technical description lillgrund 11336934

Technical Description Lillgrund Wind
Power Plant

Lillgrund Pilot Project

September 2008


Type of document

Document identification

REPORT

2_1 LG Pilot Report

Rev. No.

1.0

Report date

Project No.

September 29, 2008

21858-1

Author

Project name

Joakim Jeppsson
Poul Erik Larsen
Åke Larsson

Lillgrund Pilot Project

Customer

Reviewed by

Vattenfall Vindkraft AB
Approved by

The Reference Group
Distribution

No. of pages

78

The Swedish Energy Agency

No. of appendices

0

PREFACE
Vattenfall’s Lillgrund project has been granted financial support from the Swedish Energy
Agency and Vattenfall will therefore report and publish experiences and lessons learned

from the project. This report is compiled in a series of open reports describing the
experiences gained from the different aspects of the Lillgrund Wind Farm project, for
example construction, installation, operation as well as environmental, public acceptance
and legal issues.
The majority of the report authors have been directly involved in the Lillgrund project
implementation. The reports have been reviewed and commented by a reference group
consisting of the Vattenfall representatives Sven-Erik Thor (chairman), Ingegerd Bills, Jan
Norling, Göran Loman, Jimmy Hansson and Thomas Davy.
The experiences from the Lillgrund project have been presented at two seminars held in
Malmö (4th of June 2008 and 3rd of June 2009). In addition to those, Vattenfall has
presented various topics from the Lillgrund project at different wind energy conferences in
Sweden and throughout Europe.
All reports are available on www.vattenfall.se/lillgrund. In addition to these background
reports, a summary book has been published in Swedish in June 2009. An English version
of the book is foreseen and is due late 2009. The Lillgrund book can be obtained by
contacting Sven-Erik Thor at
Although the Lillgrund reports may tend to focus on problems and challenges, one should
bear in mind that, as a whole, the planning and execution of the Lillgrund project has been
a great success. The project was delivered on time and within budget and has, since
December 2007, been providing 60 000 households with their yearly electricity demand.
Sven-Erik Thor,
Project Sponsor, Vattenfall Vindkraft AB
September 2009
DISCLAIMER
Information in this report may be used under the conditions that the following reference is
used: "This information was obtained from the Lillgrund Wind Farm, owned and operated by
Vattenfall."
The views and judgment expressed in this report are those of the author(s) and do not
necessarily reflect those of the Swedish Energy Agency or of Vattenfall.

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Technical Description Lillgrund Wind Power Plant

SUMMARY

Lillgrund offshore wind power plant comprises 48 wind turbines, each rated at 2.3 MW,
bringing the total wind farm capacity to 110 MW. The Lillgrund offshore wind power plant is
located in a shallow area of Öresund, 7 km off the coast of Sweden and 7 km south from
the Öresund Bridge connecting Sweden and Denmark. An average wind speed of around
8,5 m/s at hub height, combined with a relatively low water depth of 4 to 8 meters makes it
economically feasible to build here.
Vattenfall Vindkraft AB is the owner and operator of Lillgrund offshore wind power plant.
Lillgrund is a Swedish pilot project supported by the Swedish Energy Agency (STEM). The
bidding process was completed during 2005 and the offshore power plant was constructed
in the period 2006 to 2007.
Vattenfall awarded the contract for foundation and seabed preparation work to the DanishGerman joint venture of Pihl & Sohn A/S and Hochtief Construction AG, and the contract for
wind turbines and electrical systems to Siemens Wind Power A/S.
The Lillgrund project is considered a success story not only from a technical point of view
but also from a social point of view. The wind farm was constructed on time and has now
been successfully operational since December 2007. The project team, composed by
specialists from different parts of Sweden and Denmark, have truly lived up to the vision
“One Vattenfall”.
There is, however, always potential for improvement and the aim of this report has been to
determine and highlight these areas. It is worth pointing out that only the electrical system
and the foundations are tailor made at offshore wind power plants. The wind turbines are
more or less standard products with none or very limited possibilities for project specific
design changes.
Geotechnical investigations are expensive and it can be difficult to balance the risks as well

