On- and Offshore Wind Turbines. Technology and Design


Master's Thesis, 2019

95 Pages


Free online reading

Contents

Abstract

Acknowledgement

Dedication

CHAPTER 1 INTRODUCTION
1.1 BACKGROUND
1.2 PROBLEM STATEMENT
1.3 OBJECTIVES
1.4 METHODOLOGY
1.5 OUTCOME
1.6 ORGANIZATION OF CHAPTERS
1.6.1 Chapter
1.6.2 Chapter
1.6.3 Chapter
1.6.4 Chapter
1.6.5 Chapter
1.6.6 Chapter

CHAPTER 2 WIND ENERGY
2.1 INTRODUCTION
2.2 WIND ENERGY IN THE WORLD
2.2.1 WIND A RENEWABLE ENERGY SOURCE
2.2.2 WIND ENERGY BASICS
2.2.3 WIND ENERGY EXTRACTION
2.2.4 GLOBAL EXPLOITATION OF RENEWABLE ENERGY SOURCES
2.2.5 WIND VERSUS SOLAR ENERGY
2.2.6 GLOBAL INTEREST IN WIND ENERGY
2.3 WIND ENERGY IN PAKISTAN
2.3.1 DEMAND AND SUPPLY
2.3.2 POWER GENERATION BY SOURCE
2.3.3 WIND POWER PRODUCTION IN PAKISTAN
2.3.4 FUTURE OF WIND POWER IN PAKISTAN

CHAPTER 3 WIND TURBINE
3.1 INTRODUCTION
3.2 WIND TURBINE
3.2.1 WIND SPEED
3.2.2 ROTOR SWEPT AREA
3.3 TYPES OF WIND TURBINE
3.3.1 LOCATION OF IMPLEMENTATION
3.3.2 ROTATION AXIS OF TURBINE
3.3.3 TYPES OF TOWER USED
3.4 COMPONENTS OF WIND TURBINE
3.4.1 NACELLE
3.4.2 ROTOR
3.4.3 TOWER
3.4.4 TRANSITION PIECE
3.4.5 FOUNDATION
3.5 TYPES OF FOUNDATIONS FOR OFFSHORE WIND TURBINE
3.5.1 GRAVITY BASED FOUNDATION
3.5.2 TRIPOD / TRIPILE FOUNDATIONS
3.5.3 JACKETS FOUNDATIONS
3.5.4 FLOATING FOUNDATIONS
3.5.5 MONOPILE
3.6 INTERNATIONAL COMPANIES
3.7 COMPANIES QUOTATIONS
3.7.1 SKYWIND, GERMANY
3.7.2 RIGHT RENEWABLE TEK, INDIA
3.7.3 VAIGUNTH, INDIA
3.7.4 AEOLOS, DENMARK
3.7.5 HUMMER, CHINA
3.7.6 COMPARISON
3.8 GRID CONNECTION
3.8.1 On-Grid WIND TURBINES
3.8.2 Off-Grid WIND TURBINES
3.9 TRANSPORTATION OF COMPONENTS

CHAPTER 4 METHODOLOGY
4.1 INTRODUCTION
4.2 LIMIT STATES
4.3 DESIGN LOADS
4.3.1 PERMANENT LOADS
4.3.2 VARIABLE FUNCTIONAL LOADS
4.3.3 ENVIRONMENTAL LOADS
4.3.4 ACCIDENTAL LOADS
4.3.5 DEFORMATION LOADS
4.3.6 DYNAMIC LOADS
4.4 QUANTIFICATION OF WIND LOADS
4.4.1 GENERAL CONSIDERATIONS
4.4.2 DATA COLLECTION
4.4.3 FROYA WIND PROFILE
4.4.4 WIND FORCES ON TOWER
4.4.5 WIND FORCES ON ROTOR
4.5 QUANTIFICATION OF WAVE LOADS
4.5.1 WAVE
4.5.2 WAVE CHARACTERISTICS
4.5.3 LINEAR AIRY WAVE THEORY
4.6 QUANTIFICATION OF CURRENT LOADS
4.6.1 OCEAN CURRENTS
4.6.2 TYPES OF CURRENTS
4.6.3 OCEAN CURRENT VELOCITY
4.7 TOTAL HYDRODYNAMIC FORCES
4.8 LOAD COMBINATIONS
4.9 FOUNDATION DESIGNS
4.9.1 P-Y Curve (Winkler Model)
4.9.2 P-Y Curve for Piles in Sand
4.10 DESIGN METHODOLOGY
4.10.1 SERVICEABILITY LIMIT STATE (SLS)
4.10.2 ULTIMATE LIMIT STATE (ULS)
4.10.3 LATERAL PILE CAPACITY
4.10.4 LATERAL CAPACITY OF SOIL
4.10.5 AXIAL CAPACITY OF SOIL

CHAPTER 5 CASE STUDY FOR KARACHI
5.1 INTRODUCTION
5.2 WIND DATA
5.3 WAVE DATA FOR FORCES
5.4 SUMMARY OF WIND, TIDE AND WAVE DATA
5.4.1 WIND DATA
5.4.2 WAVE DATA
5.4.3 TIDE DATA
5.5 WIND TURBINE MODEL
5.6 FROYA WIND PROFILE
5.7 WIND FORCES
5.7.1 WIND FORCES ON TOWER
5.7.2 WIND FORCES ON ROTOR
5.8 HYDRODYNAMIC LOADS
5.8.1 WATER PARTICLE VELOCITY AND ACCELERATION PROFILE
5.9 DESIGN
5.9.1 SERVICEABILITY CHECK
5.9.2 LOAD DISTRIBUTION ON SELECTED SECTION
5.9.3 SUPER STRUCTURE STRENGTH CHECK
5.10 SUB-STRUCTURE DESIGN CHECKS

CHAPTER 6 SUMMARY AND CONCLUSIONS
6.1 Conclusions
6.2 Calculations
6.3 Recommendations

References

Abstract

Wind power production is an emerging industry because of green power production and restriction placed on previous environmental hazardous power production technologies. Due to advancements in this field in the recent decades more energy production with smaller turbines has become possible. This aspect of wind turbines has encouraged the world to develop megawatt scale turbines. Bigger turbines that were constructed onshore brought their new problems with them like noise pollution and wind turbulence and requiring higher towers. Due to this the wind industry is rapidly shifting to installing offshore structures that give more uniform winds at shorter heights and has no noise pollution problems. That’s why the design of offshore wind turbine structure has been explained in this thesis.

This thesis also aims to carry out a study of the design of the foundation most frequently used in offshore wind structures, i.e. the monopile.

The design of this type of wind structures implemented at sea where, the wind action combined with the action of the waves and sea currents. The quantification of these actions mainly followed the DNV (Det Norske Veritas) standards, while using and referencing the contribution provided by other design standards. This thesis presents the design of hypothetical wind turbine supporting structure in Karachi (Arabian Sea), as well as the theoretic basis and calculations that were performed in order to obtain the loads and safety verifications for a specific design.

Acknowledgement

We are very thankful to our greatest Almighty Allah for providing us the stamina and the courage to complete our research work.

We would like to express our sincere gratitude to our research supervisor Prof. Dr. Irshad Ahmad for providing the opportunity to work under his supervision. We would especially like to acknowledge his encouragement, tolerance, guidance and technical support throughout the research work.

Dedication

We dedicated this work to our great teacher, parents and all those peoples who prayed for our achievements and success.

LIST OF FIGURES

Figure 1-1 Global Cumulative Installed Wind Capacity 2001-2017

Figure 2-1 Energy sources over time

Figure 2-2 Installed Global wind capacity 2017

Figure 2-3 Global installed wind capacity

Figure 2-4 Global installed wind capacity by region

Figure 2-5 Wind power production in Pakistan

Figure 2-6 Wind power projects in Pakistan

Figure 3-1 Panemone windmill

Figure 3-2 Wind turbine

Figure 3-3 Variation in wind speed with height

Figure 3-4 Rotor swept area

Figure 3-5 Onshore wind farm

Figure 3-6 Offshore wind farm

Figure 3-7 HAWT and VAWT

Figure 3-8 Types of tower

Figure 3-9 Components of wind turbine

Figure 3-10 Types of foundation

Figure 3-11 Gravity based foundation

Figure 3-12 Triple and Tripod foundation support

Figure 3-13 Components of tripod foundation

Figure 3-14 Jacket Foundation

Figure 3-15 Floating foundation

Figure 3-16 Distribution of offshore WT foundations (2014)

Figure 3-17 Monopile foundation

Figure 3-18 Installed (2016) vs Total capacity of International Companies

Figure 3-19 Wind speed vs Annual yield

Figure 3-20 On-Grid-connected System

Figure 3-21 Off-Grid-connected System

Figure 3-22 Transportation by Road

Figure 3-23 Transportation by boats

Figure 4-1 Design Process for a typical offshore wind turbine (Malhotra, 2007c)

Figure 4-2 Design Loads for Offshore Wind Turbine

Figure 4-3 Air Density and Specific Weight

Figure 4-4 Wind speed and pressure variation in an ideal wind turbine model

Figure 4-5 Most unfavorable condition of rotor

Figure 4-6 Forces on Stationary Rotor Blade

Figure 4-7 Wind Flow through turbine blades

Figure 4-8 Lift Force Mechanism

Figure 4-9 System of forces acting on the blade; b) Resulting lift and drag loads in the x-axis direction

Figure 4-10 CL and CD values for NACA N63-212

Figure 4-11 Wave period vs Wave length

Figure 4-12 Wave period vs Wave celerity (DNVGL-RP-C205)

Figure 4-13 Wave Parameters

Figure 4-14 Sinusoidal waveform in time domain

Figure 4-15 Wave forms according to Airy theory

Figure 4-16 Wake amplification factor as function of KC number for smooth (solid line) and rough (dotted line)

Figure 4-17 Hydrodynamic loads on a slender member

Figure 4-18 Flexible Vs Rigid Pile

Figure 4-19 Winkler model of the pile response to lateral loads

Figure 4-20 Variation of the factor As with normalized depth z/D

Figure 4-21 Variation of the factor Bs with normalized depth z/D

Figure 4-22 p-y curve shape for pile in sand under static loading (after Reese et al., 1974)

Figure 5-1 Annual wind speed data (taken at 61 meters height) histogram for the year 2002

Figure 5-2 wave heights in Arabian Sea

Figure 5-3 Wind Turbine Structure

Figure 5-4 Froya wind speed profile

Figure 5-5 Water particle velocity and acceleration profile

Figure 5-6 Tower Top Deflection vs Diameter vs Thickness

Figure 5-7 Hydrodynamic and Aerodynamic force profile

Figure 5-8 Depth vs Deflection, Shear and Moment

Figure 5-9 P-Y Curves

Figure 5-10 Design Details

Figure 5-11 Designed Sections Detail

LIST OF TABLES

Table 3-1 Wind Turbine Companies

Table 3-2 Skywind 1000W specifications

Table 3-3 Right Renewable Tek 3000W Wind Turbine

Table 3-4 Right Renewable Tek 20,000W Wind Turbine

Table 3-5 Vaigunth Wind Turbine Specifications

Table 3-6 Aeolos-H 500W Wind Turbine Specifications

Table 3-7 Aeolos-H 2000W Wind Turbine Specifications

Table 3-8 Aeolos 5kW Wind Turbine Specifications

Table 3-9 Aeolos 10kW Wind Turbine Specifications

Table 3-10 Aeolos 30kW Wind Turbine Specifications

Table 3-11 Hummer Wind Turbine Specification

Table 3-12 Quotation of different companies

Table 4-1 Load factors for the Ultimate Limit Stat

Table 4-2 kpy of the p-y curve for piles in sand above and below the water table

Table 4-3 Equations for y and P for different sections of P-Y curve

Table 5-1 Wave Data

Table 5-2 Case Study Wind Turbine Specifications

Table 5-3 Sea Bed Soil Profile

Table 5-4 Serviceability check sections

Table 5-5 Wind Turbine Profile

Table 5-6 Economical Sections

CHAPTER 1 INTRODUCTION

1.1 BACKGROUND

The world has been producing energy from coal and other burning fuels for more than a century. The sources used were harmful to environment and had shown adverse effects in the history. The use of them was easy and fast and the power produced was more with small components. This generation of electricity played a key role in the development of world industries. More and more industries and factories came into being and the world urbanized very fast within short time. The way the people lived changed. These all big changes were beautiful when looked from outside but in essence it drew the mankind away from the environment they have lived in for centuries. The change of everything in a very short time transferred its effect to the environment. The environment also became polluted in a short time and to such an extent that has never been reached.

