Van Oord installs world’s first submerged slip joint

Van Oord has managed to install the 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.

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 this week. 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 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 a rapid, simple and safe installation solution in combination with reduced maintenance for the duration of the project.

Borssele V

The Borssele Wind Farm Zone consists of five sites that will collectively generate 1400 MW of energy by 2020. Site V (20 MW) has been specifically designated as an Innovation Site and is located some 20 kilometres off the Dutch coast. The Borssele Site V project consists of two 9.5-megawatt wind turbines.

Two Towers BV, consisting of Van Oord, Investri Offshore and Green Giraffe, has been awarded the project and was given the 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. It is expected that the demonstrated innovations will set a new standard for future foundation designs.

Author: Mariska Buitendijk

Mariska Buitendijk is one of SWZ|Maritime's journalists as well as the magazine's copy editor.