1. Research Proposal on Optimal Arrangement of
Floating Offshore Wind Farm
Mohammad Azim Fikri Zakaria
MEng Mechanical Engineering
with Sustainable Energy Systems
University of Southampton
United Kingdom
Email: azim fikri23@yahoo.com
This is a summary page
Abstract—This report proposes a PhD research on Floating
Offshore Wind Turbine (FOWT). It was clear from literature that
study on dynamic analysis with coupling effect is sufficient and
supported by the deployment of 7MW Trifloater near Fukushima.
Hence, it is only natural that the next study should focus on
the implementation of FOWT for large scale wind farm. The
combination effect of ambient incident wave with scattered and
radiated waves when out of phase will create a contour of destruc-
tive intersection points. Placing a floating device on this point will
result in a reduction in wave induced motion. It is suggested that
an optimal farm arrangement can encourage this phenomena
created by the FOWT hence improving the overall performance
of the system. Floating platform and mooring/anchoring system
can be made cheaper.
I. INTRODUCTION
The rapid progress of offshore wind energy is now advanc-
ing to deep water but with the challenge of costly floating
platforms. Together with mass production, it is hypothesized
that a reduction in total cost for floating wind farm can utilize
the effect of close proximity hydrodynamic interaction. This
paper does not only propose a new study on optimal FOWT
arrangement but also presents the thought process that lead to
the concept.
A. Project Aim
To study the optimal arrangement of FOWT for large scale
application in terms of both cost and yield efficiency.
B. Objectives
1. Determine the hydrodynamic interaction effect by am-
bient incident and interaction waves (radiated and scattered
waves by arrays) on close proximity devices.
2. To identify the best arrangement of FOWT that minimises
wave induced motion with considerations from the first objec-
tive.
3. To compare the performance of optimal arrangement
established in this project with fixed bottom wind farm.
II. THOUGHT PROCESS
It was found out that the study on fully coupled dynamic
analysis is abundant with almost all possible approach and
considerations. The presence of conflicting drivers within the
design of FOWT requires a new way of reducing wave motion
without increasing the total costs. Taking from the concept
of Wave Energy Device (WED) farm arrangement, the idea
is to place the devices as such to encourage the creation of
destructive intersection points by ambient incident and arrays
interaction waves. For large regular arrays, these hydrody-
namic interaction effects are reinforced as more scattered
waves from several cylinders may arrived at a given point.
It is also decided that this research project will focus on the
Trifloater design for future integration with the Fukushima
project and the slightly better overall performance of the
concept.
III. METHODOLOGY
The simulation method proposed will use a similar under-
lying process as Kagemoto (1991) but with a much larger
implementation and taking into account aerodynamic contribu-
tion on the floating structure. Several complexities are pointed
out for example the possibility of dynamic destructive contour
due to floating structure translational motion. Trade-off study
is required to determine the best course of action between
reductions in downwind or wave induced motion with respect
to FOWT arrangement. The computational power required to
simulate large arrays of FOWT pose a potential difficulty
due to the large number of unknowns involved. Gantt chart
is included to ensure knowledge development and simulation
results can be obtained in a considerable time.
IV. CONCLUSION
A study on this research area will be beneficial for the
overall progression of this technology towards large scale
deployment in the future. Overall, more self-development is
required to determine the best simulation method in terms or
algorithm and numerical simulation. With the support of senior
researchers from University of Tokyo and joint industries,
research objectives will be achievable.

2. I. INTRODUCTION
Offshore wind is one of the renewable sources that are
becoming feasible and more reliable within the coming years.
In this paper, a study on optimal wind farm arrangement for
floating offshore wind turbine (FOWT) is proposed. The effect
of incoming incident waves on floating structure will produce
scattered and radiated waves. These interaction waves travel
and when in a certain phase with ambient incident waves,
caused a contour of destructive intersection points where wave
motion is minimal. It is therefore suggested that arrangement
of FOWT for a wind farm can be made to encourage this phe-
nomenon and located at these intersection points to minimise
load acting on them [1]. As a result, platform structure and
mooring/anchoring system can be made cheaper and hence
reduces the overall costs. The thought process of this concept
is explained in greater detail in the next section along with a
brief literature review on floating wind turbine designs.
