Very large Floating Structures

VERY LARGE FLOATING STRUCTURES (VLFS)

2012-2013 Seminar Report Submitted by

Sahil K.A

DEPARTMENT OF CIVIL ENGINEERING

SCHOOL OF ENGINEERING, THRIKKAKARA COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY

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CERTIFICATE Certified that this is a bonafied record of the seminar report titled

VERY LARGE FLOATING STRUCTURES

Submitted by

SAHIL K.A

of VII semester Civil Engineering in the year 2012 in partial fulfillment of the requirements for the award of Degree of Bachelor of Technology in Civil Engineering of Cochin University of Science & Technology.

Dr. Benny Mathews Abraham

Dr. Deepa G Nair

Head of the Division

Seminar Guide

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ABSTRACT Incoming years, the world is facing new problems such as the lack of land, due to the growing population and fast urban developments. Many developed island countries and countries with long coastlines in need of land have for some time now been successfully reclaiming land from the sea to create new space and, correspondingly, to ease the pressure on their heavilyused land space. In response to the aforementioned needs and problems, researchers and engineers have proposed an interesting and attractive solution the construction of very large floating structures. In recent years, an attractive alternative to land reclamation has emerged – the very large floating structures technology. Japan is the world’s leader in VLFS (Very large floating structures).VLFS can and are already being used for storage facilities, industrial space, bridges, ferry piers, docks, rescue bases, airports, entertainment facilities, military purpose, and even habitation in many countries. In this seminar paper we can discuss about the types of VLFS, Components of VLFS, the advantages, Disadvantages Applications of VLFS in detail. VLFSs can be speedily constructed, exploited, and easily relocated, expanded, or removed.

Keywords – VLFS,Applications of VLFS,Megafloat

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TABLE OF CONTENTS Page no. i.

CHAPTER I 1. Introduction

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2. Components in VLFS ii.

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CHAPTER II 1. Creating VLFS

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1. Analysis and Design

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2. Approval of Government agencies

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3. Fabrication and towing works

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4. Joining of parts at sea

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5. Maintaining the structure

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2. VLFS in details

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1. Advantages of VLFS

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2. Disadvantages of VLFS

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3. Applications of VLFS

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1. Floating Bridges

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2. Floating docks, piers, berths and

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container terminals 3. Floating Plants

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4. Floating Emergency Bases

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5. Floating Storage Facilities

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6. Floating Airports

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3. Developments in VLFS

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1. Mooring systems

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2. Mitigating the hydro elastic responses

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3. Connector designs

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4. Other developments

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4. Minimizing deflection in VLFS SCHOOL OF ENGINEERING, CUSAT

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5. Minimizing Motion in VLFS iii.

CHAPTER 3

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1. Conclusion

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2. Reference

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ACKNOWLEDMENT It is matter of great pleasure for me to submit this seminar report on “Very Large Floating structures “, as a part of curriculum for award of degree in “Bachelor of Technology” in Civil Engineering Cochin University of Science and Technology.

First and foremost, we would like to thank to our supervisor of this project, Dr. Deepa G Nair, Civil Engineering Department for the valuable guidance and advice. She inspired us greatly to work in this project. Her willingness to motivate us contributed tremendously to my topic. It gave me an opportunity to participate and learn about the Very Large Floating Structures. Finally, an honorable mention goes to our families and friends for their understandings and supports on us in completing this project.

SAHIL KA

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CHAPTER 1 : 1. INTRODUCTION The total land area of the Earth’s surface is about 148,300,000 square kilometers, while the Earth’s surface area is 510,083,000 square kilometers. Thus, the main part of the Earth’s surface is covered by sea, lakes, rivers, etc, which takes up 70 percent of the Earth’s total surface area. Therefore, the land that we lived on forms only 30% of the Earth’s surface. A large part of the Earth, which is the ocean, remains unexploited.

VLFSs can be constructed to create floating airports, bridges, breakwaters, piers and docks, storage facilities (for oil), wind or solar power plants, for military purposes, industrial space, emergency bases, entertainment facilities, recreation parks, spacevehicle launching, mobile offshore structures and even habitation (it could become reality sooner than one may expect). In certain applications of VLFS such as floating airports, floating container terminals and floating dormitories where high loads are placed in certain parts of the floating structure, the resulting differential deflections can be somewhat large and may render certain equipment non operational. Therefore, it is important to reduce the differential deflection in VLFS.