The advancements in the world changed people’s lifestyle and urged them to advance further. The race between different developed countries to reach beyond their targets and the struggle of developing countries to copy these standards further made the situation worse.

In such a time if there had been need to produce more energy and harness new sources, the people would point out the nuclear power extraction. This will further pose new problems if got out of control. However, due to advancements in every field of science, some renewable energy sources were also introduced. The active forces for environment preservation struggled to point out the difference between good and bad sources of energy and were successful in some countries. The production of wind and solar power and the effects of climate change forced the people to opt for green energy. This energy started at local scales but with new technologies the powerful investors started to introduce this technology on large and commercial scale.

The wind energy got its populace due to large amount of energy produced from individual units. This energy could be used along with other sources or individually. Though the initial cost of projects were high but with time the researchers optimized the machines and the engineers found new and economic ways of its installation. The industry grew more and more due to being a feasible option and motive to produce green energy. The graph shows the trend in electricity generation through wind turbines in the recent years. The graph rises in an exponential fashion and the future of this industry is justified from it.

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Figure1-1 Global Cumulative Installed Wind Capacity 2001-2017 [1]

1.2 PROBLEM STATEMENT

The advancements of wind turbine in the power industry is prominent. The green energy produced with small units is not only easily accessible in remote areas but also have less effects on ecology. The old ways of power production like coal burning is harmful to our environment. Even if we talk about the clean energy coming from dams, they also have the potential to change the ecology and put many species in danger of extinction [3].

The fast-growing advancements in this field has grasped the attention of many researchers. These machines are being installed in the sea many kilometers away from the shore. Due to many investments and demand their sizes are getting bigger. These bigger turbines installed on the sides of valley, in open plains, on the top of hills and in the mid sea are exposed to bigger and intense forces due to large sizes and hence there supporting structures need to be properly designed. In order to design the supporting structure of turbines we have done some research work.

This thesis discusses different types of turbines and their foundation. This also shows the different companies involved in the business at small and large scales. The data required for the structure analysis and design and their conversion to forces is also discussed. At the end this thesis shows all calculations for a specific site in Pakistan.

1.3 OBJECTIVES

Pakistan is a developing nation and heavily spends on the development of conventional power plants to meet the national electricity demand. The national oil imports have been on rise at 3.8% per year since 1991. The total consumption of fossil fuels increased to 67 million tons of oil equivalent (TOE) in 2014 compared to 28.6 million TOES in 1990 [4]. The total natural gas and oil consumption accounted for 72%. In the meantime, the international oil prices increased up from US$23 barrel to US$50.05 barrel in 2001. This is almost 115% rise of oil price in a short span of time. Natural gas is a local abundant source of energy but started to decrease due to increased dependence upon it.

In order to take part in the stability of social, economic and environmental development we decided to take part in this race. Our objective was to design a monopile foundation for offshore wind turbine. This work let us study the preliminary knowledge and at the end the analysis and design of one type of foundation. But this let us explore many things that needs to be studied. These aspects that we discovered are added in the recommendation at the end of this thesis. So, this is an initiative. This thesis discusses the Karachi coastal area. This cites some data from the researchers that have worked on the Pakistan wind energy capacity. So being a work on Pakistan, this will be most helpful to encourage other people to research on this region. Slowly we will become self-sufficient in this industry and will be able to harness the huge wind energy present in this country.

1.4 METHODOLOGY

The design process involves an initial site selection followed by an assessment of external conditions, selection of wind turbine size, subsurface investigation, assessment of geohazards, foundation and support structure selection, developing design load cases, and performing geotechnical and structural analyses.

In this thesis the tower and monopile of offshore wind turbine is designed by trial and error method for serviceability limit state using SAP 2000 and ALLPILE and then checked for ultimate limit state, sub soil and pile checks are also applied to confirm the safety of the structure.

1.5 OUTCOME

The outcome of the research ranges from the basic know how to analysis and design of the structures. It gives the definitions of various terms that are necessary for the understanding of the turbine industry. It gives an overview of the wind energy industry and the efforts being carried out by different countries. The power production curves and the previous and current trends are also added with the references and citations.

The turbine companies that are involved in the industry are also discussed. there relevant data and contacts are given. The comparison of their machines on the basis of performance and prices is also provided. The data was collected through emails and quotations from these companies. The wind speed and its requirement for different turbines is also given which shows the working wind speeds of these machines.

After that the thesis discuss different types of turbines. The classification is based on onshore and offshore. Also, another classification on the basis of axis of rotation of the blades is given. The supporting structure of turbine is also discussed. The foundation classification is also given.

All types of foundations that are used onshore and offshore are discussed up to date. The uses of them in different circumstances and their comparison is given in the form of advantages and disadvantages of them.

Then comes the different wind related knowledge. The wind interaction with turbine blades is discussed. The wind and the horizontal axis turbine blades are discussed. Slowly and gradually the work converges to horizontal axis wind turbines. The drag and lift coefficients of the blades are discussed. The different parameters that need to be taken into account are discussed in order to find net forces on the blades. The effect of angle of attack and other angles are discussed and their ranges are shown in which the blades give maximum lift and minimum drag. A plot between angle of attack and drag and lift coefficient is also provided that was taken form a turbine manufacturing company.

Different turbine limit states are overviewed. The application of different loads is discussed. The important limit states are discussed. The decision is made on which limit states are fundamental and must be taken for the design process.

After these different theories and their application is discussed that are used for the calculation of environmental loads. The airy theory for wave analysis and froya wind profile is discussed.

The load calculation is explained in detail. The discussion limits slowly to the offshore wind turbines. The foundation type selected is monopile foundation. At this stage only one type of structure and foundation type is explained i.e offshore horizontal axis wind turbine with monopile foundation. All force calculation techniques are explained in detail. at the end use of p-y curves is shown.

Towards the end the turbine design and all calculations are done for the Karachi, Pakistan. The wind data and wave data are collected from some sources on the internet.

The last chapter discuss the better ways to analyze and design the turbine structures. It discusses the shortcomings in the design and assumptions made. It also discusses the new techniques that are employed in the design of the structures. In the form of recommendation, it give suggestions to make the current work more accurate and optimized.

This work unveiled the different aspects of turbine structure design to us. We were introduced to wind and wave data processing. We visited and contacted some institutes and companies regarding the data. This exposed us to interaction with them in a professional way.

1.6 ORGANIZATION OF CHAPTERS

1.6.1 Chapter 1

This chapter gives the introduction to our work. This explains our objective that what is this document meant to explain. It shows the outcome of our work, the things we learnt and the things we did.

1.6.2 Chapter 2

This chapter is about the wind energy production in the world and in Pakistan. This chapter also shows the importance of wind energy with respect to other energy sources.

1.6.3 Chapter 3

In this chapter the wind turbine, its types and its components are discussed in detail. The different types of foundations used for offshore wind turbine are also discussed in this chapter. The international wind turbine manufacturing companies and their quotations are also given in this chapter.

1.6.4 Chapter 4

This chapter is about the methodology used for the analysis and design of the tower and foundation of monopile offshore wind turbine.

1.6.5 Chapter 5

This chapter discuss the previous calculations for the site in Karachi, Pakistan. The data is taken from some sources and processed and then forces obtained. The end result is the dimensions of the turbine structure.

1.6.6 Chapter 6

This chapter shows the assumptions made and the simplifications in the work. It also gives recommendation and show a better path to analyze and design a structure (offshore).

CHAPTER 2 WIND ENERGY

2.1 INTRODUCTION

After the industrial revolution of 18th century the industries continued to develop and are still developing with increasing rate. One who has more industries will be more economically stable and globally dominant. Power production is one of the key factors for its development and its now a key pillar for human wellbeing. However, ensuring the power need of all the sectors are satisfied is getting more and more challenging.

Specially after the adaptation of sustainable development goals. Energy is no longer the only requirement but its sustainable energy that the world seek. For decades coal and fossil fuel were the only means of producing power but these resources are non-renewable and not environment friendly, so the world started looking for the alternatives.

2.2 WIND ENERGY IN THE WORLD

According to the climate change, the international agreement of Kyoto protocol has an important rule to reduce the emission of greenhouse gases. The objective of this protocol is to reduce the consumption of energy and increase the production of renewable energy worldwide. The world is now conscious that the problems caused by the dependence on oil and increasing carbon emissions must be solved. However, current non-renewable resources like oil, natural gas, nuclear power and coal are still the primary energy sources of many countries around the world. Nevertheless, the sources’ supplies are limited, the burning of fossil fuels is very harmful to the environment, inserting carbon dioxide into the atmosphere. For these reasons, it is necessary to reduce society’s dependence on fossil fuels and focus on efficiency and green sustainable energy sources, so that our emissions will not increase so fast or even stabilize or decline. The most commonly renewable energies used are the hydro, the wave, the solar and the wind energy.

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Figure2-1 Energy sources over time [2]

2.2.1 WIND A RENEWABLE ENERGY SOURCE

All above-mentioned renewable energies have advantages and disadvantages, but in recent years, wind energy is the one that has had the most development and investment as shown in the given figure.

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Figure2-2 Installed Global wind capacity 2017 [1]

2.2.2 WIND ENERGY BASICS

Wind energy has been used by farmers and ranchers for more than two thousand years for pumping water and grinding grain, so it was mainly used for transforming wind energy into mechanical energy. Nowadays, wind energy is utilized to generate electricity through wind turbines requiring a huge development in engineering techniques, focusing in high efficiency.

Wind is caused by the uneven heating of the atmosphere by the sun. So basically, wind energy is another form of solar energy. As without the solar energy the variation in atmospheric temperature and pressure which are the basic need for the wind to flow is negligible.

Variations in the earth's surface, and rotation of the earth. Mountains, bodies of water, and vegetation also influence wind flow patterns of the wind.

2.2.3 WIND ENERGY EXTRACTION

Now as we know that energy possessed by anything due to its motion is kinetic energy. To convert this kinetic energy into more usable form of energy i.e. most likely electrical energy so that it can be transmitted over larger distances and can be used by everyone. we need a mechanism, a device that efficiently extracts the energy from wind and convert that energy to electric energy such device is called as wind turbine

2.2.4 GLOBAL EXPLOITATION OF RENEWABLE ENERGY SOURCES

The coal and fossil fuel’s resources of the world are depleting faster than ever due to the aberrant use of such resources for energy and heat production this goes against the rules of sustainability as resources must be used to meet the needs of the present without disabling the future generations to meet their own. If the current behavior continued, the coal and oil deposits will run out in 50 to 100 years.

This leaves us with only one option i.e. renewable energy sources. Now renewable energy sources can be either solar energy, wind energy, hydropower, geothermal energy etc. the most renowned ones are solar and wind energy.

2.2.5 WIND VERSUS SOLAR ENERGY

Both offers clean, reasonably-priced alternatives to the financially, environmentally and escalating costs, of fossil fuels. Their affordability doesn't mean that they're on equal ground.

Both wind and solar have a key role to play in our movement towards sustainable living on our planet. Often it is the case that wind and solar can work in well with each other because it is often windy when it is cloudy, and the wind can blow at night. Future grids will have wind and solar, but the role of wind will mainly be on the utility scale as it is not a great technology for people to own who do not have the skills to maintain them. While solar is more feasible on residential scale.

2.2.6 GLOBAL INTEREST IN WIND ENERGY

So far, we can elicit from the above discussion that wind energy is a new hope for sustainable development and its one off the most invested renewable energy sources figure 2-3 and 2-4 delineates the global interest in this technology to achieve sustainable development.

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Figure2-3 Global installed wind capacity [1]

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Figure2-4 Global installed wind capacity by region [1]

2.3 WIND ENERGY IN PAKISTAN

The country is facing power shortage problem for decades now and it’s getting worse with each passing year as the population is increasing rapidly and with that the demand is increasing dramatically too but compare to all that a little increase in capacity leads to a situation where an economic development is slowed down, and prosperity index is not improving at all.

