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Revised design practice for offshore monopile wind turbine foundations

A joint industry project (JIP) led by Det Norske Veritas (DNV) has looked at offshore monopile wind turbine structures to improve the calculation of the axial load capacity and to review current design practices.

By Renewable Energy Focus staff

The project found that existing design practices do not properly describe the physical behaviour of large diameter grouted connections without shear keys in offshore wind turbine monopile structures.

Existing design

A grouted connection is used to connect the transition piece to the monopile (see Figure 1). A transition piece is placed on top of the monopile, resting on temporary supports. During installation, the transition piece is then jacked up to the correct verticality before grouting. After curing, the jacks are removed, leaving a gap of a few cm between the temporary supports and the monopile.

Settlement down to the temporary supports may result in a different force flow in the structures than that intended at the design stage. An unintended force transfer through the temporary supports has led to concern about fatigue cracking in the structures which would lead to repair needs.

A report resulting from the project has concluded that a cylindrical shaped design of grouted connections without additional support arrangements for axial load is not recommended as the axial capacity is found to be lower than previously assumed, due to the effect of large diameters, the lack of control tolerances contributing to the axial capacity. And the abrasive wear of the grout due to the sliding of contact surfaces when subjected to large bending movements from wind and waves.

Where cylindrical shaped grouted connections are already used offshore and where the design is such that settlement may be expected, additional support arrangements should be considered for the transfer of axial loads, DNV says. Such mitigation methods for existing installations have been deployed and are already implemented on several structures.

Shear keys

The findings of the project report are expected to influence the design of large diameter grouted connections with shear keys.

Shear keys are circumferential weld beads on the outside of the monopile and the inside of the transition piece in the grouted section. They increase the sliding resistance between the grout and the steel so that no settlement occurs.

However, before this solution can be recommended, a design practice for shear keys should be developed and incorporated in design standards, DNV says. The risk management firm has therefore initiated a complementary JIP with the aim of updating existing knowledge of and design practices for grouted connections with shear keys.

JIP partners

  • Ballast Nedam Engineering
  • BASF Construction Chemicals Denmark A/S
  • Centrica Renewable Energy Ltd
  • Densit A/S
  • DONG Energy
  • DNV
  • GustoMSC
  • MT Højgaard A/S
  • Per Aarsleff A/S
  • RWE Innogy GmbH
  • Statoil ASA
  • Statkraft AS
  • Vattenfall Vindkraft A/S

Conical connections

Based on the JIP, a design practice for large dynamic bending moments on monopiles has been developed using conical shaped connections.

The monopile and transition piece are made with a small cone angle in the grouted section (see Figure 2).

If the bonds between the steel and grout are broken during in-service life, some slight settlement of the transition piece will occur. This settlement will introduce compressive contact stresses between the steel and the grout which, together with some friction, will provide sufficient resistance against further settlement, DNV says.

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