as the benefits of this expense in the early phases of a large infrastructure project. As a
whole, the geotechnical surveys at Lillgrund proved to be useful. They identified potential
issues, such as the fact that extra excavation was required for two of the foundations. It
also revealed the location of a small number of boulders that would have to be removed.
Vattenfall requested a complete study of the electrical system for Lillgrund to be delivered
with the bids. That request was not met. Instead Siemens Wind Power began a complete
electrical system study after being awarded the Contract. The electrical system study was
completed during the construction period and revealed that (i) the insulation level in the
main transformer was too low, (ii) surge arresters needed to be installed in all 48 wind
turbines and (iii) some large transients occurring when the 130 kV main circuit breaker was
switched on. This caused extra costs and the experience shows that it is vital to perform an
electrical systems study in good time before the construction period begins.
In general, the working conditions at the Lillgrund site have been good. However, late
autumn and winter 2006 the combination of harsh winds and inconsistent current directions
made it impossible to perform the offshore work. Situations like these need to be taken into
consideration when writing the contract to ensure that the appointment of risk between
owner and contractor is clearly defined.

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Many minor problems and disputes with the contractors can be avoided if the owner has a
site representative present on-site during the whole project. This must be required both for
production sites for the foundations, concrete or steel, as well as for the offshore work.
The foundation contractor and designer underestimated the reinforcement needed to fulfil
the requirements from the agreed design code. Experience from earlier projects designed
after other codes were not valid.
Different kinds of cement can be used for the foundations. If a long lifetime is required the
choice of cement can be of importance. A Portland cement with a higher amount of alkali
can make cracks self heal, which is beneficial. The characteristic is not present in cement

with micro silica, which was the cement chosen for the Lillgrund project.
It is recommended that anodes be used as cathode protection system on all foundations,
including the transformer station foundation. The influence of the cable armouring should
also be taken into consideration in the design.
Due to corrosion problems, hand railings are preferably made of aluminium as opposed to
painted or galvanised carbon steel.
Boat landings should be as simple as possible, if ice is a problem, consider a solution
where you accept that some of them disconnect during hard winters. This might be the
overall cheapest solution.
Cable laying should be avoided during wintertime. At Lillgrund a propeller breakdown on
the vessel resulted in the cable being placed on the seabed ±15 meter within the trench
line. After repair of the vessel and waiting for proper weather conditions the cable was
picked up from the seabed and re-laid in the trench. During this delay of almost 2 months
the pre-excavated trench was partly backfilled by natural causes. After re-laying the cable
in the trench, water jetting had to be used to bring the cable to the bottom of the preexcavated trench.

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TABLE OF CONTENTS

1

INTRODUCTION................................................................................................... 6
1.1 Purpose ....................................................................................................... 6
1.2 Background and limitations.......................................................................... 6

2

GENERAL DESCRIPTION ................................................................................... 7

2.1 General ........................................................................................................ 7
2.2 Location ....................................................................................................... 7
2.3 Park layout................................................................................................... 8
2.4 Site conditions ............................................................................................. 8
2.4.1
General .......................................................................................... 8
2.4.2
Wind resources .............................................................................. 8
2.4.3
Water depth ................................................................................... 9
2.4.4
Wave conditions............................................................................. 9
2.4.5
Current ......................................................................................... 10
2.4.6
Ice ................................................................................................ 10
2.4.7
Other ............................................................................................ 10
2.5 Discussion ................................................................................................. 11

3

FOUNDATIONS .................................................................................................. 12
3.1 Technical specification............................................................................... 12
3.1.1
Geometry ..................................................................................... 12
3.2 Geotechnical investigations ....................................................................... 14
3.2.1
Year 2001; Phase I Geotechnical investigation ........................... 14
3.2.2

Year 2002, Hydrographical survey............................................... 14
3.2.3
Year 2003, Phase II Geotechnical investigation .......................... 14
3.2.4
Year 2005, Geophysical investigation.......................................... 15
3.3 Certification of design ................................................................................ 15
3.4 Design requirements.................................................................................. 16
3.5 Construction method of the main structure................................................ 16
3.5.1
Onshore ....................................................................................... 16
3.5.2
Offshore ....................................................................................... 18
3.6 Foundation tower interface ........................................................................ 22
3.7 Discussions................................................................................................ 24

