The Monopiles

The Transition Pieces

THE BUILDING AND INSTALLATION OF

The monopiles and the transition pieces form the foundations for the turbines.

They are drilled into the seabed with absolute precision. All of the 150 monopiles were placed

in record-breaking time, in the summer of 2015.

Less room on deck

The placement of jackets on the seabed is also not a sinecure: the risk of damages is evident and getting the position exactly right is difficult. One more advantage of monopiles is that they take up less room on deck, when shipping out of sea. Adding to this, Siemens made it clear in the design phase that the new generation of aerodynamic turbine blades could withstand a higher wind pressure than before. This made the use of a slimmer monopile feasible. And finally, the wharf terrain in Hoboken/Antwerp would not have been large enough to store and transport 150 turbine jackets. The choice was clear: monopiles it would be. 75 of them were manufactured near Rostock (Germany), the other 75 in Antwerp. As the images show, it is all about steel; bending it and welding the huge rings together. After the installation, the monopile sticks out above the sea level for 6 metres. On top of this, the transition piece is mounted.

The Monopiles

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THE BUILDING AND INSTALLATION OF

Evidently not the most spectacular part of a wind turbine: the monopile. Nevertheless, this is the foundation that keeps everything in place – for 25 years or longer. In the development phase of the project two alternatives for the foundation were discussed: jackets or monopiles.

Both have their pro’s and con’s. For example, monopiles use up more steel. On the other hand, they are less labour-intensive to produce. Jackets have to be welded together and their cost of maintenance is higher. Furthermore, main contractor Van Oord already had extensive experience with the use of monopiles. The Van Oord-vessels are completely set up for this system.

Mother Nature

Onshore Mockup

Protecting the foundations

Connecting contractors

Gemini has put a lot of effort into testing materials and construction details to discover those which would perform best in the North Sea.

By experimenting ‘on land’, failure at sea was able to be prevented. One perfect example was the mock-up for the interface between a monopile and a transition piece, built in Antwerp.

The Gemini Team put a lot of focus on allowing Van Oord and Siemens to operate next each other with as little disturbance as possible.

Siemens was commissioned to produce the turbines, while Van Oord was responsible for the onshore and high-voltage stations and the electrical connections to the foundations. That interface had to be managed as effectively as possible. Everything had an impact. For example, a cable ship must not get in the way of a turbine installation vessel.

The Gemini Wind Park was built at sea, surrounded by maritime flora and fauna. How do you manage that process as carefully as possible? Gemini has paid particular attention in the project to the matter of permits and environmental monitoring. Gemini’s ambition? To minimise the wind park’s impact, now as well as later. The project also makes a special contribution to expanding knowledge of nature and the environment.

The foundations are continuously exposed to the salt water of the North Sea. How to protect the monopiles from corrosion, over the entire lifecycle of the project? An ingenious system was installed and tested before adoption.

Construction

A very important aspects during the production of this steel constructions is that all the welding work is monitored accurately. The main design of the structure was directly commissioned by Van Oord. The engineering teams worked closely to optimize the design for particular platforms, ladders and other facilities (the so-called secondary shaft). Every transition piece weighs 260 tons. At the end of the production process, a fresh layer of special paint is applied to protect the steel from the harsh influence of the North Sea climate.

The Transition Pieces

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THE BUILDING AND INSTALLATION OF

The transition pieces (TP’s) form, together with the monopiles, the foundation of the turbines. The transition piece includes various functionalities such as access for maintenance, cable connection for the energy of the turbine and the corrosion protection of the entire foundation.

This piece is fixed on the monopile. On top of this foundation is still a steel tower of 80 m ultimately placed on which the turbine is mounted. The transition piece consists of a thick-walled tube of more than 5 m in diameter, with a wall thickness of 40 to 80 mm. The tube has a length of about 20 and is constructed from rolled plates of an offshore steel quality. In order to weld the plates of this size to one another, the welding processes are highly automated.

The Transition Piece

Shipping out Eemshaven

Managing construction

Precision is everything

The last ‘station’ before being shipped out to the open sea: Eemshaven, a seaport in the upper north of the Netherlands. From here, the monopiles and transition pieces started on their final journey: some 85 kilometres off the coast, the location of the Gemini Wind

Park.

The North Sea is a big place with lots of space. Nevertheless, the process must be undertaken with utmost precision. State-of-the-art technology was used to make sure that all of the 150 Gemini turbines were placed in exactly the spot designated for them.

The transition piece binds the monopile and the turbine together. It is the vital link in the entire construction. The transition piece is also the main access point when it comes to maintenance. Therefore, it’s a very important part of the wind park operation.

Didi te Gussinklo Ohmann was the Project Director for Gemini with main contractor Van Oord, and she headed an impressive operation. At its highpoint in 2015, there were around 800 employees at work. What does Didi remember most about the five intensive years she was

involved with the project?