Double Slip Joint - Conical rings for connection of transition pieces

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TKI Wind op Zee
Double Slip Joint (Source KCI)

Pioneering SME: KCI

The size of turbines and the water depths for future KCI Engineering has developed an equally simple and efficient construction of conical steel rings. Installation is 30% faster and special maintenance is unnecessary. After successful tests, the engineering company is looking for a wind farm developer who dares to use them.windfarms continue to increase. As a result of the loads on the monopile foundations increase which in turn results in larger monopiles (diameter), thus creating challenges for the critical connection between a monopile and the transition piece.


For what problem did you find a solution?

"For the transition piece that connects monopiles to the foundation, two connection methods are in use. The most common are grouted connections where cement (grout) is poured between the tubes. Cement, however, suffers from the constantly varying dynamic loads. The strength decreases and the risk of failure increases. Despite all improvements with conical sections and shear keys, the ultimate, cost-effective solution has not yet been found. A second way is to anchor the tubes with hundreds of bolts in double rows. The problem with such heavy bolt connections is that once one bolt is tightened, the previous bolt gets loose slightly. In addition, the bolts will loosen over time so you have to monitor and tighten them. Corrosion is also a risk. And there is a limit on the usage of the bolts for future connections. You cannot scale up endlessly. "

What’s the core of your solution?

"We have been involved in the search for better transit pieces since the Prinses Amalia Wind Park. The Double Slip Joint is, according to us, the ultimate solution. This connection consists of two sets of  conical steel rings, welded to the monopile and the transition piece. The steel rings with diameter between 6 and 10 m are about 80-90 centimeters wide and taper in an angle of 2˚. At the moment of positioning the transit piece and the monopile, the rings slip seamlessly a few centimeters by their own weight.

What is so pioneering about your solution?

"Our solution is simple but very effective. The secret is in the small angle of the rings. The rings slip over each other for about 90% immediately. Due to the dynamic load of storms and waves, the rings are increasingly slipping down, until they are as tight as if they were welded. "

What are the advantages?

"The rings don’t need not be maintained nor are particularly sensitive to corrosion above or below water. In addition, the rings can be manufactured with standard offshore steel accurately and in large series. Furthermore, we estimate installing is about 30 percent faster because there is no time lost in pumping cement into the transit or in tightening bolts. You can sail away immediately after placement of the transition piece. This is essential given the rate at which wind farms should be built and the limited period of time that conditions at sea allow installation. Finally, if the connection is made below sea level, the weight of the heaviest part, the foundation pile, becomes a lot of lighter, making it possible to use lighter cranes."

What are the challenges?

"The challenge is not so much in technology. Finding a park owner who dares to go for it is much more difficult. It remains risky, even though there is a business case; otherwise we would never have started. For an SME entrepreneur it is financially impossible to do it on your own. You need to connect all parties, without losing your IP. We are therefore looking for the Borssele innovation site at, a fantastic idea, and hope to find other innovative parties through the Offshore Wind Innovators community among others, to jointly submit a tender. "

How far are you now?

"We have developed the innovation with the vital financial support of TKI Wind on Sea and with our partners Van Oord, who is interested in the installation of the construction, and manufacturer SIF, who is looking at serial production of monopiles with a DSJ .

In 2016 we started building the prototype, on a scale of 1 to 5 of an 8 MW turbine, or a full scale 500 W turbine. A truck brought the parts to the knowledge institute and test institute WMC in Wieringerwerf this spring, where it was tested. The tests have been completed. The report is nearly finished, but the slip behavior seems to be as we have predicted."


This article has been prepared in cooperation with the Offshore Wind Innovators.

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