The German Offshore-Metmasts Amrumbank West and Arkonabecken

THE GERMAN OFFSHORE-METMASTS AMRUM BANK WEST and
ARKONA BECKEN SÜDOST
Jörg Bendfeld, Ralf Ditscherlein, Michael Splett, Jürgen Voss
University of Paderborn / WUZ
Contact:
University of Paderborn / WUZ, Prof. Dr.-Ing. Juergen Voss, Pohlweg 55, 33098 Paderborn,
Germany,
E-Mail: Bendfeld@nek.uni-paderborn.de
Summary:
If Europe is to be able to achieve the pledged goal of 20 % renewable energy by the year
2020, implementation of offshore windparks is an essential prerequisite, but because the
costs for offshore constructions are very high due to restricted accessibility of the locations
as well as the large distances from the coastline and the nearest seaport, entailing extremely
expensive logistics, nearly all potential windpark operators depend on bank loans. To reduce
the financial risk for the lenders and investors, it is essentially necessary to make local
measurements for designing the windparks before actually building them.
Research platforms presently permit the most precise assessments of energy yields at
offshore locations. This plays a decisive role for the implementation and funding of wind
plants. Research platforms furthermore provide the possibility of contributing to the
understanding of the relevant parameters and their interactions for offshore wind energy
utilisation.
1 The situation
With a capacity of almost 20 000
Megawatts (MW), more wind energy
power is installed in the German Federal
Republic than in any other country.
Unfortunately the situation out on the open
sea still lags behind this: Not even a single
offshore windpark has been installed in
German waters so far. So far only two
prototype plants have been implemented
in the immediate vicinity of the coast. One
reason for this is the great distance of the
prospective windparks from the coast. The
greater water depth out there calls for a
definitely more sophisticated technology,
entailing corresponding higher costs. This
is aggravated by the huge price increase

of the raw materials for steel, copper and
aluminium since about the year 2002.
Apart from all this, entrepreneurial initiative
is discouraged by the very exacting,
tedious and expensive approval
procedure.
Nevertheless, wind energy is destined in
the long run to play a central role in the
extension of renewable energy sources.
The construction of offshore windparks not
only contributes towards protecting our
climate, it also provides innovative drive
for the economy, and in addition thereto
industrialised countries can make
themselves a bit more independent of
energy imports from politically unstable
regions. In the German part of the North
Figure 1: Offshore windpark areas in the North Sea [1]
Sea and the Baltic Sea the distance of the
planned offshore windparks from the
coastline is considerably greater than in
the case of other countries. On account of
shipping, touristic and nature reserve
interests – supported by political attitudes
– the planning corporations have sited
their parks far out (30 – 100 kilometres)
away from the coast (Figures 1 and 2
show the locations of the windpark areas
in the German North Sea and Baltic Sea).
This has the consequence that for
economically profitable operation the wind
energy plants and their foundations should
be as large as possible.

Figure 2: Offshore windpark areas in the Baltic Sea [1]

The measurements make it possible to
study many important parameters already
in the preliminary phase. To meet the
technological challenges offshore,
measuring platforms are an ideal data
source and an important means for
gathering vital information for subsequent
operation of the windparks already before
beginning to construct them. The
measurements should be extended over
numerous parameters for this purpose.
Some parameters are important, for
example, for constructing the wind power
plants, whereas other parameters are
decisive for designing the foundations.
Knowledge regarding the accessibility of
the platforms can later be applied for
devising appropriate maintenance
concepts. To investigate the windpark
behaviour at an offshore site it is first of all
necessary determine the wind potential
and therewith the energy yield. In the
German waters measuring buoys have
been collecting wave and wind data on the
sea surface since more than a decade.
Such data lack representative
interpretability in the sense that they do
not originate at the elevation of the
machine hubs of future offshore wind
energy plants (90 m). A mere extrapolation
of sea surface buoy data to machine hub
elevation leads to false estimates of the
wind potential and thus to incorrect results
of energy yield calculations. Only
measurements actually made at machine
hub height of the future wind energy plants
are reliable for determining the energy
yield at the respective sites. In the years
2005 ("Amrum Bank West") and 2006
("Arkona Becken Südost"), two measuring
stations were erected in the German North
Sea and Baltic Sea.
2 The Metmasts
2.1 Amrumbank West:
Since April 2005 the metmast
“Amrumbank West” has been constructed
and at work.
Water depth
The water depth is about 24 m. The water
depths around the station vary within
narrow limits.
Fetch distances
Figure 3: Locations and fetches
Figure 3 shows the fetches for the
research station Amrumbank West. They
vary from 35 km in the East to over
500 km in the West. For the main wind
directions the fetch is far more than
100 km.

