessel is launched.



A concrete barge for a floating phosphate plant was built at grade, on the edge of aharbor, and then slid sidewise, jacked up to an angle, and slid downways into the water.As the barge left the ways due to buoyancy, it temporarily imposed a concentrated loadunder its upper edge, while causing a transverse sag bending moment in the barge. Bothwere properly designed to resist this loading.

It is planned that the prefabricated navigation dam units for the Olmsted Dam on theOhio River will be constructed at grade and slid sidewise on pile-supported beams ontoa cradle. The cradle will then be slid down the ways, allowing the dam unit (3500 T) tobe floated off at any stage of the river. This lift will be partly assisted by lift from acatamaran crane barge which will support it during transport and later lower itinto position.

A vessel or prefabricated element constructed at grade level may also be moved onto aship lift, such as a Syncrolift, and lowered by hoists into the water. If a permanent installa-tion is not available, it can be constructed as an overhead gallows frame and multiplehoists. The element being lowered must be structurally able to be lifted from the ends, orelse a structural barge or frame must be placed under the element. Lifting by a highcapacity gantry crane, which runs on trestles over the water, is an alternative schemebeing considered for the Olmsted Dam segments.

A vessel or segment can be slid onto a floating dry dock for launching or onto a launchbarge, that is, a barge capable of deep submergence, which can stand the external head andstill structurally support the structure or vessel. To allow it to submerge deeply enoughwithout losing stability, two or more columns are erected at the corners or one end, thusgiving it water-plane stability and residual buoyancy.

Of course, if only one element or vessel is involved, it may be built initially on the launchbarge. Float-off from a launch barge is normally attained at an even hull. However, thefloat-off may be assisted by tipping and sliding off the end of the barge. Then the launchingis carried out in known depth of water, so that the end of the barge grounds at about 308

angle, where the lower end of the element begins to lift off. As the load rotates, a tug pullsthe launch barge forward, allowing the load to float off.

Many other ingenious methods have been proposed, taking advantage of tides or ice.Sand jacking to lower the vessel through sand has been proposed. Structural integritymust be assured at all stages.

Mooring fittings, such as posts and bollards, should be post tensioned to a thickenedand heavily reinforced seat in the hull. Usually, prestressing bars will be used instead ofstrands, since seating losses of prestress can be eliminated by the use of bars.

Fairleads and/or hawse pipes are either post tensioned on or embedded in the concretewith proper anchorage. Steel plating should be placed alongside to eliminate the wearfrom the rubbing of anchor chains. Piping and mechanical systems should, as far as ispracticable, be integrated into the hull design, with embedments preinstalled. Corrosionprotection should be provided with sacrificial anodes, especially at seawater intakes anddischarges. There are many other design and construction details applicable to concretevessels. Reference is made to Gerwick Construction of Prestressed Concrete, 2nd edition(Gerwick 1996).

A very interesting hull design has been developed as a result of testing at both theUniversity of Manchester, U.K., the University of California at Berkeley and in Japaneseconstruction laboratories. It is a double-steel shell, made to work compositely withconcrete infill. Thus, it can be designed for shell action to resist high hydrostatic pressuresand, at the same time, benefit from the ductile behavior of the steel. Additional testing ofthe concept for ice-resisting structures has been carried on in Japan. As noted in Section4.3.2, a prefabricated shell using shear stud connectors is commercially available. Amongthe potential applications are submerged tunnels (tubes) and suction anchors.



Constructibility of large vessels requires consideration of access and crane reach, sincework will need to be pursued concurrently at many locations. Construction at grade isthus more efficient and hence less costly than construction in a basin.

13.5 Floating Concrete Bridges

Floating bridges go back to their origins in about 1500 B.C. It was the floating bridge thatDarius built in 400 B.C. across the Bosporus that failed due to dynamic excitation by stormwaves, yet engineers have not heeded its lesson. The Hood Canal Bridge and later, the I-40Lacey P. Morrow Bridge, both sank under dynamic wave action and the floating bridges atHobart, Tasmania, and Evergreen Point, Washington, were both damaged by storms andhad to have extensive retrofit or replacement.

Most concrete bridges have been designed and constructed as a series of long, rectangu-lar barges, with interior egg-crate bulkheads, both longitudinally and transversely. Theearly ones were conventionally reinforced but later floating bridges have had increasingamounts of longitudinal prestress. Joints have generally been large mild steel bolts but thelast two replacement bridges have been post-tensioned to about 7 MPA precompressionfor their full length. In the case of the Hood Canal Bridge, both original and replacement,the deck has been elevated on short columns.

Moorings have been spread moorings to large concrete block weights. The mooringlines have been led through glands in the sides just below the deck and are jacked forequalization of load. They are cathodically protected.

