"Converting Offshore Wind into Electricity" (complete book 2011)

Convertingoffshore
wind into Electricity
The Netherlands’contribution
to offshore wind energy knowledge
We@Sea research programme 2004-2010

Preface
The authors and Acknowledgements
1. The We@Sea programme in time perpective 7
1.1 State of the art of offshore wind energy at
the start of the We@Sea programme 000
1.2 Bridging the gap between the wind energy
and the offshore industry 000
2. Offshore wind energy technology 000
2.1 Introduction; the difference between
onshore and offshore design drivers 000
2.2 Dedicated concepts and design challenges 000
2.3 We@Sea research: new concepts and
components 000
2.3.1 The DOT system; a radical change
of concept 000
2.3.2 The use of thermoplastics in blades 000
Box: System identification 000
2.3.3 Support structures 000
2.4 We@Sea research: Analysis and design tools
for wind turbines 000
Box: Integration of design tools 000
2.4.1 Wind shear 000
2.4.2 Extreme loads 000
Box: Frequency domain analysis of loads 000
2.4.3 Remote measurements of blade
deflections 000
2.4.4 Fatigue loading of large blades 000
2.5 We@Sea research: Analysis and design tools
for offshore wind farms 000
3 Transport and Installation 000
3.1 Transport and installation 000
3.2 Logistics 000
Box: Foundations and installation methods 000
4 Up keeping offshore wind farms 000
4.1 Introduction 000
4.2 We@Sea research: Operation and maintenance 000
4.2.1 O&M cost modeling 000
4.2.2 Low cost load monitoring 000
4.2.3 Design for redundancy and fault tolerant
control 000
Box: Access technology: Ampelman 000
5 Grid integration of offshore wind power 000
5.1 Introduction how to accommodate large
amounts of electricity from the sea 000
5.2 We@Sea research: the offshore wind farm
electrical modeling 000
5.3 We@Sea research: Interconnection 000
5.4 We@Sea research: Energy and power balancing 000
6 Ecological impacts of offshore wind energy 000
6.1 Ecology 000
6.2 We@Sea research: Marine mammals 000
6.2.1 Introduction 000
6.2.2 Impacts on grey seals 000
6.2.3 Impact of the Offshore Wind Farm
Egmond aan Zee (OWEZ) on harbor
porpoises 000
6.2.4 Impact of pile hammering during
construction of the OWEZ wind farm 000
6.3 We@Sea research: Birds 000
6.3.1 Introduction 000
6.3.2 Bird sensitivity map 000
6.3.3 Explanation of bird distribution in
the Frisian Front 000
6.3.4 General conclusions 000
6.4 We@Sea research: Fish and benthos 000

6.4.1 …. 000
6.4.2 Conclusions 000
6.5 We@Sea research: Acoustic noise effects 000
6.6 Cumulative effects 000
7 Realising the ambitions 000
7.1 The need for a comprehensive overall policy 000
7.2 Contribution of the Dutch industru 000
7.3 Benefits and cost for society 000
Box: Two scenarios 000
Box: Map of offshore wind energy plans in
the Netherlands Exclusive Economic Zone 000
8 Concluding remarks 000
References 000
Annex 1 Participants of We@Sea 000

Preface
This book describes the main results of the Netherlands’
research programme We@Sea, which was carried out in
the period between 2004 and 201
1. This programme was
developed in 2003, a time during which very little experi-
mental evidence from fully exposed offshore wind farms on
the North Sea was available, while the national ambitions
for offshore in various countries were sky high. To embark
on the road towards thousands of megawatts of installed
wind power on the North Sea much knowledge was needed
to reduce risks, increase reliability and to reduce cost of
energy. Roughly wind from the sea was about twice as ex-
pensive as wind electricity from land. We@Sea was the first
comprehensive dedicated national wind offshore research
and development programme in the world. The We@Sea
programme was designed to contribute to the broad design
base, needed to successfully accelerate the implementation
speed of offshore wind power in the Netherands and to
facilitate the emergence of a new industrial branche, the
offshore wind power industry.
We@Sea’s objectives were derived from assumed needs of
the industry as they were known in 2003. At the time the
programme started, 592 MW of offshore wind power was
installed. In the course of time more and more operational
experience became available which showed evidence of
real problems rather than anticipated problems. Options
for improvement also arose. As a consequence the research
priorities of We@Sea were subject to changes and thus the
originally formulated deliverables do not completely match
with the results actually achieved.
This book will deal with the results actually achieved and
will not address the why’s and what’s of the differences
between the reference situation of 2003 and the present
(201
1) situation. The specific results of the We@Sea pro-
gramme are being described within the wider framework of
offshore developments worldwide with respect to meeting
present day’s research needs.
A large part of the content of this book is based on the
summary reports of the research areas of the We@Sea pro-
gramme. In Annex 1, more information about the We@Sea
programme can be found.
This book covers a very broad spectrum of scientific and
technical disciples on which wind energy technology is
based upon. Although the team of authors have tried to
explain technical terms as much as possible, it is virtually
impossible to explain all specialist terms. Despite the fact
that I accept that biologists will have difficulties in fully un-
derstanding the electrical engineering topics in this book,
and the structural engineers in understanding the chapter
about ecological impacts, I hope that this book will leave
a thorough understanding about progress in offshore wind
energy technology to which the We@Sea programme’s re-
searchers have contributed.
Jos Beurskens,
Scientific Director We@Sea

The authors
Editor and author:
Jos Beurskens (ECN, We@Sea)
.
Authors:
Eize de Vries (Rotation), Chris Westra (ECN, We@Sea)
Co auteurs:
Michiel Zaaijer (TUDelft).
Luc Rademakers. (ECN)
Jakob Asjes, Han Lindeboom (IMARES)
Frans van Hulle (XP Wind)
Acknowledgements
The team of authors wishes to thank the engineers,
scientists and project leaders of the more than 60
projects of the We@Sea programme who provided
inputs for this book through their reports and com-
ments.

7
Chapter 1
The We@Sea programme
in time perspective
1.1 State of the art of offshore wind energy at the start of the We@Sea programme
Since the oil crisis of 1973, renewable energy has quickly become a viable option for a sustainable energy
system. Wind energy technology was the first to make a significant impact on the energy balance. By the end of
2010, wind energy covered 5.3 % of Europe’s electricity demand and 4.1 % of the Netherlands’. From the begin-
ning of the emerging popularity of wind energy technology, energy specialists and policy makers were aware that
offshore wind energy was to play a significant role in the renewable energy’s share of our supply system. Without
exploiting offshore wind energy to its fullest extent, it would be impossible to meet the national and European
sustainable energy targets.
The first offshore wind turbine was put into operation in 1990 near Nogersund, Sweden (0.22 MW, 25 m rotor
diameter, figure 1.2), followed by the first wind farm in 1991 nearby Vindeby, Denmark (1
1 x 0.45 MW). This by
no means meant that offshore wind energy was an entirely new development at that time. Offshore wind energy
had been researched since the early 1970s [Heronemus]. In 1978 within the IEA (International Energy Agency)
LS WECS (Large Scale Wind Energy Conversion Systems) progamme, a systematic approach was launched to
assess the technical feasibility of offshore wind technology (see Figure 1.1.) All relevant technical aspects, from
wind turbine concepts to foundations and logistics, were addressed. The results, however, remained limited to
reports. It was not until 1990 and 1991 before the first tangible results were achieved. However, the first wind
turbines, which were installed offshore, were not the results of dedicated concepts like those investigated in
the IEA programme. They were merely derivatives of land-based machines. The construction was adapted to
meet offshore conditions. New design elements included transport, installation and foundations. The installed
power of the first wind turbines was equal to that of the commercial onshore wind turbines. The Vindeby Bonus
machines, e.g., had rotor diameters of 35 m and the installed power was 0.45 MW, which in terms of capacity is
smaller than the presently (201
1) used offshore turbines by about a factor of 10. The first-generation wind farms
were built close to shore, in shallow and relatively sheltered waters. Table 1.1 gives an overview of all wind farms
built before 2004.

8 Converting offshore wind into Electricity The We@Sea programme in time perspective
Figure 1.1. Within the
IEA LS WECS pro-
gramme since 1978
offshore technol-
ogy was studied. The
participating coun-
tries were Denmark,
the Netherlands,

Sweden, Great Britain
and the USA.
Figure 1.2. Ronneby wind turbine, Sweden

Converting offshore wind into Electricity The We@Sea programme in time perspective 9
Since the best wind sites on land in Northwest Europe were
gradually being used up, and as public resistance against
using large land areas for wind energy development grew,
industry and policy makers became increasingly aware that
offshore deployment of wind turbines was the only way to
achieve the ambitious national and European goals.
The sheer size and capacity of the anticipated wind farms
and the associated operations, financing, planning and
legislation required a comprehensive and balanced exten-
sion of the knowledge base. Offshore wind energy would
become the motor for innovations and scaling up the di-
mensions of wind turbines. During this evolution new physi-
Table 1.1. Offshore wind farms constructed in the period 1991-2003
Commissioned Location Number of turbines
and capacity [MW]
(water depth [m]) /
(distance to coast [km])
Type of foundation
1990 Nogersund (S) 1 x 0.22 = 0,22 Tripod on solid rock
(abandoned)
1991 Vindeby (DK) 11 x 0.45 = 4.95 (2.5 – 5) /
(1.5 – 3)
Gravitation
1994 Lely park, IJsselmeer (NL) 4 x 0.5 = 2.00 (2 – 3) /
(1)
Monopile in fresh water
1995 Tunø Knob (DK) 10 x 0.5 = 5.00 Gravitation
1996 Dronten, Ijsselmeer, (NL) 28 x 0.6 = 16.8 (2 - 3) /
(0.02)
Monopiles in sand
1998 Bockstigen (S) 5 x 0.5 = 2.5 (5.5 – 6.5) /
(4.5)
Monopile
2000 Blyth, North Sea (GB) 2 x 2 = 4 (6 + 5m tides) Monopile
2000 Utgrunden (S) 7 x 1.43 = 10 (7.2 – 9.8) /
(12.5)
Monopiles
2001 Middelgrunden (DK) 20 x 2 = 40 (3 – 5) /
1.7 – 3.5)
Gravitation
2001 Yttre Stengrunden (S) 5 x 2 = 10 (xx) ?
(5)
Gravitation
2002 Horns Rev I (DK) 80 x 2 = 160 Monopiles
2003 Nysted, Rødsand (DK) 72 x 2.3 = 165.6
2003 North Hoyle (GB) 30 x 2 = 60
2003 Frederikshavn (DK) 6 x 3,6 = 22.6
2003 Ronland (DK) 8 x 2.15 = 17.2
2003 Sansø (DK) 10 x 2.3 = 23
2003 Arklow Bank (EIR) 7 x 3.6 = 25.2

