Effect on flange waviness on the C1 Wedge Connection

Abstract

Wind turbines constitute a sustainable and effective solution to produce energy using wind power. Offshore wind turbines are becoming of special interest and more demanding with continuously increased diameters. However, their design poses great challenges, since the installation and fabrication tolerances in combination with high cyclic loading play an important role on the fatigue assessment of the wind turbine structure.One part of specific interest for the fatigue assessment of a wind turbine is the way that the transition piece is connected to monopile. Since now, grouted and bolted connections are commonly used, but they have many drawbacks. These disadvantages regarding their performance inspired by C1 Connections BV to invent a new type of connection to connect large diameter tubulars, the C1 Wedge Connection. This innovation allows to increase the fatigue life of the wind turbine structures and decrease simultaneously their construction and installation cost. The work that will be presented here deals with the fatigue assessment of the C1 Wedge Connection, and how the presence of imperfections may affect the fatigue life of it.After analysis of the load path and the innovative preload mechanism, a reference wind turbine has been selected and the 3D CAD software of Solid Edge is used to design all the components of the wedge connection and assemble them to their final position. The stiffness of the main components of the wedge connection (TP-flange, TP-shell and MP) is calculated analytically, based on the method proposed by Seidel and adapted for the C1 Wedge Connection. An analytical model using Timoshenko beam theory has been developed for TP-flange stiffness calculation. This model and all analytical calculations were verified against FEM results built on ANSYS Workbench and mechanical for that purpose.Once the components’ stiffnesses have been calculated based on analytical formulas, an approximation of the maximum closeable gap is plotted for different gap sizes for this specific geometry of the connection and friction coefficient μ=0.15 which was proved being conservative after calibration using results from tests taken place at TU Delft lab.Secondly different scenarios have been investigated using FE models regarding the position of the gap among the different components of the connection. From this analysis, in terms of magnitude of the remaining gap and the reaction force at the interface of both flanges, was concluded that the worst scenario is the presence of imperfection only at MP side.This worst scenario (only imperfect MP flange) is selected and examined for compression to tension under ULS overturning moment. The fatigue damage of the imperfect structure is calculated and compared with a perfect one. For the purpose of this thesis, preloading has been applied by once at all connections of the ring. Peak stresses are calculated and based on selected S-N curve from DNV-GL codes and by making use of the Miner’s rule, the fatigue damage at the critical positions of both perfect and imperfect connection is calculated. For the same imperfect structure, the fatigue damage is calculated again based on nominal stresses measured underneath each hole of imperfect MP segment. In that case the specific for the C1 Wedge Connection S-N curve is used, based on fatigue single segment test results taken place at TU Delft lab, in 2018.From this research, it is concluded that based on peak stress method the selected imperfect structure is not able to withstand the fatigue damage loads. On the other hand, the same cannot be stated using nominal stress method which is more realistic based on test results and not sensitive to mesh details. In that case the fatigue damage for every hole is <<1.