The dynamic excitation referred to earlier is due to the waves traveling obliquely to theaxis of the bridge, thus lengthening the effective span between crests to much longer thanthe wave length. This in turn leads to harmonic response by the bridge. Any crackingbecomes a through crack. The alternate opening and closing sucks in water, then closes,leading to hydraulic fracture. Therefore there must be sufficient steel area crossing allpotential cracks to stay below yield at the ultimate tensile strength of the concrete.

Flowing concrete is especially well-suited for application to floating bridges because ofthe thin sections and congestion of reinforcement.

A curved floating bridge in Norway is supported on transverse pontoons ofprestressed concrete.

13.6 Floating Tunnels

A submerged floating tunnel has been proposed for the crossing of the deep Lysefjord, alsoin Norway, near Stavanger. It would combine the technology of immersed tunnels (tubes),as presented in Section 9.5, with that of floating bridges and offshore platforms.

The concept is to design and construct a concrete tunnel of cylindrical external cross-section, which would be pulled down by tethers to the seafloor. The tethers were selectedin the case of the Lysefjord. While dynamic interaction with currents remains the principaldesign consideration, assembly of segments and deployment are the dominantconstruction problems.

Because the walls will necessarily be thick, heat of hydration must be minimized. Jointsbetween segments will probably follow the pattern developed for immersed tubes.Assembly may be made in an adjacent bay, and the entire string towed and moored to



the final location. Jointing and sealing at the tunnel entrances on either side must accom-modate peak bending moments and shears.

13.7 Semi-Submersibles

The Troll Olje is a concrete semisubmersible installed on the Troll field offshore Norway toserve as an FPSO. It is moored by sixteen catenary mooring lines in 325 m water depth andsupports a topside operating weight of 32,500 tn. and thirty-three risers and tubes. Thehull consists of four rectangular pontoons connected to four columns by four corner nodes.These nodes are the most complex zones of the platform. It is here that the external fair-leads of the catenary moorings are attached, thus subjecting the elements to high localloading. The columns consist of two concentric concrete columns, thus giving damagestability. The columns are designed to resist ship and beat impact and to have ballastingand pumping equipment. At the top of the columns, a steel corbel ring is post tensioned. Itsupports the module support frame with a moment-free pin connection. The entireconstruction was performed in a graving dock near Bergen.

13.8 Barges

The N’Kossa barge is a large spread-moored barge that supports an offshore processing,storage, and shipping facility. It was installed offshore the Congo in water depth of about200 m. It is 220 m long, 46 m beam, and 16 m deep, and supports 33,000 tn. of equipmentand structures. It was constructed in a graving dock in Marseilles and fully outfitted withthe topside modules. Unusual was the use of reactive powder concrete to achieve a dense,

FIGURE 13.9Assembly of precast concrete segments for inner walls of N’Kossa floating barge. (Courtesy of BuoyguesOffshore.)


FIGURE 13.10Completing construction of N’Kossa barge. (Courtesy of Buoygues Offshore.)

FIGURE 13.11N’Kossa barge being towed out of dry dock. (Courtesy of Buoygues Offshore.)



FIGURE 13.12Installing superstructure and outfitting of N’Kossa barge. (Courtesy of Buoygues Offshore.)


high strength, concrete hull. Internal bulkheads were made of precast slabs while theexterior hull was cast-in-place, using slip forms.

Concrete barges have been employed to support floating concrete batching and mixingplants, because their mass provides stability and minimum motion (Figure 13.9 throughFigure 13.12).

13.9 Floating Airfields

Floating airfields, both runways and taxiways, have been proposed for a number ofprojects, including one in the Thames, another in San Diego, California, and one atOsaka, Japan.

When the San Francisco Airport was planning the expansion of its runways, one ofthe alternatives studied in detail was a floating structure. Its 50!50 m sections would beconstructed in an offsite construction yard and towed to the site, where they would be joinedby prestressing, both longitudinally and transversely. The sections would be designedas semisubmersibles, i.e., columns through the tidal zone. The completed structurewould be held in fixed position and tied down against tidal changes by ground anchors.

Although this concept requires three horizontal slabs, the bottom and the top of the hulland the airfield deck itself, it met the important environmental requirement that the waterwould be free to flow through.

The U.S. Navy has intensely studied concepts for construction of its Mobile OffshoreBasin System, a floating airfield runway, plus extensive repair and servicing facilities, theobjective being a mobile support base for naval operations at a distance, thus minimizingthe need for land bases. This would have to be able to be deployed and moored so as tosurvive a major storm in the open sea. The semisubmersible concept is favored because ofits low response to waves and swells. Since the length of several thousand meters isbeyond reasonable shipyard production capabilities, deployment has assumed fabrication



and deployment in steel segments of less than 1000 m each, with mating to be carriedout afloat.

