At Ampelmann, our motto is “Offshore access as easy as crossing the street”. We take this very seriously: on a day to day basis hundreds, sometimes thousands of people go to work safely using our systems all across the world. They leave their vessel, wait for the green light and transfer to their workplace, be it an offshore rig, a turbine or an FPSO. We want that experience to be equivalent to crossing the street: be aware of the action you are taking, follow the procedures and complete the transfer easy, safe and fast.

1. AMPELMANN
THE NEW OFFSHORE ACCESS SYSTEM
D.J. Cerda Salzmann, MSc
Delft University of Technology – DUWIND
Stevinweg 1, 2628 CN Delft, The Netherlands
Tel.: +31 15 27 85077, E-mail: d.j.cerdasalzmann@tudelft.nl
J. van der Tempel, PhD
Ampelmann Company
Rotterdamseweg 380, 2629 HG Delft, The Netherlands
Tel.: +31 15 27 86828, E-mail: j.vandertempel@ampelmann.nl
SUMMARY
To provide safe ship-based access to offshore wind turbines, the Delft University of
Technology has developed 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 hydraulic
cylinders. The Stewart platform is fixed on the ship deck. To keep the transfer deck
motionless, a sensor continuously measures the motions of the ship deck. The
cylinders of the Stewart platform are controlled in such a way that all ship motions are
counteracted, thereby creating a stable and motionless transfer deck.
The main driver within the design and development of the Ampelmann system was
safety. The Ampelmann safety philosophy led to a fully redundant system design
enabling full motion compensation, and thus safe access, in sea states up to 3 meters.
This design was made into a full-scale prototype with all redundancies thoroughly
tested. Finally, the Ampelmann system was taken offshore to prove its function: to
provide safe access to an offshore wind turbine. The Ampelmann system is the first
system ever to provide a full motion-compensating platform to enable safe offshore
access. The system has been thoroughly tested in offshore conditions and proved to
provide safe access in sea states up to 3 meters. After its successful test results, the
Ampelmann system has become commercially available.
1
INTRODUCTION
Due to the increasing amount of offshore wind farms that have been installed over the last years,
near-shore locations to place new wind farms are getting scarce. As a result, wind farms are
gradually being placed farther offshore, where wind speeds are higher and the available locations
have a larger areal extent allowing for wind farms with a larger number of turbines. However, such
sites are commonly in deeper water and subject to rougher wave conditions than the currently
operational wind farms. When regarding operations and maintenance, this presents a practical
problem: accessibility, which is defined as the percentage of time that a turbine can be accessed.
Whenever an offshore wind turbine requires a corrective maintenance action, the turbine will
remain unavailable for electricity production until it is repaired. Lack of accessibility, most probably
due to rough wind and wave conditions, can cause long downtimes thereby reducing the turbine’s
availability. A decreased availability results in a decrease in power production, which will ultimately
lead to revenue loss.
Over 90% of all maintenance actions only require the transfer of personnel and of parts which can
be carried by man or lifted by a turbine’s permanent internal crane [1] [2]. In the offshore wind

2. industry personnel transfers by helicopters are usually not applied due to safety related arguments,
high costs and the fact that a hoisting platform is required on each turbine. Offshore wind turbines
are therefore generally accessed by vessels. Safe transfers are enabled by intentionally creating
frictional contact between the vessel’s bow and the turbine’s boat landing aiming to have no vessel
translations at the point of contact. The main downside of this access method is that it is limited to
moderate wave conditions. Based on industry comments, a fair estimate of the limiting wave
conditions appears to be a significant wave height Hs of 1.5 meter.
Future wind farms at locations with heavier sea conditions will have a significantly decreased
accessibility when using the current ship-based access method, due to the maximum significant
wave height that limits transfers. To examine the accessibility of typical offshore wind farm sites as
a function of the limiting sea state, two Dutch offshore locations with wave data available from [3]
have been selected: the IJmuiden Munitiestortplaats (YM6) and the K13a platform (K13). The
former is situated approximately 37 km offshore, the latter at a distance of about 100 km from shore.
Scatter diagrams with the yearly distribution of sea states of both locations were used to determine
the year-round accessibility of fictive wind farms at these two sites. The YM6 location is
representative for sea conditions at currently operational wind farm sites: the Offshore Windpark
Egmond aan Zee (OWEZ) and the Prinses Amaliawindpark (previously named Windpark Q7) are
situated nearby thus exposed to similar wave conditions. At this site, current access methods
limited to a significant wave height of 1.5 meter result in an accessibility of 68% as shown in Table 1.
At the location farther offshore, K13, this number reduces to 60% for the same access limit. It is
also shown in Table 1 that when the access-limiting significant wave height can be increased to 2.0
or 2.5 meters, a very large increase in accessibility can be achieved at both sites. An increase from
2.5 meters to 3.0 meters has a relatively smaller effect and one can question whether this justifies
the probable additional costs involved.
Table 1 Year-round accessibility for different limiting sea states at two offshore sites
Location
Year-round accessibility [%]
for different limiting sea states
Distance to
shore
Hs,lim =
1.0 m
Hs,lim =
1.5 m
Hs,lim =
2.0 m
Hs,lim =
2.5 m
Hs,lim =
3.0 m
YM6
37 km
45
68
83
91
95
K13
100 km
36
60
76
87
93
With the anticipated increase in number of offshore wind farms in mind, especially at locations
farther offshore with rougher wave climates, it can be stated that there is a clear industry need to
develop a safe ship-based access system for wind turbine maintenance with a high accessibility,
preferably up to a significant wave height of 2.5m.
2
THE AMPELMANN
To create a safe transfer system, it would be ideal to have on a vessel a transfer platform for
which the vessel motions can be compensated in all six degrees of freedom in order to make it
stand still in comparison to the fixed world, in this case the offshore wind turbine. A gangway
between the transfer platform and the turbine will then enable personnel to walk safely from the
vessel to the offshore structure and vice versa.
Systems that can create motions in all six degrees of freedom exist in the form of flight simulators.
The moving part of these simulators is an assembly of a cockpit and video screens. This assembly

