ESTUDO COMPARATIVO DE UMA PLATAFORMA FLUTUANTE...

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Universidade Federal do Rio de Janeiro ESTUDO COMPARATIVO DE UMA PLATAFORMA FLUTUANTE OFFSHORE PARA GERAÇÃO DE ENERGIA EÓLICA NAS REGIÕES NORDESTE E SUDESTE DO BRASIL Carolina Langenegger Videiro 2019

Transcript of ESTUDO COMPARATIVO DE UMA PLATAFORMA FLUTUANTE...

Universidade Federal do Rio de Janeiro

ESTUDO COMPARATIVO DE UMA PLATAFORMA FLUTUANTE

OFFSHORE PARA GERAÇÃO DE ENERGIA EÓLICA NAS REGIÕES

NORDESTE E SUDESTE DO BRASIL

Carolina Langenegger Videiro

2019

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ESTUDO COMPARATIVO DE UMA PLATAFORMA FLUTUANTE OFFSHORE

PARA GERAÇÃO DE ENERGIA EÓLICA NAS REGIÕES NORDESTE E

SUDESTE DO BRASIL

Carolina Langenegger Videiro

Rio de Janeiro

Setembro de 2019

Projeto de Graduação apresentado ao Curso de

Engenharia Naval e Oceânica da Escola Politécnica,

Universidade Federal do Rio de Janeiro, como parte dos

requisitos necessários à obtenção do título de

Engenheiro.

Orientadores:

Marcelo Igor Lourenço de Souza

Milad Shadman

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ESTUDO COMPARATIVO DE UMA PLATAFORMA FLUTUANTE OFFSHORE

PARA GERAÇÃO DE ENERGIA EÓLICA NAS REGIÕES NORDESTE E

SUDESTE DO BRASIL

Carolina Langenegger Videiro

PROJETO DE GRADUAÇÃO SUBMETIDO AO CORPO DOCENTE DO CURSO DE

ENGENHARIA NAVAL E OCEÂNICA DA ESCOLA POLITÉCNICA DA

UNIVERSIDADE FEDERAL DO RIO DE JANEIRO COMO PARTE DOS

REQUISITOS NECESSÁRIOS PARA A OBTENÇÃO DO GRAU DE ENGENHEIRO

NAVAL E OCEÂNICO.

Examinado por:

RIO DE JANEIRO, RJ – BRASIL

SETEMBRO 2019

___________________________________________

Prof. Marcelo Igor Lourenço de Souza, D.Sc.

___________________________________________

Milad Shadman, D.Sc.

___________________________________________

Prof. Segen Farid Estefen, Ph.D.

___________________________________________

Ana Paula França de Souza, D.Sc.

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Videiro, Carolina Langenegger

Estudo Comparativo de uma Plataforma Flutuante Offshore

para Geração de Energia Eólica nas Regiões Nordeste e Sudeste do

Brasil/ Carolina Langenegger Videiro – Rio de Janeiro: UFRJ/

Escola Politécnica, 2019

VII, 55 p.: il.: 29,7 cm

Orientador: Marcelo Igor Lourenço de Souza e Milad

Shadman

Projeto de Graduação – UFRJ/ Escola Politécnica/ Curso de

Engenharia Naval e Oceânica, 2019

Referências Bibliográficas: p. 51-53.

1. Energia Eólica. 2. Plataforma Flutuante. 3. Geração de

Energia. 4. Comportamento Hidrodinâmico. 5. Análise Estrutural.

I. Lourenço de Souza, Marcelo Igor. II. Shadman, Milad. III.

Universidade Federal do Rio de Janeiro, Escola Politécnica, Curso

de Engenharia Naval e Oceânica. IV. Título

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Acknowledgements First, I would like to thank Professor Marcelo Igor and D.Sc Milad Shadman for their

expert advice and encouragement throughout this work.

I would also like to thank the Professors from the Department of Naval Architecture for

all teachings during my years at UFRJ as well as the professors from Ecole Centrale

Marseille who introduced me the domain of renewable energies.

Also, I would like to thank all the people, from several laboratories, who helped me in

this project, especially in getting the input data and understanding the FAST code.

I’m also thankful to all friends that I have made during these years at university and who

have always been there for me.

Finally, thanks to my family, especially to my mother, my father and my brother. Thank

you for always supporting me and for always being an example to me.

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Abstract of Undergraduate Project presented to POLI/UFRJ as a partial fulfilment of the

requirements for the degree of Engineer.

COMPARATIVE STUDY OF AN OFFSHORE FLOATING PLATFORM FOR WIND

POWER GENERATION IN THE NORTHEAST AND SOUTHEAST REGIONS OF

BRAZIL

Carolina Langenegger Videiro

September/2019

Advisors: Marcelo Igor Lourenço de Souza and Milad Shadman

Course: Naval Architecture

The increasing demand for energy as well as the concern of global warming stimulate the

search for renewables energies. One of the most developed green energy today is wind

power.

Wind power development has started with land-based structures. However, concerns

about visual and noise impacts as well as difficulties to find appropriate spaces for it have

boosted the transition to shallow waters with bottom fixed structures. Currently, it can be

observed the transition to deep waters with floating structures. This can be explained by

the better quality of wind at a considerable distance from the shore as well as the limitation

related to water depth for the installation of bottom fixed structures.

Thus, this work aims to compare the behaviour of a wind turbine supported by a floating

platform in two different regions near the shore of Brazil to evaluate its feasibility. The

power production of the turbine, the hydrodynamic behaviour of the platform and an

extreme structural analysis in the connection between the tower and the platform are

considered in this comparison.

All simulations are done with the FAST code, in which the hydrodynamic and

aerodynamic effects on the structure are considered. The platform and the mooring system

considered are the one available at the FAST library. The wind condition as well as the

wave condition are determined according to the studied regions.

Keywords: Wind Energy, Floating Platform, Wind Power Generation, Hydrodynamic

Behaviour, Structural Analysis.

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Resumo do Projeto de Graduação apresentado à Escola Politécnica/UFRJ como parte dos

requisitos necessários para a obtenção do grau de Engenheiro Naval e Oceânico.

ESTUDO COMPARATIVO DE UMA PLATAFORMA FLUTUANTE OFFSHORE

PARA GERAÇÃO DE ENERGIA EÓLICA NAS REGIÕES NORDESTE E

SUDESTE DO BRASIL

Carolina Langenegger Videiro

Setembro/2019

Orientadores: Marcelo Igor Lourenço de Souza e Milad Shadman

Curso: Engenharia Naval e Oceânica

A crescente demanda por energia assim como a preocupação com o aquecimento global

estimulam a busca por energias renováveis. Uma das energias verdes mais desenvolvidas

atualmente é a energia eólica.

O desenvolvimento da energia eólica começou com turbinas terrestres. No entanto, as

preocupações com os impactos visuais e sonoros assim como as dificuldades em

encontrar espaços apropriados para sua instalação, impulsionaram a transição para águas

rasas com estruturas fixas no fundo do oceano. Atualmente, pode-se observar uma

transição para águas profundas com estruturas flutuantes. Isto pode ser explicado pela

melhor qualidade do vento a uma distância considerável da costa assim como pela

limitação relacionada à profundidade da água para a instalação de estruturas fixas.

Assim, este trabalho tem como objetivo comparar o comportamento de uma turbina eólica

sustentada por uma plataforma flutuante em duas regiões diferentes próximas à costa do

Brasil para avaliar sua viabilidade. A produção de energia da turbina, o comportamento

hidrodinâmico da plataforma e uma análise estrutural extrema na conexão entre a torre e

a plataforma são considerados nesta comparação.

Todas as simulações são feitas com o código FAST, no qual são considerados os efeitos

hidrodinâmicos e aerodinâmicos na estrutura. A plataforma e o sistema de ancoragem

considerados são os disponíveis na biblioteca do FAST. As informações do vento assim

como as informações das ondas são determinadas de acordo com a região estudada.

Palavras-chave: Energia Eólica, Plataforma Flutuante, Geração de Energia,

Comportamento Hidrodinâmico, Análise Estrutural.

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Sumário Acknowledgements ...................................................................................................................... v

1. Introduction ..........................................................................................................................1

1.1. Renewable Energies .....................................................................................................1

1.2. Wind Energy ................................................................................................................1

1.3. Types of Floating Platforms .........................................................................................3

1.3.1. Spar ......................................................................................................................3

1.3.2. Tension Leg Platform (TLP).................................................................................3

1.3.3. Semisubmersible...................................................................................................4

1.4. Comparisons of Floating Platforms ..............................................................................5

1.5. Floating Concepts .........................................................................................................7

1.5.1. Spar - Statoil Hywind ...........................................................................................8

1.5.2. Semisubmersible - Principle Power WindFloat ....................................................9

2. Objective ............................................................................................................................11

3. Methodology ......................................................................................................................12

4. FAST ..................................................................................................................................14

4.1. Theoretical Background .............................................................................................14

4.1.1. Aerodynamic Model ...........................................................................................14

4.1.2. Hydrodynamic Model .........................................................................................15

4.1.3. Structural Model .................................................................................................16

4.1.4. Mooring Model ...................................................................................................16

5. Input Data ...........................................................................................................................17

5.1. Floating System - OC4 DeepCwind .......................................................................17

5.2. Turbine ...................................................................................................................20

5.3. Study Regions.........................................................................................................21

5.4. Design Load Cases .................................................................................................22

5.5. Environmental Loads ..............................................................................................25

6. Results and Discussion .......................................................................................................38

6.1. Power Production .......................................................................................................38

6.2. Hydrodynamic Behavior.............................................................................................41

6.3. Structural Analysis .....................................................................................................47

7. Conclusion .........................................................................................................................50

Referências Bibliográficas .........................................................................................................51

Annexes .....................................................................................................................................54

Annex I: Summary of Simulation Cases.................................................................................54

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1. Introduction

1.1. Renewable Energies Nowadays, a quickly increasing demand for energy as a consequence of the rapid global

economic growth can be observed. Since the industrial revolution, the main sources of

energy have been the conventional fossil fuels such as coal, oil and natural gas. However,

these sources are facing depletion besides becoming a concern due to their effects on our

environment [1]. The main culprits of global warming are considered the greenhouse

gases emitted by burning these fossil fuels, especially carbon dioxide [2].

The Kyoto Protocol has established limits of greenhouse gases emissions. So, in order to

reduce these emissions as well as to search for energy security, the development of

renewable energies has been pushed. In addition to that, the aversion to the traditional

fission nuclear power and the lack of progression in the fusion nuclear power has also

contributed to that development [3]. The renewable energy is expected to play an import

role in handling the demand of the energy required along with environmental pollution

prevention [2], because it uses the own resources of a region and it has a non-significant

contribution to the emissions of carbon dioxide [3].