Similarly, preliminary engineering studies were carried out in Japan for a floatingairfield, both for generic application and for specific application for the new KansaiAirport in Osaka. These were based on the semisubmersible concept, with pairs oftubular columns formed into underwater Us by tubular pontoons.

13.10 Structures for Permanently Floating Service

Floating oil storage vessels of steel have been used for many years. Design and construc-tion follows shipbuilding practice, except that, being stationary vessels, they are not facedwith the necessity to reduce drag; hence in protected seas such as the Persian Gulf, theycan be very simple large boxes, with vertical sides and ends.

FPSOs of steel are permanently moved in the North Sea and elsewhere. Designed toweathervane to minimize wave forces, they are molded like typical seagoing vessels. Mostrecently, turret moorings have been employed, to weathervane about their center ofrotation and to permit oil and product lines to be swiveled in the turret.

Recently, the SPAR has re-emerged as a potentially optimum vessel for offshore oildrilling, production, storage, and offloading in the very deep ocean, subject to severestorms. These are much larger than previous spars and have drafts of 150 m or more.They are built in shipyards in the horizontal position and launched like a ship. Concreteversions of the spar have been proposed, utilizing the favorable ability of concrete to resisthydrostatic loads (see Chapter 22).

The Red Hawk Oil Drilling and Production Platform is a relatively small steel SPAR builtup of seven steel cylindrical cells, each 26 ft. in diameter. Four of these cells are 280 ft. inlength; three extend a full 560 ft. The upper four are open for 7 ft. at their bottom andcompressed air will be injected to regulate trim and adjust ballast. To prevent vortex

FIGURE 13.13Floating LNG export terminal concept. (Courtesy of Mobile Technology.)



shedding, spiral strakes are affixed to each of the cells. In addition, metal fins encircle theextended legs at sharp angles. Three sets of heave dampers are installed. Polyester lineswill be used to moor the SPAR to suction anchors in 5300 ft. water depth (Figure 13.13).

13.11 Marinas

Small floating piers, both steel and concrete, have been used for many years. These includemarinas for small boats, boat docks, and seaplane docks.

Marinas have utilized pontoons constructed of lightweight concrete and also compositessuch as fiberglass. Moorings have been single piles, one at each end, with neoprene andstainless-steel sliding surfaces. Concrete and polyester piles have also been used. Compo-sites suffer from ultraviolet degradation, although progress has been made in incorporationof UV resistance. Lightweight concrete utilizes galvanized mesh reinforcement. Shearresistance in the thin walls may be enhanced by 3-D mesh or by inclusion of fibers.

13.12 Piers for Berthing Large Ships

Large floating piers have been constructed. The Valdez Alaska container terminal has beenin service for over ten years. It was constructed in two 100 m long segments so as to reducemoments during towing in the open sea. On arrival at Valdez, it was joined together bypost tensioning to form a 200 m long pier. A floating pier and breakwater was built inMonaco and joined to the shore bulkhead by a universal articulated joint. The U.S. Navy iscurrently testing a Modular Pier segment at San Diego (see Section 13.2).

13.13 Floating Breakwaters

Floating breakwaters of concrete pontoons have been successfully used in partiallyprotected installations to give further protection to small boat harbors and critical shorefacilities. Proposals have been made to use larger versions in the open sea but have not sofar proven viable, due to the extreme motions that they would undergo in storms and thedifficulty in providing long-term moorings.

Floating breakwaters are joined in segments, generally using chains. Thus, the attach-ment to the pontoons must be designed to prevent fatigue and abrasion. Some of thoseproposed are configured to divert the incoming energy to vertical jets and dissipate it,rather than reflect the waves. Others have been designed with sufficient mass to avoidresonance with the larger waves. A large floating breakwater has been constructed ofprestressed concrete to protect the harbor of Monaco. It is moored by an articulatedjoint to a fixed abutment.

13.14 Mating Afloat

Many concrete and steel floating structures have been mated afloat in inland waters. Theseinclude concrete bridges in Washington and the floating concrete container terminal in



Valdez. Steel vessels have been jumboized, adding a midbody while the bow and stern areafloat. In Tokyo Bay, the two halves of a test module for the Okinawa Offshore Heliportwere mated by a method described similar to that described below.

The process of jointing shallow-draft floating pontoons and barges in inland water hasbeen to pull the structure together with deck winches. Mating spuds extending on thedeck of one segment engage the mating cones on the other. The barges are usuallyballasted to tilt slightly down at the joint. Timber bumpers and/or rubber fenders areused to cushion the impact. As soon as the top edges are in contact, they are locked thereby stressed wire rope or steel members. Then, the ballast is shifted so that the hulls rotateto press the bottom edges together.