VLFS may be classified under two broad categories namely the pontoon-type and the semi-submersible type (Fig. 1.1). The latter type has a ballast column tubes to raise the platform above the water level and suitable for use in open seas where the wave heights are relatively large. VLFS of the semi-submersible type is used for oil or gas exploration in sea and other purposes. It is kept in its location by either tethers or thrusters. In contrast, the pontoon-type VLFS is a simple flat box structure and features high stability, low manufacturing cost and easy maintenance and repair. However, it is only suitable in calm sea waters, often near the shoreline. Pontoon-type VLFS is also known in the literature as mat-like VLFS because of its small draft in relation to the length dimensions.

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Fig 1.1 VLFS may be classified under two broad categories namely the pontoon-type and the Semi-submersible type

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2.COMPONENTS IN VLFS Japanese calls VLFS as Megafloat also. The components of a VLFS system (general concept) are shown in Fig. 1.2. The system comprises 1. a very large pontoon floating structure, 2. an access bridge or a floating road to get to the floating structure from shore, 3. a mooring facility or station keeping system to keep the floating structure in the specified place, and 4. a breakwater, (usually needed if the significant wave height is greater than 4 m) which can be floating as well, or anti-heaving device for reducing wave forces impacting the floating structure 5. structures, facilities and communications located on a VLFS.

Fig 1.2 Components of a VLFS system

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CHAPTER II 1.Creating VLFS

Fig 1.3 Realization of VFLS project

Fig 1.3 represents the whole VLFS construction process. The whole VLFS project comes to after these steps 1.

2.

3.

4.

5.

Analysis and Design Approval of Government agencies Fabrication and towing works Joining of parts at sea Maintaining the structure

1.1 Analysis and designing The analysis and design of floating structures need to account for some special characteristics when compared to land-based structures; namely:

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1. Horizontal forces due to waves are in general several times greater than the (nonseismic) horizontal loads on land-based structures and the effect of such loads depends upon how the structure is connected to the seafloor. It is distinguished between a rigid and compliant connection. A rigid connection virtually prevents the horizontal motion while a compliant mooring will allow maximum horizontal motions of a floating structure of the order of the wave amplitude.

2. In framed, tower-like structures which are piled to the seafloor, the horizontal wave forces produce extreme bending and overturning moments as the wave forces act near the water surface. In this case the structure and the pile system need to carry virtually all the vertical loads due to selfweight and payload as well as the wave, wind and current loads.

3. In a floating structure the static vertical selfweight and payloads are carried by buoyancy. If a floating structure has got a compliant mooring system, consisting for instance of catenary chain mooring lines, the horizontal wave forces are balanced by inertia forces. Moreover, if the horizontal size of the structure is larger than the wave length, the resultant horizontal forces will be reduced due to the fact that wave forces on different structural parts will have different phase (direction and size). The forces in the mooring system will then be small relative to the total wave forces. The main purpose of the mooring system is then to prevent drift-off due to steady current and wind forces as well as possible steady and slow-drift wave forces which are usually more than an order of magnitude less than the first order wave forces.

4. A particular type of structural system, denoted tension-leg system, is achieved if a highly pretensioned mooring system is applied. Additional buoyancy is then required to ensure the pretension. If this mooring system consists of vertical lines the system is still horizontally compliant but is vertically quite stiff. Also, the mooring forces will increase due to the high pretension and the vertical wave loading. If the mooring lines form an angle with the vertical line, the horizontal stiffness and the forces increase. However, a main disadvantage with this system is that it will be difficult to design the system such that slack of leeward mooring SCHOOL OF ENGINEERING, CUSAT

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lines are avoided. A possible slack could be followed by a sudden increase in tension that involves dynamic amplification and possible failure. For this reason such systems have never been implemented for offshore structures.

5. Sizing of the floating structure and its mooring system depends on its function and also on the environmental conditions in terms of waves, current and wind.

The design may be dominated either by peak loading due to permanent and variable loads or by fatigue strength due to cyclic wave loading. Moreover, it is important to consider possible accidental events such as ship impacts and ensure that the overall safety is not threatened by a possible progressive failure induced by such damage.