cal phenomena were encountered which previously were
ignored in the development of smaller onshore turbines.
Examples of such phenomena are the impact of waves on
structures, dynamic behaviour of foundation and impact
on wind turbine loading, turbulence characteristics offshore
and rotor blade aerodynamics.
In the Netherlands, a comprehensive programme was
developed in 2003 by some 30 parties, representing
the stakeholders in the offshore wind energy industry, in
order to contribute to a dedicated offshore knowledge
base. The programme, called We@Sea, was to focus on
those aspects that are crucial for the acceleration of the
use of offshore wind energy in the Netherlands. It was
based on the research needs of the wind energy indus-
try. Consequently, the scope of the programme was wider
than that of the earlier traditional technology develop-
ment programmes. Besides wind turbine and wind farm
technology, it addressed separate research areas on en-
vironmental and nature issues, grid integration, financing,
operation and maintenance (O&M), and integration and
knowledge transfer.
.Some time before the launch of the We@Sea programme,
the government entered into an initiative to build a demon-
Figure 1.3 ‘NL op weg naar 2000 MW in 2000’. The energy unit [PJ] indicates fossil fuels being replaced.

stration wind farm of over 100 MW in the North Sea, inside
the 12-mile zone. The reason for this initiative was to accel-
erate the reduction of CO2
emissions in order to meet Eu-
ropean and national targets. The wind farm, commissioned
in 2006 and later called ‘OWEZ’ (Offshore Windpark Eg-
mond aan Zee) had a capacity of 108 MW. It consisted
of 36 Vestas wind turbines with a rotor diameter of 90 m
each. OWEZ was not the first offshore wind farm situated in
the unsheltered waters of the North Sea. Before the We@
Sea proposal was finished at the end of 2003, a 530 MW
offshore wind farm was commissioned – 280 MW of which
were operating in the sheltered and shallow waters of the
Balic Sea and the IJsselmeer in the Netherlands. It took until
2002 before the first wind farm (Horns Rev), with a capacity
Figure 1.4. 160 MW Horns REV offshore wind farm in the North Sea, at Esbjerg, Denmark. Photograph: ELSAM.

of over 100 MW, was realised in the North Sea waters. It was
located between 14 and 17 km from the coast near Esbjerg,
Denmark, and was installed in water 6 to 14 meters deep.
Previously, the offshore wind farms had a maximum capac-
ity of 40 MW. Knowledge about large-scale wind farms in
the North Sea was limited and the existing information was
virtually not available for public use.
The knowledge base for embarking on a cost-effective and
time-efficient way of implementing large-scale wind farms
in the North Sea in 2003 was limited. At the same time,
projects with a total capacity over 7000 MW were in the
planning stage. Thus investing in offshore wind energy was
a risky business and financing was not a routine affair. Proj-
ect developers and financiers faced uncertainty of govern-
ment incentives for offshore projects and lack of spatial
plans.
1.2 Bridging the gap between the wind
energy and the offshore industry.
The two major groups of industrial stakeholders in the off-
shore sector are the wind turbine manufacturers and the
offshore operators and contractors. The wind turbine man-
ufacturers have a long history of designing and manufactur-
ing wind turbines for onshore applications.
The offshore industry used to serve the offshore oil and
gas sector, but was not acquainted with the typical re-
quirements of wind energy technology. Wind turbines are
relatively light and highly fragile structures compared to
components for oil and gas rigs. These are heavy, mostly
compact and robust to handle. Also, the number of units
to be installed in the wind versus the gas and oil sectors
differed significantly.
Basically it was the traditional wind energy industry that
took the lead in realising the first generation of offshore
wind projects. Both the offshore sector and the wind sector
had their own proven design practices. These worlds, how-
ever, were completely separated. Although there is quite
some overlap between the two, there was insufficient time
to integrate the design tools before the first wind energy
offshore projects were built. In particular, there was not
enough time to study integral structural dynamics, incorpo-
rating the dynamics of the support structure and the wind
turbine on top with considerations of wind and wave loads.
Doing so could result in less conservative design loads. Ul-
timately this would lead to lighter and thus cheaper integral
wind turbine structures.
Another area where synergies were anticipated constituted
of combined foundation, transportation and installation
approaches.
Merging design practices and logistics of the classical and
offshore wind technologies form the basis to bridge the
gap between the two sectors. This would offer the oppor-
tunity to establish new industrial activities for the offshore
industry with ample economic opportunities in light of the
ambitious national and European future offshore scenarios.
Facilitating the creation of new large-scale economic activi-
ties was one of the objectives of We@Sea. In practice bridg-
ing the gap would mean:
– the integration of design practices, creating missing
knowledge and verification,
– equal participation of both sectors in project design
(wind farm layout, electrical infrastructure, O&M, logis-
tics, protection of environment and nature),
– gaining confidence from the financial sector.

1.3 Evolution of research needs
Although all national dedicated offshore programmes claim
to cover the whole spectrum of offshore issues, financial
and human resources of these national programmes are in
reality too limited to address all aspects in full detail. Each
programme focuses on areas in which a country has unique
knowledge, experience, hardware or services to offer – or
where region-specific problems need to be resolved, such
as possible ecological impacts and grid infrastructure issues.
The research topics of the national We@Sea programme
resulted from two systematic approaches: the basic differ-
ences between onshore and offshore wind energy and the
research needs of the participants as perceived in 2003.
The fundamental differences between onshore and offshore
wind energy are summarized in Table 1.2.
Figure1.5.European wind energy development scenarios. Cumulative power as a function of time. Note that the uncertainty in the offshore
forecasts is larger than the onshore ones.. [1]

Based on the review of those differences the following re-
search areas were defined for We@Sea.
– Offshore Wind Power Generation (Technology)
– Spatial Planning and Environmental Aspects
– Energy Transport and Distribution
– Energy Market and Finance
– Installation, Operations and Maintenance
– Education, Training and Education
– Integration and Scenarios
From the numerous aspects addressed in these research
areas, the ones in bold from Table 1.3 were selected and
will be described in this book. From this table, it can also
be seen how these topics are situated in the overall R&D
landscape
The distribution of the available funds for the research areas
did not only reflect the overall need for know-how, but also
the focal points of the Netherlands’ companies and institu-
tions, participating in the programme.
The relative distribution of financial resources at the be-
ginning of the programme is shown in Figure 1.5. Most of
the funds were reserved for technology development and
ecology. After completion of the programme, however,
it appeared that the funds for technology development
remained partly unused. As the Netherlands has a rela-
tively small wind turbine manufacturing sector, the de-
mand for resources for wind offshore wind turbine tech-
nology lagged behind the initial planning levels of We@
Sea. Confidentiality constraints were another reason why
manufacturers were reluctant to use public funds from
the programme, as this would imply publication of results.
Topic Description
Cost break down The cost of transport, installation, grid connection and support structure of off-
shore wind turbines/farms is significantly higher than for onshore installation.
External conditions Along with wind speeds (average and extreme values) and turbulence, the follow-
ing parameters have to be addressed for offshore circumstances:
waves, marine currents, scour, salinity of the atmosphere. Further values for
extreme winds and for turbulence intensity.
Support structures Part of the support structure is below sea level
Transport Transport by ship, with associated dynamic behaviours
Installation and Commissioning Time and weather window for installation offshore is small and installation times
should be minimised. Commissioning is preferably before transport.
Operation and Maintenance (O&M) Access to wind turbines limited due to adverse weather conditions
Grid Integration No electrical infrastructure is available offshore (yet)
Environmental and ecological issues Offshore wind turbines operate in a completely different marine environments
(habitats)
Scale and risk For cost reasons, capacities of offshore wind farms will be much larger than for
land-based projects. Combined with technical risks this will lead to significantly
larger risks, and to higher cost of financing.
Table 1.2. Fundamental differences between on- and offshore wind energy technology.

The larger part of the available resources, however, was
spent on the development of design tools, which builds
on the relatively strong position of the Dutch R&D com-
munity.
More resources were needed for O&M-related research
compared to the original plan. See also Figure 1.5. This
trend reflects what was happening in practice. Considering
the construction of more wind farms in unsheltered waters
during the We@Sea programme (see Table 1.4), it appeared
that O&M issues were causing unforeseen expenditures.
R&D Objective Method Topic
Reduction of generation
cost of wind electricity at
wind turbine level
Up scaling
Fatigue properties of materials and components
New (blade) materials, including LCA
Components, support structure included
Rotor aerodynamics en aero-elasticity
Tools for design en analysis; reliability
External design conditions
Electrical conversion system en grid interface
Control- en safety concepts
Learning by doing & As-
set management
Transport
Installation
Operation & Maintenance
Reduction of generation
cost of wind electricity at
wind park level Optimisation of output
and load factor
Wake interaction
Interaction between wind turbines; electrically
Interaction between large scale wind farms and impact on macro wind
regime
Integration in the elec-
tricity supply system
Balancing electricity
supply and demand
Park control, safety, remote control
Improving predictability of wind farm output within 24 hours
Strorage systems
Load management
Interaction market and security of supply
Maximise utilisation of
variable (wind) power
supply
Grid improvement, intelligent grids, international ‘super grid’ offshore
Minimalise effects on
environment and nature Knowledge of potential
effects
Safety, in particular ships,
Birds
Marine life (sea mamals, fish, benthos)
Reduction cost-benefits
institutional processes
EIA guidelines
Basics for efficient providing concessions
Table 1.3. Major topics of the We@Sea research areas.

