DEVELOPING THE AMPELMANN
Safe and easy access to offshore wind turbines
D.J. Cerda Salzmann and J. van der Tempel
Duwind, Faculty of Civil Engineering and Geosciences, Delft University of Technology
Stevinweg 1, 2628 CN Delft, The Netherlands
Tel.: +31 15 27 85077, Fax: +31 15 27 85124, E-mail: Cerda@offshore.tudelft.nl
Tel.: +31 15 27 86828, Fax: +31 15 27 85124, E-mail: J.vanderTempel@offshore.tudelft.nl
SUMMARY
In order to provide safe ship-based access to offshore wind turbines, the Delft University of
Technology is currently developing a system named "Ampelmann". This system enables safe
transfer of personnel and goods by providing a motionless transfer deck on a vessel. This deck is
mounted on top of a Stewart platform, a mechanism (often used for flight simulators) that can
provide motions in all six degrees of freedom using six extendible legs. The Stewart platform is
fixed on the ship deck. To keep the transfer deck motionless, a sensor is constantly measuring the
motions of the ship deck. With the use of custom-made software, the legs of the Stewart platform
are controlled in such a way that all ship motions are continuously compensated, thereby creating
a stable and motionless transfer deck. The working of the concept of instantaneously
compensating vessel motions was proven in the beginning of 2005 with a scale model. The next
step in the development of the Ampelmann system is to build a full-size prototype. In preparation
of this phase, new focus points have arisen: the preliminary design procedure, the dynamic
behaviour of the entire system and safety aspects are currently being studied. This paper presents
the current status and latest findings within the development of the Ampelmann.
ACCESSING OFFSHORE WIND TURBINES
While the offshore wind energy industry is rapidly growing, it is still facing many challenges. One of
the larger hurdles is how to perform the maintenance on an offshore turbine: personnel and equipment
need to be transferred to and from the turbines in a safe and easy way.
Currently, most access to offshore turbines is being facilitated through boat-to-ladder transfers.
Although this method is relatively simple and cheap, it only allows transferring personnel with pocketsize equipment and the physical transfer from vessel to turbine and back can be quite challenging.
Moreover, this method is limited to calm weather conditions: normal service is said to be possible in
significant wave heights up to 1.3m. In rougher environmental conditions helicopters can be used.
Helicopter hoisting provides fast site access, but is very expensive and again allows for little other than
personnel transfer. This option also requires the installation of a hoist platform on the top of each
turbine. Most offshore wind turbines are not equipped with such a platform. The use of a jack-up vessel
provides a stable and secure base for the transfer even of large components (e.g. rotor blades), albeit at
high cost. The jacking operation becomes nearly impossible above 1 m significant wave height. The
three different access methods are shown in figure 1.
While wind turbines continuously increase in size and power output, turbine maintenance is becoming
more important: downtime of turbines leads to larger revenue loss. The trend of putting new bigger
models of turbines offshore also brings new complications; more maintenance operations can be
expected. Finally, wind farms in the near future are expected to be built farther offshore and in more
exposed sites. This means harsher weather conditions, higher wave heights and thus decreasing turbine
accessibility through the conventional boat-to-ladder access method.
Figure 1: Access methods: boat-to-ladder, helicopter hoisting and jack-up
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THE AMPELMANN
In order to provide safe and easy ship-based access to offshore wind turbines with a high workability,
the real problem of ship-based access was defined: the ship is moving while the structure is not.
Therefore, to create a safe transfer system it would be ideal to have a platform on top of the vessel
cancelling all motions to make it stand still in comparison to the fixed world; the turbine. These
systems exist in a reversed form: flight simulators. The moving part of these simulators consists of a
contraption of 6 cylinders, known as a Stewart platform. These platforms can move in all six degrees of
freedom and are therefore ideally suited to cancel all ship motions. By using a motion sensor that can
measure all ship motions accurately and in real-time, the Stewart platform can be programmed to
counteract these motions exactly. This system will create an upper deck that is motionless relative to
the turbine, canceling out the relative motions by means of active motion compensation. This concept
has been visualized in figure 2.
Figure 2: Active motion compensation using a Stewart platform
This active motion compensation system has been patented by the Delft University of Technology and
was named "Ampelmann" after the little green man in the Berlin traffic lights, making offshore access
"as easy as crossing the street". An artist's impression of the Ampelmann in operation is shown in
figure 3.
Figure 3: Artist's impression of the Ampelmann in operation
PROOF-OF-CONCEPT
In the beginning of 2005, the basic concept of the Ampelmann system was successfully demonstrated
at the Delft University of Technology after months of intensive research, development and testing. This
resulted in a proof-of-concept by means of a perfectly working scale model Ampelmann. The system
consisted of basically three parts: a motion sensor, a motion system and the control software. The
motion sensor used in this system was an Octans. This device contains three gyrocompasses and three
accelerometers to provide the translations and rotations in all six degrees of freedom. A Micro Motion
System (MMS) was used as the motion system. This is a small-size Stewart platform with a neutral
height of 67.9 cm. Finally, in-house software was developed for the communication between motion
sensor and motion platform.