Among many renewable resources, the importance of wind power can be explained by

the combination of two factors: the availability of resources and the maturity of the

technology in term of cost efficiency [3]. Wind power is the only one that offers a mature

technique as well as promising commercial prospects and is already applied in large-scale

electricity generation [1]. Due to that, wind energy is the one that presents the fastest

development by all the renewable energies [3].

Other advantages of wind energy are that it is an infinitive type of energy that can be

gathered either in the mainland with land-based turbines or on the ocean with bottom-

fixed structures at shallow waters or floating structures at deep waters [2].

1.2. Wind Energy The wind energy technology has started with land-bases structures, followed by bottom-

fixed offshore structures at shallow waters and now floating types are being developed

for deep waters [4].

Although wind power plants have relatively little impact on the environment compared

to fossil fuel power plants, concerns have been raised over the noise produced by rotor

blades, visual impacts and deaths of birds and bats that fly into the rotors [2]. Because of

this, it is becoming increasingly difficult to find appropriate sites for the future growth of

onshore wind turbines that are near their limits [5]. So, space for turbines is becoming

scarce [1], leading to the investigation of offshore site locations, where most of these

issues are eliminated [6].

Offshore wind turbines have greatly reduced visual impacts, less turbulence and lower

noise constraints because they are far enough away from the shore and human life,

allowing higher rotor speeds. Besides, offshore wind power plants can produce up to 50%

more electricity than onshore wind power as the wind, normally stronger and more

uniform at sea than on land, allows the installation of larger turbines [5] [1]. Therefore,

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offshore wind power has recently been widely focused on and developed, as it is reliable,

intensive and its source is abundant and offers vast offshore areas [1]. Besides, comparing

to its onshore counterpart, the offshore wind energy exhibits advantages such as security

of supply, stability of output electricity and less environmental impacts [7].

The first advantage of offshore wind installations is the better quality of the wind resource

in the sea, where wind speed is usually bigger, even increasing with the distance to the

coast, and more uniform, leading to less turbulence effects; therefore, the fatigue is less

important and let to increase the lifetime of the offshore wind turbine generator [3].

The second advantage becomes from the bigger suitable free areas in the sea where

offshore wind farms can be installed, leading to greater installations. Their placement, far

from population areas, lets to reduce the environment regarding the noise emission. Also,

this large distance allows to reduce the visual impact from the coast [3].

It is also relevant to mention a few disadvantages of offshore installations, which with

technological development can be improved and become effective in the near future.

The first disadvantage is the cost of the permitting and engineering process, and of the

construction and operation phases due to the high costs of the sea operations. Besides,

unlike onshore wind farms, there are not usually marine electrical infrastructures that

connect the highest wind resource areas with the consumers centres, leading to the

construction of longer electrical networks [3].

A second disadvantage is the necessity of a more developed offshore wind farms

technology. This is essential for the wind turbine generators which will be subjected to

high loads and must adapt themselves to the marine environment and therefore to be

prepared for the corrosion conditions [3].

And the last disadvantage is that, due to the limited roughness of the sea surface, the

turbulences propagation are higher offshore than onshore; then wake effects provoked by

the own wind turbine generators are very important leading to a big impact over the

lifetime of the wind turbines. To reduce it, the wind turbines must be disposed obeying a

minimum separation among them [3].

It should be taken into account that, along with the development of offshore wind energy

industry, it is inevitable that offshore wind farms would move from shallow waters to

deep waters for harnessing the abundant wind resource available at a considerable

distance from the shore [7].

The cost of fixed mounted offshore wind turbines increases with water depth. Their use

is consequently not economical in some locations. In deep water areas, floating wind

turbines can be the most cost-effective and reasonable solution [5].

Several important benefits will be delivered when utilizing the floating foundation to

support the offshore wind turbine in deep waters. The first one is the less cost compared

with the fixed-bottom configuration in deeper waters (water depth range: 50-300 m). The

second one is the better flexibility with regards to construction and installation

procedures, easy removal and decommissioning. The third one is the wide varieties of

technical solutions available in terms of floating platform design. The last one is the

feasibility of a large number of countries and regions [7].

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1.3. Types of Floating Platforms Floating wind turbines are supported by floating structures. They have six degrees of

freedom, which can be excited by wave, wind and current loads. The entire system should

be moored and can be ballast, mooring or buoyancy stabilized [8].

Any kind of stabilized and moored floating body can be considered as a base structure for

a wind turbine. However, the base cases are spar, tension-leg platform (TLP) and

semisubmersible [8]. They differ in their source of stability.

1.3.1. Spar

Spar platform is a large circular cylinder made from steel or concrete with low water

plane area [6]. At the top of it, the tower and the rotor/nacelle assembly are put [8], as it

can be seen in Figure 1.

Figure 1: Spar offshore wind turbine layout [8]

This large cylinder is used to stabilize the spar structure [7]. The lower compartments are

ballasted using water, metal or concrete [8]. The upper compartments remain light. This

configuration lows the centre of mass below the centre of buoyancy of the structure [5].

In this case, the stability is obtained with the difference between the vertical distances

from the centre of buoyancy and the centre of gravity, which leads to a recovering

moment when the floating cylinder declines from the present axis [7]. Moreover, the high

metacentric height helps to increase the stability of the structure [8].

For stability requirements, the draft of the floating spar is usually larger than the hub

height. This makes a spar more likely for deeper water with a restriction of the minimum

water depth for application [4].

1.3.2. Tension Leg Platform (TLP)

A tension leg platform consists of a central column with the tower and rotor/nacelle

assembly mounted at the top of it [8] moored onto the seabed through a set of tension legs

[7], as it can be seen in Figure 2. The tendons can be attached using stiff arms or pontoons,

depending on the design. The use of pontoon will reduce the size of the central column

and consequently reduce the hydrodynamics loads.

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Figure 2: Tension Leg Platform offshore wind turbine layout [8]

Tension Leg Platforms are mooring stabilized. The stability is conditional and derived

from the restoring moment due to the tension in the tethers [4]. The tension legs

compensate the force difference between buoyancy and total weight [8]. In other words,

the TLP achieves stability through the equilibrium of tensioned taut mooring lines and

the excess buoyancy of the platform [5].

It is important to notice that the installation of this system can be a challenge. Generally,

the system is ballasted before transportation and installation and then deballasted prior to

installation of tension legs. Another important point is that the metacentric height of such

a system is likely to be negative. This means that the structure is not stable if the tendons

are removed [8].

1.3.3. Semisubmersible

Combining the two designs of the spar and the TLP, a semisubmersible foundation is

introduced to achieve static stability [7].

Usually, the semisubmersible foundation is composed of three or four slender columns

that are connected to each other through braces, as it can be seen in Figure 3. The wetting

surface area of a single column, the height of the buoyance centre and the distances

between two columns affect the forces acting on the floating foundation. These three

elements recover the original location and posture of floating foundation. Moreover, the

increase of the wetting surface provides more hydrodynamic stability and more structural

stiffness to sustain the wave load. For the connections between columns, steel braces/bars,

which increase the stiffness of the foundation, are widely used [7].

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Figure 3: Semisubmersible offshore wind turbine layout [8]

This type of platform is buoyancy or water-plane stabilized. The water-plane moment is

considerably increased by the spacing of the water-piercing columns, ensuring stability

for the platform [4]. Moreover, the columns also provide ballast and flotation stability

[6].

Semisubmersibles are suggested for supporting offshore wind turbines thanks to the wave

cancellation effect, which could improve wave-induced dynamic responses of the

offshore structure [7].

1.4. Comparisons of Floating Platforms Ref [7] presents a comparison among the three main types of floating platforms. As it can

be seen in Table 1, TLP is advantageous in the matter of natural periods, coupled motions,

moorings and maintenance. On the other hand, spar presents advantages of wave

sensitivity and anchors. Lastly, semisubmersible has advantages in what concern turbine

weights, anchors, construction and installation. Other advantages and disadvantages of

which type of platform are presented in Table 2 and Table 3, respectively.

Table 1: The comparisons of mainstream floating foundations [7]

+ Relative Advantage

TLP Spar Semi-submersible 0 Neutral

- Relative Disadvantage

Pitch Stability Mooring Ballast Buoyancy

Natural Periods + 0 -

Coupled Motion + 0 -

Wave Sensitivity 0 + -

Turbine Weight 0 - +

Moorings + - -

Anchors - + +

Construction and

Installation - - +

Maintenance + 0 -

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Table 2: Advantages of floating foundations

ADVANTAGES

SPAR TLP SEMISUBMERSIBLE

Little volume close to free

surface – small wave forces

[5]

Little volume close to free

surface – small wave forces

[5]

Better hydrodynamic

behaviors to the wind load

excitations [7]

Advantageous in natural

period, anchors, operation

and maintenance [5]

Advantageous in the natural

period, couple motions,

operation and maintenance

[5]

Relatively lower cost

associated with the

installation of mooring

system [7]

Better heave performance

than semisubmersibles due to

deep draft [9]

Lower fatigue loads in

tower and blades than

semisubmersibles and in

the tower base than spar [6]

Easily towed back to shore in

case of major repairs due to

stability and low draft [6]

Catenary mooring: low cost

and easy to install [5]

Can be fully assembled in a

dry-dock [6]

Better surge response than

TLP [7]

Inherently hight stability

structure [6]

Very good heave and

angular motions [9]

Smaller pitch motion than

spar [7]

A relatively simple structure

to manufacture, minimum

amount of welds, possibility

to use concrete [6]

Simple structure to inspect;

Few active systems and

components; Low amount

of welds that will require

inspections [6]

Good stability in operational

and transit conditions –

significantly cheaper to tow

out, install and commission

[9]

Few active systems or

complicated components [6]

Most flexible design with

regard to water depth with a

typically low draft [6]

Inexpensive platform

geometry [5]

Construction, assembly,

outfitting and commissioning

of the foundation and even

the installation of wind

turbines can be done on the

dock [7] [6]

Reduced vertical wave

exciting forces [9]

Table 3: Disadvantages of floating foundations

DISADVANTAGES

SPAR TLP SEMISUBMERSIBLE

Small water line area:

buoyancy far below the free

surface, stability relies on

buoyancy/weight distribution

[5]

Small water line area:

buoyancy far below the free

surface, stability relies on

positive mooring tension [5]

Challenging design due

to complex dynamic

responses induced by

the wind-wave

combined loads [7]

Installation need special

procedures [5]

Vertical moorings: positive

tension needed, expensive

anchors, weight-sensitive [5]

Difficult to control

vertical motions; the

heave responses is the

prime concern [7]

More pitch and roll motions

since the water plane area

contribution to stability is

reduced [9]

Complexity of platform

depends upon design [5]

Might have larger wave-

induced motions that

may impact the rotor,

tower and blades [6]

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Not suited for shallow water

[5]

Challenging in anchors and

construction [5]

More complex structure

to manufacture [6]

The hydrodynamic motion

characteristics in a stormy

seastate are superior

compared to the

semisubmersible [4]

Not suited for shallow water

[5]

Might be more subject

to corrosion and ice-

loads since much of the

structure is close to the

water surface [6]

Fatigue load in tower base

might be higher than for TLP

[6]

Less technological experience

[6]

The large draft may limit the

possibility for in-shore

assembly which would add

several offshore operations

[6]

Tendon tensioning and

transitioning from a free-

floating phase to a TLP phase

could be challenging [6]

The large draft may limit the

possibility for tow-back to

shore in case major

maintenance is required [6]

Can be challenging to

disconnect for tow-to-shore in

case of major repairs. Tendon

termination points (and

possibly active tensioning

system) needs attention [6]

Challenging in weight and

mooring [5]

Complexity and cost of the

mooring installation [9]

Change in tenden tension due

to tidal variations [9]

Structural frequency coupling

between the mast and the

mooring system [9]

1.5. Floating Concepts

A summary of various floating concepts around the world is summarized in [10] and the

list of them is presented in Table 4. Then, some of them are going to be presented in this

work.

