Most offshore wind turbines in the North Sea are composed of a monopile, transition piece, tower, nacelle, and rotor blades. The installation of each of these components takes time and therefore installation is a significant part of the total project costs. Significant cost savings can be achieved by reducing weather sensitivity, the number of steps and installation time.
A bolted connection is the most used type of connection between the turbine and the monopile. This type of connection requires considerable installation time and is vulnerable to failures. The Slip Joint is a new type of connection between the wind turbine and the monopile. Installation is done by simply sliding the wind turbine over the monopile, creating a friction-based connection without the use of grout or bolts. To use the Slip Joint, it is necessary to equip the tower part of the wind turbine with a conical shape that matches the conical shape of the foundation monopile.
The Slip Joint simplifies connection and therefore reduces installation time and costs, as well as maintenance costs. This type of connection has never been used offshore before.
The SJOR project consisted of three different campaigns: installation of the monopile, installation of the turbine and, after a year of operation, the removal of the wind turbine. The innovative Slip Joint was tested under offshore conditions in the North Sea within Princess Amalia Wind Park owned and operated by Eneco. As a first step a dedicated monopile was installed using a Vibro installation tool in combination with a floating installation vessel operating on Dynamic Positioning (DP). This meant that no griper frame or impact hammer was used, a world’s first!
A couple of months later Heerema Marine Contractors’ installation vessel Aegir transported a retrofitted 500 kW turbine to this location. Installation was once again done on DP in combination with a single lift approach. Installation was completed within the hour. Immediately afterwards DOT was able to send engineers on board the wind turbine to start commissioning. The turbine was operational within 24 hours, and during the next 9 months of operation the project team made extensive measurements on the Slip Joint behaviour by using various high-quality sensors.
The time required for installation using conventional bolted flange-to-flange connections is several hours. This project showed that by using the Slip Joint, this time can be reduced to less than an hour. The results also suggest that the Slip Joint makes installation using floating vessels easier as the installation tolerance is greater than when using other types of joints. Both improvements lead to significant time savings and thus cost savings. Further cost savings will be achieved through lower maintenance costs.
The analysis of the measurements showed that the Slip Joint connection is stable and reliable also in real life conditions. The project team concluded that no extra action is required once the installation of a Slip Joint connection is completed. Besides the offshore campaign TNO also performed measurements to increase knowledge on friction behaviour inside the Slip Joint. The team also investigated fretting fatigue, which may occur because of local contact forces as well as corrosion effects. In this way, insight is gained in the lifetime of the joint. The results of all tests were incorporated in Slip Joint design tools to be able to develop a cost-effective Slip Joint for next offshore wind turbine projects. All these results led to a DNV-GL type certificate paving the way to commercialization of this innovative connection.
The Slip Joint technology is currently being demonstrated on full-scale turbines at the Borssele V Innovation site. The Two Towers consortium, consisting of Van Oord, Investri Offshore and Green Giraffe, has installed two turbines of 9.5 MW in 2020. In contrast to the tests in the SJOR project, the Slip Joint is placed underwater with an extended transition piece to the monopile. This will allow also installation at greater depths in the future.
Finally, the Slip Joint can probably also be used in other parts of the wind turbine, for example between the nacelle and the tower. This is subject to further research.
May 2018: Installation DOT Monopile
September 2018: Installation DOT Wind Turbine
August 2019: Removal DOT Wind Turbine
Jan van der Tempel
Slip Joint Offshore Research project (SJOR) (presentation at GROW side-event 2019)
Dutch Innovations Promise Lower Maintenance Costs For Offshore Wind (article in Clean Technica 2018)
This project is supported by the Netherlands Enterprise Agency (RVO) and TKI Wind op Zee. Project information at the TKI Wind op Zee website