6. Unlike land-based constructions with their associated foundations poured in place, very large floating structures are usually constructed at shore-based building sites remote from the deepwater installation area and without extensive preparation of the foundation. Each module must be capable of floating so that they can be floated to the site and assembled in the sea.

7. Owing to the corrosive sea environment, floating structures have to be provided with a good corrosion protection system.

8. Possible degradation due to corrosion or crack growth (fatigue) requires a proper system for inspection, monitoring, maintenance and repair during use.

1.2 Government approval The plan of Megafloat must be evaluated and approved by the authority. The general plan of Megafloat must be compliant with both the Port and Harbor Law and the Fishing Port Law. Buildings on the Megafloat shall be regulated by both the Building Standard Law and the Fire Defenses Law. Floating Structures are regulated by the Ship Safety Law. Approval processes differ from law to law. A Megafloat Safety Evaluation Committee must be proposed and accepted by the government. Experts and all government bureaus in charge of the approval gather in the committee and evaluate the application. Once the plan is judged to be acceptable, each bureau approves the plan. The Fig 1.4 represents approval process.

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Fig 1.4 The approval process

1.3 Fabrication and towing of units Units of a Megafloat are simple structure and construction itself is not a difficult task.

Most of the technology developments in the construction phase are related to construction operation at sea. Experiments to test the towing of Megafloat units should be carried out during the construction of onsite experimental models.

1.4 Joining of units at sea Megafloat is constructed by joining unit structures that were fabricated in shipyards. Unit structures are fabricated in the well-controlled environments of shipyards but the joining of the units take place at sea and are exposed to the natural environment of the installation site. Construction by dry welding with a water draining device and wet welding at sea must be investigated. The influence of both wave conditions and unit joining sequences on the responses of structure and performance of construction were investigated.

1.5 Maintaining the structure The VLFS structure should be well maintained for at least 100 years. Environmental impact studies should also be conducted. Inspection and maintenance must be done regularly.

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2. Very Large Floating Structures in detail View 1. Advantages of VLFS 

Very large floating structures have the following advantages over traditional land reclamation in creating land from the sea:

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They are easy and fast to construct (components may be made at shipyard and then be transported to and assembled at the site), thus, the sea space can be quickly exploited.

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They are cost effective when the water depth is large or sea bed is soft.

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They are environmentally friendly as they do not damage the marine ecosystem or silt-up deep harbors or disrupt the ocean currents.

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They can easily be relocated (transported), removed, or expanded.

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The structures and people on VLFSs are protected from seismic shocks since VLFSs are inherently base isolated.

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They do not suffer from differential settlement as in reclaimed soil consolidation.

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Their positions with respect to the water surface are constant and thus facilitate small boats and ship to come alongside when used as piers and berths.

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Their location in coastal waters provide scenic body of water all around making them suitable for developments associated with leisure and water sport activities.

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There is no problem with rising sea level due to global warming.

2. Disadvantages of VLFS: 

Mat-like VLFSs are only suitable for use in calm waters associated with naturally sheltered coastal formations (solution: use of breakwaters, anti-motion devices, anchor or mooring systems)

 (might be) not sufficient stability for the airport control systems (solution: keeping these systems on a shore) 

Low security (bombing, terroristic attacks).

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3. Applications of VLFS VLFS can be applied to 1. Floating Bridges 2. Floating docks, piers, berths and container terminals 3. Floating Plants 4. Floating Emergency Bases 5. Floating Storage Facilities 6. Floating Airports and other offshore bases

3.1 Floating Bridges

Fig 1.5 Yumeshima-Maishima Floating

Yumeshima-Maishima Floating

Bridge in Osaka, Japan

Bridge in Osaka, Japan

In 1874, a 124-m long floating wooden railroad bridge was constructed over the Mississippi River in Wisconsin and it was repeatedly rebuilt and finally abandoned.

Brookfield Floating Bridge is still in service and it is the seventh replacement structure of a 98-m long wooden floating bridge (Lwin 2000). In 1912, the Galata steel floating bridge was built across Istanbul’s Golden Horn where the water depth is 41 m. The 457m long bridge consists of 50 steel pontoons connected to each other by hinges. However, in 1992, soon after a new bridge was erected just beside the original bridge, a fire broke out and the old Galata floating bridge was burned down (Maruyama et al. 1998). The sunken bridge is placed upstream after having been raised from the seabed. The lesson that one can learn from this steel bridge is its amazing resilience against the corrosive sea

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environment, contrary to engineers perception that corrosion would pose a serious problem to such floating steel structures.

