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

To circumvent the 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. Slip-stick equations of motion are derived for a discrete mass model of the joint. The slip distances under self-weight, with an initial velocity and under a harmonic loading are calculated numerically. The results show that the desired settlement under a harmonic loading can be achieved for various combinations of amplitudes and frequencies. In the second part of this contribution, the static capacities and the dynamic response of the slip joint under extreme loads are determined by means of a finite element (FE) model in which the two parts of the joint are initially in full contact. In these analyses, small cone angles and large overlaps are identified as most conducive for a successful transferral of the loads in the slip joint. The dynamic assessment of the FE model shows that the slip joint continues to stick and the stresses remain well below the plastic limit throughout the duration of an extreme operating gust.