Table 4: Global Floating Offshore Wind Foundation Development [10]

Global Floating Offshore Wind Foundation

Development

Project Name Technology

1 Principle Power WindFloat Semisubmersible

2 Mitsui SemiSub Semisubmersible

3 DeepCwind Semisubmersible

4 HiPR Wind Semisubmersible

5 Mitsubishi Semisubmersible

6 DIWET Semisub Semisubmersible

7 Gusto TriFloater Semisub Semisubmersible

8 Shimizu Semisub Semisubmersible

9 Wind Lens Floater Semisubmersible

10 Hitachi Semisub Semisubmersible

11 MODEC (Mitsui) Skwid Floater

12 Nass et Wind WinfFlo Floater

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13 Technip Vertiwind Floater

14 Pelagic Power Floater

15 Poseidon Floating Power Floater

16 IDEOL Floater (Floatgen) Floater

17 Technip INFLOW Floater Floater

18 Hexicon Floater Floater

19 WindSea Floater Floater

20 Statoil HyWind Spar

21 Toda Construction Hybrid Spar Spar

22 Japan Marine United Advanced Spar Spar

23 Nautica AFT Spar

24 Sea Twirl Spar

25 Sway Floating Tower Spar

26 GICON TLP

27 Blue H TLP TLP

28 Pelastar TLP TLP

29 Iberdrola ETORGAI FLOTTEK TLP

30 Mitsui TLP TLP

31 Ocean Breeze TLP

It can be noticed that semisubmersible is the most popular floater at the moment. This can

be explained by all the advantages presented by this floating platform combined with its

small number of disadvantages.

1.5.1. Spar - Statoil Hywind

Hywind was the world’s first full-scale floating wind turbine. It was installed in 2009 in

the North Sea, 10 km from the shore. It was a 2,3 MW spar type floating wind turbine

moored to the seabed using three sets of catenary mooring [8], as it can be seen in Figure

4.

Figure 4: Hywind Demo [11]

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After 8 years of testing of the full-scale prototype offshore in Norway, the Hywind Demo

has shown to function well in all wind and wave conditions [12].

The next step was the Hywind Scotland, the world’s first fully operational floating wind

farm. The wind farm is composed of five 6MW turbines with a total installed capacity of

30 MW located 25 kilometres offshore Scotland [12].

Figure 5: Hywind Scotland [12]

Figure 5 shows how the wind turbines will be positioned in the water offshore Scotland.

They are anchored up with three suction anchors each and linked together to send the

electricity produced onshore [12].

The floating platform is a spar type and it consists of a steel cylinder filled with ballast

water and rock or iron ore. Updates in the design have been made based on the experience

from the Hywind demo prototype [12]. The technical information about Hywind demo

and Hywind Scotland are presented in Table 5.

Table 5: Technical information from Hywind Demo and Hywind Scotland [12]

Dimension

Hywind Demo

2,3 MW

Hywind Scotland 6

MW

Mass 5300 tonnes 11200 tonnes

Draught 100 m 78 m

Hub Height 65 m 98 m

Water Depth 220 m 105 m

Substructure

Diameter 8,3 m 14,4 m

Rotor Diameter 85 m 154 m

Anchor

Drag embedded

anchor Suction anchor

Mooring Wire/Chain Chain

1.5.2. Semisubmersible - Principle Power WindFloat

WindFloat is a semisubmersible type floating offshore wind turbine. The prototype

project was located 6 km offshore Portugal, at a depth of about 42 m [8]. The full-scale

pilot was launched into the water at the end of 2011 [10].

The semisubmersible platform consists of three columns that provide buoyancy to support

the turbine. The columns are connected by braces to each other and the 2MW turbine is

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located above one of the columns, as it can be seen in Figure 6. The structure is anchored

to the seabed by four mooring lines made of conventional components. Two are attached

to the column which supports the turbine and the other two are attached to the other

column [8].

Figure 6: WindFloat [13]

Its stability is due to the use of water entrapment plates on the bottom of each column

combined with a static and dynamic ballast system [13].

This project constitutes the first offshore wind deployment worldwide which did not

require the use of heavy-lift equipment offshore. Moreover, this is the first offshore wind

turbine in open Atlantic waters and the first deployment of a semisubmersible platform

supporting a multi-megawatt wind turbine. WindFloat operated for 5 years with high

availability [13].

The next step in the development of WindFloat will be the WindFloat Atlantic. This wind

farm will consist of three turbines that will collectively deliver an installed capacity of 25

MW. It will be located 20 kilometres off the coast of Portugal, where the water depth

reaches 100 meters [13].

11

2. Objective This work aims to compare the behavior of a 5 MW wind turbine supported by a semi-

submersible floating platform in two regions near the shore of Brazil: one in the southeast

and another in the northeast.

The comparisons are made based on the power production of the turbine, the

hydrodynamic behavior of the platform and an extreme structural analysis in the

connection between the tower and the platform.

12

3. Methodology The first step was the choice of the wind turbine platform. Based on the perspective of

the evolution of wind energy, a floating system was selected. Then, according to all

advantages and disadvantages presented by the main types of floating platforms as well

as the existing floating concepts, a semisubmersible platform was considered the best

option. Afterwards, the floating system OC4 DeepCwind was selected because it was

available at FAST library.

The second step was the choice of the two study regions. A northeast region and a

southeast region were targeted, based on their great energy potential. Then, the regions

were selected according to their water depth and their distances from the shore.

The third step was the definition of which design load case would be considered in the

analysis. This was based on “DNVGL-ST-0437: Loads and site conditions for wind

turbines” [14]. Afterwards, with the design load cases defined, wind, wave and current

information were searched.

The wind information required was the mean wind speed at the hub height and the

extreme wind speed. The wave information required was the significant wave height and

the wave frequency. The current information required was the current speed. It was

possible to calculate all required design load cases due to this information.

The wind models were defined based on the wind information. Then, the wind files were

generated by FAST using TurbSim and IECWind. Afterwards, the input files for FAST

were prepared based on the required design load cases. The files from hydrodynamic and

aerodynamic models were also available at FAST library as well as the files with tower,

blades and mooring properties. Finally, all simulations were done by running FAST.

The power production and the capacity factor of the turbines were calculated. To calculate

them, the average power production was estimated by the wind distribution and the power

curve from the turbine.

The main movements of the platform (surge, pitch and heave) were considered to

compare its hydrodynamic behavior. The extreme values of the platform movements were

calculated by FAST as well as the critical design load cases. Then, a comparison of these

movements during the time series of the simulation was made.

An extreme structural analysis was made in the connection between the tower and the

floating platform. The design load case considered was defined according to the

recommendations of DNVGL rule.

Based on the power production of the turbines and the hydrodynamic behavior of the

platforms, a comparative result analysis was made. The extreme structural analysis

validated the resistance of the structure.

A flow chart illustrating this methodology can be seen in Figure 7.

13

Figure 7: Flow Chart

14

4. FAST FAST (Fatigue, Aerodynamics, Structures and Turbulence) is an open-sourced

aeroelastic computer-aided engineering (CAE) tool for horizontal axis turbines developed

by National Renewable Energy Laboratory (NREL). The coupled dynamic response of

wind turbines can be simulated in it. [15]

FAST is divided into several models to enable coupled nonlinear aero-hydro-servo-elastic

simulation in the time domain. There are aerodynamics models, hydrodynamics models,

control and electrical system (servo) dynamics models and structural (elastic) dynamics

models. [15]

FAST tool enables the analysis of different wind turbine configurations as two or three-

blade horizontal-axis rotor and upwind or downwind rotor, for instance. Besides that, the

wind turbine can be modelled on land or offshore on fixed-bottom or floating

substructures. [15] An overview of FAST for floating platforms can be seen in Figure 8.

The aerodynamic models use wind-inflow data and solve blade-element aerodynamic

loads, including dynamic stall. The hydrodynamics models simulate the regular or

irregular incident waves and currents and solve for hydrostatic, radiation, diffraction and

viscous loads on the offshore substructure. The servo dynamics models simulate control

devices as well as the generator and power-converter components of the electrical drive.

The elastic dynamics models apply the control and electrical system reactions as well as

the aerodynamic and hydrodynamic loads. They also add gravitational loads and simulate

the elasticity of the rotor, drivetrain and support structure. [15]

Figure 8: Overview of FAST for floating structures [16]

4.1. Theoretical Background

4.1.1. Aerodynamic Model

AeroDyn is the time-domain wind turbine aerodynamics module in FAST that enables

aero-elastic simulation of horizontal-axis wind turbines. [17]

The aerodynamic calculations are based on the principles of actuator lines. The three-

dimensional flow around a body is approximated by local two-dimensional flow at cross-

15

sections and the distributed pressure and shear stresses are approximated by lift and drag

forces and pitching moments grouped at a node in a 2D cross-section. [17]

The 2D forces and moment at each node along the length of each blade and tower are

computed as distributed loads per unit length. Then, the total 3D aerodynamic loads are

found by integrating the 2D distributed loads along the length. [17]

AeroDyn is divided into four submodels: rotor wake/induction, blade airfoil

aerodynamics, tower influence on the wind local to the blade nodes and tower drag. [17]

The influence of the wake is calculated via induction factors based on the quasi-steady

Blade Element Momentum (BEM) theory. [17] This is an iterative approach that based

on the conservation of momentum coupled with the lift and drag coefficients. [18]

The flow field expands radially after passing the rotor-plane. Then, it is not possible to

calculate the resulting forces and torques on a rotating wind turbine rotor directly from

the rotor and fluid velocity. Based on the Froude-Rankine theorem, the axial fluid velocity

in the rotor-plane is the average between the wind speeds far ahead and far behind it. [18]

