Van Oord Installs First Slip-Joint Turbine Foundation


Van Oord has successfully installed a slip-joint foundation at the Borssele Wind Farm Site V. It is the first time anywhere that a submerged slip joint was used on a full-sized offshore wind turbine on a fully commercial basis, the company says. 

After years of developing the system in cooperation with its technology partners, Van Oord deployed the offshore installation vessel – Aeolus – to install the slip-joint foundation. The design and production of the slip joint was certified by DNV GL in the autumn of 2019, with MottMcDonald assessing it on bankability aspects. 

The Borssele Site V project consists of two 9.5 MW wind turbines. Van Oord has been awarded the balance of plant scope for the project.

The slip joint is an alternative connection between foundation elements, a monopile and a transition piece. It consists of two conical sections placed on top of each other. The slip joint is based on friction, with the weight ensuring firmness and stability. Installation takes place by sliding the wind turbine’s foundation elements over the monopile, without having to use grout or bolts. 

Unlike standard solutions, the slip joint makes a submerged connection possible, allowing for a more balanced weight distribution between monopile and transition piece. It therefore opens up the possibility of manufacturing larger foundations for the next generation of wind turbines with existing manufacturing facilities and installing them at deep-sea locations using existing vessels. The conical sections that make the connection are produced using standard manufacturing methods. The slip joint provides an installation solution in combination with reduced maintenance for the duration of the project.

The Borssele Wind Farm Site V is a new sustainable energy zone approximately 20 km off the Dutch coast, which is designated as an innovation site within the Borssele Wind Farm Zone. Two Towers BV, consisting of Van Oord, Investri Offshore and Green Giraffe, has been awarded the project and was given an opportunity to introduce advanced technologies for commercial application in future offshore wind farms with larger turbines and in deeper waters and to demonstrate the practical value. 

The 19 MW offshore wind farm is expected to reach commercial operation in 2021.

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