The Transition Piece

This material forms a rock pad approximately 30 metres in diameter and 1.5 metres deep that prevents erosion around the monopile.
The transition piece includes various functionalities such as access for maintenance, a cable connection for the current from the turbine and the corrosion protection for the entire foundation. This piece is fixed onto the monopile.


The foundations are an important part of a wind turbine foundation at sea. The foundation consists of two parts: the monopile and the transition piece.


The monopile is a steel tube that is anchored in the seabed. This tube is hammered (driven) into the seabed until the top reaches 5 m above the lowest astronomical tide. Guided by GPS and carrying a load of 26,000 tons, Van Oord’s flexible fall-pipe vessel, Stornes, deposited 2,000 tons of scour protection material onto the seabed before each monopile was hammered in.



The last ‘station’ before being shipped out to the open sea: Eemshaven, a seaport in the upper north of the Netherlands. It seems like the end of the world to most Dutch people. But it’s not the end of the line for the monopiles and transition pieces which started on their final journey from here: 85 kilometres off the coast, the location of the Gemini Wind Park. With immaculate precision, they were loaded in equal quantities onto the transport and installation vessels. Impressive!

Sail out Monopiles and Transition Pieces

The construction phase was not the end of the Gemini involvement in Eemshaven, the harbour that is increasingly playing an important role in the maintenance of wind parks in the North Sea. The Siemens-service vessel “Windea la Cour” operates out of Eemshaven. Every two weeks a fresh crew of servicemen heads to the wind park from here.



With the experience of earlier wind parks (Prinses Amalia, Belwind, Teesside, and Luchterduinen) on board, the construction phase started for marine contractor Van Oord. Didi te Gussinklo Ohmann, looking back: ‘A wind park bigger than we had ever built before and that was much further offshore. In itself it was quite risky, but with thorough preparation and good people and equipment we weren’t afraid to take on the challenge. And there was a team with like-minded specialists on the customer side.’ The Van Oord project team was multidisciplinary, with many different nationalities plus a good mix of young and old. We had seniors in the key positions, but a large group of young people also worked on Gemini.’
Didi te Gussinklo Ohmann concludes that a lot of new knowledge was developed during the project. ‘But the learning curve hasn’t stopped, and now we’re taking that experience to other projects. One of the main lessons learned is that you really must keep thinking out of the box. The technology is developing all the time: the only certainty we have is that in a few years, things will be very different. But that’s how it has to be if we want to turn wind into a really competitive energy source without Government subsidies. And that’s the task we have to keep focusing on with the entire sector.’


Managing construction



Precision is everything

Installing a monopile is a complex procedure that has to be performed with great precision. The state-of-the-art equipment used made it possible to position the monopiles in exactly the right location in the wind park – the red dot marks the spot.