4

ELECTRICAL SYSTEM ...................................................................................... 25
4.1 Electrical system study .............................................................................. 25
4.2 130 kV system ........................................................................................... 27
4.2.1
General ........................................................................................ 27
4.2.2
Onshore substation “Bunkeflo” .................................................... 27
4.2.3
Techniques to reduce switching transients .................................. 30
4.2.4
130 kV Onshore cable ................................................................. 31
4.2.5
130 kV Sea cable......................................................................... 33

4.3 Offshore substation.................................................................................... 36
4.3.1
General ........................................................................................ 36
4.3.2
Electrical system .......................................................................... 41
4.4 Main transformer........................................................................................ 42
4.4.1
General ........................................................................................ 42
4.4.2
Technical data.............................................................................. 43
4.4.3
Gas-in-oil transmitter.................................................................... 43
4.4.4
Oil collector .................................................................................. 44
4.4.5
FAT .............................................................................................. 44

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4.5

4.6

4.7

4.8

Internal grid................................................................................................ 45
4.5.1

General ........................................................................................ 45
4.5.2
33 kV sea-cables ......................................................................... 45
4.5.3
J-tubes ......................................................................................... 51
4.5.4
33 kV Switchgear ......................................................................... 51
System grounding...................................................................................... 53
4.6.1
General ........................................................................................ 53
4.6.2
130 kV system ............................................................................. 54
4.6.3
33 kV system ............................................................................... 54
4.6.4
Wind Turbines.............................................................................. 55
4.6.5
Energization ................................................................................. 55
Relay protection......................................................................................... 56
4.7.1
General ........................................................................................ 56
4.7.2
130 kV system ............................................................................. 58
4.7.3
33 kV system ............................................................................... 59
4.7.4
33 kV wind turbine feeders .......................................................... 59
4.7.5
Wind turbines ............................................................................... 59
Discussion ................................................................................................. 60

5

WIND TURBINES ............................................................................................... 62
5.1 General information and technical data ..................................................... 62
5.2 Power Curve and Energy Production ........................................................ 64
5.3 Noise.......................................................................................................... 65
5.4 Electrical Layout ........................................................................................ 65
5.4.1
Wind Turbine Transformer ........................................................... 66
5.4.2
Wind Turbine Generator .............................................................. 67
5.4.3
Wind Turbine Converter............................................................... 67
5.5 Mechanical Layout..................................................................................... 68
5.5.1
Rotor ............................................................................................ 68
5.5.2
Transmission system ................................................................... 69
5.5.3
Tower ........................................................................................... 71
5.6 Lightning protection system ....................................................................... 72
5.7 Coating ...................................................................................................... 73
5.8 SCADA ...................................................................................................... 73

6

COMMENTS AND CONCLUSIONS ................................................................... 76

7

REFERENCES ................................................................................................... 78

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1

1.1

INTRODUCTION

Purpose

This report summarises the design and gives a technical description of Lillgrund Offshore
Wind Power Plant. The report was written on behalf of the Swedish Energy Agency. It aims
at providing valuable practical information based on experience gained during the
construction of the Lillgrund offshore wind power plant. This experience cab be used to
ensure that the construction of future offshore wind power plants are more cost efficient.
1.2

Background and limitations

Vattenfall AB has received governmental support for the construction of Lillgrund offshore
wind power plant. A requirement for the financial support was that experiences gained
during the project development and installation phase are outlined in a report to the
Swedish Energy Agency. The full report will include areas such as economy, design and
technical solutions, installation and commissioning, environmental impact, operation and
maintenance, production analysis and communication. This report is limited to design and
technical solutions.

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2

GENERAL DESCRIPTION

2.1

General

The Lillgrund offshore wind power plant is comprised of 48 wind turbines, each rated at 2,3
MW, resulting in a total wind power plant capacity of 110 MW. The wind power plant
system also includes an offshore substation, an onshore substation and a 130 kV sea and
land cable for connection to shore.
2.2

Location

The Lillgrund offshore wind power plant is located in a shallow area of Öresund, 7 km off
the coast of Sweden and 9 km off the coast of Denmark. The wind power plant is situated
7 km south of the Öresund bridge, which connects Copenhagen and Malmö.

