Dutch trio drive double joint

Dutch trio KCI, Van Oord and Sif have developed a new technique known as a double slip joint to connect monopile foundations to transition pieces.

The new technique would replace grouted or bolted connections between monopolies and TPs and could be tested at full-scale at the 20MW Borssele 5 offshore test zone off the coast of the Netherlands.

The double slip joint uses two sets of two conical rings that can be placed inside each other to make the connection.

The partners have developed a 1:5 scale prototype, which has a diameter of 1.6 metres and weighs 12 tonnes. The TP is 15 metres in length and has a foundation pile of six metres.

The prototype has been built horizontally instead of vertically because of its size, the developers said. During testing it has been suspended between two wires with counter weights used to compensate for gravity.

An axial cylinder was then used to apply a constant force of 60 tonnes on the transition piece inside the foundation pile, the developers said.

The tests also simulated the impact of waves on the joint, with a maximum force of 30 tonnes applied.

Test plans and test results will be reviewed by DNV GL in the first step towards certification of the design.

The project has been supported by TKI Wind op Zee, a Dutch government-backed initiative to help reduce the cost of offshore wind.

Image: TKI Wind op Zee