3. is set in motion by a configuration of six hydraulic cylinders known as a hexapod or a Stewart
platform, as shown in Figure 1. Due to the use of six cylinders, these platforms can move in a
controlled manner in all six degrees of freedom. This principle seems to be ideally suited to cancel
all motions when mounted on a ship, after replacing the cockpit and video screens by a transfer
deck. A prerequisite for compensating motions is to have accurate real-time measurements of the
ship motions and a control system to convert the motion sensor data into control signals for the
Stewart platform. Thus by combining the technologies of a Stewart platform and motion sensors
active motion compensation can be achieved in all six degrees of freedom. This concept was
invented during a Wind Energy Conference in Berlin in 2002, and was therefore named
“Ampelmann” after the typical little man with the hat in the former East Berlin traffic lights, “das
Ampelmännchen” (Figure 2) making offshore access as easy as crossing the street. An artist’s
impression of the Ampelmann system is shown in Figure 3.
Figure 1 Flight simulator with
Stewart platform
Figure 2 Das
Ampelmännchen
Figure 3 Artist’s impression of the
Ampelmann system
Prior
stated:
•
•
•
•
to the development of the Ampelmann system, the following system requirements were
3
SCALE MODEL TESTS
Highest safety standards
Ship-based system, applicable on a wide range of vessels
No adaptation to wind turbines necessary
Provide accessibility Hs ≥ 2.5m
It was to be examined whether the different technologies combined in the Ampelmann system, i.e.
the Stewart platform and motion sensor, would allow for a motion control fast and accurate enough
to minimize create a motionless upper deck on a moving vessel in order to enable safe transfers. To
research this, a series of scale model tests were performed using a small sized Stewart platform
(cylinder stroke of 20 cm) in combination with an Octans motion sensor (consisting of three
accelerometers and three fibre-optic gyros) and custom-made software. This proof of concept was
conducted by first placing the system on top of a larger Stewart platform (Figure 4) to test and
enhance its performance by fine-tuning of the controls. Finally, the system was mounted on a 4
meter vessel which was placed in a wave basin to excite the vessel with regular and irregular waves
(Figure 5). These scale model test proved the Ampelmann concept: enabling a motionless transfer
deck on top of a moving vessel.
The dry and wet tests performed with the small scale Ampelmann model gave good insights in the
use of the combined technologies of an Octans motion sensor and a hydraulic Stewart platform for
active motion compensation. In random wave fields, the Ampelmann scale model managed to keep
the upper platform of the small Stewart platform nearly motionless with residual motions less than
1cm. The results of this proof-of-concept phase justified continuing with the next phase: creating a
prototype.

