floating breakwaters: sustainable solution for creating
TRANSCRIPT
Springer Nature Singapore Pte Ltd. 2021
J.N. Reddy et al. (eds.), ICSCEA 2021, Lecture Notes in Civil Engineering,
https://doi.org/...
Floating Breakwaters: Sustainable Solution for Creating
Sheltered Sea Space
C.M. Wang and H.P. Nguyen*
School of Civil Engineering, The University of Queensland, St Lucia, QLD 4072, Australia
Abstract. This paper is concerned with floating breakwaters which provide a sustainable solution
for creating a sheltered sea space from strong waves. We shall first present motivations for using
floating breakwaters, their advantages and some example applications. This is followed by recent
advances in materials, modelling, analysis and design of floating breakwaters, and recommendations
for future research and developments.
Keywords: Floating breakwater, wave attenuation performance, hydrodynamic analysis, modelling
1. Introduction
Oceans cover more than 70% of the Earth’s surface, and play a vital role in the global socio-economic development.
Oceans have contributed about US$ 3 trillion to the global GDP by 2016, created opportunities for tourism for
almost 200 countries, been responsible for nearly 10% of the global trade, and provided humans with enormous
supply of seafood, clean water and energy [1]. Ocean developments involve the construction of very large floating
structures such as floating roads and bridges, floating energy plants, floating aquaculture platforms and floating
buildings and cities. The safety of these structures under wave action is of utmost importance in order to avoid
significant economic losses, and negative environmental and social impacts. A conventional solution for protecting
marine structures and fragile shorelines is to use bottom-founded breakwaters which can be constructed by using
quarried rocks or concrete caissons resting on a foundation at the seabed. While the bottom-founded breakwater
solution is effective in blocking ocean waves, it may be not economical for water depths larger than 6 m or for soft
seabed due to the enormous foundation costs [2].
For sea sites with soft seabed and deep waters, floating breakwaters may be a more promising solution because
this type of breakwaters does not require a huge amount of filled materials and a long construction time for seabed
foundations like the traditional bottom-founded ones. When compared to bottom-founded breakwaters, floating
breakwaters also possess the following advantages:
• they allow for water circulation through the gap between the seabed and the lower hull of the breakwater,
which can result in improving water quality and minimizing impacts on marine ecosystems in the lee side
of the breakwater;
• they have a low profile relative to the water surface for all tidal periods, which minimizes their presence
on the horizon;
• they can be rearranged, removed, relocated, expanded and downsized more easily.
These advantages of floating breakwaters have stimulated extensive research and developments in this space
in recent decades, and floating breakwaters have been used for a variety of purposes including:
• protecting harbors and acting as a berth for vessels. Examples for such applications are the Holy Loch
breakwater in Scotland and the floating breakwater in Fezzano, SP, Italy (see Figs. 1 and 2).
• protecting offshore fish farms. For example, a 450 m steel floating breakwater was used to protect an
offshore fish farm that is exposed to wave heights up to 11 m in Uwajima, Japan [3].
• providing space for congested coastal cities. The 350 m long floating breakwater in Monaco has been
used for not only to attenuate ocean waves but also to provide space for car park and shopping center [4].
2 C.M. Wang and H.P. Nguyen
• forming a storm barrier for fragile coastlines. Fig. 3 presents a design concept of a floating forest which
is a mega floating breakwater and a windbreak to protect vulnerable shorelines and coastal infrastructures
from waves and winds in extreme storm events [5].
• providing tourism attractions. For example, the weather side of a V-shaped floating breakwater with high
waves is suitable for surfing, the lee side having sheltered waters can be used for swimming, and the
lower part of the breakwater can be designed as an artificial reef to attract divers [6].
• creating a calm patch of ocean space for a floating city, as shown in Fig. 4.