Köpenhamn

Saltholm

Dragör
Malmö

Flintrännan

Pepparholm
Öresundsbron
9 km
7 km

Lernacken
Bunkeflostrand

7 km

Klagshamn

14 km
10 km

Skanör
Figure 2.1

Höllviken

Location of the Lillgrund offshore wind power plant.

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2.3

Park layout

The wind power plant incorporates 49 foundations in total, of which 48 are turbine
foundations and one is for the offshore substation. The substation is placed at position W01 in Figure 2.2. The turbines are connected to each other and to the substation through
five radials as shown below.

Figure 2.2

2.4

2.4.1

Park layout showing the radials. Note the “hole” in the park; this is due to the shallow
water, which prevents vessels from being able to manoeuvre in this area.

Site conditions

General

Site conditions, with respect to wind, waves, water depths, water levels, ice and current
have been studied and numerically modelled in order to establish expected values along
with design values [4]. The data is used for the design of the foundations and the combined
foundation and wind turbine structure. The information is also used as a basis on which
contractors can develop their bids during the bidding process. Site conditions such as
expected wind speed, waves and currents are of great importance when contractors decide
on suitable equipment and methods.
2.4.2

Wind resources

The wind resources for the site were estimated in different ways. The expected wind

resource was presented in a report from Risø [1]. Extreme winds were analysed in [2] and
observed results from the onsite wind measurement mast was presented in [3]. Mean wind
speed for the site is estimated to be 8,5 m/s at 65 meters height and a prevailing wind
direction of 225 to 255 degrees (Figure 2.3).

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Figure 2.3

2.4.3

Wind frequency for Lillgrund offshore wind power plant.

Water depth

The water depth at the site is studied for more than one reason. It is important to know the
depth to the seabed in order to design the foundations. It is also important to know the
variation of the mean sea level since there is a minimum depth in which the sea vessels
can operate.
Table 2.1 from [4] is an example of the results from the study of the water level.
Table 2.1

Return
period
(year)
10
50
100

Estimated water levels (m) from mean sea level [4].

Low-water level
Skanör
-1,25
-1,40
-1,45

Drogden
-1,30
-1,60
-1,70

High-water level
Skanör
1,30
1,55
1,65

Drogden Köpenhamn Drogden Klagshamn
1,25
1,26
1,25
1,35
1,40
1,46
1,45
1,59
1,45
1,54

1,54
1,68

The sea bottom is not all flat and for this reasons five types of foundations with different
shaft heights were used. Table 2.2 shows the difference between design seabed level,
caisson bottom level and excavation level.
Table 2.2

Design seabed level, caisson bottom level and excavation level for the Lillgrund
site, all depths in m.

Type
1
2
3
4
5
2.4.4

Design seabed
level
-4,7
-5,7
-6,7
-7,7
-8,7

Caisson
bottom level
-6,8

-7,8
-8,8
-9,8
-10,8

Excavation
level
-7,1
-8,1
-9,1
-10,1
-11,1

Wave conditions

Numerical modelling of the wave conditions at the site was done in [4] in order to establish
the expected wave conditions for the site. This information is useful for contractors when
choosing suitable equipment for the project. The wave height is dependent on wind
direction, direction of the current and wind speed, and in order to establish the expected
wave conditions a number of data sources were studied and numerical simulations
performed for more than 240 different scenarios.

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2.4.5

Current

The speed of the current is very important for many offshore activities and must be

specified correctly. At high currents, diving can be prohibited causing delays for the project.
Diving work is often a critical part of the foundation work, the installation of secondary
structures and cable laying.
2.4.6

Ice

Estimation of ice conditions is of importance for the design of the foundations. Ice
conditions are also of importance from a maintenance point of view.
2.4.7

Other

During the construction work special consideration was taken with regards to the existing
gas pipe located to the north of the site. Transport was prohibited over the gas pipe
prolonging the transportation time for staff and supplies from Limhamn harbour. Positioning
of the most northern foundations along with dredging in vicinity of the gas pipe was done
with great care.