4. A
B
C
D
Mooring Lines
Roll Dampers
Octans
Wave height
Measurement
E Hydraulic Pump
F Stewart Platform
Figure 4 Dry test set-up
4
Figure 5 Wet test set-up in wave basin
SAFETY PHILOSOPHY AND DESIGN CONSEQUENCES
After the scale model tests, the next step was to develop a prototype to prove the Ampelmann
concept in real offshore conditions for the purpose of transferring personnel. This objective
presented three new main challenges:
•
•
•
Make the integral Ampelmann system inherently safe for personnel transfer
Create a system that can counteract the motions of a sea-going vessel in Hs = 2.5m
Prove its full operational use in offshore conditions: easy access
Although Stewart platforms with cylinder strokes exceeding 1m are commonly used as flight
simulators, the application of such a platform in offshore conditions is new. To tackle the first
challenge the following safety philosophy was decided upon:
•
•
Operation must continue after a single component failure
This ride-through-failure must work for at least 30 seconds
A safety based design procedure was developed to create a system that is inherently safe whilst
meeting the other two challenges: full motion compensation in predefined sea states and easy
access. This procedure is shown in Figure 6 and defined four sub-objectives:
•
•
•
•
Verify strength of all structural components
Create a Stewart platform that can compensate ship motions in Hs = 2.5m
Ensure full redundancy of the Ampelmann system (motion control)
Prevent possible failures due to human errors

5. Safety Based
Design
Structural
Strength
Ship Motion
Compensation
Motion
Control
Operational
Procedure
No failure
of structural
components
Full motion
compensation
No failure
of critical
components
No failure
due to
human errors
Lloyd’s Register:
• Design Appraisal
• Fabrication
Survey
• Load Test
• Optimized Stewart
platform design
for full motion
compensation in
Hs = 2.5m
• All main
components
redundant
• Trained operators
• Easy access
• Ampelmann Safety Management
System (ASMS) monitors all system
functions, takes mitigation measures
and warns operator
Figure 6 Safety based design procedure
The Stewart platform design is elaborated in Section 5. The structural strength verification was
done by Lloyd’s Register and is treated in Section 6.
To ensure the full redundancy of the Ampelmann system, a Failure Modes and Effects Analysis
(FMEA) was performed to identify the possible failures on all system components and examine the
effect of each failure. For all effects that can result in malfunctioning of the Stewart platform or any
other hazardous situation, directly or indirectly, a measure was taken to either reduce the
occurrence of failure or reduce the effect. This was done for all components until a system design
emerged where component or computational failures could no longer cause unsafe effects. This
meant that after any failure the Ampelmann system is able to continue its functionalities for at least
30 seconds. As a result of the FMEA it was concluded that all critical components in the system had
to be made redundant; all redundancies were tested to prove the ride-through-failure capacity.
To connect all possible component failures to the operational procedures, several HAZID (Hazard
Identification) meetings were held with all stakeholders in the development of the Ampelmann
prototype. The outcome of these meetings led to the drafting of the ASMS: the Ampelmann Safety
Management System. In this extensive spreadsheet based model, all possible failures were
connected to a warning level. These warnings are only visible to the operator, who can assess
whether the person transferring can finish his operation before the system is returned to its settled
position. Only the occurrence of a double failure is relayed to all of the crew: alarm lights will flash
and sirens will sound. A person transferring has 5 seconds before the system will retract itself from
the structure and can either complete the transfer or step back and hold on tight. During these 5
seconds the operator also has the option to abort the operation manually.
To prevent failures due to human errors all Ampelmann operators will be trained properly.
Transferring personnel will receive a safety induction, but will basically only need to look at a traffic
light mounted on the transfer deck. In case the green light is switched on by the operator, it is safe
to walk over the gangway to access the offshore wind turbine: Offshore access as easy as crossing
the street.

6. 5
STEWART PLATFORM DESIGN
The purpose of the Ampelmann system’s Stewart platform is to provide motions in all six degrees
of freedom large enough to keep the platform’s transfer deck motionless on a moving vessel in
predefined sea states. The design of the Stewart platform architecture should therefore enable
compensation of vessel motions in sea states of Hs=2.5m. In addition, the axial forces in the
hydraulic cylinders caused by the transfer deck and gangway should be kept low in order to have
low power requirements and low costs. Finally, mechanical singularities of the platform should be
avoided. Mechanical singularity in a platform can be defined as the configuration or pose of a
mechanism that causes unpredictable behaviour; this is a situation that can and must be prevented
by examining all possible platform poses.
A design process was developed to determine the Stewart platform’s architecture best apt for the
Ampelmann system. This was done by first determining many possible architecture options, limited
by different boundary conditions: cylinder stroke length and size limits. Furthermore, the ultimate
load cases for the Ampelmann application were to be determined. Then a calculation procedure
was performed for all proposed platform architectures to determine the motion envelope, calculate
the cylinder forces and check for singularities. From vessel motion simulations, it was found that
within the motion envelope the vertical excursions were a determining criterion. After different
design considerations the best platform architecture could be selected. The process for determining
the platform architecture is shown in a flowchart in Figure 7. This process yielded the final Stewart
platform architecture as well as a clear procedure for determining future Stewart platform
architectures for Ampelmann systems in case design requirements are altered.
Size Limits
Stroke length
Calculation Procedure
Load cases
Other
Architecture
Parameters
Design Considerations
Preferred Architecture
Figure 7 Flowchart to determine Stewart platform architecture
Subsequently, the motion compensation capacity of the selected Stewart platform architecture
was examined for three different vessel types. For this, vessel motions in all six degrees of freedom
were simulated for these vessels in different sea states. The results are presented in Figure 8,
showing that the objective of enabling motion compensation in a sea state of Hs=2.5m is reached
when the Ampelmann is mounted on a 50m vessel.
Type vessel:
Dimensions:
Displacement:
Max. sea state:
Workability:
Anchor handling tug
24m x 10m x 2.75m
120 tons
Hs = 2.0m
85% (S. North Sea)
Type vessel:
Dimensions:
Displacement:
Max. sea state:
Workability:
Multi purpose vessel
50m x 12m x 3.80m
900 tons
Hs = 2.5m
93% (S. North Sea)
Type vessel:
Dimensions:
Displacement:
Max. sea state:
Workability:
Offshore support vessel
70m x 16m x 5.60m
4000 tons
Hs = 3.0m
97% (S. North Sea)
Figure 8 Motion compensating capacity of the Ampelmann system on different vessels