Figure 4: Research platform Amrumbank
West
Surrounding land
The station Amrumbank West is
surrounded by the small Islands of Sylt,
Amrum and the mainland of Schleswig
Holstein to the East, the island of
Heligoland and the mainland of
Niedersachsen in the South. Most of the
areas are flat. The orographic effects on
the airflow are expected to be small.
Station design
The Station can be considered as
consisting of three major sections:
• The monopile of approximately 60
m length and about 290 metric tons
weight has a maximum diameter of
approximately 3.50 m.
Figure 5: The monopile
• The transition piece with the
measuring container has a weight
of about 50 metric tons and a
height of 8 m.
Figure 6: The transition piece
• The lattice mast of about 68 m
length and 40 metric tons weight
narrows from 4.50 m * 4.50 m to 1
m * 1 m.

Figure 7: The lattice mast
Figure 8: Top view
Station history
The station was erected in April 2005.
Operation started in April 2005 and it is
still going on.
Data acquisition system
A set of reliable identical conventional cup
anemometers has been arranged on the
Northwest side facing away from the
platform. The anemometers measure the
windspeed at seven different elevations.
The lowest level is at 35m, the highest
level is at 90m. The wind direction is
recorded using conventional wind vanes.
Furthermore, the wind is measured three-
dimensionally by an ultrasonic
anemometer. Data concerning
atmospheric humidity, air temperature and
air pressure complement these readings.
The windspeed data is collected in several
altitudes. The height difference between
successive measuring altitudes is about
10 m. This permits the determination of
windspeed gradients. In order to exclude
systematic errors, measuring instruments
of different types or with different
measuring procedures are used. Only well
proven measuring instruments are utilised.
2.2 Arkona Becken Südost
Since November 2006 the metmast
“Arkona Becken Südost” has been
constructed and at work.
Water depth
The water depth is about 25 m. The water
depths around the station vary within
narrow limits.
Fetch distances
Fgure 9: Location and fetches
Figure 9 shows the fetch distances for the
research station. The fetches vary from
40 km to the Islands of Rügen (Germany)
and Bornholm (Denmark), and 100 km to
the Island of Sealand (Denmark) in the
West.
Surrounding land
The station Arkona Becken Südost is
surrounded by the islands of Rügen and
Bornholm and the mainland of
Mecklenburg Vorpommern to the South,

Most of the areas are flat. The orographic
effects on the airflow are expected to be
small.
Station design
The station can be considered as
consisting of three major sections:
• The gravity foundation has a weight
of approximately 1300 metric tons
and a maximum diameter of
approximately 30 m.
Figure 10: The gravitiy foundation
• The monopile with the transition
piece and the measuring container
has a weight of about 150 metric
tons and a height of 40 m.
Figure 11: The monopile
• The general design of the lattice
tower was changed to a triangular
cross-section. The lattice tower has
a length of about 84 m and weighs
50 metric tons.
Figure 12: The lattice mast
Figure 13: Top view
Data acquisition system
A set of reliable identical conventional cup
anemometers has been arranged on two
booms around the mast. The
anemometers measure the windspeed at
eight different elevation levels. The lowest
level is at 18 m and the highest level is at
95 m. The height difference between

successive measuring levels is about 10
m. The wind direction is recorded using
conventional wind vanes. Furthermore, the
wind is measured three-dimensionally by
two ultrasonic anemometers.
3 Underwater measurements
Utilisation of an oceanographic sensor
ADCP is highly appropriate for monitoring
numerous oceanographic parameters.
ADCPs work by transmitting beams of
acoustic energy and listening for the
backscattered energy along the beam
directions from small bubbles or particles
in the water. In principle, ADCPs combine
the functionality for measuring waves and
currents in a single package.
The installation of the ADCP is a special
challenge. In many cases installation at
the monopile has to be abandoned
because the emitted signals would be
reflected by the monopile, and the current
to be measured would be strongly
disturbed by the monopile. Therefore it is
appropriate to lower the sensor to the
seabed about 50m away from the
monopile. However, this leads to further
difficulties such as the risk of losing the
protection against scouring at the ADCP or
rather at its support, but this problem can
be minimised by appropriate
constructional design. It must also be
taken into consideration that the support is
constructed in a way that the sensor
cannot be moved even by a strong
current.
Figure 14: The research platform Arkona
Becken Südost

