ABSTRACT rating of 4.8 megawatts (MW) per turbine

ABSTRACT

Wind turbines
are complex engineering systems, subjected to high fluctuations and irregular
loading. These mechanical systems are located in high demanding environment
which poses challenges in design of complete structure, including substructure
and foundation. This paper reviews the fundamental aspects and major issues
related to the modern designs in practice now a days. The optimal design of
wind turbines, particularly its foundation structure is worthwhile. Due to
limited guidelines for design and analysis of foundations; insufficient
strategies and alternative techniques; and high construction and maintenance
cost, offshore wind energy structures require more advance engineering
techniques than for onshore power turbines. Different advance design approaches
are discussed in terms of loading criteria, natural frequency and power
generation. A review of design safety considerations and structural reliability
studies are mentioned. The ultimate challenges and possible approaches are
highlighted. Some new recommendations are given for future work in this high
relevant field of research.

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Keywords: offshore wind turbines, structural analysis, design,
components

1.    
Introduction:

The wind
industry is thriving worldwide, both offshore and onshore, for energy
production as it requires a natural force (wind) for power generation. Wind
turbines are categorized by axis of rotation of the main rotor shaft (either
horizontal or vertical axis) and whether they are located onshore or offshore
(Tong, 2010). Due to less land availability, the offshore wind turbines sites
are of key interest to many countries. In the United States, roughly 50% of
total population lives in coastal areas to include counties directly on
shoreline and counties that drain to coastal watersheds. In 2016, 361 offshore
wind turbines (OWTs) of an average capacity rating of 4.8 megawatts (MW) per
turbine were constructed in Europe (Pineda and Tardieu, 2016). Abundant
offshore energy resources have the potential to supply immense amount of
renewable energy to coastal areas.

OWTs operate at
the same basic principle, wind blows and flows over the air foil blades of wind
turbines, causing the blades to spin. The blades are connected to a drive shaft
that turns an electric generator to produce electricity.  Besides being characterized by reduced visual
impact since they are placed far from coast, OWT can take advantage of high
velocity wind with intensive force, which could increase the regularity and
amount of power generated by each machine. From the general point of view, OWT
is formed by mechanical and structural elements. As a consequences, it is not a
common “civil engineering structure”; it behaves differently according to its
various functional phases like (idle, power production etc), and it is
subjected to highly variable loads like winds, waves, sea currents etc. Now a
days, development of newer turbine and foundation technologies will allow wind
turbines to be built further offshore in deep waters.

For modern
commercial wind turbines, the main rotor shaft is aligned horizontally. Rated
power generation capacity is largely dependent on rotor diameter and wind speed
(IRENA, 2012); e.g., if wind speed
increases two fold, its energy content increases eight fold. Two key speed
terms are ‘cut-in speed’ at which the wind turbine begins to produce power, and
‘cut-out speed’ at which the turbine must be shut down to protect the rotor and
drive train machinery from damage (Sørensen et al., 2009; Tong, 2010).

In order to
generate more electricity, modern offshore wind turbines are built with large
rotor diameter and at greater water depth, which significantly increases the cost
of an offshore project. A recent trend, however is the return of development
interest to new production lines for the size ranges most relevant to land
based turbines, from 800kW up to about 3MW. Of the other main components,
larger rotor diameters have been introduced in order to enhance exploitation of
low wind speed sites. Reinforced structures, relatively short towers and
smaller rotor diameters in relation to rated power are employed on extremely
high wind speed sites.

Between 2000 and
2011, global wind-power capacity approximately doubled every 3 years, with an
estimated total power generation of 238 GW achieved by the end of 2011; China, the
USA and Germany are the top industry players (GWEC, 2011). Although the market
is still dominated by onshore, with significant onshore wind resources yet to
be explored, the offshore wind market is growing rapidly. Global total
installed capacity for offshore of 3.12 GW was generated by the end of 139 2010,
with 1.16 GW added in 2010 alone – a 59.4% increase on the previous year (WWEA,
2011). Total offshore wind-power capacity in Europe reached 2.90 GW by the end
of 2010, with 0.88 GW added in 2010; again this represents a significant increase
of 43.6% on the previous year. This occurred at the same time as onshore
new-capacity additions declined by 13% (WWEA, 2011). The size of offshore wind
farms is also increasing, with 2010 data indicating that the average size of an
offshore wind farm in terms of power output was 155 MW – more than double the
average wind farm size of 72 MW for 2009 (EWEA, 2011).