Problems occurred mainly with electric and electronic com-
ponents, gearboxes, generators, cables and transition pieces
for monopole foundations.
The matter was complex as it was not always clear what
the exact cause of problems was: poor manufacturing or
incorrect design requirements. In general service and repair
actions proved expensive as dedicated vessels for accessing
wind turbines during stormy weather conditions were not
available. This experience led to an increased demand for
research on O&M strategies and remedying solutions.
Research into the ecological aspects are in part regionally
specific. As such, they form an important part of each pro-
gramme. In shaping the programme it appeared that refer-
ence knowledge about the marine ecosystem in order to
assess the potential impact of wind turbines offshore was
virtually absent. Most of the ecological research was fo-
cused on conditions prior to the construction of wind farms
to create a reference for post wind farm construction evalu-
ations. This is why many research activities in this field were
not wind-energy specific. In a way, one could conclude that
the offshore wind energy research programmes have con-
tributed to the generic knowledge of the marine ecosystem.
In hindsight, it can be concluded that relatively large, long-
term research programmes being carried out synchronously
with large-scale commercial activities – like the We@Sea
Figure 1.6. Budget distribution before and after completion of the programme, showing the evolution of priorities.

programme – should be set up in a flexible way in order to
accommodate emerging research needs. This was the case
in the We@Sea programme.
A point that deserves special interest is formed by research
needs of general interest for the entire sector rather than
issues that are important to strengthen the competitive po-
sition of individual companies. Examples of this category
are ecological research and research in logistics (harbours).
Those activities should, however, be fully financed by (local)
governments or by common funds available to all relevant
companies who have a commercial interest in offshore wind.
The evolution of wind turbine technology is characterized
by gradual up-scaling (both in terms of wind turbine size
and wind farm capacity), a high level of reliability (typically
over 95%) and a high degree of controllability that makes
modern wind turbines grid-friendly. Extrapolating the suc-
cessful development to offshore sites requires completely
new disciplines in order to cope with the typical conditions
in the ocean.
Apart from the technical issues other aspects of realizing an
offshore wind farm need to be investigated. Operation and
maintenance were mentioned briefly. Optimizing these in-
terventions provides a significant option of decreasing cost
and ensuring operation at the highest efficiency during the
typical design life of 20 years.
Another essential element is envisioning a comprehensive
cross-national offshore electrical transport infrastructure at
sea for the integration of large peak power levels of electric-
ity into European grids.
Table 1.4 Development of offshore wind power in the period 2004
– 2010 From [1].
2004 605 MW
2005 695 MW
2006 787 MW
2007 1106 MW
2008 1479 MW
2009 2061 MW
2010 2944 MW

Chapter 2
Offshore wind energy technology
2.1 Introduction: The difference between onshore and offshore design drivers
The first offshore wind farm was built in 1991. This marks a gradual evolution of the wind turbine design base,
which had been under development since the late seventies. It now needed to be extended to address typical off-
shore issues. These topics were gradually incorporated into various national, EU and IEA research programmes.
The best way to achieve a comprehensive overview of the key issues related to offshore projects is to systemati-
cally review the differences between onshore and offshore wind energy.
Figure 2.1 gives an overview of these issues. It is evident that all these differences fall into two categories of
dedicated wind turbines: one for land and a second for offshore applications. It might take up to two decades to
develop, test and commercialise a fully dedicated optimized offshore turbine. The development of the Multibrid
wind turbine, for instance, was purposefully designed for offshore circumstances and has been under develop-
ment since 1995. More radical designs, deviating significantly from the present-day designs, only started some
5 years ago. One example is the Sway turbine from Norway. Thus in the future we will see more radically different
concepts appear next to the more conventional systems, which resulted from an incremental design evolution.
Figure 2.1.What
make onshore
wind turbines
different from
offshore turbines

Taking the scale and associated risks of offshore projects
into consideration, it is no surprise that the focus of mod-
ern research has shifted rapidly toward offshore technology.
Offshore has become the motor of research and innovation
in wind energy technology. Innovations resulting from off-
shore-oriented research might possibly find their way into
the class of land-based wind turbines in order to further
increase the cost competitiveness. The two classes, offshore
and land-based machines respectively, might further show
deviating rotor dimensions, driven by the differing cost
breakdowns. See Figure 2.5.
The ultimate goal of all research and development activities
is cost reduction, either straightforwardly through reducing
the initial investment cost of equipment and machines, and/
or indirectly by decreasing operational and lifecycle costs.
All technical and non-technical aspects dealt with by the
developer of a wind turbine or wind farm can be translated
into a risk factor and finally into a cost component. The
cost of capital is partly determined by the total risk taken on
during the course of the project.
Without a specific design, it is not possible to quantify the
risks. However, in general terms it is possible to identify the
risks and present them in a map which indicates the mutual
relationships of the risk factors and how they contribute
to the total risk of the project. Figure 2.2 illustrates such a
map, which has been used in the We@Sea project to quali-
tatively assess R&D activities in terms of their significance
in reducing the total risk. In such a way, risk considerations
have been used to establish project priorities and bring co-
herence to the programme. The risks are divided into two
main categories: the endogenous risks and the exogenous
risk reduction. Endogenous risks are those risks that can be
avoided or minimized by setting up proper design condi-
tions and execution of the activities accordingly. Exogenous
risks are risks associated with external factors that cannot be
influenced. However, actions can be taken to reduce or miti-
gate the possible effects of those conditions on the project.
It was the objective of the We@Sea programme to make
significant additions to the design base for offshore wind
turbines and farms. The contributions were based on avail-
able expertise and the needs of the partners. The actual de-
sign of wind turbine systems was considered as belonging
to the confidential strategic domain of the manufacturers
themselves, and thus was not included in the programme.
In this chapter, specific topics from three categories will be
addressed:
1. Wind turbine concepts and components.
2. Design analysis and tools for wind turbines
3. Design analysis tools for wind farms.
2.2 Dedicated concepts and design
challenges
Present practices of wind turbine design for offshore appli-
cations are based on existing designs for land-based wind
turbines. For applications in marine conditions, modifica-
tions were added to the basic design. This resulted in tur-
bines that, after a period of consequent ‘debugging,’ reach
an acceptable reliability level. However, it is unlikely that
these designs will reach the lowest possible cost after a
certain development period. The available time-to-market
period allowed by the present market for new offshore
wind turbines is too short to make it tempting to embark
on the development of completely new dedicated off-
shore wind turbines. Creating a vision of the main features

Figure 2.2
presents the
risk factors that
were addressed
by the We@Sea
programme, as
indicated by red
boxes.

of the ‘perfect’ offshore wind turbine – a “lighthouse” vi-
sion so to speak - is essential for paving the way to new
offshore turbines via a more incremental development
route. Such a view is also necessary in planning research
activities for the medium- and short- term. The danger of
this trajectory is that machines tend to become complex –
a property that does not prove very favourable for offshore
circumstances.
In order to sketch out a future offshore turbine (the ‘light-
house’), we must practice back-casting rather than forecast-
ing. Back-casting in our case means conceiving of a wind
turbine system that can extract energy from sea winds at
the lowest possible cost from scratch, rather than making
extrapolations from existing concepts. The available base of
knowledge and experience should be utilized to its full ex-
tent when designing a new dedicated wind turbine system.
Such an exercise should, however, fit into a strategic market
approach. Changing a concept radically each time a serious
problem is faced implies that a new learning process must
be established each time. After the initial phase of debug-
ging and solving the easier problems, the actual learning
curve can be continued under the condition that the con-
cept is not changed. Debugging and learning by doing in
this phase are the major mechanisms for achieving reliability
and cost effectiveness.
New designs encompass both the overall concept and nov-
el components that do not yet exist. This process could lead
to rather radical changes in concepts compared to existing
technology. General offshore design requirements have to
be applied to radical concepts. Table 2.1 includes a general
overview of these requirements.
Many ideas of radical concepts were and are being devel-
oped. Most of them are characterised by a very visual fea-
ture: the integrated foundation, wind turbine and installa-
tion approach. Some examples are shown in Figure 2.3. One
of the examples, the DOT concept, has been the subject of
a feasibility study carried out in the framewiork of the We@
Sea programme; it will be described in this chapter. The gen-
eral properties of the offshore turbine of the future, namely
size and extremely effective RAMS performance (Table 2.1),
require components that are not yet available at all; some
are under development. Some of these are also shown in
Figure 2.4.
Offshore wind turbines have to operate under severe exter-
nal conditions. The number of parameters required for the
design, the transport, the installation and the O&M system
is significantly larger than for land-based wind turbines.
It is quite a challenge to conceive a design that meets the
extremely demanding operational offshore requirements.
Moreover, designing wind turbines for typical offshore ex-
ternal conditions is a challenge. Table 2.2 gives an overview
of the external conditions that have to be taken into ac-
count. Some of these conditions change over the course of
time in such a way that the dynamic properties of the wind
turbine may change. An example is the transportation of
Table 2.1 General requirements to offshore wind turbines.
–Size: as large as possible
– Consequences of up scaling (rotor control, materials)
– Reliability (reduced number of components – direct drive)
– Availability
– Maintainability
– Servicebility
– Accessebility

