357.2r-88 state-of-the-art report on barge-like concrete structures · 2019-05-14 · design of...

ACI 357.2R-88

Report on Barge-Like Concrete StructuresReported by ACI Committee 357

Jal N. Birdy Anthony E. FioratoChairman Secretary

William J. CichanskiEditor

Irvin B. BoazAnthony D. BoydV. M. BuslovRoger A. CampbellGeorge F. DavenportJoseph A. DobrowolskiJ. Michael Duncan

Svein FjeldGeorge A. FotinosHarvey H. HaynesGeorge C. HoffWilliam A. IngrahamRichard W. LittonAlan H. Mattock

John S. PriedemanKarl H. RungeB. P. Malcolm SharplesRam G. SisodiyaCharles E. SmithArthur L. WalittAlfred A. Yee

This report addresses the range of current engineering experience for thedesign and construction of floating, barge-like concrete structures. A briefdiscussion of past and present structures and design concepts is presented toestablish both the versatility and technical viability of concrete barge-likemarine structures.

Barge-like concrete structures are used at both sheltered and exposedsites. The marine environment can be both severe and highly unpredictable,necessitating unique design requirements for floating concrete structures. Inaddition, barge-like structures serve a wide variety of uses such as industrialplantships, floating bridges, floating docks, parking and hotel structures, andother applications, and as such, further attest to the wide range of possibledesign requirements.

Design loads and recommended design criteria are presented. Design proce-dures and methods of analysis are discussed to better acquaint the reader withthe design considerations that are unique to barge-like marine structures.

Methods used to construct barge-like concrete structures play a major rolein the success of each application. Construction methods and materials usedfor recent applications are presented to demonstrate the importance of the con-struction process during the planning and design of marine concrete structures.Important aspects of delivery from the construction site and installation atthe deployment site are presented.

The durability and serviceability of barge-like structures at remote sitesare important considerations to project planners and developers. Constructionmaterial selection and inspection, maintenance, and repair techniques arediscussed.

Keywords: abrasion; accidents; admixtures; aggregates; barges; concreteconstruction; concrete durability; corrosion; detailing; dynamic loads; fatigue(materials); finite element method; floating bridges; floating docks; freeze-thaw durability; installin ; inspection; lightweight concretes; limit designmethod; loads (forces ntenance; moorings; permeability; post-tensioning;precast concrete; prestressed concrete; prestressing steels; quality control;reinforced concrete; reinforcing steels; repairs; serviceability; stability;structural design; surveys; towing.

Copyright @ 1997, American Concrete Institute ACI Committee Reports, Guides, Standard Practices, and Com-mentaries are intended for guidance in designing, planning,

All rights reserved including rights of reproduction and use in executing, or inspecting construction, and in preparingany form or by any means, including the making of copies by any specifications. Reference to these documents shall not bephoto process, or by any electronic or mechanical device, printed made in the Project Documents. If items found in theseor written or oral, or recording for sound or visual reproduction documents are desired to be part of the Project Documents,or for use in any knowledge retrieval system or device, unless they should be phrased in mandatory language and incorporat-permission in writing is obtained from the copyright proprietors. ed into the Project Documents.

357.2R-1

jcg

(Reapproved 1997)

ACI COMMITTEE REPORT

CONTENTS

1.0 INTRODUCTION

2.0 APPLICATIONS

2.1 Introduction2.2 Historical Background2.3 Barge Structures2.4 Industrial Plantships2.5 Floating Piers and Docks2.6 Floating Bridges2.7 Other Structures2.8 Summary

3.0 MATERIALS AND DURABILITY

3.1 Introduction3.2 Testing and Quality Control3.3 Structural Marine Concrete3.4 Reinforcing and Concrete Cover3.5 Special Considerations3.6 Summary

4.0 EVALUATION OF LOADS

4.1 Introduction4.2 Load Definitions4.3 Load Determination4.4 Summary

5.0 DESIGN APPROACHES

5.1 Introduction5.2 Design Codes5.3 Analysis Methodology5.4 Design and Detailing5.5 Summary

6.0 CONSTRUCTION

6.1 Introduction6.2 Construction Methods6.3 Concrete Construction6.4 Construction Afloat

357.2R-3

CONTENTS (continued)

)

7.0

8.0

9.0

6.5 Segmental Construction -- Joining While Afloat6.6 Summary

TOWING AND INSTALLATION

7.1 Introduction7.2 Design Considerations7.3 Tow Route7.4 Summary

MAINTENANCE, INSPECTION, AND REPAIR

8.1 Introduction8.2 Structural Deterioration8.3 Surveys and Periodic Inspection8.4 Repairs8.5 Summary

SPECIFIED REFERENCES

9.1 American Concrete Institute (ACI)9.2 American Petroleum Institute (API)9.3 American Society for Testing and Materials (ASTM9.4 Det norske Veritas (DnV)9.5 FGdhation Internationale de la Prhzontrain (FIP)

ABBREVIATIONS USED IN TEXT

1.0 INTRODUCTIONThis state-of-the-art report is intended to further the development offloating concrete structures. By presenting the state of the art in design,materials, construction, installation, maintenance, and repair of floatingbarge-like concrete structures, the technology is demonstrated as availablefor additional applications. Existing applications are reviewed as a meansof demonstrating that the technological risks are at a known and acceptablelevel.

The durability of concrete in a marine environment is demonstrated in fixedstructures by the wide use of concrete construction in waterfront and harborfacilities. The rationale for selecting concrete fixed harbor structures isdirectly transferable to floating structures. Barge-like structures areaddressed because numerous commercial applications use a configuration whichis barge-like in shape. Shipshaped hulls and bottom-founded oil explorationand production platform configurations are beyond the scope of this report.For additional information on the subject of concrete bottom-foundedstructures, see ACI 357.1R. Additional information on shipshaped vesselsmay be found in "Design and Construction of Concrete Ships" by theFederation Internationale de la Precontrainte (FIP).

For this report, the definition of a barge-like structure is a floatingvessel with near vertical walls and a near rectangular plan. The bow andstern may be raked or shaped as required. "Floating" relates to structuresthat are temporarily, intermittently, or continuously afloat. Certainvessels included within the definition of barge-like structures are designedfor towing and subsequent grounding, and afterward function as fixed gravity-type structures. Later, these structures may be refloated and transportedto a new location. Other structures are designed to remain continuouslyafloat, with or without permanent mooring. An example of a barge-likestructure is a floating bridge, of which several exist. The oldest floatingconcrete bridge has a length of over 1.5 miles (2.4 km) and has remained inservice for more than 40 years. It spans Lake Washington from Seattle toMercer Island in the state of Washington, USA.

Future applications include floating piers, breakwaters, industrial plants,LNG and LPG processing and storage vessels, oil storage structures, andairports [1.1, 1.2]. Industrial plants have been constructed on barges,typically steel barges, in fabrication yards and towed to remote locations.Similar examples now exist using concrete barges. Given certain conditions,the cost of such a plant is lower, and construction time is shorter whencompared to building the plant on site? For concrete barges, addi-tional benefits are obtained from improved motion characteristics duringtransit and an increased service life. The market for floating industrialplants is evolving and is likely to have a significant future. Otherapplications have been developed and are currently in service. These willbe discussed in Chapter 2.0.

1 Cichanski, W.J. and Priedeman, J.S., "The Technological Versatilityof Floating Plants," presented at the American Concrete InstituteFall Convention, New York City, October 1984.

BARGE-LIKE STRUCTURES 357.2R-5

Reinforced, prestressed concrete and composite concrete-steel structures areused and will be discussed. In 1943, the first prestressed concrete bargewas built by the U.S. Navy. Today, the preferred construction approach forlarge structures is to use prestressed concrete instead of ordinary rein-forced concrete. The ability of prestressed structures to control nettensile stresses and to close cracks that develop from temporary overloadsituations enhances watertightness and durability. Composite concrete-steelconstruction is also becoming popular. Concrete is used in the exteriorbulkheads and base to provide durability, and steel is used for the internalframing and deck to provide weight savings.

The cost economies of floating concrete structures are not addressed in thereport. Specific case-by-case analyses are necessary to yield the appropri-ate cost comparison to alternative construction materials. For very largestructures, life-cycle analyses suggest considerable advantage to concretestructures because of low maintenance costs resulting from the use of durablematerials. Dry-docking for inspection and repair is costly. At this time,regulatory agencies do not require periodic dry docking for floating concretestructures. In the past,insurance costs to cover towing and delivery havebeen more expensive for concrete structures than for steel barges becauseexperience was less for floating concrete structures. As experience hasgrown, this cost penalty has disappeared. Today, barge-like structures canbe "classed" by regulatory agencies using procedures similar to those usedfor steel vessels.

Design of concrete barge-like structures requires the knowledge of manydisciplines. The designer must have a thorough understanding of concretedesign principles, concrete as a material, and construction practice. Also,the designer must have an understanding of environmental loadings, marineoperations, requirements for vessel certification, and the importance ofstructure inspection, maintenance, and repair. All of these aspects havebeen addressed in this report to provide the reader with a comprehensivebackground in the subject of concrete barge-like structures.

The text of this state-of-the-art report is intended as a technical overviewof the subject. The seven chapters which follow provide current informationon the subject. This report is not a design specification nor a designcriteria document, but rather a discussion of major aspects of concretebarge structure design, construction, and service performance. Specificreferences are provided, where appropriate, to refer the reader to addi-tional detailed, technical data and design formulae.

REFERENCES FOR CHAPTER 1.0

1.1 Gerwick, Jr., Ben C., "A Presentation of the Expanding Use ofPrestressed Concrete for Ocean Structures and Ships with Guides toEffective Design and Construction Practice," Prestressed ConcreteOcean Structures and Ships, Prestressed Concrete Institute, Chicago,IL, September 1975.

357.2R-6 ACI COMMITTEE REPORT

1.2 Gerwick, Jr., Ben C., Mansour, A.E.,Price, Edward, and Thayamballi, A.,Feasibility and Comparative Studies for the Use of Prestressed Concrete- - - -in Large Storage/ProcessingMarine Engineers,

Vessels, Society of Naval Architects andNew York, NY, 1978.


2.0 APPLICATIONS2.1 IntroductionThis chapter addresses selected applications of the use of concrete inbarge-like floating structures. The selection is not intended to provide acomprehensive list of applications, but rather to illustrate the wide varietyof marine applications for which concrete has provided safe, functional,durable, and economical solutions. These applications not only illustratethe versatility of floating concrete structures, but also highlight somecreative and novel engineering solutions to complex design problems.

This chapter first presents a brief historical background on the use ofconcrete for floating structures, and then describes examples of concreteships, barges, plantships, storage facilities, piers, docks, and breakwatersthat have been constructed or are being developed.

2.2 Historical BackgroundThe first use of reinforced concrete in floating vessels is attributed toLambot who, in 1848, constructed a boat by applying sand-cement mortar overa framework of iron bars and mesh [2.1]. The first self-propelled reinforcedconcrete ship was launched in 1917. This was the M.S. Namsenfjord, built byN.K. Fougner in Norway. Fougner went on to build several larger self-propelled reinforced concrete vessels.

The first self-propelled concrete ship in the United States was the S.S.Faith, which was launched in 1918. It was built in San Francisco and was atthat time the largest concrete ship in the world with a design deadweight of5000 tons (4540 tonnes). It had an overall length of 320 ft (97.5 m), abeam of 44.5 ft (13.5 m), and a depth of 30 ft (9.1 m) [2.1].

A principal impetus for continued development of concrete ships was theshortage of steel that occurred during World Wars I and II. In 1918, theUnited States Emergency Fleet Corporation instituted a program that resultedin the construction of 12 reinforced concrete vessels with deadweights up to7500 tons (6800 tonnes) [2.1]. These vessels used lightweight concreteextensively. Expanded clay and shale aggregates were developed to obtainconcretes with 28-day compressive strengths in excess of 4000 psi (28 MPa)and unit weights of approximately 110 pcf (1760 kg/m3).

Although a few concrete vessels were built after World War I, it was notuntil World War II that another major concrete ship program was undertaken[2.1]. The U.S. Maritime Commission initiated a project in mid-1941 whicheventually resulted in construction of 104 vessels, 20 of which were self-propelled. These vessels had concrete strength requirements of 5000 psi(35 MPa) at 28 days. Concrete produced at the different yards had freshunit weights ranging from 108 to 128 pcf (1730 to 2050 kg/m3) and 28-daycompressive strengths ranging from 5085 to 6920 psi (35 to 48 MPa). Noneof the World War I or II vessels saw extensive service, but many wereeventually used for storage barges or breakwaters.

Not all of the reinforced concrete vessels constructed during World War IIwere ships or barges. A number of large floating concrete dry docks werealso constructed. One had a length in excess of 400 ft (120 m) and could


dry-dock a 7000-ton (6350-tonne) vessel. It is still in use for the Port ofBellingham, Washington, USA.

Two prestressed concrete vessels were also constructed during World War II[2.2]. One was a landing craft and the other was a barge. These wereconstructed of precast "egg-crate" cells with prestressing steel tensionedalong the space between cells (Figure 2.1). The steel was then covered witha layer of shotcrete.

Since World War II, the primary applications for floating concrete structureshave been barges, oil drilling and storage platforms, floating bridges,docks, floating breakwaters, and pontoons.

2.3 Barge StructuresAlong the United States Gulf Coast in Louisiana and Texas, concrete bargesserve as float-in-place foundations for oil production, processing, andstorage facilities [2.3, 2.4]. The barges are used to support pump andcompressor installations, processing equipment, water and wastewatertreatment plants, settling basins, skimmer tanks, and living quarters (Fig-ure 2.2). They can also serve as floating docks. One type of barge, theBelden system, consists of an "egg-crate" hull made up of precast reinforcedconcrete panels. The assembled panels are "wrapped" with welded wire fabricreinforcement and shotcrete is applied to the exterior. Various topsideconfigurations can be constructed on the hull, depending on the intended useof the barge. Once constructed, the fully outfitted facility can be towedto its destination and ballasted into position. These structures aredesigned to be deballasted, refloated, and relocated. Over 400 of thesestructures are in use.

Prestressed concrete transport barges constructed in the Philippines havebeen in service since 1964 [2.5, 2.6]. Sixteen were built for general cargouse and three for bulk petroleum transport (Figure 2.3). The typical con-figuration includes an inner steel framing system supporting a mild steelreinforced and prestressed concrete hull. Deadweights range from 700 to2000 tons (630 to 1810 tonnes). The vessels have seen considerable service,including transport of ammunition, explosives, and petroleum products duringthe Vietnam war. The concrete in these smaller barges has resisted severeexposure from cargoes such as industrial salts and fertilizers. The interiorstructural steel framing, however, has been susceptible to corrosion.

To overcome the durability and maintenance limitations of the transversestructural steel framing, a patented concrete honeycomb framing system hasbeen developed that provides an efficient strength-to-weight ratio [2.7,2.8]. The total volume of internal framing occupies only a small fractionof the enclosed volume of the hull.

2.4 Industrial PlantshipsA floating terminal facility for liquefaction and storage of liquid petroleumgas (LPG) was built for Atlantic Richfield Indonesia [2.9]. The 65,000-ton(59,000-tonne) vessel, the Ardjuna Sakti, was designed as a post-tensionedsegmental structure (Figure 2.5). Segments were individually match-cast andthen post-tensioned together to form the hull. This structure was con-structed in Tacoma, Washington, USA, and towed 10,000 miles (16,000 km)

357.2R-9
BARGE-LIKE STRUCTURES
Fig. 2.1--Prestressed Concrete Landing Craft

Fig. 2.2--Floating Concrete Barge


DECK PLAN

MIDSHIP SECTION

Fig. 2.3--General Cargo Barge

Fig. 2.4--Circular Honeycomb Structural System used for PhosphateBarge


Fig. 2.5a--LPG Processing and Storage Facility (Ardjuna Sakti)

136 .0 ’ I

Fig. 2.5b--Cross Section of Ardjuna Sakti


across the Pacific Ocean to the Ardjuna oil and gas fields in the Java Sea.It has been in service since 1976.

Another example of a concrete plantship is a barge-like hull designed forthe Johns Hopkins University/Applied Physics Laboratory [2.10, 2.11]. Thedesign of this offshore thermal energy conversion (OTEC) plantship includesa post-tensioned concrete hull (Figure 2.6). The OTEC principle uses thewarm surface waters in the ocean to vaporize a low boiling point fluid. Thevapor is used to drive a turbine for power generation. Cold ocean water ispumped up from great depths and is used to condense the vapor, and the cycleis repeated. The OTEC plantship concepts allow operation in two differentmodes. In a stationary moored configuration, it could provide electricityfor industrial process plants. Alternately, in the mobile mode, energy-intensive products such as ammonia could be manufactured onboard and thentransported via carrier vessels for onshore use. The ammonia-producingplantships would "graze" to maintain optimum thermal gradients. As yet, noconcrete OTEC plantships have been constructed, but steel-hulled plantshipsand land-based systems are being tried on a developmental scale. Concretewas selected for as the Johns Hopkins University plantship hull materialbecause it is highly durable and requires little, if any, maintenance. Theconfiguration and size of this OTEC plant will preclude regular dry-docking.

Floating prestressed concrete hulls supporting industrial process equipmenthave been proposed by Swedyards Development Corporation for ammonia ureaplants [2.12]. Concrete hulls, typically 350 x 100 x 20 ft (100 x 30 x 6 m)in overall dimensions, can be constructed in a dry dock, outfitted, andtransported on semisubmersible barges. Once on location, the hull modulescan be off-loaded, floated to their operating site, and permanently groundedon a prepared seabed (Figure 2.7). Such vessels make it possible forindustry to operate economically in remote, often underdeveloped areas ofthe world.

The honeycomb framing system, as previously described, has been used forconstruction of a prestressed concrete barge for use as a floating platformto support a phosphate processing plant off the coast of Baja, California(Figure 2.4). The barge was constructed in Singapore and towed to the site.Construction included both cast-in-place and precast concrete. For anadditional application of this framing system, the reader is directed toReference 2.1.

Basically, there are three primary types of concrete floating plantships[2.12]:

(a) Type 1 -- Wet-towed and moored at an installation site(b) Type 2 -- Wet-towed and grounded at an installation site(c) Type 3 -- Dry-towed by semisubmersible barge, off-loaded and

floated into position at the installation site, and grounded

Each type may be capable of redeployment.

