Finding the optimal solution for the offshore wind turbine and foundation interface
On 10 November 2016, the seminar ‘Grout, Bolt, Slip’ took place at De Oude Bibliotheek.
During the introduction participants were asked to discuss with their neighbor which connection form between monopile and transition piece would be most successful in the future: Grout, Bold or Slip? Three options, many more opinions, of which some very strong ones.
The fact that the answer to the topical question was not straightforward was made very clear by Stefan Erents from Enersea, by outlining the major TP design requirements and challenges. The question remained: grout, bolt or slip?
Eneco has vowed not to use grouted connections anymore after their experiences at the Prinses Amalia wind farm. At Luchterduinen they had positive experiences with bolted connections. Gemini however experienced severe problems with the bolted connection of their 150 turbines. As it turns out, the physics behind the bolted connection are not yet fully understood. A dedicated research project is required!
The slip joint already has a history in onshore wind, where it was applied between tower sections.
Monopile manufacturer SIF made no secret of their preference for the slip joint connection.
KCI presented their innovative design of the double slip joint. After successful tests with a small-scale prototype, the next step is to build a medium scale test setup.
The final presentation was by DOT who presented promising results from their full scale onshore test campaign with a slip joint connection between a monopile and medium size wind turbine (44m rotor diameter) at the Maasvlakte II. The settlement and resulting stresses in the joint were carefully monitored. As it turned out, the hoop-stresses during operation were within the expected range (far below yield strength!). The next step is to test this connection offshore.
What made this seminar a great success was the no-nonsense approach by the speakers, who were willing to share detailed information on TP connection problems they had encountered. The audience, which included several experts, was fully engaged and discussions took place well after the seminar was formally ended. So much so that a strong case was made for a follow-up session in 2017!