Other floating bridges include Seattle’s three Lake Washington Bridges, i.e. (i) the 2018m long Lacey V. Murrow Bridge which uses concrete pontoon girders and opened in 1940, (ii) the 2310-m long Evergreen Point Bridge completed in 1963, and (iii) the 1771m long Homer Hadley Bridge in 1989; the 1988-m long Hood Canal Bridge built in 1963 , the Canadian 640-m long Kelowna Floating (concrete) Bridge which was opened to traffic in 1958, the Hawaiian’s 457-m long Ford Island Bridge which was completed in 1998. Fig 1.5 represents the Yumeshima-Maishima Floating Bridge in Osaka, Japan.

3.2 Floating docks, piers, berths and container terminals There are in existence many floating docks, piers and wharves. For example, the 124 m x 109m floating dock in Texas Shipyard built by Bethlehem Marine Construction Group in 1985. Floating structures are ideal for piers and wharves as the ships can come alongside them since their positions are constant with respect to the waterline. An example of a floating pier is the one located at Ujina Port, Hiroshima (see Fig. 1.6). The floating pier is 150 m x 30 m x 4 m. Vancouver has also a floating pier designed for car ferries. Car ferry piers must allow smooth loading and unloading of cars and the equal tidal rise and fall of the pier and ferries is indeed advantageous for this purpose. A floating type pier was also designed for berthing the 50000 ton container ships at Valdez, Alaska. The floating structure was adopted due to the great water depth.

Fig 1.6 Floating Pier at Ujina, Japan

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VLFSs are ideal for applications as floating emergency rescue bases in seismic prone areas owing to the fact that their bases are inherently isolated from seismic motion. Japan has a number of such floating rescue bases parked in the Tokyo Bay, Ise Bay and Osaka Bay.Another advantage of VLFS is its attractive panoramic view of the water body.

Waterfront properties and the sea appeal to the general public. Thus, VLFSs are attractive for used as floating entertainment facilities such as hotels, restaurants, shopping centers, amusement and recreation parks, exhibition centers, and theaters.

3.3 Floating Plants In 1979, Bangladesh purchased from Japan a 60.4 m x 46.6 m x 4 m floating power plant.

The power plant is located at Khulna, Bangladesh. In 1981, Saudi Arabia built a 70 m x 40 m x 20.5 m floating desalination plant and towed to its site where it was sunk into position and rests on the seabed. In 1981, Argentina constructed a 89 m x 22.5 m x 6 m floating polyethylene plant at Bahia Blance. In 1985, Jamaica acquired a 45 m x 30.4 m x 10 m floating power plant. This plant was built in Japanese shipyards and towed to Jamaica and moored by a dolphin-rubber fender system. Studies are already underway to use floating structures for wind farms, sewage treatment plant and power plant in Japan.

Fig 1.7 represents Concept design of clean energy Plant by Floating structure association of Japan.

Fig 1.7 Design of a Clean Energy Plant,Floating Structure Association of Japan

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3.4 Floating Emergency Bases As floating structures are inherently base isolated from earthquakes, they are ideal for applications as floating emergency rescue bases in earthquake prone countries. Japan has a number of such floating rescue bases parked in the Tokyo Bay, Ise Bay and Osaka Bay. and Figs.1.8 show the emergency rescue bases at Tokyo bay and Osaka bay, respectively.

Fig 1.8 Emergency Rescue Base In Tokyo Bay 3.5 Floating Storage Facilities Very large floating structures have been used for storing fuel. Constructed like flat tankers (box-shaped) parked side by side, they form an ideal oil storage facility, keeping the explosive, inflammable fluid from populated areas on land. Fig 1.9 represents an Oil storage base.

