Afolaranmi Amodu MSc Project

8/3/2019 Afolaranmi Amodu MSc Project

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University of Strathclyde

Department of Naval Architecture and Marine

Engineering

Development Concepts for the Integrated

Design of Marine Renewable Energy and

Aquaculture Platform

By

Afolaranmi Ajibola Amodu

This research thesis presented in partial fulfilment of the requirements

for the degree of Master of Science in Subsea Engineering from the

University of Strathclyde, Glasgow

August 2011


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Copyright statement

This thesis is the result of the author's original research. It has been composed by the

author and has not been previously submitted for examination, which has led to the

award of a degree.

The copyright of this thesis belongs to the author under the terms of the United

Kingdom Copyright Acts as qualified by University of Strathclyde Regulation 3.50.

Due acknowledgement must always be made of the use of any material contained in,

or derived from, this thesis.'

Signed: Date:


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Abstract

This research report introduces renewable energy and aquaculture farming and shows

the integrated design of wind turbines, their means of installation, challenges

surrounding their development and ways of overcoming this challenges. It alsoinvolves development into offshore aquaculture, its technical challenges and

different means of eradicating problems experienced with both industries by moving

them offshore. Reasons for the selection of wind energy instead of other alternative

sources were highlighted as well as salmon farming rather than other types of fish

farming.

The main objective of this report shows ways or possible reasons for combining the

development of both challenging and extremely rewarding sectors. The

developments of this sectors offshore provides alternatives to the numerous setbacks

encountered in their respective sectors individually onshore and though their

progress might be stunted developing individually, the combination of both sectors

might just be the edge needed to make both sectors investment viable and

commercially achievable. Reasons for combining both sectors have been specifically

highlighted. Mathematical modelling for the mooring line static and dynamic

analysis that can be used for the combination of both the wind turbine and the fish

cages was also developed.

The report also highlights areas that need further study for future development as

well as indicating other stakeholders required for consideration during the research

process to ensure the success of such a project. Finally decommissioning methods

were briefly mentioned.


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Dedication

This thesis is dedicated to the greatness of God Almighty and to my late mother

(Olatoun Opeoluwa) my last thoughts of you are words of inspiration. I miss and

love you mum.


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Acknowledgement

I wish to offer my sincere gratitude to my supervisors, Professor A. Incecik (Head of

Department) and Dr A. H. Day (Reader/Director of laboratories) who have supported

me throughout my thesis with their patience and knowledge whilst allowing me the

room to work in my own way. I attribute the level of my Masters degree to their

encouragement. I also want to thank my father Abimbola Amodu, you have been a

friend, a father and a mother. The support you gave me has aided me thus far and the

wisdom you shared has guided me greatly, so thanks dad.

I also want to thank the entire staff of Strathclyde university and my colleagues, the

experience of meeting you, spending time together and living in a foreign home is an

experience I will cherish forever. Our sojourn together has inspired me to continue

even when the odds seemed impossible. To my siblings you have been a force to rely

upon and even though your presence was not felt physically, your prayers were felt

always and I thank you.

Finally I would like to thank all those who have contributed to the achievement of

my goals in anyway and to my best friend kemy, you have made a year seem like a

month and the thought of disappointing you has inspired me to excel.

Thank you all.


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Table of Contents

Abstract ............................................................................................................................................................. iii

Dedication......................................................................................................................................................... iv

Acknowledgement .......................................................................................................................................... v

Table of Contents ........................................................................................................................................... vi

List of Figures .................................................................................................................................................. ix

List of Tables ..................................................................................................................................................... x

List of Symbols Abbreviations and Nomenclature ...................................... ...................... .............. xi

Chapter One ...................................................................................................................................................... 1

1.1 Introduction to Renewable Energy ................................................................................................. 1

1.2 Wind Farms and Offshore Technology ................... ..................... ..................... ..................... ......... 4

1.2.1 Offshore Wind Power ......................................................................................................... 5

1.3 Aquaculture ............................................................................................................................................... 6

1.3.1 Salmon Farming ................................................................................................................... 7

1.4 Review on Marine Energy and Aquaculture Development.................................... ................ 9

Chapter Two ................................................................................................................................................... 11

2.1 Project objectives .................................................................................................................................. 11

2.2 Solution Methodology ......................................................................................................................... 112.3 Background behind Wind Energy selection .................... ..................... ..................... ................. 11

2.3.1 Wind vs. Solar .................... ..................... ..................... ..................... ..................... .............. 12

2.3.2 Wind vs. Biomass .................... ..................... ..................... ..................... ..................... ....... 12

2.3.3 Wind vs. Hydropower .................. ..................... ..................... ..................... ..................... 12

2.3.4 Wind vs. Geothermal .................... ..................... ..................... ..................... ..................... 13

2.3.5 Summary ............................................................................................................................... 13

2.4 Background behind Salmon selection .................... ..................... ..................... ..................... ....... 13

Chapter Three ................................................................................................................................................ 15

3.1 Wind Energy Harnessing (Turbine design) .................... ..................... ..................... ................. 15

3.2 Design specifications ........................................................................................................................... 15

3.2.1 Temperature ................... .................... ...................... ..................... .................... .................. 16

3.2.2 Aerodynamics ..................................................................................................................... 16

3.2.3 Power control .................... ..................... ..................... ..................... ..................... .............. 16


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3.2.4 Turbine size ......................................................................................................................... 17

3.2.5 Generator .............................................................................................................................. 17

3.2.6 Blade design ......................................................................................................................... 18

3.2.7 Blade count........................................................................................................................... 19

3.2.8 Blade Materials and Design Considerations ..................... .................... .................. 20

3.2.9 Tower ..................................................................................................................................... 21

3.2.10 Foundations ...................................................................................................................... 22

3.3 Floating Wind Turbine Concepts .................... ..................... ..................... .................... .................. 22

Chapter Four................................................................................................................................................... 27

4.1 Aquaculture Cage systems (offshore)......................... ..................... .................... ..................... .... 27

4.2 General approaches to offshore cage design ..................... ..................... ...................... ............. 27

4.3 Floating Cages ......................................................................................................................................... 29

4.3.1 Floating flexible cages...................................................................................................... 29

4.3.2 Floating rigid cages .................... ..................... ..................... ..................... .................... .... 30

4.4 Semi-submersible cages ..................................................................................................................... 31

4.4.1 Semi submersible flexible cages ................................. ..................... .................... .... 32

4.4.2 Semi submersible rigid cages ..................... ..................... ..................... ..................... 34

4.5 Submersible rigid cages ..................................................................................................................... 35

Chapter Five ................... ..................... ..................... ..................... ..................... ..................... .................... .... 37

5.1 Introduction to Mooring Lines ........................................................................................................ 37

5.2 Design Calculations/Mathematical Modelling of Mooring Lines (Static Analysis) .... 37

5.3 Dynamic Analysis of mooring lines ................................ ..................... ..................... ..................... 41

5.4 Mooring line response ........................................................................................................................ 44

Chapter Six ...................................................................................................................................................... 46

6.1 Reasons and Advantages of Combining both Aquaculture and Offshore (Wind

Energy) ............................................................................................................................................................. 46

6.2 Problems associated with wind turbines ..................... ..................... ..................... ..................... 51

6.3 Wind Energy Challenges .................................................................................................................... 52

6.4 Problems associated with Offshore Aquaculture ................... ..................... ..................... ....... 55


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Chapter Seven ................................................................................................................................................ 56

7.1 Discussions and Conclusions............................................................................................................ 56

7.2 Future Recommendations ................... .................... ...................... ..................... .................... ........... 57

References and Bibliography................................................................................................................... 58


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List of Figures

Figure 1: Wind Primary Energy Production (Courtesy of Good transportation

journal) ......................................................................................................................... 2

