Van Oord has installed a submerged slip joint at the Borssele 5 offshore wind farm off the Netherlands.
The Dutch company said this is the first time such a joint has been installed at a full-sized offshore wind farm on a commercial basis.
Van Oord project engineer Gilles Bresser said the company had been working for two to three years on the project.
The company worked with client Two Towers to install the joint on a foundation at the wind farm.
“The slip joint is a connection between the monopile and the transition piece, between two conical sections which slide over each other and by gravity and friction they become tight,” said Bresser.
He added that there are several advantages with a slip joint over other methods of connecting monopiles and TPs, such as bolting or grouting.
For example, for bolting 160 bolts are required, while with a slip joint “you just slide them over each other and you are finished”, he said.
There are also economic benefits with a slip joint as less maintenance is required during the project’s lifetime.
“Slip joints make it possible to build bigger wind turbines because you don’t need the bolts which can become limiting at the moment of connection and the slip joint can become larger,” Bresser added.
Design and production of the slip joint was certified by DNV GL last autumn, with Mott MacDonald assessing bankability.
Van Oord installation vessel Aeolus installed the slip joint foundation.