The induction calculation as well as the resulting inflow velocities and angles are based

on local flow to each analysis node of each blade, according to the relative velocity

between the wind and the structure. 3D corrections can be optionally applied in the BEM

solution as Prandtl tip-loss, Prandtl hub-loss and Pitt and Peters skewed-wake. [17]

The blade airfoil aerodynamics can be steady or unsteady. In the first model, the supplied

static airfoil data are used directly to calculate nodal loads. The second one accounts for

flow hysteresis and can be considered as 2D dynamic corrections to the static airfoil

response as a result of time-varying inflow velocities and angles. [17]

The influence of the tower on the wind local to the blade is based on a potential-flow and

a tower shadow model. The potential-flow model uses the analytical potential-flow

solution for flow around a cylinder to model the tower dam effect on upwind rotors. The

tower shadow model considers the tower wake deficit on downwind rotors. Both tower-

influence models are quasi-steady models and are applied within the BEM iteration. [17]

The wind load on the tower is based on the tower diameter and drag coefficient as well

as the local relative velocity between wind and structure at each tower analysis node. The

tower drag load calculation is quasi-steady and independent from the tower influence on

wind models. [17]

4.1.2. Hydrodynamic Model

HydroDyn is the time-domain hydrodynamics module in FAST that enables aero-hydro-

servo-elastic simulation of offshore wind turbines. [19]

HydroDyn allows multiple approaches for calculating the hydrodynamic loads on a

structure: a potential-flow theory solution, a strip-theory solution or a hybrid combination

of these two. The potential-flow solution is applicable to substructures or members of

substructures that are largely relative to a typical wavelength while the strip-theory

simulation is used for substructures or members of substructures that are small in diameter

relative to a typical wavelength. [19]

16

The waves are analytically generated for finite depth using linear Airy theory or first plus

second-order wave theory. The wave kinematics are only computed in the domain

between the flat seabed and still water level (SWL). [19]

The hydrodynamic forces on floating structures can be divided into three independent

parts. One part is the hydrostatic displacement-dependent forces. The two other parts are

wave-related problems. One of them is the radiation and the other is the diffraction. The

radiation problem is due to structure-generated radiating waves and describes the memory

effect that those waves have on the platform. The diffraction problem assumes the waves

are diffracted when passing the structure and includes the frequency-dependent wave-

excitation vector. [18]

Commonly, the marine engineering problems are solved in the frequency domain.

However, the FAST code works in a time-domain simulation. Then, the frequency-

domain wave properties are transformed into time-domain through an Inverse Fast

Fourier Transform (IFFT). [18]

4.1.3. Structural Model

ElastoDyn is the time-domain structural-dynamics module for floating substructures. It

combines multi-body and modal dynamics.

The structural model divides the system into small separate bodies that are dynamically

linked with each other. Flexible multibody systems allow the description of bodies and

associated degrees of freedom that account for body deformation. Then, numerical

reduction methods are applied to reduce the high order of ordinary differential equations.

[18]

FAST code models the three turbine blades and the tower as flexible bodies, representing

dynamics up to the second mode in two directions as well as variable rotor-speed. [18]

For the reduced model a rigid multibody system approach is chosen. This system consists

of four rigid bodies with an associated lumped inertial mass and a mass moment of inertia.

The spring-damper element that represents the tower deformation is the only elastic part

whereas the platform, nacelle and rotor are fully indeformable. [18]

The mathematical model has been elaborated using the Newton-Euler formalism which

takes the dynamics effects of local rotations like Coriolis and gyroscopic forces into

account. [18]

4.1.4. Mooring Model

The code FAST uses the quasi-static model. This approach assumes that the motion of

the system is linear and uniform between two static positions during a given time step for

which the loads on the system are assumed constant. The dynamic effects on the mooring

system is not considered, disregarding the motion dependency of mass, damping and fluid

acceleration on the system. The mooring line shape and tension are derived from the

catenary formulations, based on the three assumptions. First, it is assumed that the line is

in static equilibrium in each time step. Then, it is also assumed that inertia effects can be

neglected and that the line profile is well described by the catenary equations. The

mooring force consists only of the position-dependent static restoring force. The

hydrodynamic and inertial forces on the line are neglected. [20]

17

5. Input Data

5.1. Floating System - OC4 DeepCwind

For this work, the semisubmersible floating system OC4 DeepCwind was chosen,

because it was already available at FAST library. As it can be seen in Figure 9, the upper

column length is 26 m, the base column length is 6 m and the tower length is 77,6 m. The

tower is 10 m above the sea water level and the platform draft is 20 m.

The total metal mass of the structure is around 4000 ton (3,8522E+6 kg) and the total

water mass is around 9500 ton (9,6208E+6 kg) [21].

Figure 9: OC4 DeepCwind [21]

5.1.1. Platform

The floating platform consists of a main column attached to the tower and three offset

columns that are connected to the main column through a series of smaller diameter

pontoons and cross members. At the base of the three offset columns is a base column

with a larger diameter. They help to suppress motion particularly in the heave direction,

but also in surge, sway, roll and pitch [21]. The platform can be seen in Figure 10 and in

Figure 11.

The total mass, including ballast, of the floating platform is around 13500 ton. This mass

was calculated such that the combined weight of the rotor-nacelle assembly, tower and

floating platform plus the weight of the mooring system in water balances with the

buoyancy of the platform. The centre of mass of the floating platform is located at 13,46

m along the platform centreline below the still water level (SWL) [21].

18

The information of the floating platform geometry can be seen in Table 6 and the floating

platform structural properties can be seen in Table 7.

Figure 10: OC4 DeepCwind platform for 1/50th scale tests [21]

Figure 11: Plan and side view of the DeepCwind semisubmersible platform, respectively [21]

Table 6: Floating platform geometry [21]

Floating Platform Geometry

Depth of platform base below SWL (total draft) 20 m

Elevation of main column (tower base) above SWL 10 m

Elevation of offset columns above SWL 12 m

Spacing between offset columns 50 m

Length of upper columns 26 m

Length of base columns 6 m

Depth to top of base columns below SWL 14 m

Diameter of main column 6,5 m

Diameter of offset (upper) columns 12 m

Diameter of base columns 24 m

Diameter of pontoons and cross braces 1,6 m

19

Table 7: Floating platform structural properties [21]

Floating Platform Structural Properties

Platform mass, including ballast 1,3473E+7 kg

CM location below SWL 13,46 m

Platform roll inertia about CM 6,827E+9 kg-m²

Platform pitch inertia about CM 6,827E+9 kg-m²

Platform yaw inertia about CM 1,226E+10 kg-m²

5.1.2. Tower

The base of the tower is coincident with the top of the main column of the

semisubmersible platform and the top of it is coincident with the yaw bearing. The base

is located at an elevation of 10 m above the still water level (SWL) and the top is located

at an elevation of 87,6 m above the SWL, as it can be seen in Table 8Erro! Fonte de

referência não encontrada. [21].

The base of the tower has a diameter of 6,5 m and a thickness of 0,027 m. This diameter

is the same as the diameter of the main column of the semisubmersible platform. The top

of the tower has a diameter of 3,87 m and a thickness of 0,019 m. The diameter and

thickness of the tower are assumed to decrease linearly from the base to the top [21].

The total tower mass is around 250 ton and the centre of mass of the tower is located at

43,4 m along the tower centreline above the SWL, as it can be seen in Table 8 [21].

Table 8: Tower properties [21]

Undistributed Tower Properties

Elevation to tower base (platform top) above SWL 10 m

Elevation to tower top (yaw bearing) above SWL 87,6 m

Overall (integrated) tower mass

249718

kg

CM location of tower above SWL along tower centerline 43,4 m

Tower structural-damping ratio (all modes) 1%

5.1.3. Mooring

The OC4 DeepCwind platform is moored with three catenary lines spread symmetrically

about the platform Z-axis. The mooring lines are attached to the platform at the top of the

base columns at a depth of 14 m below the still water level (SWL).

The mooring system properties can be seen in Table 9.

Table 9: Mooring system properties [21]

Mooring System Properties

Number of mooring lines 3

Angle between adjacent lines 120 °

Depth to anchors below SWL 200 m

Depth to fairleads below SWL 14 m

Radius to anchors from platform centerline 837,6 m

20

Radius to fairleads from platform centerline 40,868 m

Unstretched mooring line length 835,5 m

Mooring line diameter 0,0766 m

Equivalent mooring line mass density

113,35

kg/m

Equivalent mooring line mass in water

108,63

kg/m

Equivalent mooring line extensional stiffness 753,6 MN

Hydrodynamic drag coefficient for mooring lines 1,1

Hydrodynamic added-mass coefficient for mooring lines 1,0

Seabed drag coefficient for mooring lines 1,0

Structural damping of mooring lines 2%

5.2. Turbine

The NREL offshore 5-MW wind turbine has three blades and presents an upwind

orientation. The rotor diameter is 126 m and the hub height is located 90 m above the

mean sea level (MSL). The rotor speed varies from 6,9 rpm to 12,1 rpm. The cut-in wind

speed is 3 m/s, the cut-out wind speed is 25 m/s and the rated wind speed is 12 m/s. The

rotor-nacelle assembly (RNA) weights 350 ton [22].

The properties of the NREL 5-MW wind turbine can be seen in Table 10. The structural

properties of the blades can be seen in Table 11. The properties from the nacelle and hub

can be seen in Table 12.

Table 10: NREL 5-MW Wind Turbine Properties [22]

Gross Properties Chosen for the NREL 5-MW Baseline Wind Turbine

Rating 5 MW

Rotor orientation, configuration Upwind, 3 blades

Control Variable speed, collective pitch

Drivetrain High speed, multiple-stage gearbox

Rotor, hub diameter 126 m, 3 m

Hub height 90 m

Cut-in, rated, cut-out wind speed 3 m/s, 11,4 m/s, 25 m/s

Cut-in, rated rotor speed 6,9 rpm, 12,1 rpm

Rated tip speed 80 m/s

Overhang, shaft tilt, precone 5 m, 5°, 2,5°

Rotor mass 110000 kg

Nacelle mass 240000 kg

Tower mass 347460 kg

Coordinate location of overall CM (-0,2 m, 0,0 m, 64 m)

Table 11: Blade structural properties [22]

Undistributed Blade Structural Properties

Length (w.r.t. root along preconed axis) 61,5 m

21

Mass scaling factor 4,536%

Overall (integrated) mass 17740 kg

Second mass moment of inertia (w.r.t. root)

11.776.047 kg-

First mass moment of inertia (w.r.t. root) 363.231 kg-m

CM location (w.r.t. root along preconed axis) 20,475 m

Structural-damping ration (all modes) 0,48%

Table 12: Nacelle and hub properties [22]

Nacelle and Hub Properties

Elevation of yaw bearing above ground 87,6 m

Vertical distance along yaw axis from yaw bearing to shaft 1,96256 m

Distance along shaft from hub center to yaw axis 5,01910 m

Distance along shaft from hub center to main bearing 1,912 m

Hub mass 56.780 kg

Hub inertia about low-speed shaft 115.926 kg-m²

Nacelle mass 240.000 kg

Nacelle inertia about yaw axis 2.607.890 kg-m²

Nacelle CM location downwind of yaw axis 1,9 m

Nacelle CM location above yaw bearing 1,75 m

Equivalent nacelle-yaw-actuator linear-spring constant 9.028.320.000 N-m/rad

Equivalent nacelle-yaw-actuator linear-damping constant 19.160.000 N-m/(rad/s)

Nominal nacelle-yaw rate 0,3 °/s

5.3. Study Regions

This work aims to compare the power production and the hydrodynamic behavior of a

semisubmersible platform in two different locations near the shore of Brazil. Thus, one

region in the northeast and another in the southeast were selected.