Design criteria selected for the different vessel types must address whetheror not the hull is to be considered as a ship [2.12]. This is generally thecase for Types 1 and 2. When "classed" as a ship by a certification agency

357.2R-13

COLD WATER PUMP!37 r-HE. LIFT FRAME

COLD WATER H.E.--\ ‘,\\

WARM WATER HE. ‘\

IA~0 ‘,,

PA /-CREW QUARTERS AND HELOPAD

0?f

GANTRY CRANE

pi-I‘vi

r

MOORING FENDERS --.-u0 - DTEC EQUIPMENT/

..__.~. 378'- o:-.._. _ _

PLAN -

- W A R MTHRUSTER WATER

INTAKEA - ASECTION

‘cJ\ COLD WATER PIPE

PROFILE

Fig. 2.6--OTEC Concrete Plantship

Fig. 2.7--Ammonia Urea Plantships


such as the American Bureau of Shipping (ABS), Det norske Veritas (DnV), orLlayds Register of Shipping (LRS), criteria for intact and damage stability,seaworthiness, crew safety, etc., are similar to those of conventionalvessels. For Type 3, the hull may be designed as a temporary floatingstructure. However, care must be taken during design to fully account forthe interaction of the concrete vessel and the supporting semisubmersiblebarge when afloat in the seaway.

Process applications for concrete plantships include [2.13]

(a) Fertilizer production(b) Manufacturing plants(c) Refineries(d) Desalination plants(e) Electric power stations(f) Chemical treatment facilities(g) LNG and LPG terminals

This list of applications illustrates the high potential for continueddevelopment.

2.5 Floating Piers and, DocksA floating precast prestressed concrete container dock is now in service inValdez, Alaska [2.14]. The system consists of a 100-ft-wide x 700-ft-long x30-ft-deep (30- x 210- x 9-m) prestressed concrete floating dock, a mooringsystem to hold the dock in position, and a fender system that protects boththe dock and ships during berthing operations (Figure 2.8). Constructioneconomy and a tight schedule required the dock to be prefabricated off siteand towed to the deployment location. Two dock pontoon sections were towedfrom Tacoma, Washington, to Valdez and joined together on site. The dockprovides a low-maintenance, high-capacity marine facility that rises andfalls with tidal changes, providing an efficient interface with surfacevessels during cargo transfer. Because it is floating, it can be redeployed.

Another example of a concrete floating dock facility is the twin ferryterminals on either side of Burrard Inlet in Vancouver, British Columbia,Canada [2.15]. Each terminal, E-shaped in plan, consists of four cellularconcrete modules post-tensioned together to form a single integrated unit(Figure 2.9). The floating system facilitates vessel berthing and easespassenger transfer. The concrete modules were assembled and prestressed ina graving dock and floated out upon completion. While afloat, decking ofthe individual modules was completed. They were then joined by post-tensioning through cast-in-place joints. Normal weight concrete with a28-day design compressive strength of 7000 psi (48 MPa) was used throughout.

The Camas Mill floating dock represents a unique industrial use of a concretesystem [2.16]. The 60- x 180-ft (18- x 55-m) dock is longitudinally andtransversely post-tensioned. It was constructed in Tacoma and towed toCamas, Washington. Four recesses in the deck accommodate hydraulic transferramps which are used to transfer mill products to river barges (Figure 2.10).

U.S, Navy requirements have led to the development of double deck concretefloating pier systems [2.17]. Generic designs for a double deck floating


Fig. 2.8--Valdez Floating Container Pier

Fig. 2.9--Concrete Floating Dock, Vancouver, British Columbia,Canada

Fig. 2.10--Floating Dock for Crown Zellerbach Mill, Camas,Washington, USA

Fig. 2.11--Concept of a Double-Decked Floating Navy Pier


pier [2.17, 2.18] have overall dimensions of 75 ft (23 m) in width by 33 ft(10 m) in depth including a pontoon depth of 18 ft (5.5 m). A specificapplication has an overall length of 1200 ft (365 m), which is made up oftwo 600-ft (182-m) sections. The segmental configuration is used to facili-tate construction in a dry dock and subsequent towing (Figure 2.11). Pilesallow vertical movement yet provide anchorage to prevent excessive horizontalmovement. Preliminary designs specify concrete with a density of 125 pcf(2000 kg/m3) and a design compressive strength of 5000 psi (34 MPa).

The U.S. Army Corps of Engineers has also worked on design of portableconcrete docks for off-loading containerships. One concept developed isthat of a self-contained off-loading lighter, SCOL (Figure 2.12). The SCOLis designed to move out to containerships at their discharge site, lower itslegs to the seafloor, stabilize, and then elevate the dock structure to alevel suitable for rapid off-loading of containers from the containership.Elevation is accomplished using rack and pinion drive systems on steel spudlegs. When the containers have been transferred, the dock structure islowered, the legs are raised, and the SCOL propels itself to shore foroff-loading. An elevated causeway system is used to bridge between the SCOLand shore. The SCOL system design provides the military with rapid materialdeployment on undeveloped shorelines.

2.6 Floating BridgesThe Hood Canal floating bridge crossing Puget Sound in Washington state,USA, is probably the best-known example of a concrete floating bridge struc-ture [2.19]. The bridge consists of a floating structure, fixed structureapproaches to each end, and east and west road approaches (Figure 2.13).The structure was originally constructed in 1960. The bridge is 7863 ft(2347 m) long including approach ramps, and the floating concrete portion is6471 ft (1973 m) long. The original floating structure was designed to opento provide a 600-ft (180-m) wide channel for ship traffic. Prestressedconcrete guide pontoons flanked the central floating pontoon in its openposition. The pontoons were built in a graving dock in Seattle and towed tothe site in 1959. Normal-weight concrete for the pontoons had a designcompressive strength of 3000 psi (21 MPa) at 10 days. A schematic of thepontoon and elevated roadway structures is illustrated in Figure 2.14.

Storms in the winter of 1959/60 damaged the Hood Canal bridge pontoons whilethe bridge was still under construction and necessitated strengthening byadding post-tensioning. An epoxy-resin grout connection was used betweenthe pontoons [2.19]. The bridge was opened to traffic in August 1961. TheWashington State Department of Transportation (WSDOT) established a mainte-nance and inspection plan for the bridge, and maintained the bridge until1979.

In 1979, a severe winter storm destroyed the western portion of the bridge[2.20]. A number of the pontoons were damaged and sunk. WSDOT conducted aninvestigation of the cause of the sinking and determined that the bridgepontoons and anchor system had been loaded by a combination of waves andwinds in excess of the original design criteria.

A three-stage plan was developed for the reconstruction of the bridge(Figure 2.15). The plan included reconstruction of many pontoon segments

357.2R-18

Fig. 2.12a--SCOL Discharging Containers at Shore

Fig. 2.12b--SCOL Moored with Containership


Fig. 2.13--Hood Canal Floating Bridge

Fig. 2.14--Cutaway Drawing of Standard Pontoon for Hood CanalBridge


New Western Portion

Stage I

Stage I StructureNew LiftDraw Span Existing Structures

Stage IIRelocatedTemporaryConnection N

Stage III

Fig. 2.15--Three-Stage Plan for Hood Canal Bridge Construction

Fig. 2.16--Hood Canal Bridge Span Replacement Concrete GravityAnchor During Installation


[2.21], reuse of some existing bridge segments, and a new anchoring schemeto secure the bridge. This anchoring scheme called for 26 concrete gravityanchors that are 46 ft (14 m) in diameter and 27 ft (8 m) in height (Figure2.16). The anchors are weighted with crushed slag and connected to thebridge segments by l-3/4-in. (45-mm) cables.

The reconstructed portion of the bridge uses larger pontoons and a widerroadway, more extensive use of prestressed concrete, and higher strengthconcrete. The reconstructed bridge has been opened to traffic since October1982.

2.7 Other StructuresReinforced concrete segments were used to construct an underwater tunnel toconnect Amsterdam with surrounding areas [2.22]. The reinforced concretesegments were constructed in a dry dock in 70-ft-wide x 73-ft-long (21- x22-m) pieces that were assembled into 440-ft (135-m) long units before beingfloated to the tunnel site (Figure 2.17). Each unit was outfitted withinstrumentation to permit survey control during placement. The underwatertunnel length is 4840 ft (1475 m). Because of the unique nature of theproject, special care was taken to carefully control the unit weight of thereinforced concrete segments. This permitted accurate control of launching,sinking, and final positioning.

Other potential uses of concrete barge-like floating structures relate tooil storage terminals [2.23]. Such terminals can be turret moored to agravity anchor system on the seafloor to allow alignment with dominant wind,wave, and current conditions (Figure 2.18). The concept of rotating mooringstorage provides for storage of large quantities of oil and facilities formooring and loading tankers.

An underwater oil storage concept has been developed for the Arabian Gulfwhere each storage unit would have a capacity of approximately 200,000barrels of crude oil. Units are made of prestressed concrete and can berefloated and used at another site after the oil reservoir has been depleted(Figure 2.19).

In Japan, a floating oil storage facility has been proposed for Kyushu. Thefacility utilizes seven prestressed concrete storage barges connected to amarine terminal (Figure 2.20). Total storage capacity is 40 million barrelsof crude oil.

Another unique application of concrete barge-like structures is the slopingfloating breakwater that was conceived to protect dredges used for bypassingsand across the Oregon Inlet in North Carolina. The system consists ofmoored inclined pontoons made of prestressed concrete (Figure 2.21), whichare ballasted and deballasted to provide needed wave protection. The pon-toons can be fabricated in a dry dock and towed to the site for connectionto mooring lines.

2.8 SummaryThe structures described above indicate the wide variety of applicationsthat are embodied by the concrete barge-like structure concept. For theseapplications, the economic disadvantage of the low deadweight-to-displacement


Fig. 2.17--Tunnel Elements Being Towed Into Position

Communication holes

Fig. 2.18--Rotating Mooring Storage System

357.2R-23

Fig. 2.19--Concept for Underwater Storage Tanks, Arabian Gulf

Fig. 2.20--Concept for 40-Million-Barrel Floating Crude OilStorage at Kyushu, Japan


Fig. 2.21--Sloping Float Breakwater Moored Element

357.2R-25

ratio of concrete has been partially overcome by the simple geometry of thestructures, the selected internal framing and, in some cases, the use oflightweight concrete. Concrete structures provide advantages which includestability during sea operations, low vibration levels, noncorrosive cargoenvironments, adaptability to all types of cargo, durability, fire resistance,low maintenance, ease of repair, and extensive use of common constructionmaterials.


2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

2.10

2.11

U.S. Department of Transportation, U.S. Coast Guard, Inspection Guidefor Reinforced Concrete Vessels, Vol. 2, Commentary, Report CG-M-11-81by A.E. Fiorato, Washington, DC, October 1981, 133 pp.

Anderson, A.R., Prestressed Concrete Floating Structures (State of- -the Art), Society of Naval Architects and Marine Engineers, New York,NY, 1975, pp. 123-136.

"Concrete Barges Multiply in Gulf," Concrete Products, Vol. 70, No. 1,January 1967, pp. 56-58.

"Marine Concrete Structures," Marine Concrete Structures, Inc., NewOrleans, LA, 8 pp.

Yee, A.A., Lum, K.B.T, and Golveo, V.S., "Design and Construction ofOceangoing Prestressed Concrete Barges," Journal of the AmericanConcrete Institute, Vol. 72, No. 4, April 1975, pp.125-134.

Sare, P.N. and Yee, A.A., "Operational Experience with PrestressedConcrete Barges," Concrete Afloat, Thomas Telford Ltd., London, 1977,pp. 71-81.

Yee, A.A., "Honeycomb Units for Barges and Floating Platforms,"Structural Engineering Practice, Vol. 1, No. 1, 1982, pp. 89-93.

"Concrete Barge Supports Mining Facility," Concrete International,Vol. 4, No. 3, March 1982, pp. 35-38.

Anderson, A.R., "World's Largest Prestressed LPG Floating Vessel,"Journal of the Prestressed Concrete Institute, Vol. 22, No. 1,January/February 1977, 21 pp.

Magura, D.D. and Mast, R.F., "Ocean Thermal Energy Conversion (OTEC),10 MWe Preliminary Plantship Design," Proceedings, Offshore TechnologyConference, OTC Paper No. 3593, Houston, April 30-May 3, 1979,pp. 2059-2066.

Litvin, A. and Fiorato, A.E., "A Lightweight Concrete for OTEC ColdWater Pipes," Concrete International, Vol. 3, No. 3, March 1981,pp. 48-55.


2.12

2.13

2.14

2.15

2.16

2.17

2.18

2.19

2.20

2.21

2.22

2.23

Birkeland, P.W., LaNier, M.W., Magura, D.D., and Mast, R.F.,"Prestressed Concrete Hulls for Barae Mounted Plants." ChemicalEngineering Progress, November 1979, pp. 44-45.

Haynes, H.,ProceedingsEngineering

"Future Applications for Concrete Ocean Structures,"of the XVII UPADI Convention, Pan American Federation of- - -Societies, San Juan, Puerto Rico, August l-7, 1982, 5 pp.

"Floating Container Terminal, Valdez, Alaska," Journal of thePrestressed Concrete Institute, Vol. 27, No. 4, July-August 1982,9 pp.

"Floating Ferry Terminals, Burrard Inlet, Vancouver, BC," ABAM EngineersInc.,Federal Way, WA, 2 pp.

"Camas Mill Floating Dock, Crown Zellerbach Corporation," ABAM EngineersInc.,Federal Way, WA, 1 p.

Chow, P. and Haynes, H., "Innovative Designs for Navy Piers,"Proceedings of the Ports '83 Conference, March 1983, pp. 442-455.- - - -

"Navy's Floating Pier Meets Tough Demands," Engineering News-Record,January 27, 1983, pp. 15-16.

Nichols, C.C., "Construction and Performance of Hood Canal FloatingBridge," Symposium on Concrete Construction in Aquaeous Environments,ACI Special Publication SP-8,American Concrete Institute, Detroit,MI, 1964, pp. 97-106.

Abrahams, M.J. and Belvedere, J.A., "Hood Canal Bridge," ACISymposium on Offshore Structures, New York, NY, October 1984.

"Bridge Pontoons ae Cast Singly in Tight Order," Engineering News-Record,September 29, 1983, p. 70.

Zallocco, G., "A Sunken Segments Railway Tunnel in Amsterdam,"L'Industria Italiana Del Cemento, Vol. 14, Rome, September 1984,pp. 506-523.

Hersent, A. and Andrier, B., "Floating Dry Dock and Prestressed Con-crete and Rotating Mooring Storage (RMS)," Proceedings of FIP Symposium,--Concrete Sea Structures, Tbilisi, September 1972, Federation Inter-nationale De La Precontainte, Wexham Springs, Slough, England, 1973,pp. 49-54.

357.2R-27

3.0 MATERIALS AND DURABILITY3.1 IntroductionThe development of both normal-weight and lightweight marine concretesparallels the history of concrete ships and floating barge-like structures.Normal-weight concretes having design compressive strengths of 9000 psi(62 MPa) and lightweight concretes having design compressive strengths of7000 psi (48 MPa) are now achievable.

To obtain improved buoyancy and resulting cargo economy, lightweight aggregateconcrete was introduced for ship and barge construction during World War I.In recent years, prestressed lightweight concrete has been used for marinestructures to allow additional weight reduction, with accompanying shallowerlaunch draft, and construction benefits through segmental construction.Reduced weight also aids construction speed and economy when handling precastsegments. The use of floating barges for liquefied gas and petroleum productshas created additional concrete durability requirements. Documentation ofprevious satisfactory performance for the proposed materials or, in lieu ofsuch documentation, sufficient backup test data are necessary in reviews forvessel certification. The American Bureau of Shipping has accepted certifi-cates from suppliers as proof of quality.

Research is continuing on materials for barge-like structures. In general,Chapter 2, Materials and Durability, of ACI 357R is applicable to concretebarge-like structures. Additional specific discussion can be found inACI 357.1R.

3.2 Testing and Quality ControlTests for concrete and other construction materials should be in accordancewith the applicable ASTM standards cited in ACI 318. These methods providesatisfactory guidance for the research and construction industry. Due tothe varied use of barge-like structures, complete records should be madeavailable for inspection during construction and retained by the owner forthe lifetime of the structure. Such records may prove valuable in theformulation of maintenance and repair procedures.

As new materials are developed, new standard tests may also need to bedeveloped to assess compliance with specified durability and quality speci-fications. A case in point is the increased use of silica fume as a mineraladmixture for reduced permeability, increased durability, and higher strengthconcretes [3.1]. This material has demonstrated by test a high potentialfor enhancing the quality of marine concrete structures. Although used inEurope, the quality and performance standards of silica fume need to beassessed by standard test methods (such as ASTM) to further enhance universalacceptance in the United States.

Providing the day-to-day quality control functions during construction of amarine concrete structure is normally the responsibility of the constructioncontractor. Within the contractor's organization, the overall management ofthe quality control program is often assigned to a professional engineer whohas specific skills in materials technology and construction methods, andwho also has a clear understanding of materials test methods, acceptancestandards, and the statistical nature of acceptance testing of in-place


concrete. This individual typically reports directly to upper management ofthe construction firm.

Frequently, a barge-like structure is built that is new in concept or func-tion and may require construction methods which are used for the first time.In such instances, the owner will want to specify additional quality assurancefunctions at the job site to augment the quality control function providedby the contractor. Such quality assurance may be necessary to meet ownerrequirements for vessel certification by a regulatory agency, or simplybecause of the complex nature of the structure assembly, or because of theload sensitivity of certain portions of the structure. In such instances,it is common to expect that representatives of the regulatory agency(American Bureau of Shipping, U.S. Coast Guard, Det norske Veritas, andothers) and the barge design consultant will be in residence during con-struction. The function of these individuals is to provide the contractorwith an in-depth understanding of acceptable marine construction standardsand a more complete understanding of the design intent and service operationof the vessel, and to assure the owner's financial and insurance intereststhat the vessel has been constructed to the required standards.

A summary of testing and quality control considerations for barge-likestructures under construction can be found in Reference 3.2.

3.3 Structural Marine Concrete3.3.1 GeneralBoth normal-weight and lightweight concretes have been used in the construc-tion of barge-like concrete structures. Lightweight concretes have beenused for vessels such as ships and barges where maximizing payload andreducing power requirements are important. For stationary vessels such asmoored barges, floating bridges, breakwaters, and floating docks, normal-weight concretes are commonly used. High-quality normal-weight concretesare generally more available than equivalent strength lightweight concretes.With proper selection of constituent materials (including supplementarycementitious materials), specification of low water-cement ratio, use ofhigh-range water-reducing admixtures, acceptance criterion at 56 or 91 days,and moist curing, concrete compressive strengths exceeding 9000 psi (62 MPa)are attainable.