Fig 1.9 Shirashima Floating Oil Storage Base, Japan (Photo courtesy of Shirashima Oil Storage Co Ltd) SCHOOL OF ENGINEERING, CUSAT

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3.6 Floating Airports and other offshore bases In more recent times, a different sort of problem arose. Land costs in major cities have risen considerably and city planners are considering the possibility of using the coastal waters for urban developments including having floating airports. As the sea and the land near the water edge is usually flat, landings and take-offs of aircrafts are safer. In this respect, Canada has a floating heliport in a small bay in Vancouver (Fig 1.11). Moreover, this busy traffic heliport is built for convenience as well as noise attenuation. Japan has made great progress by constructing a large airport in the sea. Kansai International Airport at Osaka is an example of an airport constructed in the sea, albeit on a reclaimed island. The first sizeable floating runway is the one-km long Mega-Float test model built in 1998 in the Tokyo bay (Fig 1.10)

Fig 1.10Runway at Tokyo bay

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Fig 1.11 Vancover Helipad

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4 Development of VLFS technology Presented herein are the technological developments of VLFS, focusing on the design of mooring systems, methods for mitigating the hydro elastic responses and connector designs.

4.1 Mooring systems The mooring system ensures that the VLFS is kept in position so that the facilities installed on the floating structure can be reliably operated as well as to prevent the structure from drifting away under critical sea conditions and storms. A freely drifting very large floating structure may lead to not only damage to the surrounding facilities but also to the loss of human life if it collides with ships. The station keeping system of a floating structure may be grouped into two main types: (1) the mooring lines the caisson or pile-type dolphins with rubber fender system .The former type uses chains, wire ropes, synthetic ropes, chemical fiber ropes, steel pipe piles, and hollow pillar links. These mooring systems are used for VLFS operating in deep sea such as the tension leg floating wind farm and the floating salmon farm (see Figure 1.12). However, the motions of a floating structure become large when the length of mooring line is rather long. Especially in deep seas, the tension leg system (see Figure 1.12(b)) is adopted to which the pretension is applied to the mooring line in order to restrain heaving motion. In such a station keeping system, it is difficult to restrain. The horizontal motion and usually the mooring lines experience significant tension forces.

Fig 1.12 Mooring systems are used for VLFS

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The rubber fender-dolphin mooring system was first adopted for the two floating oil storage bases at Kamigoto and Shirashima islands in Japan. The mooring system has since been used for other facilities such as floating piers, floating terminals, floating exhibition halls, floating emergency bases, and floating bridges. The rubber fenderdolphin type is very effective in restraining the horizontal displacement of the floating structure. As the large size rubber fenders are able to undergo a large deformation (of up to approximately one-third of their lengths), a considerable amount of the kinetic energy of the floating structure can be absorbed.

4.2 Mitigation of Hydro elastic response Various methods have been proposed by engineers to minimize the hydroelastic response of the VLFS. One of the earliest methods is by constructing bottom-founded breakwater close to the VLFS as was done for the Mega-Float. Studies by Utsunomiya et al. (2001) and Ohmatsu (1999) showed that the bottom founded type breakwater is very effective in reducing the hydroelastic response as well as the drift forces. However, such type of breakwater still possesses some drawbacks that include massive construction material requirements, difficulty in construction, occupying precious sea space, difficulty in removing the breakwater if the VLFS is to be relocated elsewhere, not environmentally friendly, and the reflected waves from the breakwater could result in coastal erosion. The floating box-like breakwater moored with mooring lines has been proposed as an alternative to the conventional bottom-founded type breakwater for protecting VLFS from a severe sea. Floating breakwaters do not disrupt the ocean current flow and cause relatively little damage to the seabed. Furthermore, the floating box-like breakwater (being the most common type) constructed around the FFSF as shown in Figure 4 could also function as collision and oil spill barriers.

4.3 Connector Designs VLFS is usually constructed in modules due to its massive size. The modules are fabricated in shipyard, and then connected on site in the sea by welding or by using rigid connectors. More recently, Fu et al. (2007) and Wang et al. (2009) proposed the use of hinge or semi-rigid connectors instead because they found that the non-rigid connectors SCHOOL OF ENGINEERING, CUSAT

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are more effective in reducing the hydroelastic response as compared with the rigid connectors. There have been various connector designs proposed and a review paper by Lei (2007) gave a wide range of these connector systems. However, there is still work to be done on developing a robust and economical connector system for very large floating modules.