Figure 2.4

Marked area is the working area; the drawing also shows the existing gas pipe just
north of the site and the existing optic cable passing through the working area.

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2.5

Discussion

In general, the working conditions at the Lillgrund site have been good. However, late
autumn and winter 2006 and 2007 the combination of harsh winds and turbulent current
directions made it impossible to perform the offshore work. Situations like these needed to
be taken into consideration when writing the contract to ensure that the apportionment of
risk between owner and contractor is clearly defined.
It has yet to be seen if the design of the wind power plant layout is optimal. There were a
number of constraints, including the gas pipe, the proximity to the fairway for the ships
through Öresund and proximity to the Danish border. The above-mentioned constraints
combined with the constraints from the environmental permit with regards to total height of
the turbines, have given rise to a layout where the wind turbines are situated in close
proximity to each other. If turbines are placed too close to each other, park efficiency will
be reduced.

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3

FOUNDATIONS

3.1

Technical specification

The foundations for the Lillgrund wind turbines are gravity based. They are made of
reinforced concrete and filled with ballast to ensure they become heavy enough to
withstand the overturning moment created from the turbine.
3.1.1

Geometry

Figure 3.1 shows the overall geometry and dimensions of the bottom slab. In Figure 3.2 a
section of the foundation is shown along with the heights of the five different foundation
types that were used. Finally Figure 3.3 shows how the foundation is placed in a pit
excavated in the seabed, and how the erosion protection is solved.

Figure 3.1

The six-sided bottom slab of the foundations. The six pockets and the circular section
in the shaft are filled with ballast. Units are in millimetres.

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Figure 3.2

Figure 3.3

Section of the foundation. Five different shaft heights were used to adjust the
foundations for different water depths. Units are in millimetres.

Section showing dredging depth, ballast in the ballast pockets, ballast fill in the shaft
and scour protection.

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The weights of the foundations are shown in Table 3.1.

Table 3.1

Type
1
2
3
4
5

3.2

Weight of the foundation with and without ballast.

Weight without
ballast [tons]
1299
1318
1337
1356
1375

Weight
Ballast [tons]
803
822
841
860
879

Total weight

[tons]
2102
2140
2178
2216
2254

Geotechnical investigations

The Danish Geotechnical Institute (GEO) performed the geotechnical investigations for
Lillgrund in two phases. The first phase was carried out in autumn 2001 and the second
phase was carried out in autumn 2003.
In 2002 a hydrographical survey and sub bottom seabed profile investigations were carried
out and in the autumn 2005 a geophysical survey was carried out for the wind power plant
area as well as the internal cable routes between the wind turbines.
3.2.1

Year 2001; Phase I Geotechnical investigation

In this phase the sea floor was inspected and seabed sediment samples were taken at ten
locations. In seven of the locations soil/rock coring was done to 40 m below the sea floor
and a geophysical profile was logged from the boreholes before they were grouted.
Static triaxial compression and oedometer tests were used to determine the strength and
deformation properties of the core samples.
3.2.2

Year 2002, Hydrographical survey

Results from the hydrographical survey are presented in section 2.4, detailing information
about water depth, wave conditions, currents and ice occurrence.

3.2.3

Year 2003, Phase II Geotechnical investigation

For this phase CPT’s (Cone Penetration Test) were carried out at three points at each of
the 49 locations for the foundations. At five of the windmill locations, geotechnical
boring/rock coring was carried out to a depth of 20 m below the seabed.
Laboratory testing of samples from the five locations was carried out in order to establish
the material characteristics. In addition, strength and deformation tests, primarily static
triaxial compression and shear box tests, were performed on the samples.

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3.2.4

Year 2005, Geophysical investigation

Seismic mapping was carried out in order to identify the depth to the bedrock and the
thickness of the overlaying sediment. This was done for the all foundation locations as well
as for the cabling routes between the foundations. An example of the results from these
investigations is presented in Figure 3.4.

Figure 3.4

3.3

Depth to bedrock colour chart.