Figure 2: "GWEC, Global Wind Report Annual Market Update (Courtesy of

GWEC) ......................................................................................................................... 3

Figure 3: "GWEC, Global Wind Energy Outlook" (Courtesy of GWEC) .................. 3

Figure 4: Worldwide installed capacity 19972020 [MW], developments and

prognosis. (Courtesy of WWEA) ................................................................................. 5

Figure 5: Aerial view of Lillgrund Wind Farm, Sweden (Courtesy of Mariusz

Padziora)..................................................................................................................... 6

Figure 6: Salmon farm in the archipelago of Finland. (Courtesy of Wikimedia

Commons) .................................................................................................................... 8

Figure 7: Floating Wind Turbine Concepts (Courtesy of Institute for Wind Energy)

.................................................................................................................................... 23

Figure 8: Floating Cage Nets (Courtesy of aquaculture.com) ................................... 29

Figure 9: Open Ocean Aquaculture or Offshore Aquaculture (Courtesy of care2.com)

.................................................................................................................................... 30

Figure 10: Graphic image of Subsea Aquaculture Cage (Courtesy Cage Aquaculture).................................................................................................................................... 33

Figure 11: Composure of Farmocean Facilities (Courtesy of www.farmocean.se) ... 33

Figure 12: A photo-illustration composite image of an Aquapod fish-farming cage.

(Courtesy of www.Nationalgeographic.com). ........................................................... 34

Figure 13: Inelastic Hanging Cable............................................................................ 38

Figure 14: Expression of Mass spring constants for Analysis ................................... 42

Figure 15: 3D preview of MpOP, Wind farm and Fish farm. (Courtesy of Nuno

Santana) ...................................................................................................................... 50

Figure 16: Offshore Wind Platforms (Courtesy of www.renewablepowernews.com)

.................................................................................................................................... 51


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List of Tables

Table 1: Design Challenge Trade-offs for Stability Criteria ...................................... 25

Table 2: Offshore cage types...................................................................................... 28

Table 3: Advantages and Disadvantages of Floating Flexible Cages. ....................... 29

Table 4: Advantages and disadvantages of Floating Rigid Cages. ............................ 31

Table 5: Advantages and disadvantages of Semi-submersible flexible Cages. ......... 32

Table 6: Advantages and disadvantages of Semi-submersible Rigid Cages.............. 35

Table 7: Advantages and disadvantages of Submersible Rigid Cages. ..................... 36


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List of Symbols Abbreviations and Nomenclature

Abbreviations

DNV - Det Norske Veritas

EU - European Union

FAO - Food and Agriculture Organisation

GWEC - Global Wind Energy Council

HAWTS - Horizontal Axis Wind Turbine

ICZM - Integrated Coastal Zone Management

IMTA - Integrated Multi-trophic Aquaculture

JONSWAP - Joint North Sea Wave Analysis Project

NGOs - Non-Governmental Organisations

O&M - Operation and Maintenance

OWECS - Offshore wind Energy Conversion Systems

TLP - Tension Leg Platform

UDN - Ulcerative Dermal Necrosis

WFD - Water Framework Directive

WWEA - World Wind Energy Association


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Nomenclature

CO2 - Carbon Dioxide

CO - Carbon Monoxide

Hs - Mean Significant Wave Height

U - Wind Speed

Tz - Period

H Horizontal Component of Cable Tension

L Length

m - Mass

g - Acceleration due to gravity

X1, X2 - Displacements

K1, K2, K3Spring Constants


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Chapter One

1.1 Introduction to Renewable Energy

Renewable energy is power that comes from natural resources such as sunlight,

wind, rain, tides, and geothermal heat, which are renewable (naturally replenished).

In 2008, about 19% of global final energy consumption came from renewables, with

13% coming from traditional biomass, which is mainly used for heating, and 3.2%

from hydroelectricity. New renewables mainly small hydro, modern biomass, wind,

solar, geothermal, and biofuels accounted for another 2.7% and are growing very

rapidly. The share of renewables in electricity generation is around 18%, with 15%

of global electricity coming from hydroelectricity and 3% from new renewables (1).

These figures are expected to increase significantly to a combined total of about 30%

in the next 10 years. While many renewable energy projects are large-scale,

renewable technologies are also suited to rural and remote areas, where energy is

often crucial in human development (2). Some of the factors acting as incentives for

the rapid development of this sector despite the large cost of infrastructure are

Climate change concerns, coupled with high oil prices, peak oil as well as increasing

government support, incentives and commercialisation (3). The major uses of

renewables are power generation, heating and transport fuels (4).

Renewable energy provides 19 percent of total power generation worldwide.

Renewable power generators are spread across many countries, and wind power

alone already provides a significant share of electricity in some areas. Solar heating

in form of hot water makes an important contribution in many countries, most

notably in China, which now has 70 percent of the global total (180 GWth). Most of

these systems are installed on multi-family apartment buildings and meet a portion of

the hot water needs of an estimated 5060 million households in China. The use of

biomass for heating continues to grow as well. In Sweden, national use of biomass

energy has surpassed that of oil. Transport fuels such as biofuels have contributed to

a significant decline in oil consumption in the United States since 2006. The 93

billion litters of biofuels produced worldwide in 2009 displaced the equivalent of an


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estimated 68 billion litters of gasoline, equal to about 5 percent of world gasoline

production (4).

Figure 1: Wind Primary Energy Production (Courtesy of Good

transportation journal)

In figure 1 above, it can be observed that increased production is observed in Wind,

solar, geothermal and coal production with these trends its no surprise the world has

cast a searchlight on renewables and alternative means of power generation. This text

will therefore be focussing on wind energy.

The sun unevenly heats the earth, such that the poles receive less energy from the sun

than the equator. Along with this, dry land heats up (and cools down) more quickly

than the seas do. The differential heating drives a global atmospheric convection

system reaching from the earth's surface to the stratosphere that acts as a virtual

ceiling. Most of the energy stored in these wind movements can be found at high

altitudes where continuous wind speeds of over 160 km/h (99 mph) occur.

Eventually, the wind energy is converted through friction into diffuse heat

throughout the Earth's surface and the atmosphere. Wind power is the conversion of

wind energy into a useful form of energy, such as using wind turbines to make

electricity (5).


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Figure 2: "GWEC, Global Wind Report Annual Market Update

(Courtesy of GWEC)

Figure 3: "GWEC, Global Wind Energy Outlook" (Courtesy of GWEC)


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Large-scale wind farms are connected to the electric power transmission network

while smaller facilities are used to provide electricity to isolated locations and the

utility companies increasingly buy back surplus electricity produced by small

domestic turbines. Wind energy, as an alternative to fossil fuels, is plentiful,

renewable, widely distributed, clean, and produces no greenhouse gas emissionsduring operation. The construction of wind farms is not universally welcomed

because of their visual impact, but any effects on the environment from wind power

are generally less problematic than those of any other power source (5). Airflows can

be used to run wind turbines. Modern wind turbines range from around 600 kW to 5

MW of rated power, although turbines with rated output of 1.53 MW have become

the most common for commercial use. The power output of a turbine is a function of

the cube of the wind speed, so as wind speed increases, power output increasesdramatically (7). Areas where winds are stronger and more constant, such as offshore

and high altitude sites, are preferred locations for wind farms. Typical capacity

factors are 20-40%, with values at the upper end of the range in particularly

favourable sites (8)(9). Globally, the long-term technical potential of wind energy is

believed to be five times total current global energy production, or 40 times current

electricity demand. This could require wind turbines to be installed over large areas,

particularly in areas of higher wind resources. Offshore resources experience mean

wind speeds of approximately 90% greater than that of land, so offshore resources

could contribute substantially more energy (10)(11).