II. LITERATURE REVIEW
A. Floating Wind Turbine
Vast portion of offshore wind resource is in the deep water
region where depth is generally deeper than 30 meters. This
open up more possibilities to other regions for wind energy
generation for example Japan with estimated potential value of
141GW [2]. Without noise or visual restriction, wind turbines
can operate at optimum configuration such that a higher Tip
Speed Ratio (TSR) can be used producing higher energy yield
with less load acting on the blades [3].
The structural viability of floating offshore structures have
been demonstrated by the oil and gas industry. This however
are made possible by their strong economic potential. For
the deployment of wind turbines on floating platforms, the
system must therefore be made cheaper for it to be market
effective. This is indeed a challenge as most of the total
costs are contributed by the floating mechanism which include
platform, mooring and anchoring systems. These components
made up an important role to the overall structure by providing
the buoyancy force needed and the dynamic stability [4].
Nevertheless, unlike the oil and gas rigs, the application of
wind farm allows the benefit of mass production for floating
offshore wind turbines. This will drive down price significantly
and from Musail (2004), if the floating mechanism can be
made at 25% of the total costs, then energy cost of $0.05/kWh
is possible [5].
Generally, there are 3 established design concepts for
FOWT which are Tension Leg Platform (TLP), Spar Buoy,
and Semi-Submersible Trifloater. A further description of
these designs as well as their strengths and weaknesses are
thoroughly discussed in many literatures [4] [6] [7]. Most of
these literatures refer to the importance of coupled tower-
platform dynamic analysis with respect to aero and hydro
induced motion. However, it was found out that research
within those area are too abundant with various approach
and considerations. It would be beneficial to focus research
on new areas that may add significant contribution to the
development of this technology. Hence in the next section,
the thought process that was undertaken which lead to this
research proposal are discussed in sufficient detail.
Fig. 1. Spar buoy(left) and TLP(right) [4].
Fig. 2. Trifloater [4].
B. Thought Process
In order to narrow down research field to a single area of
interest, it would be best to focus on one design concept which
has the highest potential. Although a lot of literatures are
available on this matter, it is still hard to determine the absolute
best design due to their tendency to be site specific and lack
of real-life data on operation and maintenance. However, it is
safe to say that the Trifloater system is slightly better in terms
of dynamic response and cost compared to the rest [6] [7] [8].
Due to this and the fact that the FOWT prototype deployed
near Fukushima is a Trifloater, it is decided that this research
project should focus on this design. Afterwards, attention was
given to the case of coupled tower-platform dynamic analysis
in order to identify any possible study. The dynamic response
contributed by wave, wind and sea current on the platform are
important to effectively simulate the whole system towards
actual offshore environment. Some of these are summarised
in Table 1.

3. TABLE I
SUMMARY ON SOME OF THE STUDIES CONDUCTED ON COUPLED DYNAMIC ANALYSIS
Paper Description Remarks
[9] Ruoyu Zhang (2012) conducted a coupled dynamic analysis in the
time and frequency domain. The influence of mooring to the overall
response of the system was also taken into account.
Finite element models were established and subjected to different
combinations of turbulent wind, constant current and irregular
waves. By achieving this, the ultimate and fatigue loads can be
determined and hence the feasibility of such design can be justified.
This is an important approach as the use of time domain analysis
allowed all subjected loads to be taken into account.
Meanwhile frequency domain only considers wave load.
[10] According to Waris (2012), the dynamic response of a floating
wind turbine depends on these several factors:
1. Aerodynamic and Hydrodynamic effects.
2. Restoring and Resonance effects.
3. Mooring effects and Control system.
Ishihara and Phuc (2007) investigated the importance of aero-
dynamic and hydrodynamic damping supported with water tank
experiment. Study on resonance effects were also conducted.
The use of linear model by them for mooring system and restoring
force however may be influential for a small floating platform with
large response. Here, linear relationship may no longer applies and
non-linearity needs to be considered.
In this paper, non-linear model was applied to both catenary and
tensioned moorings as well as the restoring force. Dynamic analysis
was done with coupled tower-platform-mooring.
It was found out that both the catenary and tensioned moorings has
similar dynamic response when levelised at the same magnitude.