4.4 Other Developments The shapes of the VLFS may take on more arbitrary geometries such as the irregularshaped floating island in the han river instead of the conventional rectangular shape VLFS. Various researchers have also considered VLFS of different shapes that could reduce the hydroelastic responses. For example, okada (1998) has investigated VLFS with different edge shapes and confirmed that the notched edge is able to reduce the propagation of deformation over the VLFS. With the view to reduce the hydroelastic response, VLFS with moonpools and different stiffness are proposed and they are found to be very effective in reducing the hydroelastic response of the VLFS when the wave length is small. Wang et al. (2006) have also introduced the innovative gill cells in very large floating container terminal in order to provide an effective solution for reducing large differential deflections of a VLFS under uneven static loading.

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5.Minimizing differential deflection in VLFS Although a lot of studies have contributed much to the theory and analysis of VLFS, there is relatively little work carried out in studying the problem of reducing the differential deflection and stress-resultants in VLFS when subjected to a heavy central load.Wang et al. (2006) proposed an innovative solution to reduce the differential deflection by having compartments in the floating structure with holes or slits at their bottom floors that allow water to flow in and out freely. The free flowing of water through these holes and slits resembles the gills of fish and thus these compartments have been referred to as gill cells. At the gill cells, the buoyancy forces are eliminated. By appropriate positioning of these gill cells, it is possible to reduce the differential deflection significantly as well as the stress-resultants. It is worth noting that the holes in the gill cells have negligible effect on the flexural rigidity of the floating structure. The presence of gill cells leads to a slight loss in buoyancy for the floating structure, which is a small price to pay for the advantage gained in minimizing the differential deflection and stress-resultants. Wang et al. (2006) analyzed a rectangular super-large floating container terminal by using the gill cells. In the studies of Wang and Wang (2005), the hydroelastic of a super-large floating container terminal under sea state of Singapore was analyzed and they found that the deflections due to the wave loads are very small when compared to the static deflections due to the large live loads from the containers resting on the floating structure. Therefore, Wang et al. (2006) neglected the action of wave and they found that when gill cells are appropriately designed and located, the differential deflection and stress-resultants in VLFS are reduced significantly. Fig 1.13 Represents the Cross sectional portion of the floating structure with gill cells.

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Fig 1.13 Cross sectional portion of the floating structure with gill cells

6. Minimizing motion of VLFS There are some ways to reduce the effect of wave on the VLFS. The traditional way is using breakwaters which reduce the height of incident water waves on the leeward side to acceptable level. However, in many cases such as for consideration of environmental protection and economics in open sea, breakwaters with high wave transmission are adopted and the response at the ends of VLFS may become an obstacle to the facilities mounted on the floating structures. Recently, anti-motion devices have been proposed as alternatives for reducing the effect of waves on VLFS where the wave dissipation effect of breakwaters is small or there are no breakwaters. An anti-motion device is a body attached to an edge of VLFS so it does not need mooring system like floating breakwaters and the time needed for construction is also shorter. Ohkusu and Nanba (1996) proposed an approach that treats the motion of VLFS as a propagation of waves beneath a thin elastic-platform. According to this approach the motion of VLFS is presented as waves. That means the anti-motion coincides with a reduction of wavetransmission from the outside to the inside of VLFS. Following this idea, some simple anti-motion devices have been proposed and investigated. Takagi et al. (2000) proposed a box-shaped anti-motion device and investigated its performance both theoretically and SCHOOL OF ENGINEERING, CUSAT

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experimentally. They found that this device reduces not only the deformation but also the shearing force and moment of the platform. The motion of VLFS with this device is reduced in both beam-sea and oblique sea.

A horizontal single plate attached to the fore-end of VLFS was proposed and investigated experimentally by Ohta et al. (1999). The experimental results showed that the displacement of VLFS with this anti-motion device is reduced significantly not only at the edges but also the inner parts. They suggested that it would be possible to eliminate the construction of breakwaters in a bay where waves are comparatively small.

Utsunomiya et al. (2000) made an attempt to reproduce these experimental results by analysis. The comparison of the analytical results with the experimental results has shown that their simple model can reproduce the reduction effect only qualitatively. A more precise model considering rigorously the configuration of the submerged horizontal plate within the framework of linear potential theory is constructed in the study of Watanabe et al. (2003a) and has successfully reproduced the experimental results.