Preliminary data
for 2011 suggest that offshore wind power capacity in Europe increased by 0.86
GW (EWEA, 2012), with the offshore market likely to be driven by mainly the UK
and Germany, although France and Sweden also have significant projects imminent.
As the end of 2015, a cumulative total of at least 990000 wind turbines were
installed all over the world. This is an increase of 5% (8.3% in 2014) compared
to previous year, when 945000 units were registered. The recorded small wind
capacity installed worldwide has reached more than 945MW at the end of 2015.
This is growth of 14% compared with 2014, when 830MW were registered. (WWEA,
2015)

 In 2016, 21GW of new installations took place
worldwide. Wind capacity has reached 456GW, where Germany, India and Brazil
leading in market growth (WWEA, 2016). Interest in offshore wind power is also
increasing in other regions of the world, with, for example, China, the USA and
South Korea planning to generate 6.0 and 3.0, 2.5 GW, respectively, by 2020.
Building on this, China and the USA have ambitious plans to generate 65 and 54
GW, respectively, from offshore wind by 2030 (AWEA, 2012; Musial and Bonnie,
2010).

 A significant hurdle for the offshore market,
however, is the high initial capital investment costs of the project, which is related
to: inadequate and (or) potentially unreliable design guidelines for offshore
wind-turbine (OWT) installations, especially foundation structures; more
stringent requirements for durable construction materials to withstand the
harsh marine environment; high-tech equipment requirements for on-site
operation and also shortage of trained manpower (Musial and Bonnie, 2010).

2.    
Design considerations of modern offshore wind
turbines

The main components of typical wind turbines (figure1) include foundation,
support structure, tower, rotor blades and nacelle. The foundation system and
support structure, used to keep the turbine in its proper position while being
exposed to the forces of nature such as wind and sea waves, are now made
stronger using materials such as reinforced concrete or steel. Support
structures connect the transition piece or tower to foundation at sea bed
level. The tower also provide a means to correct any misalignment of the foundation
that may have occurred during installation.

Electric current generated through
wind power is converted to higher voltage via a transformer at the base of the
tower. The power that can be harnessed from the wind is proportional to the
cube of wind speed up to a theoretical maximum of about 59 percent. However,
today’s wind turbines convert only a fraction of the available wind power to
electricity and are shutdown beyond a certain wind speed due to structural
limitations and concern for wear and tear (Malhotra, 2011). Modern OWTs are
installed with either pitch regulated blades or variable rotational speed
systems in order to allow optimization of power production over a wide range of
prevailing wind speeds. The rotational speed of main rotor shaft is typically
10 to 20rpm (Alderlieste, 2010; Malhotra, 2011).

The optimum tip speed depends on the
number of blades and profile type used.

Figure
2: Effect of number of blades
on power performance (Burton, 2001)

 

The fewer the
number of blades, the faster the rotor needs to turn to extract maximum power
from the wind. Three bladed rotors have a higher achievable performance
coefficient which does not necessary mean that they are optimum. Two bladed
rotors might be suitable alternative because although the maximum capacity is a
little lower, the width of peak is higher and that might result in larger
energy capture. Reducing the number of blades reduces the weight of the rotor
and subsequently the weight of the support structure. In addition it shortens
the time required for transportation and installation which directly decrease
the cost of energy.

2.1      FOUNDATION SYSTEMS AND SUPPORT STRUCTURES

The offshore bottom mounted support
structures are classified according to three basic properties that are:
installation principle, structural configuration and foundation type (Ferguson
et al. 1998). In this paper, classification based on water depth that each
concept can be used economically, gravity base, monopile, tripod and floating
support structures are reviewed.

