Introducing gill cells in pontoon type floating structures

1. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30 – 31, December 2014, Ernakulam, India
66
INTRODUCING GILL CELLS IN PONTOON-TYPE
FLOATING STRUCTURES
Pankaj Krishnan V.B1
, Meera C.M2
1
M-Tech Computer Aided Structural Engineering, Sree Narayana Gurukulam College of Engineering, Kadayiruppu,
Ernakulam, Kerala
2
Assistant professor, Sree Narayana Gurukulam College of Engineering, Kadayiruppu, Ernakulam, Kerala
ABSTRACT
Nowadays, as the demands for container shipment increases, there is a trend to build larger container ships. But
these mega-container ships can only be anchored at several ports. Future ports must be able to accommodate these mega-
container ships. The lack of fill materials for land reclamation needed in the construction of the mega ports in deep
waters and the need to preserve the coastal environment and current flow is another matter of concern. In this study,
research is conducted on a very large floating container terminal. Based on the functional and operational requirements a
preliminary sizing of the floating container terminal with dimensions 270m x 210m x 10m is proposed. The floating
container terminal is constructed by using high performance concrete. Using a finite element model, static analyses were
performed for the floating container terminal under immense live load due to the 7-tier container loading on its central
stacking yard. As a result of such a central loading, the floating container terminal undergoes a dish-like deformation. A
differential deflection occurs between the central portion and the edges and corners of the floating container terminal and
is relatively large. This causes problem for the smooth operation of the cranes. An innovative and cost effective solution
was found in the form of “gill cells” in order to reduce the deflection of the edge areas and the central portion. These gill
cells are compartments in the floating structure where the bottom surface is perforated to allow water to flow freely in
and out. At the locations of these gill cells, the buoyancy forces are eliminated. When placed appropriately, it removes
the buoyancy forces and hence creates hogging moments that reduce the central deflection. It will be shown here that the
deflection is indeed considerably minimized. The analysis is done using ANSYS Workbench 15.
Keywords: Floating Container Terminal, Gill Cells, Pontoon Type Floating Structures.
I. INTRODUCTION
Before the 20th Century, human activities such as mining, farming and energy production have been basically
confined to land, which only takes 29 per cent of the earth surface area whereas 71 per cent of the earth surface is
covered by water. In the last few decades, continuous exploitations, growing populations and developing economies have
stretched land resources to their limits. This limitation has forced people to colonize and exploit the ocean for space,
energy, water food and even to store carbon dioxide to mitigate global warming. There are many directions in modern
sea space utilization. Besides the traditional harbour engineering, offshore jack-up rigs foroil drilling and the maritime
transportation, new sea space utilization focuses on the construction of offshore artificial cities, offshore power stations,
marine parks, offshore airports, submarine tunnels and submarine warehouses and so on. People are currently
constructing or designing various artificial islands, very large floating structures and submarine engineering structures
used for offshore oil and natural gas production, working and living environment. For instance, the most famous Burj al-
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2. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
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Arab, a luxurious hotel on a man-made island in Dubai, United Arab Emirates and the Treasure island in San Francisco
Bay, USA.
In the exploitation of ocean space, as we all know, Netherlands, Japan, Norway and USA are the pioneers in the
utilization of sea space. The sea reclamation of Netherlands was once regarded as a miracle. Netherlands has an area of
41,528 square kilometres and a population of about 16 million. As it is a lowland country (approximately half of the
country’s land is below or at the sea level), the Dutchmen have been fighting against sea throughout all these years. As
early as the 13th century, Dutchmen constructed the dykes to hold back sea water. Through hundreds of years, the
Dutchmen constructed as long as 1800 kilometres dykes and at the meantime, increased their land area by six thousand
square kilometres. Now, 20 per cent of the land area of Netherlands is created from land reclamation. Japan is an island
country with a total land area of 0.38 million square kilometre. Of the land areas, 80% is mountainous and most human
activities are concentrated on the plains along the shoreline. Ocean spaceutilization has been a key priority for Japan.
Until the 20th
century, reclamation of shallow waters has been the only technology available to expand human activities
onto the sea. Kansai International Airport was a wonder built on the reclaimed land. In the late of 1950s, architects in
Japan had proposed the concept of ocean space utilization using a floating structure and in the following several decades,
more and more research on the very large floating structure technology had been performed. Following this trend, the
technology of very large floating structure has been being developed at a high speed. The construction of the Kamigoto
and shirashima oil storage base and other floating structures had shown the advanced state of the Japanese in the large
floating structures technology.
The same as Netherlands and Japan, in the last several decades, Norway and USA developed very fast in
theocean space utilization, especially on the designing and constructing floating structures for various kind of purpose.
II. VERY LARGE FLOATING STRUCTURES
A very large floating structure (VLFS) is a relatively new technology for creating land from the sea. As
population and urban development expand in the land-scare island countries (or countries with long coastlines), city
planners and engineers of these countries have resorted to land reclamation from the sea in order to reduce the pressure
on existing land shortage problem. However, there are some disadvantages in land reclamation, such as the negative
environmental effect on the country’s and neighbouring country’s coastlines and marine eco-system, soil settlement
problems and the huge economic costs in reclaiming land from deep coastal waters. Because of the requirement of land
usage and the problems in land reclamation work, engineers have proposed the construction of very large floating
structures (VLFS) for industrial space, airports, and storage facilities.
VLFS may be classified to two categories, i.e. the pontoon-type and the semi-submersible-type. The pontoon-
type of floating structure is a simple flat box structure, like a giant plate floating on the water. It features high stability,
low manufacturing cost and easy maintenance and repair. However, the pontoon-type structures can only be constructed
in calm waters associated with naturally sheltered coastal formations. In contrast, the semi-submersible-type of floating
structure is raised above the sea level using column tubes or ballast structural elements. It is suitable for open seas where
there are large waves since it is able to minimize the effects of waves while maintaining a constant buoyant force. The
super-large floating container terminal that we are concern about in this thesis is to be constructed in calm waters, and it
is a pontoon-type floating structure. A pontoon-type floating structure system consists of the following components:
• A very large pontoon-type floating structure
• Station keeping system to keep the floating structure in place
• An access bridge or a floating road to get to the floating structure from shore
• A breakwater for reducing wave forces impacting the floating structure
Fig 1: Components of a floating structure

3. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30 – 31, December 2014, Ernakulam, India
68
III. FLOATING CONTAINER TERMINALS
There is a trend to build larger container ships as the demand for container shipment increases. In the
presenttime, these mega-container ships with 10,000 to 15,000 TEUs capacity can only call at several ports in United
States and Europe.
All the ports are built on reclaimed land due to the acute shortage of firm land. The land reclamation solution is
cost effective provided that the water depth is shallow and fill materials are available at a reasonably cheap price.
However, when faced with large water depths and very costly fill materials, the land reclamation option becomes an
expensive solution. The alternative solution is to construct a floating structure to provide the artificial piece of land in
deep waters for the mega ship container terminal.
These very large floating container terminals have advantages over their traditional land reclamation
counterparts in the flowing respects:
• They are cost effective when the water depth is large and the seabed is soft.
• Environmental friendly as they do not damage the marine ecosystem, or silt-up deep harbours or disrupt the
tidal/ocean currents.
• They are easy and fast to construct (components may be made at different shipyards and then brought to the site
for assembling) and therefore sea-space can be speedily exploited.
• They can be easily removed (if the sea space is needed in future) or expanded (since they are of a modular form).
• Their positions with respect to the water surface are constant and thus facilitate ship to come alongside.
IV. STATIC ANALYSIS
Consider a super-large floating container terminal of dimensions 270x210x10m. An isometric view and a plan
view of the floating container terminal considered are shown in Fig.3.1. The proposed floating structure is to be
constructed using light weight, high strength concrete or (high performance concrete in short). Tables summarizes the
dimensions and construction material properties of the proposed floating structure, the structure’s self-weight and the
weight of quay cranes that have been adopted for the static analysis. The VLFS comprises of a large number of
watertight compartments/cells to provide the buoyancy needed. A typical cross-section of the cells is shown in Fig.2. The
FEM software ANSYS workbench is adopted for the static analysis.
Fig 2: Isometric and plan view of container terminal
TABLE I: DIMENSIONS OF FLOATING STRUCTURE
Total length 270m
Total width 210m
Total height 10m
Thickness of top and bottom slabs 0.4m
Thickness of intermediate level slab 0.2m
Thickness of vertical walls 0.3m
Height of beam stiffeners for top and
bottom slabs
1m
Width of beam stiffeners for top and
bottom slabs
0.5m

4. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30 – 31, December 2014, Ernakulam, India
69
TABLE II: MATERIAL PROPEERTIES
Density of high
performance concrete
1900 kg/m3
Modulus of high
performance concrete
22.9 GPa
Poisson’s ratio of high
performance concrete
0.2
TABLE III: DEAD LOADS
Total selfweight of container terminal 165167.6346 ton
Weight of one typical container 30 ton
Weight of one quay crane 1360 ton
Number of quay cranes 6
Fig 3: Typical cross-section of 10x10m watertight cell
Fig 4: 270 x 210 m floating structure in Ansys Workbench
Loading conditions considered are;
Load Case 1. Self-weight of the floating structure
Load Case 2.Self-weight of the floating structure plus 50kN/m2
uniformly distributed load (equivalent to about 8-tier
containers) on the entire top surface 270x210m.
Load Case 3. Self-weight of the floating structure plus 50kN/m2
uniformly distributed load on the central portion 250m×
190mwith no load on the 20mwide road around the perimeter

5. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30 – 31, December 2014, Ernakulam, India
70
Fig 5: Deformation for 1 tier loading
Fig 6: Deformation for 8 tier loading
TABLE IV: DEFLECTION OF FLOATING STRUCTURE FOR CENTRALLY LOADED STRUCTURE
Tier loading Deflection
1 2.3416
2 2.6854
3 3.0261
4 3.3604
5 3.6979
6 4.048
7 4.3827
8 4.7298
The deflections of the floating structure are computed for various tier numbers of containers in the loaded area.
In sum, the analysis shows that the deflection is significant when the floating container terminal is loaded to 3-tier or
more tier container loading. This undesirable ‘‘dishingeffect’’ must be overcome for smooth operation of the container
terminal, especially when the terminal is fully loaded.
For overcoming this problem a new technique is introduced. The solution involves perforating the bottom
surface of the perimeter compartments (or cells) to allow sea-water to freely flow in and out. By so doing, the perforated
cell region has zero buoyancy. The elimination of buoyancy force at the edges of the floating structure simulates
“hogging moments” at the edges. The hogging moments bend the edges of the structure backwards and due to the
flexural rigidity of the structure, the central portion gets lifted up. The overall effect is a flatter surface. Note that the
floating structure maintains its structural stiffness integrity since the holes or slits made are small. Apart from the gill cell
region of the floating structure, we have buoyancy force acting on the bottom surface of the structure. By adjusting the
number and location of gill cells, the floating container terminal surface can be kept as flat as possible under varying
loadings due to changing tiers of containers. At the gill cell locations, the elastic spring attached to the bottom surface of
the structure model are removed as there are no buoyancy forces.

6. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30 – 31, December 2014, Ernakulam, India
71
Fig 6: Cross-sectional portion of the floating structure with gill cells
Fig 7: Side view of floating structure
Fig 8: 1 Tier loading(with gill cells)
Fig 9: 8 Tier loading(with gill cells)

7. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30 – 31, December 2014, Ernakulam, India
72
TABLE IV: DEFLECTION OF FLOATING STRUCTURE FOR CENTRALLY LOADED STRUCTURE WITH GILL CELLS
Finite element analyses were performed on the floating structure with gill cells. Compared to the analyses
results without gill cells it can be seen that when a very large floating structure is loaded heavily in its central portion, the
gill cells provide an effective solution to mitigate the deflection.
V. CONCLUSION
From the analysis of the floating container terminal, it is found that the floating container terminal suffers from
the problem of “dishing effect”. The large differential deflection between central portion and the edge areas of the
container terminal due to the heavy container loads on the central stacking yard cause some operational problems. In
order to solve this large differential deflection problem, we introduce the innovative concept of “gill cells” and
performed some analyses of the floating container terminal with gill cells. Gill cells are compartments in the
floatingstructure where the bottom surface is perforated to allow water to flow freely in and out. The removal of
buoyancy forces at appropriate gill cell locations induces a negative hogging moment at the edges to counter the sagging
response of the floating structure under container loads. This results in minimizing the deflection, thereby flattening the
structural deflections. This point was illustrated by conducting a static analysis on a floating container terminal using the
finite element software ANSYS. The gill cells turn out to be very effective as the deflection of the floating container
terminal is considerably reduced.
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Tier loading Deflection
1 1.5071
2 1.7266
3 1.9461
4 2.1656
5 2.3851
6 2.6046
7 2.8241
8 3.0436