Fig. 1. Holy Loch breakwater in Scotland
<https://www.maritimejournal.com/news101/marine-
civils/port,-harbour-and-marine-
construction/breakwater_beats_the_weather_at_holy_lo
ch>
Fig. 2. Floating breakwater at Fezzano, SP, Italy,
courtesy of Ingemar srl
Fig. 3. Design concept of floating forest Fig. 4. Floating breakwater for protection of floating
city
<https://www.seasteading.org/>
2. Materials for Floating Breakwaters
Floating breakwaters can be made of timber, plastic, reinforced or prestressed concrete, or steel. Timber was a
preferred material due to its large availability in the 1800s when floating breakwater concepts were first proposed,
e.g. for use in the Plymouth Sound [7]. Timber floating breakwaters are subject to severe degradation due to large
mechanical loads from ocean waves and wood-degrading organisms in coastal zones [8]. Nowadays, timber is
usually used in combination with plastic and steel. A common design for such a combination comprises: (i) plastic
tubes or boxes that play a role as buoyant structures and are free from corrosion caused by sea environments and
organisms; (ii) a timber platform that is used to provide safe sidewalks; and (iii) a galvanized steel frame that is
used to provide additional strength to the breakwaters to withstand wave loads and improve abrasion resistance
[9].
3
Another common material for floating breakwater is plastic such as high-density polyethylene (HDPE) [10].
Plastics have a high resistance to biofouling and corrosion [11]. Plastic floating breakwaters are usually light, and
can be formed into various configurations. They can also be easily constructed in-land, towed to deployment sites
by small vessels, assembled and anchored to the seabed. The setbacks of plastic floating breakwaters are that they
suffer from large deformation under wave action due to their flexibility and shallow drafts because of light weights,
and also their limited effectiveness in attenuating in long waves where sufficient drafts are required for blocking
water particle motions.
Steel has also been used for floating breakwaters (e.g. see Fig. 5). Its main advantage lies in their high tensile
strength that allows floating breakwaters to be used in high energetic sea environments. For example, a steel
floating breakwater was used at an offshore site (with wave heights up to 11 m) in Uwajima, Japan [3]. Steel
breakwaters have another advantage that they can be designed and constructed by utilizing the extensive
knowledge, experience, available technologies and supply chains for offshore steel structures and ships. However,
steel structures require regular maintenance to reduce the rate of degradation caused by corrosion and fatigue
cracking [12], leading to high maintenance costs. Experience in the maritime sector shows that maintenance costs
may account for about 20%-40% of the total operating expenditure [12].
An alternative material for floating breakwaters is concrete. Floating concrete breakwaters are usually heavier
than their steel counterpart. The heavy selfweight of concrete breakwaters results in a larger draft that increases
wave attenuation performance. Concrete breakwaters also require less maintenance, and have a longer design life
than steel breakwaters because well designed concrete exhibits better corrosion resistance and higher durability.
Some estimates (see for e.g. [13]) showed that maintenance costs can be reduced by 10%, and the design life be
increased by 20 to 40 years by using concrete for marine structures. These enormous benefits have resulted in
common use of concrete for floating breakwaters in practice. Two examples of massive floating breakwaters using
concrete include the Monaco breakwater and the Kan-on breakwater (Fig. 6) [14]. A main drawback of concrete
is with its low tensile strength, which may cause considerable cracks in the structure leading to corrosion of steel
bars and the loss of buoyancy. To overcome this drawback, prestressed tendons and/or steel plates should be used
for reinforced concrete floating breakwaters (e.g. see the design of Kan-on floating breakwater in Ujina, Japan
[14]).
Fig. 5. Floating steel breakwaters in Burlington
marina, Canada
<http://www.kropfindustrial.com/marine/floating-
breakwaters>
Fig. 6. Kan-on floating prestressed concrete
breakwater in Ujina, Japan [14]
3. Design of Floating Breakwaters
In designing floating breakwaters, the following issues need to be considered [3,4]:
• Attenuation performance in long waves (e.g. relative to the breakwater width). Floating breakwaters allow
for more transmitted wave energy when compared to bottom-founded breakwaters. Their applications are
usually limited to wave periods up to 5 s (e.g. [3]). Such wave conditions are found in nearshore and in
4 C.M. Wang and H.P. Nguyen
relatively calm areas. For more exposed sites, breakwater dimensions have to be increased to achieve a
required level of wave attenuation performance. This translates to an increase in costs for materials,
mooring systems, manufacturing, and installation.