 

Table 1: Concept for support
structure (Ashuri and Zaaijer, 2007)

Support
structure concept

Water
depth (m)

Gravity Base

0-10

Monopile

0-30

Tripod

>20

Floating

>50

 

2.1.1       
Gravity base structures (GBS)

From
structural point of view, a GBS is a monotower that is fixed at the top of a
gravity base foundation (figure 3). The foundation has a flat base to resist
overturning loads imposed by the wind and wave, and a conical part at the water
surface level to break the ice and reduce ice load by causing the ice sheets to
bend downwards and break-up as they contact the conical section (Zaaijer,
2003).

Figure
3: A tupical Gravity based
support structure (Ashuri and Zaaijer, 2007)

In order to keep
the attachment between the GBS and sea bed, ballasts are laid on the flat base.
In this way, the foundation always remains in compression under all
environmental conditions and cannot be detached from seabed.

2.1.2       
  Monopile

Monopile support
structure consists of a steel pipe as a foundation which is driven or drilled
in to the soil. The monopile is equipped with a transition piece to absorb
tolerances on the inclination of the monopile and to reduce the assembling time
required at sea and the tower which is mounted offshore on the top of the
transition piece.

The steel pipe
transfers all the loads by means of vertical and lateral earth pressure to the
ground. Therefore, both uncertainties in the ground properties and scour holes
can lead to a structure with different frequency than designed for. Designing a
monopile support structure is a challenging task.

Figure
4: A monopile support structure
(Ashuri and Zaaijer, 2007)

2.1.3       
 Tripod

The tripod
consists of a central steel shaft and three cylindrical steel tubes with driven
steel piles. The central part distributes the loads to the cylindrical tubes
and acts as a transition piece for tower. The cylindrical tubes give additional
stiffness and strength and increase the capacity of the structure to support
additional overturning moments (Zaaijer, 2003).

The foundation
has the advantage that it requires less protection against scour than the
monopile, which generally has to be protected against scour in sandy sea beds.

Figure
5: A typical tripod structure (Ashuri
and Zaaijer, 2007)

2.1.4       
 Floating

Current fixed bottom technology has seen limited deployment to water
depths of around 30-m thus far. A floating support structure increases the
flexibility in locating the turbine in water depths of up to 200 meters and is
well known from oil and gas industry. The floating support structure consists
of a floating platform and a platform anchoring system. The platform has a
transition piece to install the tower on top of that.

The platform can have several topologies such as single and multiple
turbine floaters. The anchoring system fixes the platform and can be gravity
base drag embedded driven pile.suction anchor type (Musial, Butterfield and
Boone, 2006)

Figure 6: Candidate Floating support
structure  (Ashuri and Zaaijer, 2007)

3.    
Environmental loading on offshore wind turbines

The relationship
between the environment and offshore wind turbines is rather unique: offshore
wind turbines are especially designed to catch as much wind load as possible.
However, the economical aspect of wind loads on structural design must be
strictly separated. The calculations for a successful project are critical and
rely heavily on the quality of environmental data available.

It is observed that
environmental loading is composed of a mean or slowly variable part and a
stochastic part. In the case of the aerodynamic and hydrodynamic
actions, the first component is generated by the mean wind velocity and by the
sea current, while the stochastic component is generated by the turbulence wind
velocity and by the non-rotational (exception made for breaking waves) waves
(Francesco, Hui and Franco, 2010). Aerodynamic loading results
from interaction of the rotor and parts of the tower with the turbulent wind
field, with generated wind power directly proportional to the cube power of
mean wind speed. However aerodynamic loading conditions for offshore and
onshore scenarios are markedly different, with considerably lower fluctuation
in loading experienced by the former on account of free-flow conditions and
lower surface roughness, although advantages of reduced dynamic loading are
partly undone by higher mean wind speeds (Fischer, 2011).

Since wind is the primary
energy source for ocean waves, higher wind speed may produce marginal increases
in turbulence on account of ensuing increases in roughness of the ocean surface
(Letchford and
Zachry,  2009). Another aspect of fluctuating wind speed is turbulence
induced in wake conditions. Ambient non-obstructed turbulence is the ‘normal’
turbulence that would be experienced by a single stand-alone turbine at a
particular site (Frandsen and Thøgersen, 1999).

