Slip joint for wind turbine

Name: Maxim Segeren (29)
Nationality: Dutch
Supervisor: Professor Andrei Metrikine (Civil Engineering and Geosciences)
Subject:  Modeling a slip joint connection for monopile support structures for offshore wind turbines
Thesis defense: In two years

“Since the first offshore wind farms were built, virtually all turbines have had the same support structure: the monopole, which is driven into the seabed, and, after that, a transition piece is grouted on top of it.

Finally, the wind turbine is connected to the transition piece via a bolted connection. The transition piece has a larger diameter than the monopile and is attached by filling the ‘gap’ between the two overlapping cylinders with grout- a very strong concrete. Recently, it was observed that, in most North Sea wind farms, these grouted connections are settling. Consequently, an alternative way to connect the support structure and the turbine must be found.

To solve this problem, I will investigate the use of an alternative connection, called slip joint, which consists of two conical sections of measuring 5 meters in diameter, with one attached to the top of the monopile and the other to the bottom of the transition piece. The two sections must fit in the same way as two inverted cups.

The slip joint’s main advantage is that all the complicating factors related to the grout are eliminated. Also, since the lengthy grouting process is removed, the installation of the whole structure may be accelerated. There are, however, still many interesting questions to solve, especially regarding the installation of the slip joint connection. For example, which factors influence the settlement and the resulting overlap length? Pile driving may also deform the cone on top of the monopile. In order to take these influences into account, we will measure the geometry of a conical monopile before and after pile driving.

I have high hopes that the slip joint connection will be used in future. My research will contribute to the use of a slip joint connection in a new offshore wind farm, of which construction is planned for 2014-15. That industry is interested in this research makes it even more exciting.”

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