External condition Sub division Input for design
Wind Wind speed distribution Load spectrum
Extreme wind speeds Ultimate loads
Turbulence intensity Fatigue loads and variations electrical output
Wind shear Fatigue loading, tower height
Waves & Currents Ocean waves Access, Fatigue loading, Installation & maintenance
Wind waves Access, Fatigue loading, Installation & maintenance
Extreme waves Extreme loads
Breaking waves Extreme loads
Ocean currents Loads, Installation & Maintenance, Scour
Salinity of the atmosphere Corrosion protection components
Humidity Protection electric equipment
Lightning Protection blades and electric equipment
Morphology Sand dunes Dynamic behavior entire construction, cable loads
Sand ripples Cable loads
Earth quakes Extreme loads
Table 2.2 External design conditions for offshore wind turbines
sand near the wind turbine foundation. The stiffness of the
monopole might change, and so then will the resonance
frequency of the wind turbine structure as a whole. This
again might have an impact on the behavior of the control
system.
Because of the cost breakdown of offshore turbines, there
is a strong pressure to scale up wind turbines where the
cost of support structures is dominant. Even now, offshore
wind turbines constitute the largest rotating machines
ever built, as seen in Figure 2.5. No matter whether we will
pursue radical designs or more conventional ones, one
factor will remain the same: large components, such as
blades, will face physical phenomena that cannot be over-
looked, as was the case with smaller machines. Local flow
conditions of a blade section are one example of these
phenomena. It means that completely new materials, acti-
vators, manufacturing methods, control methods and gen-
erator designs have to be developed. Those developments
will take at least 5 years to materialise. Since only abstract
descriptions of the conceptual selection are fixed, further
R&D must lead to detailed solutions. The feasibility of new
technologies must be established. For one or more de-
tailed solutions, the properties have to be determined and
again the novelty of the technologies may require exten-
sive research. Before the components can be applied to re-
mote and difficult-to-access locations, thorough tests un-
der simulated offshore conditions have to be performed.
Whether for a more evolutionary development and a radi-
cal approach, design tools and new components are need-
ed. Within the We@Sea programme, a selection was made
from the entire spectrum of research needs. See also Table
1.3. First the activities that might contribute to new wind
turbine concepts will be addressed, followed by analy-
ses and design tools for wind turbines and wind farms.

2.3 We@Sea research: new concepts
and components
The following topics will be described in greater detail:
– The DOT system – a radical change of concept.
– The use of thermo-plastics in blades – necessary for op-
timising manufacturing of complex blade parts and for
efficient recycling
– Support structures, trends, new developments and eco-
friendly design.
– Box: System identification; how to keep the wind turbine
stable under changing circumstances.
2.3.1. The DOT system – a radical change of
concept
Out-of-the-box thinking about radical designs for off-
shore wind farms resulted in the Delft Offshore Turbine
(DOT). The DOT system includes a robust and simple
wind turbine that directly drives a water pump located
in the nacelle. Pressurised water flow is channelled from
all wind turbines in a wind farm to a single offshore sta-
tion, where ‘hydro’ power is converted into electricity.
This results in an offshore wind farm that is significantly
simpler than a traditional wind farm. By reducing the
Figure 2.5 Up scaling of wind turbines in historical perpective.

number of critical components, a higher level of reliabil-
ity is obtained and cost will be reduced (see Figure 2.6.)
Conventional offshore wind farms typically include a gen-
erator platform (Offshore High Voltage Station or OHVS)
where the electricity is accumulated. This OHVS input volt-
age is stepped up to the required medium-voltage level be-
fore feeding it into sea transport cables for onshore high-
voltage grid connection.
The DOT system uses seawater as an energy medium in-
stead of electricity. Hydraulic transmission systems applied
in wind turbines as such are not new, but using seawater as
hydraulic fluid is a novelty. An additional advantage of us-
ing seawater is that any leakages in the system do not cause
environmental damage.
The proposed wind turbine has a two-bladed rotor directly
connected to the water pump. The blades are fixed to the
nacelle. The rated power of the system is set at 5 MW. This
concept leads to lower tower head mass compared to pres-
ent systems and does not require electronic components,
which frequently cause failures in present-day turbines.
The DOT system’s claims include:
–Minimal maintenance requirement
–Superior availability resulting from reliability levels
–High overall efficiency
–Easy installation
–Reduced power generation costs compared to present
technology.
The heart of the DOT system is the hydraulic transmission
system. Investigations have focused on the design of a high-
pressure hydraulic energy transmission system that extends
from the rotor shaft to the generator platform. The power
transmission of the DOT system is caused by high torque
being converted into high-pressure flow.
Figure 2.6.
DOT system.

Pumps can be divided into two general categories: kinetic or
hydrodynamic, and positive displacement types. In hydro-
dynamic pumps like centrifugal pumps, flow is continuous
from pump inlet to pump outlet. From a physics point of
view, flow is created by kinetic momentum applied to a fluid
stream. System output is characterized by low pressure and
high volume. Overall pump inefficiency and easy stalling
are known to be negative effects linked to backpressure oc-
currence. This makes this category of pumps unsuitable for
DOT application.
With positive displacement pumps, by contrast, the fluid
moves from an inlet opening into a chamber. As the pump
shaft rotates, a (positive or definite) fluid volume is sealed
from the inlet part and transported to the outlet part where
it is subsequently discharged. An essential difference be-
tween these two main categories is that kinetic pumps are
designed for transport of fluids and positive displacement
(PD) pumps for fluid power systems.
Hydraulic drives power-to-weight ratio is substantially
higher compared to gearbox-generator combinations ap-
plied in wind turbines, even without taking into account
all extra components required for making full use of a gen-
erator.
In terms of dynamic performance, seawater is a preferred en-
ergy transport medium over hydraulic oil, due to the higher
bulk modulus. Operating an open loop system means that
the water temperature will remain well within its liquid
range. However, low seawater viscosity also implies poor
lubrication performance and high risk of premature wear,
due to both erosion and corrosion.
Potential DOT candidate pumps include vane types and ra-
dial piston pump types. Vane-type pumps enable coping
with low viscosity fluids like water but are limited in terms
of pressure (< 100bar). A radial piston pump, by contrast, is
capable of generating high pressure (> 500bar) and can be
purposely designed to operate at high efficiency (> 95%)
at rotational speed rates, matching those of wind turbines.
Current maximum power rating of suitable pumps is below
2MW; these pumps require hydraulic oils as power trans-
port fluid. Larger systems are technically feasible but are not
being manufactured yet due to lack of demand.
An optional added feature of the DOT system is that it
allows for water storage/accumulation capacity aimed at
smoothing power output variations.
More details can be found in references [3-5].
2.3.2. The use of thermo plastics in blades
The long-term objective of the project Thermoplastic
Blades is to enable the replacement of currently used
thermoset blade material with thermoplastics. Thermo-
plastics reduce the impact of waste material at the end
of the blades’ lifetime. Economics are improved because
thermoplastics enable recycling of the resin and enable
new topologies. Although this application is certainly
also of interest for wind turbines on land, the overall
goal for the project is to enable the use of thermoplas-
tics in blades of ideal sizes. The potential advantages of
thermoplastic are related to two properties of this ma-
terial. First, the polymerisation process can be reversed,
enabling the separation of the blade into the original
raw materials. Secondly, thermoplastics can be welded,
enabling new manufacturing techniques. The specific
aim of this project is to overcome two disadvantages of
thermoplastics. First, current manufacturing processes for

thermoplastics have undesirable restrictions and are ex-
pensive. The project investigates how to overcome this
with the use of vacuum-infusion, which is common for
thermosets but not for thermoplastics. Secondly, the
fatigue performance of thermoplastics is low, due to a
poor fibre-to-matrix interface. The project aims to find
the reasons for this poor interface and to develop im-
provements.
Despite a significant reduction in energy consumption and
waste during the blade manufacturing process in the past
years, further effort is required to improve the recycling pro-
cess of decommissioned blades. Current recycling foci in-
clude more efficient incineration procedures and applying
natural fibres for composite reinforcement.
The fact that thermoplastic composites can be melt-pro-
cessed numerous times offers material reuse opportunities
in less-demanding new applications. Particularly when ap-
plying expensive carbon fibres, such reuse offers substantial
economic and environmental benefits.
Superior impact properties
Additional advantages of thermoplastic composites include
superior impact properties. That is, they do not turn brittle
at low temperatures due to higher toughness and their un-
limited shelf life.
Given all the potential advantages, the actual application
list of fibre-reinforced thermoplastic composites is surpris-
ingly short. This is attributed to several constraints:
A. Fatigue performance is often disappointingly low due to
poor fiber-to-matrix interface
Figure 2.7 Specimen of a blade section made from thermoplastics. …. (TUDelft)

B. Rotor blade manufacturing requires new processing
methods and expensive equipment
C. Material costs are significantly higher compared to ther-
moset composites due to the necessary intermediate
materials like extruded polymer films, semi-pregs or pre-
consolidated laminates
D. Melt-processing is generally performed at temperatures
in excess of 200ºC, which requires expensive tempera-
ture-resistant tooling and introduces thermal stress that
degrades material properties
E. The need for heavy presses limits achievable component
thickness, size and level of integration as compared with
melt processing. In particular, blade spar manufacturing
with a laminate thickness up to 100mm near the blade
root poses a key technological bottleneck hampering
large rotor blade development.
Vacuum infusion
The main considerations for actively engaging in the devel-
opment of vacuum infusion technology for thermoplastic
composite structures were:
I. Applying vacuum as a driving force for fiber impregna-
tion so as to eliminate heavy presses and to offer pos-
sibilities for manufacturing thermoplastic-composite
blades with similar size and thickness compared to
state-of-the-art thermoset-based equivalents
II. In situ thermoplastic matrix polymerisation around the
fibers, providing exiting opportunities for a chemical
fiber-to-matrix interface, which is much more difficult
to achieve with melt processing methods. This technol-
ogy feature is expected to greatly enhance thermoplas-
tic composites’ fatigue performance
III. Vacuum infusion is a commonly applied state-of-the-
art technology for manufacturing wind turbine blades
and consequently does not require completely new
processing methods and technologies
IV. Omitting the need for expensive intermediate mate-
rials, like extruded polymer films, semi-pregs or pre-
consolidated laminates, significantly reduces material
costs.
The thermoplastic casting resin applied for vacuum infusion
is an anionic polyamide-6 plastic, which was modified at
Delft University for achieving an elongated mould filling pe-
riod. This modification is also a necessary precondition for
infusing a dense fiber pre-form. At temperatures of around
180°C, the caprolactam monomer polymerizes into highly
crystalline polyamide-6 with the addition of an anionic ini-
tiator and an activator. After 60 minutes, demoulding is
possible, resulting into a composite with 50% fiber volume
content.
Initial fatigue assessment
Fatigue performances of three different composite laminates
with equal fibre content and glass reinforcement type have
been compared in sample tests under standardised condi-
tions whereby temperatures reached a maximum of 31ºC:
1. Vacuum-infused APA-6 composite
2. Vacuum-infused epoxy composite
3. Melt processed PA-6 composite manufactured by a tra-
ditional method utilising a hot platen press.
In high-cycle fatigue situations, materials performance is
commonly characterized by an S-N curve, also known as
a Wöhler curve. This is a graph of the magnitude of a cyclic
stress (S) against the logarithmic scale of cycles to failure
(N). Figure 2.8 shows the three derived S-N curves.