2.3.1 DEMAND AND SUPPLY

According to the press release of spokesman power division at 25th of June 2018 the overall power demand was assessed to be 23,055 MW compared to electricity generation of 22,700 MW showing a shortfall of 355 MW.

2.3.2 POWER GENERATION BY SOURCE

- Furnace oil: 14% of total x Natural gas: 31% of total x Coal: 16% of total
- Hydroelectric: 29% of total
- Nuclear: 4% of total
- Renewable (Solar & Wind): 5% of total
- Others (Bagasse, Waste Heat Recovery etc.): 1% of total

2.3.3 WIND POWER PRODUCTION IN PAKISTAN

As shown above almost 61% of the country’s need is fulfilled by oil, gas and coal despite of their disastrous effects on the environment a somewhat bigger portion is of hydel power but the problem with hydel power is that it requires a huge investment and so far, the economic condition of the country does not seem very good

The country is highly indebted to several organizations and borrowing more for construction of dams can worsen the current economic conditions. Moreover, as mentioned earlier dams required huge investment because of extremely high initial cost so the loans required for such purpose may be too much that it will make the project infeasible. So solar and wind are the only options left.

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Figure2-5 Wind power production in Pakistan [1]

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Figure2-6 Wind power projects in Pakistan [1]

2.3.4 FUTURE OF WIND POWER IN PAKISTAN

Lower technology prices have led Pakistan’s government to increase the share of wind and solar in the power mix of the country. 2017 saw a major shift in the wind energy sector with the decision to develop new wind power projects through competitive bidding instead of feed- in tariffs. Based on available grid infrastructure in the country, the government has set a target for adding 1,200 MW of wind power by 2020. The share of wind energy is intended increase in line with future power demand growth.

Pakistan is beginning to reap the benefits of Chinese investment in renewable energy infrastructure, with the opening of the first wind power projects constructed as part of the huge China-Pakistan Economic Corridor (CPEC). Just under 200MW of wind capacity was added during 2017 at sites located at the so-called Gharo-Jhimpir wind corridor in Sindh province, which, according to the Pakistan Meteorological Department, has the potential for 11,000 MW of wind power development.

In addition, the International Finance Corporation (IFC) plans to finance three 50MW wind power projects in the Gharo-Jhimpir wind corridor. The World Bank has started mapping Pakistan’s wind potential, looking at wind corridors in Punjab as well.

The Pakistan Government has initiated the development of solar-wind hybrid projects. The scheme combines solar with wind power at existing wind farms to increase project capacity factors and maximize the use of the existing grid. The key barrier to wind development in Pakistan is the insufficient grid capacity and transmission capabilities in the country. The government is working to address this issue to harness Pakistan’s rich wind resources and move toward decarbonizing the energy sector.

CHAPTER 3 WIND TURBINE

3.1 INTRODUCTION

People used wind energy to propel boats along the Nile River as early as 5,000 BC. By 200 BC, simple wind-powered water pumps were used in China, and windmills with woven-reed blades were grinding grain in Persia and the Middle East.

In 1st century AD for the first time in known history, a wind-driven wheel was used to power a machine. A Greek engineer, Heron of Alexandria, creates this wind wheel. By 7th to 9th century wind wheels were used for practical purposes in the Sistan region of Iran, near Afghanistan [7]. The Panemone windmills shown in figure 3-1 were used to grind corn, grind flour, and pump water. By 1000 AD Windmills were used for pumping seawater to make salt in China and Sicily. During 1180s Vertical windmills were used in Northwestern Europe for grinding flour.

The first known wind turbine used to produce electricity was built in Scotland in 1987. The wind turbine was created by Prof James Blyth of Anderson's College, Glasgow (now known as Strathclyde University). The turbine was 10 m high and was installed in the garden of his holiday cottage and was used to charge accumulators developed by the Frenchman Camille Alphonse Faure, to power the lighting in the cottage, thus making it the first house in the world to have its electricity supplied by wind power.

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3.2 WIND TURBINE

Figure3-1 Panemone windmill

Wind turbine is a device that converts kinetic energy of the wind into electrical energy. It consists of different parts as shown in the figure. The wind rotates the rotor which converts the wind energy to mechanical energy this mechanical energy is then carried by the gear box to the generator which converts the mechanical energy to electrical energy. Finally, this electric energy is carried by metallic wires to the nearby grid station from where it is distributed to various places.

Three key factors that affects the amount of energy a turbine can harness from the wind are wind speed, air density, and rotor swept area.

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Figure3-2 Wind turbine

3.2.1 WIND SPEED

Wind speed increases as we move away from the surface of earth. As shown in figure 3-3. so, the tower height is kept higher to increase the power generation of wind turbine. The amount of energy in the wind varies with the cube of the wind speed, in other words, if the wind speed doubles, there is eight times more energy in the wind. Small changes in wind speed have a large impact on the amount of power available in the wind.

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Figure3-3 Variation in wind speed with height

3.2.2 ROTOR SWEPT AREA

The larger the swept area (circular area normal to the direction of wind) as shown in figure 3-4, the more power the turbine can capture from the wind. As area depends on the square of radius so a small change in the blade length cause a larger change in the swept area.

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3.3 TYPES OF WIND TURBINE

Figure3-4 Rotor swept area

As the time passed the technology improved and a better and improved with higher efficiencies wind turbines were developed. The technology is still improving but with production of high capacity wind turbines, but the basic principle remains the same and due to that there some basic types of wind turbines following these sets of basic principles

Now the wind turbines could be either onshore or they are offshore. Onshore wind turbines are those which are located on land while offshore wind turbines are located at sea. No matter where the wind turbines are present the basic principle behind its working remains the same. So broadly wind turbines can be characterized based on.

- Location of implementation
- Rotation axis of turbine
- Types of tower used

3.3.1 LOCATION OF IMPLEMENTATION

This involves onshore and offshore wind turbines.

3.3.1.1 ONSHORE WIND TURBINE

Traditionally onshore turbines have dominated the wind market, with the first turbine constructed in the late 1800’s.

People are familiar with onshore wind. We can point to many examples around the world of how successful onshore wind can be. Denmark is receiving over 40 percent of their electricity from wind and 75 percent of that comes from onshore turbines.

The infrastructure necessary to transmit electricity from onshore turbines is considerably less expensive than that of offshore. Onshore wind is also competitive in the greater renewable market, as it is the cheapest form currently available.

Onshore turbine production could act as a boost to local economies. If turbines are installed closer to their manufacturing sites, their value is likely to stay closer.

There would be less emissions from transporting wind structures if they are installed closer to the manufacturing site.

However onshore wind speeds are more unpredictable than offshore. Because turbines are optimized at a specific speed, they could lose efficiency if wind is too slow or too fast. Similarly, onshore wind direction changes much more often. Turbines must be facing the direction of the wind to operate efficiently. Advances in technology have led to new turbines that have some ability to pivot towards the wind.

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Figure 3-5 Onshore wind farm [5]

3.3.1.2 OFFSHORE WIND TURBINE

Offshore wind technology is much less developed than its predecessor. It was first implemented almost a century later than onshore. The first offshore wind project went into effect in the early 1990’s near Denmark.

Offshore wind turbines are tending to be more efficient than onshore because wind speed and direction are more consistent. Conceivably, less turbines are needed to provide the same amount of electricity as onshore turbines and it does not interfere with land use.

Offshore wind could benefit a marine ecosystem in which it is constructed. Some studies suggest that offshore wind farms protect sea life by restricting access to certain waters and increasing artificial habitats [9].

However, the technology necessary to transmit energy from turbines in a body of water is expensive. This could change as the industry matures, but this makes it hard to justify offshore over onshore [8].

Offshore turbines endure more wear and tear from wind and waves than onshore. This brings up operation and maintenance costs, further distancing the price from onshore.

Because offshore turbines are harder to get to, it could take longer to fix problems and restore them to function properly.

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Figure3-6 Offshore wind farm [6]

3.3.2 ROTATION AXIS OF TURBINE

The wind turbines, due to their axis of rotation, can be classified as turbines with a vertical axis of rotation or turbines with horizontal rotation axis.

3.3.2.1 HORIZONTAL AXIS WIND TURBINE (HAWT)

Horizontal axis wind turbine dominates most of the wind market. As the name implied in HAWT the blades or rotor rotates about an axis that is parallel to the ground on which the wind turbine is located.

Advantages of HAWT is that it produces more power from a given amount of wind as all the blades are always normal to the direction of wind. However, the structure gets heavier as all the assemblies are places on top of the tower. Another disadvantage is that additional system is required to align the rotor automatically against the wind as the direction of wind does not remain constant.

3.3.2.2 VERTICAL AXIS WIND TURBINE (VAWT)

In VAWT the axis of rotation is perpendicular to the ground. VAWT are used in small wind projects and residential application.

VAWT performs well in tumultuous wind conditions and does not require any specialized mechanism for wind in different directions as some of the total blades are always aligned against the wind however, this leads to less power production as all the blades are not facing the wind simultaneously.

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Figure3-7 HAWT and VAWT

3.3.3 TYPES OF TOWER USED

There are several types of towers for wind turbines, differing in the type of material used and the type of structure of the tower as shown in figure 3-8. Nowadays, the wind turbine market is dominated by the tubular steel towers. These towers are constituted by cylinders made of steel plates welded longitudinally. The cylinders are all connected by transverse welds, to obtain one tower section. Each section then finishes with a steel flange on both end, which bolts the sections to each other. In this type of towers, the increase of the diameter corresponds to a reduction of the plate thickness, thus increasing the tension on the tower, but decreasing the buckling.

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Figure3-8 Types of tower [10]

3.4 COMPONENTS OF WIND TURBINE

As the wind blows, it flows over the airfoil-shaped blades of wind turbines, causing the turbine blades to spin. The blades are connected to a drive shaft inside the nacelle, the shaft goes into a gearbox which increases the rotation speed and the generator converts the rotational energy into electrical energy figure 3-9 gives a brief overview of wind turbine components [11] [12].

3.4.1 NACELLE

It contains the electromechanical components of the wind turbine, including the generator which converts the mechanical rotational energy from the wind into electrical energy. Above the nacelle, an optional helicopter pick-up area is used for an easier maintenance

3.4.2 ROTOR

It contains the hub and the blades. These can be made of plastic reinforced with fiberglass or manufactured in steel for bigger turbines. The blades are connected to the hub, which transmits the rotational energy to the gearbox via the main shaft. The blades’ size usually have between 80 and 100 meters in diameter [11], and their rotation speed is between 10 to 30 rpm [13]. The bigger they are, the more energy it is possible to obtain.

3.4.3 TOWER

It provides support to the assembly of the nacelle, blades and hub. Dependent on the emplacement location and height, it is a tubular structure made of steel or cement, and it is through several sections. Typical tower heights range from 80 to 130 meters and it contains a ladder or elevator inside of it to reach the nacelle.

3.4.4 TRANSITION PIECE

It connects the tower to the driven pile foundation. This component is provided with a boat landing, a ladder and platform which gives access to the entrance of the tower. This element is only used in monopile support structures.

3.4.5 FOUNDATION

It contributes for the support of the wind turbine. Different types of foundation structures exist and will be presented in this chapter.

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Figure3-9 Components of wind turbine

3.5 TYPES OF FOUNDATIONS FOR OFFSHORE WIND TURBINE

As time passed the need for bigger turbines was felt so better and bigger turbines were produced but that also increased the demand of loads on the foundations. The foundation used for certain wind turbine in sea depends on more than one factor. Loads, distance from the shore and depth of water etc. these factors collectively decide about the foundation type. Below are some commonly used types of foundation of offshore wind turbines.

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Figure3-10 Types of foundation [14]

3.5.1 GRAVITY BASED FOUNDATION

Gravity based substructures are the second most used in offshore wind farms foundations, but in recent years, the applications of this type of substructures have been decreasing, due to the necessity of going more into deep waters and their use is not so profitable (EWEA, 2014). The limitation becomes more evident for water depths greater than 15 meters. During the year of 2013, the Karehamn Offshore Wind Farm (48 MW) in Sweden was the only project installed. According to EWEA, gravity- based foundations corresponded only to 0.1% of the installed foundations in that same year. In 2014, there is no record of any emplacement of this type of foundations support, unlike in the early days of offshore wind energy in Denmark, when they were very popular. These types of substructures are not as used in the recent days, because of the construction method, the spent time for curing the concrete, the dredging requirements for seabed preparation, and the heavy lift vessels that are needed, so it is not profitable to use. Typically, gravity-based foundations are a huge concrete structure designed to support the moments and forces generated by the turbine and by the environment conditions, and they are used for shallow water depths.