More importantly, the effect of heaving plates with respect to
diameter were also investigated.
This was initially thought as a potential research study based on a
paper by Simon Lefebure (2012). At this point, research on control
system for FOWT might be beneficial.
More research studies on this area are available but will
not be discussed in much detail for simplicity. For example,
Jonkman (2011) conducted a fully coupled time domain dy-
namic analysis on the 3 FOWT design concepts [13]. There
may still be possible research interests within this area that
are not identified due to the lack of expertise in this matter.
But it is arguable that current literatures have covered most
of the essentials and what is most needed now are full scale
experiment data to support the simulation results.
From here onwards, it would be best to look back into
the fundamental problems of FOWT in order to identify the
complications that need to be solved. It is proven that the
deployment of FOWT in hope for better access to wind
resource comes with greater complexities and total costs.
Overall, the conflicting design drivers for a platform can be
summarised as in Figure 2 [11].
From the diagram, it is apparent that to minimise pitch and
avoid wave periods require an increase in vessel mass. This
is the case for spar buoy design where ballast weight is used
to shift the structure natural frequency and centre of gravity
to be below centre of buoyancy [6]. This however conflicts
with cost. It is also worth mentioning another conflicting
drivers which are the mooring and anchoring system. Pitch
and heave motion can be limited reliably with more moorings
and anchors but at the expense of high cost. Conclusion that
can be drawn here is that a different approach needs to be
considered where minimisation of structure motions can be
done at a lower cost and without conflicting each other.
At this point, not many areas within the Trifloater are
left unexplored. The comprehensive study on coupled tower-
platform-mooring dynamic response towards environmental
loads were done to the point that full scale test prototype
was deployed with enough confidence. Optimal studies on
components sizing or even wind turbine specifications can be
conducted in response to real test data obtained. But, a study
on this would not add a sense of originality to the knowledge
base of this field. This should rather be left for engineering
firms to develop the framework of design and construction
according to a specific site.
As dynamic and static analysis are now reaching substantial
amount for full scale prototype to be tested, the idea of
large floating wind farm deployment is within reach. Although
optimal arrangement of fixed bottom offshore wind turbines
are well established and implemented in various sites, the case
would be different for FOWT. Due to the presence of hydro-
dynamic interaction effect on floating structures, one cannot
assume that all the devices will behave in the same dynamic
manner for the whole farm. The loading would be different
for each individuals and hence total power output cannot be
simply proportional to the number of devices [12]. As the
case for Wave Energy Device (WED), optimal arrangement
is needed to prevent a reduction in energy extraction due to
wave force shadowing or destructive interactions. However,
this conditions would actually be useful for the case of FOWT.
Kagemoto (1991) studied the effects of scattered and radi-
ated waves (interaction waves from close proximity structures)
to the device of interest [1]. The presence of hydrodynamic
interaction as a result of incoming interaction waves and
ambient incident wave when in certain phase will create a
contour of destructive and constructive wave forms. As a
result, it would be beneficial to arrange floating structures to
either be on the destructive or constructive points depending on
the purpose of the device. Constructive for the case of WED,
whereas FOWT on the destructive points. In small numbers,
this effect will not bring major impact. However for larger
arrays, these hydrodynamic interaction effect are reinforced
due to the availability of more scattered waves from many
cylinders arriving at a given point [14].

4. Fig. 3. Conflicting design drivers [11].
The principle idea for this study is not new, but simulations
were simplistic even for WED which is the source idea for
this research project. For example, the simulation studies by
Kagemoto (1991) were only done on a basic cylinder struc-
ture with 2 maximum arrays consisting of 4 cylinders each.
Furthermore, the dynamic response of a FOWT is different
as the presence of high wind tower subjects the structure to
aerodynamic load. As a result, the coupling of aero and hydro
induced motion will produce a different response of scattered
and radiated waves. Hence the research interest proposed here
is to study the optimal arrangement of a floating wind farm
with interest on minimising the wave induced motion via
hydrodynamic interactions.
Fig. 4. Basic representation of hydrodynamic interaction resulted from
ambient incident and interaction waves (scattered and radiation).
