Investigation of a slip joint connection between the monopile and the tower of an offshore wind turbine

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To circumvent current industry problems related to the settling of grouted connections, a steel-to-steel or slip joint connection is proposed for fitting a transition piece onto an installed monopile foundation. In the first part of this contribution, a simplified dynamic analysis of the installation of such a joint is considered. Assuming velocity dependent Coulomb friction, slip-stick equations of motion are derived for a simplified 1-D model of the joint, and the slip distance under self weight and resulting overlap lengths are calculated for different initial cone angles and friction coefficients. It is concluded that even for small initial cone angles, small angle differences between the top and bottom cone, and low friction coefficients, the tangential displacement caused by the self weight is insufficient to reach the desired contact overlap. In the second part of this contribution, the static capacities (axial and bending) of the joint in the in-place situation are determined by means of a FE model. Contact analyses are performed based on the ideal situation in which the two parts of the joint is initially in full contact. Of the parameters varied in these analyses, small cone angles and large overlaps are identified as most conducive to a succesfull transferral of the loads from the transition piece to the monopile. Given the uncertainty on the friction coefficient, it is then also recommended to use a cone angle of 1 ◦ and preferably an overlap > 1.5D.