• Safety and durability of the structure. To maintain buoyancy, the structure must be designed to not only
meet the strength requirements against environmental forces and accidental ship collision, but also to
minimize corrosion and cracking. In addition, stress concentrations at connections between floating
modules, and between the modules and mooring systems may affect the cost effectiveness and safety of
the connector design.
• Robustness of mooring systems. To increase the wave attenuation performance, the structure needs to be
designed to: (i) increase the portion of incident waves being reflected; and/or (ii) increase frictional and
turbulent dissipation [3]. The former case directly leads to an increase in the horizontal forces acting on
the structure and hence on the mooring system.
Addressing these issues in a cost-effective manner requires innovations in the design of breakwater’s cross-
sectional and plan shapes, connector systems and mooring systems. An alternative solution is to integrate floating
breakwaters with other purpose marine structures in order to share costs for mooring, operation and maintenance.
Next sub-sections will briefly present advances in designs of concrete and steel breakwaters, and integrated
systems to be used for wave attenuation and other purposes. We focus on concrete and steel floating breakwaters
as they have greater potential to be used in exposed sites to support future developments for ocean space creation
and blue economy activities.
3.1. Breakwater cross-sectional shapes/configurations
A traditional cross-sectional shape of a concrete or steel floating breakwater is a box shape as shown in Fig. 7a.
Analysis results for this traditional design showed that the width-to-wavelength ratio may need to be larger than
0.35 to attenuate up to 50% of incident wave energy [2]. In order to increase the breakwater performance in wave
attenuation, a simple modification is to extend the outer side walls as shown in Fig. 7b. This modified design
allows for larger wave reflection and turbulent energy dissipation due to the extended side walls, while minimizing
the increase in the material volume [15]. Another solution is to have horizontal ‘wings’ (or bilge keels) at the
bottom of floating breakwaters as shown in Fig. 7c. These attachments help reduce the roll motion of floating
breakwaters, and the amplitude of radiated waves [16]. The wave attenuation performance may also be increased
by using two pontoons placed side by side (i.e. dual-pontoon floating breakwaters) as shown in Fig. 7d [7,17]. A
rather similar solution is to have a floating breakwater comprising two pontoons, but with a confined air chamber
(see Fig. 7e). The air pressure inside the chamber can be controlled to maximize energy dissipation [18]. Recently,
attention is given to floating breakwaters comprising porous elements (e.g. Fig. 7f). The porous elements can
provide increased turbulence, reduce breakwater motions and forces on mooring systems as compared to
impermeable elements. Example of physical models of porous floating breakwaters can be found in [19,20].
5
(a) (b) (c)
(d) (e) (f)
Fig. 7. Illustrations of cross-section designs for: (a) box-type breakwaters, (b) breakwaters with extended side
walls, (c) breakwater with horizontal ‘wings’, (d) dual-pontoon breakwaters, (e) breakwaters with confined air
chambers, (f) breakwaters with porous elements.
3.2. Breakwater plan shape
In addition to cross-sectional shapes, the plan shape of floating breakwaters is of concern in breakwater design. In
order to examine the effect of the breakwater plan shape on wave attenuation, we conducted a numerical study on
heave-only box-type floating breakwaters of the same material volume, but with different plan shapes. Fig. 8 shows
the wave transmission coefficients for different plan shapes. The wave fields show that the use of a V-shaped
floating breakwater can result in increasing the wave attenuation performance by 60%, as compared to the
conventional straight breakwater. Zhang and Magee [21] recently numerically investigated floating breakwaters
with different plan shapes for protecting a floating hydrocarbon storage facility (see Fig. 9). They found that the
plan shape of floating breakwater have considerable effects on the transmitted wave field and hydrodynamic
motions of floating storage tanks [21].
(a) Plan shape #1 (b) Plan shape #2 (c) Plan shape #3
Fig. 8. Contours of wave elevation amplitude normalized with respect to incident wave amplitude for wave
period 4 s, water depth 10 m, draft 2 m.
Fig. 9. Contours of wave elevation amplitude normalized with respect to incident wave amplitude for L-shaped
and U-shaped breakwaters protecting hydrocarbon storage facility [21].