Until recently, high-strength lightweight concretes have not been widelyspecified for floating marine structures. Lack of demonstrated high-strengthcharacteristics, assumed poor resistance to marine corrosion and deteriora-tion due to high permeability, availability of quality aggregates, and costshave discouraged specification of higher strength lightweight concretes.Today this is changing because high-strength, low-unit-weight, and high-durability (low-permeability) lightweight concretes are available.

3.3.2 Marine Lightweight ConcretesIt is now common to consistently produce high-quality marine lightweightconcretes having fresh unit weights of 120 to 125 pcf (1920 kg/m3 to2000 kg/m3) and design compressive strengths (fc) in excess of 6500 psi(45 MPa). See ACI 213R. Current research in the United States and Japanindicates that marine lightweight concretes having a unit weight of 110 to120 pcf (1760 to 1920 kg/m3) and compressive strength of 9000 psi (62 MPa)


can be made commercially [3.3, 3.4, 3.5]. These concretes typically havewater-cement ratios of 0.40 or less and cement contents of 690 to 830 lb/cy(410 to 490 kg/m3). Batching these concretes using prequalified materialsand exercising proper construction supervision (having a comprehensivequality control program) can assure the production of durable barge-likeconcrete structures. Ships, barges, and jetties constructed using conven-tional, lower strength lightweight concretes during World War II are stillin service showing little deterioration.

Given a normal-weight and a lightweight concrete of equal strength andpermeability, the high-strength lightweight concrete may offer advantagesover the normal-weight concrete for reasons which go beyond those associatedwith reduced vessel draft. High-strength lightweight concretes can offerthe following advantages for marine structures:

(a) Higher resistance to microcracking due to the reduced modulus ofelasticity of the aggregates

(b) Lower stress concentrations within the matrix to partiallycompensate for the reduced aggregate strength

(c) Lower modulus of elasticity, which results in reduced stressescaused by shrinkage, creep, and in thermal effects

(d) Lower values of thermal conductivity and thermal expansion,which provides improved resistance to thermal cracking

The Federation Internationale de la Precontrainte (FIP) report, State-of-the-Art Report: Lightweight Aggregate Concrete for Marine Structures,contains considerable discussion on these subjects:

Lightweight aggregate concretes can also present disadvantages to both thedesigner and constructor. The tensile strength of lightweight concrete isfrequently a reduced proportion of the compressive strength when compared tonormal-weight concretes. Hence, care must be taken during design to providefor additional mild steel reinforcing in areas of high flexure and shear(diagonal tension). During construction, aggregate stockpiling must becontrolled to prevent variations in moisture content of the aggregates thatcan affect uniformity of batching. Also, concrete consolidation and placingmethods must address the tendency for the aggregates to absorb water duringmixing and handling and to "float" during placing.

There are several precedents for the use of lightweight concretes in marinestructures [3.2]. The U.S.S. Selma was constructed in 1919 using lightweightconcrete with a dry unit weight of 119 pcf (1910 kg/m3) and a 28-day com-pressive strength of 5000 psi (35 Mpa). The concrete mixture had a water-cement ratio of 0.49 and a very high cement content of 1034 lb/cy (613 kg/m3).In 1953, this hull was inspected and the core samples taken indicate concretestrengths exceeding 8000 psi (55 MPa). The hull was again inspected in 1980and concrete test samples indicated a compressive strength of 10,000 psi(69 MPa). More recent examples include the Tarsiut Arctic caisson, con-structed for Dome Petroleum and deployed in the Canadian Beaufort Sea in


m

A

m

Tc

gc

a

Tm

aacd

w

1981 [3.6]; and the Global Marine "Super-CIDS" arctic caisson, constructedin Japan in 1984 [3.7].

Typical marine concretes have relatively high cement contents [650 to 950 lbper cubic yard (385 to 565 kg/m3)], and many contain mineral admixtures(slag, fly ash, silica fume). Mixtures such as these can have low cementpaste permeability, a definite benefit for marine concrete structures.However, high cement factors and high reactivity of cementitious materialsmay produce undesirable heat of hydration when curing the concrete, whichcan lead to potentially detrimental thermal cracking of the concrete. Forthese reasons, selection of curing techniques for marine concrete structuresneeds careful consideration and strict control during construction. Extendedoist curing or use of insulated formwork is often considered.

CI 221R and 213R provide information on aggregates; ACI 211.1 and 211.2provide data on mix designs for normal-weight and lightweight concretes.

3.3.3 Constituent Materials3.3.3.1 CementSpecial cements are not required for marine concretes. Cement Types I, II,and III corresponding to ASTM C 150 have been widely used. Blended cementsin accordance with ASTM C 595 have also been used. ACI 225R provides infor-ation on the selection and use of hydraulic cements.

o provide resistance to sulfate attack in the marine environment, specifi-ations commonly limit the tricalcium aluminate (C,A) content of the cement.Suggested limits for allowable C3A content of the cement vary but, ineneral, range from 4% to 10% [3.2]. The minimum limit is used to provide aorrosion-resistant environment for embedded reinforcing. The maximum limitis used to reduce the detrimental effects of sulfate on the matrix. Theddition of pozzolans such as fly ashes has been found to be beneficial inreducing sulfate attack [ACI 225R].

he alkali content of the cement may also be limited using optional require-ents of ASTM C 150 if alkali-aggregate reactivity is a consideration forthe combination of constituent materials being used [ACI 225R].

3.3.3.2 AggregatesNormal-weight aggregates should conform to the specifications of ASTM C 33nd lightweight aggregates to ASTM C 330. Lightweight aggregates thatbsorb significant quantities of mixing water during batching and placingan cause rapid slump loss and poor workability. This can lead to extremeifficulty in placing the concrete in congested reinforcing areas common tolight draft marine barge-like structures. To maximize resistance to freezingand thawing, aggregate moisture content is frequently controlled to 8% orless at mixing. To provide uniform batching control in the recent con-struction of an Arctic concrete structure, dry lightweight coarse aggregatesere used. The batch water was adjusted to compensate for the initialabsorption of the aggregates. This was done in an attempt to maximizefreezing and thawing resistance of the structure [3.7]. Lightweight aggre-gates that are fired after crushing or pelletizing generally have a sealedsurface that reduces water absorption.


Aggregates should be free of contaminants and reactive agents. Excess freewater in stockpiled aggregates should be drained, and the aggregate stock-piles sampled to assess how the batching proportions should be modified tocompensate for the moisture content of the aggregate.

Current industry trends include the development of coated surface light-weight aggregates. These aggregates, once fully tested and developed, mayprovide valuable enhancement of durability of lightweight concretes byproviding additional protection against freezing and thawing. Suchaggregates can have moisture absorption levels equivalent to that for normal-weight aggregates. Additional information on aggregates is available inACI 221R and 213R.

3.3.3.3 Mixing WaterMixing water should be clean and should meet requirements of ACI 201.2R and318. In some very controlled instances, using nonpotable water is allowedprovided trial batches and sample mortar cubes indicate satisfactory strengthcapacity, and as long as the accepted limits on the chemical composition ofthe mixing water are met. Chemical composition limits address the chlorideand sulfate content of the water. These limits are established to ensuresufficient durability and resistance to chemical attack and corrosion.

3.3.3.4 AdmixturesAdmixtures are used in concrete structures for a variety of reasons. Anadmixture is any material other than hydraulic cement, aggregate, or mixingwater or fiber reinforcement which has been intentionally added to theconcrete to modify its properties or performance either in the fresh orhardened state. A list of commonly used admixture types follows:

(a) Pozzolans (including fly ash and silica fume) [3.1](b) Retarding admixtures(c) Air-entraining agents(d) Water reducers(e) Accelerating agents(f) Workability agents(g) Miscellaneous admixtures (air-detrainers, waterproofing

agents, etc.)

Careful consideration should be given before selecting an admixture toenhance concrete properties. Some admixtures may contain excessive amountsof chlorides or other corrosion-contributing components, and it is suggestedthat such admixtures only be used to the extent that suggested limits fordurability, such as the overall chloride content of the concrete, are notexceeded. ACI 212.2R provides guidance for admixture use.

Because concrete durability in the marine environment is a critical designconcern, many marine concretes are air-entrained and have a low water-cementratio. Air-entraining and water-reducing admixtures are commonly used inmarine concretes. Use of these admixtures successfully during constructionrequires a thorough understanding of their effects on the time-dependentproperties of concrete.


3.4 Reinforcing and Concrete CoverFor most general-purpose barge-like vessels, requirements for reinforcingand prestressing steel systems are identical to those used in more commonconstruction (buildings, bridges, piers, etc.). Even barge-like structuresintended for Arctic service are being designed using existing standards.

There are exceptions regarding requirements for concrete reinforcing. Formoderately low-temperature applications such as for Arctic service, mechan-ical couplers of reinforcing bars or threadbar systems should be tested forductility when loaded beyond yield stress at service temperatures. Thedesigner may wish to prepare a specification outlining requirements for suchtests on systems which are otherwise not substantiated by test or prequali-fied for Arctic low-temperature use.

For cryogenic applications, use of cold-drawn wire strands for prestressingtendons and mild-steel reinforcing per ASTM A 706 should be considered. Asis the case for other prestressing systems, the supplier should demonstratethe capacity of the anchorage system to sustain a load of at least 90% ofthe ultimate strength of the strand or bar for bonded systems, and 100% forunbonded systems. For marine concrete structures, bonded post-tensioningsystems should be used.

Concrete cover requirements are important and must be considered duringdesign of a concrete barge-like vessel. Concrete cover is that amount ofconcrete which surrounds or overlays the reinforcing and establishes abarrier between the concrete and the environment. Requirements for minimumcover are frequently listed as mandates by reviewing agencies (AmericanBureau of Shipping, Det norske Veritas) and design codes. These requirementsvary depending upon where the reinforcing is located in the vessel such asthe splash zone, the atmospheric zone, or the submerged zone. All of theseapparent mandates are based on the common goal to prevent corrosion of thereinforcing steel, to ensure proper concrete/steel bond behavior, and toprevent future reduced function of the vessel.

The amount of necessary cover depends on crack control considerations, thepermeability of the concrete itself, and the likelihood of surface degrada-tion of the concrete during normal service. Undamaged, low permeabilityconcrete can provide very adequate corrosion protection for the reinforcingwith cover as low as l/2 to 3/8 in. (15 to 10 mm). Indeed, old marinestructures such as harbor facilities and vessels have been inspected andfound to be virtually free of corroded reinforcing while having surprisinglylittle concrete cover. Conversely, poorer quality concretes overlayingreinforcing with substantial cover [as much as 3 in. (75 mm)] may not provideadequate protection. Industry recommendations need to be followed regardingconcrete cover to minimize costly repairs, and a comprehensive tabulation ofthese recommended cover limits can be found in ACI 357.1R. Current industrytrends include the development of very low permeability concretes. Usingthese concretes may allow reduction in minimum cover requirements. Testingis needed to quantify these requirements.

3.5 Special ConsiderationsFor deck structures, areas subjected to spillage of caustic or corrosivematerials, or other areas of a barge-like vessel where heavy wear is


anticipated, epoxy-coated reinforcing bars, in accordance with ASTM A 775,are frequently used.

For thin sections, or sections subjected to abnormal loadings, steel fibers,mixed with the fresh concrete, may be used to enhance toughness and increasemember shear resistance.

3.6 SummaryConsiderable precedents have been established for the use of various mate-rials for concrete barge-like vessels. High-quality, highly durablematerials are available economically in large quantities for marineconstruction. Virtually all of the materials in use today (concretes,reinforcing, coatings) have well-established performance records. Themarine concrete construction industry has developed cost-effective methodsfor using these materials for barge-like vessel construction. Designers andcertifying agencies have established comprehensive guidelines to be used inpreparation of construction specifications for industry use. As with anycomplex structure, comprehensive and rigorous quality assurance and qualitycontrol programs are required to ensure the successful construction.

The materials industry and the technology base for this industry are growing.Higher-strength, lighter, and more durable concretes are being developed andtested which will allow construction of improved marine structures in thefuture.

t

3.1

3.2

3.3

3.4

3.5

3.6

REFERENCES CHAPTER 3.0

ACI Committee 226, "Silia Fume in Concrete," ACI Materials Journal,Vol. 84, No. 2, March-April 1987, pp. 158-116.

U.S. Department of Transportation, U.S. Coast Guard, Inspection Guidefor Reinforced Concrete Vessels, Report CG-M-11-81 by A.E. Fiorato,Washington, DC, October 1981.

Fiorato, A.E., Person, A., and Pfeifer, D.W., "The First Large ScaleUse of High-Strenth Lightweight Concrete in the Arctic Environment,"TP-040684, Second Arctic Offshore Symposium, Houston, TX, April 1984.

Holm, T.A., "Physical Properties of High Strength LightweightAggregate Concretes," Second International Congress on LightweighConcrete, London, April 1980.

Seabrook, P.T. and Wilson, H.S., "High-Strength Semi-LightweightConcrete for Use in Offshore Structures: Utilization of Fly Ash andSilica Fume," CSCE/CANMET International Workshop on Concrete forOffshore Structures, St. John's, Newfoundland, September 10-11, 1986.

Fitzpatrick, J. and Stenning, D.G., "Design and Construction ofTarsiut Island in the Canadian Beaufort Sea," Proceedings, OffshoreTechnology Conference, OTC Paper 4517, Houston, TX, 1983.


3.7 Wetmore, S.B., “The Concrete Island Drilling System: Super Series(Super CIDS)," Proceedings, Offshore Technology Conference, OTC Paper4801, Houston, TX, 1984.


4.0 EVALUATION OF LOADS4.1 IntroductionThis chapter addresses the identification, definition, and determinationof the loads to which a barge-like concrete structure may be exposed. Thevarious loads which should be identified and accounted for in the design andoperation of the structure include dead loads, live loads, deformationloads, accidental loads, construction loads, and environmental loads.

The definition of loads, with the exception of deformation loads, does notvary between a concrete structure and those constructed of other materials.However, the structure's response to these loads may differ substantially.Obviously, temperature differences will cause structural responses unique tothe characteristics of the construction material. In addition, the dynamicresponse and distortion of the vessel is, in part, a function of the massdistribution of the vessel. This distribution may vary significantly betweenconcrete and steel construction. For simple cargo-carrying applications,dynamic response is generally not considered. However, if the structure isto carry motion-sensitive equipment and slender, tall appurtenances such asmay be used in process systems, or if sophisticated structural analysisprocedures are to be followed, the dynamic response of the vessel should beassessed.

For floating structures, the predominant environmental consideration in thestrength of the vessel is wave effects. For vessels used in general trans-portation services, no other environmental load is generally considered forthe structural analysis. Vessels for certain specialized services, such asfor ice-breaking or transporting low-temperature cargoes such as liquefiedgases, are considered unique and would require specific load definition on acase-by-case basis. While there have been full ocean-service self-propelledconcrete vessels [4.1] and a recent design [4.2] for such a vessel, it islikely that concrete vessels will be limited to use as barges (i.e., nopropulsion machinery) for the near future. Barges are generally intendedfor use in a specific service and, accordingly, marine practice is todifferentiate between barges used in full ocean, short coastwise, or riverservice with regard to the applicable wave criteria.

Site-specific structures (i.e., structures intended to remain moored onstation for one year or longer) are also designed predominantly consideringwave load; however, considerations of current, wind, and tidal range arealso taken into account. In ice regions, ice loads and thermal effects arealso considered. Some site-specific structures such as floating nuclearpower plants or generating stations are also designed for unique verticalpressures and accelerations caused by undersea earthquakes, called "seaquakes."

Section 4.2 discusses the load definitions for vessel design. The usualprocedure in determining global moments and shears is to consider a "still-water" condition and a transient condition. The still-water conditionrepresents the balance of the dead and live load and buoyancy in calm seas.The transient condition is caused by deformation, environmental, or acci-dental loads. Calculating the still-water loads is a simple procedureconsidering static conditions of load, upward buoyancy, and hydrostaticpressure. The transient component of load can be considered either as a


quasi-static load or, using statistical procedures, as a time or frequency-dependent load.

4.2 Load Definitions4.2.1 Dead LoadsDead loads associated with the structure are loads that do not change duringthe mode of operation under consideration. Dead loads include

(a) Weight in air of the barge

(b) Weight of permanent ballast and the weight of permanent machineryincluding liquids at operating levels

(c) External hydrostatic pressure and buoyancy in calm sea conditions,assuming the vessel is submerged to the design waterline

4.2.2 Live LoadsLive loads associated with the normal operation of the structure are loadsthat could change during the mode of operation considered and are con-trollable through operating procedures. Live loads include

Weight of production equipment that can be removed

(b) Weight of crew and consumable supplies

(c) Weight of liquids in storage tanks

(d) The forces exerted on the structure during the operation ofcranes and vehicles

(e) The forces exerted on the structure from moorings or towing

(f) Anticipated cargo

When applicable, the dynamic effects on the structure from Items (d) and(e), above, should be considered.

4.2.3 Deformation LoadsDeformation loads are those resulting from temperature variations leading tothermal stresses in the structure; effects of prestress, creep, and shrinkage;and, where appropriate, those resulting from soil displacements (e.g.,differential settlements or lateral displacements). Topside structures forfloating vessels will be affected by the global hull deflections. Thesedeflections are considered as boundary conditions in the design of thetopside structures.

4.2.4 Accidental LoadsAccidental loads occur as a result of an accident or exceptional conditions,such as

(a) Impact caused by vessels of the size anticipated to be in thevicinity of the structure


(b) Impact caused by dropped objects and floating debris

(c) Loss of internal overpressure required to resist hydrostaticloading and to maintain buoyancy

(d) Explosion

(e) Fire ’

(f) Ice collision

4.2.5 Construction Loads(a) Launching(b) Topside erection(c) Equipment installation

42.6 Environmental LoadsEnvironmental loads consist of static, quasi-static, or dynamic loads causedby various environmental phenomena. These are

(a) Waves(b) Winds(c) Currents(d) Earthquake(e) Ice and snow(f) Tides

4.3 Load DeterminationDead and live loads are determined from weight and load distributions on thehull. These loads are compiled during the design process and are balancedby hydrostatic pressure distributions. Deformation loads due to thermaleffects are determined from a steady-state condition considering air, water,and internal space temperature patterns and the thermal characteristics ofthe construction material. Deformation loads resulting from prestress andmaterial effects are calculated following standard concrete design practice.Accidental loads are, in general, estimates of possible impact loads oroverpressures which could result from anticipated conditions.

The possible approaches for calculating environmental loads include a quasi-static procedure and a time or frequency domain dynamic procedure. For mostapplications, a suitably formulated quasi-static approach is sufficient andwill result in a safe design.