Certification of design

A consultant subcontracted by the foundation contractor carried out the design of the
foundations. The foundation contractor was also responsible for the certification of the
design and had therefore a contract with a certification body. It is standard procedure to
use a certification body but the contract can be setup in different ways.
The above setup was not ideal. The certification body and the consultant doing the design
were unable to prepare certified drawings prior to the start of the construction of the
foundations in Poland. More than 40 of the 49 foundations were placed at Lillgrund before
the design of the primary structure was certified. The certification of the secondary
structures was delayed in a similar manner.

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The reason for the delay is not fully clear to Vattenfall. Vattenfall did not have any contract
with the certifying body and was, because of the setup, not included in the correspondence
between the designer and the certifying body. Two meetings have been held in order to
discover what the reason for the delay was and why this situation developed. One issue
identified was that a prolonged iteration took place; design proposals were sent back and
forth between the designer and the certifying body. This indicates that the designer was not
as skilled as required. It also became clear that the certifying body was not sufficiently
staffed to handle this prolonged iteration phase.
One lesson learned is that the organisational setup, with one contract between the designer
and the foundation contractor and another contract between the foundation contractor and
the certifying body, needs to be revised. This set up meant that Vattenfall, as the owner,
could not influence the process.
3.4

Design requirements

The design was carried out according to [5], “Design of offshore wind turbine structures,
OS-J101, June 2004” from Det Norske Veritas. This project is one of the first to be
designed according to this standard. The crack width requirements in combination with the
fatigue criteria, proved to the hardest to fulfil.
3.5
3.5.1

Construction method of the main structure
Onshore

The foundations were cast directly on barges in Poland, see figure 3.5. Tower bolts (Figure
3.6), bolts for bollards and other secondary structures were cast into the foundation. After
completion of four foundations, the maximum number per barge, the foundations were then
towed to the Lillgrund site, see figure 3.7.

Figure 3.5

Form and reinforcement work, Swinoujscie Poland.

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Figure 3.6

Tower bolts to be cast in the concrete. Total height of the bolts is 1600 mm.

Figure 3.7

Barge with foundations leaving Poland.

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3.5.2

Offshore

During the casting in Poland, the dredging work had started at Lillgrund (Figure 3.8).

Figure 3.8

Dredging work at Lillgrund.

In the first phase the dredging was only carried out for the foundations. After dredging, a
crushed bed of rock was placed to level the floor of the excavation. This was done using a
centrepiece standing on the seabed. The centrepiece was long enough to reach above the
water in order to facilitate the positioning. A steel frame was placed around the
centrepiece, and this was carefully levelled horizontally. A vessel then delivered crushed
bedrock into the excavated pit, where a diver levelled the crushed material using a beam
connected to the centrepiece and dragged on top of the frame (Figure 3.9). No compacting
of the material was carried out.

Centrepiece

Vessel with crushed
bedrock

Crushed bedrock

Beam

Figure 3.9

Frame
Outline of the work with the crushed bedrock.

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After finalising the dredging and placing the crushed bedrock in the excavated pit, the
foundations were positioned using a crane barge see figure 3.10.

Figure 3.10

Positioning of foundations at the Lillgrund site.

After placing the foundations, dredging was started for the J-tube extensions. J-tube
extensions are pipes mounted to the side of the foundations to provide access for the inter
array cables. Figure 3.11 shows how the J-tubes are placed in the foundations and Figure
3.12 shows a J-tube extension lying on the quay.

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Figure 3.11

Drawing of a typical foundation showing the J-tube position as dashed line.

Figure 3.12

J-tube extension on the quay.

Next step was to place filter rock (see Figure 3.3 for the placement of filter rock). When the
placement filter rock was finalised, J-tube extensions were fitted using divers. The next
step was to place ballast rock, and armour rock I and II together with filter rock around the
J-tube extensions. Thereafter, ballast fill was placed in the shaft of the foundation and a
concrete slab was cast on the top of each foundation as shown in figure 3.2.