1.2 Wind Farms and Offshore Technology

A wind farm is a group of wind turbines in the same location used for production of

electric power. A large wind farm may consist of several hundred individual wind

turbines, and cover an extended area of hundreds of square miles, but the land

between the turbines may be used for agricultural or other purposes. A wind farm

may also be located offshore. In 2010, more than half of all new wind power was

added outside of the traditional markets in Europe and North America. This was

largely from new construction in China, which accounted for nearly half the new

wind installations (16.5 GW) (12). Global Wind Energy Council (GWEC) figures


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show that 2007 recorded an increase of installed capacity of 20 GW, taking the total

installed wind energy capacity to 94 GW, up from 74 GW in 2006. Despite

constraints facing supply chains for wind turbines, the annual market for wind

continued to increase at an estimated rate of 37%, following 32% growth in 2006. In

terms of economic value, the wind energy sector has become one of the importantplayers in the energy markets, with the total value of new generating equipment

installed in 2007 reaching 25 billion, or US$36 billion (13).

Figure 4: Worldwide installed capacity 19972020 [MW], developments

and prognosis. (Courtesy ofWWEA)

1.2.1 Offshore Wind Power

Offshore wind power refers to the construction of wind farms in bodies of water to

generate electricity from wind. Better wind speeds are available offshore compared

to on land, so offshore wind powers contribution in terms of electricity supplied is

higher. As of October 2010, 3.16 GW of offshore wind power capacity was

operational, mainly in Northern Europe. According to BTM Consult, more than 16

GW of additional capacity will be installed before the end of 2014 and the UK and

Germany will become the two leading markets. Offshore wind power capacity is
http://www.wwindea.org/http://www.wwindea.org/


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expected to reach a total of 75 GW worldwide by 2020, with significant

contributions from China and the US (14).

Figure 5: Aerial view of Lillgrund Wind Farm, Sweden (Courtesy of

Mariusz Padziora)

1.3 Aquaculture

Aquaculture, also known as aquafarming, is the farming of aquatic organisms such as

fish, crustaceans, molluscs and aquatic plants. Aquaculture involves cultivating

freshwater and saltwater populations under controlled conditions, and can be

contrasted with commercial fishing, which is the harvesting of wild fish (15).

Aquaculture produces one half of the fish and shellfish that is directly consumed by

humans (16). Particular kinds of aquaculture include fish farming, shrimp farming,

oyster farming, algaculture (such as seaweed farming), and the cultivation of

ornamental fish. Emphasis will be on fish farming for the purpose of this report and

particularly the aquatic salmon fish farming.

Global wild fisheries are in decline, with valuable habitat such as estuaries in critical

condition (17). The aquaculture or farming of piscivorous fish, like salmon, does not

help the problem because they need to eat products from other fish, such as fish meal


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and fish oil. Studies have shown that salmon farming has major negative impacts on

wild salmon, as well as the forage fish that need to be caught to feed them (18)(19).

Fish that are higher on the food chain are less efficient sources of food energy. Apart

from fish and shrimp, some aquaculture undertakings, such as seaweed and filter-

feeding bivalve molluscs like oysters, clams, mussels and scallops, are relativelybenign and even environmentally restorative (20). Filter-feeders filter pollutants as

well as nutrients from the water, improving water quality (21). Seaweeds extract

nutrients such as inorganic nitrogen and phosphorus directly from the water (22), and

filter-feeding molluscs can extract nutrients as they feed on particulates, such as

phytoplankton and detritus (23). Farmed salmon can be contrasted with wild salmon

captured using commercial fishing techniques (24).

1.3.1 Salmon Farming

Salmon, along with carp, are the two most important fish groups in aquaculture. In

2007, the aquaculture of salmon and salmon trout was worth US$10.7 billion. The

most commonly farmed salmon is the Atlantic salmon. Other commonly farmed fish

groups include tilapia, catfish, sea bass, bream and trout. Salmon are usually farmed

in 2 stages and in some places maybe more. First, the salmon are hatched from eggs

and raised on land in freshwater tanks. When they are 12 to 18 months old, the smolt

(juvenile salmon) are transferred to floating sea cages or net pens anchored in

sheltered bays or fjords along a coast. This farming in a marine environment is

known as mariculture. There they are fed pelleted feed for another 12 to 24 months,

when they are harvested (25).


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Figure 6: Salmon farm in the archipelago of Finland. (Courtesy of

Wikimedia Commons)

Salmon aquaculture production grew over ten-fold during the 25 years from 1982 to

2007. Leading producers of farmed salmon are Norway with 33 percent, Chile with

31 percent, and other European producers with 19 percent (26). There is currently

much controversy about the ecological and health impacts of intensive salmon

aquaculture. The population of wild salmon declined markedly in recent decades,

especially North Atlantic populations which spawn in the waters of western Europe

and eastern Canada, and wild salmon in the Snake and Columbia River system in

north-western United States. The decline is attributed to the following factors:

Sea lice - transfer of parasites from open-net cage salmon farming, especiallysea lice, has reduced numbers. It is reported that wild salmon on the west

coast of Canada are being driven to extinction by sea lice from nearby salmon

farms.

Overfishing in general has reduced populations, especially commercialnetting in the Faroes and Greenland

Warming in ocean and river water can delay spawning and accelerate thetransition to smolting.
http://commons.wikimedia.org/wiki/Main_Pagehttp://commons.wikimedia.org/wiki/Main_Page


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Ulcerative dermal necrosis (UDN) infections of the 1970s and 1980s severelyaffected adult salmon in freshwater rivers.

Habitat - the loss of suitable freshwater habitat, especially degradation ofstream pools and reduction of suitable material for the excavation of redds

has caused a reduction in spawning.

Other environmental factors such as light intensity, water flow, or change in

temperature dramatically affect salmon during their migration season.

1.4 Review on Marine Energy and Aquaculture Development

Previously Hydroelectric has been the dominant source of marine energy however

with new and existing offshore engineering technologies, wave, wind and tidal

energy are now been produced with a significant contribution to the power

generation grid hence justifying the huge capital investments required. These cutting

edge technologies can be implemented into the design of a platform for both

aquaculture farming and wave energy generation. During the course of my review, I

realised that considerable research has been done in the area of marine renewables

and aquaculture farming with both industries looking for innovative ways to advance

significantly however little or no research has been done in the combination of these

two viable and potentially lifesaving and renewable industries. The combination of

these two ideas may be the much-needed incentive to open the gate of capital

investments required for both renewable energy sustainment and protection of

aquatic life in the form of wild salmon while also producing adequate food supply.

Aquaculture can be done in various ways 2 of which are mariculture aquaculture or

integrated multi-trophic aquaculture. Mariculture is the term used for the cultivation

of marine organisms in seawater, usually in sheltered coastal waters. In particular,

the farming of marine fish is an example of mariculture while Integrated Multi-

Trophic Aquaculture (IMTA) is a practice in which the by-products from one species

are recycled to become inputs i.e. fertilizers, food for another. "Integrated" in IMTA

refers to the more intensive cultivation of the different species in proximity of each

other, connected by nutrient and energy transfer through water while the Multi-


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Trophic" refers to the incorporation of species from different trophic or nutritional

levels in the same system. Some of the issues of this is that the process can be more

environmentally damaging than exploiting wild fisheries on a local area basis but has

considerably less impact on the global environment on a per kg of production basis.

Local concerns include waste handling, side effects of antibiotics, competitionbetween farmed and wild animals, and using other fish to feed more marketable

carnivorous fish. However, research and commercial feed improvements during the

1990s & 2000s have lessened many of these.