4.3.1 Quasi-Static Procedure4.3.1.1 GeneralThe quasi-static approach has been widely used for designing vessels ingeneral cargo service. The assumption made in this procedure is that thevessel can be analyzed at some instant when local dynamic effects are maxi-mized and the global loads are determined considering the vessel poised witheither the design wave crest or trough amidships.

This approach can also be used directly when applied to site-specific vesselswhich are moored in such a way that they will "weather vane" around a mooring;


where

Mt = total bending moment

M =SW still-water bending moment

Mwi = wave-induced bending moment

Similarly, the total barge hull shear can be

v, = vsw + vwib

where

Vt = total hull-girder shear

VSW

= maximum still-water hull-girder shearing force

Vwi= maximum shearing force induced by waves

The longitud nal distribution of both bending moment and shearing forces canbe expressed mathematically. Expressions for the above forces and momentscan be found in References 4.6 through 4.10. Common parameters used inthese expressions are vessel length, beam and block coefficient, and nominaldesign wave height. In addition to longitudinal hull-girder loads, trans-verse and torsional loads are often considered directly in design. Combininglongitudinal, transverse, and torsional loads requires special attention bythe designer to provide a safe yet economical design.

expressed as

(4-2)

The empirical formulas provided by the classification societies are appro-priate for determining the still-water components for preliminary design;however, it is generally recommended that the calculation of final designstill-water bending moment and shear be based on the actual load distribution.It is accepted practice to use the empirical formulas for wave effectsthroughout the design process. It is also acceptable to reduce the wavecomponents, depending on the anticipated area of service. Vessels intendedfor use on rivers and in harbors are often designed for one third of thefull wave-induced values; designs for short coastwise service [within12 miles (19.3 km) of shore] often use two thirds of the full value. Thesereductions recognize the lower wave heights of these locations and theincreased proximity to harbors as compared to the open ocean. Utilizingthese reductions in the design will of course limit the use of the bargeduring its life.

4.3.1.3 Local LoadsLocal loads, which include the effects of external water pressure, cargoloads, and equipment loads, can also be determined by superimposing staticand dynamic load components.

The external water pressure is generally calculated as the static pressureconsistent with the vessel floating at its design waterline in combinationwith a dynamic component dependent upon the vessel design. Reference 4.11provides a discussion of the experimental work undertaken to determine the


dynamic component of external pressure, and References 4.3 through 4.10provide practical design guidelines for estimating external design pressures.

Internal liquid pressures are calculated as a static component taken to theheight of the overflows plus a dynamic component which is a function of thedynamic response of the vessel. References 4.3, 4.4, and 4.5 provide designmethods for predicting the dynamic component. Large tanks are also subjectto sloshing loads. The provision of slosh bulkheads can limit this phenome-non; however, the possibility of large dynamic pressures caused by sloshing,as discussed in Reference 4.12, should also be investigated.

Bulk dry cargoes (i.e., grain, coal, and iron ore) are assumed to load thestructure similarly to liquid cargoes; however, the height of cargo islimited by the height of the cargo space, the angle of repose of the cargo,and an assumed amount of shifting related to the dynamic characteristics ofthe vessel. Cargoes carried in containers or independent tanks load thestructure similarly to machinery and the local structural design followsusual foundation design procedures.

4.3.2 Dynamic Procedure4.3.2.1 GeneralProcedures are available to conduct wave-induced dynamic response analyseson floating structures. Generally, such analyses are performed usingfrequency domain procedures; however, time domain solutions are also avail-able. Both methods involve the use of computer programs which should bevalidated for accuracy using model test results or applicable closed formtheoretical solutions.

A frequency domain analysis can involve

(a) Establishing a statistically described definition of the waveconditions

(b) Calculating system mass, damping, and the varying pressure onthe vessel for a range of unit wave heights

(c) Determining the dynamic response of the structure (i.e., thedisplacements, velocities, accelerations, and inertial loadsof the structure)

(d) Computing the influence of moorings on system response, asapplicable

(e) Completing a load analysis which establishes a response compo-nent, such as vertical hull-girder bending moment over a rangeof wave frequencies in the form of a response amplitude operator(RAO)

(f) Combining the RAO with the statistical representation of thewave climate to establish the design loads on the structure

This procedure was originally developed for naval vessels and unique designssuch as liquefied natural gas carriers and high-speed container vessels.


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Its application to a particular design will be based on the function of thevessel rather than the construction material. See References 4.11 through.27.

.3.2.2 Environmental Conditionsver the past 30 years, considerable work has been undertaken to collectdata and develop statistical definitions of the ocean wave environment. Inaddition, hindcast techniques which define oceanographic conditions based oneteorological conditions have been developed. As these are specializedareas of study, it is generally prudent for the designer to utilizeeteorologists, oceanographers, and other specialists in developing the waveenvironment. The wave conditions are generally represented by the probabil-ity of occurrence of various significant wave height groups classified byirection and range of characteristic periods. In addition, the average

storm duration for various significant wave-height groups can be estimated.

he appropriateness of the statistical methods used in a specific analysisan be demonstrated by relevant statistical tests, confidence limits, andther measures of statistical significance. To compute the dynamic loadingscting on a vessel, a spectral representation of the wave data is necessary.If spectral data are not available in adequate quantities for the intendedpplication, appropriate mathematical formulations such as those attributedo Pierson-Moskowitz, Bretschneider, JONSWOP, ITTC, etc., are used [4.28,.29, 4.30).

s previously mentioned, vessels in general cargo service are generallyesigned for the effects of waves, independent of wind and current. However,he design environmental conditions for site-specific designs should repre-ent some rational combination of the environmental loads produced by waves,ind, current and, where appropriate, ice.

ind intensity and direction are commonly assumed to be directly correlatedith wave conditions. Accordingly, site-specific designs considering aertain wave probability of occurrence include wind intensity of the samerobability.

ind velocities are classified on the basis of their duration. Windelocities having a duration of less than 1 minute are referred to as gustinds. Wind velocities having a duration equal to or greater than 1 minutere referred to as sustained winds. The standard reference elevation forind measures is 33 ft (10 m) above still-water level (SWL). Wind predic-ions can also be made by statistical methods. However, a quasi-staticepresentation of wind load is generally employed in analysis even thoughhe waves are characterized by a wave spectrum.

idal current, wind-generated current, density current, circulation current,nd river-outflow current are often combined on the basis of their probabil-ity of simultaneous occurrence to determine a design current velocity.urrent velocity profiles are estimated on the basis of site-specific studiesr defensible empirical relationships. Unless a detailed study of currentirections is made, currents are generally assumed to run in any direction.ufficient data generally do not exist for a spectral definition of currentsand accordingly the current load is often treated quasi-statically.


cv

tt

TetM

s

supo

Ice loads are considered for a number of situations. Predicted extreme iceonditions with a probability of occurrence consistent with the life of theessel are general considered independent of wave, wind, and current.Lesser levels of ice conditions are combined with wave, wind, and currentload levels having probabilities of occurring in conjunction with these iceloads. The statistical representation of these conditions is specific tohe site and, accordingly, special studies are generally necessary to definehe load conditions associated with these events.

he design environmental condition for site-specific designs is a statisticalstimate of the most probable extreme environmental conditions to occur athe installation site within some chosen return period. The Mineralsanagement Service [4.13] specifies a return period of five times the designlife of the structure. Therefore, a structure with a 2O-year design lifehould be designed for conditions having a return period of 100 years. Thisis considered an extreme event and relates to the ultimate strength limittate. For ocean-going vessels, the spectral analysis procedures are basedpon a wave with a probability of exceedance of 10'8, which corresponds to arobability of one occurrence in 20 years, assuming an average wave periodf 6.3 seconds.

.3.2.3 Hydrodynamic Pressure Calculationsor barge configurations that will substantially alter the undisturbed,ncident flow field, forces due to the incident, diffracted, and radiatedaves should be included. Available hydrodynamic theories used to computehe wave forces on a barge can be grouped into two categories: two-imensional strip and three-dimensional exact methods. Two-dimensionalheories available at the present time are

(a) Two-dimensional Lewis transformation method(b) Frank close-fit method using two-dimensional source distribution(c) Generalized mapping technique(d) Two-dimensional fluid finite element method

mong the three-dimensional methods, three-dimensional source distributionethods and three-dimensional finite element methods are available.

se of procedures based on two-dimensional theory is generally acceptableor design, particularly for vessels and barges with ship-like length-to-beamL/B) ratios, 7:1 to 4 : 1 When such a method is used, the wave field isescribed by a wave spectra characterization appropriate to the wave heights,ave periods, and water depth at the location being considered. Wave impactoads on structural members above and below the design waterline elevationre accounted for by theoretical methods, or by using relevant models orull-scale data. The inertial load component due to rigid body motion ofhe structure due to flow-induced cyclic loading is also accounted for.

In an analysis using the two-dimensional strip theory, the barge is idealizeds having many two-dimensional transverse sections. The wave forces on thearges are then calculated section by section assuming that there is no

hydrodynamic interaction between them. The sectional forces are then summedlong the vessel's length to get the total forces and moments. Detailed

4Fiwtdt

Am

Uf(dwlaft

ab

a


4T

b

T

w

descriptions of the various two-dimensional procedures can be found in theReferences 4.14 through 4.18.

For a nonslender barge, that is, one with a small L/B ratio (i.e., beamgreater than 0.25 length), the hydrodynamic interaction between sections andthe three-dimensional effects near bow and stern sections may not be negli-gible. In this case, a three-dimensional hydrodynamic theory is moreappropriate for calculating the hydrodynamic forces. Existing three-dimensional methods can be separated into two computational methods: one isthe three-dimensional source distribution method, while the other employs afluid finite element technique. The three-dimensional source method utilizesthree-dimensional wave sources distributed over the submerged part of thebarge. By solving the boundary value problem, the velocity potential andpressure field are determined. Many references [4.19, 4.20, and 4.21] areavailable for the three-dimensional source method.

The fluid finite element method solves the fluid field equation, therebysatisfying the boundary conditions on the vessel's surface. In general, thesize of the matrix used to solve the boundary value problem is much largerthan the source distribution method. Details can be found in References 4.22and 4.23.

.3.2.4 Determination of Rigid Body Motions and Inertial Loadinghe dynamic component of the vessel's motion represents the oscillatorydisplacement due to wave loads about a static mean position. The inertialloads are produced by the acceleration of the vessel. The oscillatory rigidody motion of the barge in six degrees of freedom (surge, sway, heave,roll, pitch, and yaw) can be evaluated by frequency or time domain solutions.

he equations of motion are described by a set of six second-order differ-ential equations for the translational and rotational displacements of thevessel from the mean reference position. These equations may be written inthe following form:

(Mml + Mm2)ii + p;< + yx = F(t)

here

= vessel mass

Mm2 = added mass

P = linearized damping coefficient

Y = hydrostatic restoration coefficient

F(t) = external force due to waves

X = displacement of the motion

(4-3)

i = velocity of the motion

iz = acceleration of the motion


The external forces which cause the motion of the barge are those due to therelative motion between the structure and the surrounding fluid. The greatestcomputational difficulty lies in the calculation of the fluid forces on thestructure, which can be accomplished using the techniques discussed inSubsection 4.3.2.3.

The motion analysis of the barge involves various nonlinear phenomena. Themajor nonlinear effects are produced by viscous drag (which is a nonlinearfunction of the fluid velocity), finite motions, and wave amplitude effects.In comparison to a linear frequency domain technique, the time domain tech-nique involves a direct numerical integration of the equations of motion,accounting for those nonlinear effects of the relevant wave and motionvariables. Therefore, the time domain solution can be used to investigatenonlinear and finite amplitude phenomena. However, time domain analysis ofmotion is time consuming and costly. Accordingly, the equations of motionare generally solved in the frequency domain which assumes a linear relation-ship between the forces and the resultant motion of the vessel. Thissolution is considered adequate for applications where a certain level ofpast experience can be relied upon for design guidance. Relatively noveldesign concepts, such as tension leg platforms, should be analyzed usingtime domain procedures. Reference 4.24 details the frequency domain analysisfor vessel motions.

4.3.2.5 Response Amplitude Operator CalculationRAOs are computed for each of the six degrees of freedom. Once the rigidbody motion of the barge is determined by solving the equations of motion,the inertial forces due to the acceleration of the barge motion are calcu-lated in conjunction with wave-induced and motion-induced hydrodynamicloading along the length of the barge. This loading is applied to thevessel model for subsequent structural analyses.

The shear forces and torsional and bending moments at any section of thebarge can be calculated from the ship motion analysis if the weight distri-bution along the vessel length is known. These load effect RAOs are usedto predict the root mean square (RMS) value of the random process for theload effect.

4.3.2.6 Response SpectrumThe response spectrum is calculated by multiplying the wave spectral ordinateby the square of the value of the RAO at the corresponding wave frequency.Once the response spectrum is determined, it is integrated to compute thevarious statistical parameters of the response spectrum. From theseparameters, the initial probability density function of the response can bedetermined for a given wave spectrum. Since there are many wave spectra ofdifferent wave periods for a design significant wave height level, thestatistical parameters, RMS, and probability density function of responseshould be determined for the range of zero crossing wave periods up to 95%confidence level.

4.3.2.7 Design ValuesTo predict the extreme value of the response, two methods are commonly used.The first is the short-term extreme value based on the worst single stormwhich could occur in the lifetime of the structure. In order to predict the


probability of extreme value, the response is assumed to be a stationaryrandom process for which statistical parameters do not change during thestorm. The second method is based on a long-term prediction technique forthe lifetime of the structure. The response to each wave spectrum is deter-mined and weighed according to the probability of occurrence of each wavespectrum. See References 4.25, 4.26, and 4.27 for discussions of determiningextreme values.

4.4 SummaryBarge-like concrete structures can be designed for both transportation andsite-specific service. Although most of the existing design tools (classi-fication agency rules and dynamic analysis theories) were initially developedfor steel ships and barges, these tools are applicable to concrete vesselsas described herein. The designer has a choice of which of these tools toapply. The quasi-static design approach of load determination is generallyconsidered conservative but may be adequate if structural weight considera-tions are not critical. The probabilistic dynamic procedures will generallyresult in a more optimal design but are expensive and time consuming.


4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

Morgan, R.G., "Development of the Concrete Hull," Concrete Afloat,Thomas Telford Ltd., London, 1977, pp. 122.

Stanford, A.E., "LNG Concrete Ship Economics," Concrete Afloat,Thomas Telford Ltd., London, 1977, Pp. 83-96.

Cornstock, J. (ed.), Principles of Naval Architecture, Society ofNaval Architects and Marine Engineers, New York, NY, 1975.

Taggert, R. (ed.), Ship Design and Construction, Society of NavalArchitects and Marine Engineers, New York, NY, 1980.

Hughes, O., Ship Structural Design, John Wiley & Sons, New York, NY,1983.

Rules for Building and Classing Steel Vessels, American Bureau of- -Shipping, Paramus, NJ.

Rules and Regulations for the Construction and Classification of- -Steel Vessels, Bureau Veritas, Paris.

Rules for the Construction and Classification of Steel Ships, Det- - -norske Veritas, Oslo.

Rules and Regulations for the Classification of Ships, Lloyd's Registerof Shipping, London.

- -

Rules and Regulations for the Construction and Classification of- -Ships, Nippon Kaiji Kyokai,Tokyo.


4.11

4.12

4.13

4.14

4.15 Ogilvie, T.F., and Tuck, E.O., A Rational Strip Theory of ShipMotion: Part I, Report No. 013, Department of Naval Architecture,The University of Michigan, Ann Arbor, MI, 1969.

4.16 Salvesen, N., Tuck, E.O., and Faltinsen, O., "Ship Motions and SeaLoads," Transactions, Vol. 78, Society of Naval Architects and MarineEngineers, 1970, pp. 250-287.

4.17 U.S. Coast Guard, Ship Structures Committee, Program SCORES -- ShipStructures Response in Waves, Report SSC-230, Washington, DC, 1972.- -

4.18 Kim, C.H., Chou, F.S., and Tien, D., "Motions and Hydrodynamic Loads

4.19

4.20

4.21

4.22

Kim, C.H., "Hydrodynamic Loads on the Hull Surface of a SeagoingVessel," Proceedings, STAR Symposium, Society of Naval Architects andMarine Engineers, New York, NY, 1982, pp. 103-124.

U.S. Department of Transportation, Maritime Administration, A NumericalAnalysis of Large Amplitude Liquid Sloshing in Baffled Containers,Report No. MA-RD-940-82046 by T.C. Su, et al, Washington, DC, 1982.

U.S. Department of the Interior, Minerals Management Service, OuterContinental Shelf Order No. 8, Platforms and Structures, U.S. Depart-- - - _ment of the Interior, Washington, DC, 1979.

Korvin-Kroukovsky, B.V., and Jabob, W.R., "Pitching and HeavingMotions of a Ship in Regular Waves," Transactions, Vol. 65, Societyof Naval Architects and Marine Engineers, New York, NY, 1957, pp. 590-632.

of a Ship Advancing in Oblique Waves," Transactions, Vol. 88, Societyof Naval Architects and Marine Engineers, New York, NY, 1980, pp. 225-256.

Kim, W.D., "On a Free Floating Ship in Waves," Journal of ShipResearch, Society of Naval Architects and Marine Engineers, 1966,pp. 182-191.

Faltinsen, O.M., and Michelson, F.C., "Motions of Large Structures inWaves at Zero Froude Number," Proceedings, International Symposium onthe Dynamics of Marine Vehicles and Structures in Waves, London,-1974, pp. 99-114.

- -

Shin, Y.S., Three Dimensional Effects on the Hydrodynamic Coeffi-cients and Wave Exciting Forces Used in Predicting Motions of Ships,Report No. 210, Department of Naval Architecture, The University ofMichigan, Ann Arbor, MI, 1979.

_

_

Bai, K.J., and Yeung, R.W., "Numerical Solutions to Free SurfaceProblems," Proceedings, 10th Symposium on Naval Hydrodynamics, Office- -of Naval Research, Arlington, VA, 1974, pp. 609-648.


4.23

4.24

4.25

4.26

4.27

4.28

4.29

4.30

Bai, K.J., "A Localized Finite Element Method for Three-DimensionalShip Motion Problems," Proceedings, Third International Conference onNumerical Ship Hydrodynamics, Office of Naval Research, Arlington,-VA, 1981, pp. 449-464.

Lewis, E.V., "The Motion of Ships in Waves," Chapter IX, Principlesof Naval Architecture, J.P. Comstock (ed.), Society of Naval Architects- -and Marine Engineers, New York, NY, 1966.

Ochi, M., "On Prediction of Extreme Values," Journal of Ship Research,Vol. 17, No. 1, Society of Naval Architects and Marine Engineers, NewYork, NY, March 1973, pp. 29-37.