Figure 2.8 S-N curves of the three composites.
Main differences include:
– The vacuum-infused APA-6 composite test piece has
higher fatigue resistance compared to the melt-pro-
cessed PA-6 counterpart, most likely caused by a much
stronger fibre-to-matrix interface
– With regard to fatigue damage, the epoxy-based com-
posite outperforms both thermoplastic composites,
most likely due to a superior fibre-to-matrix bond
Crack growth in composite materials crack is largely influ-
enced by the presence of voids. Test results demonstrate
that there is ample room for improving fatigue performance
of APA-6 composites since their voids content is signifi-
cantly higher. Because an unreactive monomer has a simi-
lar detrimental effect on composite material properties like
voids, additional improvements can be expected from in-
creasing the degree of polymerization.
Conclusions
APA-6 resin is cheaper compared to typical epoxy resins
grade when applied for rotor blade manufacture. This is not
surprising because thermoplastic resin predominantly con-
sists of a caprolactam monomer – a basic ingredient for pro-
duction of one of the most commonly applied production
engineering plastics worldwide: polyamide-6 or Nylon®
-6.
However, the low cost of PA-6 is somewhat deceiving since
granule price is given per kilogram. But before these granules
can be processed into textile fibre-reinforced PA-6 compos-
ites, they have to be extruded first into polymer films or even
preconsolidated laminates. These additional processing steps
significantly add to cumulative material costs and explains the
cost advantage of the APA-6 resin over both other matrices.
The study further demonstrates the feasibility of manufactur-
ing thermoplastic composites through a vacuum infusion
process, enabling the manufacture of larger and thicker ther-
moplastic composite components and assemblies that are
also achievable with traditional melt processing. This offers
opportunities for fully recyclable thermoplastic composite-
based rotor blades that can be processed more rapidly,
while capable of more easily meeting structural design re-
quirements necessary for future smart blade generations,
reinforced by lower matrix costs. It has also been shown
that fatigue performance of thermoplastic composites can
be optimised by reactive processing. That in turn enables a
stronger fibre-to-matrix bond. And although epoxy compos-
ite materials still outperform vacuum-infused thermoplastic
composites with respect to fatigue life performance, further
improvements are expected for the foreseeable future.
Finally, thermoplastic composites with up to 20mm com-
ponent thickness have been manufactured at Delft under
controlled laboratory conditions.
More details of this topic can be found in [6-40].
400
350
300
250
200
150
100
50
0
1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07
N
Epoxy composite
APA-6 composite
PA-6 composite
Smax
[MPa]

System identification – how to keep the wind turbine stable while operational conditions change
Controllers in wind turbines are used to optimize power performance, avoid overloading of components and secure
structural stability during operation. Conventional controllers use dynamic models and properties that have been
determined analytically in advance of the actual installation of a wind turbine. This means that controllers have
to be insensitive to errors made in this dynamic model. The behaviour of offshore wind turbines is subject to more
changes in the course of its operational life than that of land-based turbines. Among others this is due to uncer-
tainties in foundation properties and foundation stiffness variations resulting from scour or removal of sand in a
wider area. The changes of sea bed patters was subject of a PhD study at the Twente Universtity [132]. The figure
below shows an example of sand movement at the Dutch North Sea coast caused by a wind turbine cluster.
Because offshore wind turbines involve complex loading, they could benefit from better-tuned controllers that
incorporate the variations in wind turbine properties in the control process. As a consequence, the controllers
become less robust. The determination of the key properties of the wind turbine is called system identification.
In order to identify a system in operation, excitation signals have to be added to the operational controller signals.
From the response of the wind turbine to these excitations, the key properties can be determined. Excitation sig-
nals are simultaneously applied to blade pitch angle and generator torque. In this project the following input-output
relations are investigated: collective pitch to rotor speed, collective pitch to tower top fore-aft speed, generator
torque to rotor speed, generator torque to tower top sideways speed, and several combinations of these inputs and
outputs. System identification for these inputs and outputs was performed with simulated data from a linear wind

turbine model (with the ECN simulation programme TURBU) and from the non-linear models PHATAS from ECN
and Bladed from GH-GL.
Results
System identification methods, which are scientifically well developed, identify the system from time series data or
frequency domain data. For this project, only time domain methods were considered, which match better with the
non-linear behaviour of wind turbines and the online availability of time domain data in the turbine.
To aid the identification process, a signal is added to the blade pitch angle and/or generator torque demand that is
provided by the standards controllers in the wind turbine. These signals provide additional and known excitations
to the system and are only effective if all frequencies of interest for control design are included. This signal, a so-
called Pseudo Random Binary Signal (PRBS), has properties similar to white noise – but the values of the signal
are actually precisely known. Furthermore, the signal can only have two values, which ensures that no excessive
excursions of blade pitch and generator torque are caused. It becomes possible to choose the two values of the
signal in a way to create sufficient excitation for system identification, while keeping the additional loads on the
wind turbine acceptable. The excitation signals for pitch and torque needed to be filtered so as to avoid excitation of
high-resonance frequencies and to avoid overly high demands on the pitch actuators. Furthermore, the pitch signal
is filtered to avoid resonance of the tower-bending vibration. The generator torque demand signal did not need
additional filtering to avoid drive train resonance, because the wind turbine in this case study appeared to have suf-
ficient drive train damping provided by the controller. The amplitudes for the unfiltered signals were set at 1.5º for
pitch and 3% of rated torque for the generator. When both excitations are applied at the same time, the amplitude
for pitch is reduced to 1º, because the combined excitation otherwise leads to overly high loads.
Table 1 shows the results of the comparison of different identification methods (first column) for three simulation
methods. The results are also based on validation tests.
Table 1. Potential of system identification methods for use in wind turbines, as determined from three types of simulations
TURBU (linear) PHATAS (non-linear) BladedA (non-linear)
Direct most potential most potential potential
Indirect not promising less promising
Joint I/O very potential potential
CLIV potential, with caution potential potential
Tailor-made IV not potential not potential
CL-N4SID very potential potential
PARSIM potential very potential potential
SSARX very potential very potential potential
PBSID effective effective

A
Not all methods were applied because some methods require knowledge about the controller for closed-loop
identification. This knowledge is not used in these tests with Bladed.
System Identification; example of blade pitch control
If the response properties of the wind turbine are not exactly known, it is impossible to separate the response due
to the system’s behaviour and response to external excitations by wind and waves. This problem can be solved by
adding a known excitation signal, i.e., collective pitch adjustments, on top of actions demanded by the controller.
This excitation has to meet the following three limitations: 1) The operation of the turbine has to remain close to
normal; 2) The additional loads caused by the additional excitation may not be significant; and 3) The acceleration
and speed required for pitching may not exceed the capabilities of the pitch actuators. This signal has two values, in
this case +1.5º and -1.5º. The variation in the signal between its high and its low value appears random, but is pre-
cisely defined. This signal contains a wide range of frequencies, with almost equal amplitude. The left-hand graph
shows the variation of this signal over time in blue and the right-hand graph shows its spectrum, also in blue. The
sharp transitions in the time series are related to the high frequency content of the signal, and they lead to unac-
ceptable demands on the pitch actuators. These high frequency components in the signal are not relevant for sys-
tem identification for control design. Therefore, a low-pass filter is used, which only keeps the lower frequencies
in the spectrum. The resulting spectrum is the red dashed line in the right-hand graph, and the red dashed time
series clearly show a smoother variation. This signal is added to the operational pitch demand in a simulation of
an offshore wind turbine. It is shown that this signal meets the three limitations. The responses of the rotor speed
and the tower top for-aft speed to this excitation signal are also sufficiently large for good system identification.
More information can be found in [41-44].

2.3.3. Support structures: trends, new
developments and eco-friendly design.
A support structure consists of the foundation, tower and
possibly a transition piece in between. A support structure
leads the axial forces, acting on the rotor, into the sea bot-
tom. Contrary to oil and gas rigs, where the structure is
mainly loaded by vertical gravitational forces, wind turbine
support structures are mainly loaded by horizontal axial
forces acting on the rotor. These axial forces can be very
high. If the wind turbine is operating at its maximum power
point, the axial forces on the rotor are approximately equal
to the force the wind would apply to a closed disk with an
area slightly less (10%) than the area swept by the rotor
blades. (On a wind turbine rotor, for example, operating
under optimal conditions, with a diameter of 126 meters,
the axial force at 10m/s is about 600 kN.)
There are different ways of leading these forces into the sea
bottom. In the first approximation, the type of foundation
depends on the water depth. On the one hand, the towers
above the water level can be somewhat shorter than on
land, since the vertical wind shear under offshore condi-
tions is less than on land. On the other hand, the total
tower heights are much larger because of the water table.
The deeper the water, the larger the moment the support
structure applies at the point of the sea bottom. Methods to
lead the forces into the seabed vary, including using gravity,
driving piles into the bottom, and fixing the support struc-
ture onto the seabed surface. Apart from these methods, the
water itself can be used to support the wind turbine struc-
ture by using uplift caused by the displaced mass of water
from a floating body. The most common types are shown
in the taxonomy of foundations in Figure 2.10. Figure 2.12
shows various types of these foundations in relation to wa-
ter depth.
Figure 2.10. Taxonomy of offshore foundations
The geometric details of a specific design of foundations depend
on many more parameters than water depth alone. These param-
eters are shown in Figure 2.11.
In the framework of the We@Sea programme, an analysis of
the cost of different foundations, including the installation
and transport activities, has been performed. An alterna-
tive environmentally friendly monopole, including installa-
tion method, has been developed as well. The comparative
analysis of the foundation and installation methods is ad-
dressed in Chapter 3. Here we will describe the innovative
monopole foundation.
Drilled concrete monopiles
A clear majority of all offshore wind turbines operational
today consist of steel monopile substructures. These are
thick-walled pipes of about 3-6m in diameter and 50-60
metres in length, with actual dimensions mainly depend-
ing upon water depth, turbine size and mass. Monopiles
are typically rammed 20-40 metres into the seabed, again
depending on soil conditions, water depth and turbine size.
The second installation step is placing a ‘transition piece’

