Slip Joint Connection by Van Oord

The Slip Joint is an innovative connection for offshore wind substructures and WTG towers as alternative to grouted or bolted connections. It works the same way as two upside-down paper cups stacked on top of each other. The Slip Joint connection provides a fast, simple and safe installation solution in combination with reduced maintenance requirements. Also, the Slip Joint can be used submerged allowing for an optimized design of the transition piece and the monopile in terms of length and weights.

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Van Oord has been involved in the development of the Slip Joint connection since 2012 in several R&D projects. In 2018 Van Oord was awarded as Balance of Plant contractor for the offshore innovation site Borssele V by Two Towers. Borssele V will demonstrate several innovations including a Slip Joint foundation. This is the first time that a Slip Joint is implemented in the foundation of a full size 9.5MW wind turbine in a commercial wind farm with a lifetime of 25 years.

An extensive design and testing program was performed, together with industry and science partners, to qualify the slip joint as a safe technology. The program included an extensive parameter study with hundreds of calculation models, development of design and analysis tools including validation, laboratory tests on friction and fatigue behaviour, corrosion tests and scale tests including an offshore test with a slip joint in a foundation of a 500kW wind turbine.

The Borssele V wind farm including the submerged Slip Joint foundation designed by Van Oord, received a statement of compliance by DNVGL Hamburg in their project certification scheme. In parallel, together with partners, a more general A-level component/type certification process is also finished. This qualifies the Slip Joint as a new connection type to the same level as conventional bolted and grouted connections.

The manufacturing of the Borssele V Slip Joint foundation is finished and will the Slip Joint was successfully installed offshore in April 2020. With the demonstration of the Borssele V Slip Joint, a new and improved certified substructure connection is available for the offshore (wind) industry.

More information related to the Slip joint can be found in video 2, 3 and 5 via here.


  • CAPEX reduction: As the split between MP and TP can be anywhere in the foundation, including submerged, an optimum split in length and mass of the MP and TP can be made, allowing smaller and cheaper installation vessels to be used. Compared to other connections, the installation of a slip joint is much faster as there is no delay due to bolting or grouting, leading to lower installation vessel costs.
  • OPEX reduction: The Slip Joint does not require maintenance, reducing O&M costs and project risks.
  • EHS: Other connections require workers to be on the foundation during installation, standing under a suspended load. A Slip Joint installation can be done without workers on the foundation

How will this innovation contribute to the industry?

The Slip Joint connection adds a new type of connection to the offshore industry. While focused on the offshore wind industry, the Slip Joint also has potential in other offshore markets where (monopile) foundations are used. The Slip Joint connection leads to faster installation, allows for submerged installation and is maintenance free. With foundation getting bigger and bigger, the Slip Joint could leads to a lower LCOE, making sure offshore wind remains the energy source of the future.

By completing the certification program, both for a specific project and as a general component, the Slip Joint is now fully certified by DNVGL for the complete lifetime of the foundation. This certification includes the effects of corrosion, even when used submerged, and ultimate- and fatigue loads.

The implementation of the a Slip Joint connection is not limited to a foundation. This connection also shows potential for use in (offshore) wind turbines, were currently tower segments and the nacelle are connected using bolted connections. This requires manual work, under suspended loads, at great heights. The Slip Joint does not required any personal to be present when making this connection, eliminating these safety risks.