Ochi, M., "Wave Statistics for the Design of Ships and Ocean Struc-tures," Transactions, Vol. 86, Society of Naval Architects and MarineEngineers, New York, NY, 1978, pp. 47-76.

Liu, D., Chen, H., and Lee, F., "Application of Loading Predictionsto Ship Structure Design: A Comparative Analysis of Methods,"Proceedings, Extreme Loads Response Symposium, Society of NavalArchitects and Marine Engineers, New York, NY, 1981, pp. 249-260.

Comstock, 3. (ed), Principles of Naval Architecture, Society of Naval- -Architects and Marine Engineers, New York, NY, 1967.

Michel, W.H., "Sea Spectra Simplified," Marine Technology Societyof Naval Architects and Marine Engineers, New York, NY, January 1968,pp. 17-30.

Mansour, A.E. and Faulkner, P., "On Applying the Statistical Approachto Extreme Sea Loads and Ship Hull Strength," Transactions, RoyalInstitution of Naval Architects, London, 1973.


5.0 DESIGN APPROACHES5.1 IntroductionThe design of barge structures is controlled by the nature of the marineenvironment, and is particularly affected by the global and local effects ofrepeated wave loading, and the deleterious effects of the seawater. Inaddition, marine concerns for compartmentation, watertightness, and tankventing must be considered to assure the integrity of the ocean-going vessel.Hence, the design and arrangement of the framing of a barge-like vessel willbe dependent upon both structural requirements and requirements for stabilityand safety.

5.1.1 Global Load ConsiderationsThe floating structure's response to wave encounter is similar to that of asimple beam, with the distribution of buoyancy and load causing shear,moment, and torsion in the hull. Reference 5.1 provides an explanation ofthe method for determining these loads. The method described assumes thatthe vessel is poised on an idealized wave; then the buoyancy distributionalong the vessel is determined and the resulting shears,-moment, and torsionloads are calculated. Using this methodology in conjunction with a con-servatively selected sea state will generally result in a conservative design.References 5.2 and 5.3, as well as Chapter 4.0 of this report, introduce thespectral approach to design of the hull, which can be used to predict designloadings by taking into account the random nature of ocean waves and thevarious joint probabilities of vessel heading and wave encounter.

5.1.2 FatigueBecause of the random and repetitive nature of loads applied to floatingstructures, potential fatigue of critical hull framing members should beconsidered. The number of waves encountered in a 20-year structure life isaccepted as 108, which corresponds to an average wave period of 6.3 seconds.Typical distributions are shown in Table 5.1. From this table, it can bededuced that the majority of the waves occur in the low wave height range.Such small waves lack sufficient energy to cause large loads on large float-ing structures and should not control the structural design for ultimatestrength. However, the cyclical stresses caused by these repetitivealternating wave loads, referred to as "hogging" and "sagging," can resultin a fatigue condition that reduces allowable design stresses and may requireanalysis.

One commonly used fatigue analysis method relies on the Miner's summationapproach, which linearly combines individual damage events to calculatecumulative damage. Reference 5.4 discusses the application of this methodto vessel design, and Reference 5.5 discusses the fatigue characteristics ofconcrete structures, with Reference 5.6 specifically addressing concretevessel design.

Low cycle fatigue loads will be critical for the overall framing of thehull. Both low cycle and higher cycle repetitive loads should be consideredfor the design of mooring hardware and attachments to the hull.

5.1.3 ServiceabilityAs discussed in Chapter 3.0, the durability of concrete in seawater can beachieved provided the structural framing design and concrete mix design

TABLE 5.1 NORTH ATLANTIC WAVE HEIGHT DISTRIBUTIONS

H1/3 H1/3

Range (ft) Range (m)

H. Walden*

% Cumulative

H. Walden

% Cumulative

Hobgen & Lumb

% Cumulative

0 - 3

3 - 6

6 - 9

9 - 12

12 - 16

16 - 21

21 - 27

27 - 34

34 - 42

> 42

0 - 0.91 8.75

0.91 - 1.83 23.75

1.83 - 2.74 30.70

2.74 - 3.66 20.35

3.66 - 4.88 6.90

4.88 - 6.40 4.95

6.40 - 8.23 2.69

8.23 -10.36 1.70

10.36 -12.80 0.25

> 12.80 0.05

91.25

67.50

36.80

16.45

9.55

4.60

2.00

0.30

0.05

8.75

23.75

30.70

20.35

6.90

4.95

3.350

1.060

0.168

0.022

91.25

67.50

36.80

16.45

9.55

4.60

1.25

0.190

0.022

11.2100 88.79

36.5240 52.27 is25.9160 26.35

E13.6899 12.66 j

Fl7.5440 5.12 R

B2.2325 2.88 4

2.1258 0.076

0.7428 0.015

0.0121 0.0029

0.0029

* Modified


account for the effects of the marine environment. Proper concrete mixdesign will result in concretes having low permeability and low seawaterreactivity. Proper detailing of internal reinforcing can preclude rein-forcement corrosion due to seawater reactivity (insufficient concrete cover),and excessive cracking (insufficient reinforcing quantity and distribution).It is generally recommended that when bulkheads, top decks, sides, and baseslabs of barge structures are subjected to membrane stresses, the membershould be designed such that through thickness tension will be limited tozero or very low values under normal service conditions [5.7]. In addition,crack widths and corresponding reinforcing steel stresses should be con-trolled for all types of service loading [5.8]. Watertight members, forexample, are designed for maximum crack widths not exceeding 0.010 in.(0.25 mm) and corresponding maximum reinforcing steel stresses of 17 ksi(117 MPa). In addition, Recommendations for the Design, Construction and- -Classification of Floating Concrete Structures (Det norske Veritas) suggeststhat through cracking be prevented by requiring that a portion of the memberremain in compression at all times. Also, in watertight members subject tocyclic shear, the effect that principal stresses acting at an angle to thereinforcing may have in increasing crack widths is considered in the design.

Currently, ACI Committee 357 is investigating design approaches regardingcracking in marine structures. Special attention is being placed on differ-entiating service "zones" on the hull and using different design criteriafor each zone. Zone categories include atmospheric, splash, and submergedareas.

5.1.4 Hull ArrangementsThe arrangement of a floating structure must account for maintainingequilibrium when intact and, in most cases, when damage causes flooding.Accordingly, compartmentation of the hull may be dictated more by thenecessity for intact and damage stability rather than purely structuralconsiderations. The need for ballast spaces to maintain a desired trimduring various loading conditions and the need for maintaining adequatestability in the event of damage are considered. In addition, watertightclosures, tank fills and vents, and internal piping are arranged to limitprogressive flooding. References 5.1 and 5.9 discuss these considerationsas applied to ship design.

5.2 Design CodesThe recognized standard agencies for the marine industry are called "classi-fication" societies. There are several such societies in existence, withthe major ones being: American Bureau of Shipping (ABS), Bureau Veritas(BV), Det norske Veritas (DnV), Germanischer Lloyds (GL), Lloyd's Registerof Shipping (LR), and Nippon Kaiji Kyokai (NKK). When a structure isdesigned, constructed, and inspected in accordance with established codes,called "rules" of a particular classification society, it is said to be"classed" with the society. Several of these societies have established"rules" or design guides for concrete barge structures. FIP's Recommenda-tions for the Design and Construction of Concrete Sea Structures is oneexample. Also, ACI Committee 357 has established guidelines that can beutilized for barge structural design. Two alternative design approaches


have been established in the industry: namely, the limit state designapproach and the permissible stress design approach.

5.2.1 Limit State DesignStructural codes of practice usually provide margins of safety by adoptingpermissible stresses for the materials defined as fractions of their strength.In the limit state design, the designer examines all the possible conditionsthat might lead to structural failure or failure to perform the intendedservice. When a structure or part of a structure ceases to render itsproper function or no longer satisfies the conditions for which it wasdesigned, a limit state condition is said to be reached. Four limit stateshave been adopted for use in the design of barge-like concrete structures.A description of each follows. DnV's Rules for the Design, Construction,- -and Inspection of Offshore Structures offers further definition of theselimit states.

5.2.1.1 Ultimate Limit State (ULS)This limit state corresponds to the maximum load carrying resistance of thestructure. Design for the ULS is concerned with providing adequate struc-tural strength to mitigate thedesign loads.

probability of collapse when subjected to the

For the ultimate limit state,load combinations. ACI 357R;struction, and Classification

the load factors are specified for variousDnV's Recommendations for the Design, Con-- -of Floating Concrete Structures; and FIP's

Recommendations for the Design and Construction of Concrete Sea Structurescan be used to obtai ese load factors. The resistance or capacity reduc-tion factors to use in the design are also specified in these guides.

Due to the random nature of the loadings and the variable material proper-ties common to concrete barge-like structures, designers may consider usingprobabilistic theory to assess the various limit states. In this approach,the structural safety is assessed by verifying that the design load will notexceed the design resistance (capacity) of the structure. The design resis-tance, which is related to a specified limit state, is the product of thecharacteristic strength and a material coefficient (strength reductionfactor less than unity). Material coefficients are specified for variousloading conditions (flexure, shear, compression) in applicable design codes.The design load for a given type of load and limit state is obtained as theproduct of the characteristic load and the corresponding partial safetyfactor (load factor). The reliability or safety of the structure withregard to a specified limit state for the design life is indicated by areliability index, which in turn is related to the probability of failure.In the semiprobabilistic approach, the structure is to be designed such thatthe probability of attaining a particular state is sufficiently small.References 5.14, 5.15, and 5.16 discuss this approach.

5.2.1.2 Fatigue Limit State (FLS)This limit state addresses the effects of repeated loading due to the oftencyclic nature of environmental loads. Design for the FLS addresses selectionof preferred element geometry and providing structure ductility (often byproper confinement of the concrete). The design strengths of constructionmaterials (concrete and reinforcement) are defined by characteristic S-N or


Wohler diagrams. S represents a characteristic stress of a loading cycleand N is the number of cycles to failure.the material factor.

The S-values are divided by y,,

5.2.1.3 Progressive Collapse Limit State (PLS)Design for this limit state is intended to ensure a sufficient margin toresist progressive collapse in the event of partial damage to the structure.The analysis process typically involves modeling the structure and assuminglocal damage, then assessing the capacity of the structure to resist furthercollapse. Models are based on elastic,. plastic, and yield-line theories.

5.2.1.4 Serviceability Limit State (SLS)This limit state addresses the performance of the structure during normaluse. When designing for the SLS, the structure is proportioned to prevent

(a) Large deflections of the structure or member part, which mayin turn cause reduced function and efficiency of the operationof the structure

(b) Reinforcement corrosion and deterioration of the concrete

(c) Cracking of sufficient severity, which may cause progressivestructure deterioration (Requirements for minimum reinforcementand reinforcing spacing are often used to ensure satisfactorycrack distribution.)

(d) Excessive vibration, which may result in discomfort, damage,or interference with other functions

(e) Leakage, corrosion, and pollution in areas where containmentis required

Serviceability requirements are often controlled by specifying the permiss-ible stresses for load combinations which may be encountered in the life ofthe structure.

5.2.2 Permissible Stress DesignSafety in permissible stress design is provided because the stress, ascalculated in accordance with the standard methods of elastic theory, isless than a fixed proportion of the mean strength of the material. Loadsacting on the structures are defined under the assumption that they are notlikely to be exceeded in service. Variations in loads and strength ofmaterials are implicitly accounted for in the selection of permissiblestresses. Stress permissibles in the concrete are checked for compression,tension, bearing, bond, shear and diagonal tension. Stress permissibles inthe reinforcing steel are checked for tension. In prestressed structures,particular attention is often given to control of specific applied stressesrelative to

(a) Permissible stresses in the highly compressed concrete fibersunder extreme maximum and minimum loading conditions

(b) Permissible compressive stresses at the time of initial prestress


(c) Permissible tensile stresses in the tendons at various stages ofprestressing

(d) Permissible bond and anchorage stresses in the concrete

Depending upon the intended vessel purpose and service conditions, stressescaused by vibration, thermal gradients, and fatigue are also taken intoconsideration. Conformity of permissible stresses does not necessarilyensure the adequacy of structural strength (ultimate limit state), but it isa positive measure of ensuring serviceability.

The proposed Method for Analysis of Prestressed Concrete Vessels as given byABS (1967) [5.10] and the Provisional Rules for Prestressed Concrete Bargesby NKK (1975) [5.11] can be used as a guide for designing concrete barge-likestructures using an permissible stress approach.

5.3 Analysis MethodologyThe structural design of a vessel is, for the most part, controlled by thenature of the environmental loads. A barge-like vessel at rest in stillwater is a relatively lightly loaded structure, being similar to a con-tinuously supported beam having a relatively uniform load. The occurrenceof waves alters this condition, causing the support conditions to be highlyvariable and causing variations in the local design loads induced by vari-ations of hydrostatic pressures.

The structure can be designed by superimposing the various modes of struc-tural response on each load condition (e.g., local bending plus globalbending, or by modeling the structure as a complete unit utilizing computeranalyses to calculate the overall structural response to the loading).Reference 5.12 provides a discussion of both design methods.

5.3.1 Superposition of LoadsA simplified design method treats the structural response of a vessel asthree phenomena, computing each of the three responses, and then superimpos-ing the results. Reference 5.9 defines these phenomena as follows:

(a) "Primary response" is the response of the entire hull whenloaded as a beam. This is caused by nonuniform load and support(i.e., wave conditions, or asymmetric still-water loading).

(b) "Secondary response" comprises the stress and deflection of thestructure contained between major supports. Bulkheads or sideshells may act as support points for this structural response,the response frequently caused by local hydrostatic, hydrodynamic,or cargo loads. Internal loading conditions on the bulkheadsare also considered when determining "secondary response."

(c) "Tertiary response" is defined as the out-of-plane deflection ofan individual shell panel supported along its edges by girdersor stringers. Concrete construction attempts to limit the useof girders, and hence this response may not be applicable. Ingeneral, tertiary response is caused by local loads on thepanel.

357.2R-53

Cracking caused by secondary and tertiary response modes is normally flexurecracking where one face of the member remains in compression.

The most severe likely combination of these responses can be utilized todesign the various structural components. The major shortcoming of thisprocedure is that end conditions are approximated by assumptions of fixity.The tendency by designers is to make conservative assumptions and, thus, aconservative design generally results. The use of computer-aided analysiswill generally provide greater accuracy in obtaining the structural response,because the otherwise time-consuming process of assessing the structure'sstiffness and load distribution capacity can be accomplished rapidly usingavailable computer software. These methods are especially appropriate forcomplex structures.

5.32 Finite Element Modeling and Analysis TechniquesCurrent advances in the state of the art in finite element analysis ofreinforced concrete structures allow the analyst/designer a wide range ofanalysis sophistication which can be applied to particular applications.The level of sophistication is generally dictated by the design stage andload level and type of structure under investigation, but can also beselected and varied for economic reasons.

In preliminary design stages, first-order estimates of general behavior areused for initial sizing. Economical linear modeling is normally justified,especially for low load levels such as operational loads. For ultimatelimit state design, use of nonlinear techniques which model the brittleplastic behavior of the constituent materials in a structure (including theincreased compressive strength of confined concrete) can be included and maybe essential. A comprehensive presentation of the various finite elementsand their material models is found in Reference 5.13. A brief summary ofthe reference is presented below for various levels of modeling sophistication.

The most common modeling procedure for reinforced concrete structures assumesthat the composite material behaves as a linear elastic, isotropic material.Figure 5.1 illustrates some examples. A standard linear finite elementmodel is straightforward to prepare and relatively inexpensive to solveusing a wide variety of available computer programs. For low levels ofloading and for evaluating the system of internal forces in the global orcomplete structural system, the linear analysis is effective and efficient.

However, the results of the linear analysis can be difficult to interpret oreven inappropriate if the load levels are high and significant nonlinearbehavior exists. If significant nonlinear response is inherent in thedesign, a more accurate representation of the local behavior is required.

Specialized finite elements can be used which are capable of simulating manyof the complex nonlinear characteristics of concrete. Elements exist whichcan simulate the effects of cracking in the concrete, commonly the majorsource of nonlinearity within the concrete (see Figure 5.2). The stiffnessof these finite elements is automatically modified when the maximum principaltensile stress within the finite element has reached a user-specified concreterupture strength.

I-D TRUSS ELEMENT I-D BEAM ELEMENT

E L E M E N T

Fig. 5.1--Standard Linear Finite Elements

b.

C.

Fig. 5.2--Behavior of Typical Concrete CrackingElement


w

Other finite elements are available which can simulate the complex nonlinearcompressive behavior of concrete. At compressive stresses above approxi-mately 0.30 times the uniaxial compressive strength of concrete, fc, concretebegins to behave in an increasingly nonlinear manner characterized by pro-gressive softening. This continues until the concrete reaches its ultimatecompressive strength. Additionally, the strength and the stress-straincharacteristics of concrete in compression have been quantified by experi-ment to be highly dependent upon the state of multiaxial stress existingwithin the concrete. Concrete under conditions of triaxial compression, forinstance, exhibits strengths many times higher than its uniaxial compressivestrength. Finite elements now exist which are able to simulate much of thisbehavior, making use of the available experimental data (see Figure 5.3).The simplest of these elements use nonlinear elasticity as the basis fortheir information, while other, more complex elements make use of plasticitytheory or the more recently developed endochronic theory of concrete behaviordescribed in Reference 5.13.

Linear elastic elements can be used effectively if the concrete is subjectedto monotonic, proportional loading. Proportional loading implies that theload pattern is fixed. Nonlinear elements can be more accurate in caseshere-the concrete is subjected to nonproportional loading and unloading.Creep and shrinkage of concrete can also be modeled using specialized con-crete elements.

Steel reinforcement within the concrete can be modeled in two primary ways.One way is to make use of a representation wherein the effects of steelreinforcement are distributed over the surface or volume of a concreteelement. Alternatively, reinforcement can be modeled using a discreterepresentation utilizing truss bars or beam elements. These elements arethen superimposed upon the existing plain concrete mesh (see Figure 5.4).The type of representation that is best for an analysis depends primarilyupon the nature of the structure. Heavily reinforced beams normally requirediscrete modeling of the reinforcement in order to obtain good prediction ofbehavior, whereas shear panels with uniform reinforcing grids can be modeledusing a distributed representation of the reinforcement.