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Chapter Two

2.1 Project objectives

This project will focus mainly on design of marine renewables (Wind energy), its

design and how it can be significantly harnessed in combination with an Aquaculture

system (Salmon farming) to produce energy and food supply. Previously

Hydroelectric has been the dominant source of marine energy however with new and

existing offshore engineering technologies, wind energy is now been produced with a

significant contribution to the power generation grid hence justifying the huge capital

investments required. These cutting edge technologies can be implemented into the

design of a platform for both aquaculture farming and wave energy generation.

2.2 Solution Methodology

This will be done by assessing the technical, economical and environmental

feasibility of constructing, installing, operating, servicing, maintaining and

decommissioning of the integrated platforms proposed and also by discussing the

drawbacks of both industries individually and ways of eradicating them with a

possible combination of both industries.

2.3 Background behind Wind Energy selection

A renewable energy source is defined as any energy source that comes from natural

resources that can be continually renewed, leaving no worry of running out. A lot of

countries today are gradually adopting renewable energy as the threat of global

warming worsens. With renewable energy, there is no danger of releasing harmfulchemicals to the atmosphere since nearly all of them do not have harmful waste

products during energy conversion.

There are currently 5 popular kinds of renewable energy sources for power

generation: wind, solar, biomass, hydropower and geothermal energy. Among these

5, wind power has been the one thats growing the fastest. There are various reasons


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why wind power is the renewable energy resource that many countries are

developing. Lets compare wind energy with each of the other four energy sources to

see the big picture:

2.3.1 Wind vs. Solar

There is one basic reason why wind and solar are the two most popular renewable

energy resourceswind and sunlight can be found nearly everywhere. There are two

catches why wind power is much better though.

First, sunlight can only be gathered half of the time. Second, sunlight cant be

gathered during bad weather conditions. This makes location selection hard when

dealing with solar energy.

2.3.2 Wind vs. Biomass

Biomass is a renewable energy taken from recently alive plants. The conversion

process of biomass to electricity has air emissions though, some of which can

influence global warming like CO (carbon monoxide) and CO2 (carbon dioxide).

The main advantage wind power has over biomass is that it doesnt emit harmful

gases during conversion. Another con of biomass is that it sometimes takes more

energy to harvest biomass crops than it ever produces after. Wind turbines on the

other hand pay off the energy spent in its construction after only 3-5 months of

energy generation.

2.3.3 Wind vs. Hydropower

Hydropower generates electricity through the use of falling waters gravitational

force. Its the most widely used renewable energy resource for good reasons. It

doesnt produce any direct waste like carbon dioxide and dams have a variety of uses

besides electricity generation, such as flood control.

The problem with hydropower is that building dams can damage the surrounding

aquatic ecosystem and can permanently alter some species behavior. Also, siltation

can occur which can permanently damage the dam and leave it nonoperational. Other


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than that though, it really depends on a countrys natural resources if whether a

hydro power plant or wind farm would be more beneficial.

2.3.4 Wind vs. Geothermal

Geothermal energy is heat energy stored in the Earth. Its a great source of energy

since its renewable and cost-effective. It also does not create any pollution in

production, so it helps slow down climate change. The cost of building a geothermal

power plant is also distinctively lesser than building oil, nuclear and coal power

plants.

Wind power is still preferable over geothermal energy though, since costs of drilling

are extremely high whilst scouting for a good location would take a lot of time. Also,

sites have a tendency of suddenly running out of steam, leaving the plant

nonoperational. Wind, on the other hand, is always present as long as the sun shines

and is more effective in the long run.

2.3.5 Summary

Currently, wind power is the fastest-growing renewable energy resource. Its also

very cheap, and may steadily replace gas as the cheapest energy resource per unit

KWh. Choosing between wind power and other renewable energy resources isnt

really about which is good and which is bad, but is more of a choice between whats

good and whats best.

2.4 Background behind Salmon selection

Experts develop practical recommendations for decision-makers, scientists and

producers for a sustainable development of Mediterranean aquaculture. Humandemand for fish is growing steadily. With fisheries decreasing worldwide,

aquaculture is becoming an important socio-economic alternative and a source of

proteins and healthy oils. According to FAO, aquaculture production is already

reaching almost 50% of the total fish production for human consumption, including

marine and freshwater species. Some even say that the future of fish production lies

with aquaculture.


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Aquaculture practices are quickly developing. But they raise many concerns too. The

impact of aquaculture facilities and infrastructure may affect the local fauna and flora

negatively, including threatened species. The effluents from aquaculture farms

containing undesired chemicals (e.g. from antifouling products) and therapeutants

might distress the local ecosystem. Farm escaped organisms can also have an impact.The use of exotic species in aquaculture is even more important, as they bring some

risks such as the introduction of associated forms of life that come together with

them (e.g. algae or microorganisms) or new pathogen agents that can spread out to a

new environment. The source of food for cultivated fish, which normally consists of

fish meal and fish oil, is another question to consider, as these primary products are

made from small pelagic fishes whose origin might not be sustainable and even

increase the already exaggerated pressure on existing fisheries.

In their natal streams, Atlantic salmon are considered a prized recreational fish,

pursued by avid fly anglers during its annual runs. At one time, the species supported

an important commercial fishery and a supplemental food fishery. However, the wild

Atlantic salmon fishery is commercially dead after extensive habitat damage and

overfishing, wild fish make up only 0.5% of the Atlantic salmon available in world

fish markets

Atlantic salmon is, by far, the species most often chosen for farming. It is easy to

handle, it grows well in sea cages, commands a high market value and it adapts well

to being farmed away from its native habitats.


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Chapter Three

3.1 Wind Energy Harnessing (Turbine design)

The selection of a wind turbine suitable for an offshore floating system raises some

basic design issues. It is reasonable to assume that to justify all the balance of plant

costs beyond the direct costs of the wind turbine support structures themselves, large

units must be used. Evidently a minimum tower height and associated machine size

will also be dictated by wave heights and hydrostatic stability considerations. Rather

than simple marinisation of land-based versions, the turbine should be specifically

designed to exploit the benefits of offshore locations some of which are:

Higher wind speeds

No Noise Pollution - Wind Turbines emit a slight whirring noise, which hasled to problems with people living nearby. Some farmers have also

complained that the livestock like sheep get affected by the moving of the

Wind Blades. Offshore Wind Farms are located far off the coast cause no

such noise problems for humans or wildlife

No Injuries to Birds Older Wind Farms on Land frequently cause deathsand injuries to birds though newer wind turbines dont cause too much

problems. Offshore Wind Farms do away with this problem entirely as they

are located in the Ocean where birds dont fly frequently if at all.

No loss in scenery though near shore offshore wind farms have come intocontroversy because of this, the Cape Wind Project is attracting a lot of

protests.

3.2 Design specifications

The design specification for a wind-turbine will contain a power curve and

guaranteed availability. With the data from the wind resource assessment it is

possible to calculate commercial viability. The typical operating temperature range is

-20 to 40 C (-4 to 104 F). In areas with extreme climate (like North sea) specific

cold and hot weather versions are required. Wind turbines can be designed and

validated according to IEC 61400 standards. Special considerations have to be given


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to the following for an effective design i.e. temperature, aerodynamics, power

control, turbine size, generator, blade design, tower, foundation etc.

