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.
An exciting new step has been taken 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.
Jan Willem van Bloois of KCI: “The Double Slip Joint will be a game changer in offshore wind. In comparison with previous connection-methods of transition pieces to monopiles, the most important advantages of the Double Slip Joint are mainly the reduced installation-time and a significant better quality of the connection. Based on the need for cost reductions for offshore wind farms, the Double Slip Joint will play an important role in the coming years”
The Double Slip Joint 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 prototype diameter is 1.6 m and the longest component (the Transition Piece) is about 15 m long. The total weight is 12 ton. The small main part (Foundation Pile) 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, the set-up was built horizontally instead of vertically. While the prototype was being tested it is suspended by two wires and counter weights were used to compensate for gravity. An axial cylinder pushed 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 were simulated in the tests by the two horizontal cylinders which can apply cyclic loads at the tube at 9 meters distance apart. The maximum force was 30 ton and varied with a cycle period of about 60 seconds.The total slip is about 50 mm.
Regular load patterns were applied as well as irregular loads. Each pattern resulted in a typical settling behaviour, which were simulated numerically to validate the design model. Reproducing the mini-tests by simulations were successful, and now KCI tackles 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 Double Slip Joint will participate in the Borssele V tender for the first full scale demonstration offshore in 2020.