Foundation designs under the spotlight
A barge carrying monopile foundations arrives at Ramsgate for the Thanet Offshore Windfarm.
Classification society Det Norske Veritas (DNV) has recently published a Joint Industry Project (JIP) report into offshore wind turbine foundation structures.
This follows concerns that the design of large diameter grouted connections between the monopile and the transition piece does not properly describe the physical behaviour of the connection.
The JIP comprised a group of 12 owners, operators, grout producers and designers and involved revising and improving the basis for calculating the relevant axial load capacity and to review current design practices through physical testing in DNV’s laboratory, structural analyses, field monitoring and the sharing of experiences.
Construction of the foundations involves the transition piece, with temporary supports fixed to the inside being lowered over, and resting on, the top of the monopile. The transition piece is then jacked up to the correct height and the grouting added, making a permanent connection between the two components. Once the grouting has set, the jacks are removed leaving a small gap between the supports and the monopile. A flow of forces in the structure that can result from settlement of the transition piece onto the monopile through the supports has led to concerns about fatigue cracking in the structures.
It was found that due to the effects of the large diameter, the lack of control of tolerances and the effects on the abrasive wear of the grout from wind and wave induced bending moments, the axial load capacity is lower than previously assumed. The report recommends that where such settlement may be expected in existing structures, additional support arrangements should be considered, something that has already been implemented on several turbine foundation structures.
Two solutions are outlined in the report. The grouted connection itself can be modified, through the addition of shear keys in the form of weld beads on the outside of the monopile, and the inside of the transition piece in the grouted section to increase the sliding resistance between the grout and the steel to avoid settlement. This solution is however based on limited test data and DNV have set up a complementary JIP to update the existing knowledge and design standards.
The second solution involves fabricating both components with conical shaped connections in the form of a small cone angle in the grouted section. Should the steel and grout bonds be broken, the expected slight settlement will introduce compressive contact stresses between the steel and grout providing sufficient resistance against further settlement.
Participants in the JIP were: 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 and Vattenfall Vindkraft A/S.
By Peter Barker
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