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Figure3-11 Gravity based foundation [15]

The gravity-based structure is hollow to reduce the weight for an easier transportation and emplacement. After the seabed preparation, the substructure is placed in position and its interior is filled with ballast providing the overall design weight. Scour protection is essential for this type of foundation due to the large diameters involved, which can be as great as 22m. These structures normally do not penetrate the seabed, but are supported by it, and they can also be designed with a flat base.

3.5.1.1 ADVANTAGES

- The material that it is made of is not expensive (concrete), and it is readily available in terms of raw material [16].
- The complete structure has a lower weight due to the hollow section that it is built on, being easier to handle.
- Smaller wind turbines can be manufactured onshore and then transported to offshore site, reducing the global costs [16].

3.5.1.2 DISADVANTAGES

- Once the gravity-based foundation does not penetrate the seabed ground, the overturning moments must be considered and designed.
- The substructure is placed directly on top of the seabed, so its superficies needs to be prepared, to level the ground for the correct positioning and completely upright. This process increases the installation costs.
- Due to the larger base it requires an extensive scour protection, more than a monopile foundation [16].
- Installation is limited to deeper waters.

3.5.2 TRIPOD / TRIPILE FOUNDATIONS

Tripod, as the name suggests, is a three-legged support, capable of providing greater stiffness and lateral stability than a single monopile [16]. The emplacement of this type of substructures requires a pre-installation of three monopiles driven into the seabed using a vibratory hammer technique to a depth of 21 meters, while the remaining depth is achieved by a hydraulic hammer. The last part of the piles is hammered to prove the required bearing capacity of the piles.

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Figure3-12 Triple and Tripod foundation support [16]

The central column in the tripod transfers the loads from the tower through the diagonal braces to the pile sleeves which carry the load to the driven piles. The pile sleeves and the driven piles are connected between a grouted connection to allow the verticality correction through the tripod and the driven piles. The tower is connected to the top flange of the tripod structure by bolts. Further on, the nacelle, hub and braces are assembled to the tower. This foundation support is designed for water depths from 25 to 50 meters.

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Figure3-13 Components of tripod foundation [15]

3.5.2.1 ADVANTAGES

- Can be placed in water depths up to 50 meters, much deeper than a standard monopile.
- Better in transferring the loads from the tower, providing a greater lateral stability and stiffness.
- Uses less material to be manufactured than a single monopile for greater depths.

3.5.2.2 DISADVANTAGES

- The process of installing the three driven piles needed by this structure increases the costs of this solution.
- All the three driven piles must be designed for the extreme load case, because of the weather conditions, wind and waves that come from every direction and are constantly changing. This makes the structure heavier and more expensive.
- The transportation of this type of structures is more complex than a simple monopile, requiring bigger vessels.

3.5.3 JACKETS FOUNDATIONS

Based on the oil and gas industry technology, jacket support structures consist of a combination of circular hollow sections welded together with fabricated nodes at the joints, in other words, it uses the basic truss structure to provide stability and strength and it is easier to manufacture into large sizes. Jackets were the preferred offshore support structures, but as the water depths increases, the placement of the offshore rigs requires other types of solutions that are more profitable. The wind energy sector is improving, and the turbines are getting bigger, heavier, and required to be assembled in deeper waters, so the designers opted to the jacket support structure. The jacket foundation is fixed to the seabed using piles that are driven through pile sleeves. These piles are installed through impact and vibratory hammers [16].

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Figure3-14 Jacket Foundation [15]

Jackets can be fabricated in three or four legs, comprising tubes of diameter between 864 mm and 1200 mm, depending on the structure configuration, site conditions and the weight of the turbine applied. As with the tripod foundation, a grouted joint is used to connect the jacket to the driven piles and to permit the correction of the verticality between them, to improve the axial capacity at the pile head. A transition piece is fitted above the jacket, allowing the connection of the tower to the jacket by bolts to the top flange. Dynamic load actions resulting from the wind, waves and operations are divided into single axial pile forces that are transmitted to the driven piles. Depending on the stiffness of the soil, scour protection may or may not be required.

3.5.3.1 ADVANTAGES

- Easy to design to improve the structure stiffness. However, to obtain more stiffness on the monopile structure is necessary to increase the diameter or the thickness wall of the pile (additional steel is needed).
- The complete support structure is not so heavy, being better for transportation, and using less quantity of steel when compared to the monopile [16].

3.5.3.2 DISADVANTAGES

- Higher manufacturing costs due to the complexity of the structure, many connections must be done between components (legs and braces).
- In the situations where it is necessary to use a scour protection, it is not easy to install, due to the inner parts of the jacket being difficult to reach [16].

The design and analysis of a jacket is more complex than a monopile’s, and it is necessary to make an additional stress check for the joints and members, leading to more time consumption.

3.5.4 FLOATING FOUNDATIONS

Local zones with better wind conditions are generally found in deeper water zones, usually with more than 60 meters depth (Navigant Consulting Inc., 2014). Therefore, the necessity of designing new types of foundation support structures such as floating solutions appeared, to reach greater depths. This new concept of foundation, compared to the traditional support structures that have been applied until the recent days, reduces the quantity of material needed to manufacture the complete substructure, eliminates the complex installation process until the seabed, and the decommissioning process is much easier. Applying this new type of foundations in the available area for wind energy production oversea increases the power capacity, and efficiency will be maximized in deep sea locations [17]. After the construction of floating oil rigs, the new concept of offshore floating wind turbines appeared. These structures are more complex than a floating oilrig because of the huge mass of the nacelle and blades that are supported only by a unique tower, being very difficult to sustain the complete structure. Unlike the floating oil rig, it is much easier to be laterally stabilized due to the large platform area. Therefore, the mass supported by the single tower must be balanced with a huge mass submerged underwater to obtain the desirable stability.

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Figure3-15 Floating foundation [19]

3.5.4.1 ADVANTAGES

- Easy to transport, due to the floatable base of the wind turbine. It can be towed to the site, reducing the process costs of loading and unloading all the components into barges.
- Possible to install in deeper waters.
- No scour protection is required.
- These substructures can be manufactured and assembled completely onshore and then towed to the correct location, allowing to reduce the production and installation costs. Regarding maintenance, it is also possible to bring the substructure to shore for repairs.
- The installation process of these structures does not cause noise [16].

3.5.4.2 DISADVANTAGES

- In shallow waters, it is not rentable to install this type of substructures due to the expensive technology used.
- The stability of the wind turbine is a huge concern [16].

3.5.5 MONOPILE

Monopile foundations have been used for offshore oil and gas platform foundations for decades. They are the most common support structures for offshore wind turbines, around 76 % of all turbine foundations installed to date are founded on monopiles as shown in figure 3-16. These substructures have proven to be an efficient solution in reasonable ground conditions and in water depths up to 35 meters. These piles resist lateral wind and wave loading (and resulting moments) by mobilizing horizontal earth pressures in competent near- surface soils [20]. The complete structure consists of a single large-diameter, thick walled, steel cylindrical tube (pile) driven into the seabed (using hammering or vibration techniques), a transition piece with a grouted connection that joins the pile, and a tower in which the turbine is mounted on top. The substructure includes a boat landing and a work platform for the maintenance of the structure and turbine components. The J tubes, which can be external or internal, transport the cables from the nacelle until the seabed. the foundation needs a scour protection that is usually estimated at 2.5 times the diameter of the tube, which can be composed by rocks or geotextiles around the circumference of the pile

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Figure3-16 Distribution of offshore WT foundations (2014)

3.5.5.1 ADVANTAGES

- Simple design compared to the other types of support structures.
- Easy to be manufactured in serial production due to the simple geometry. Also, very convenient to transport and install in series with specialized installation vessels.
- Technology used for many years that proves the efficiency and cost effective of this solution
- It is possible to be installed in almost all kinds of soils conditions, due to the installation techniques and the shape of the pile, being a versatile solution.
- Efficient in transferring the forces from the turbine to the ground [16]

3.5.5.2 DISADVANTAGES

- Is not so profitable at greater depths due to the huge quantity of steel that is needed, larger diameters, thickness and length, increasing the costs of the structure because the steel is expensive. So, monopile is not the best solution financially for larger scales, but research carries on optimizing the monopile to be more economically feasible on greater depths.
- The installation process and the project in deeper waters are more complex, the structure is bigger and heavier due to the stiffness that is needed, requiring huge machinery for the implementation.
- After service lifetime, the structure is not totally removed, the standards require the support to be catted 1.5 meters below the seabed. This process over the years will originate the corrosion of the rest of the pile under the seabed level, being dangerous for the sea life, and causing pollution of the sea waters [16].

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Figure3-17 Monopile foundation [15]

3.6 INTERNATIONAL COMPANIES

Following are some top international companies that are manufacturing wind turbine.

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Figure3-18 Installed (2016) vs Total capacity of International Companies

3.7 COMPANIES QUOTATIONS

Quotations from following countries are available below in this article.

Table 3-1 Wind Turbine Companies

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3.7.1 SKYWIND, GERMANY

Table 3-2 Skywind 1000W specifications

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Figure3-19 Wind speed vs Annual yield

3.7.2 RIGHT RENEWABLE TEK, INDIA

Table 3-3 Right Renewable Tek 3000W Wind Turbine

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Table 3-4 Right Renewable Tek 20,000W Wind Turbine

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3.7.3 VAIGUNTH, INDIA

Table 3-5 Vaigunth Wind Turbine Specifications

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3.7.4 AEOLOS, DENMARK

Table 3-6 Aeolos-H 500W Wind Turbine Specifications

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Table 3-7 Aeolos-H 2000W Wind Turbine Specifications

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Table 3-8 Aeolos 5kW Wind Turbine Specifications

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Table 3-9 Aeolos 10kW Wind Turbine Specifications

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Table 3-10 Aeolos 30kW Wind Turbine Specifications

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3.7.5 HUMMER, CHINA

Table 3-11 Hummer Wind Turbine Specification

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3.7.6 COMPARISON

Table 3-12 Quotation of different companies

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3.8 GRID CONNECTION

3.8.1 On-Grid WIND TURBINES

A home that is currently connected to a locally provided power source is considered on-grid. In this scenario a power-conditioning unit (inverter) that makes the turbine output electrically compatible with the utility grid is installed. Your turbine works in tandem with your electric utility to power your house. When the wind isn’t blowing, the utility supplies your electricity. But when it’s windy out, your personal wind turbine pivots to catch the best wind and provides clean, quiet electricity. When it generates more electricity than you need, your meter can spin backwards—which means you’re selling electricity back to the utility

An on-grid system can be practical if the following conditions exist:

- You live in an area with an average annual wind speed of at least 10 mph (4.5m/s)
- Utility-supplied electricity is expensive in your area
- The utility’s requirements for connecting your system to its grid are not prohibitively expensive
- There are good incentives for the sale of excess electricity for the purchase of wind turbines

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Figure3-20 On-Grid-connected System

3.8.2 Off-Grid WIND TURBINES

Systems not connected to a local utility supplier are known as “off-grid” systems. A hybrid system that uses both solar photovoltaic panels and a wind turbine will make the perfect complement to provide minimal interruptions in power to a remote home or business that is off-grid. In much of the U.S, wind speeds are low in the summer when the sun shines brightest and high in the winter when less sunlight is available. Due to the alternating nature of peak operating times hybrid systems are ideal for producing consistent power. In case of emergency, off-grid systems normally have an engine-generator on hand.

- An off-grid hybrid system may be practical if:
- You live in an area with an average annual wind speed of at least 9 mph (4.0m/s)
- A grid connection is not available or can only be made through an expensive extension
- You would like to gain energy independence from the utility

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Figure3-21 Off-Grid-connected System

3.9 TRANSPORTATION OF COMPONENTS

Assessments of wind energy site suitability rely primarily on resource potential and distance to transmission infrastructure, while the challenges of transporting wind energy components from the manufacturer to a potential site receives less attention (Cotrell et al. 2014a; Cotrell et al. 2014b). The dimensions and weight of wind components often exceed the limits of U.S. infrastructure, making them difficult to transport. Because of the difficulties associated with transportation logistics, sites that are extremely remote or that are on complex terrain are more costly sites to develop. As wind components continue to get larger, the challenges of transporting these components are likely to intensify.