Figure 7: wind and wave action
configurations (Francesco, Hui and Franco, 2010)

 

4.    
Failure analysis of wind turbine components

There is a
limited literature on failure analysis of wind turbines before 1990s. Veers
noticed the random and uncertain parameters involved in the component design of
wind turbines and first performed reliability analysis for a vertical axis wind
turbine blade. Around the 2000s, Ronald et al. applied reliability methods to
analyses of rotor blades of horizontal axis wind turbines. Afterwards, more
probabilistic models of wind turbine structural components were proposed, and
advanced wind turbine simulation tools came to use. In the past few years,
interesting studies of reliability and failure analysis of different assemblies
of wind turbines have been carried out. Thus we classify the survey into
following categories: rotor blades, bottom fixed spport structures, floating
systems and mechanical and electrical components.

Table 2
summarizes representative failure modes of these components. The failure modes
should be interpreted in a broader sense. For example, large deformation of
blades does not necessarily cause damage on blade itself, but an interference
with the tower should be deemed unacceptable. Suitable reliability methods
should be used to avoid these failures.

Table 2: List of failure modes of
wind turbine components

Category

Component

Failure Modes

References

Rotor Blades

blade

excessive bending stress, fatigue, buckling, large deformation

(Veers,1990) and (Ronald and Larsen, 2000)

Bottom-fixed support structures

tower

excessive deformation, fatigue, yielding and plastic collapse

(Jin, Ju and Zhang, 2016) and (Philipidis and Bacharoudis, 2013)

grouted connection

loss of bearing capacity, soil failure

Lee and Choi et al., 2014

gravity based foundation

loss of bearing capacity, soil failure

Vahdatirad et al., 2014

tubular structure

fatigue, large displacement

(Dong and Moan, 2012) and (Wei, Arwade and Myers, 2014)

Mechanical components

shaft

fatigue

Tarp, 2003

gear

contact fatigue, bending fatigue

(Dong and Moan, 2014) and (BSI, 2006)

bearing

rolling contact fatigue, white etching crack, skidding

(Musial, Butterfield and McNiff, 2007) and (Jiang, Xing, Guo, Moan and
Gao, 2015)

Electrical components

solder elements

creep and fatigue, bond wire lift-off

(Kostandyan and Sorenson, 2011) and (Blaabjerg, Ma and Zhou, 2012)

Floating system

mooring lines

extreme load and line breakage

(Wandji, Natarajan, Dimitrov, 2016) and (Hallowell et al., 2017)

 

Numerous
reliability methods are suggested for each components such as response surface
methodology (RSM), incremental wind-wave analysis (IWWA) and peak response
factor (PRF) etc. (Ronal and Larsen, 2000), (Veers, 1990), (Teixeira, Connor
and Nogal, 2016).

 

5.    
considerbale factors in designing owt

This paper also reviews the main
factors on which modern designs of offshore wind turbines are based which
includes,

–       
Capacity Factor

–       
Reliability

–       
Challenges in Offshore Production

2.2  Capacity Factor

The previous
focus of the industry was increasing the total nameplate capacity of wind
turbines, the focus has now shifted to the capacity factor of the turbine,
which helps keeps energy cost low by providing the most possible power.

Keith Longtin
said that about 10 years ago, the capacity factor of typical turbine was 25
percent, today it is over 50 percent. This improvement in capacity factor also
improves the cost of energy which enables to go into more and more locations
where wind is lower.

One of the
deciding forces so far to increase capacity factor has been an increase in size
of the rotors used on wind turbines. In U.S, a turbine with 1.6MW capacity
comes with a 100 meter rotor, compared to 70 meter rotor in the past.
Increasing the size of rotors creates new challenges for manufacturers, however
rotors scale poorly with size, as a result the cost goes up faster than the
revenue generated by the increased capacity factor.

Turbine rotors
are affected by two different forces, torque and thrust. Torque turns the
rotors and create energy while thrust pushes against the turbine. Dealing with
thrust can be difficult when designing a rotor. There are tremendous loads of
up there, and it goes to great engineering technology to be able to create
these very reliable turbines.

