Interview with Prof. Dr. Peter Schaumann ForWind, Leibniz University Hannover, Institute for Steel Construction in The Windfair Newsletter

Windfair: Prof. Schaumann, you are currently working on grouted joints for offshore wind turbine support structures. Where exactly are these connections applied?

Prof. Schaumann: Yes, within several research projects conducted by two institutes at the Leibniz University Hannover, we have investigated the load bear-ing capacity of grouted joints exposed to axial loads and bending moments on extreme and fatigue load level. As a tube-to-tube con-nection, grouted joints are used in various support structures of offshore wind turbines (OWT). This type of connection is applied between the foundation piles and rising support structure of the OWT.

Windfair: How long has the University been doing this?

Prof. Schaumann: Offshore wind turbines with monopile foundations, which have been installed since the beginning of the 90s, are equipped with these hybrid connections, where the plain cylindrical tubes of pile and transition piece are slid on top of each other. The annulus between pile and the so called sleeve is cast with high-strength grout.

Windfair: In light of recent events, which problems have arisen?

Prof. Schaumann: During regular maintenance intervals, vertical slippages of tube-to-tube connection have been detected in monopiles located in Denmark, UK and the Netherlands. These vertical settlements indicate a failure of the adhesive bond between the steel pipes and the grout layer.

Windfair: What does this mean for the offshore industry?

Prof. Schaumann: Practical solutions are required, which on the one hand improve the grouted joints, and on the other hand, provide repairing methods for affected wind turbine structures. Only by this, a widespread error can be averted from the future-oriented offshore wind energy industry. Solely the repair of the affected grouted joints in monopile foundations of offshore wind turbines, in the UK, experts have estimated the costs at several million pounds. Deservedly, the concerns referred to other operating and planned wind parks are quite high. But it has to be kept in mind that a repair of an existing structure always means a change of the static system. In the worst case, the repair could lead to further unexpected consequences, and thus, influence the system negatively.

Windfair: Which improvements can be made to prevent grouted joints from shifting?

Prof. Schaumann: The load-bearing and fatigue behavior of grouted joints are influenced significantly by the dominating type of loading. It is necessary to clarify the true bearing behaviour under ultimate and fatigue loads of grouted joints with plain pipes. To prevent unscheduled shifts in future, an increase of the axial load-bearing capacity is crucial. Actually, a change of the construction type by using mounting parts to increase the load-bearing capacity is discussed. With regard to future structures using grouted joints, one possible solution is the use of shear keys as an additional mechanical interlock which influence the load-bearing capacity of grout connections positively. These and other outcomes of our research works are topics of the 2-day expert seminar Offshore-Windenergie Design und Installation von Tragstrukturen in der Nordsee (seminar on German), taking place on the 7th and 8th of September 2011 in Bremerhaven, Germany. In this experts forum, my colleagues Prof. Ludger Lohaus and Mr. Stephan Lochte-Holtgreven will join me in introducing the mutual examinations concerning load-bearing capacity, maintenance and optimization of grouted joints of OWT.
In this regard, I especially look forward to the exchange and discussion with the participants concerning the challenges in offshore wind energy as well as the progress reports.

Thank you Prof. Schaumann!