Transportation makes up about 3%–8% of the total land-based wind capital costs in the United States, and with projected increases in turbine sizes, these percentages are expected to increase significantly (Cotrell et al. 2014a; Cotrell et al. 2014b; Zayas et al. 2015). Despite this projected increase, little is known about the state of U.S. transportation infrastructure for transporting these large wind components. Yet, understanding transportation infrastructures’ influence on wind development is important to eliminate hurdles that are preventing wind development.

Coupled with the physical issues of transporting large wind components are the hurdles involved with the required permitting process. For most state and local regulatory authorities, permits are required for truck transport of oversized and overweight (OSOW) wind components through a given jurisdiction’s boundaries. Often each regulatory jurisdiction requires unique transporting requirements and restrictions as part of the permitting process, and such permits must be negotiated individually with each regulatory unit (Cotrell et al. 2014a; Cotrell et al. 2014b).

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Figure3-22 Transportation by Road

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Figure3-23 Transportation by boats

CHAPTER 4 METHODOLOGY

4.1 INTRODUCTION

In this chapter the method, formulations and limit states used for the analysis and design of tower and monopile of Monopile Offshore Wind Turbine are discussed.

The design process involves an initial site selection followed by an assessment of external conditions, selection of wind turbine size, subsurface investigation, assessment of geohazards, foundation and support structure selection, developing design load cases, and performing geotechnical and structural analyses. A flow diagram for the design process of a typical offshore wind turbine is shown in figure given below:

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Figure4-1 Design Process for a typical offshore wind turbine (Malhotra, 2007c)

In this chapter the discussion is limited to the limit states and design loading for the tower and foundation of monopile offshore wind turbine.

4.2 LIMIT STATES

An offshore wind turbine can be designed using the following limit states:

- Ultimate Limit State (ULS) ֜ Corresponds to the maximum load carrying capacity of the structure supporting actions and influences that may occur during its life span.
- Serviceability Limit State (SLS) ֜ Associated to the deflections and vibrations of the structure.
- Fatigue Limit State (FLS) ֜ Corresponds to the effect of cyclic loading.
- Accidental Limit State (ALS) ֜ Accidental loads, like fire, explosions and impacts. Wind turbine is generally designed using Ultimate Limit State (ULS) and then check is applied by evaluating the Serviceability Limit State (SLS) of the turbine.

4.3 DESIGN LOADS

Loads due to wind on the blades and tower of an offshore wind turbine structure are dominant, it results in dynamics characteristics that are different from the wave and current loading. While the loads on wind turbine foundations is characterized by relatively small vertical loading which is due to the mass of structure and larger horizontal and moment loads due wind, wave and current loads which are also dynamic.

The different types of loads that acts on the blades, tower and foundation of wind turbine are classified into Permanent Loads (G), Variable Functional Loads (Q), Environmental Loads (E), Accidental Loads, Deformation Loads (D) and Dynamic Loads.

4.3.1 PERMANENT LOADS

Permanent loads refer to loads that are constant in magnitude, position or direction during the life span of the structure. This type of loads refers to the mass of the whole structure of wind turbine like rotor, hub, blades, nacelle, tower, transition piece and monopile, including the mass of grout and ballast, equipment, or attachments which are permanently mounted onto the access platform. The hydrostatic pressure (external and internal) that acts on the monopile and transition piece, is also considered as permanent load on the structure.

4.3.2 VARIABLE FUNCTIONAL LOADS

Variable loads are loads that may vary in magnitude, position and direction during the period considered. These include loads that are related to fabrication and installation operations, ship impacts from service vessels and maintenance of wind turbine, so it is necessary to consider personnel, crane operational loads, loads from fendering, access ladders, platforms and variable ballast and actuation loads. Actuation loads result from the operation of the wind turbine. Generally, permanent and variable loads can be quantified with some certainty. These types of loads were not considered on this study due to the lack of information.

4.3.3 ENVIRONMENTAL LOADS

Environmental phenomena that can cause damage to the structural components of wind turbine are classified as Environmental Loads. These loads depend on the site climate and have a greater degree of uncertainty associated with them. Unlike permanent loads these loads are time dependent and may vary in magnitude position and direction during the time period considered. These loads act on the wind tower through different load combinations and directions under different design conditions and are then resolved into an axial force, horizontal base shear, an overturning moment and torsional moment to be resisted by the foundation.

These loads include loads due to [21]:

- Earthquakes
- Soil Conditions
- Temperature
- Snow and Ice
- Tides
- Marine Growth

Throughout this chapter, due to the limited resources available on time, the three most important environmental loads for the design of offshore wind turbines will be presented and studied. These are:

- Wind
- Waves
- Current

4.3.4 ACCIDENTAL LOADS

Accidental loads are associated to technical failure or abnormal operations, caused by:

- Collision impact from vessel, helicopter or other objects
- Dropped objects
- Load from rare, large breaking wave
- Explosions
- Fire etc.

Due to unavailability of information these loads are not considered in the design.

4.3.5 DEFORMATION LOADS

Loads due to unwanted or unaware events that the structure is subjected to are considered as Deformation Loads. These loads are due to [17]:

- Settlement of foundation
- Temperature loads

Deformation loads are also not considered in this design due to the reasons mentioned above.

4.3.6 DYNAMIC LOADS

Dynamic loads are due to an effect caused by some sort of excitation of cyclic nature. The cyclic loading produce vibration which may cause serviceability damage or failure of the structure.

The excitation of a wind turbine that leads to a dynamic response might be caused by the following events:

- Earthquake
- Waves
- Wind
- Operation of wind turbine (rotor operation).

Some of these events/loads will be further analyzed and discussed, in the next subchapters. However, complex phenomena, such as earthquakes, weren’t studied, since they could lead to a whole new research topic.

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Figure 4-2 Design Loads for Offshore Wind Turbine [23]

4.4 QUANTIFICATION OF WIND LOADS

4.4.1 GENERAL CONSIDERATIONS

Wind speed varies with time as well as with the height above the ground or the height above the sea surface. For these reasons, the averaging time for wind speeds and the reference height must always be specified.

A commonly used reference height is H = 10 m. Commonly used averaging times are 1 minute, 10 minutes and 1 hour. Wind speed averaged over 1 minute is often referred to as sustained wind speed.

4.4.2 DATA COLLECTION

Wind data is collected over sufficiently long periods at the specific site. This data is usually required to develop the wind speed statistics to be used as the basis of design. This data is also available with various departments like Meteorological department. For this design the wind data required is [21]:

- 10 minutes mean wind speed at the height of 10 meters (for normal wind conditions)
- 1 hour mean wind speed at the height of 10 meters (for extreme wind conditions)

The normal wind conditions and extreme wind conditions are distinguished by DNV (DET NORSKE VERITAS) Offshore Standard. According to this guideline the normal wind conditions are used to evaluate the primary fatigue loads on the structure while the extreme wind conditions are used to evaluate extreme loads on wind turbine structure and foundation. So, here the extreme wind conditions are used because it gives conservative results [24].

4.4.3 FROYA WIND PROFILE

Froya wind profile is used to determine the wind speed at the requested height and time. For normal wind condition Froya wind profile is given as [21]:

Abbildung in dieser Leseprobe nicht enthalten

Where:

U10: 10-meter mean wind speed at 10-meter height

h: 10 meters

T10: 10 minutes

Z: Requested height above sea level

T: Requested height.

For extreme mean wind speeds and strong guests, considering the wind turbulence (IU) Froya wind profile is given by:

Abbildung in dieser Leseprobe nicht enthalten

Where:

U0: 1-hour mean wind speed at 10-meter height (m/s)

h: 10 meters

T0: 3600 seconds and T<T0

Z: Requested height above sea level

T: Requested height.

4.4.4 WIND FORCES ON TOWER

Due to the higher wind speeds available over the sea offshore wind farms have an advantage over onshore projects in context of power generation but in design perspective, offshore wind turbines have disadvantage over onshore wind turbine.

The wind induced loads are time dependent due to the fluctuation in wind velocity. Wind pressure loads act in the direction normal to the surface.

After the evaluation of wind speed at requested heights by Froya wind profile, it is easy to determine the wind load on the tower and the transition piece of wind turbine using the following equation [25]:

Abbildung in dieser Leseprobe nicht enthalten

Where:

ρ: Air Density [kg/m3] (see figure 4-3)

CS: ShapeCo-efficient (see DNV 30.5)

A: Projected area of the member, normal to force direction [m2]

U: Wind velocity, according to Froya wind profile [m/s]

For Air density (ρ) the following graph is used:

4.4.5 WIND FORCES ON ROTOR

A German physicist Albert Betz published a law known as Betz law in 1919 to determine the maximum power that can be extracted from the wind, independent of the design of wind turbine in open flow. He proposed that the kinetic energy in wind is not extracted totally and converted to mechanical energy by wind turbine rather a part of this energy is expelled back to the environment [26].

The law is derived from the principles of conservation of mass and momentum of the air stream flowing through an idealized "actuator disk" that extracts energy from the wind stream. The turbine is represented by a uniform “actuator disk” which creates a discontinuity of pressure in the stream tube of air flowing through it, as shown in the figure.

Abbildung in dieser Leseprobe nicht enthalten

Figure4-3 Air Density and Specific Weight

Abbildung in dieser Leseprobe nicht enthalten

Figure4-4 Wind speed and pressure variation in an ideal wind turbine model [27]

Inthe above figure at section A (upstream), V1 is the actual velocity of wind acting on turbine rotor before disturbance. At section B just to the left, due to the rotor the wind is compressed so the speed (V) decreases as the pressure (P) increases (Bernoulli's Theorem).

At section B just to the right the wind got past the rotor and area of tube increases so, pressure drop occurs, but velocity doesn’t increase because of kinetic energy conversion to mechanical energy.

At section C (downstream), the velocity of wind is V2 which is less than V1 .

This decrease in wind speed between upstream and downstream of rotor due to the transformation of kinetic energy into mechanical energy is called Axial Induction Factor (a) given by:

Abbildung in dieser Leseprobe nicht enthalten

Now the force on rotor is computed for two conditions of rotor:

1. Working condition (i-e rotor in operation)
2. Stopped Condition.

4.4.5.1 WORKING CONDITION

When the wind speed is below the Cut-out speed for the turbine, then the turbine is in operation and the forces on the rotor during this condition due to wind pressure is determined by the following equation:

Abbildung in dieser Leseprobe nicht enthalten

Where:

A: Rotor area [m2]

ρ: Air density [kg/m3]

V: Acting wind speed on the turbine rotor [m/s]

a: Axial induction factor [-]

4.4.5.2 STOPPED CONDITION

Usually, for safety, the aero-generator stops when the wind speed exceeds the Cut-out wind speed for wind turbine. The cut-out wind speed is normally taken as 25 m/s, so when speed of wind exceeds this velocity, the rotor is stopped slowly by a built-in security mechanism in order to do not induce significant stresses on the structure [23].

The rotor blades may stop in any configuration but from experiments the most unfavorable condition is when one blade of the rotor is in parallel with the tower in upward direction as shown below. As this produces the worst condition it will be considered in the study [28].

Abbildung in dieser Leseprobe nicht enthalten

Figure 4-5 Most unfavorable condition of rotor [28]

The mechanism through which the wind rotates the blades of the rotor of wind turbine is discussed below. The amount of energy extracted from the wind depends upon the geometry of the blade and its orientation. The following figures illustrate the forces that acts on wind turbine blades and the cross section of common wind turbine blades.

Abbildung in dieser Leseprobe nicht enthalten Figure 4-6 Forces on Stationary Rotor Blade [29]

Abbildung in dieser Leseprobe nicht enthalten

Figure4-7 Wind Flow through turbine blades [26]

As shown in the figure, Drag and Lift forces are applied on blades by wind flowing through the wind turbine blades. The drag force is because of the direct impact of wind on the blade while the Lift force is due to the phenomena shown in Figure 4-8 below:

Abbildung in dieser Leseprobe nicht enthalten

Figure4-8 Lift Force Mechanism [29]

As shown in figure, the blades are so designed and oriented that the distance to be traveled by the wind between the leading and trailing edges are different for upper side and lower side of the blade. So, when the wind flows through the turbine blades, the upper side and lower side wind streams have to cover different distances in the same time i.e upper wind stream distance is greater than lower wind stream distance, due to this on the upper side of blade the wind speed is high and it’s pressure is low while on the lower side the wind speed is slow and it’s pressure is high, producing Lift force on the lower side of the blade, causing the blades to rotate.