Figure 2.11. Design parameters for offshore foundations
Figure 2.12. Various types of offshore support structures in relation to water depth. [45]
Tekening moet nog aangepast worden

over the pile top section that extends several metres above
the water. Its main function is to compensate for a slight
pile ramming process inclination error. In a final installation
step, the ‘top head’ comprised of tower, nacelle and rotor
is assembled. This project stage is usually subdivided into a
number of separate hoisting and assembly operations.
Marine health effects
At close distance, monopile ramming causes a loud sound
that is initially dull. As ramming progresses, this sound
gradually changes into an intense sharp noise carried
over a long distance. The underwater sound is potentially
harmful for the hearing ability of sea mammals and fish
larvae in close vicinity of the construction site. Alternative
methods like pile drilling are mainly applied for seabed
with soil types that are difficult to penetrate by ramming.
Other well-known yet less frequently applied alternative
substructure designs include steel tripods, jackets, and tri-
piles. However, these three alternatives also involve pile
ramming.
Another alternative is a hollow concrete gravity-based
structure lowered directly onto the seabed, where it is filled
Structure Examples Use Notes
Monopile
Utgrunden (SE), Blyth (UK), Horns Rev
(DK), North Hoyle (UK), Scroby Sands
(UK), Arklow (IE) Ireland, Barrow (UK),
Kentish Flats (UK), OWEZ (NL), Pricess
Amalia (NL)
Shallow to
medium wa-
ter depths
- Made from steel tubes, typically 4-6 m in diameter
–Installed using driving and/or drilling methods
· Transition piece grouted onto top of pile
Jacket Beatrice (UK), Alpha Ventus (DE)
Medium to
deep water
depths
· Made from steel tubes welded together, typically 0.5-1.5
m in diameter
· Anchored by driven or drilled piles, typically 0.8-2.5 m in
diameter
Tripod Alpha Ventus (DE)
Medium to
deep water
depths
· Made from steel tubes welded together, typically 1.0-5.0
m in diameter
· Transition piece incorporated onto centre column
· Anchored by driven or drilled piles, typically 0.8-2.5 m in
diameter
Gravity base
Vindeby (DK), Tuno Knob (DK), Mid-
dlegrunden (DK), Nysted (DK,) Lilgrund
(SE), Thornton Bank (BE)
Shallow to
medium wa-
ter depths
· Made from steel or concrete
· Relies on weight of structure to resist overturning; extra
weight can be added in the form of ballast in the base
· Seabed may need some careful preparation,
· Susceptible to scour and undermining due to size
Floating struc-
tures
Karm øy (NO)
Deep to very
deep water
depths
· Still under development
· Relies on buoyancy of structure to resist overturning
· Motion of floating structure could add further dynamic
loads to structure
· Not affected by seabed conditions
Table2.3[2] provides an impression of the application of various types of foundations in offshore projects in Europe

with sand, gravel, or rocks. Individual supplier preference
often seems to determine specific substructure/foundation
choices, but these are again influenced by main variables
such as turbine size, soil conditions and water depth.
Current installation methods are generally time-consuming
and therefore expensive, while the number of workable days
or weather windows for offshore construction are limited.
Aiming to reduce underwater noise emission during in-
stallation, Ballast Nedam Offshore developed a concrete
drilled monopile to be installed using the Svanen installa-
tion vessel. This 100-metre-high floating ‘Heavy Lift Vessel’
was initially developed for heavy bridge construction. The
self-propelled catamaran-type vessel can hoist loads up to
8,700 tonnes, a factor of 6-30 times more compared to
common state-of-the-art jack-up vessels and towed barge
maximum crane capacities. For wind turbine installation, a
jack-up vessel in an elevated position has a key advantage of
providing a fixed and thus stable working platform to about
40 metres water depth. However, the process of converting
Figure 2.13 shows a number of
actually realized foundations

from floating to fixed operations and visa versa is rather
time-consuming and never without risk. Being a floating in-
stallation vessel, the Svanen (Figure 3.z) faces no real water
depth restriction, and the stability due to its size has proven
more than sufficient for installing of all types of foundations.
Generally, no heave compensation is applied. This includes
monopiles, gravity-based structures, tripods and jackets.
Prefab rings
The drilled monopile consists of multiple prefab concrete
cylindrical shape rings and its development has formed
part of a major offshore wind research project. The actual
number of concrete rings depends on the required length,
whereas the structural assembly is secured by multiple post-
tensioned steel cables integrated within the concrete wall
material. Furthermore, the complete manufacturing and as-
sembly process takes place at an onshore construction site
under controlled conditions.
Putting in plugs closes off the normally exposed concrete
pile sides and allows the structures to float. That in turn
enables towing of these piles to an offshore construction
site. A traditional barge transport solution always involves
unavoidable risks of component loss during bad weather
or bad marine conditions. These risks are eliminated by the
floating-towing solution. After arrival at the construction
site, the Svanen upends each pile for plug removal. Prior to
actual installation proceedings, the structure is placed in a
fixed vertical position with the aid of a positioning frame.
Steel nosecone
Each pile bottom is fitted with a steel add-on and a slightly
bigger-diameter steel nose cone, which tapers downward
into a rather sharp edge at the soil contact area. During pile
penetration, a self-hardening drilling fluid (a kind of grout)
is injected inside the annular gap between the concrete pile
and the bored hole created in the seabed. Both measures
aim at easing pile seabed penetration.
At the heart of the installation procedure is a specially de-
veloped rotating drill head with adjustable diameter that is
lowered inside the pile. In cases where the soil structure is
relatively soft, the pile sinks several metres into the seabed
through its sheer mass. Under these soil conditions, the drill
head diameter is adjusted so that it is slightly smaller than
the nose cone diameter. At the contact area, the drill head
serves mainly to loosen the soil, which is then internally
disposed of. As the process continues, the pile gradually
lowers toward its predefined seabed depth.
If the drill head hits a difficult-to-penetrate hard soil layer, it
is then lowered slightly further inside the pile, past the steel
nosecone. This position enables the drill head diameter to
be enlarged so that it matches the outer nosecone diameter,
allowing the pile to easily pass through this soil obstruction.
Once the drilling process is completed and the monopile
protrudes roughly 3.5 metres above sea level (MSL), the drill
head unit is removed. Finally a conical shaped ‘anti-icing’
top flange is placed on top of the pile. It also serves as a
wind turbine tower-mounting flange.
While the concept is new for the wind industry, it is based
upon a combination of proven design and working methods.
The drilling technology has been applied in recent years un-
der the Amsterdam railway central station, but the application
of concrete monopiles with multiple rings dates back to the
Saudi Bahrain Causeway project in the 1980’s. The methods
for transportation and offshore placement of floating steel
monopiles have been practice-proven at several wind farms
including Rhyl Flats (UK), Belwind (B) and Walney 2 (UK).

2.4. We@Sea research: Analysis and design
tools for offshore wind turbines
Considering the wide spectrum of topics that make up the
design basis, within the We@Sea framework a limited num-
ber were addressed. These consisted of those topics that
were considered to be essential and not fully understood.
Before describing these activities, it is useful to understand
more about the design philosophy in general.
A design process is an iteration in which design solutions
are generated and updated, based on an assessment of
how well the solutions behave. For a design on the drawing
board, the assessment is typically made with software tools
that simulate or predict properties of the turbine. For tur-
bines that are already built, analysis of measurement serves
to verify design conditions and predicted values of loads
and energy output parameters, among others. The analy-
sis of dynamics is especially relevant, with an emphasis on
structural properties such as deformations, stresses and fa-
tigue damage accumulation under the conditions set by the
offshore environment. The projects aim to improve either
speed or accuracy in the determination of these param-
eters. In the early stages of the design process, speed of the
analysis is one of the more important aspects. In this phase,
the concepts and overall dimensions are determined, for
which many variations in the design solutions are analysed.
In later stages and for certification, accuracy of the analysis
becomes more important. Table 2.4 provides an overview of
the phases for which each project is most relevant. Next, the
objective of each project is described individually.
Figure 2.14.
Drilling
monopiles re-
duces under-
water noise
and thus
avoids dam-
age to hearing
ability of sea
porpoises.