III. METHODOLOGY
It was found out that resonance effects play a significant
role in large arrays causing enhanced hydrodynamic effect on
individual structures and large free surface elevations. These
bring serious implications for large arrays deployment and
hence it is important to understand how these effects occurred
and interact with other variables [14]. The initial methodol-
ogy proposed here will use the same underlying process of
Kagemoto (1991). First step would be to perform a simplified
simulation on heaving single cylinders (not triple columns as
of a Trifloater). The purpose is to determine how scattered and
radiated waves interact with ambient incident wave and how
these in turn affect other cylinders within the farm. Changes to
the overall performance of the system can then be identified.
Although this has been done by Kagemoto (1991), it is still
a crucial step as to verify the validity of simulation method
used in this research project. New information may also arise
and further improvements can be made according to current
technology.
Secondly, pitching and surging motion would be applied to
the cylinders in conjunction to the presence of aerodynamic
response to the wind turbine (unlike WED, pitching and
surging are assumed to be small). The effect of surging is
thought to have a significant impact as the FOWT would
then have the possibility to move out of the destructive points
identified from the simulation. It is assumed for now that the
destructive contour might be dynamic i.e. changing with time.
This would prove rather challenging to determine the optimal
contour and hence wind farm arrangement with respect to time.
A trade-off is probably required and to make sure that FOWT
would not be on the constructive points. The arrangement
should also consider the effect of downwind on the wind
turbines. Optimal positioning of devices with respect to wave
motion and wind conditions are hence needed. For example,
the positioning of the FOWT to reduce wave motion via
destructive intersections might cause a reduction in downwind.
Simulation will then proceed to actual Trifloater model.
Initially, a single isolated model should be studied first to de-
termine the Trifloater interaction waves charateristics produced
by the incoming incident wave. Note that real data from the
Fukushima FOWT prototype can be used to determine the

5. Fig. 5. Research timeframe.
scattered and radiated waves. Contour results obtained from
simulation can be compared to real data observations from
Fukushima Trifloater. Once validated, proceed to implementa-
tion of simulation method to multiple Trifloater as the case for
a wind Farm (e.g. 100 devices). Large arrays computation are
possible by conventional integral equation methods. However,
high computational power is still required simply because of
the large number of unknowns involved [14].
Later on, a comparison can be made between the optimal
arrangement produced in this project with the already estab-
lished offshore fixed bottom wind farm. The relevance of this
is that the performance of the derived FOWT arrangement can
be evaluated in terms of both cost and energy yield.
Indeed the methodology proposed here is still rudimentary
and more research studies are needed in terms of simulation
method i.e. numerical model or algorithm. Although both
Kagemoto (1991) and B.F.M Child (2010) presented in their
paper thorough numerical method for wave hydrodynamic
interaction, some of them require higher level of understanding
than just the standard linear wave and hydrodynamic theory
[12]. Also, the coupling effect of aerodynamic loading on
hydrodynamic interaction must be well understood. In general,
a time plan was drafted to assist in acquiring the required skills
and ensure research objectives can be achieved.
The Gantt chart provided here is only a rough idea of how
the research project will proceed. Plenty of time is allocated
at the beginning for knowledge and skills development. More
discussion with supervisors are needed for the overall progres-
sion of the project, simulations and field work data acquisition.
It is also worth mentioning that all the ideas put on here are
not mandatory and the writer is open to any suggestions given
by potential supervisors.
IV. CONCLUSION
As a conclusion, the aim of this research project is to study
the effect of hydrodynamic interaction on Trifloater FOWT
and the optimal arrangement for the implementation of a
wind farm. It is hypothesized that the presence of destructive
points as a result from these hydrodynamic interaction can
be beneficial for reducing induced wave motions on FOWT.
Although this concept has been introduced a long time ago
for all sorts of offshore industry, application on FOWT are
fairly new. Further considerations are needed for example on
pitching and surging effect from aerodynamic loading and
sea current. A study on this research area is believed to
be beneficial for the overall progression of this technology
towards large scale deployment in the future. Agreeably,
further understanding is still required namely on the numerical
and simulation method for this research project. An initial
time plan is given and with the possible support from senior
researchers from University of Tokyo and the joint industries
on this project, self-improvements can be made in order to
achieve the research objectives.

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