However, a risk exists that the
instrument could be damaged by trawl
nets or even lost.
4 Energy supply
Renewable energies are essential
contributors to a solution for the energy
supply problem, because they give energy
supply security, reducing dependency on
Diesel fired generators. Our goal is to
provide nearly 99% of the energy demand
by solar and wind power.
Small wind generators in combination with
photovoltaic (PV) devices are an
economically efficient way of obtaining a
secure energy supply.
A PV system consists of arrays of framed
solar modules under a certain tilt angle.
The generated energy is stored into a
rechargeable battery (accumulator cells) in
order to tide over periods of darkness and
low irradiation intensity. The battery is also
used to provide the energy required for
operating all measuring equipment, the
lights and the transponder (AIS) under
worst case conditions. The battery unit is a
combination of a battery with battery circuit
breakers. These components have to be
suitable for the harsh offshore
environment. The units can incorporate
different types of batteries, such as nickel
cadmium and lead acid accumulator
batteries for solar applications. Connecting
cells and batteries in parallel and series
provides the desired combination of
battery voltage and capacity. Selecting the
correct battery type is always crucial for
the system.
The backup for all these systems is the
Diesel generator, chosen for this purpose
because Diesel generators have no
ignition systems, no carburetors, and no
sparking plugs. The required maintenance
of these generator types is small if they
are used only as backup.
Lessons learned
Research platforms presently permit the
most precise assessments of energy
yields at offshore locations. This plays a
decisive role for the implementation and
funding of wind plants. Research platforms
furthermore provide the possibility of
contributing to the understanding of the
relevant parameters and their interactions
for offshore wind energy utilisation.
The design of the data acquisition system
functions very well [2].
Lightning strikes can be a problem for
offshore installations.
The environmental conditions offshore can
be very hard in some cases.
Underwater measuring technology
requires special maintenance and is
expensive.
Redundancy is very important.
It is very important to have many
measuring instruments.
Only proven instrumentation is utilised [2].
Data evaluation should take place in real
time in order to detect any problems at an
early stage.

The data memory device must have
sufficient capacity for 6 months of data
collection.
A regenerative energy supply suffices in
the case of judicious selection of the
measuring instruments (burst mode is not
necessary) [2].
Literature references:
[1]: www.bsh.de
[2] J.Bendfeld, M.Splett, J.Voss, A.
Higgen, J. Krieger: Two Years Operation
of the Offshore Metmast Amrum Bank
West in the German North Sea,
European Wind Energy Conference &
Exhibition (EWEC 2007), Milan, May 2007
Translation Figure 1: Offshore windpark
areas in the North Sea [1]
Grenzen Boundaries
Festlandsockel Continental shelf
12-Seemeilenzone
/ Küstenmeer
Internationale
Grenze
12 nautical miles
zone / coastal sea
International
boundary
Offshore windparks Offshore
windparks
in Betrieb in operation
genehmigt approved
nicht genehmigt not approved
geplant planned
Netzanbindung Grid system
connection
genehmigt approved
geplant planned
Externe
Datenquellen
External data
sources
Elsam A/S
Denmark
Geodätisches
Datum: WGS 84
Kartenprojektion:
Mercator (54° N)
Elsam A/S,
Denmark
Geodetic data:
WGS 84
Map projection:
Mercator (54°N)
Translation Figure 2: Offshore windpark
areas in the Baltic Sea [1]
Externe
Datenquellen:
Ministerium für
ländliche Räume
Ministerium für Bau
und Arbeit
Bezirksregierung
Kalmar (Schweden)
External data
sources:
Ministry for rural
areas
Ministry for
building and
labour
District
government of
Kalmar (Sweden)