6 C.M. Wang and H.P. Nguyen
3.3. Module connections
Another important task in designing floating breakwaters is to engineer durable and cost-effective connection
solutions for floating breakwater modules. The length of floating breakwaters may stretch over a few hundreds or
even kilometers depending on the sea space to be sheltered. Such a huge breakwater causes significant difficulties
for manufacturing and installation if it is made in one piece. To facilitate the construction process, a modularity
approach has been widely adopted in practice. For example, floating concrete breakwaters can be assembled from
modules with lengths of about 20 m [22]. The connector system for adjacent modules can be rigid or flexible.
• Flexible connections between breakwater modules can be made by using post-stressed tendons/wires,
chains or rubber-bolt systems to connect the top parts of modules. The use of chains has been adopted in
early designs of floating breakwaters as given in a design guideline [3], but it seems to be less common
nowadays possibly to avoid collision between breakwater modules from relative motions. Post-stressed
tendons/wires, or rubber-bolt systems [22–24] can be used to tighten the top parts of adjacent modules,
and better control the relative motions. In order to avoid collision, breakwater modules need to be
separated at a sufficient distance (e.g. by a rubber cylinder, and by trimming the bottom part of the
modules [22,24]).
• Rigid connections can be made by connecting both the top and bottom portions of adjacent modules;
using joining bolts as in Fig. 10, or by welding of the modules as in the case of the Mega-Float [25], or
using corner and side connectors as for the Marina Bay performance stage [26]. Floating breakwaters
with rigid connections are free from floating module collisions, and do not have spacing between modules
where waves can be transmitted. However, a long breakwater assembled from many rigidly connected
modules may experience large bending moments causing considerable elastic deformations (as seen for
very large floating structures [27]). In addition, installation of rigid connections may be difficult, e.g.
divers may be required for installation of connections as shown in Fig. 10b.
(a) (b)
Fig. 10. Rigid connection between modules of Kan-on floating breakwater [14]: (a) jointing bolts; (b) work of
divers to assemble breakwater.
3.4. Mooring systems
Common mooring systems for floating breakwaters include: piles, mooring dolphin systems and mooring lines.
• Pile and mooring dolphin systems completely restrain horizontal motions of floating breakwaters, but
they allow for free motions perpendicular to the water surface [14,28]. When compared to piles, mooring
dolphin-rubber fender systems are more robust and applicable for large floating breakwaters (e.g. see
[14]). These solutions are preferred for shallow waters, usually up to 10 m depth [2]. For deeper waters,
large wave forces and water depths may cause enormous horizontal forces and bending moments on the
piles and mooring dolphin systems, which can significantly increase the costs of the mooring design.
• Mooring lines are normally used for deeper waters [2,3]. They allow floating breakwaters to move in all
directions to a certain extent, which results in reducing wave forces acting on the structure and mooring
7
system. Mooring lines can be catenary or taut [29]. Catenary mooring lines are easier for installation, and
less affected by tidal variations [30]. However, they are longer (about 3 times of water depth [29]), and
thus may have larger impacts on underwater environments and seabed. In addition, catenary mooring
lines provide lower effectiveness in mitigating breakwater motions. Taut and catenary mooring lines are
usually made of chains, wires and synthetic ropes. Chains provide catenary effects due to their heavy
weights. They also possess good abrasion properties, and are usually preferred for seabed and fairlead
segments [29,31]. Wires and synthetic ropes can provide greater elasticity which is of importance for taut
mooring lines. Combination of chains, wires and synthetic ropes may potentially be an optimal solution
in terms of costs and safety [29]. An example of such combination can be seen in designs of floating
aquaculture platforms [32], where chains are used for seabed and fairlead segments, and synthetic ropes
are used for the middle segment that is under certain pretension.
3.5. Integration with other purpose marine structures
Innovations in ocean engineering promise to provide human with solutions to increase renewable energy
production, create more tourism attractions, and deal with the negative effect of climate change on marine
ecosystems. Integrating these sustainable climate-resilient developments with floating breakwaters has potential
to significantly reduce capital and operational costs, through cost sharing for construction, installation, mooring,
foundation, and maintenance. Such an integration approach also enables an efficient use of ocean space, as a given
area can be used for multiple purposes. An example of integrated systems is between floating breakwaters and
wave energy converters (WECs) [33]. Such an integrated system may help wave energy industry unlock its
significant potential that is estimated to be around 2GW only for the region within 50 km from the shore [34].