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Figure 3.13 shows the connection between the concrete slab cast after the ballast fill
offshore and the shaft wall. Figure 3.13 also shows a cast in hang off flange. At the lower
part of the slab, between the wall and the slab, there is an expanding water stop. On some
foundations the water stop was placed close to the top surface of the concrete slab. When
the water stop started to expand, as it should when subjected to water, the concrete
cracked (Figure 3.14). The repair work had to be executed in hurry to avoid problems with
the installation of the wind towers. It is suggested that more site inspections are carried out
to verify the correct installation if this solution is used on future projects.

Figure 3.13

Connection between slab cast offshore and foundation shaft.

Figure 3.14

Damages due to expansion of the water stop.

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3.6

Foundation tower interface

The physical interface between the tower and the foundation is of course of outmost
importance. If a mismatch occurs during the installation, with a fully loaded vessel, the costs
for the delay that follows will be significant. The installation vessel can only unload in a
specified sequence. If this is interrupted, the ship needs to seek harbour, and then unload
and reload on quay.
To avoid this potential problem, narrow tolerances was set up for the tower bolts. This
tolerance was however only defined and followed locally, i.e. for each bolt. A problem was
discovered when it became clear that it was not sufficient even if the tolerance
requirements for one singe bolt was fulfilled.
Before placing the tower on the foundation, shims are used to make sure that a horizontal
surface is available, making the tower stand vertical, see Figure 3.15. When this was done
it was discovered that the top surface of the foundation was so uneven that the bolts on the
lower parts proved too short (Figure 3.16). A certain distance was required for the jacket
used to pretension the bolts before fastening the washer. The situation was resolved by
removing the concrete locally at the affected parts, see figure 3.17.

Figure 3.15

Shims placed on the foundations to achieve a horizontal surface for the tower.

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Local tolerance
X±y

Global tolerance
To short

Figure 3.16

Figure 3.17

Sketch showing the problem with the tower bolt tolerances.

Solution of the problem: the careful observer can see that the concrete has been
lowered locally give enough height above the shims.

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3.7

Discussions

Different kinds of cement can be used for the foundations. If a long lifetime is required the
choice of cement can be of important. An offshore wind power turbine is exposed a high
ratio of dynamic loads. This means that fatigue in the reinforcement bars is the main
design factor when determining the appropriate amount of reinforcement. Fatigue loads
also indicate that there will be cracks on the concrete surface that open and close. The
cement type chosen influences how these cracks behave. A Portland cement with a higher
amount of alkali can make the cracks self heal, which is beneficial. The characteristic is not
present in cement with micro silica, which was the cement chosen for the Lillgrund project.
The contractor and designer underestimated the reinforcement needed to fulfil the
requirements from the agreed design code. Experience from earlier projects designed after
other codes could not be taken into consideration. It could be argued that the design

requirements used are too rigorous, however the criteria is designed for an endurance of 50
years, so the requirements are reasonable from a durability point of view.
Geotechnical investigations are expensive and it can be difficult to balance the risks as well
as the benefits of this expense in the early phases of a large infrastructure project. As a
whole, the geotechnical surveys proved to be useful. They identified potential issues, such
as the fact that extra excavation was required for two of the foundations. They also
revealed the location of a small number of inconveniently positioned boulders that would
have to be removed. Adjustments regarding the excavation levels had to be made on more
than one foundation at Lillgrund. If unfortunate, the soil conditions could have been so bad
that the turbines could not have been put in the chosen positions. The relocation of the
turbines could be difficult since it may influence the efficiency of the surrounding turbines in
a negative way. The economy of the project could then be in jeopardy.
Many minor problems and disputes with the contractor can be avoided if the owner has a
site representative during the whole project. This is required both at the site for the
production of the foundations, concrete or steel, and for the offshore work.
It is recommended that anodes be used as cathode protection system on all foundations,
including the transformer station foundation, and that the influence of the cable armouring is
taken into consideration in the design.
Hand railings are preferably made of aluminium, as opposed to painted or galvanised
carbon steel.
The need for Davit cranes shall be carefully investigated for each project, if not needed by
the operation and maintenance crew they can be omitted. If used, ensure that they have a
locking device for the boom.
Boat landings should be as simple as possible, if ice is a problem, consider a solution
where you accept that some of them disconnect during hard winters. This might be the
overall cheapest solution.

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