5.4 Design and Detailing5.4.1 Weight ControlIn general terms, the capacity of a vessel to carry cargo is the differencebetween the weight of the water displaced by the vessel and the lightshipweight (i.e., the weight of the vessel without cargo aboard). Once thevessel's external dimensions have been established, increasing local struc-tural sizes will reduce the cargo-carrying capacity. For certain applica-tions, in particular oil storage, this could result in an actual limitationin the amount of cargo capacity. Accordingly, weight control is consideredas an important design parameter by the marine structural designer. Detaileddesign weight estimates should allow for concrete placement tolerances andweight growth due to formwork spreading. Concrete mix designs are propor-tioned to achieve weight control. Batching of the concrete at the construc-tion yard is monitored closely to assure conformance with design intent.


3.0

2.0

1.0

00.01 0.02

Fig. 5.3--Finite Element Capabilities in Modeling Triaxial StressEffects on Compression Behavior

Fig. 5.4--Use of One-Dimensional Truss and Beam Elements to ModelBeam Reinforcing


A

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5DfaHmcrbc

5Utfadddldsla

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5.4.2 Inspection major advantage of concrete vessels is their low maintenance requirementsin comparison with steel. To fully benefit from the advantage, the concreteessel should be designed to dispense with requirements for periodic dryocking for inspection. For this to be acceptable to various regulatoryodies, the vessel must be fully inspectable internally. This will requirehat the design incorporate safe access to all compartments, with adequaterovisions for venting during inspection.

.4.3 Detailingurability and trouble-free operation in the marine environment is often aunction of proper detailing. For example, an epoxy patch over a tendonnchor point which deteriorates does not immediately jeopardize the structure.owever, in the long term it may, and its repair, especially if underwater,ay be difficult and expensive. Similar examples such as seepage throughonstruction joints and local damage due to impact from supply boats canequire expensive repairs. The designer should strive to avoid these problemsy emphasizing the importance of maintenance and repair when detailing theonstruction drawings.

.5 Summaryse of established design codes (rules) and attention to detail are essentialo ensure a safe, highly serviceable floating structure. Where allowanceor a reasonably conservative design approach provides performance withinccepted standards with little or no appreciable cost penalty, use of existingesign codes or classification society "rules" can provide rapid, competentesign formulations. Where it may be necessary to optimize a structureesign to meet very stringent conditions (for example, the need for a veryightweight structure subjected to high magnitude loads), more rigorousesign techniques such as dynamic response analyses, and finite elementtress analyses may be warranted. Both approaches are acceptable, and it iseft to the discretion of the designer to select the design methodology mostppropriate.

n addition to concerns for accurate assessment of design loads and stressesmposed on structures exposed to a highly variable set of service conditions,he designer must pay close attention to structural detailing as a means ofnhancing the service performance of a vessel. The structural serviceabilitynd, in some cases, the ultimate strength performance of a floating concretetructure will be greatly affected by details such as

(a) Reinforcing steel lap splice and bond lengths in fatigue criticalareas of the structure

(b) Control of concrete crack width and induced reinforcing steelstresses under service conditions

(c) Adequate preparation of construction joints in the concretestructure

(d) Adequate concrete cover over reinforcing and prestressing steel


(e) Concrete mix designs which emphasize low permeability and highcement content

(f) Proper grouting and bonding of post-tensioning tendons, andproper preparation of post-tensioning blockouts and anchorages

Much experience exists in the use of concrete in the marine environmenttoday to provide attention to details as noted above. Perhaps the mostcomprehensive compilation of such experience is available through the vari-ous marine classification societies. Designers, constructors, and owners ofconcrete barge-like structures should develop a dialogue with such societiesand agencies to assure the best possible control of structure performance.


5.1

5.2

5.3

5.4

5.5

5.6

5.7

5.8

5.9

5.10

5.11

Cornstock, J. (ed), Principles of Naval Architecture, Society of Naval- -Architects and Marine Engineers, New York, NY, 1967.

Michel, W.H., "Sea Spectra Simplified," Marine Technology Societyof Naval Architects and Marine Engineers, New York, NY, Januay 1968,pp. 17-30.

Mansour, A.E. and Faulkner, P., "On Applying the Statistical Approachto Extreme Sea Loads and Ship Hull Strength," Transactions, RoyalInstitution of Naval Architects, London, 1973.

Hughes, O., Ship Structural Design, John Wiley & Sons, New York, NY,1983.

Fatigue of Concrete, Publication SP-41, American Concrete Institute,Detroit, MI, 1960.

Fatigue Behavior of Prestressed Concrete in a Ship Hull, Technicaland Research Bulletin 2-24, Society of Naval Architects and MarineEngineers, New York, NY, 1979.

Mast, R.F., "The ARCO LPG Terminal," Proceedings of Concrete Shipsand Vessels, University of California, Berkeley, CA, 1975.

Gerwick, B.C., "Practical Methods of Ensuring Durability of PrestressedConcrete Ocean Structures," Durability of Concrete, Publication SP-47,American Concrete Institute, Detroit, MI, 1975, pp. 317-324.

Ship Design and Construction, Society of Naval Architects and MarineEngineers, New York, NY, 1980.

"Proposed Method of Analysis of Prestressed Concrete Vessels,"American Bureau of Shipping, Paramus, NJ.

"Provisional Rules for Prestressed Concrete Barges," Nippon KaijiKyokai, Tokyo.


5.12

5.13

5.14

5.15 Ellingwood, B.R., Reliability Basis of Load and Resistance Factors

5.16

Gerwick, B.C., Mansour, A.E., Price, E., Thayamballi, A., "Feasibilityand Comparative Studies for the Use of Prestressed Concrete in LargeStorage/Processing Vessels," Transactions, Society of Naval Architectsand Marine Engineers, New York, NY, 1978, pp. 163-196.

Finite Element Analysis of Reinforced Concrete, State-of-the-ArtReport, American Society of Civil Engineers, New York, NY, 1982.

Ellingwood, B.R., Galambos, T.V., MacGregor, J.G., Cornell, C.A.,Development of a Probability Based Load Criteria for American NationalStandard A 58, National Bureau of Standards Special Publication 577,National Bureau of Standards, Washington, DC, June 1980.

for Reinforced Concrete Design, Building Science Series No. 110,National Bureau of Standards, Washington, DC, February 1978.

Wen, Y.K., "Methods of Reliability of Structures Under Multiple TimeVarying Loads, Journal of Nuclear Engineering Design, Vol. 60,1970, pp. 61-71.

ACI COMMlTTEE REPORT

6.0 CONSTRUCTION6.1 IntroductionThe state-of-the-art for construction of barge-like structures is welldeveloped, and regulatory agencies have established comprehensive codes andguidelines for the design and construction of such structures. Numerousstructures have been built, and construction techniques are well established.Large, marine concrete structure fabrication yards exist throughout theworld.

Barge-like structures can be constructed in accordance with high-technologyconstruction industry standards. The construction process is often monitoredby the design agent and inspection is provided by appropriate cognizantreviewing agents. A construction plan, which includes but is not limited toplans for materials control, quality control and quality assurance, formingtechniques, post-tensioning and prestressing techniques, repair techniques,and launching, is often prepared in advance of construction.

Large floating concrete structures can be constructed in dedicated facilities(graving docks and slipways) specifically constructed for the purpose. Suchfacilities are available. Because these structures will have large displace-ments, structure draft will be an important design/construction parameter.Construction often allows for partial completion on a slipway or in a gravingdock facility with completion afloat after launch. Hence, significantconstruction may take place over water.

6.2 Construction Methods6.2.1 Construction on SlipwaysConstruction of a floating concrete structure on a slipway requires specialattention not commonly associated with construction in a dry dock or gravingdock facility. Some of the important considerations are

Construction of the Slipway -- The slipway support structuremust have the capability to support the structure loads andloads due to construction equipment. It must also have suffi-cient stiffness and rigidity to prevent large distortions whichmay induce racking and distortions in the concrete formwork orin the completed concrete structure.

Moving the Structure -- The slipway must be designed to allowfreedom of motion between the structure base slab and slipwayduring repositioning and launching. High contact pressures andpossible suction forces should be considered. The slipwaysurface should be designed to minimize concentrated loadings onthe structure base slab.

Structure Launching -- An analysis should be made which con-siders the bridging effects of a structure partially afloat andpartially on the slipway during launch. Slipway angle and thebuoyancy distribution of the structure should be considered.The calculations should be included as part of the structuredesign using applicable construction load factors with allowancefor the variableconditions.

357.2R-61

6.2.2 Construction in a Graving DockBarge-like structures may be constructed in a large graving dock or dry dockfacility. Should the barge be sufficiently small, it may be constructed asa single piece within the dock. Alternatively, large barges can be made insegments having dimensions compatible with dock dimensions and draft restric-tions, then launched and joined or mated with other segments. Concretestructures can be designed for segmenting in the plan dimension to avoidhorizontal construction joints in exterior bulkheads.

The decision to segment the structure is made early during the design stage.Key factors to be considered are

(a) Construction facility length, width, and draft restrictions

(b) Draft and width restrictions in waterways and estuaries awayfrom the construction facility

(c) Weight distribution of the segments (still-water loading)

(d) Stability characteristics of the segments and of the joinedstructure

(e) Design wave characteristics at the construction facility andduring tow to the integration site

(f) Structure internal framing arrangements

(g) Allowable deflections of the structureand wave-loaded conditions

(h) The method for joining the segments

in both the still-water

(i) The construction schedule and its relation to concrete designstrength requirements

(j) The effects of construction loading both in the dry and whileafloat

The graving or dry dock facility used for state-of-the-art concrete bargeconstruction should be equipped with

(a) A level, structural floor with sufficient strength to resistheavy, local construction loads from equipment and heavy precastelements

(b) A floor that will not adhere to the barge structure duringlaunch

(c) A dewatering system to prevent seawater contamination of thework


(d) Sufficient land space around the dock to allow for a concretebatch plant and construction materials stockpiling and storingprecast panels and concrete formwork

(e) A batch plant equipped with quality assurance and testingfacilities

6.3 Concrete Construction6.3.1 Precast/Cast-in-Place ConstructionA high degree of production repetition and reduced construction time can beachieved by fabricating the structure segments from joined, similar precastwall and deck elements [6.1, 6.2]. The precast elements can be fabricatedto close tolerances in formwork erected in precast plant facilities wherematerial and labor quality control is easily maintained (see Figures 6.1 and6.2).

Precast wall panels can be erected in the graving dock using standard liftingequipment. Bracing of members is usually required. The walls or bulkheadsof a large structure are subdivided into a series of precast panels separatedby cast-in-place (CIP) closure pours. The closure pours are sized to allowsplicing of mild steel reinforcing and post-tensioning ducts. Closure poursare cast to the full height of the panel joints, usually after casting thestructure base slab (see Figure 6.3). As an alternate to closure pours,precast members can be match cast (see Figure 6.2) and epoxy bonded prior topost-tensioning [6.3].

A high integrity closure pour joint between the as-cast precast elements andthe CIP pour requires special consideration. The as-cast faces of theprecast panels need roughening before erection. Allowance must be made toprevent joint separation and distortion of formworkformwork during concrete placement.The mating surfaces of the precast panels can be coated with an epoxy bondingagent prior to casting the closure pour. Figures 6.4 and 6.5 illustratethis type of hull construction in a graving dock.

6.3.2 Cast-in-Place ConstructionMajor portions of the structure may be cast-in-place concrete construction.Base slabs, closure pours, and top deck overlays are common examples.Allowances are made during concrete mix design for transporting the concretefrom the batch plant to the job site, possibly over long distances. Sincecritical elements (exterior bulkheads and base slabs) may be heavily rein-forced, concrete slump and compaction during placement deserve specialattention. Pumping may also be required. When pumping is warranted, theconcrete mix design should be reviewed to assess possible deleterious effectson performance caused by the pumping operation.

The effects of heat of hydration must be considered, not only for possibleimpact on concrete quality, but also on possible introduction of unwantedstresses in the structure. The heat generated by the concrete in a masspour may be sufficient to induce significant stresses in adjacent memberswhich are already restrained by the surrounding structure. The stressesinduced due to such conditions are frequently assessed during design. Toprevent extensive surface cracking which may occur due to rapid heat loss in

BARGE-LIKE STRUCTURES357.2R-63

Fig. 6.1--Precast Pontoon Element, Valdez Terminal

Fig. 6.2--Precast Hull Element, ARC0 LPG Barge

357.2R-64

Fig. 6.3--Precast Hull Panel, Braced, Showing Closure Pour

Fig. 6.4--Hull Construction, ARC0 LPG Barge


Fig. 6.5--Hull Construction, Valdez Container Terminal

Fig. 6.6--Valdez Container, Upper Mating Fittings

357.2R-66 ACI COMMlTTEE REPORT

such pours, consideration is also given to the use of insulated formwork andprotection of exposed surfaces.

6.3.3 Post-TensioningBarge structures are frequently prestressed by post-tensioning about morethan one principal structure axis. Typically, the top deck and base slabare post-tensioned both longitudinally and transversely to account fordelivery voyage hog/sag loading, base slab hydrostatic and grounding forces,and deck loadings. In addition, external bulkheads may be post-tensionedvertically to control cracking due to hydrostatic pressures and externallyapplied environmental forces (i.e., ice and waves). Precast members may beprestressed to control cracking during lifting and handling.

During design, allowance is often made for the possible occurrence of blockedtendon ducts at critical sections of the barge structure. If possible,additional tendon ducts are often specified to account for this eventuality.As a minimum, a certain percentage of the tendon ducts are oversized toallow for incorporation of tendons having more strands than those specified.

An analysis is commonly undertaken to assess the effects of the proposedstressing sequence on the structure. Consideration is given for post-tensioning the partially completed structure below waterline before launchfrom the graving dock and to the effects on the final stressed condition bypost-tensioning conducted after float-out. An inspection of sensitive areasis often made before and after the post-tensioning operation.

6.3.4 Grouting of AnchoragesBonded post-tensioning systems are often specified for barge-like concretestructures. A bonded system will require injecting grout into the post-tensioning ducts. The procedures for grouting of post-tensioning ducts andanchorage blockouts are outlined below:

6.3.4.1 Grouting Tendon DuctsGrout is composed of portland cement, water, and admixtures proportioned toensure a flowable grout with minimum bleeding or separation. The maximumrecommended water-cement ratio for the grout is often 0.44. for vertical ornear vertical tendon ducts, a combination of thixotropic admixtures andspecial grouting procedures may be necessary to prevent formation of trappedair pockets which will inhibit complete filling of the duct. Horizontal ornear horizontal ducts should be fitted with vents to allow removal of trappedair and bleedwater, and to allow complete filling of the duct. FIP's Recom-mendations for Acceptance and Application of Post-Tensioning Systems offersguidance on this subject.

6.3.4.2 Tendon AnchoragesTendon anchorages are frequently recessed in precast "blockouts." Followingprestressing, these blockouts need to be filled to protect the tendon anchor.After the post-tensioning duct grout has cured, the anchorages are cleanedand roughened, commonly by sandblasting. Epoxy bonding agents are oftenused to cover the blockout, the anchor, and the ends of tendon strand. Apositive mechanical means of retaining the entire blockout patch is neces-sary. Hooking rebar into the anchorage blockout or providing a cast-inkeyway in the blockout perimeter are techniques used. If a keyway is used,


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it is proportioned so that it will be uniformly filled during the blockoutrouting operation. Blockouts can be filled with grout using forms toretain the grout and to ensure expansion of the grout into the keyway duringuring. If cement grout is used to fill the anchorage zone, it should beade with the same cement used for the concrete structure containing therestressing.

.3.5 Suction Bondrecautions should be taken to provide a mechanism to effectively break theuction bond that may occur when the base slab is cast directly against thery dock or graving dock floor. One method is to discharge compressed airr water through ducts in the structure framing which exit at the dry dockloor interface. This method was used during construction of the Valdezontainer Terminal floating dock pontoons. The compressed air or water issed as a bond breaker to facilitate structure liftoff during graving docklooding. Alternately, the dry dock floor may be overlain with a permeableembrane to reduce the suction forces. This method was used during con-truction of the concrete caisson for the Glomar Beaufort Sea I Arcticrilling structure.

.4 Construction Afloatompleting the structure while afloat presents unusual design and construc-ion challenges [6.4]. Precautions are taken to assure a high-integritytructure. Several important considerations are

(a) Still-water weight distribution and wind-, wave-, and current-induced loadings

(b) Stability (intact and damage)

(c) Post-tensioning sequences

(d) Resolution of local, temporary construction loads

(e) Concrete mix design which considers the marine environment

(f) Concrete curing methods

(g) Protection of concrete materials, reinforcing steel, and pre-stressing steel and anchorages from seawater contamination

(h) Construction joint preparation to remove laitance, marine growth,and salts

he freeboard of the floating structure is an important construction parameter.In some cases, minimum freeboard requirements are specified by regulatorygencies. The freeboard, trim, and stability characteristics of the struc-ure are monitored throughout the construction process to assure stabilityin the still-water condition and to prevent wave overtopping.

.5 Segmental Construction -- Joining While Afloatany methods for joining large precast floating structure segments arevailable. Each embodies some type of construction splice joint. The


g,

details vary generally because of the specific nature of the splice jointdesign. However, these methods share common features which are outlinedbelow.

(a) Two or more similar, but not necessarily identical floatingconcrete segments or pontoons

(b) A splice joint consisting of a cast-in-place closure pour sur-rounding grouted post-tensioning tendons, mild steel reinforcinand mating/alignment fittings

(c) A method of sealing and dewatering the closure pour duringmating

(d) A method of differentially ballasting the floating segments tocontrol loads

A typical segment may be outfitted with cast-in-place steel male/femalepin/socket fittings near the top deck and steel rocker bearing assembliesnear the base slab below waterline. The pin socket fittings provide forpositive, visual, and mechanical alignment of the segments. Figure 6.6illustrates the top fittings for the Valdez Container Terminal. A typicaljoining sequence is outlined below:

(a)

(b)

(c)

(d)

(e)

(f)

Inspect alignment hardware cast in the segments at the joininginterface bulkhead.

Ballast each segment to a level trim and clean all mating sur-faces to remove salts, marine growth, and laitance.

Ballast each segment to a trim by the stern to bring only therocker bearings into contact.

When contact is made, hold segments in place by cables andtemporary deck winches or by post-tensioning tendons spanningthe joint.

Ballast the segments to "close" the joint and mate the alignmenthardware.

Dewater the joint to allow for uncapping of previously sealedpost-tensioning tendon ducts. Providing a sealed closure thatcan be dewatered can be accomplished either by constructing thesegments with deformable perimeter seals, or by affixing aportable sealed cofferdam assembly to the segment joint. Fig-ure 6.7 shows the cofferdam in place at the Valdez Terminal.

(g)

(h)

After the closure is sealed and dewatered, the mating surfacesare inspected and cleaned by jetting or sandblasting.