These regions were selected based on their water depth and their distance from the shore.

As the total cost varies with the distance from the shore and the water depth [7], these

regions should not be too far from the coast. Besides that, they should have a water depth

above 50 m to justify the use of a floating platform. The two regions were selected based

only on their water depth and their distance to the shore. A comprehensive study to select

the regions with the biggest power capacity was not made.

The southeast region selected is near Cabo Frio, Rio de Janeiro. The region is 100 km by

100 km, located around 100 km from the coast. The water depth in this region varies from

50 to 300 m. This region can be seen in Figure 12, highlighted by a black square.

22

Figure 12: Southeast region selected

The northeast region selected is near São Luís, Maranhão. The region is also 100 km by

100 km, located around 100 km from the coast. The water depth in this region varies from

40 to 300 m. This region can be seen in Figure 13, highlighted by a black square.

Figure 13: Northeast region selected

5.4. Design Load Cases

Based on DNVGL-ST-0437 [14], there are eight design situations that should be

considered to verify the structural integrity of wind turbines components. These situations

are divided into:

1) Power production: that is when the wind turbine is in operation and connected to

the electrical grid. It is assumed that no-fault situation occurs and that the control

system is active;

2) Power production with the occurrence of fault: that is when a significant fault for

wind turbine loading is assumed to occur during power production. It can be any

fault in the control or safety systems or any internal fault in the electrical system;

23

3) Start-up: that is when the loads during the transitions from any standstill or idling

situation to power production on the wind turbine are considered;

4) Normal shutdown: that is when the loads during the normal transitions from power

production to stand-by condition on the wind turbine are considered;

5) Emergency stop: that is when a manual actuation of the emergency stop

pushbutton is considered;

6) Parked: that is when the rotor of the wind turbine in stand-by mode is at standstill

or idling;

7) Parked and fault conditions: that is when the non-stand-by state (standstill or

idling) resulting from the occurrence of a fault is considered;

8) Transport, installation, maintenance and repair: that is when situations and loads

due to transport, installation, maintenance and repair from the wind turbine are

considered.

The purpose of this work is to compare the power production as well as the hydrodynamic

behavior of a semisubmersible in two different regions. Thus, it was considered that only

some design situations were necessary for this first study of comparison.

The comparison of the hydrodynamic behaviour of the platform will initially be made

only on power production and parked situations. Thus, transition situations as start-up

and shutdown were not analysed. Similarly, transport, installation, maintenance and

repair situations were also disregarded.

5.4.1. Power production

Design situation “Power Production” has seven design load cases (DLC) associated

with it, as it can be seen in Figure 14.

DLC 1.1 is required only for calculation of the ultimate loads acting on the RNA and on

tower and foundation of onshore wind turbines [14]. As this load case is not related to an

offshore wind turbine, it will be disregarded.

DLC 1.2 is used for fatigue analysis. The discretization of the wind speed intervals within

the wind speed ranges to be investigated shall not be chosen to be larger than 2 m/s [14].

At the beginning, a fatigue analysis would be included in this work. However, an extreme

structural analysis was considered more appropriate as this work presents a first

evaluation of the behavior of an offshore wind turbine. Nevertheless, it is known that a

fatigue analysis will be considered in further work of structural analysis.

DLC 1.3 embodies the requirements for the ultimate loading resulting from extreme

turbulence conditions. These conditions include both environmental turbulence as well as

turbine wake extreme turbulence [14]. Only environmental turbulence is being considered

in this work, so this load case was also disregarded.

DLC 1.4 and DLC 1.5 consider different rotor azimuth positions [14].

DLC 1.6 considers an offshore wind turbine in the event of a combination with a severe

sea state [14].

DLC 1.7 considers humid weather conditions with ice formation on the rotor blades [14].

As both study regions are located near Brazil’s shore, it wasn’t considered the formation

of ice on the rotor blades. So, this load case was also disregarded.

24

Figure 14: Design load cases for power production situation [14]

5.4.2. Parked

Design situation “Parked” has five design load cases (DLC) associated with it, as it can

be seen in Figure 15.

DLC 6.1 considers an average oblique inflow of ± 8° and also that the average yaw

misalignment does not lead to larger values and, consequently, that slippage of the yaw

system can be excluded [14].

DLC 6.2 considers a grid failure in an early stage of the storm with the extreme wind

situation [14]. As the connection of the platform in a grid wasn’t part of this work, this

load case was disregarded.

DLC 6.3 considers the extreme wind speed with a recurrence period of one year together

with an extreme oblique inflow or average extreme oblique inflow. An average oblique

inflow of up to ± 20° shall be assumed [14], then an average oblique inflow of 20° was

considered.

DLC 6.4 considers significant fatigue damage at any component due to a fluctuating load

appropriate for each wind speed in a non-power production time [14].

DLC 6.5 includes weather conditions with ice formation on the offshore wind turbine

structure. For the same reason as DLC 1.7, this load case was disregarded.

In the design load cases DLC 6.1, 6.2, 6.3 and 6.5, the extreme wind speed model (EWM)

shall be applied and in DLC 6.4, the normal turbulence model (NTM) shall be used [14],

as it can be seen in Figure 15.

Figure 15: Design load cases for parked situation [14]

25

5.4.3. Parked and fault conditions

Design situation “Parked and fault conditions” has two design load cases (DLC)

associated with it, as it can be seen in Figure 16.

DLC 7.1 considers a fault condition combined with the extreme wind speed model and a

recurrence period of one year [14]. A fault on blade number 1 pitch-drive was considered

[23].

DLC 7.2 considers significant fatigue damage in any component due to faults of the

electrical network [14]. For the same reason presented for DLC 6.2, this load case was

disregarded.

In design load case DLC 7.1, the extreme wind speed model (EWM) shall be applied and

in DLC 7.2, the normal turbulence model (NTM) shall be used [14], as it can be seen in

Figure 16.

Figure 16: Design load cases for parked and fault conditions situation [14]

All design load cases simulated with their information can be seen in Annex I: Summary

of Simulation Cases at the end of this work.

5.5. Environmental Loads

5.5.1. Wind Condition

Based on the design load cases explained above, all wind conditions will be presented in

this section. Besides, the classification of a wind turbine in the two study regions will be

made.

5.5.1.1. Classification of Wind Turbines

According to IEC Classification of Wind Turbines [24], a wind turbine can be classified

as class I, II or III and the turbulence intensity can be classified as A, B or C (higher,

medium or lower, respectively).

The wind turbine class is defined according to the annual mean wind speed at hub height

(Vave), the 50-year extreme wind speed over 10 minutes (Vref) or the 50-year extreme gust

over 3 seconds (V50,gust). Besides that, the category of turbulence intensity is defined

according to the mean turbulence intensity at 15 m/s (Iref), as it can be seen in Table 13.

26

Table 13: IEC Classification of Wind Turbines [24]

Wind Turbine Class I II III

Vave (m/s) 10 8,5 7,5

Vref (m/s) 50 42,5 37,5

V50,gust (m/s) 70 59,5 52,5

Iref

A 0,16

B 0,14

C 0,12

5.5.1.1.1. Mean wind speed

The mean wind speed was obtained through the website “Research Data Archive” [25].

The database “NCEP Climate Forecast System Version 2 (CFSv2) Selected Hourly Time-

Series Products” was used. It is possible to select the variables wanted with a temporal

selection and a spatial selection.

Here, the variables of interest are u- and v-components of the wind. The temporal

selection was chosen to be the past five years, from December 2013 to December 2018,

and the spatial selection was made based on the study regions.

The parameters of the wind are measured at 10 m above the water level. The wind speed

at the hub height will be estimated based on the measure mentioned by the following

formula:

𝑣2

𝑣1= (

ℎ2

ℎ1)

𝛼

Here, v1 is the wind speed at height h1 and v2 is the wind speed at height h2. α is the

shear exponent, according to Table 14 [23]. For tropical offshore regions, the wind shear

exponent range is 0,07 to 0,1. The value of 0,1 was chosen.

Table 14: Typical shear exponents for different site conditions [23]

Typical Shear Exponents for Different Site Conditions

Terrain Type

Land

Cover

Approximate Range of Annual

Mean Wind Shear Exponent α

Offshore, temperate Water 0,10 - 0,15

Offshore, tropical Water 0,07 - 0,10

There are many options to the grid size of the spatial selection. The smaller option was

selected in order to have a bigger number of points in the study region. Thus, the grid

0,205° x 0,204°, which is approximately 25 km x 25 km, was selected. As the study

regions are 100 km x 100 km, this selection guarantees 16 elements inside the study

region.

The spatial selection was made using a bounding box. It is important to notice that

latitudes and longitudes must be specified in whole degrees. Therefore, the southeast

bounding box is a little bit bigger than the study region selected. The spatial selection as

well as the bounding box used for them can be seen in the figure below.

27

Figure 17: Southeast region

Figure 18: Bounding box for southeast region

Figure 19: Northeast region

Figure 20: Bounding box for northeast region

The output file is converted to netCDF and they are grouped by month. In order to

calculate the monthly mean wind speed, a code using “Jupyter” was created as it can be

seen in Figure 21. The monthly wind speeds for the southeast region are shown in Table

15 and for the northeast region in Table 16.

28

Table 15: Southeast region - wind speeds 10 m above the ground

Table 16: Northeast region - wind speeds 10 m above the ground

Then, the total mean wind speed and the standard deviation were calculated, as it can be

seen in Table 17.

Table 17: Total mean wind speed and total standard deviation for both regions

Southeast Northeast

Mean (10 m) 3,86 Mean (10 m) 6,67

StdDeviation 1,77 StdDeviation 0,58

Lastly, the wind speed at the hub height was estimated based on the wind measured 10 m

above the water level by the formula:

𝑣2

𝑣1= (

ℎ2

ℎ1)

𝛼

For the southeast region:

𝑣2

3,86= (

90

10)

0,1

𝑣2 (90 𝑚) = 4,8 𝑚/𝑠

For the northeast region:

𝑣2

6,67= (

90

10)

0,1

𝑣2 (90 𝑚) = 8,3 𝑚/𝑠

Then, according to Table 13, the wind turbine class from the southeast region is class III

and the one from the northeast region is class II.

