Offshore personnel transfer dissertation

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: [email protected]

J. van der Tempel, PhD Ampelmann Company

Rotterdamseweg 380, 2629 HG Delft, The Netherlands Tel.: +31 15 27 86828, E-mail: [email protected]

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

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

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

Year-round accessibility [%] for different limiting sea states

Location 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

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.

Prior to the development of the Ampelmann system, the following system requirements were

stated: • Highest safety standards • Ship-based system, applicable on a wide range of vessels • No adaptation to wind turbines necessary • Provide accessibility Hs ≥ 2.5m

3 SCALE MODEL TESTS

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.

Figure 1 Flight simulator with Stewart platform

Figure 2 Das Ampelmännchen

Figure 3 Artist’s impression of the Ampelmann system

4 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

Figure 4 Dry test set-up Figure 5 Wet test set-up in wave basin

A Mooring Lines B Roll Dampers C Octans D Wave height Measurement E Hydraulic Pump F Stewart Platform

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.

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

Structural Strength

Ship Motion Compensation

Motion Control

Operational Procedure

Safety Based Design

No failure of structural components

Full motion compensation

No failure of critical

components

No failure due to

human errors

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.

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.

Figure 8 Motion compensating capacity of the Ampelmann system on different vessels

Calculation Procedure

Design Considerations

Stroke length

Preferred Architecture

Load cases

Other Architecture Parameters

Size Limits

Type vessel: Anchor handling tug Dimensions: 24m x 10m x 2.75m Displacement: 120 tons Max. sea state: Hs = 2.0m Workability: 85% (S. North Sea)

Type vessel: Multi purpose vessel Dimensions: 50m x 12m x 3.80m Displacement: 900 tons Max. sea state: Hs = 2.5m Workability: 93% (S. North Sea)

Type vessel: Offshore support vessel Dimensions: 70m x 16m x 5.60m Displacement: 4000 tons Max. sea state: Hs = 3.0m Workability: 97% (S. North Sea)

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.

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

Figure 9 Motion test Figure 10 Motion compensation test

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.

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 Rademakers, L, and H. Braam. O&M Aspects of the 500 MW Offshore Wind Farm at NL7 – Optimization Study. DOWEC 10090 rev 1. 2003. 2 Rademakers, L, and H. Braam. O&M Aspects of the 500 MW Offshore Wind Farm at NL7 – Baseline Configuration. DOWEC 10080 rev 2. 2002. 3 www.golfklimaat.nl, January 2009.

Figure 13 Ampelmann at platform decommissioning

Figure 14 Ampelmann at transition piece installation