Another example of integrated systems is to use breakwaters for not only attenuating ocean waves, but also for
regenerating marine life or as tourism attractions (as for the proposal of the Webber reefs [6]).
4. Modelling of Floating Breakwaters
A floating breakwater comprises two main components: a floating structure and a mooring system. These
components are subject to wave loads which result from fluid motions of undisturbed waves, and the fluid-structure
interaction. Modelling the structure, mooring system, fluid motions, and fluid-structure interaction is required for
accurate estimates of wave loads acting on the structure, stresses in the structure, mooring forces, and wave
attenuation performance.
4.1. Modelling of structure
Most studies have been carried out by modelling floating breakwaters as infinitely long and rigid bodies in a two-
dimensional domain. The hydrodynamic problem of two-dimensional breakwaters can be solved analytically, and
the wave attenuation performance can be easily estimated by some approximation formulas (e.g. see [15]) which
provide great simplicity for practical engineers. However, two-dimensional modelling may lead to an
overprediction of wave attenuation performance; and hence an under-design of floating breakwaters because it
does not account for diffraction and radiation effects at the breakwater ends [35,36]. The two-dimensional
modelling approach should only be adopted for a large ratio between the breakwater length and wavelength.
Numerical examples in our previous study [35] indicate that such a ratio should exceed 10.
In order to improve the accuracy of the estimates of breakwater hydrodynamic performances, recent efforts
have been given to develop three-dimensional modelling of floating breakwaters, e.g. by Diamantoulaki et al. [28].
This modelling approach is also able to account for non-straight floating breakwaters, and three-dimensional
interaction between floating breakwaters and the protected marine structure. For example, Wang et al. [37]
examined an arch-shaped floating breakwater (see Fig. 3). Tay [38] investigated a floating breakwater with an L-
shape in plan. Zhang and Magee [17] studied the wave attenuation performance of breakwaters in consideration
of their interaction with the protected floating hydrocarbon storage tanks.
8 C.M. Wang and H.P. Nguyen
Modelling of floating breakwaters has been developed to account for the flexibility of floating breakwaters that
may result from the following sources:
• flexible connections between breakwater modules. As floating breakwaters are frequently built in form
of a modular structure with flexible connections between floating modules, their behaviors cannot be
regarded as a single rigid body, especially under oblique wave action (see Fig. 11a and Fig. 11b).
• large horizontal dimensions of floating breakwaters with shallow depths, and/or materials with low
elastic modulus such as high-density polyethylene. An example is that the Mega-float [39] with 1 km in
length, 60 m to 120 m in width and 3m depth, and comprising rigidly connected steel modules may be
considered as a flexible structure whose responses under applied loads are dominated by elastic
deformations, instead of rigid-body motions (see Fig. 11c).
The flexibility of floating breakwaters can be accounted for by using various modelling approaches available in
structural engineering, such as those based on plate or shell theories [40,41]. The shell theory is more general, and
is able to account for detailed structural components including side walls, top/bottom slabs, and stiffeners of the
floating structure (e.g. see Fig. 12). However, the complexity of the shell theory cause difficulties for ocean
engineers and researchers in the modelling process. In addition, enormous computational resources required for
the associated simulations make the shell theory to be resorted to at a detailed design stage. At a preliminary design
stage where the main aims are to estimate the wave attenuation performance and hydrodynamic properties, floating
flexible breakwaters may be modelled as equivalent plates according to the Kirchhoff or Mindlin plate theories
[40]. The equivalent plate has almost the same vibration properties (i.e. natural frequencies and mode shapes) as
the realistic flexible breakwater [42]. When compared to the Kirchhoff plate theory, the Mindlin plate theory is
able to account for the effects of transverse shear deformation and rotary inertia which are more important for
thicker plates. The Mindlin plate theory also provides more accurate stress-resultants as its formulation involves
only first derivatives of the transverse displacement and bending rotations.
(a) (b) (c)
Fig. 11. Global response of: (a) a conventional rigid breakwater, (b) a breakwater with a flexible module
connection, and (c) a flexible breakwater under a concentrated static load [18].