Astlom Wind
North America took its eco100, a 3MW turbine with a 100 meter rotor and
upgraded it to 110 meter rotor in 2010. Last year, the company increased that
to 122 meter. The director of innovation for this company said that increasing
nearly 40 percent the area of rotor will deliver more efficient wind turbine to
the costumers because this will tend to produce more energy at lower wind
speed.

2.3  RELIABILITY

While the focus
on increasing the power produced from wind turbines may be on capacity factor,
another way is to make sure wind turbines are operational and available.
According to Keith Longtin, the availability of wind turbines 10 years ago was
about 80-85 percent and the wind industry was doing fine with that because in
past it was about 70 percent. But for running steam, gas and nuclear plants, 98
percent availability is required. Lot of investment is required to improve the
overall availability of a wind turbine and now it has increased to around 98
percent.

To help achieve
this kind of reliability and to continue improving on it, it is required to
improve individual components used in turbines both electronics and gearboxes.

2.4  CHALLENGES
IN OFFSHORE PRODUCTION

While the
onshore wind turbine industry is going strong, the wind industry is looking
toward the possibility of adding offshore wind capacity in the future. The need
of offshore wind production require different solutions than onshore. The use
of different technologies for onshore and offshore wind power projects is
another change that has occurred over the past 10 years, while companies used
to take the same wind turbine used on land and installed it offshore but now a
different approach is used with current generation of offshore wind turbines.

Floating wind
turbines are more significantly used which require floating structure instead
of requiring wind towers to be set into foundation under water. The modern
offshore wind turbines are now in the phase of demonstration which would result
in producing wind turbines with high capacity factor, more reliable and
economical as well.

One of the immediate design challenges
is the ability to accurately predict applied loads and resulting dynamic
response of the coupled wind-turbine and support structure under the action of
combined stochastic wave and wind loading (Musial and Butterfield, 2006). At present, analysis/design and installation of monopile
foundations for wind-turbine structures usually rely on general geotechnical standards, combined with more
specific guidelines and semi-empirical formulas developed by the offshore
oil/gas industry (API, 2000; DIN, 2005;
DNV, 2011;
Germanischer
Lloyd, 2005).

 

6.    
recommendation for future prospects

Wind power is
more efficient and affordable than it has ever been, it is an easy and quick
energy solution for increasing power generation. Technical modification and
system upgrades are required for extensive energy and environmental
adaptability of offshore wind turbines which includes,

–       
Strengthening the tower to cope with loading
forces from waves and ice flows, pressurizing the nacelles to keep corrosive
sea spray away from electrical components.

–       
Automatic greasing systems could be installed to
lubricate bearings and blades as well as heating and cooling system within a
specific range.

–       
Use of more sophisticated electromechanical
parts in OWTs (e.g. direct-drive units that eliminate the requirement for a
gearbox, thereby removing one of the key components prone to failure) will
increase the efficiency and hence energy yield and also reduce O costs
for the project.

–       
Computer simulation can be useful to companies
as they look to increase the capacity factor of turbines. These software allows
companies to help design blades that allow for attached flow across a range of
flow velocity without continuously make to rotors larger.

–       
The technology can be used efficiently by using other
simulations including manufacturing components along with monitoring the
potential performance of components, performing structural analysis and maintenance
of different parts.

7.    
summary and conclusion

Offshore wind
power generation appears to be a promising solution to overcome the universal
demand for clean, cost-effective and sufficient amount of energy. There is a
room for improvement in all areas of wind farm development; in design, through
innovative use of composite materials, support structures and foundations; and
in construction processes, through improvement in installation techniques, fabrication
and transportation. General consideration in designs of modern offshore wind turbines
have been reviewed. Wind farm developers and engineers should identify various
issues that are likely to arise in the development phase of an offshore wind
turbines.

In the design
process, verification of design solution is mandatory. Because of the complex behavior
of OWTs, design codes should be used to analyze the design solutions. Although some
guidelines or recommendations for wind turbine design are scattered in the
literature, an accepted best practice that takes a new designer step by step
through process of designing a wind turbine is missing.