Joints for concrete structures have rubber gaskets that are compressed to enable dewa-tering of the joint. Ducts are then run from holes in connecting blocks in one segment tomating holes in the other segment and prestressing tendons installed. The gap in the jointis now grouted. Full moment and shear transfer is developed by prestressing tendons topand bottom and by the grouted shear keys.

When external access to the below-water outside of the joint is needed (for example, toweld steel plates for connecting two steel barges) then a temporary box cofferdam isplaced underneath and on the sides. Larger segments may employ more-sophisticatedgaskets like the gaskets used in jointing submerged prefabricated tunnel segments.

When mating large structures in the open sea, the dynamic motions of the two segmentsbecome dominant. These huge masses develop very large inertial responses in all sixdegrees of freedom. Forces are often beyond the capacity of conventional mooringsystems to control. Presumably the two segments will be joined in favorable sea andwind condition, so the large segments may be headed into the swells. The barges can bejoined by mooring lines, one on each side, leading to constant-tension winches. The twosegments should be “pulled apart” at the same time, either by anchor lines in the water ofmoderate depth or by tugs, thus keeping the joining lines under tension, yet allowing thetwo barges to be slowly pulled together, overcoming the differential surge. Long-strokehydraulic jacks and commercially available dock fenders, particularly the buckling-type,could be utilized to cushion the final impact between two vessels.

Recognizing the inherent problem of joining floating structures in the open sea,constructors have adopted two solutions. The first is to join all the floating units in acalm harbor, with articulated joints, and then tow the entire string to the site offshore inthe same manner described for the tension legs of tension leg platforms. Tension is main-tained on the entire string of floating elements by a tug at the stern of the string. Uponarrival, the individual elements are moored, still maintaining shear capacity at their joints.If the system is inherently a flexible system, similar to that of a tubular steel pile or tensionleg, then the moment connections can also be made in the calm harbor. Differential heaveand associated pitch are the most difficult to control.

The second system obviates joining in shear and moment by spacing the individualelements with a gap between. The gap is crossed by an articulated steel bridge. Articulatedarms and deck allow independent heave. This is the system tentatively adopted for theMobile Offshore Basing System of the U.S. Navy (MOBS) but not pursued beyondpreliminary engineering.

Where continuity of barge-shaped structures is required, long-stroke hydraulic cylin-ders are engaged when they come within reach. These not only can overcome the surgemotions, but the yaw as well. Crossed wires can bring the two large segments together asfar as sway is concerned. Next is to match the heave and roll motions. Hydraulic cylinders,acting in a near-vertical orientation, can be brought into play. Shock absorbers, up to5000 tn. capacity and 1 m stroke, have been developed. Alternatively, heavy walled



pipe, turned on its side, can be crushed, giving inelastic consumption of energyand momentum.

The moment transfer between large floating structures is very difficult. The differentialpitch at the joint is countered by the ballasting of the two segments to rotate to closure atbottom. The moment is developed by very large post-tensioning strands, both top andbottom. Matching cones and spuds are used for final jointing. All jointing systems musthave ductility so that if they are overloaded, they will yield while still maintaining a force.Use of a heavy vegetable oil in small amounts, dripped from an upwind tug, will calm thewaves (but not the swells) during this jointing operation.

There is an obvious need to develop a system for effectively mating floating structures inthe open sea. Not only airfields and piers are involved, but also large floating processingplants. Possible avenues to pursue might include that involved in the mating of a seagoingpusher tug to a large barge; a procedure that is currently state-of-the-art, showing that asmaller module can be mated to a large, massive floating structure in the sea.

The successful float-over installation of the topsides of offshore platforms in the opensea, by using long-stroke hydraulic jacks, shows that it theoretically is possible to developsuch a system for mating two floating structures in the open sea under favorable con-ditions. Much more developmental work and testing remains to be done.

q 2007 b

His heart was mailed in oak and triple brassWho was the first to commit a frail bark to the rough seas.

He was not afraid of the swooping sou’westerBattling it out with the winds of the north,

Nor the weeping Hyades, nor the madness of the south wind,The supreme judge of when to raise and when to lay the Adriatic.

He did not fear the approaching step of Death,But looked with dry eyes on monsters swimming, on ocean boiling,

And on the ill-famed Acroceravnian rocksIn vain in his wise foresight did God sever the lands of the Earth

By means of the dividing seasIf impious ships yet leap across waters

Which they should not touchBoldly enduring everything, the human

Race rushes to forbidden sin.

Horace, 65–68 BC, Ode I:III (9–25)

y Taylor & Francis Group, LLC

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