The rotation velocity of the rotor depends on the lift force [31]. To obtain higher velocity, it is necessary for the blades to generate more lift force, which depends on the “attack angle (α)”. The lift force increases with the increase in the attack angle under certain limit, because above that limit the blade of the turbine “stalls” and the lift force decreases with the increase in the attack angle [29].

The Drag and Lift forces and the respective angles is represented by the following figure:

Abbildung in dieser Leseprobe nicht enthalten

Figure4-9 System of forces acting on the blade; b) Resulting lift and drag loads in the x-axis direction [30]

Here Vrel is the Relative velocity that is due to the wind velocity and the rotational velocity of the blade.

The relative wind speed at the blade section is calculated as:

Abbildung in dieser Leseprobe nicht enthalten

The attack angle (α) is generally given by the manufacturer of the turbine blades. To obtain the flow angle (φ), the tip speed ratio (λ) is needed, as well as the rotational induction factor (a´) that describes the wind turbine flow field, which is due to the rotation flow in the wake zone. The equations are expressed as:

Abbildung in dieser Leseprobe nicht enthalten

The Lift Force is given by:

Abbildung in dieser Leseprobe nicht enthalten

Where:

CL: Aerodynamic lift coefficient

CD: Aerodynamicdrag coefficient

ρ: Air density [kg/m3]

Ca: Airfoil chord length [m]

∆r: Radial length of the blade [m]

α: Attack angle [deg]

θ: Pitch angle [deg]

φ: Inflow angle [deg]

The CL and Cd coefficients are provided by the manufacturer of wind turbine blades. For example, for NACA N63-212 airfoil the coefficients can be selected from the figure 4-10 for greater production of wind turbine the attack angle is where the CL has greater value and CD has smaller value and here it is approximately 10 degrees with CL value of about 1.2 and CD value of about 0.01 [31].

Abbildung in dieser Leseprobe nicht enthalten

Figure4-10 CL and CD values for NACA N63-212 [30]

Now the total force in X-axis direction per blade element is:

Abbildung in dieser Leseprobe nicht enthalten

4.5 QUANTIFICATION OF WAVE LOADS

4.5.1 WAVE

These are the surface disturbances mostly formed due to friction of wind over the surface of water resulting in circular movement or water particles which forms waves. Wave is formed as the energy is transferred from wind through the water.

Ocean waves are irregular and random in shape, height, length and speed of propagation. Waves do not have regular direction of propagation. Its direction changes continuously. Water particles have velocity component in vertical and horizontal direction that repeat itself about the mean position. The vertical velocity component does not apply force on vertical shaft and pile so only horizontal direction is considered.

Following are the steps for the design purpose:

- Ocean surface data is needed.
- From the above data velocity is calculated.
- Using certain formulations, the forces are determined from the above velocity.

4.5.2 WAVE CHARACTERISTICS

Waves are disturbances that travel through a fluid medium. A regular travelling wave has permanent form. Several common wave characteristics include frequency, period, wavelength, and amplitude.

The design of any structure in ocean water depends on above mentioned characteristics [21].

4.5.2.1 WAVE LENGTH

The wave length (L) is the distance between successive crests or troughs of a wave. Figure 4-11 gives the wave length as a function of wave period for various water depths for linear wave [21].

Abbildung in dieser Leseprobe nicht enthalten

Figure4-11 Wave period vs Wave length [21]

Abbildung in dieser Leseprobe nicht enthalten

4.5.2.2 WAVE CELERITY

The propagation velocity of the wave form is called phase velocity, wave speed or wave celerity (C) and is denoted by:

Abbildung in dieser Leseprobe nicht enthalten

Figure gives the wave celerity as a function of wave period for different water depths and for linear wave [21].

Abbildung in dieser Leseprobe nicht enthalten

Figure4-12 Wave period vs Wave celerity (DNVGL-RP-C205) [21]

4.5.2.3 WAVE FREQUENCY

Wave frequency (f) is defined as the number of waves passing through a given point in one second. Wave frequency is the inverse of wave period.

Abbildung in dieser Leseprobe nicht enthalten

4.5.2.4 WAVE ANGULAR FREQUENCY

Wave Angular Frequency (ω) is defined as the wave angular displacement per unit time. Wave angular frequency is given by:

Abbildung in dieser Leseprobe nicht enthalten

4.5.2.5 WAVE AMPLITUDE

Wave Amplitude (a) is defined as the maximum distance of wave from its mean position. It is a measure of the height of the crest of wave from midline (still water level). It is equal to one half the vibration path [21].

Abbildung in dieser Leseprobe nicht enthalten

4.5.2.6 WAVE NUMBER

The number of waves per unit distance is called Wave Number (k). It is given by:

Abbildung in dieser Leseprobe nicht enthalten

Figure4-13 Wave Parameters [32]

4.5.2.7 SEA SURFACE ELEVATION

The surface elevation η(x,y,t) is the distance between the still water level and the wave surface.

For regular linear waves the wave crest height AC is equal to the wave trough height AH and is denoted the wave amplitude A, hence H = 2A. The surface elevation is given by:

Abbildung in dieser Leseprobe nicht enthalten

4.5.2.8 PHASE ANGLE

Abbildung in dieser Leseprobe nicht enthalten

Phase Angle (θሻ of a periodic wave is defined as the number of suitable units of angular measure between a point on the wave and a reference point.

Water particle velocity, acceleration and pressure depends on phase angle of the wave, which ranges from 0 radians to 2π radians. Forces are determined for the phase angle which causes higher stresses (shear force, bending moment and pressure) [22].

It is given by:

Abbildung in dieser Leseprobe nicht enthalten

Figure4-14 Sinusoidal waveform in time domain

4.5.3 LINEAR AIRY WAVE THEORY

Linear Wave Theory is presented by Airy to describe the motion of wave particles. This theory gives a linear description of the propagation of gravity waves on the surface of the homogenous ocean surface.

In this theory the wave height is assumed to be much smaller than both the wave length and the water depth while the depth is assumed to be uniform and the water is assumed to be incompressible, inviscid and irrotational.

This theory is commonly used for the determination of water particle velocities and accelerations due to wave and is often used in ocean engineering and coastal engineering.

Airy theory is applicable only to intermediate depth waters and deep waters. In deep waters the water particles move in circles in direction of wave propagation and the circle diameter decreases with the increase in water depth. Due to the influence of sea bed, the water particles motion changes from circular to elliptical in intermediate and shallow depth waters as shown in the Figure 4-15.

The wave propagation is positive along the x-axis and along the z-axis the mean is at the ocean surface, positive upwards, as it presented in Figure 4-15. The horizontal water particle is the most important for the design of the monopile while its vertical component is not considered because the tower is not inclined [33].

4.5.3.1 DEEP WATER

For waters where

Abbildung in dieser Leseprobe nicht enthalten

The horizontal particle velocity is given by:

Abbildung in dieser Leseprobe nicht enthalten

The horizontal particle acceleration is given by:

Abbildung in dieser Leseprobe nicht enthalten

The hydrodynamic pressure is given by:

Abbildung in dieser Leseprobe nicht enthalten

1.1.1.2 Intermediate Depth Waters:

For intermediate depth waters where

Abbildung in dieser Leseprobe nicht enthalten

The horizontal particle velocity is given by:

Abbildung in dieser Leseprobe nicht enthalten

The horizontal particle acceleration is given by:

Abbildung in dieser Leseprobe nicht enthalten

The hydrodynamic pressure is given by:

Abbildung in dieser Leseprobe nicht enthalten

In the above equation the first term reflects the static part and second term is the dynamic part.

Where:

a: Amplitude of wave [m]

ω: Wave angular frequency [rads/s]

k: Wave number

Z: Desired depth [m]

d: Water depth [m]

g: Gravity acceleration [m/s2]

θ: Phase angle [rad]

ρ: Water density [kg/m3]

Abbildung in dieser Leseprobe nicht enthalten

Figure4-15 Wave forms according to Airy theory [32]

4.6 QUANTIFICATION OF CURRENT LOADS

4.6.1 OCEAN CURRENTS

The movements of ocean water caused by the difference in temperature, density and salinity of water at different places in an ocean are called Oceans Currents.

Ocean currents are large streams of ocean water that flows through the ocean that is why currents should be considered for the design of offshore structures.

4.6.2 TYPES OF CURRENTS

The most common categories of ocean currents are [21]:

- Wind generated currents
- Tidal currents
- circulation currents
- Loop and eddy currents
- Soliton currents
- Longshore currents

But here we will consider only wind generated currents and tidal currents due unavailability of data.

4.6.2.1 WIND GENERATED CURRENTS

Wind generated currents are generated at still water level by the wind stress and atmospheric pressure gradient throughout a storm.

4.6.2.2 TIDAL CURRENTS

The daily rise and fall of earth’s water on its coastlines, caused by the combined effects of gravitational forces exerted by the sun, moon and rotation of the planet.

4.6.3 OCEAN CURRENT VELOCITY

According to DNV-OS-J101 the ocean current velocity (V(Z)) is given by [21]:

Abbildung in dieser Leseprobe nicht enthalten

Wind generated current’s velocity at level z is given by:

Abbildung in dieser Leseprobe nicht enthalten

Tidal current’s velocity is given by:

Abbildung in dieser Leseprobe nicht enthalten

Wind-generated current at still water level may be evaluated by the following expression:

Abbildung in dieser Leseprobe nicht enthalten

Where:

h0: Reference depth from wind-generated current, h0=50 [m]

z: Vertical distance from still water level, positive upwards [m]

h: Water depth from still water level (always taken as positive) [m] K: 0.015 to 0.03

U0: One-hour mean wind speed at 10 m height [m/s]

vwind0: Wind-generated current at still water level [m/s]

vtide0: Tidal current at still water level [m/s].

4.7 TOTAL HYDRODYNAMIC FORCES

The forces exerted by fluid due to the combined effect of waves and currents on a structure in the fluid are called Hydrodynamic Forces.

Morison Equation is used for the determination of hydrodynamic forces. The wave kinematics like wave velocity [u(x,z,t)] and wave acceleration [u”(x,z,t)] from the Linear Airy Theory and the current velocity [v(z)] is used to calculate the forces (F) and Moments (M) applied on the structure present in fluid. The Morison Equation is given by [21]:

Total Hydrodynamic Force per unit length is:

Abbildung in dieser Leseprobe nicht enthalten

Hydrodynamic Drag Force is given by:

Abbildung in dieser Leseprobe nicht enthalten

Hydrodynamic Inertial Force is given by:

Abbildung in dieser Leseprobe nicht enthalten

Where:

Cd: Hydrodynamic drag coefficient

Cm: Hydrodynamic inertiacoefficient

ρ: Water density [kg/m3]

D: Pile diameter or another projected cross-sectional dimension of the member [m]

u(x,z,t):Horizontal wave particle velocity [m/s]

v(z): Ocean current velocity {m/s]

u"(x,z,t): Horizontal wave particle acceleration [m/s2].

As obvious from the above equations the currents only effect the drag force, it has no effect on the inertial force.

Everything is known or already discussed in the above equations except Drag and Inertial coefficients. According to DNV-OS-J101 the drag and inertial coefficients depends on the Reynold’s number (Re), the Keulegan-Carpenter number (Kc) and on the relative roughness (K). For a cylindrical section with diameter “D” the Reynold’s number (Re)and the Keulegan-Carpenter number (Kc) are given below [21]:

Abbildung in dieser Leseprobe nicht enthalten

Where:

umax: Maximum particle velocity at still water level [m/s]

D: Pile diameter [m]

v: Kinematic viscosity of seawater [m2/s]

T: Wave period [s].

