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Raised Flange Connection

Public summary

For the connection between the foundation and wind turbine tower different options are possible. The conventional approach has so far been a transition piece fixed to the foundation by a grouted connection. Although the grout connection is considered to be the industry standard to date, grout connections have failed, leading to slippage and unacceptable stresses in the structure. While research is being done by many parties in the industry, the issues with grout are not fully understood and have not yet been solved. The issue originated in the DNV standard which prescribed a design procedure which did not adequately predict the vertical load bearing capacity. In 2009 DNV revoked this guideline and has since reissued updated guidelines for grouted connections.

The objective of this project was to verify feasibility, design, and certify a monopile foundation without a transition piece and therefore without a grouted connection. The main objective was to directly bolt the turbine tower onto the monopile. The challenges to be investigated during the feasibility and design phases for this research and demonstration project are to understand and find feasible solutions for the main issues of the bolted connection:

• Design of a flanged connection, capable of withstanding the ultimate and fatigue limit state loads over the lifetime of the wind farm
• Piling directly on a flange without damaging the flange or supports for secondary steel;
  • In order to limit damage due to impact load, the monopile foundation flange was designed with an inclined face at the inner part of the flange, and a horizontal face at the outermost part.
• Piling within the inclination tolerances due to the fact there is no transition piece anymore to correct inclination;
  • The overall inclination was remarkably good with an average tolerance less than 0.1 degrees. Only one monopile, which was the second monopile installed, had an inclination of 0.58 degrees, which was exceeding the tolerance limit of 0.25 degrees. This exceedance was found to be due to a measuring equipment error, and after re-assessment of the measurement equipment all remaining piles were installed well within tolerance.
• Installing secondary steel (offshore) afterwards and on monopile instead of welding it onshore on the transition piece. Secondary steel consists of:
  • Boat landing including resting platforms
  • External ladder
  • Main external platform
  • Airtight platform
• Assessment of risks and investigation into mitigating measures;

A number of design studies have been undertaken to assess the feasibility of design alternatives to the non-grouted connection (see Figure 1, left). Two design alternatives were further detailed after which the raised flange connection proved to have significant cost and risks benefits compared to the conical grouted connection. Therefore it was decided to select the raised flange connection (see Figure 1, right) as the monopile design for Eneco Luchterduinen. The design and underlying studies of the monopile and the secondary steel have demonstrated that it is possible to design monopile foundations which can be installed by direct piling on the flange, which meet all design and certification criteria. 

Prior to the installation of the monopiles Van Oord prepared method statements for the monopile and secondary steel installations to provide a detailed work description and ensure effective and efficient execution of the specified work activities. One of the lessons learned during installation is that the Zinga anti-corrosion protection coating had to be applied to the flange very accurately in order to prevent issues with the bolt tension of the bolts connecting the monopile to the tower.

The movements of the foundations are being monitored during operation to verify the design assumptions and to provide insights for future foundation designs. The monitoring has commenced on November 2015 and will continue for three years.

The project has demonstrated that the raised flange concept is a cost-effective design option (potential cost reduction potential of 3%) for the design and installation of monopile foundations for offshore wind turbines within the boundaries set by turbine type and specific site conditions.