Slip Joint foundation makes debut off the Netherlands

Van Oord has installed a Slip Joint foundation at the Borssele Wind Farm Site V offshore the Netherlands.

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, Van Oord said.

After years of developing the system in cooperation with its technology partners, Van Oord deployed offshore installation vessel Aeolus to install the Slip Joint foundation. The foundation, manufactured by Sif, was installed on 25 April.

The design and production of the Slip Joint was certified by DNVGL in the autumn of 2019, with MottMcDonald assessing it on bankability aspects.

The Borssele Site V project consists of two MHI Vestas 9.5 MW wind turbines and Van Oord has been awarded the Balance of Plant scope for this project.

The Slip Joint is an alternative connection between 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 a monopile and a transition piece, Van Oord said.

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 Borssele Wind Farm Site V is a new sustainable energy zone some 20 kilometres 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 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 other technologies demonstrated at the Borssele V Site are Thermally Sprayed Aluminium (TSA), Impressed Current Cathodic Protection (ICCP) optimisation, oval cable entry holes, and Eco-friendly scour protection. Once operational in 2021, the 19 MW offshore wind farm will provide sustainable energy to 25,000 households.