Riding out the waves

A major challenge facing offshore wind energy developers is figuring out how to board wind turbines for maintenance and repair during periods when the oceans (or lakes) are turbulent and wave actions create unsafe conditions. Attempting to disembark a ship being bounced around by rolling waves to climb onto the ladder of a wind turbine tower can be treacherous to say the least.

One solution to this problem is to design and engineer turbines to such a high degree of precision that the necessity of physical maintenance visits are minimized. However, it's unlikely that the need for such visits will ever be completely eliminated. Blades snap, drive trains fail, metals corrode. Some things cannot be fixed from land-based control rooms.

That's where innovations like the Ampelmann can make the difference between economic success or failure. (GW)

Waiving the rules on platform motion

By Tom Shelly
Eureka Magazine
February 8, 2008

A combination of control and hydraulics is removing a major problem for those who live and work in a marine environment. Tom Shelley reports

By using a motion platform in reverse, a ship-mountable platform and gangway can be created that allows people to step safely from a vessel, heaving on the waves, on to a wind turbine or other offshore structure.

The Ampelmann – named after the East German pedestrian crossing symbol that indicated it was safe to cross the street – is the brainchild of Dr Jan van der Tempel, spun out of the Technical University of Delft in the Netherlands. The idea took root during an informal get-together between van der Tempel and colleague David Molinaar after the World Wind Energy Conference in 2002 in Berlin – in full sight of one of the actual Ampelmann equipped crossings.

The concept was first unveiled to the UK at working model stage on the Eureka website in March 2007, and was successfully demonstrated at sea in December 2007, when the first man was transferred from an Ampelmann, fitted to a vessel called the Smit Bronco, to wind turbine No.3 in the offshore wind farm Egmond aan Zee. Significant wave height (the mean of one third of the highest waves in a wave field) was only 0.5m at the time, but the system has since been tuned to deal with larger waves.

Since the technology is based on flight simulators, the Ampelmann team has been working with Simona, the International Research Institute for Simulation, Motion and Navigation, also part of the Technical University of Delft. Motion measurement is with the OCTANS-4 fibre optic gyrocompass and motion sensor made by the French company IXsea, which can output true heading, roll, pitch, surge, sway, heave, velocities and rate of turn.

IXsea, Bosch Rexroth and Dutch company Boskalis have donated more than 100,000 Euros to the project so far.

Two systems have been designed, boasting a 1m and 2m cylinder stroke respectively. In theory, the first can cope with a 1m significant wave height on a 25m vessel and the second with a 1.5m significant wave height on a 25m vessel, or 2m on a 50m vessel.

As well as allowing easier and safe access to offshore wind turbines and other structures, the team see potential applications in anti-seasickness platforms on ferries and cruise ships, and stabilisation of helicopter decks on both civilian and naval vessels. The platform could also make it easier to launch remotely operated vehicles (ROVs), and be used to support communication and measuring equipment in such a way as to keep them on target with satellites.

Stabilising parts of ship decks would ease transfer of equipment to offshore installations, as well as people. And, by having platforms on two vessels, it should make it simpler to transfer personnel and equipment between them. By placing offshore topsides on multiple Ampelmann platforms, it should also be possible to ease float over installation – a technique where the topside is placed on an installation vessel, or barge, and sailed in between the pre-installed legs of a sub structure. The installation vessel is then ballasted until the weight of the topside is transferred to the sub structure.

Elaborate FMEA (Failure Mode Effect Analysis) and Hemp (Hazard Effect Management Process) studies ensure safe working in all conceivable circumstances.

Pointers

• By using a six degrees of freedom motion simulator in reverse, it is possible to remove the effects of wave motion on a vessel to ease the transfer of people or goods to an offshore structure
• The idea has reached the stage of full-scale trials involving the transfer of people