Tendon ducts are uncapped and duct splices put in place. Mildsteel reinforcing is spliced to bars protruding from the floatingprecast segments. Figure 6.8 shows spliced tendon ducts.

357.2R-69

Fig. 6.7--Dewatering Cofferdam, Valdez Container Terminal

Fig. 6.8--Tendon Duct Splices, Valdez Container Terminal


(i) Post-tensioning tendons are pulled through the ducts and anchoragehardware put in place. All post-tensioning can be accomplishedfrom inside the floating segments. Abutment and anchoragedetails need to be included in the structure design effort.

(j) Formwork for the cast-in-place closure pour is put in place andthe pour is made.

(k) When the concrete has reached acceptable strength, the joint ispost-tensioned using a predetermined sequence developed tocontrol induced loads and to account for the time-dependent gainin concrete strength.

(1) The temporary mating equipment and cofferdam are removed.

(m) The post-tensioning ducts are grouted.

Important design checkpoints for segmental construction are as follows:

(a) Temporary mating equipment should be affixed and preloaded torestrain the two structures while afloat.

(b) A detailed post-tensioning procedure should be established.

(c) A prejoining and postjoining inspection should be made.

(d) Forces in the mated joint should be identified at each step inthe procedure. Allowances are made for wind, wave, and currentforces; forces from temporary construction equipment; effectsdue to ballasting; weight of cast-in-place closure pour; and theeffects of buoyancy due to the dewatering cofferdam, if used.

6.6 SummaryTechniques for constructing floating barge-like concrete structures are wellestablished. Historically, many barge-like structures have been built thatcurrently serve owners well. Larger, more complex structures can beexpected to be designed and constructed in the future. The success of thesestructures will require a unique blend of coordination among owner, designer,constructor, andwork together tocient to provideto work togetherrequirements and

regulatory agent. Designer and regulatory agents need-toupdate design codes and quality assurance standards suffi-safe, economical structures. Constructors and owners needto establish construction schedules compatible with theirresources.

Figures 2.5a and 2.8 through 2.10 illustrate completed floating structuresfabricated using the techniques described herein.


REFERENCES FOR, CHAPTER 6.0

6.1 Zinserling, M.H. and Cichanski, W.J., "Design and Functional Require-ments for the Floating Container Terminal at Valdez, Alaska, Proceed-ings, Offshore Technology Conference, OTC Paper No. 4397, Houston,TX, 1982.

6.2 Mast, R.F., Cichanski, W.J., and Magura, D.D., "Cost-EffectiveArctic Concrete Structures," Proceedings of the Conference Arctic 85,- -American Society of Civil Engineers, March 1985, pp. 1229-1242.

6.3 Anderson, Arthur R., "World's Largest Prestressed LPG Floating Vessel,"Journal of the Prestressed Concrete Institute, Vol. 22, No. 1,January-February 1977.

6.4 Gerwick, Ben C., Jr., "Construction Considerations II, Assembly andLaunching of Concrete Ships," Proceedings of the Conference on Concrete- -Ships and Floating Structures, University of California, Berkeley, CA,September 15-19, 1975, pp. 175-179.


7.0 TOWING AND INSTALLATION7.1 IntroductionTowing a large displacement, barge-like structure requires that specialattention be given to design considerations for strength, maneuverability,and stability. These concerns are especially important when the vessel isto be constructed at one location, then towed, possibly over great distancesin an ocean environment, to a remote location. For some recent applicationsof the barge concept (Valdez Container Terminal, the Rofomex concrete hullphosphate plant, and the ARC0 LPG barge), the anticipated delivery voyagetowing loads were highly influential in sizing the primary barge hullscantlings (dimensions). Barge hull strength (bending and shear) andfreeboard requirements can be more critical during the delivery voyage thanduring service at more sheltered locations. For these reasons, towing andinstallation considerations such as selection of tow route and time of yearfor the tow, towing configuration, associated towing and rigging forces, andattachment to mooring systems at the service site need close attentionduring the barge design process.

7.2 Design Considerations7.2.1 Intact and Damage StabilityRecommendations for minimum intact and damage stability characteristics areprovided by the classification societies and published in various designguideline documents by professional societies. DnV's Rules for the Design,- -Construction, and Inspection of Offshore Structures and FIP's Recommendationsfor the Design and Construction of Concrete Sea Structures are examples.Reference 7.1 is a further example. All of these references discuss bothdamage and intact stability and require that the vessel possess an adequaterange of static stability during conditions of expected loading, plus reserverighting energy to withstand the overturning moment due to a horizontal windcondition of specified magnitude.

For barge-like concrete vessels that are towed infrequently, single-compartment damage conditions are often assumed. Various codes and regula-tions require that the damaged vessel have sufficient minimum rightingenergy to withstand a short-duration (seasonal storm) wind force withoutloss of stability, and that compartmentation be arranged to preclude pro-gressive flooding if damaged.

For a barge which is intended for frequent towing or for Arctic service,two-compartment damage stability is recommended. Additionally, the designmust ensure that when the barge is damaged, the resulting heel or trim ofthe vessel with one or more compartments flooded to the equilibriumwaterline does not cause progressive flooding of adjacent compartmentsthrough vents, hatches, or other nonwatertight openings.

7.2.2 StrengthAll aspects of towing should be investigated to ensure that the structure isnot exposed to loadings greater than those for which it was designed. Thedesign bending moment capacity should be sufficient to withstand a wavecomparable to a lo-year return significant wave height, and for a range ofwave periods for the towing season. For a rectangular-shaped barge struc-ture, wave lengths nearly equal to the length of the barge will produce


critical wave bending loads at the assumed design wave height. Strengthen-ing at the waterline is an important consideration for operations involvingcontact with ice during transportation and installation of the vessel. Forconcrete barges, special attention is given to control of cracking, particu-larly membrane cracking due to the design delivery voyage wave conditions.In addition, consideration is given to the control of barge hull deflectionsdue to combined dead load and wave effects to assure that structures orsystems affixed to the barge are not excessively loaded. For a more completedefinition of these strength-related matters, see Chapter 5.0.

7.2.3 Response to MotionThe response of the structure to motion in all degrees of freedom can bedetermined for the assumed towing conditons. Many computer programs areavailable to make these determinations. These programs typically require asinput data overall vessel size and shape, added mass, center of gravitylocation, and radii of gyration about the three axes of rotation. Computerprograms are often verified against model test results. Where the shape ofthe structure makes it sensitive to undesirable motions (e.g., dynamicuplift, nosediving, and yaw), the hydrodynamic control of the structureunder tow should be adequate to minimize this effect. Checks are made toensure that the motions of the unit in the most severe expected environmentalconditions do not result in unacceptable stresses or increases in draft.

Adequate tiedowns are provided to prevent damage to the superstructure andequipment caused by dynamic accelerations during the tow. Again, motionanalyses computer programs are capable of providing anticipated accelerationswith respect to the six degrees of freedom at any point on the floatingstructure. Several existing programs provide the "coupled" motion responses.These data can be used to design required additional holddowns and bracing.The resulting tiedown design loads should consist of the coupled analysisloads multiplied by a suitable factor to account for slamming and high-frequency oscillation effects.

7.2.4 Towing ConnectionsAn adequate number of towing connections, suitably placed, are normallyfitted to the structure to permit the towing vessels (tugs) to be easilysecured and released. Design of the towing connection to the barge providessufficient load capacity such that the strength of the connection exceedsthe strength of the tow line with a suitable factor of safety.

Towing contractors generally recommend that two separate sets of quick-release clench plates, chain bridles, and wire pennants be available forward,with one complete set as a backup. The pennant line is used to allow tugsto make fast without approaching the hull too closely. If the unit is to betowed through ice-laden waters, two sets of quick-release clench platesshould be available at the stern in case additional tugs are needed toassist with steering and stoppage. This arrangement can prevent the towedunit from overrunning the towing vessels should the lead vessel become besetby ice. For towing in ice conditions, towing connections should be strate-gically placed in order not to interfere with ice movement around thestructure.


Towing connections often have an ultimate strength of at least two times thebreaking load of the tow wire, equal to four times the bollard pull, of thelargest tug which would be envisaged for such tow. This allows for substi-tution of larger towing vessels if necessary. Emergency towing arrangementsshould include a spare pennant wire connected to an emergency bridle. Thisbridle, in turn, may be lashed back along the side of the unit and connectedto a polypropelene floating rope of bright color connected at the end of abright colored marker. If a unit is manned during the tow, the emergencyarrangements are secured onboard and streamed overboard as directed by thetow master. In the case of an unmanned tow, the floating rope and markerbuoy trail behind the tow.

7.2.5 MooringsVessel mooring design is considered either as part of a contingency planduring towing or at the intended final location. It is often consideredtogether with the effect of ice if such exposure is anticipated. The envi-ronmental extremes used to design the mooring system often take into accountthe length and season of exposure. In the absence of further guidance, al0-year seasonal return period is commonly recommended for temporary instal-lations (less than four weeks), and a 100-year return period for permanentinstallations, including ice conditions where a 100-year return period isalso recommended. It is not normally required to include the combinedeffects of ice and waves. Slow drift forces are an important considerationin the design of a mooring system.

The mooring system is often designed for mooring loads that will not exceed60% of the breaking strength for wire rope and 70% for chain. The design ofmoorings is covered in the American Petroleum InstituteAPI RP-2P.

7.2.6 Other ConsiderationsConsiderations are also given to improving the safety ofsuitable manning, emergency anchor(s), access ladders,

(API) specification

the tow by providingflood detection

systems, emergency generators, and navigation lights in accordance withInternational Regulations for Preventing Collisions at Sea, published by theIntergovernmental Maritime-Consultation Organization-(Reference 7.2). Thesafety of the tow will be enhanced by providing accompanying vessels,developing contingency plans, providing soundings and adequate pumpingarrangements, and using onboard weather facsimile equipment and weatherrouting. For some specific applications, outfitting the barge with skegsand/or a false bow may be necessary to provide directional control.

7.3 Tow Route7.3.1 Depth of WaterA properly selected towing route ensures that the structure is afloat at alltimes with an adequate underkeel clearance. Calculations of the underkeelclearance should take into account the increase in draft due to roll, pitch,heave, squat effect (the tendency for a vessel to increase draft while underway in very shallow water), towline forces, and wind heel. Attention shouldbe focused on the fact that shallow water [less than 165 ft (50 m)] bottomclearance may affect computations for drag, sway, and dynamic surge.


w

W

Where the soundings shown on the largest scale navigational chart availableare of questionable accuracy (e.g., where the charts are based on incompletehydrographic surveys or they are of areas subject to changes in seabedtopography), a pretow survey of the selected route should be performed. Theplanning and conduct of surveys are based on the accuracy and repeatabilityof the navigation of the survey vessel and on the size and draft character-istics of the towed object. Where sand waves occur which could obstruct thetow, their shapes and locations are determined by a sufficient number ofsurveys to enable their movements to be predicted. The effect of storms onsand waves is often considered.

Typical nautical charts indicate spot water depths which have been measuredbelow the instantaneous water level by lead line or echo-sounder. In someareas of the world, these data points may be a mile apart, and may be out ofdate. Verification of data using methods such as side-scan sonar or barsweeps is a prudent way to map a proposed tow route.

7.3.2 Towing in Restricted WatersTowing in restricted water often receives special attention. Such watersmay offer shelter from the wind, and the fetch may be such that waves aresmall. However, currents, the proximity of navigational hazards, shippingdensity, and the need to tow with shortened tow lines may necessitate ahigher towing horsepower for safety than would be required in open water.Assistance is often obtained from a pilot having knowledge of the localaters.

The selected route should provide adequate sea room for the maneuvering oftugs, and the width of channel at the seabed is carefully considered inrelation to the width of the structure being towed and to the tidal, current,and weather conditions. In restricted waters, the selected navigable channelshould permit the passage to be made with a minimum number of course changes.here available information on the strength and direction of tidal streamsand currents may be inadequate, a survey should be made to determine suchcurrents at the surface and its variation with depth.

7.3.3 Towing at SeaFor towing at sea, towing vessels are selected that have sufficient power tosafely hold the unit in gale force winds and associated waves, together witha current of at least 1 knot. This can be calculated using Reference 7.3.Greater current or tidal stream velocities are considered in certain locali-ties. The calculated forces on the structure imposed by wind, waves, andcurrents should be verified by model tests, if necessary.

Tugs should be adequately equipped with spare towing hardware, chain andline, and have sufficient reserve fuel for any reasonable contingency. Afull complement of replacement supplies should be provided if the tow entersa remote area where emergency assistance is unavailable.

7.3.4 Environmental Criteria and Weather LimitationsTows are generally scheduled to be completed within a period covered by areliable weather forecast, and are often scheduled to take place within aperiod of mild weather. Tows generally do not commence other than in fine


weather and with a favorable weather forecast. It is common to identifypotential "safe harbors" along the tow route before commencing the tow.

When the tow is expected to last beyond the period covered by a reliableweather forecast, it should be planned to deal safely with the most severewind speed, wave height, and current which would be reasonable to expectduring the tow. A l0-year statistical return period is often considered forthe season of the tow. Special consideration may be given to towages ofshorter duration. The climate along the tow route should be investigatedand towing avoided during seasons or periods when unacceptable weather isfrequent or when solid ice or pack ice may be expected.

7.4 SummaryThe technical feasibility and, in some extreme cases, the insurability of abarge-like structure may depend heavily on the attention given during designto towing and installation considerations. Although damage is unlikely,vessels are commonly designed assuming that damage could occur during thetow. The consequences of intercompartmental flooding must be considered,and appropriate attention must be given to the dynamic accelerations of thevessel in the seaway. Towing forces and towline redundancy along withvessel manueverability are also considered. Experienced towing contractors,consultants, and regulatory agencies can provide the best source of suchinformation to the vessel designer.

In spite of the necessary concerns noted herein, the towing and mooring ofbarge-like concrete structures can be accomplished by the same state-of-the-art methods used for steel vessels. The major difference between steel andconcrete vessels is one of displacement. The larger displacement of concretevessels means increased towing horsepower requirements, but also decreasedmotions in the seaway.


7.1 Rules for Building and Classing Mobile Offshore Drilling Units,- -American Bureau of Shipping, Paramus, NJ, 1985.

7.2 Proceedings of the International Conference on Revision of theInternational-Regulations for Preventing Collisions at Sea, 1972,Intergovernmental Maritime Consultative Organization,-London, 1973.

7.3 Remery, G.F.M., and Hermans, A.J., "The Slow Drift Oscillations of aMoored Object in Random Seas."


8.0 MAINTENANCE, INSPECTION, AND REPAIR8.1 IntroductionFloating concrete structures, and indeed marine concrete structures ingeneral, continue to demonstrate low maintenance and high serviceabilitycharacteristics. The serviceability performance can be greatly enhanced byattention to detail during design and preparation of construction specifica-tions, followed by implementation of high-quality concrete constructiontechniques and methods. Regular, well planned surveys of the in-situ struc-ture can greatly mitigate the need for extensive maintenance and can beinstrumental in preventing costly repairs.

However, the occurrence of accidents (impact, collision, fire), use ofsubstandard construction materials and techniques, abrasion, or a variety ofother causes may require that the structure be repaired. Should a repair benecessary, measures should be taken to enhance the ease with which therepair can be made to ensure the highest quality end result. In any event,all repairs should be performed as closely as possible in accordance withrecommendations for new construction. A comprehensive guide to the subjectof maintenance, inspection, and repair can be found in FIP's "Maintenance ofPrestressed Concrete Structures."

Professionals involved with the repair work should have full knowledge ofthe following related facts where applicable:

(a) The causes of concrete structure deterioration

(b) The design purpose or service requirements of the part of thestructure to be repaired

(c) The extent and urgency of the required repair

(d) The composition and intended purpose of the materials used forthe repair

(e) Available repair methods and techniques

Because of the heterogeneous composition of a concrete structure, the basicprocedure for any repair is to remove the damaged material until soundparent material is expoed, replace the damaged structure with selectedreplacement materials, and cure the repaired concrete properly to ensurebond and integration with the undamaged structure.

The proper implementation of a repair can be accomplished only after thecause of the deterioration is thoroughly understood. High-quality repairsare not a guarantee against continued deterioration. Every repair shouldbegin with an assessment of the cause for the deterioration.

8.2 Structural DeteriorationThe following is a list of major sources of deterioration in a concretestructure. Any one of these, or a combination thereof, may suggest the needfor a repair.


(a) Poor design or substandard construction practice, leading tocracking, spalling, or collapse of a framing member

(b) Excessive overload

(c) Fire damage

(d) Accidental impact

Abrasion by tidal action andwaters or industrial waterways

currents, especially in Arctic

Sulfates in seawater reacting with certain cements and causingsurface softening (The softened surface would then be more susceptible to abrasion damage.)

(g) Cyclic wetting and drying in the splash zone, which can underminethe bond between the aggregate and cement paste

(h) Freezing and thawing, which may cause spalling and scaling ofthe concrete due to the expansion of freezing water in theconcrete matrix

(i) Saline intrusion and expansion upon recrystallization, causing apitting, scaling, and spalling condition (see Reference 8.1,page 513)

(j) Corrosion of reinforcing caused by cracking or by the penetra-tion of the combined effects of chloride and sulfate solutionsthrough the concrete cover, which can lead to further formationof cracking, which in turn may cause progressive reinforcingcorrosion and deterioration of the concrete

Other causes and forms of deterioration may exist, but can be associatedwith one or more of the types listed above. Many of the above forms ofdamage can be prevented by proper selection and use of construction mate-rials, concrete mix design, and high-quality construction practices.

8.3 Surveys and Periodic InspectionPeriodic inspections and preventive maintenance are effective methods forkeeping a structure in serviceable condition. It is often recommended thatfloating structures be surveyed annually to determine if any damage ordeterioration has occurred. Special surveys of primary load-carrying membersor watertight boundaries are advisable after an accident, exposure to extremeenvironmental conditions (storms), or a sudden noticeable change in the rateof structural deterioration or a reduced capacity of the structure to performin an acceptable, serviceable manner. Results of annual surveys should bereviewed every five years to determine if undesirable trends in structuralperformance or condition have occurred.

An important adjunct to any survey is a reliable set of as-built drawingsfor the structure. These drawings should define reinforcing and prestressing


locations, positions of embedments and penetrations, and requirements forpost-tension tendon stressing, as appropriate.