2.4.1 Wind shear
The wind speed in the atmospheric boundary layer chang-
es with altitude. The shape of this so-called ‘wind shear’
profile affects the loads on a wind turbine and its power
production. In a vertical position, a rotor blade pointing
upwards is exposed to higher wind speeds than a blade
pointing downwards. The wind shear profile offshore is sig-
nificantly different from onshore. Both the roughness of
the surface and the temperature difference between surface
and air play a role in this difference. The temperature dif-
ference between sea surface and the air has a large effect
on stability of the atmospheric boundary layer, which in
turn affects the wind shear profile. Since seawater and land
surfaces cool and heat at different rates, the stability ef-
fect is different when offshore rather than onshore. Typi-
cally, a wind speed offshore reaches its final constant value
at much shorter heights above the surface than the wind
speeds above the land surface.
There are various models that describe the wind shear
profile for various stability conditions, but it is unknown
whether these adequately describe wind conditions over
the North Sea. It is also uncertain which of the suggested
methods is suitable to estimate the parameters that are used
in the models. Furthermore, it is unknown to what degree
analysis of loads and power production is affected by errors
in the wind shear model. The purpose of this research was
to remove these uncertainties and to provide guidance for
wind shear modeling as part of the analysis of offshore wind
turbines, particularly in the North Sea.
Figure 2.15 Comparison of measured and calculated wind shear
profiles at the OWEZ site in the North Sea.
Early design stage
(speed)
Detailed design and
certification
(accuracy)
Improvement of built
turbines
(actual properties)
Wind shear X
Extreme loads X
Frequency domain analysis of loads (Box) X
Remote measurements of blade deflections X
Blade fatigue X
Table 2.4 Phase of development where results for each project are most appropriate

Wind data collected at the meteorological mast at Egmond
aan Zee Offshore Wind Farm was used to assess how well
wind shear can be predicted by different models. Tempera-
ture and wind speed at sea level and at a 21 m height were
used to determine stability conditions of the atmosphere.
The stability conditions are expressed in the Obukhov
length, L, which is used as parameter on the horizontal-axis
of the at the last row of turbines in the easurementse the
.ition is e used to of erformed best, with the smallest graph.
L is a measure for atmospheric stability. In Figure 2.15 a rep-
resentative result of this project is shown. The left-hand side
of the horizontal axis corresponds to very unstable condi-
tions, the centre to neutral conditions and the right-hand
side to very stable conditions. For each set of measured
data, the stability condition is determined and data for the
same stability conditions are grouped.
The vertical axis, displaying the wind speed at 116 m above
sea level divided by the wind speeds at 21 m, is an indicator
of the magnitude of the wind shear effect. With a uniform
wind speed, without wind shear, this parameter has a value of
Integration of design tools
The possibility of assisting ‘offshore wind energy’ designers with a new method or tool was explored. One of the
leading observations is that many different design processes are involved and that these processes are inherently
asynchronous. The left-hand drawing illustrates how engineers are thinking about the hardware and procedures
that constitute the offshore wind farm and its operation. They sit together at the table so as to work together on an
integrated solution that best suits the project developer in the middle. However, this suggestion of integration is
an illusion. In practice, components such as wind turbines and installation
equipment are developed separately and long before they are considered for
use in a specific wind farm. This sequence of events is inescapable, because
these components are to be used for many different wind farms. As a conse-
quence, designers of, say, wind turbines are limited in the ability to assess
how their product will eventually contribute to the overall performance of an
offshore wind farm. The hypothesis was formulated that the design process
of such suppliers can benefit from a design emulation of offshore wind farms.
A tool that designs an offshore wind farm can be used as a mock-up, to test
the effect of design alterations of a supplied component on other parts of the
wind farm and on the overall performance. This facilitates trade-offs that
involve aspects of the wind farm that are normally out of the scope of the
supplier. In other words, the design emulation brings integration knowledge
into the design process, without the need to physically work together with
other contributors of a wind farm. An approach was formulated to assess the
effectiveness of this tool.
The results of this project are presented in [46-50].
Asynchronous design processes
Design emulations as ‘mock up

1. The measured wind speed at 21 m and at 1
16 m were aver-
aged over groups with the same stability conditions to obtain
the black line of the measured profile. The more stable the
conditions, the greater the measured wind shear. Temperature
and wind speed at sea level and at 21 m height were used to
predict the wind speed at 1
16 m, based on different models.
The red line for the predicted profile shows the averaged re-
sults for the so-called Bulk Richardson Number method. This
method includes the effect of stability conditions on the wind
shear profile. Two other methods shown in the graph, the
logarithmic law and the power law, do not represent this ef-
fect and therefore show horizontal lines. Of five methods that
include the effect of stability on wind shear, this Bulk Richard-
son Number method performed best, with the smallest error
between predicted wind shear and measured wind shear.
References [51-53] provide more details of this research
project.
2.4.2 Extreme loads
The estimation of the extreme loads that an offshore wind
turbine will experience is hampered by two fundamental
problems. Firstly, the actual future conditions during the life-
time of the wind turbine are unknown and can only be repre-
sented by their statistical properties. Secondly, the probability
of occurrence of extreme events under vulnerable conditions
of the wind turbine is so low that it is very unlikely that such
events will occur during acceptable measuring periods, rep-
resenting the wind turbine’s entire life span. The current prac-
tice is to analyse the loads for certain specified conditions
– under the assumption that these specified conditions ad-
equately represent reality and that they include those that
result in the largest loads. Alternatively, it is possible to per-
form a statistical analysis to determine the highest load and
its probability of occurrence. This approach is a direct answer
to the two fundamental problems. It is therefore expected to
provide additional insight not only into the loading of off-
shore wind turbines but also for offshore platforms of the oil
and gas industry. This method is included in the IEC standard
for onshore and offshore wind turbines.
However, the method is not unambiguous. One of the open
issues is that results of load simulations can fit with different
mathematical functions to describe the statistics, yet it is un-
known which function is most appropriate. The fit is needed
to extrapolate the extreme loads in the simulations, which
have a high probability of being exceeded, to extreme loads
with a low probability of being exceeded. This project aimed
to identify the effect on probabilistic load predictions of
different fitting functions. Another issue is the large amount
of simulations, and consequently the computation time that
is needed for statistical evaluation. This project also aimed
at establishing the effectiveness of reducing the necessary
number of simulations by applying constraint simulations.
In this technique, a dynamic model of the wind turbine is
used to compute in advance which environmental condi-
tions cause a predefined extreme load.
Figure 2.16

Frequency domain analysis of loads.
Before the design of a wind turbine structure can be completed successfully, calculations have to be cross-checked
regularly during the design iterations. It is common practice to carry out the first detailed design calculations in the
time domain and do verifications often in the frequency domain. In such a way a fast verification of the elaborated
time domain simulations can be achieved. Another advantage is that the effects of small changes in the design, for
instance on power production or fatigue loading, can be assessed quickly, without repeating the time-consuming
simulations in the time domain. A disadvantage of the frequency domain analysis is that it cannot handle non-
linear behavior; as a result system parameters have to be linearised.
Due to the varying seabed properties, offshore structures have to be designed for each individual site specifically.
The time-consuming design process thus has to be carried out many times for a large wind farm. Frequency do-
main simulations help to reduce time in carrying out these multiple calculations.
Since the use of frequency domain analysis is new for offshore wind turbines, it is unknown how it can effectively
be applied in the design process and how well that accuracy compares with time domain simulation. The objective
of the research was to remove these uncertainties.
As part of the research, the validity of the linearisation of wind turbine dynamics was tested by comparing results
from TURBU (an ECN simulation tool for loads and displacements, designed for frequency domain analysis) with
results from PHATAS (an ECN tool for time domain analysis). A 6-MW wind turbine was used as a reference ma-
chine for comparative simulations.
Although the project showed the effectiveness of
combining the two simulation methods in the de-
sign process, differences were observed between
the results of the time domain simulation and fre-
quency domain calculations. In a number of cases,
the equivalent loads calculated by TURBU were
higher than the PHATAS results. One of the causes
for the difference likely lies in the linearization pro-
cess of the hydrodynamic loads.
The traditional method to obtain an overview of the
dynamic response of an offshore wind turbine is
to generate a Campbell diagram. A Campbell dia-
gram shows the natural frequencies and excitation
frequencies as a function of rotational frequency of
the rotor. In this diagram, the coincidence of natural

Data collected at the ECN test station EWTW in the Wier-
ingermeer polder were used to assess the characteristics
of obtaining extreme loads by extrapolation of statistics
of a limited data set. Data regarding blade root bending
moments were selected for wind speeds with a 10-minute
average of 15.5 (±0.5) m/s and a wind direction in which
the measured turbine was not in the wake of other turbines.
The loads were made dimensionless by means of the mea-
sured maxima for the free stream condition at rated wind
speed. For each 10-minute time series, the maximum bend-
ing moment was determined and stored. These maxima are
sorted in descending order. This resulted in a list of 150
values, since 150 time series were selected. The probability
that the maximum in a fictitious 151st
time series would
exceed the largest maximum in the list of 150 values equals
1/151 ≈ 6.6*10-3
. Therefore, the lower-right blue square
of the largest maximum (of about 1
12%) has a probability
of exceedance of 1/151. The probability of exceeding the
largest or the second largest value in the list is twice as high,
and therefore the second blue square has a probability of
exceedance of 2/151. The second largest maximum has a
value of about 1
1
1%. All blue squares are similarly plotted,
up to the smallest maximum in the list with a value of about
88% and a probability of exceedance of 150/151. Four
functions that are commonly considered to fit well through
the plotted data points are used to extrapolate to lower
values of probability of exceedance. Blade root bending
moments with a probability of exceedance of about 1*10-6
are of interest, since these loads are expected to occur, on
average, once every 50 years during average wind speeds of
15.5 (±0.5) m/s. The exact probability of interest depends
on how often the selected average wind speed occurs. The
frequencies and excitation frequencies indicate resonance, which leads to large stresses and consequential fatigue
damage. Such coincidence should be avoided by avoiding the rotational frequencies where they occur, or by adapting
the structure to achieve other natural frequencies. Although this is a useful method, it lacks quantitative information
about the magnitude of the dynamic response. Computations of the response of an offshore wind turbine in the fre-
quency domain were used to obtain a more detailed overview of the dynamic behaviour. The offshore wind turbine was
modeled with the frequency domain analysis tool, using parameter values that represent the linearised behaviour of
the turbine during the average wind speed of interest. The linearised behaviour was required for frequency domain
calculations. The magnitude of the response was determined as a function of the frequency of the response. This pro-
cess was repeated for different average wind speeds with intervals of less than 0.5 m/s. The resulting response data as
a function of both frequency and average wind speed are presented in one plot, using a colour index for the magnitude
of the response. The black dashed lines show rotation (1P), blade passing (3P) and harmonic frequencies (6P, …) for
reference. The plot shows the excitation of the first tower bending mode between 0.2 and 0.3 Hz and the second tower
bending mode at about 1.5 Hz. It also shows the magnitude of the response to the 3P excitation, even though this is
not at a particular natural frequency of the system. Unlike a Campbell diagram, the plot also shows how much the
response increases in accordance with increasing wind speed, and how large the effect of low frequency response due
to turbulence becomes. To generate this plot took only several minutes of computation time.
The work of this project is presented in [58 and 59].