29

Figure 21: Jupyer code

30

5.5.1.1.2. Turbulence intensity

Based on [23], the turbulence intensity (Iref) can be calculated by the following formula:

𝜎1 = 𝐼𝑟𝑒𝑓(0,75𝑉ℎ𝑢𝑏 + 5,6)

𝐼𝑟𝑒𝑓 =𝜎1

0,75𝑉ℎ𝑢𝑏 + 5,6

Here, σ1 is the standard deviation and Vhub is the mean wind speed at the hub height.

Based on the values presented in Mean wind speed, the turbulence intensity for both study

regions was calculated. Then, based on Table 13, they were classified.

The southeast region presents Iref = 0,19 and its turbulence intensity is classified as A.

The northeast region presents Iref = 0,05 and its turbulence intensity is classified as C.

5.5.1.2. Extreme wind speed model (EWM)

The extreme wind speed model (EWM) is characterized by the 50-year extreme wind

speed over 10 minutes (Vref) and the 1-year extreme wind speed(V1-yr) [23].

The 50-year extreme wind speed over 10 minutes (Vref) was estimated with NBR 6123

[26]. Based on Figure 22, the extreme wind speed in the southeast region is 37 m/s and in

the northeast region is 39 m/s.

Based on Table 13, the wind turbine class in the southeast region is class III and the one

in the northeast region is class II. This classification is in accordance with the one made

based on the mean wind speed.

Figure 22: Extreme wind speed in Brazil

31

5.5.1.3. Normal turbulence model (NTM)

In the normal turbulence model (NTM), the wind speed at the hub height will vary from

2 m/s to 70% of the Vref [23]. The discretization of the wind speed intervals within the

wind speed ranges to be investigated shall not be chosen to be larger than 2 m/s [14].

Then, in the southeast region the range of wind speed in the NTM will vary from 2 m/s

to 26 m/s and in the northeast region the range will vary from 2 m/s to 27 m/s. In both

cases, the increment is of 2 m/s.

5.5.1.4. Extreme Coherent Gust with Direction Change (ECD)

In the extreme coherent gust with direction change (ECD) the wind speed at the hub

height will be considered as the rated speed from the wind turbine as well as this speed ±

2 m/s, denoted (Vr, Vr+2 and Vr-2) [23].

As the rated speed is 12 m/s, Vr+2 is 14 m/s and Vr-2 is 10 m/s.

5.5.1.5. Extreme Wind Shear Model (EWS)

The extreme wind shear model (EWS) only considers that the wind speed at the hub

height will be equal to the rated speed from the wind turbine [23].

5.5.2. Wave Condition

Based on the design load cases explained in Design Load Cases, all wave conditions will

be presented in this section.

5.5.2.1. Extreme Sea State (ESS)

The extreme sea state (ESS) is characterized by a significant wave height and a wave

period. The ESS is used for return periods of 50-years and 1-year. The range of wave

period appropriate to each of these significant wave heights shall be considered [14].

5.5.2.1.1. Significant Wave Height

Based on DNVGL-RP-C205 [27], a 2-parameter Weibull distribution can be assumed for

the marginal distribution of significant wave height (Hs) by the formula:

𝐹(𝐻𝑠) = 1 − 𝑒𝑥𝑝 {−𝐻𝑠

𝛼}

𝛽

Here, α is the scale parameter and β is the shape parameter. These distribution parameters

are determined in appendix C according to the nautic zones presented in appendix B from

the recommended practice [27].

By definition, the cumulative distribution function of a real-valued random variable X

evaluated at x is the probability that X will take a value less than or equal to x.

𝐹𝑋(𝑥) = 𝑃(𝑋 ≤ 𝑥)

If N sea states are considered during a period of time, the probability of occurrence of

each of these sea states will be 1/N. Then, the probability to have a significant wave height

bigger than these sea states will be 1 – 1/N.

𝐹(𝐻𝑠) = 1 −1

𝑁

An expression to determine the significant wave height can be calculated equating the

two expressions from F(Hs):

32

𝐹(𝐻𝑠) = 1 − 𝑒𝑥𝑝 {−𝐻𝑠

𝛼}

𝛽

= 1 −1

𝑁

𝑒𝑥𝑝 {−𝐻𝑠

𝛼}

𝛽

=1

𝑁

{𝐻𝑠

𝛼}

𝛽

= − ln1

𝑁

𝐻𝑠 = 𝛼 [− 𝑙𝑛 (1

𝑁)]

1𝛽⁄

Based on DNVGL-RP-C205 - Appendix B [27], the nautic zones were defined to the two

study regions. The southeast region is zone 74 and the northeast region is zone 66, as it

can be seen in Figure 23.

Figure 23: Appendix B Nautic zones for estimation of long-term wave distribution parameters [27]

Based on DNVGL-RP-C205 - Appendix C [27], the distribution parameters were defined

to the two study regions, as it can be seen in Table 18.

Table 18: Distribution parameters for Hs

Southeast Northeast

Area 74 Area 66

α 2,23 α 2,33

β 1,69 β 2,15

33

The number of sea states in one year is calculated by the formula:

𝑁1 = 365 𝑑𝑎𝑦𝑠 × 24 ℎ𝑜𝑢𝑟𝑠

3 ℎ𝑜𝑢𝑟𝑠= 2920 𝑠𝑒𝑎 𝑠𝑡𝑎𝑡𝑒𝑠

Here, it is assumed that a sea state takes 3 hours.

The number of sea states in 50 years is calculated multiplying the number of sea states in

one year by 50:

𝑁50 = 50 × 𝑁1 = 146000 𝑠𝑒𝑎 𝑠𝑡𝑎𝑡𝑒𝑠

Then, the significant wave height for return periods of 1-year and 50-years can be

calculated by the formulas below. The results are presented in Table 19.

𝐻𝑠 1 − 𝑦𝑒𝑎𝑟 = 𝛼 [− 𝑙𝑛 (1

𝑁1)]

1𝛽⁄

𝐻𝑠 50 − 𝑦𝑒𝑎𝑟𝑠 = 𝛼 [− 𝑙𝑛 (1

𝑁50)]

1𝛽⁄

Table 19: Significant wave heights

Southeast Northeast

Hs 1-year 7,62 m Hs 1-year 6,12 m

Hs 50-years 9,65 m Hs 50-years 7,37 m

5.5.2.1.2. Wave Period

Based on DNVGL-RP-C205 [27], joint environmental models are required for a

consistent treatment of the loading in reliability analysis and for assessment of the relative

importance of the various environmental variables during extreme load/response

conditions and at failure.

Different approaches to establishing a joint environmental model exist. The maximum

likelihood model (MLM) and the conditional modelling approach (CMA) [27].

The following CMA joint model is recommended when the significant wave height is

modelled by a 3-parameter Weibull probability density function [27]:

𝑓𝐻𝑠(ℎ) =𝛽𝐻𝑠

𝛼𝐻𝑠(

ℎ − 𝛾𝐻𝑠

𝛼𝐻𝑠)

𝛽𝐻𝑠−1

𝑒𝑥𝑝 {− (ℎ − 𝛾𝐻𝑠

𝛼𝐻𝑠)

𝛽𝐻𝑠

}

And the zero-crossing wave period conditional on Hs is modelled by a lognormal

distribution [27]:

𝑓𝑇𝑧|𝐻𝑠(𝑡|ℎ) =1

𝜎𝑡√2𝜋𝑒𝑥𝑝 {−

(𝑙𝑛𝑡 − 𝜇)2

2𝜎2}

Here, the distribution parameters μ and σ are functions of the significant wave height.

34

For the nautic zones defined in Appendix B [27], the distribution parameters are given in

Appendix C [27], where:

𝛾𝐻𝑠 = 0

𝜇 = 0,70 + 𝑎1𝐻𝑠𝑎2

𝜎 = 0,07 + 𝑏1𝑒𝑏2𝐻𝑠

The coefficients a1, a2, b1 and b2 for the two study regions can be seen in Table 20.

Table 20: Coefficients for Tz

Southeast Northeast

Area 74 Area 66

a1 1,143 a1 1,115

a2 0,148 a2 0,183

b1 0,1148 b1 0,1192

b2 -0,0087 b2 -0,0203

Then, to calculate the wave period the program CONTHT, developed by Paulo Mauricio

Videiro at LACEO Coppe/UFRJ, was used. This program calculates the extreme contour

of an ocean area based on the Winterstein procedure.

The inputs needed by the program are the Weibull distribution and the lognormal

distribution as presented above.

𝑊𝐸𝐼𝐵(𝐻𝑠; 𝛾𝑠; 𝛼𝑠; 𝛽𝑠)

𝐿𝑂𝐺𝑁𝑂𝑅𝑀𝐴𝐿(𝑇𝑧; 𝜇; 𝜎)

For the southeast region:

𝑊𝐸𝐼𝐵(𝐻𝑠; 0; 2,23; 1,69)

𝐿𝑂𝐺𝑁𝑂𝑅𝑀𝐴𝐿(𝑇𝑧; 0,70 + 1,143 ∗ 𝐻𝑠 ∗∗ 0,148; 0,07 + 0,1148 ∗ 𝑒𝑥𝑝(−0,0087 ∗ 𝐻𝑠)

For the northeast region:

𝑊𝐸𝐼𝐵(𝐻𝑠; 0; 2,33; 2,15)

𝐿𝑂𝐺𝑁𝑂𝑅𝑀𝐴𝐿(𝑇𝑧; 0,70 + 1,115 ∗ 𝐻𝑠 ∗∗ 0,183; 0,07 + 0,1192 ∗ 𝑒𝑥𝑝(−0,0203 ∗ 𝐻𝑠)

Then, for the two return periods chosen (1-year and 50-years), the output of the program

is the extreme contour. The contour related to the southeast region can be seen in Figure

24 and the one related to the northeast region can be seen in Figure 25. As the significant

wave height was already calculated in Significant Wave Height, it is possible to find the

correspondent wave period in the result graphs. The results are presented in Table 21.

35

Table 21: Wave periods

Southeast Northeast

Tz, 1-year 9,8 s Tz, 1-year 9,65 s

Tz, 50-years 10,5 s Tz, 50-years 10,2 s

Figure 24: Contour from the southeast region

Figure 25: Contour from the northeast region

5.5.2.2. Severe Sea State (SSS)

The severe sea state (SSS) is also characterized by a significant wave height and a wave

period [14].

The same values calculated for the extreme sea state (ESS) were considered as

conservative values [23].