After establishing communication between the Octans and the Micro Motion System through the new
software, the system was tested in two different ways. First the scaled system was placed on a larger
Stewart platform in order to test the motion compensation of all separate degrees of freedom in
different frequencies. During this phase, filters were added and tuned for enhanced system performance.
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Subsequently, the system was mounted on a 4 meter vessel and placed in a wave basin to examine the
motion compensation on a moving ship. The different stages of this proof-of-concept are shown in
figure 4; the testing of this scale model is described in detail in [1].
Figure 4: Establishing communication, dry testing and wet testing
With respect to the tests performed on the scale model Ampelmann, the following conclusions were
drawn:
 The Octans is accurate and fast enough to function within the Ampelmann system.
 The MMS Stewart platform is accurate and fast enough to function within the Ampelmann
system.
 The Converter software works perfectly and includes filters that enable proper functioning at
wave frequencies from 0.2 to 0.55 Hertz, equal to periods from 1.8 to 5 seconds.
 Waves with periods shorter than 1.8 second barely occur and when they do it is in
combination with significant wave heights no higher than 0.25 meters.
 Waves with longer periods than the 5 seconds mentioned caused insignificant vessel motions
in this particular test set-up and scale; they could therefore not be examined.
DYNAMIC ANALYSIS
During the proof-of-concept phase, a substantial problem appeared immediately when the system was
engaged: resonance occurred in the roll direction. When preparing the boat, its keels were removed to
prevent damage to the bottom of the basin; the wave basin was shallow compared to the draught of the
vessel. Unfortunately this caused the boat to be less stable around its length axis, allowing the MMS to
cause the resonance. Since the MMS has about the same weight as the boat, the cylinders of the MMS
could easily push the boat too far during motion compensation. This problem was solved by welding
roll-dampers on each side of the boat. This solution proved to be simple but perfect for the rest of the
testing procedure. However, this problem raised questions about the dynamics of any vessel carrying an
Ampelmann: the damping of the vessel in the roll direction has to be large enough to ensure a stable
system. To understand the detected resonance of the scale model, a dynamic analysis must be
developed.
To enhance the insight of the system dynamics, a new series of tests was performed. The goal of this
test series was to measure the vessel's response to waves of different frequencies and from different
directions, while different loads were placed on top of the Ampelmann. The results are currently being
processed; a preliminary conclusion is that the response amplitude operator for heave is barely
influenced by the top load. The Ampelmann appears to have a positive effect on the system dynamics:
since the mass on top is not moving, this mass will not contribute to the system's inertia. The final goal
of the dynamic analysis is to create a model that can simulate Ampelmann and vessel in waves. To
achieve this, a study is currently being conducted to incorporate all systems elements into a multi-body
program. A schematic representation of this model is shown in figure 5.
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platform + load
vessel
a, b, c
hydrodynamic coeff.
wave forces
PID-control
multi-body analysis
response of system
Figure 5: Model of dynamic analysis
PRELIMINARY DESIGN PROCEDURE
One of the main advantages of an Ampelmann system is that it can be designed fit-for-purpose. For a
given location and a specific vessel, the Ampelmann dimensions can be designed to reach any
demanded workability. At this moment, a preliminary design procedure is being developed to automate
the design process in order to accelerate this process and get a quick insight in all design alternatives
for different vessels and different workability. This will simplify the selection of vessel and
Ampelmann. The preliminary design procedure is illustrated in figure 6.
site selection
resulting 3D-envelop
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Heave
vessel selection
0
-2
0.5
1
0
Sway
scatter diagram
response time series
water time series
0
-0.5
-1
Surge
motion envelope
platform
RAO’s
wave spectrum
Figure 6: Preliminary design procedure
SAFETY-BASED DESIGN
The safety aspects of the Ampelmann system are of prime importance. The workability can only
increase while the safety of the personnel is ensured: no safety, no work. The safety does however
depend on many different aspects, the main aspects being: the transfer procedure, the control system,
the motion system and the user interface.
For the transfer procedure, concept designs have been made for means of access from ship deck to
transfer platform and for the gangway between transfer platform and wind turbine. These designs will
be worked out in further detail in the very near future. Hazard studies have also been planned to
investigate the entire access procedure.
The safety study for control and motion system is currently being conducted. To minimize failure,
design philosophies from different industries have been examined. It can be concluded that the aircraft
industry has a leading position in safety-based design: the procedures for landing on automatic pilot
have been developed and optimized for over 30 years. A system architecture will be made using their
philosophy, to ensure the highest safety-level suitable for the Ampelmann system. The first findings of
this study will be published in [2].
REFERENCES
[1]
[2]
Tempel, J. van der et al., Scale model testing of the Ampelmann, Copenhagen Offshore Wind
Conference Proceedings, 2005
Cerda Salzmann, D.J. et al., A safety-based design philosophy for the Ampelmann, European
Wind Energy Conference & Exhibition, Athens, 2006
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