(a) (b)
Fig. 12. Example cross sections of: (a) steel pontoon and (b) concrete pontoon.
4.2. Modelling of mooring systems
Mooring systems affect motions of floating breakwaters, and they need to be treated in floating breakwater models.
Floating breakwaters restrained by piles or mooring dolphin systems usually have only heave motions (i.e. the
motions perpendicular to the water surface). However, some designs of connections between piles (or mooring
dolphin systems) and the breakwater platform may allow for pitch and roll motions of the breakwater (e.g. see
[39,43]). A common modelling approach for piles and mooring dolphin systems is to impose boundary conditions
on the plate model of freely oscillating floating breakwaters to reflect the constraints on the breakwater motions,
whereas the interaction of piles and mooring dolphin systems with the fluid is often neglected for simplicity.
If mooring lines are adopted for the station keeping purpose, they can be modelled as equivalent linear springs.
The spring stiffnesses for a given configuration of mooring lines can be obtained by using some formulas given in
9
[44,45]. The linear spring model is simple, but it is unable to account for dynamic behaviors of mooring lines, e.g.
time-dependent changes in mooring configurations due to the breakwater motion, and the interaction between
mooring lines and the fluid. A more advanced modelling approach was proposed by Loukogeorgaki and Angelides
[46], where a procedure was established to integrate a hydrodynamic model of freely oscillating floating
breakwaters with a mooring line model for predicting mooring line forces and drag damping. The established
procedure requires multiple iterations to accurately account for the changes in breakwater motions due to the
mooring line forces and drag damping, and vice versa. However, this modelling approach [46] does not account
for inertia of mooring lines which is important for catenary mooring lines. The inertia may be considered by using
a lumped-mass model [47] or a finite element (FE) model [29] of mooring lines.
4.3. Modelling of fluid motions
Fluid motions are commonly described by using the linear potential wave theory where the velocity potential exists
and satisfies the Laplace equation and boundary conditions on the free surface, seabed, at infinity, and the wetted
surface of the breakwater (e.g. see [35]). This theory is able to give reasonable accuracy for relatively small wave
steepness (i.e. the ratio between wave height and wavelength). A study [48] for wave steepness of about 0.04 and
for 2D box-type floating breakwaters showed a good agreement between measured and estimated transmission
coefficients. Another study [49] for a larger wave steepness of 0.07 showed that the transmission coefficients were
overestimated by up to 35% by using the linear wave theory, and the lowest accuracy of the estimated transmission
coefficients was seen when the resonance occurs leading to large motions of the breakwater. The inaccuracy of
the linear wave theory for such a case results from its limitation that it cannot account for turbulence and viscosity.
This can be overcome by using the Navier-Stokes equations (e.g. see [50]). However, the use of the Navier-Stokes
equations requires more computational resources when compared to the adoption of the linear wave equations;
and thus, the trade-off between accuracy and computational resources for different wave theories need to be
considered carefully.
4.4. Modelling of fluid-structure interaction
For floating breakwaters without porous elements, the interaction between fluid and the structure is commonly
represented by the boundary condition that on the wetted surface of the structure, and the fluid velocity is the same
as the structure velocity (e.g. see [35]). For porous floating breakwaters, a special attention needs to be given on
how to illustrate the fluid flow through the porous medium. Such an illustration is usually based on semi-empirical
formulas that show the relation between the fluid motions on the two sides of the porous medium and the medium
properties. Some modelling approaches for floating structures with porous elements can be found in [51].
5. Analysis of Floating Breakwaters
Designing floating breakwaters involves hydrodynamic, structural, and mooring analyses. The hydrodynamic
analysis focuses on the wave attenuation performance of floating breakwaters and their motions under wave action
and applied loads. Mooring analysis is carried out to examine forces and moments acting on the mooring system,
which is needed for designing a cost-effective and robust station keeping system. Structural analysis aims to study
the stresses in the floating structure including at module connections.