3.2.1 TemperatureUtility-scale wind turbine generators have minimum temperature operating limits

which apply in areas that experience temperatures below 20 C. Wind turbines must

be protected from ice accumulation, which can make anemometer readings

inaccurate and which can cause high structure loads and damage. Some turbine

manufacturers offer low-temperature packages at a few percent extra cost, which

include internal heaters, different lubricants, and different alloys for structural

elements. If the low-temperature interval is combined with a low-wind condition, thewind turbine will require an external supply of power, equivalent to a few percent of

its rated power, for internal heating. For example, the St. Leon, Manitoba project has

a total rating of 99 MW and is estimated to need up to 3 MW (around 3% of

capacity) of station service power a few days a year for temperatures down to 30

C. This factor affects the economics of wind turbine operation in cold climates.

3.2.2 Aerodynamics

The aerodynamics of a horizontal-axis wind turbine is not straightforward. The

airflow at the blades is not the same as the airflow far away from the turbine. The

very nature of the way in which energy is extracted from the air also causes air to be

deflected by the turbine. In addition the aerodynamics of a wind turbine at the rotor

surface exhibit phenomena that are rarely seen in other aerodynamic fields.

3.2.3 Power control

A wind turbine is designed to produce a maximum of power at wide spectrum of

wind speeds. All wind turbines are designed for a maximum wind speed, called the

survival speed, above which they do not survive. The survival speed of commercial

wind turbines is in the range of 40 m/s (144 km/h, 89 MPH) to 72 m/s (259 km/h,


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161 MPH). The most common survival speed is 60 m/s (216 km/h, 134 MPH). The

wind turbines have three modes of operation:

Below rated wind speed operation Around rated wind speed operation (usually at nameplate capacity) Above rated wind speed operation

If the rated wind speed is exceeded the power has to be limited. There are various

ways to achieve this, some of which are stall, pitch control, yawing, electrical

breaking, mechanical breaking etc.

3.2.4 Turbine sizeFor a given survivable wind speed, the mass of a turbine is approximately

proportional to the cube of its blade-length. Wind power intercepted by the turbine is

proportional to the square of its blade-length. The maximum blade-length of a

turbine is limited by both the strength and stiffness of its material. Labour and

maintenance costs increase only gradually with increasing turbine size, so to

minimize costs, wind farm turbines are basically limited by the strength of materials,

and siting requirements.

Typical modern wind turbines have diameters of 40 to 90 metres (130 to 300 ft) and

are rated between 500 kW and 2 MW. As of 2010 the most powerful turbine is rated

at 7 MW.

3.2.5 Generator

For large, commercial size horizontal-axis wind turbines, the generator is mounted ina nacelle at the top of a tower, behind the hub of the turbine rotor. Typically wind

turbines generate electricity through asynchronous machines that are directly

connected with the electricity grid. Usually the rotational speed of the wind turbine is

slower than the equivalent rotation speed of the electrical network - typical rotation

speeds for a wind generators are 5-20 rpm while a directly connected machine will

have an electrical speed between 750-3600 rpm. Therefore, a gearbox is inserted


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between the rotor hub and the generator. This also reduces the generator cost and

weight. Commercial size generators have a rotor carrying a field winding so that a

rotating magnetic field is produced inside a set of windings called the stator. While

the rotating field winding consumes a fraction of a percent of the generator output,

adjustment of the field current allows good control over the generator output voltage.Older style wind generators rotate at a constant speed, to match power line

frequency, which allowed the use of less costly induction generators. Newer wind

turbines often turn at whatever speed generates electricity most efficiently. This can

be solved using multiple technologies such as doubly fed induction generators or

full-effect converters where the variable frequency current produced is converted to

DC and then back to AC, matching the line frequency and voltage. Although such

alternatives require costly equipment and cause power loss, the turbine can capture asignificantly larger fraction of the wind energy. In some cases, especially when

turbines are sited offshore, the DC energy will be transmitted from the turbine to a

central (onshore) inverter for connection to the grid.

3.2.6 Blade design

The ratio between the speed of the blade tips and the speed of the wind is called tip

speed ratio. High efficiency 3-blade-turbines have tip speed/wind speed ratios of 6 to

7. Modern wind turbines are designed to spin at varying speeds (a consequence of

their generator design as explained above. Use of aluminium and composite

materials in their blades has contributed to low rotational inertia, which means that

newer wind turbines can accelerate quickly if the winds pick up, keeping the tip

speed ratio more nearly constant. Operating closer to their optimal tip speed ratio

during energetic gusts of wind allows wind turbines to improve energy capture from

sudden gusts that are typical in urban settings. In contrast, older style wind turbines

were designed with heavier steel blades, which have higher inertia, and rotated at

speeds governed by the AC frequency of the power lines. The high inertia buffered

the changes in rotation speed and thus made power output more stable. The speed

and torque at which a wind turbine rotates must be controlled for several reasons:

To optimize the aerodynamic efficiency of the rotor in light winds.


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To keep the generator within its speed and torque limits. To keep the rotor and hub within their centrifugal force limits.

(The centrifugal force from the spinning rotors increases as the square of the

rotation speed, which makes this structure sensitive to overspeed).

To keep the rotor and tower within their strength limits.(The power of the wind increases as the cube of the wind speed, turbines

have to be built to survive much higher wind loads (such as gusts of wind)

than those from which they can practically generate power. Since the blades

generate more torsional and vertical forces (putting far greater stress on the

tower and nacelle due to the tendency of the rotor to precess and nutate) when

they are producing torque, most wind turbines have ways of reducing torque

in high winds). To enable maintenance.

(Since it is dangerous to have people working on a wind turbine while it is

active, it is sometimes necessary to bring a turbine to a full stop).

To reduce noise.As a rule of thumb, the noise from a wind turbine increases with the fifth

power of the relative wind speed (as seen from the moving tip of the blades).

In noise-sensitive environments, the tip speed can be limited toapproximately 60 m/s (200 ft/s).

3.2.7 Blade count

The determination of the number of blades involves design considerations of

aerodynamic efficiency, component costs, system reliability, and aesthetics. Noise

emissions are affected by the location of the blades upwind or downwind of the

tower and the speed of the rotor. Given that the noise emissions from the blades'

trailing edges and tips vary by the 5th power of blade speed, a small increase in tip

speed can make a large difference. Aerodynamic efficiency increases with number of

blades but with diminishing return. Increasing the number of blades from one to two

yields a six percent increase in aerodynamic efficiency, whereas increasing the blade

count from two to three yields only an additional three percent in efficiency. Further


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increasing the blade count yields minimal improvements in aerodynamic efficiency

and sacrifices too much in blade stiffness as the blades become thinner.

Component costs that are affected by blade count are primarily for materials and

manufacturing of the turbine rotor and drive train. Generally, the fewer the number

of blades, the lower the material and manufacturing costs will be. In addition, the

fewer the number of blades, the higher the rotational speed can be. This is because of

blade stiffness requirements to avoid interference with the tower limit on how thin

the blades can be manufactured, but only for upwind machines; deflection of blades

in a downwind machine results in increased tower clearance. Fewer blades with

higher rotational speeds reduce peak torques in the drive train, resulting in lower

gearbox and generator costs. System reliability is affected by blade count primarily

through the dynamic loading of the rotor into the drive train and tower systems.

While aligning the wind turbine to changes in wind direction (yawing), each blade

experiences a cyclic load at its root end depending on blade position. This is true of

one, two, three blades or more. However, these cyclic loads when combined together

at the drive train shaft are symmetrically balanced for three blades, yielding

smoother operation during turbine yaw. Turbines with one or two blades can use a

pivoting teetered hub to also nearly eliminate the cyclic loads into the drive shaft and

system during yawing.

Finally, aesthetics can be considered a factor in that some people find that the three-

bladed rotor is more pleasing to look at than a one- or two-bladed rotor.