figure shows that three extrapolations predict extreme blade
root bending moments in a small range between 1
17% and
126%, but that the Gumbel function extrapolates up to
150%. The selection of fitting function and the interpreta-
tion of the extrapolated extreme value should therefore be
treated with care.
The details of this project are presented in [53-56].
2.4.3 Remote measurements of blade
deflections
Taking measurements from operating wind turbines in order
to verify design assumptions with respect to structural load-
ing is of utmost importance. This does not only apply to
verification, validation or calibration design tools, but also
to determine the structural properties of the wind turbine
structure for design improvement and to estimate lifetime
consumption. This is of particular interest for difficult-to-
access offshore wind turbines, as their structural behavior is
more complex and difficult to assess than land-based ma-
chines. Developing a remote sensing measuring and evalua-
tion system is of particular interest for offshore applications.
The analysis of in-situ behaviour entails a data collection
step and a data processing step. Both steps are addressed
in the project ‘Optical Measurements’. Many common data
collection techniques require the application of sensors.
This may be costly, and may interfere with normal functions
and layout of the turbine. The sensors often have a limited
lifetime and may need to be implemented during manu-
facturing of the turbine. Optical measurement techniques
have been developed that impose very limited requirements
on the objects to which they are applied. These techniques
have been successfully tested in other applications.
LDV measuring campaign
Vibrations of two reflecting markers near the tip of the blade were measured in seven periods of 294 s, with a
parked rotor. During the measurements, wind speeds in the rotor plane averaged around 5 m/s. Eleven natural
frequencies, between 0.34 Hz and 6.13 Hz, were identified with OMA. All of these frequencies were detected by the
LDV signals, while in five cases not all of the six strain gauges contained sufficient information to detect the full
frequency spectrum. For each of the seven measurement periods, the damping ratio was determined for each of
the 11 natural frequencies. For the same natural frequency, the damping at two measurement periods was found
to show significant differences, up to more than a factor of 2. These differences might be caused by non-random
properties of the wind and by differences in wind directions between two measurements. For each natural fre-
quency, the damping determined with measurements from the LDV and one of the strain gauges was always close
to the damping determined with the six strain gauges only. For each of the natural frequencies, a mode of vibra-
tion was identified, as compared to typical values found in the existing literature. Damping of tower vibrations and
edgewise vibrations corresponded well with values found in literature. Damping of several modes of the rotor in
a flap-wise direction is far less than the damping reported in literature for these modes. The values reported in
literature considered operational turbines, which have much higher aerodynamic damping than parked rotors.
This may explain the difference.

The objective of this project was to determine the suit-
ability of these techniques for collecting data about the
system dynamics of a wind turbine in operation. To es-
tablish the behaviour of the wind turbine, the collected
data needs to be processed. Properties that describe the
behaviour of a wind turbine include its natural frequencies
and damping coefficients. Such properties are not easily
extracted from measurement data, which contain the re-
sponse of the system to many different inputs that are not
even known – as well as noise that is introduced in the
measurement process.
One of the available data processing techniques is the
Least Square Complex Exponential (LSCE) method for
Operational Modal Analysis (OMA). A second objective
of this project was to determine the issues that need to
be considered when this system identification technique
is applied to wind turbine data from optical measure-
ments.
Two optical measurement techniques were selected for com-
parative research: Laser Interferometry with a Laser Doppler
Vibrometer (LDV) and photogrammetry. To test the suitability
of these systems, we used a wind turbine with a rotor diameter
of 80 m and hub height of 80 m, located at the ECN test site
in the Wieringermeer polder. Reflecting markers were placed
on the blades and tower, to provide high-contrast images for
photogrammetric measurements and to provide high-energy
reflections for the LDV. The LDV, installed at a distance of
220 meters from the wind turbine, was used to measure the
vibrations of a blade of a parked rotor. As no standard system
to track a moving marker is available yet for the LDV sys-
tem, the rotor was not rotating during the measurements. The
laser interferometer measurements were used to determine
natural frequencies and damping ratios for OMA. To assess
the quality of the interferometer measurements, the results
obtained with the LDV and one strain gauge were compared
with results obtained with 6 strain gauges. Photogrammetric
recordings were used to determine the position of markers on
the blades and on the tower during operation of the wind tur-
bine. To determine the displacements in a frame rotating with
the rotor, the rigid body rotation data were subtracted from
the measured data. To get an impression of the accuracy, the
distances between markers were determined and compared
with the known distances. Finally, OMA was applied to the
photogrammetric measurements.
The measuring results indicate that LDV measurements in
combination with OMA techniques indeed can be used
to identify natural frequencies and damping ratios of wind
turbines. In the box below, the measurement campaign from
which the conclusions are drawn is described.
The photogrammatic measurements yielded very satisfying
results.
Despite the short measuring time of only 21 seconds, OMA
is applied to assess the natural frequencies. Besides the
natural frequencies, the results of OMA also clearly identify
the rotational frequency and multiples of the rotational fre-
quency. Excitations of the wind turbine have a high-energy
content at these frequencies. The natural frequencies and
excitation frequencies are also identified in Power Spectral
Density (PSD) plots of the flap-wise and edge-wise vibra-
tions. Despite the insufficient data length for OMA, that
method identifies more frequencies, including all frequen-
cies visible in the power density spectrum. The measure-
ment campaign is described below.
The results of this project are presented in [59-62].

The photogrammatic measurements in detail
The photogrammetric measurements were performed with two groups of two cameras. The horizontal distance
between the wind turbine and each of the camera groups was 220 m. The distance between the two groups is 120
m and the two cameras in one group were spaced 20 m apart. Each blade was equipped with 10 markers, and 20
markers were placed on the tower. The markers were placed by two people in 6 hours. This is a short period to ob-
tain 50 measurement points on a wind turbine, when compared to installation of accelerometers or strain gauges,
for example. In first instance, the wind turbine was illuminated with a 20 kW flash system. To reduce the compli-
cations of this high-power flash, a second measurement campaign was performed with a 2 kW LED-based flash
system. Because of the distance between the cameras and the wind turbine, the contrast between the markers
and the background was found to be too low during daylight. Therefore, the measurements with this flash system
were performed during nighttime. A measurement period of 21 s at a sampling frequency of 28 Hz was achieved.
Although longer measurement periods are desirable for OMA applications, this is not possible with the utilized
hardware. After the measurements, the hardware was upgraded to enable 120 s measurements at 10 Hz; further
increases are expected in the future.
Five measurements were performed. The tip displacements of the three blades during these 5 measurements
showed a maximum variation of slightly more than 1 m. Because there was no reference information about the po-
sition of the markers, the accuracy of the method was estimated from the measured distance between two mark-
ers. Because elongation and contraction of the blades in lengthwise direction was expected to be negligible, devia-
tions of this distance from the true distance were caused by measurement errors. The variations in the measured
distance showed an offset, a stochastic variation and periodicity with the rotation of the rotor. The offset, or average
error in the distance, was below 8 mm for the outer half of the blade. Closer to the root, the offset increased up
to about 15 mm. It was expected that this increase in error was related to the increase in curvature of the blade
near the root. The curved markers reflected less light, which led to less contrast in the pictures. Nevertheless, the
results indicate very good displacement accuracy for this measurement technique.
2.4.5 Fatigue loading of large blades:
basic know-how for all future offshore
wind turbines
Offshore wind energy technology is the catalyst behind
the growing evolution of wind turbines. The largest blades
on the drawing board have a length of approximately 80
meters. With the increasing size of wind turbine rotors, the
choice of blade materials becomes more critical. Since in-
ternal stress tends to increase almost linearly with the rotor
diameter, then if the concept and materials are not altered, it
is not certain that up-scaling would lead to more cost-effec-
tive wind turbines. Harvesting the advantages of up-scaling
takes place with a balance between a wind farm’s design and
installation. These become cheaper as sizes of wind turbines
increase. The way to keep stresses at bay is to introduce

new materials in combination with less conservative design
methods. However, this requires better understanding of
fatigue properties of wind turbine materials. The We@Sea
programme has made a contribution to the design base
for wind turbine blades by determining the differences in
suitability of two fatigue damage models: the ‘Miner’s Sum’
method and a strength-based life prediction.
Fatigue is a design driver for wind turbine blades. Fatigue is the
phenomenon in which the strength of a structure will degrade
until failure occurs if it is subject to varying loads, even if these
loads are small. The variability of loads is often expressed in
the number of cycles during the structure’s lifetime; for wind
turbine blades this number is amongst the highest of all prod-
ucts made of composite material. For the design of a blade,
it is of high importance to adequately predict the variation in
the loads and the effect that this variation has on the compos-
ite material with which the blades are made.
Besides a literature survey, the project focused on the
Miner’s sum method, which is a damage method, and a
strength-based life prediction, which is a phenomenologi-
cal method.
In Miner’s sum, failure is defined as the instant when a speci-
men can no longer bear the intended load. This instant is
assessed by accumulation of the damage effect of each load
cycle. Potential load order effects are ignored in this ap-
proach. In the strength-based method, the component life
is predicted by calculating the effect of each load cycle on
strength properties, until the actual load exceeds the remain-
ing strength. The order in which loads are applied influences
both the speed of residual strength degradation and whether
failure is induced by an early high load or a later lower load.
The damage determined with Miner’s sum only indicates
whether this failure occurs; it provides no other information
about deterioration of physical material properties, as is the
Figure 2.17 Remote optical measurements of wind turbine structural dynamics.

"Converting Offshore Wind into Electricity" (complete book 2011)

"Converting Offshore Wind into Electricity" (complete book 2011)

Recomendados

Recomendados

Mais conteúdo relacionado

Semelhante a "Converting Offshore Wind into Electricity" (complete book 2011)

Semelhante a "Converting Offshore Wind into Electricity" (complete book 2011) (20)

Mais de Chris Westra

Mais de Chris Westra (16)

Último

Último (20)

"Converting Offshore Wind into Electricity" (complete book 2011)