Traditionally, hydrodynamic analysis of floating breakwaters is performed in two-dimensional regular waves
[3]. The transmission coefficient, defined as the ratio between the transmitted wave height and the incident wave
height, is used to show the wave attenuation performance of floating breakwaters. For a two-dimensional floating
breakwater, the transmitted wave height is almost constant from a certain distance behind the breakwater. This is,
however, not the case for 3D breakwaters where transmitted wave heights within a predefined surface area in the
lee side may vary significantly (see Fig. 7). Thus, for 3D breakwaters, the transmission coefficient corresponding
to the transmitted wave height at a single surface point cannot reflect the overall wave attenuation performance.
This results in the need to redefine an index showing the overall wave attenuation performance of floating
breakwaters (which may be called a representative transmission coefficient). The representative transmission
10 C.M. Wang and H.P. Nguyen
coefficient can facilitate comparison between different designs of floating breakwaters, and hence allowing one to
achieve an optimal breakwater design for maximizing the effectiveness in breaking harsh ocean waves.
Michailides and Angelides [52], and Wang et al. [37] made initial efforts by using the mean transmission
coefficient over a predefined surface area. Recently, Nguyen et al. [35] proposed an alternative definition of the
representative transmission coefficient where within the predefined surface area, the probability for having
transmission coefficients smaller than the representative transmission coefficient is equal to a user-defined
percentage, say 90%.
Owing to recent advances in modelling of floating breakwaters, a more rigorous hydrodynamic analysis can be
performed with consideration of three-dimensional effects, realistic irregular waves, mooring line configurations
and module connection properties. The need to perform such an advanced hydrodynamic analysis has been
highlighted in the literature. For example, Nguyen et al. [35] showed that using a regular analysis may result in
significantly over-estimating the wave attenuation performance, as compared to that in realistic irregular waves.
Results from hydrodynamic analysis of cable-moored floating breakwaters [53] revealed that the number of
mooring lines and hinge connections between breakwater modules has a direct effect on the breakwater
performances.
6. Concluding Remarks
This paper presents the advantages of floating breakwaters, some practical examples, and recent advances in
materials, designs, modelling and analysis of floating breakwaters. Some key advances include:
• the use of reinforced or prestressed concrete floating breakwaters for higher structural integrity than those
made of wood and plastics, and for less maintenance costs than steel breakwaters;
• the modifications to cross-sectional and plan shapes, module connections and mooring systems of floating
breakwater for cost reductions, greater safety, and better wave attenuation (especially in long waves);
• the shift from two-dimensional modelling and analysis approaches in simplified regular waves towards
three-dimensional approaches in irregular waves with the ability to account for nonlinear fluid-structure
interaction, interaction between fluid and mooring lines, porous elements, breakwater rigidity, and
flexible module connections;
• the evaluation of wave attenuation performance by using a newly-defined index called a representative
transmission coefficient, and in consideration of the interaction with the protected marine structure.
Moving to offshore sites is identified as a trend in the on-going developments in ocean space to avoid congestion
in nearshore environments, for larger space and better water quality. Floating breakwaters have potential to be
used to protect marine structures in such sites, but their safety, durability, and cost-effectiveness are of main
concern due to the lack of experience for their deployment in high energetic environments. Future research and
developments are needed to enable commercialization of floating breakwaters in such environments. Some
recommendations for future studies include:
• conducting a feasibility study including cost estimates for construction of floating breakwaters in exposed
sites. This will allow to determine economic viability of floating breakwaters, and identify key cost
components of floating breakwaters that are needed to be reduced;
• proposing innovative breakwater designs for cost reductions when compared to existing breakwater
designs, while meeting the strength, serviceability and environmental requirements;
• developing appropriate modelling approaches for some recently proposed breakwater designs that have
potential for practical applications in exposed sites, such as three-dimensional breakwaters with porous
elements as in [19];
• conducting more physical model tests for three-dimensional floating breakwaters with different
configurations to provide reliable data to validate modelling and numerical techniques, and to evaluate
the hydrodynamic and structural performances of different breakwater designs.
11
Acknowledgements. This research was supported by the Australian Government through the Australian Research
Council’s Discovery Projects funding scheme (project DP170104546), with additional funding provided by the
Hyundai Engineering and Construction and ARC NanoCOMM Hub. The views expressed herein are those of the
authors and are not necessarily those of the Australian Government or Australian Research Council.
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