3.2.8 Blade Materials and Design Considerations

New generation wind turbine designs are pushing power generation from the single

megawatt range to upwards of 10 megawatts. The common trends of these larger

capacity designs are bigger wind turbine blades. Covering a larger area effectively

increases the tip-speed ratio of a turbine at a given wind speed, thus increasing the

energy extraction capability of a turbine system. Current production wind turbine

blades are manufactured as large as 100 meters in diameter with prototypes in the

range of 110 to 120 meters. New materials and manufacturing methods provide the


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opportunity to improve wind turbine efficiency by allowing for larger, stronger

blades. One of the most important goals when designing larger blade systems is to

keep blade weight under control. Since blade mass scales as the cube of the turbine

radius, loading due to gravity becomes a constraining design factor for systems with

larger blades. Epoxy-based composites are of greatest interest to wind turbinemanufacturers because they deliver a key combination of environmental, production,

and cost advantages over other resin systems. Epoxies also improve wind turbine

blade composite manufacture by allowing for shorter cure cycles, increased

durability, and improved surface finish. Carbon fiber-reinforced load-bearing spars

have recently been identified as a cost-effective means for reducing weight and

increasing stiffness. The use of carbon fibers in 60 meter turbine blades is estimated

to result in a 38% reduction in total blade mass and a 14% decrease in cost ascompared to a 100% fiberglass design.

Smaller blades can be made from light metals such as aluminium. Wood and canvas

sails were originally used on early windmills due to their low price, availability, and

ease of manufacture. These materials, however, require frequent maintenance during

their lifetime. Also, wood/canvas constructions have design constraints that limit the

airfoil shape to that of a flat plate, which has a relatively high ratio of drag (low

aerodynamic efficiency) force captured when compared to solid airfoil designs.

Construction of solid airfoil designs is possible only through use of inflexible

materials such as metals or composites.

3.2.9 Tower

Typically, 2 types of towers exist i.e.. Floating towers and land-based towers. Wind

velocities increase at higher altitudes due to surface aerodynamic drag (by land orwater surfaces) and the viscosity of the air. The variation in velocity with altitude,

called wind shear, is most dramatic near the surface. Typically, in daytime the

variation follows the wind profile power law, which predicts that wind speed rises

proportionally to the seventh root of altitude. Doubling the altitude of a turbine, then,

increases the expected wind speeds by 10% and the expected power by 34%. To

avoid buckling, doubling the tower height generally requires doubling the diameter


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of the tower as well, increasing the amount of material by a factor of at least four. At

nighttime, or when the atmosphere becomes stable, wind speed close to the ground

usually subsides whereas at turbine hub altitude it does not decrease that much or

may even increase. As a result the wind speed is higher and a turbine will produce

more power than expected from the 1/7 power law, doubling the altitude mayincrease wind speed by 20% to 60%.

For HAWTs, tower heights approximately two to three times the blade length have

been found to balance material costs of the tower against better utilisation of the

more expensive active components.

3.2.10 Foundations

Wind turbines, by their nature, are very tall slender structures,[12] this can cause a

number of issues when the structural design of the foundations are considered.

The foundations for a conventional engineering structure are designed mainly to

transfer the vertical load (dead weight) to the ground, this generally allows for a

comparatively unsophisticated arrangement to be used. However in the case of wind

turbines, due to the high wind and environmental loads experienced there is a

significant horizontal dynamic load that needs to be appropriately restrained. This

loading regime causes large moment loads to be applied to the foundations of a wind

turbine. As a result, considerable attention needs to be given when designing the

footings to ensure that the turbines are sufficiently restrained to operate efficiently.

In the current DNV guidelines for the design of wind turbines the angular deflection

of the foundations are limited to 0.5, DNV guidelines regarding earthquakes suggest

that horizontal loads are larger than vertical loads for offshore wind turbines, while

guidelines for tsunamis only suggest designing for maximum sea waves.

3.3 Floating Wind Turbine Concepts

In view of the dynamic nature of the float structure, it is recommended that extreme

and fatigue analyses should generally be based on the spectral approach, using the


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JONSWAP wave spectrum. As a result of this a correlation formula for wind speed

with mean significant wave height, and mean wave period with wave height given as

should be used as a design condition throughout the operating range.

Harnessing much of the vast offshore wind resource potential requires the

installation of wind turbines in deeper water. Numerous floating support-platform

configurations are possible for use with offshore wind turbines. Current proposals

based on the variety of mooring systems, tanks and ballast options used on platforms

by the offshore oil and gas industry may be classified in terms of how they achievebasic static stability in the platforms pitch and roll. The three primary concepts are

the tension leg platform (TLP), spar buoy and barge respectively, as shown in Figure

6 below. These platforms provide stability primarily through the mooring system

combined with excess buoyancy in the platform: either a deep draft combined with

ballast or a shallow draft combined with water plane area. Hybrid concepts that use

features from more than one class, such as semi-submersibles, are also a possibility.

Figure 7: Floating Wind Turbine Concepts (Courtesy of Institute for

Wind Energy)


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In the case of floating structures, the offshore wind industry faces many new design

challenges. Because the offshore oil and gas industries have demonstrated the long-

term survivability of offshore floating structures, the technical feasibility of

developing offshore floating wind turbines is not in question. However, developing

cost-effective offshore floating wind turbine designs that are capable of penetratingthe competitive energy marketplace will require considerable thought and analysis.

Simply adapting offshore oil and gas technology directly to the offshore wind

industry is not economically feasible.

The feasible design should trade-off the pros and cons of each of these approaches in

an attempt to reach the lowest cost system design. Table 1 gives a list of proposed

design challenge parameters that would impact the performance and cost of a

floating wind turbine system.


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Table 1: Design Challenge Trade-offs for Stability Criteria

Floating Platform Technical Challenges

Platform stability considerations

Platform Design ChallengesBuoyancy

(Barge)

Mooring Line

(TLP)

Ballast

(Spar)

Design Tools and Methods - + -

Buoyancy Tank cost/complexity - + _

Mooring Line system cost/complexity - + _

Anchors cost/complexity + _ +

Load Out Cost/Complexity (Potential) + -

Onsite Installation Simplicity (Potential) + - +

Decommissioning & Maintainability + - +

Corrosion Resistance _ + +

Depth Independence + _ _

Sensitivity to Bottom Condition + _ +

Minimum Footprint _ + _

Wave Sensitivity _ + +

Impact of Stability Class on Turbine Design

Turbine Weight + _ _

Tower Top Motion _ + _

Controls Complexity _ + _

Maximum Healing Angle _ + _

Key: + = Relative advantage

- = Relative disadvantage

Blank = Neutral advantage


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Each design challenge is evaluated for the three methods of achieving stability using

a simple method of plus (+) and minus () symbols. The plus and minus symbols

indicate ease with which each challenge might be overcome for each class.

Turbine design is impacted by the choice of platform. Therefore, it must be included

in the table of challenge trade-offs. The TLP is likely to provide the most stable

platform and thus have the least impact on the turbine dynamics. A ballast-

dominated design such as a buoy is likely to be heavier and therefore more expensive

to build. The barge is likely to be subject to higher wave loading, which will

increase the systems response (motions) to waves. Therefore, a turbine design that is

tolerant of larger tower motions is needed. Turbines can be designed to tolerate

larger motions but likely at a high cost.


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Chapter Four

4.1 Aquaculture Cage systems (offshore)

A range of cage systems is now potentially available for offshore mariculture in the

Mediterranean, though not all of these may prove to be effective in the intended

environmental conditions and production regimes. Development of offshore cages

has run approximately in parallel with that of inshore cages. Such designs have

originated from a variety of sources, including dedicated research teams, existing

cage manufacturers, net manufacturers, naval architects, ship builders, and offshore

oil hose manufacturers. Not many have involved specific inputs from fish farmers,

and as a result the cage types on offer are generally expensive and may suffer from

deficiencies of one kind or another when it comes to holding and managing fish

stocks.

4.2 General approaches to offshore cage design

While considering inshore cage development, the variety of cage designs has arisen

out of attempts to deal with a number of (at times conflicting) design objectives

trying to solve or eliminate the problems below in existing designs.