The drag coefficient Cd depends on CDS and on the KC as:

ܥௗ ൌ ܥ஽ௌ Ǥ ߰ሺܥ஽ௌ Ǥ ܭ஼ ሻ (4.42)

Abbildung in dieser Leseprobe nicht enthalten

Where CDS is the drag coefficient for steady-state flow. It depends on the roughness of the structural member surface. It is given as:

Abbildung in dieser Leseprobe nicht enthalten

Where:

D: Structural member diameter [m]

K: Surface roughness [m].

New uncoated steel and painted steel can be assumed as smooth, for concrete and highly rusted steel, K= 0.003 m, and for marine growth, k=0.005 to 0.05 m.

Now the amplification factor ψ can be computed from the graph given below:

Abbildung in dieser Leseprobe nicht enthalten

Figure4-16 Wake amplification factor as function of KC number for smooth (solid line) and rough (dotted line) [21]

The inertia coefficient follows the criteria below:

Abbildung in dieser Leseprobe nicht enthalten

So, from above mentioned equations the total Hydrodynamics forces per length is determined.

Total hydrodynamic shear force (F) and bending moment (M) exerted on a slender structure in a general fluid flow can be obtained by integrating the sectional forces acting on each strip of the structure from the seabed with z=-d until the instantaneous water surface elevation (Ʉ). It is given by:

Abbildung in dieser Leseprobe nicht enthalten

Figure below shows the shape of the wave and the current load on the monopile:

Abbildung in dieser Leseprobe nicht enthalten

Figure4-17 Hydrodynamic loads on a slender member [32]

4.8 LOAD COMBINATIONS

The load combinations depend upon the design method used, i.e. whether limit state or allowable stress design is employed.

The load combinations recommended for use with allowable stress procedures are [21]: Normal operations:

- Dead loads plus operating environmental loads plus maximum live loads.
- Dead loads plus operating environmental loads plus minimum live loads.

Extreme operations:

- Dead loads plus extreme environmental loads plus maximum live loads.
- Dead loads plus extreme environmental loads plus minimum live loads.

Environmental loads should be combined in a manner consistent with their joint probability of occurrence.

Earthquake loads, are to be imposed as a separate environmental load, i.e., not to be combined with waves, wind, etc.

But here our main concerns are the wave and wind loads. But these two forces vary independently with time (their behaviors are different over time) so the combinations of wave and wind loads have to be done separately. According to DNV standard there are two methods for the combination of these two loads.

- Linear combination of wind load and wave load.
- Combination of wind load and wave load by simulation.

Here Linear combination of wind loads and wave loads, or of wind load effects and wave load effects is done for the analysis and design of offshore wind turbine.

Here the design is for Ultimate Limit State (ULS) and then later verified for Serviceability Limit State (SLS) [21].

For SLS the load factor is taken as 1.0 for all load categories by DNV standards. For the ULS the load factor is higher, being presented in table 4-1. Therefore, the ULS is the worst scenario.

Table 4-1 Load factors for the Ultimate Limit Stat [21]

Abbildung in dieser Leseprobe nicht enthalten

4.9 FOUNDATION DESIGNS

As for wind turbine design the lateral loads are far greater as compared to the vertical loads so the monopile should be designed as laterally loaded pile. For designing the monopile the p-y curves method is used, which is provided in current design regulations, such as, the Det Norske Veritas (DNV) and the American Petroleum Institute (API) etc.

The soil-pile interaction depends on the flexibility of the pile. The offshore monopiles designed for offshore wind turbines behaves as rigid pile (pile only rotates when subjected to lateral loading in soft soil) because of large diameter related to the respective pile length, being usually called as short pile. While the piles used for the development of the p-y curves behave as flexible piles. So, for short piles the soil-pile interaction response will be higher as compared to flexible piles.

But in this study the monopile is assumed to be flexible for evaluation of soil-pile interaction. The following Figure shows the rigid and flexible behavior of pile.

Abbildung in dieser Leseprobe nicht enthalten

Figure4-18 Flexible Vs Rigid Pile [34]

4.9.1 P-Y Curve (Winkler Model)

According to simple Winkler model the soil is replaced by a series of infinitely close independent nonlinear elastic springs, with their constant K being the coefficient of subgrade reaction or soil reaction modulus which represents the soil lateral stiffness.

This is based on the semi-empirical relationships which shows the pile response {lateral displacement(y)} as result of pile pressure (p) acting against the wall of the pile.

The P-Y curve shows the relationship between the soil resistance per unit length (on y-axis) and pile deflection (on x-axis). The soil resistance increases with the increase in the depth of the soil due to overburden pressure. The soil resistance depends on soil properties (density, internal friction etc.), soil type (clay, silt, sand etc.) and pile diameter.

Referring to Figure 4-19, pile response can be described with following the differential equation [34]:

Abbildung in dieser Leseprobe nicht enthalten

Where y is the lateral displacement of the pile at a given point along the pile, D is the pile diameter, constant K being the coefficient of subgrade reaction or soil reaction modulus, EpIp is the bending stiffness of the pile with the Young’s modulus Ep and the second moment of area Ip.

Abbildung in dieser Leseprobe nicht enthalten

Figure4-19 Winkler model of the pile response to lateral loads [35]

Equation 4.47 can be solved analytically under specific boundary conditions to get the pile deflection, moment, shear force and soil reaction distribution with depth.

4.9.2 P-Y Curve for Piles in Sand

Following steps should be followed for estimating p-y curves for a pile in sand above and below the water table [35].

1. Determine the friction angle (φ') of the sand, and its unit weight (γ). Submerged unit weight (γsub) for sand is considered below water table as (γsub = γsat-γw) and total unit weight (γ) is used above water table.
2. The Ultimate soil resistance per unit length (Ps) of the pile is the minimum of below two values:

Abbildung in dieser Leseprobe nicht enthalten

3. The As factor value is estimated using Figure 4-20 and from that value the ultimate soil resistance developing for lateral displacement of ݕ௨௟௧ ൌ ͵ܦȀͺͲ is estimated using:

Abbildung in dieser Leseprobe nicht enthalten

Figure4-20 Variation of the factor As with normalized depth z/D [35]

4. The Bs factor value is estimated using Figure 4-21 and from that value the soil resistance developing for lateral displacement of ݕ௨௟௧ൌ ܦȀ͸Ͳ is estimated using:

Abbildung in dieser Leseprobe nicht enthalten

Figure4-21 Variation of the factor Bs with normalized depth z/D [35]

5. The kpy value,the slope of the initial straight-line part of P-Y curve is estimated from the Table 4-2 which depends on the relative density of the sand and groundwater table conditions.

Table 4-2 kpy of the p-y curve for piles in sand above and below the water table

Abbildung in dieser Leseprobe nicht enthalten

6. The factors m, n and C are calculated as:

Abbildung in dieser Leseprobe nicht enthalten

7. As obvious from Figure 4-22 the p-y consists of three linear and one parabolic segment, for each part the equations for deflection (y) and lateral resistance (p) are given in Table 4-3 as shown below:

Abbildung in dieser Leseprobe nicht enthalten

Figure4-22 p-y curve shape for pile in sand under static loading (after Reese et al., 1974) [35]

Table 4-3 Equations for y and P for different sections of P-Y curve [35]

Abbildung in dieser Leseprobe nicht enthalten

4.10 DESIGN METHODOLOGY

After having all the loads and P-Y curve, the tower and pile is designed for Serviceability Limit State (SLS) and then checked for Ultimate Limit State (ULS).

4.10.1 SERVICEABILITY LIMIT STATE (SLS)

To check Serviceability Limit State the tower of the wind turbine is modelled in SAP 2000 as fixed based cantilever tapered shaft and for service load the deflection is calculated at the top of the tower (say ∆1). Then the pile is modelled in ALLPILE and the deflection (∆2) and rotation (θ) of the top of the pile is find out. From the rotation the deflection of the top of the tower is calculated by the following equation [21]:

Abbildung in dieser Leseprobe nicht enthalten

Where

HT= Total heightabove top of pile.

Θ = Rotation of the top of pile.

Now the total deflection (∆) is calculated by adding the three deflections (i-e ∆=∆1+∆2+∆3). This deflection should be less than 0.01*H as per code.

4.10.2 ULTIMATE LIMIT STATE (ULS)

For the economical section from the SLS, the Ultimate Limit State of the section is evaluated to see whether the section fails or not due to the applied loads. For this the whole wind turbine is modeled in SAP 2000 for the factored loads and checked by checking the bending moment and shea-force of the turbine.

If the section fails in flexure or shear or in both, then the next economical section is checked for ULS and so the economical and safe sections for tower, transition piece and pile of wind turbine are selected by trial and error method.

4.10.3 LATERAL PILE CAPACITY

Lateral capacity of the pile cam be checked by taking plastic capacity of pile body by using following formula:

Abbildung in dieser Leseprobe nicht enthalten

4.10.4 LATERAL CAPACITY OF SOIL

Lateral capacity of soil can be checked by comparing lateral deformation of soil due to lateral load with ultimate allowable deflection given by P-Y curve

4.10.5 AXIAL CAPACITY OF SOIL

Bearing capacity of soil can be checked by Terzaghi’s bearing capacity relation with reduction factor of 0.49, using the formulas given in chapter 5.

CHAPTER 5 CASE STUDY FOR KARACHI

5.1 INTRODUCTION

In this chapter we will do all calculations for a site in Pakistan. The Pakistan has its sea in the south of it called the Arabian sea. It has two main ports: Karachi port and Gwadar’s port. The site that is selected under this task is Karachi. Karachi has a beach at south end and the adjoining offshore area can be used for the wind farm installation. This is because of the increase in average wind speeds in Karachi’s coastal areas and the more directional winds at this place. The possibility of wind farm can be roughly proved from the following data.

5.2 WIND DATA

The wind data is not recorded precisely due to small range of this work. However different resources are utilized to estimate the data of Arabian sea with the references given.

A research article shows the wind data at different heights in coastal areas of Karachi. This research also compares the data of 1995 and 2002 which shows the increase in average wind speeds in Karachi due to global climate change. This makes Karachi’s offshore area a good place for wind farm installation.

Abbildung in dieser Leseprobe nicht enthalten

Figure5-1 Annual wind speed data (taken at 61 meters height) histogram for the year 2002 [36]

The wind speed distribution shows that wind speed as high as 20 m/s @ 61m height has been experienced in coastal areas of Karachi.

Data of Tropical Cyclone 02A (NANAUK) in Arabian Sea by Pakistan Meteorological Department 2014 shows 20.58 m/s wind speed at 10m height. So, our final Uo will be 20.58 m/s.

5.3 WAVE DATA FOR FORCES

For wave data, maps will be utilized. The maps show that the climate is getting extreme. 1993 wave heights are more uniform while 2010 wave heights are very extreme, with 0.5m or less at coasts and above 8m in the mid ocean. For this work, we take 2010 data and approximate the significant wave height (Hs) for Karachi offshore sites as 2.5m. But for load calculation we will take max wave height(H) which almost twice the Hs. However, this is the average data and the extreme data will give more wave height. So, the significant wave height to be used is 3.5m. this gives maximum wave height as 7m. The time period of wave varies from 8 to 14 seconds in nearby areas. For this work we will take 11 sec average. (Wave spectral shapes in the coastal waters based on measured data off Karwar on the western coast of India).

Abbildung in dieser Leseprobe nicht enthalten

Figure5-2 wave heights in Arabian Sea

The offshore wind sites are located by NREL and another research article which locate these sites 60 km from Karachi coast. Depth information near Karachi coast is not available. Let’s roughly take average depth at this site as 20m in accordance with contour maps available on internet.

5.4 SUMMARY OF WIND, TIDE AND WAVE DATA

The data to be used for calculations is as follows.

5.4.1 WIND DATA

Abbildung in dieser Leseprobe nicht enthalten

5.4.2 WAVE DATA

Table 5-1 Wave Data

Abbildung in dieser Leseprobe nicht enthalten

5.4.3 TIDE DATA

Arabian sea tide current velocity is 1 m/s at coasts and 0.2 m/s well offshore. We take average of these two values for our calculations.

Abbildung in dieser Leseprobe nicht enthalten

5.5 WIND TURBINE MODEL

The exact turbine and its tower are not known. However, for this work, we take a 2MW turbine and a 61-meter-tall tower. The foundation type for this work is taken as monopile foundation which is a tubular steel pile type foundation. The foundation design depends on tower design and vice versa. Because the diameter of tower may govern the diameter of pile or vice versa. The initial diameter of pile is taken as 4m while initial diameter of tower is taken 4m at the base and 3m at the top. The initial thickness of pile is taken as 50mm and that of tower as 40mm.

