Dimensioning of Grouted Joints

Grouted joint an OWT [Image source: Gasch, Twele 2005]
Dimensions of the specimens with and without shear ribs

In the offshore oil and gas industry grouted joints belong to the prior art, wherein a transition piece (Transition Piece) is used, which will be charged with a certain overlap length on the monopile. There is thus a tube-in-tube connection, wherein the space between the two tubes is filled with a grout. However, the support structure requires less by normal force, but rather by bending at offshore wind turbines (OWT) as opposed to the oil platforms. Therefore, see the Institute for Steel Construction Leibniz Universität Hannover currently large model tests on grouted joints instead of a pure bending stress. 

In addition to it already axial pressure tests were performed on small-scale grouted joints, where the focus was on the support and fatigue strength when using different filling materials. In the test program also half of the specimen was equipped with thrust ribs in order to analyze its influence on the ultimate load can. The geometry of the test piece is shown in the figure to the right. More details on this series of experiments are described in the publication Bautechnik Heft 11/2009.

As part of the PhD thesis of Dipl.-Ing. C. Keindorf was developed on the basis of the experimental results, a design proposal for the determination of the load capacity of a grouted joints. The comparison in the following table shows a good agreement between the measurement results and the design proposal.

Comparison between measurement results and design proposal

Specimens Compressive strength [MPa] Load capacity [kN] Load capacity [kN]
Type Grout Test 1 and Test 2 Design proposal
E-MS-T-2 87 252 and 262 287
E-MS-T-3 42 182 and 192 178
E-MS-T-5 141 437 and 459 432

Structural behavior with shear ribs

The high load capacity of the specimens with shear ribs (name MS) is mainly due to the formation of compression struts in the grout between the overlapping steel tubes. This structural behavior thus differs fundamentally from the specimens without shear ribs (name OS). In some Grouted joints the load capacity and the fatigue strength could be increased by a factor of 10 by the arrangement of shear ribs.

In addition to the experimental and theoretical investigations in the context found the dissertation of C. Keindorf instead of numerical simulations. Based on the structure of the plot is to see how to train the struts between the shear web. The comparison with the fracture of the specimen shows a good agreement with respect to the mode of action of the load increasing shear web. 

FE-model for grouted joints with shear ribs
Mode of action of struts in the experiment (left) and model (right)