Providing a reasonably stable cage shape, to avoid stressing the stock, and toprovide a stable working environment.

Providing adequate water exchange to satisfy metabolic requirements ofstock and remove wastes from the cage area.

Absorbing or deflecting environmental forces, to maintain the structuralsoundness of the system.

Providing an efficient working environment, for routine husbandry, andwhere equipment and materials (harvested fish, feed, tanks and bins, etc.) can

be handled if necessary.

Maintaining position, to provide a secure location, free from navigationhazards, etc.

Keeping capital and operating costs as low as possible


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4.3 Floating Cages

4.3.1 Floating flexible cages

These types utilize rubber hoses originally designed for transferring oil between oil

tankers and onshore terminals. The primary commercial systems are those produced

by Bridgestone and Dunlop.

Table 3: Advantages and Disadvantages of Floating Flexible Cages.

Advantages Disadvantages

Highly resilient to wave forces with long service

life (>10 years); relatively good impact resistance

Stanchions may cause problems twisting,

turning

Effective and proven net hanging system Relatively expensive at lower volumes

Variety of configurations possible Limited walkway access

Relatively cheap at higher volumes Top net and feed systems difficult to place

Most widely used commercial offshore system Large service vessels necessary

Figure 8: Floating Cage Nets (Courtesy of aquaculture.com)


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The cage collar is essentially utilized to maintain the shape of the net, and is not

designed for working operations, which are all carried out from rafts or boats. The

most important feature of the cage is the interface between the collar and the net, as

this is where most of the stress is transferred between the two

4.3.2 Floating rigid cages

Floating rigid cages take a quite different design route from that used for floating

flexible cages. Rather than attempting to be wave compliant, these aim to be robust

enough structurally to withstand wave action, and are generally of large, massive

structure, normally of steel construction, with varying degrees of ballasting,

sometimes with mass concrete. In addition, most types also attempt to build in avariety of features to facilitate management of the fish, such as feeding systems,

harvest cranes, fuel stores and power generation, staff quarters, etc. Some systems

are also self-propelling. As a result they are typically the most expensive type of

offshore system, although this extra cost has to be weighed up against the additional

facilities that would also have to be provided for a floating flexible system.

Figure 9: Open Ocean Aquaculture or Offshore Aquaculture (Courtesy of

care2.com)


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Table 4: Advantages and disadvantages of Floating Rigid Cages.


Stable working platform for all husbandry and

management operations.

Large and heavy structures require good port

facilities and/or expensive towing to install

Relatively high capital cost. Steel structures

require protection or maintenance.

Potential for integral feeding and harvesting

systems; may be used to service other cages

May be susceptible to structural failure in

extreme conditions

Ship mortgages may be available Large mass may require heavier mooring

systems

Potentially improved operator safety and efficiency Relatively high capital cost; steel structures

require protection/maintenance

Potentially cost effective, especially in larger sizes Limited commercial track record

Construction and repair facilities may be developed

from conventional shipyard

4.4 Semi-submersible cages

This group of cage designs can be characterized by their ability to be submerged for

periods of time below the higher energy regimes of surface waters. As such they

offer the potential advantages of being lighter and simpler structures, as if submerged

appropriately during poor sea conditions, they would incur far less exposure and

hence physical stress. The reduced movement could also potentially reduce possible

damage to stocks, or motion stress. The overall consequence could be simpler, safer

and less expensive production systems. However, the deployment of these systems in

two modes, surface and sub-surface, and the need to control these effectively and at

the right times, adds potential complexity and risk. As with floating systems, there

are two structural classes, flexible and rigid, with similar design consequences.


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4.4.1 Semisubmersible flexible cages

The Refa cage (Fig. 1) is a tension leg design in which a positive circular plastic

positive buoyancy-supporting frame, held below instead of above the net pen, is held

in place by vertical mooring ropes. Mooring ropes stem from concrete blocks on the

seabed, which rise to the buoyancy ring, above which the net is kept in suspension bysubsurface buoys. An upper conical section gives access from the surface via a

traditional plastic cage collar. The cage is available in a variety of sizes up to

10,000m3. The design is simple and there are no metal structural components. The

upper cone can be removed and the cage raised for harvesting and net changing, etc.,

utilizing a full size plastic collar brought temporarily to the site. In storms or strong

currents, the cage responds automatically, the net being pulled under the water and

thus escaping the worst effects. Table 5 summarizes comparative features (see alsoLisac, this volume).

Table 5: Advantages and disadvantages of Semi-submersible flexible Cages.


Simple designautomatic response Feeding should ideally be done

subsurface, requiring separate feeding

systems, due to limited area on surface

Relatively cost effective Moorings typically concrete blocks, more

difficult to install than conventional

anchor

Small bottom area occupied by moorings

Combines features of conventional

operation with storm protection

Volume reduction no greater than 25% in

currents/storms


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Figure 10: Graphic image of Subsea Aquaculture Cage (Courtesy Cage

Aquaculture)

Figure 11: Composure of Farmocean Facilities (Courtes y of

www.farmocean.se)


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Figure 12: A photo-illustration composite image of an Aquapod fish -

farming cage. (Courtesy of www.Nationalgeographic.com).

4.4.2 Semisubmersible rigid cages

These systems are designed with rigid framework elements providing only limited

movement or volume change in response to external loads. Normally with steelframe structures, these contain adjustable buoyancy elements to raise or lower the

system. With a more rigid structure it may also possible to add service facilities such

as feeders, potentially developing self-contained systems. Primary examples of these

cage types include the Farmocean cage. The design is arranged so that the feeder and

gangway in the upper part of the system remains above the surface at all times, but

that the largest part of the cage's volume is submerged, and exposed surfaces in the

upper water column are minimized. The steel umbrella frame can be deballasted by

compressed air to bring the main structure, the lower pontoon ring, and the lower net

to the surface, for cleaning, maintenance and stock handling. A walkway is mounted

over the main pontoon ring to allow for easier access. Sacrificial anodes are attached

to lower and upper legs to minimize corrosion. Feeding is computer controlled and

temperature linked, and allows for several days feeding if access is denied due to bad

weather. A wave sensor can shut off the system if conditions become too bad.
http://news.nationalgeographic.com/news/2009/08/photogalleries/future-fish-farms-pictures/photo2.htmlhttp://news.nationalgeographic.com/news/2009/08/photogalleries/future-fish-farms-pictures/photo2.html


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Table 6: Advantages and disadvantages of Semi-submersible Rigid Cages.


Now tried and tested over 12 years in a

variety of situations and in severeconditions

High capital cost

Proven long service life Poor access for harvesting

Integrated feeding system Difficult to change/clean nets

Stable holding volume Limited surface area when submerged for

surface feeding

Good stock performance Complex steel structure; needs corrosion

protection, regular maintenance

4.5 Submersible rigid cages

For true oceanic farming of fish, where wave heights may be considerable, it is may

be proposed that the only way to avoid the worst effects of severe surface conditions

is by using fully submersible cages, whose normal operating conditions would be at a

suitable depth below the more hazardous upper water column. The presence of ice in

winter has also led developments in this direction. As required, the systems could be

raised to the surface for necessary management functions. These systems could either

be unattended by surface units, accessed only when needed, or attached by various

systems to conventional vessels.


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Table 7: Advantages and disadvantages of Submersible Rigid Cages.


