The Double Slip Joint is being developed by KCI to replace grouted or bolted connections between the transition piece and monopile of wind turbine foundations. Together with partners Van Oord and SIF a joint project has been set up with support from TKI-Wind op Zee. We took an exciting new step in designing and building the set-up at scale 1:5 at WMC. This was a serious increase in scale compared to the "mini-tests" performed in the past by 3 TU-Delft graduates using Perspex prototypes. The DSJ is based on the use of two sets of two conical rings which can be placed inside each other and have a conical angle of two degrees. The transition piece will settle down under the cyclic loads of the turbine to a stable end condition. In this condition the slip joint can no longer move and as such can be compared to a "welded" connection.
The prototype diameter is 1.6 m and the longest component (TP) is about 15 m long and the weight is 12 ton. The small main part (FP) is about 6 m long and the weight is 5 ton. Is has been quite a challenge to transport the prototype about 240 km by road from SIF Roermond to WMC Wieringerwerf.
Because of the size, we decided to build a horizontal set-up instead of vertical. While the prototype is being tested it is suspended by two wires and counter weights are used to compensate for gravity. An axial cylinder pushes a constant force of 60 ton at the "Transition Piece" (TP) inside the "Foundation Pile" (FP). In reality the axial force will be the constant weight of the turbine. Bending loads due to waves and turbine loads are simulated in the tests by the two horizontal cylinders which can apply cyclic loads at the tube at 9 meters distance apart. The maximum forces will be 30 ton and can vary with a cycle period of about 60 seconds. A total slip is about 50 mm.
Regular load patterns can be applied as well as irregular loads. Each pattern results in a typical settling behavior, which are to be simulated numerically to validate the design model. Reproducing the mini-tests by simulations was successful, and now we tackle new challenges on a larger scale and with the use of steel.
Test plans and test results will be reviewed by DNV-GL as a first step towards design certification. The DSJ will participate in the Borssele V tender for the first full scale demonstration offshore in 2020.
For inquiries please contact:
KCI the engineers B.V.
De Brauwweg 60
3125 AE Schiedam
Knowledge Centre WMC
1771 MV Wieringerwerf
phone +31(0)227-50 49 49