Growth through Research, development & demonstration in Offshore Wind


Slip Joint Offshore Research project (SJOR) Completed

A new feature in the base of the wind turbine, the so-called Slip Joint connection


Most offshore wind projects are installed in several steps: a monopile is driven into the seabed, a transition piece is put on top of this, followed by the tower, and nacelle and rotor blades. Each step takes time, is sensitive to weather conditions, and makes installation an important component in overall project costs. By reducing both the number of steps, the installation time and/or weather sensitivity of each step, significant cost savings could be achieved.


The Slip Joint is a new type of connection between the wind turbine and the monopile developed by the company Delft Offshore Turbine (DOT). The tower section of the wind turbine has a conical shape which is matched by the conical shape of the foundation monopile. Installation is done by simply sliding the wind turbine over the monopile thus creating a friction-based connection without the use for grout or bolts. This simple mechanism allows for cost reduction through both savings on required material/equipment/personnel as well as a shorter installation time.

Within the Slip Joint Offshore Research (SJOR) project the principle of this innovation will be tested under offshore conditions in the North Sea. The project is a collaboration of the GROW partners DOT, TU Delft, TNO, Van Oord and Sif Group, and the offshore test is enabled by GROW partner Eneco, the owner of the Princess Amalia offshore wind farm where the monopile and turbine will be installed. Project partner Heerema Marine Contractors provide the tools and expertise to successfully install monopile and turbine.


In September 2018 a full wind turbine has been successfully installed in the North Sea (Prinses Amalia offshore wind farm) and will stay offshore for a year to test and collect data regarding this innovative connection. The next step will be the commercialisation of this technology. The successful demonstration of slip joint technology is one of the first tangible achievements under the GROW program. DOT develops various new concepts that should contribute to a further cost reduction of offshore wind energy. Next to the slip joint technology, the monopile has been vibrated into the soil using a Vibro Lifting Tool, and the turbine itself is an example of an hydraulic turbine.


May 2018: Installation DOT Monopile

September 2018: Installation DOT Wind Turbine

August 2019: Removal DOT Wind Turbine

This project is supported by the Netherlands Enterprise Agency and TKI Wind op Zee.

Contact Details


Reinder Jorritsma
+31 (0)6 148 33 245

Technology Readiness Level

Maturity level: 4,
        4 5 6

Project duration


Foundation Logistics
Loading of the turbine magnify Loading of the turbine

Loading of the turbine


Loading of the turbine

Installation of the slip joint magnify Installation of the slip joint

Installation of the slip joint


Installation of the slip joint

Installation completed magnify Installation completed

Installation completed


Installation completed

Other information

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)

Project information at the TKI Wind op Zee website


Associate project partner