36

5.5.2.3. Normal Sea State (NSS)

The normal sea state (NSS) is also characterised by a significant wave height and a wave

period. It is associated with a concurrent mean wind speed. The significant wave height

of the NSS is defined as the expected value of the significant wave height conditioned on

the concurrent 10-minute mean wind speed [14]. The significant wave height and the

wave period will vary with the wind speed.

The significant wave height and the wave period can be calculated by the Sverdruv Munk

Bretschneider (SMB) method [23], by the following formulas:

𝐻𝑆 = 0,3𝑈2

𝑔{1 − [1 + 0,004(𝑔𝐹/𝑈2)1/2]

−2}

𝑇1/3 = 1,372𝜋𝑈

𝑔{1 − [1 + 0,008(𝑔𝐹/𝑈2)1/3]

−5}

𝑇𝑝 = 1,05𝑇1/3

Here, U is the mean wind speed at 10 m above sea level, g is the gravitational acceleration

and F is the fetch length.

5.5.2.3.1. Fetch Length

The fetch length is the length of water over which a given wind has blown. Fetch length

along with the wind speed determines the size of waves produced. The longer the fetch

length is and the faster the wind speed is, more energy will be imparted to the water

surface and larger the sea state will be.

The wave generation can be limited by fetch or duration. Based in Figure 26, there is a

limit value of fetch length that divides the sea in developed or growing/developing. A

developed sea is considered when the following relation is fulfilled:

𝐹𝑒𝑡𝑐ℎ 𝐿𝑒𝑛𝑔𝑡ℎ > 2,32 × 𝑈2

Here, U is the mean wind speed at 10 m above the mean sea level (MSL).

The normal sea state (NSS) will be associated with the normal turbulence model (NTM)

in which the wind speed varies from 2 m/s to 26 or 27 m/s. Then, to ensure a developed

sea, the fetch length was calculated considering the higher value of the wind speed in each

study region.

In Table 22, it can be seen the inferior limit of fetch length according to the wind speed as

well as the fetch length chosen for each study region. As it can be observed, the fetch

length chosen is bigger than all values of (2,32 x U²), ensuring a developed sea in all

conditions.

37

Figure 26: Fetch Limited Sea

Table 22: Fetch Length

5.5.2.4. Current

The current speed can be divided into two parts: a tide speed part and a wind speed part

[23].

𝑈(𝑧) = 𝑈𝑡𝑖𝑑𝑒(𝑧) + 𝑈𝑤𝑖𝑛𝑑(𝑧)

The wind speed part is calculated based on the mean wind speed at 10 m height, by the

formula:

𝑈𝑤𝑖𝑛𝑑 = 𝑘𝑉𝑚(10𝑚)

Here, Vm(10m) is the mean wind speed at 10 m height and k is 0,016 [23].

The tide speed part was calculated based on tides and currents map available at the

website from the Brazilian navy [28] [29].

Based on the charts, the three required values from tide speed were calculated and they

can be seen in Table 23.

Table 23: Tide speeds

Southeast Northeast

Utide, mean 0,3 m/s Utide, mean 0,1 m/s

Utide, 1-year 0,5 m/s Utide, 1-year 0,4 m/s

Utide, 50-years 0,7 m/s Utide, 50-years 0,7 m/s

38

6. Results and Discussion

6.1. Power Production

The average power production can be estimated by the wind distribution and the power

curve from the turbine.

The wind variations in a regime can be described using Weibull distribution with an

acceptable accuracy level [30].

The probability density function (f(V)) indicates the fraction of time (or probability) for

which the wind is at a given velocity V [30]. The Weibull’s probability density function

is given by:

𝑓(𝑉) =𝑘

𝑐(

𝑉

𝑐)

𝑘−1

𝑒−(𝑉𝑐⁄ )

𝑘

Here, k is the Weibull shape factor and c is a scale factor.

The Weibull factors k and c can be estimated from the mean (Vm) and standard deviation

(σV) of wind data [30], by the following formulas:

𝑘 = (𝜎𝑉

𝑉𝑚)

−1.090

𝑐 = 𝑉𝑚 𝑘2.6674

0.184 + 0.816 𝑘2.73855

Based on the formulas above, the probability density function from the two regions

considered in this work was calculated and they can be seen in Figure 27 and in Figure

28.

Figure 27: Probability density function – Northeast

Table 24: Parameters used for the Northeast region

Northeast

Vm 8,310

σV 0,580

k 18,207

c 8,283

-1,0E-01

0,0E+00

1,0E-01

2,0E-01

3,0E-01

4,0E-01

5,0E-01

6,0E-01

7,0E-01

8,0E-01

0 2 4 6 8 10 12 14 16

Probability density function - Northeast

39

Figure 28: Probability density function – Southeast

Table 25: Parameters used for the Southeast region

Southeast

Vm 4,810

σV 1,770

k 2,973

c 5,393

The power curve of the 5MW NREL turbine can be found at [22]. It indicates the power

generated by the turbine given a wind speed. The numerical data used to make it was

given by Jason Jonkman at the NREL’s forum. Based on these data, it was possible to

draw the power curve from the turbine.

The power generated by the turbine (P) can be estimated by the following formula:

𝑃 = ∫ 𝑝(𝑉)𝑓(𝑉)𝑑𝑉

-5,0E-02

0,0E+00

5,0E-02

1,0E-01

1,5E-01

2,0E-01

2,5E-01

0 2 4 6 8 10 12 14 16 18

Probability density function - Southeast

0

1000

2000

3000

4000

5000

6000

0 5 10 15 20 25 30

Gen

erat

ed P

ow

er (

kW)

Wind speed (m/s)

5MW NREL Turbine Power Curve

GenPwr (kW)

40

Here, the p(V) is the power generated by the turbine at a given wind velocity V and f(V)

is the probability of the occurrence of wind velocity V.

Based on the formula above, the power generated in the two regions considered in this

work was calculated and it can be seen in Table 26.

Table 26: Power generated in the two regions

Power Generated (kW)

Northeast 1651

Southeast 518

The capacity factor is the average power generated divided by the rated peak power, 5000

kW in this case.

𝐶𝐹 = 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑃𝑜𝑤𝑒𝑟 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑

𝑅𝑎𝑡𝑒𝑑 𝑃𝑒𝑎𝑘 𝑃𝑜𝑤𝑒𝑟=

𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑃𝑜𝑤𝑒𝑟 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑

5000

Based on the formula above, the capacity factor in the two regions was calculated and it

can be seen in Table 27.

Table 27: Capacity factor in the two regions

Capacity Factor

Northeast 33%

Southeast 10%

As it can be observed, the capacity factor in the northeast region is triple that of the

southeast region. Based on power production, the northeast region seems to be more

interesting for the installation of a floating wind turbine.

However, since a comprehensive study to select the regions with the biggest power

capacity was not made, other areas in both regions might present a bigger capacity factor.

Besides, the capacity factors calculated above were compared to the capacity factors of

existing offshore wind farms. The aim of this comparison was to evaluate the feasibility

of the two study regions based on their power production.

The information of the capacity factors was found on the website “Energy Numbers” [31].

The average capacity factor for Belgian offshore wind farms is 36,8% and it varies in a

range from 31,6% to 43,4%. For Danish offshore wind farms, the average is 41,7% and

it varies in a range from 22% to 49%. The average capacity factors for offshore wind

farms in the UK is 37,9% and it varies in a range from 9,2% to 49,3%. Lastly, for German

offshore wind farms, the average capacity factor is 39,2% and it varies in a range from

27,4% to 48,2%.

The capacity factor for the northeast study region is similar to the average of the existing

wind farms. On the other hand, the capacity factor for the southeast study region is well

below average. This corroborates the preference for the installation in the northeast

region.

41

6.2. Hydrodynamic Behavior

The hydrodynamic behavior of the platform will be evaluated comparing the movements

of heave, pitch and surge in the two study regions. Only three of the six movements will

be compared due to their relevance. As it can be seen in Table 28, the movements of surge,

pitch and heave are much bigger than the other movements.

Initially, the extreme values of the platform movements, considering all simulated design

situations, were calculated by FAST. The extreme values can be seen in Table 28.

It is already possible to observe that most parts of the biggest movements were presented

by the platform in the southeast region.

Table 28: Extreme values of platform movements

Then, the time series of the simulation was compared considering an interval of 200 s.

The most part of the extreme values has occurred in design load case 6.1. The only

exception was the maximum of the pitch movement that has occurred in design load case

1.6 with a wind speed of 12 m/s.

DLC 6.1 considers the extreme wind speed model (EWM) and the extreme sea state (ESS)

with a period of return of 50 years. The information on this design load case can be seen

in Table 29.

Table 29: Information of design load case 6.1

Southeast Northeast

Vhub 37 m/s Vhub 39 m/s

Hs 9,6 m Hs 7,4 m

Tp 10,5 s Tp 10,2 s

Utide 0,7 m/s Utide 0,7 m/s

Uwind 0,5 m/s Uwind 0,5 m/s

DLC 1.6 considers the normal turbulence model (NTM) and the extreme sea state (ESS)

with a period of return of 50 years. The information on this design load case can be seen

in Table 30.

42

Table 30: Information of design load case 1.6 with wind speed 12 m/s

Southeast Northeast

Vhub 12 m/s Vhub 12 m/s

Hs 9,6 m Hs 7,4 m

Tp 10,5 s Tp 10,2 s

Utide 0,3 m/s Utide 0,1 m/s

Uwind 0,1 m/s Uwind 0,1 m/s

• Heave

As it can be observed in Figure 29 and in Figure 30, the amplitude of the movement of

heave from the platform in the southeast is always bigger than the one from the platform

in the northeast region.

This can be explained by the fact that the significant wave height in the southeast region

is bigger than in the northeast region.

Figure 29: Comparison of heave movement between 1000s and 1200s of simulation (DLC6.1)

Figure 30: Comparison of heave movement between 3300s and 3500s of simulation (DLC6.1)

-5,00E+00

0,00E+00

5,00E+00

1,00E+01

1,50E+01

1000 1020 1040 1060 1080 1100 1120 1140 1160 1180 1200

Heave (m)

Heave Southeast Heave Northeast

-5,00E+00

0,00E+00

5,00E+00

1,00E+01

1,50E+01

3300 3320 3340 3360 3380 3400 3420 3440 3460 3480 3500

Heave (m)

Heave Southeast Heave Northeast

43

• Surge

As it can be observed in Figure 31 and in Figure 32, the amplitude of the movement of

surge from the platform in the southeast region is always bigger than the one from the

platform in the northeast region.

This can also be explained by the fact that the significant wave height in the southeast

region is bigger than in the northeast region.