Takagi et al. (2000) and Watanabe et al. (2003a) formulated the diffraction and radiation potentials using the eigen-function expansion method which was originally proposed by Stoker (1957) for the estimation of the elastic floating break-water. This method has been widely applied in many studies such as the study of elastic deformation of ice floes (e.g., Evans and Davies 1968, Fox and Squire 1990, Melan and Squire 1993) and study of the oblique incidence of surface waves onto an infinitely long platform (e.g., Sturova 1998, Kim and Ertekin 1998). More experimental work was investigated by Ohta et al. (2002).

Typical features of anti-motion devices treated in their study are L-shaped, reverse-Lshaped and beach-type plate. They concluded that L-shaped plate is more effective against long waves whereas beach-type and reverse-L-shaped plates are more effective against short waves.

There are some other ideas in reducing the motion of VLFS under wave action. Maeda et al. (2000) proposed a hydro-elastic response reduction system of a very large floating structure by using wave energy absorption devices with oscillating water column (OWC) attached to its fore and aft ends. Their results show the effectiveness of this system in reducing the hydro elastic response of VLFS. Ikoma et al. (2005) investigated the effects SCHOOL OF ENGINEERING, CUSAT

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of a submerged vertical plate and an OWC to a hydro elastic response reduction of VLFS. They found that this system is effective especially at the wave period of 14s, it is possible to reduce the hydro elastic response up to 45%. Hong and Hong (2007) proposed a method using pin connection from fore-end of VLFS to OWC breakwater. They derived analytical solutions and obtained results showed that this anti-motion device is effective in reducing the deflections, bending moments and shear force of VLFS. With the idea to reduce vibration of VLFS under action of wave, Zhao et al. (2007) analyzed theoretically a VLFS with springs attached from fore-end of VLFS to sea bed. They found the motion of VLFS is reduced by adding this kind of anti-motion device. However this idea maybe difficult in applying to real VLFS placed at deep sea condition.

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CHAPTER 3.

1. CONCLUSION The definition, applications, analysis and design of very large floating structures have been presented. It is hoped that this report will create an awareness and interest in structural and civil engineers on the subject of very large floating structures and to exploit their special characteristics in conditions that are favorable for their applications.

VLFSs can be constructed to create floating airports, bridges, breakwaters, piers and docks, storage facilities (for oil), wind or solar power plants, for military purposes, industrial space, emergency bases, entertainment facilities, recreation parks, spacevehicle launching, mobile offshore structures and even habitation (it could become reality sooner than one may expect). VLFSs may be classified under two broad categories: the pontoon-type and the semi-submersible type. The former type is a simple at box structure and features high stability, low manufacturing cost and easy maintenance and repair. The pontoon-type/mat-like VLFS is very exible compared to other kinds of offshore structures, so that the elastic deformations are more important than their rigid body motions. Thus, hydro elastic analysis takes center stage in the analysis of the mat-like VLFSs. Large differential deflection encountered in pontoon type , Very large floating structures (VLFS) may be minimized by introducing gill cells at appropriate locations.

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2.REFERENCES [1]. Minimizing differential deflection in a pontoon-type,very large floating structure via gill cells. C.M. Wanga, T.Y. Wua, Y.S. Chooa, K.K. Anga, A.C. Tohb, W.Y. Maoc, A.M. Heec. (2005) [2]. Wang CM, Watanabe E and Utsunomiya T. Very Large Floating Structures. Taylor and Francis, New York; 2008.

[2]. Overview of Megafloat: Concept, design criteria,analysis, and design , Hideyuki Suzuki Clauss, G., Lehmann, E. and Ostergaard, C. (2005).

[3]. Very Large Floating Structures: Applications, Research and Development, C.M. Wanga, Z.Y. Taya (2011) [4]. Efficient hydrodynamic analysis of very large floating structures, J.N. Newman (2005).

[5]. Very Large Floating Structures: Applications, Research and Development , C.M. Wanga, Z.Y. Taya.

[6]. Hydroelastic analysis of a very large floating plate with large deflections in stochastic seaway Xu-jun Chen, J. Juncher Jensenb, Wei-cheng Cui,Xue-feng Tang [7] . Full list of VLFS bridges http://en.structurae.de/structures/stype/index.cfm?ID=1051 [8]. www.sciencedirect.com [9]. Sato C. Results of 6 years research project of Mega-float. In: Fourth very large floating structures,(2003).

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