Slip Joint full scale test
Jan van der Tempel
1, Thijs Kamphuis2
1DOT, Delft, The Netherlands, 2TU Delft, Delft, The Netherlands
From May to November 2016, DOT and TU Delft have installed and tested a full scale, 4m diameter monopile with slip joint connection in the port of Rotterdam. The slip joint is an innovative connection between wind turbine tower and monopile foundation in which two conical sections slide across each other, removing the need for a transition piece. Static and dynamic loads from the wind turbine are transferred through contact forces between the surface contact the of the cones, mainly via friction.
In this paper, the full scope of the project will be reported including installation measurement results, settling of the structure during installation and operation and the decommisioning. The test proved that slip joint forces and settlement can be very well predicted, giving good input for a design standard for this innovative offshore connection detail.
For the test, DOT acquired a second hand V44 turbine. The lower section of the tower was used as the top part of the slip joint. The existing flange was removed and the cone angle was measured. A 4m diameter monopile was designed with a conincal top section to match the lower tower part.
On the lower tower section and the monopile several measurement systems were installed: straingages, accelerometers and position sensors at multiple levels. A prediction of stress and settlement based on rudimentary and FEM models were made.
During installation, operation and decommissioning all parameters were recorded. The slip joint was removed in November and visual inspections were conducted to conclude these tests.
The project delivered valuable insight into the working principle and design parameters for slip joint application in offshore wind. The settlement as a result of static weight only was 148mm. Additional settlement during operation was 17mm. The setllement reached terminal value after 3 weeks of operations.
The stresses over the cone of the slipjoint were well below yield stress range: order of 10%. The values of stress measured could be well predicted in advance using manual calculations. In some of the surface areas compression was observed. This was found to be related to the non-cylindrical shape of the second hand tower, creating areas where there was no contact.
Despite the non-perfect fit of the cones, all stresses were within limits at maximum 10-15% yield stress.
The slip joint was once used in onshore turbines. This project proves the applicability of the connection type for offshore use. The accuracy of production of the conical part of the monopile is very high and does not require machining or other post-processing. Even the poor quality of the cone of the second hand turbine did not reduce the capacity of the joint. For purpose built structures, manufacturing tolerances will deliver better matching upper cone to create even better transfer of loads.
The design calculations proved to accurately predict the setllement and stresses and can be used to optimize the design further. Based on the experience from this project, the overlap length could be reduced from 1.5D (diameter) to 1.0D.
The offshore wind industry has faced serious challenges regarding the connection detail between monopile and transition piece. First grout connection failed on over 1000 structures, requiring very expensive remedial works. Recently, the bolted connection proved to be problematic: bolts that are pre stress with torq machines to not reach the required pre-tension. Again requiring serious and expensive additional work.
This paper shows the application of the slip joint, real life, real size. The solution has the potential to make offshore wind installation simpler, cheaper and faster, with less requirement for inspection and maintenance.
Delegates will learn the design steps, see the measurement outcomes and view the many pictures and movies of this facinating project.