On 25 April 2020, Van Oord successfully installed the Slip Joint foundation at the Borssele 5 wind farm using a monopile and transition piece from Sif. It is the first time that an underwater Slip Joint foundation has been installed on an offshore wind turbine on a fully commercial basis anywhere in the world. Sif also supplied a foundation treated with TSA (Thermal Sprayed Aluminium) in a fully automated process – TSA is an alternative coating that protects the pile from corrosion.

 

The design of a Slip Joint connection between a transition piece and a monopile is nothing new. For Van Oord, the time was right to test this on a large scale. The project for Sif was carried out for Borssele 5. The engineers from Van Oord visited Sif to discuss its feasibility. Producing and dimensioning the conical sections is no mean feat – the tolerances involved in making sure that the conical sections fit one another are very tight, so thorough fine-tuning is essential.

 

Critical tolerances

Producing cones with these critical tolerances requires particular attention during production. Sif knows from experience that every sheet behaves differently during rolling. To achieve the precise tolerances, tests and measurements were done to determine the correct dimensioning within the required specifications.

Automated coating

The foundation coated by subcontractor Ginkel under our responsibility has already been installed. Project Manager Ralph Lacroix explains, ‘TSA (Thermal Sprayed Aluminium) is usually applied manually. The innovation in the Borssele 5 project was the use of a robot, which is hugely promising, mainly because of the speed it allows. We learned a lot!’ 

 

Baris Kibrit, Package Manager for WTGs Van Oord:

‘In 2019, the Slip Joint design was certified by DNVGL, the result of years of modelling and design work. Before the design was completed, Sif had been involved in the project so that a design could be created that would not only simplify offshore installation, but that was also optimized for manufacture. The aim was to utilize standard manufacturing methods to the fullest extent possible. As the conical sections support one another through friction, the strict tolerances on the conical sections proved a considerable challenge in manufacture. Working closely with Van Oord’s designers and Sif's experts, we’ve been able to determine these tolerances together.’

‘Innovation projects like these have a relatively high risk profile due to the number of innovations that are still to be tested. During the project, this meant constant attention and clear communication to arrive at the expected high quality. Flexibility was required of both parties, from clarifying design details to resolving challenges in manufacture. This synergy between Van Oord and Sif has endured and ultimately resulted in a successful project.’

 

What is a Slip Joint connection?

A Slip Joint comprises two conical sections that are slid over one another. The principle is based on friction, where the weight provides strength and stability. No jointing agents or bolts need to be used for installation. In contrast to typical solutions, the Slip Joint offers the option of an underwater connection, giving more balanced weight distribution between the monopile and transition piece. As such, the Slip Joint option allows for a larger foundation and enables installation in deep-sea locations for the next generation of wind turbines, using existing production facilities and installation vessels. The (conical) elements of which the joint is constructed are produced with the aid of standard production methods. The Slip Joint offers not only a faster, simpler, and more secure installation solution, but also requires less maintenance over the lifespan of the project.