Submersible designs avoid surface debris

and ice, and passing vessels

Lack of visibility in normal state

Minimal visual impact Methods of maintenance and servicing of

cages whilst submerged are still in

development

Avoids fully the effects of storms Costs relatively high

Structural strength does not need to be as

great as a surface structure

Relatively complex to operate


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Chapter Five

5.1 Introduction to Mooring Lines

For offshore floating structures, which are exposed to environmental conditions,

station keeping above a fixed point is an important requirement. To resist the

resulting external forces these structures are, in the majority of cases, anchored to the

sea bottom. It will be clear that a proper design of the anchoring system is of great

importance. Inaccuracies in the design of the system and in the estimation of line

tensions can have serious consequences. In analysing forces and tensions in anchor

lines, static and dynamic part can be discerned. The static part covers the calculation

based upon the classic catenary theory, a method that is generally applied in the

design of systems capable to balance the external forces. This procedure yields

reliable results. Also field tests have indicated that the results of such static

calculations conform very well to the measurements, which implies that cable

dynamics have a negligible contribution to the total forces. The approximation of

anchor line forces based on statics is generally valid for lines attached to structures,

whose motions come close to the static case. To meet this condition it is essential for

the structure to have large natural periods of motion and small amplitude responses

in the frequency range of wave components. For certain structures, however, the

behaviour in waves is much different. Owing to small natural periods the relatively

high frequencies of the wave components cause large amplitudes of motion.

5.2 Design Calculations/Mathematical Modelling of Mooring Lines (Static Analysis)

For the Mooring lines, this will consist of line (chain or wire) anchors at the lower

ends and floating bodies at the upper ends. Often the line is de-coupled from the

dynamics of anchors and floating bodies and is considered alone. The response to

excitation at the upper end at the wave frequency enables prediction of motion and

stresses along the line as well as conditions for resonance.


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Stress levels are very important in selecting the type and size of mooring components

to be used. Resonance as well as large displacements can cause severe peak stresses.

Catenary equations govern the components of a mooring line such as length,

y

0

(l, 0) x

T(s)

T(s)

Figure 13: Inelastic Hanging Cable

Consider the figure above

The vertical and horizontal equilibrium of an isolated element of the cable requires

that,

{ } { }

The second equation is integrated


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Where H is the horizontal component of the cable tension. The horizontal component

is constant along the cable length as no other horizontal loading is acting.

The first equation above can be rewritten as follows

{ } { }

The geometric constrain becomes

{}Hence we have

{}

Let , we have

Integrating both sides we have the following


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Further integration gives

Where c1 and c2 are constants to be determined from the boundary conditions.

The boundary conditions are

Y = 0 when x = 0

Y = 0 when x = l

From the boundary conditions, we have

{ }

The solution is therefore

{{ }{ }}In the above expression, H is still unknown.

To find H, we need to know the cable length between the two points. It is intuitively

easy to understand that H must be related to the cable length, i.e. taut or slack cables

and this presents an important question to solve for the wind turbine and aquaculture

design.

The length of the cable along the line is given by


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{}

{}

{{ }}

{ }

{{ } { }}At the right end (x = l), s = L (L is the length of the cable between the two end

points), hence

{ }H (horizontal component of tension) is obtained from solving the above equation.

L > l if the mooring line is inelastic as assumed above. The equation above can be

solved either graphically or analytically.

The above analysis can also be extended to a situation where the two end points are

not level and the equation forms the basis of some quasi-static offshore mooring line

analysis, however this requires taking into account the following factors:

Steady current loading on mooring lines and wave loading in the dynamicanalysis.

Mooring lines are not necessarily uniform from the top to the bottom end. Stretching of the mooring line as it is elastic. Mid-water buoys are employed in some configurations. Seabed friction.

5.3 Dynamic Analysis of mooring lines

The mooring line is represented by a set of discrete masses connected to each other

by massless springs. The hydrodynamic forces are only applied at the mass nodes.


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As a result a set of second order differential equations will be derived which are then

integrated in time domain using a variety of numerical methods.

To explain the essence of this derivation (lumped mass spring method), let us

consider a vertically tensioned mooring line. Let us also assume that the mooring line

has a mass distribution of m, axial stiffness EA, length L. This can be considered as a

model TLP mooring lines (which is the major assumption this report is making for

the wind turbine design)

Figure 14: Expression of Mass spring constants for Analysis

The masses m1 and m2, and spring constants k1, k2 and k3, are therefore given by

{ } { }

L1

m1

m2

L2

L3

A sinwt

X

X

Seabed


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Let x1 and x2 represent the displacements of m1 and m2, and we assume that at the top

end of the mooring line a forced vertical motion a sinwt is applied. Hence the second

order differential equations of motion for x1 and x2, i.e.

In the above modelling, linearised fluid drag forces are assumed and mooring lineinternal structural damping is ignored.

Having modelled the problem, the system natural frequencies are ascertained and this

is done by studying the non damping and external excitation problem defined by the

equations below

The characteristic equation for the natural frequency is then shown as

[ ]

which gives two positive real roots W1 and W2:


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{ } ( )

{ } ( )

5.4 Mooring line response

For the long-term steady state response, we can assume the following solutions

It is reasonable to assume the above solution forms, as after the transient motions the

system will move at the excitation frequency and there will be phase angles due to

the presence of damping.

In the above equations, A1, A2,

and

can be found by substituting the solution

expressions into the governing equations. Further, we can calculate the maximum

mooring line tension that is one of the key design parameters.

The solution in detail is shown thus:

Substituting the above into the equation of motions,


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The following equations are then obtained

i.e.

There are four unknowns in this set of equations i.e. A1, A2, and . These

unknowns cannot be solved analytically hence numerical solutions are obtained.

Also the maximum dynamic tension in the mooring line can be found out by

calculating the maximum extension of L1, L2 and L3.


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Note that in reality mooring lines are seldom straight, drag forces are non-linear and

the solution procedure requires digital computations.

Chapter Six

6.1 Reasons and Advantages of Combining both Aquaculture and Offshore (Wind

Energy)

In the case of offshore wind farming and open ocean aquaculture, approaching new

activities jointly holds the potential for future cost-benefits, e.g. through the

combination and joint use of existing structures under the viewpoint of

What need has the marine aquaculture? or what can the wind farm operators

offer?

Here, a programme of alliances for multifunctional cooperation can be suggested.

Potentials of such alliances and how close cooperation could be implemented are

multifaceted for our case study.

The following options are proposed as likely reasons:

A. Maintenance of wind turbines and aquaculture facilities.B. Training and capacitation.C. Technological multi-use of fixed structures,.D. Environmental impact assessments.E. Maritime traffic.F. Transport and supply andG.

Economic cooperation.

A. Maintenance of wind turbines and aquaculture facilities


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Existing fishing vessels could be made available as servicing craft for wind turbines.

Additionally, the harvest of mussels and seaweed by operation vessels of the wind

farm operators, which are specifically designed to fulfil both purposes, could be a

multi-use approach.

B. Training and capacitationIn order to take advantage of the existing pool of local knowledge on the

environmental conditions of the North Sea, local fishermen, who are well familiar

with the natural offshore conditions and thus require less training, could be

employed. Next to the maintenance of the local knowledge, local economy and

alternative livelihood is promoted. Due to the continuing decline in commercial

fisheries in the long-term perspective, the training of local personnel holds the futurepotential for the establishment of their own aquaculture enterprises. By their

involvement and capacitation from the very early stage, spatial conflicts between the

fisher associations and the wind farms could be minimised.

C. Technological multi-use of fixed structuresThe technological development for detached aquaculture offshore structures is far

from being satisfactorily solved. The underwater constructions of wind turbines thus

offer themselves as a cost-effective, alternative solution to fix cages, long lines or

offshore-rings and to provide some storage for the maintenance of aquaculture

facilities. This prevents the loss of expensive culture material caused by strong

currents, heavy weather and shifting of anchor stones. Fig. 3 suggests some potential

multifunctional constructions.

D. Environmental impact assessments and ecological aspectBy German and EU law,

Documents

Afolaranmi Amodu MSc Project