Figure 31: Comparison of surge movement between 1000s and 1200s of simulation (DLC6.1)

Figure 32: Comparison of surge movement between 3300s and 3500s of simulation (DLC6.1)

-1,00E+01

-5,00E+00

0,00E+00

5,00E+00

1,00E+01

1,50E+01

1000 1020 1040 1060 1080 1100 1120 1140 1160 1180 1200

Surge (m)

Surge Southeast (m) Surge Northeast (m)

-1,00E+01

-5,00E+00

0,00E+00

5,00E+00

1,00E+01

1,50E+01

3300 3320 3340 3360 3380 3400 3420 3440 3460 3480 3500

Surge (m)

Surge Southeast (m) Surge Northeast (m)

44

• Pitch

As it can be observed in Figure 33 and in Figure 34, the negative values of the movement

of pitch from the platform in the southeast region are always absolutely bigger than the

ones from the platform in the northeast region. On the other hand, the positive values vary

in a way that they are sometimes bigger in the platform in the southeast region and

sometimes in the northeast region. This was observed in the design load case 6.1.

In design load case 1.6 with wind speed 12 m/s, the negative values always have bigger

absolute value in the southeast region and the positive values are always bigger in the

northeast region, as it can be seen in Figure 35 and in Figure 36. It can be also observed

that the amplitude of the movement is bigger in the southeast region.

Figure 33: Comparison of pitch movement between 1000s and 1200s of simulation (DLC6.1)

Figure 34: Comparison of pitch movement between 3300s and 3500s of simulation (DLC6.1)

-1,20E+01

-1,00E+01

-8,00E+00

-6,00E+00

-4,00E+00

-2,00E+00

0,00E+00

2,00E+00

4,00E+00

1000 1020 1040 1060 1080 1100 1120 1140 1160 1180 1200

Pitch (deg)

Pitch Southeast (deg) Pitch Northeast (deg)

-1,20E+01

-1,00E+01

-8,00E+00

-6,00E+00

-4,00E+00

-2,00E+00

0,00E+00

2,00E+00

4,00E+00

3300 3320 3340 3360 3380 3400 3420 3440 3460 3480 3500

Pitch (deg)

Pitch Southeast (deg) Pitch Northeast (deg)

45

Figure 35: Comparison of pitch movement between 200s and 400s of simulation (DLC1.6)

Figure 36: Comparison of pitch movement between 400s and 600s of simulation (DLC1.6)

The comparisons presented above have shown a bigger movement of heave, surge and

pitch of the platform in the southeast region.

However, the two design load cases considered above are defined according to the

significant wave height with a 50-years period of return. As this wave height is bigger in

the southeast region, it was already expected a bigger movement of the platform in this

region.

Then, to compare the movement of the two platforms in a similar situation, it was chosen

to make a comparison considering the design load case 1.2. It considers the normal

turbulence model (NTM) and the normal sea state (NSS). The wind speed of 8 m/s was

chosen.

-3,00E+00

-2,00E+00

-1,00E+00

0,00E+00

1,00E+00

2,00E+00

3,00E+00

4,00E+00

5,00E+00

6,00E+00

7,00E+00

200 220 240 260 280 300 320 340 360 380 400

Pitch (deg)

Pitch Southeast Pitch Northeast

-3,00E+00

-2,00E+00

-1,00E+00

0,00E+00

1,00E+00

2,00E+00

3,00E+00

4,00E+00

5,00E+00

6,00E+00

7,00E+00

400 420 440 460 480 500 520 540 560 580 600

Pitch (deg)

Pitch Southeast Pitch Northeast

46

Table 31: Information of DLC 1.2 with 8 m/s wind speed

DLC 1.2 _ 8 m/s

Vhub 8 m/s

Hs 0,9 m

Tp 4,7 s

Figure 37: Comparison of heave movement (DLC1.2)

Figure 38: Comparison of surge movement (DLC1.2)

0,00E+00

1,00E+00

2,00E+00

3,00E+00

4,00E+00

5,00E+00

6,00E+00

7,00E+00

8,00E+00

9,00E+00

0 100 200 300 400 500 600 700

Heave (m)

Heave Southeast Heave Northeast

-2,00E+00

-1,50E+00

-1,00E+00

-5,00E-01

0,00E+00

5,00E-01

1,00E+00

1,50E+00

2,00E+00

0 100 200 300 400 500 600 700

Surge (m)

Southeast Surge Northeast Surge

47

Figure 39: Comparison of pitch movement (DLC1.2)

As it can be seen in the figures above, there is no difference in the movement of heave of

the platform in the two regions. A small difference in the movement of surge can be

observed, in which the platform in the southeast region presents a bigger amplitude of

movement. The bigger difference can be seen in the movement of pitch, in which it is

also the platform in the southeast region that presents a bigger amplitude of movement.

At this last analysis the difference between the southeast and the northeast simulation was

only the turbulence intensity of the wind speed at the study regions. The turbulence

intensity in the southeast region (A) is bigger than the turbulence intensity in the northeast

region (C). Thus, it can be concluded that the turbulence intensity affects with more

significance the movement of pitch. Besides that, the turbulence intensity has just a small

influence on the movement of surge and no influence on the movement of heave.

The significant wave heights in the northeast region are smaller than the ones in the

southeast region. In addition to that, the turbulence intensity in the northeast region is also

smaller than the one in the southeast region. Then, based on the hydrodynamic behavior

of the platform in the two study regions, the northeast region is more suitable for the

installation of the floating wind turbine.

6.3. Structural Analysis

The transition region, where the tower base is connected to the floating platform, is a

critical region of the system. Thus, a structural analysis was made to ensure that the

maximum stress in this region is below the minimum yield stress. As this work presents

an initial analysis, only an extreme structural analysis was made. However, in further

work, a fatigue analysis shall be made.

Based on DNV-ST-0437 [14], the severe sea state (SSS) model shall be used in

combination with normal wind conditions for calculation of the ultimate loading of an

offshore wind turbine during power production. Then, DLC 1.6 was used in this analysis

and the wind speed of 24 m/s was considered.

-2,00E+00

-1,00E+00

0,00E+00

1,00E+00

2,00E+00

3,00E+00

4,00E+00

0 100 200 300 400 500 600 700

Pitch (deg)

Pitch Southeast Pitch Northeast

48

The forces and moments at the tower base were calculated by FAST. Then, to calculate

the stress in the section of the tower base, the force along the z-axis as well as the moments

about the x-axis and y-axis were considered. The tower base coordinate system can be

seen in Figure 40.

Figure 40: Tower base coordinate system [32]

The section of the tower base is an annulus (a ring-shaped object). The base diameter is

6,5 m and the thickness is 0,027 m. Eight points around this section were defined in which

the stress was calculated, as it can be seen in Figure 41.

Figure 41: Eight points to stress calculation

The stress in each point was calculated by the following formula:

𝜎 = −𝑇𝑤𝑟𝐵𝑠𝐹𝑧𝑡

𝑆+

𝑇𝑤𝑟𝐵𝑠𝑀𝑥𝑡

𝐼𝑦. 𝑠𝑒𝑛𝜃 +

𝑇𝑤𝑟𝐵𝑠𝑀𝑦𝑡

𝐼𝑥. 𝑐𝑜𝑠𝜃

Here, TwrBsFzt is the force directed along the z-axis; TwrBsMxt is the moment about

the x-axis; TwrBsMyt is the moment about the y-axis; S is the surface of the tower base

section; I is the inertia of the tower base section and θ is the angle between the point and

the x-axis.

The maximum and the minimum stress at the tower base section are presented in Table

32.

49

Table 32: Maximum and minimum stress at tower base section

Southeast Northeast

σ max 301 MPa σ max 243 MPa

σ min -299 MPa σ min -241 MPa

The steel was selected based on DNVGL-OS-C101 [33] and the minimum yield stress

was calculated based on DNVGL-OS-B101 [34].

Based on section 3.2.2 of [33], the structural category of this transition region is classified

as special due to the substantial consequences caused by a failure in this region as well as

the concentration of stress in it.

Then, based on section 4.3 of [33], structural steel can be selected. It is already known

that the structural category is special and that the strength group is high strength. Then

according to the design temperature as well as the thickness of the material, the grade of

the steel can be determined.

As it is presented above, the thickness of the tower base is 27 mm. It is supposed that the

platform is going to be constructed in Brazil, where temperature will never be below zero

degree. Then, the grade of the steel must be “E”.

Based on section 1.2 of [34], the minimum yield stress of high strength steel grade E

(DNV HS E 36) is 355 MPa.

In both regions, the maximum calculated stress is below the minimum yield stress. The

maximum calculated stress of the southeast platform is 85% of the yield stress and the

one of the northeast platform is 68% of the yield stress.

It can be observed that there is a good margin between the calculated stress and its

maximum. However, in further work, an analysis of the stress concentration factor (SCF)

shall be made in this section.

50

7. Conclusion This work presents three analyses to compare the behavior of a wind turbine supported

by a semi-submersible floating platform.

In the study of the power production of the two turbines, it was observed that the capacity

factor of the northeast region is triple that of the southeast region. This can be explained

by the higher mean wind speed in the northeast region as well as the lower turbulence

intensity presented in this region.

In the hydrodynamic behavior analysis, it was observed that the platform in the southeast

region presented bigger amplitudes of movement than the one in the northeast region.

This can be explained by the fact that the significant wave heights observed in the

southeast region is bigger than the ones in the northeast region. Besides that, it was also

observed that a higher turbulence intensity causes higher movements of pitch. The

movements of surge and heave were not significantly influenced by the turbulence

intensity.

In the structural analysis, it was defined that the structural steel for the transition region

should be DNV HS E 36 with a minimum yield stress of 355 MPa. In both regions, the

maximum calculated stress in the tower base was below the yield stress. The maximum

calculated stress of the southeast platform was 85% of the yield stress and the one of the

northeast platform was 68% of the yield stress.

It is important to point out that the analyses were made only for the two selected regions.

These regions were chosen based only on their water depth and their distance to the shore.

Besides that, only three from eight design cases defined by the DNVGL rule were

considered.

Then, based on the results presented above, the northeast region seems more interesting

for the installation of a wind turbine supported by a semi-submersible floating platform.

A summary of the results can be seen in Table 33.

Table 33: Summary of the results

Results

Northeast Southeast

Power Production (kW) 1651 518

Capacity Factor 33% 10%

Hydrodynamic Behaviour

Surge (m) 8,1 11,18

Heave (m) 9,67 11,93

Pitch (deg) -7,04 -9,62

Structural Analysis

Maximum stress (Mpa) 243 301

For further work, a study to select the areas with the biggest power capacity in the two

regions should be made. In addition to that, all design cases must be considered to define

which region is more suitable for the installation of a wind turbine supported by a semi-

submersible floating platform.

51

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54

Annexes

Annex I: Summary of Simulation Cases

All design load cases simulated with their information can be seen in Figure 42 for the

southeast region and in Figure 43 for the northeast region.

Figure 42: Design situations for southeast region

55

Figure 43: Design situations for northeast region