Because of the diverse applications for which concrete barge-like structuresmay be used, there is no single survey checklist that is applicable to allstructures. However, a partial list of common areas of concern is presentedas follows:

Visual inspection of the general condition, both external andinternal (if possible), and appearance of the structure. Inspec-tion of interior compartments, especially on vessels wherecompartments are used for ballast water, storage of consumables,or as enclosures for tankage (such as LNG and LPG) should beconducted to the fullest extent possible. Provisions should bemade during vessel design for access to such compartments andfor purging such compartments of harmful contaminants. Surfacesto be inspected should be cleaned.

(b) An assessment of the amount and location of concrete deteriora-tion and cracking. Key areas for investigation include thesplash zone, post-tensioning anchor blockout patches, areassubject to frequent berthing and mooring of supply vessels, thetop deck, materials handling areas, and areas exposed to chemicalspillage. Crack extent and width should be noted and recordedby photographs.

(c) An assessment of the condition and function of corrosion protec-tion systems.

(d) An assessment of the condition (corrosion, joint and connectiontightness, and appearance) of exposed metallic components suchas marine risers, vessel mooring and berthing fittings, towinghardware, attachments to the primary mooring system, and otherload- or function-related metallic hardware.

(e) An assessment of the nature and severity of marine growth, ifany. Adequate visual inspection of the exterior surface in thesplash zone and under water may not be possible due to accumula-tion of marine growth. Removal of growth may be required tocomplete the survey.

Surveys should be documented with adequate definitive data and filed forfuture reference (such as just prior to the next scheduled survey). Visualsigns of distress or unusual appearance which may be indicative of the needfor concern or possible repair are cracking, splitting or spalling ofconcrete, rust staining of the concrete surface by reinforcing bars, impactdamage, leaching of effluent deposits from cured concrete joints, disconti-nuity of surface condition in the area of post-tensioning anchor blockouts,or accumulation of leakage water in internal compartments. Leaching oflime-rich deposits from cracks in relatively new concrete is normal, and isnot necessarily indicative of distress. Such deposits may continue to reactwith seawater and can eventually seal cracks.


8.4 RepairsFollowing an inspection, certain areas of the structure may be identified aspossibly requiring repair. Minor distress of a large, marine structure maynot adversely affect the safety and serviceability of the vessel and may notrequire repair. Understanding the vessel design intent and service history,and combining that knowledge with common sense, may prevent unnecessary,perhaps costly repairs.

If a repair is necessary, following the systematic approach outlined belowwill assist in assuring its success:

(a) Locate (by survey or inspection) the type and extent of distressin the structure.

(b) Determine the probable cause for the deterioration and advisethe vessel operator.

(c) Evaluate the strength of the structure in the damaged conditionto assess the urgency and complexity of the repair, if required.

(d) Evaluate potential repair procedures and select a procedure withdue consideration of not only first cost and degree of difficultyand risk, but also life-cycle cost, safety, and possible remedialfuture repairs. The selected procedure should also consider theprevailing operating circumstances.

(e) Implement the repair procedure.

(f) Advise the vessel operator of recommended surveys of the repairedareas to monitor performance.

All of the above steps may influence the performance of the repaired struc-ture. Central to this approach is understanding the severity and cause ofthe damage and the selection of a cost-effective repair procedure which willnot be detrimental to the function of the structure.

The repair process itself often consists first of the development of adetailed stepwise repair procedure documented by sufficient plans andspecifications for control of the work. The procedures and documents shouldbe reviewed by the owner/operator, the repair contractor, and regulatoryagencies as applicable. Following this review, a presentation of the needfor the repair and a description of the repair procedure should be discussedwith all interested parties. Adequate planning and achieving a thoroughunderstanding of the nature of the repair can enhance the success of therepair process.

8.4.1 MaterialsIn general, the materials selected for the repair should, wherever possible,meet the same standards and specifications as used for the original con-struction. Where repairs must be undertaken at remote locations, qualitystandards for materials and workmanship may be unavoidably compromised. Forsuch cases, allowances are made in the repair by overdesign to allow forreduced strength and less than optimum performance of the repair materials.


Most important, repair materials must be selected which are compatible withthe damaged concrete structure. Compatibility is often assessed on thebasis of strength, extent of repair, modulus of elasticity, thermal proper-ties, and chemical stability in a marine environment.

Repair methods for concrete barge-like structures should embody the recom-mendations provided by ACI 357R, ACI 201, and API RP-2A, plus specificinstructions provided by suppliers and manufacturers of the repair materials.Special care is taken for the storage of repair materials, especially atremote locations where conditions may be less than optimum. A brief dis-cussion regarding important considerations which should be given in theselection of common repair materials follows.

8.4.1.1 CementCement for the repair should conform to ASTM C 150, or blended hydrauliccements per ASTM C 595. Most important, selection of cement should be tomatch the requirements of the original construction. Type I or II cementsare recommended for common repairs. Type III should be considered wherehigh early strength is desirable, and where the additional heat of hydrationwhich results is not detrimental to the repair. Cements with tricalciumaluminate (C,A) contents of 4% to 10% are recommended. Mineral admixturessuch as pozzolans may be used to enhance the complete hydration of theconcrete matrix. Where used, pozzolans should conform to ASTM C 618 andmixture proportions should be modified as needed to provide properworkability.

8.4.1.2 AggregatesAggregates should consist of natural sand and either dense coarse aggregateor suitable lightweight coarse aggregates. The selected maximum aggregatesize should be compatible with reinforcing details and forming and placingtechniques. Nominal maximum aggregate size also depends on the extent ofthe repair.

8.4.1.3 ResinsIf resins are required, moisture-resistant resins should be specified formarine concrete repairs. Such materials tend to retain bond strength andchemical stability in damp marine environments. The climate at the locationof the repair and the planned method of curing could influence the selectionof a resin. Epoxy- and polyester-base resins are generally quite suitablefor repair of floating concrete structures. Strict adherence to recommendedmixing and application procedures by resin manufacturers and development ofactual experience in handling and using the resins by performing prerepairtrials are necessary for completing a successful repair.

8.4.1.4 Concrete CompositionConventional portland-cement concrete, fiber-reinforced concrete, latex-modified Portland-cement concrete, and polymer concrete are all candidatematerials for the repair of a marine structure. Portland-cement concrete isa common material for new construction and is used where low cost, highstrength, and durability are required. As for all marine concretes, mixtureproportions should be selected to achieve high strength with low water-cementratio, low permeability, and good workability.


Latex-modified concretes and polymer concrete are becoming increasinglypopular because of their compatibility with ordinary portland-cement concreteand their good bond characteristics, high strength, and low permeabilityproperties. The consistency of latex-modified concretes in the unhardenedstate makes them suitable for application on vertical surfaces and otherdifficult locations. Not all latexes are compatible with seawater. Somewill re-imulsify upon contact with seawater and care should be taken whenselecting a specific latex. Fiber-reinforced concrete, while more difficultto work with, provides a tough material for areas subject to repeated impactand abrasion. Such concretes are more costly than ordinary concretes, butunless the repair is very extensive, requiring that considerable quantitiesbe used, differences in concrete cost may be offset by the technical orlogistic benefits of their use.

8.4.2 Repair MethodsAs much as practicable, repairs should be designed to minimize adverseworking conditions, should be attempted under favorable environmental condi-tions, and should commence only after the cause for the damage has beenidentified.

8.4.1.2 CracksCracks can be repaired in dry or submerged conditions. There is no apparentindustry consensus regarding the maximum width a crack may reach before arepair is mandatory. Reference 8.2 suggests that cracks in excess of0.005 in. (0.13 mm) in width be repaired. DnV's Recommendations for theDesign, Construction, and Classification of Floating Concrete Structuresrecommends that cracks smaller than 0.008 in. (0.20 mm) be left alone, whilelarger cracks are to be recorded and monitored with each periodic inspection.ACI Committee 357 defines narrow cracks as those less than 0.01 in. (0.25 mm)wide. Such cracks, if dormant (assumed determined by repeated surveys) mayneed only to be sealed against moisture ingress by filling the crack with alow viscosity epoxy resin. Filling can be accomplished by gravity feed, butpreferably by pressure injection.

Both ACI and FIP recommend that wider cracks [those exceeding 0.01 in.(0.25 mm) in width] and narrow cracks which are actively growing with timebe chased and sealed with an elastic material or injected and covered with aflexible membrane. Cracks exceeding 0.02 in. (0.51 mm) in width are to berepaired and should be cleaned free of dust, laitance, oil, or other foreignmatter. Cleaning narrower cracks may prove impractical.

The epoxy resin to be introduced into the crack should bond to wet concreteand should adequately cure under moist or even submerged conditions. Therate at which the epoxy is placed in the crack should be controlled todisplace any water in the crack without allowing the water to mix with anddilute or disperse the epoxy. When injection methods are used, the injectionpressure should be controlled to prevent further splitting and opening ofthe crack.

8.4.2.2 Surface DamageSurface damage can be defined as abrasion, spalling, chipping, delamination,or scaling of the concrete to a sufficiently shallow depth so as not tocompromise the function of the reinforcement. The damage may expose some


reinforcing, but should not be so severe as to compromise the reinforcingcage. This type of damage is often repaired using an overlay patch.

The initial step in making this repair is to remove all loose or unsoundconcrete to an extent to where the exposed concrete is sound. This mayinclude exposing concrete around the reinforcing. When this is required,concrete, whether damaged or sound, should be removed to a distance or depthof at least 1 in. (25 mm) around the bar. This is done to assure properbond of the patching material. After the unsound concrete is removed, thesurface should be cleaned of all loose particles and laitance. It is advis-able to apply a coating of Portland-cement mortar slurry, latex modifiedportland cement slurry, or epoxy resin to the reinforcing. Reinforcingshould be cleaned of laitance and sea salts.

The patching material can be ordinary Portland-cement concrete, latexPortland-cement concrete, fiber-reinforced concrete, or epoxy concrete.Care should be taken when applying patches during weather extremes. Thepatching material should be flowable and workable to ensure complete fillingof the void. In some cases, as when watertightness of the patch is ofspecial importance, epoxy bonding agents are applied to the sound concreteand allowed to partially cure before application of the patching material.Repair curing techniques should be selected which are compatible with thechosen patching materials. Whenever possible, the patch should be protectedfrom direct sun, wind, damage, or disturbance while curing.

8.4.2.3 Major DamageMajor damage is damage that may directly reduce the present load-carryingcapacity of the structure. Impact, fire, and structural overload are primecauses. For this type of damage, it is extremely important that the struc-tural adequacy of the vessel be assessed by inspection prior to beginning arepair. Numerous repair techniques can be used to restore the serviceabilityof the vessel.

As for all repairs, removal of damaged material is a first step. Should thedamage to the structure be extensive, analyses should be made to determineif shoring or bracing of the member is warranted to reduce the likelihood offurther damage. All concrete surfaces should be cleaned to enhance bondstrength with the replacement material. Mild steel reinforcing should beinspected for brittle cracking or yielding. All suspect mild steel rein-forcing should be replaced by lap splicing or welding of replacement steelof like specification. Bars should be lapped according to ACI 318. Whenwelding, preheating is required for the commonly used reinforcing steels.Butt welds should be avoided. Welding should be accomplished in accordancewith American Welding Society procedures. All damage to prestressing tendonsshould be considered major. An analysis should be made to assess the needfor the replacement of damaged tendons.

Prior to replacing the repair concrete, a detailed inspection of the rein-forcing bar cage and concrete surfaces should be made. Methods used toreplace large volumes of concrete are cast-in-place concrete (in formworksimilar to the original construction), using preplaced aggregate concrete,or shotcreting. In general, better consistency of concrete properties isachieved if cast-in-place concrete is used. However, certain repair

restrictions such as accessibility or available equipment may dictate thatone of the alternate methods be used.

Whatever method is used, proper curing techniques should be used to assure ahigh-quality, well-bonded repair,

8.4.2.4 Corrosion DamageConcrete suffering from reinforcing bar or prestressing tendon corrosion mayexhibit spalling, staining, and delamination damage. All such unsound orloose concrete should be removed, and the corroded reinforcement cleaned.Should the reinforcement be severely corroded, it may no longer meet ASTMspecifications and should be replaced by splicing in new reinforcing steel.Should replacement be necessary, the structural capacity should be checkedin the region where the removal of reinforcing steel is to occur. Often,epoxy-coated reinforcing bars are used in repairs necessary due to corrosiondamage.

Before replacing the concrete patch, the source of the corrosion should beascertained. A patch having a low water-cement ratio should be provided.If acceptable to the function of the structure, an increased concrete coverover the reinforcing should be considered. After placing and curing therepair concrete, it may also be advantageous to apply a protective coatingover the affected area. Vapor permeable coatings providing low resistanceto vapor transmission should be used above the waterline, except in areaswhich are never in direct sunlight. Vapor barrier coatings with a highresistance to vapor transmission should be used in the splash zone, underwater, and on the underside of the structure where sunlight is uncommon.Applying these coatings on an in-service vessel will require dewatering andcleaning of the affected areas. The effectiveness of a coating depends uponits ability to remain in place (good bond). If vapor pressures within thestructure build, the coating may be pushed off of the surface, possiblytaking with it a layer of concrete. Hence, vapor barrier coatings shouldnormally not be applied on both sides of a concrete element, since they mayencapsulate the concrete and prevent relief of vapor pressure.

8.4.2.5 Underwater RepairsRepairs can be made underwater. Certain epoxy resins can be used to patchsurfaces and seal cracks under water. Injection pumping of epoxy mortars isalso a viable method. Recently, a World War II floating concrete dry docksustained keel damage such that compartment flooding had occurred. The workwas accomplished for the Port of Bellingham, Bellingham, Washington, inAugust 1983. This vessel was repaired by placing a sealable steel cofferdamstructure over the affected area, pumping the area dry, removing the damagedkeel slab (with reinforcing), and replacing the reinforcing and concretefrom inside the compartment while the dry dock was afloat. All repair workwas done "in the dry." After the concrete had cured, the cofferdam wasremoved and the vessel put back into service. This method was far lesscostly than towing the vessel to a larger dry dock for repair.

8.5 SummaryThe state of the art for repair of marine concrete structures is advancedand many materials and methods are available for use in repair situations.A large amount of reference material for concrete structure repair can be


found in Reference 8.3. Cautionary notes include that adequate inspectionand isolation of the extent of the damage precede the actual repair process.Furthermore, a successful repair will be highly dependent upon correct useof selected materials, and manufacturer's recommendations should be followedat all times. Successful repairs are cost-effective because they extend theservice life of a vessel.


8.1 Performance ofSP-65, American

Concrete in Marine Enviroment, ACI PublicationConcrete Institute, Detroit, MI, 1980, pp. 527-556.

8.2 Campbell, R.A., Chang, K.T., and Stiansen, S.G., "Classification ofConcrete Ships: Historical Background and Current Practice," ConcreteAfloat, Thomas Telford Ltd., London, 1977, pp. 51-70.

8.3 U.S. Department of Transportation, U.S. Coast Guard, Inspection Guidefor Reinforced Concrete Vessels,Washington,

Report CG-M-11-81 by A.E. Fiorato,DC, October, 1981.


9.0 SPECIFIED REFERENCESThe documents of standards-producing organizations referred to in thisreport are called specified references and are listed below with theirserial designation, including year of adoption or revision. The listeddocuments were current at the time this report was written. Because some ofthese specified references are frequently revised, the user of this reportshould contact the appropriate sponsoring group if it is desired to refer tothe latest document revision.

9.1 American Concrete Institute (ACI)

201.2R-77 Guide to Durable Concrete.(Reaffirmed 1982)

211.1-81 Standard Practice for Selecting Proportions for(Revised 1984) Normal, Heavyweight, and Mass Concrete.

211.2-81 Standard Practice for Selecting Proportions forStructural Lightweight Concrete.

212.2R-81 Guide for the Use of Admixtures in Concrete.

213R-79 Guide for Structural Lightweight Aggregate Concrete.(Reaffirmed 1984)

221R-84 Guide for Use of Normal Weight Aggregates in Concrete.

225R-85 Guide to the Selection and Use of Hydraulic Cements.

318-83 Building Code Requirements for Reinforced Concrete.

357R-84 Guide for the Design and Construction of FixedOffshore Concrete Structures.

357.1R-85 State-of-the-Art Report on Offshore Concrete Struc-tures for the Arctic.

The above references can be obtained from the following organization:

American Concrete Institute38800 Country Club DrFarmington Hills, MI 48333

9.2 American Petroleum Institute (API)

RP-2A "API Recommended Practice for Planning, Designing,and Constructing Fixed Offshore Platforms," FifteenthEdition, 1984.

RP-2P "The Analysis of Spread Mooring Systems for FloatingDrilling Units, First Edition, 1984.



American Petroleum Institute211 North Ervay, Suite 1700Dallas, TX 75201

9.3 American Society for Testing and Materials (ASTM)

A 706-82a Specification for Low-Alloy Steel Deformed Bars forConcrete Reinforcement

A 775-81 Specification for Epoxy-Coated Reinforcing Steel Bars

C 33-82 Specification for Concrete Aggregates

C 150-83a Specification for Portland Cement

C 330-82a Specification for Lightweight Aggregates for Struc-tural Concrete

C 595-83 Specification for Blended Hydraulic Cements

C 618-83 Specification for Fly Ash and Raw or Calcined NaturalPozzolan for Use as a Mineral Admixture in PortlandCement Conccrete


American Society for Testing and Materials1916 Race StreetPhiladelphia, PA 19103

9.4 Det norske Veritas (DnV)

Recommendations for the Design, Construction and Classification ofFloating Concrete Structures, April 1978.

Rules for the Design, Construction, and Inspection of Offshore Struc-tures, 1977.


Det norske VeritasVeritasveienP.O. Box 3001322 HdvikOslo, Norway

9.5 Federation Internationale de la Precontrainte (FIP)

Design and Construction of Concrete Ships, 1985 (draft).


Design and Construction of Concrete Sea Structures, 4th edition,1985.

Recommendations for Acceptance and Application of Post-TensioningSystems, March 1981.

Maintenance of Prestressed Concrete Structures, 1978.

State-of-the-Art Report: Lightweight Aggregate Concrete for MarineStructures, 1978.

Recommendations for the Design and Construction of Concrete SeaStructures, 1977.


Federation Internationale de la PrecontrainteWexham SpringsSlough SL3 6PLEngland

CIP

ft

km

kg/m3

in.

m

mm

MPa

%

pcf

psi

RAO

rms

ABBREVIATIONS USED IN TEXT

cast-in-place concrete

feet

kilometers

kilograms per cubic meter

inches

meters

millimeters

megapascals

percent

pounds per cubic foot

pounds per square inch

response amplitude operator

root mean square

The full report was submitted to letter ballot of the committee, whichconsists of 23 members; 21 members returned ballots, 20 of whom votedaffirmatively and 1 abstained.