Table 5-2 Case Study Wind Turbine Specifications [37]

Abbildung in dieser Leseprobe nicht enthalten

Figure5-3 Wind Turbine Structure

Table 5-3 Sea Bed Soil Profile

Abbildung in dieser Leseprobe nicht enthalten

5.6 FROYA WIND PROFILE

The formulas for this are given in chapter 4. We will use equation 4.4 to calculate the wind speed profile along the structure height.

Abbildung in dieser Leseprobe nicht enthalten

5.7 WIND FORCES

Figure5-4 Froya wind speed profile

5.7.1 WIND FORCES ON TOWER

The tower is discretized at 1-meter interval. The wind speed at each meter is taken from Froya wind profile. The wind force at each meter of tower is calculated using equation 4.5. The total wind force on tower is obtained by summing forces on each meter.

The wind force on tower is dependent on diameter of tower because it changes the projected or normal area to wind. That why for each trial the force on tower will be different.

5.7.2 WIND FORCES ON ROTOR

The wind force on rotor is either calculated using formulations given in section 4.4.4 or they are acquired from the blade manufacturer.

The forces are considered for two conditions. One condition is when the turbine operating at max operating speed and the other condition is when turbine is stopped during high storm condition. The turbine stop itself automatically during storm to avoid wear and tear of its components.

The rotor will be same for every trial. That’s why the rotor forces will be same and are given below.

Abbildung in dieser Leseprobe nicht enthalten

5.8 HYDRODYNAMIC LOADS

The hydrodynamic forces will be calculated at each depth. For this purpose, we need water particle velocity and acceleration at each point. That velocity and acceleration will be used in equation 4.38 and 4.39 respectively to calculate forces.

5.8.1 WATER PARTICLE VELOCITY AND ACCELERATION PROFILE

The velocity of water has three components. Wave velocity, tide velocity and wind generated velocity. The structure is discretized at one-meter interval. The velocity is calculated on each meter. Same procedure is done for acceleration at each point. The total velocity i-e the sum of three components, and acceleration are plotted to know the water particle velocity and acceleration at each depth. Wave velocity, acceleration, tide velocity and wind generated water velocity are calculated using equations 4.27, 4.28, 4.35 and 4.34 respectively.

Abbildung in dieser Leseprobe nicht enthalten

Figure5-5 Water particle velocity and acceleration profile

5.9 DESIGN

The dimensions are not known at first. The dimensions can affect the loads on the whole structure. If diameter of tower and pile is taken more, the load of the wind and waves will also be more. The thickness should be such that it gives least area (least material) and qualify buckling and bearing capacity checks. So, an optimized diameter and thickness will be chosen by trial and error method explained below.

So, first initial diameters and thicknesses will be utilized. The serviceability will be checked against a range of diameters and thicknesses. The deformations, diameters and thickness will be plotted on graph. The sections that just qualify the serviceability criteria will be selected for further analysis. After that all the checks will be applied on each. The section that qualify the checks and has least cross section area will be selected as final design cross section.

5.9.1 SERVICEABILITY CHECK

This will be done in the following steps.

1. Select a few sections
2. Find forces on it as shown in previous topics
3. Model tower and transition piece in SAP2000
4. Find the deflection of tower
5. The analysis will give deflection of tower- 'tower
6. The analysis will also give vertical forces and moments at the base. These forces and moments will be applied on top of pile
7. Go to Allpile and model pile section
8. Assign the forces and moments on the pile top that were determined from SAP2000
9. Find the pile head deflection ('pile)
10. Find the pile rotation
11. From pile rotation, calculate the deflection due to pile rotation at tower top ('pile, rotate).
12. Sum the three rotations to get final rotation at tower top ('total)
13. Summarize the data in a table and a make a graph of it.
14. Select the sections that just fulfill the serviceability criteria and proceed with further checks.

The following sections are selected for serviceability check:

Table 5-4 Serviceability check sections

Abbildung in dieser Leseprobe nicht enthalten

Figure 5-6 Tower Top Deflection vs Diameter vs Thickness

The total height of the structure is 115 m. For serviceability criteria to be fulfilled, the lateral deflection should not exceed 1% of the total height i-e 1.15 m.

Table 5-5 Wind Turbine Profile

Abbildung in dieser Leseprobe nicht enthalten

The sections that just fulfill serviceability criteria are:

Table 5-6 Economical Sections

Abbildung in dieser Leseprobe nicht enthalten

We will check these two sections against other criteria. If these sections fail, then we will come down in the deflection Figure 5-6 to choose another two sections. But we should also keep an eye on cross sectional area of the section. Lesser the area, lesser will be the cost of the structure and lesser will be the weight or vertical force.

So, section with diameter of 3.3m and thickness 25mm is selected for further design because it gives least area and has better moment of inertia.

5.9.2 LOAD DISTRIBUTION ON SELECTED SECTION

The load distribution along height of tower can be calculated by using Froya wind profile, water particle velocity & acceleration profile and the formulas discussed earlier in this chapter. The rotor force is applied additionally at top as 144 kN which is not shown here.

Abbildung in dieser Leseprobe nicht enthalten

Figure 5-7 Hydrodynamic and Aerodynamic force profile

5.9.3 SUPER STRUCTURE STRENGTH CHECK

The section that is selected is modeled in SAP2000. The details of model are given below.

Abbildung in dieser Leseprobe nicht enthalten

The tower is tapered. The top diameter is mostly 1-1.5m less than the base diameter. In similar way the tower top thickness is 10-15mm less than tower base thickness. The transition piece connects the tower to the pile. The transition piece thickness is same as tower base thickness. Its diameter is a little more in order to fix the tower base in it at the top and pile at the bottom. The pile outer diameter is same as tower base diameter.

The section was checked in SAP2000 with load of 1.35 factor as suggested by DNV guidelines for turbine structure design. The structure was safe. The results showed that structure is loaded to 0.5-0.7 of its capacity. This verified our structure against strength limit state.

5.10 SUB-STRUCTURE DESIGN CHECKS

The Allpile software was used to check the sub-structure strength. For this purpose, we needed the loads that were transferred to pile top from the super-structure. These were obtained from the SAP2000 model previously made for design. The loads that were transferred were:

Abbildung in dieser Leseprobe nicht enthalten

These values were fed into Allpile software which gave the following output:

Abbildung in dieser Leseprobe nicht enthalten

Figure5-8 Depth vs Deflection, Shear and Moment

Abbildung in dieser Leseprobe nicht enthalten

Plastic moment capacity of pile is used in pile design. The plastic moment capacity of pile is given by:

Abbildung in dieser Leseprobe nicht enthalten

Mplastic = 64623 kN (OK)

Max shear capacity of pile can be estimated as: V = 0.6 * max tensile strength * Area

V = 0.6 * 400 MPa * 0.257

V = 61680 kN (OK)

The pile design checks are complete.

Now let’s go to soil design checks

The vertical allowable settlement in pile is 20mm > (2mm vertical deflection)

The allowable lateral deflection in soil is estimated from P-Y Curve below:

Abbildung in dieser Leseprobe nicht enthalten

Figure 5-9 P-Y Curves

Here the Yult is around 115mm

So,

Yult > (Ymax = 41.8mm) (OK)

Now let’s check the Terzaghi bearing capacity under toe

Qfout = β x σ'z x πD.L Where,

Qf = Skin Friction.

σ'z = Average Effective Stress

β = ko x tan x φi

For smooth pile surface like steel pile

φi = ½* φ' = 200

ko = (1-sin φ’) = 0.36

β = 0.13

Qfout = 2763.5 kN

Qfinside = Qf/2 =1382 kN

Qf = Qfout + Qfinside

Qf = 4145.21 kN

Now let’s check bearing capacity at bottom of pile (Qb)

Qb= Nq x σ'zb x Ab

Abbildung in dieser Leseprobe nicht enthalten

Now in order to increase the bearing of soil we must increase the thickness of pile. By taking 50mm thickness we get:

φgb x Qult = 0.49xQult = 4780 kN > (Qapplied=4645kN) (Ok)

Abbildung in dieser Leseprobe nicht enthalten

Figure5-10 Design Details

Abbildung in dieser Leseprobe nicht enthalten

Figure5-11 Designed Sections Detail

CHAPTER 6 SUMMARY AND CONCLUSIONS

6.1 Conclusions

The work done in the previous chapters gave an overview of the turbine industry. The basic definitions and the types of turbines and their foundations were discussed. This gave a broad view of the options available to work on. The new foundation types that float in the mid sea were also discussed. At the end a very simple approach was followed to give a design for the Karachi wind farm. The scope of the work did not allow us to go further and utilize the more advanced techniques in the analysis of the structures.

The projects done in the past were shown in the form of graphs. This can help recognize where and which country is more active in the turbine industry. This helps in finding the basic knowledge and literature for further research if someone intends to. From the search and data collection it came to our knowledge that European and Scandinavian countries have excelled in this field. China has also played a vital role in this industry and has the largest production form this technology. So, for further studies one should collaborate with them.

The expanding industry has now turned its way to offshore wind farms. The onshore machines carried some nuisances with them. Like noise pollution was big problem when installed near the population. The other problems are the winds and the turbulences it carried. However, the offshore condition of winds is more uniform and more directional. Speeds are greater and can be achieved at lower heights. Also, the turbulences are least. The problem of the noise pollution is not involved. Bigger size machines can be installed.

6.2 Calculations

The calculations performed were basic in nature. They were very approximate methods of analysis. These methods make thing conservative in one place effecting the economy while in some other place they underestimate things endangering the structural stability of the turbines.

The airy theory used in wave calculation was approximate. It was a linear approach to problem. But to make things simple and as a starter we selected to use it. The more sophisticated theories like Stoke’s theory that are multi degree (up to 11-degree function) should be used. Also, the wind forces calculated on the rotor was through an old technique. The simulation should have been used to calculate these forces. The simulation results closely follow the wind tunnel test results.

The accuracy of newer techniques is better but still researchers are in search of better techniques. They have modified these old theories for the turbine analysis. This is because the complexities involved in this field. The nature of the conditions of wind and waves are complex and mostly new techniques employed still give an approximation of the real condition. Now scientists are shifting to modelling of the wind turbine structures. This is becoming a new trend in this industry. Different companies have created their software that do the job of structure analysis and design. They model the real field condition and subject the structure to it. However, research is under way to make even these computer techniques better. The design guidelines that used old computations are also slowly shifting to computer-based techniques. The difficult task is the modelling of the field condition of the winds and waves

inside the computer. This advancement will enable to make the structures more optimized and the turbines more efficient. This will reduce the project cost and increase the capacity of the same size of turbine. The overall technology will become more favorable for environment and also economic. So, the future will favor the installation of more turbines. In this way cheaper energy will be produced and the environment will also be conserved.

6.3 Recommendations

The recommendations are provided in order to work on better ways of structure analysis of turbine. These better ways and techniques were not discussed in this work because of the broad extent of the work. However, if someone want to further work on this type of structure i.e offshore monopile structures then we provide the guidance in the form of the following points. These are some good topics to research on:

- The calculation of wind forces on the blades by using more accurate methods like discretizing the blade length and applying the forces on each part.
- Verifying the forces calculated in the above step by computer simulation. This will show the accuracy of the manual method.
- Modelling of the wind data in the computer simulation. This will cover the turbulence of wind as well. This data can be used for any structure subjected to forces of wind.
- Simulation of the waves in computer.
- Applying modern methods and theories for the calculation of wave forces
- Checking other limit states on the turbine structure and foundation
- Wind and wave data processing, their collection methods and use of different spectrums to analyze the data
- Working on the dynamic aspects of turbine and the interaction with dynamic forces and their frequencies to avoid the resonance with them.

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[...]

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Title
On- and Offshore Wind Turbines. Technology and Design
Author
Year
2019
Pages
95
Catalog Number
V501688
ISBN (Book)
9783346114495
Language
English
Tags
offshore, wind, turbines, technology, design
Quote paper
Muhammad Muzzamil (Author), 2019, On- and Offshore Wind Turbines. Technology and Design, Munich, GRIN Verlag, https://www.grin.com/document/501688

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