7. 6
TESTING AND CERTIFICATION
After the assembly of the Ampelmann prototype, a series of tests was performed to ensure the
proper functioning of the Ampelmann system:
•
•
•
•
•
Motion Tests
Redundancy Tests
Motion Compensation Tests
Operation and Emergency Simulation
Operational tests
During the motion tests, first the Stewart platform’s entire motion envelope was verified (Figure 9).
Subsequently the control system was fine-tuned until high motion control accuracies were achieved.
The next step was to verify the full system redundancy. This was done by simulating failures for
each component, checking if its redundant component takes over its functionality and confirming
the proper warning by the Ampelmann Safety Management System.
Figure 9 Motion test
Figure 10 Motion compensation test
After the motion and redundancy tests, the Ampelmann system was loaded on a barge to perform
motion compensation tests outside the Port of Rotterdam (Figure 10). In a sea state of Hs = 1.5 m
the residual motions measured on the transfer deck were less than 4 cm heave and less than 0.5
degrees roll and pitch, confirming the appropriate functioning of the Ampelmann system.
Back onshore, the gangway was mounted onto the transfer deck and operational procedures as
well as emergency cases were simulated as shown in Figure 11. As a final test, the Ampelmann
was installed on the SMIT Bronco to test offshore access in the OWEZ wind farm off the Dutch
coast. The demonstration of a transfer is shown in Figure 12.
Figure 11 Operation Simulation
Figure 12 First operational test

8. In addition to the extensive test sequence, the structural strength of the Ampelmann system was
to be certified. For this Lloyd’s Register performed fabrication surveys on all structural components
during the production phase. Furthermore a design appraisal on the entire design was conducted.
Finally a load test was done by applying 450 kg on the tip of the fully extended gangway, which was
witnessed by Lloyd’s Register.
7
EVALUATION AND OUTLOOK
The development of the Ampelmann idea into a fully functional prototype presented many
challenges. The most prominent challenges were first to prove the concept of active motion
compensation in all six degrees of freedom and subsequently to build a prototype to provide safe
access to offshore wind turbines. The prototype was developed while keeping a strong emphasis on
the inherent safety of the system, which resulted in a proven fully redundant transfer system.
After the transfer demonstration at OWEZ, the Ampelmann became commercially available and
has been applied in different offshore projects. Amongst these projects are the decommissioning of
an offshore platform (Figure 13), where motion compensation was achieved in sea states up to
HS=2.8m, and the installation of transition pieces of offshore wind turbines (Figure 14). During the
different projects, the Ampelmann has made approximately 350 landings and provided over 1600
personnel transfers. A second Ampelmann system has been operational since this summer.
Figure 13 Ampelmann at platform
decommissioning
Figure 14 Ampelmann at transition piece
installation
The next step for the Ampelmann is to significantly increase the accessibility of offshore wind
turbines in order to increase uptime, power production and revenues. The Ampelmann technology
has proven to be a safe method to transfer personnel to offshore wind turbines and can in fact
provide access in sea states up to 3 metres, making offshore access as easy as crossing the street.
REFERENCES
1
2
3
Rademakers, L, and H. Braam. O&M Aspects of the 500 MW Offshore Wind Farm at NL7 –
Optimization Study. DOWEC 10090 rev 1. 2003.
Rademakers, L, and H. Braam. O&M Aspects of the 500 MW Offshore Wind Farm at NL7 –
Baseline Configuration. DOWEC 10080 rev 2. 2002.
www.golfklimaat.nl, January 2009.