Investigation of the mechanical behaviour of conical cans with large slopes

Abstract

Currently, monopile foundations are the most used type of fixed-bottom structures in the offshore wind industry. The support structure of a fixed offshore wind turbine also includes a transition piece that connects the monopile to the wind tower. Both monopile and transition pieces are commonly manufactured from welded cans. Moreover, often this connection needs to overcome a diameter disparity between the pile and the wind tower. In a typical design, this transition between diameters is done by using conical cans in the monopile, in addition to the usual cylindrical ones. The greater the disparity, the greater the required slope on the conical cans. However, a non-conventional solution is offered. It consists of performing the diameter change in the transition piece by means of conical cans. In that sense, the primary steel of three different transition pieces is designed using a given available clearance based on a real case situation. In addition, slope angles larger than the current industrial limit of 5° are considered. The proposed transition piece concepts are investigated in the face of the applicable standards for the limit states. The aim is to obtain a minimum thickness of the cans that complies with all the criteria established by the design standards. Initially, the ultimate limit states are assessed analytically. Later, a computational analysis using the finite element method provides comparable results. Moreover, the fatigue limit state is also studied, especially the stress concentration factors generated by the conical cans and the effect that the slope angle has on the fatigue life of the cans. The results indicate how the large slope cans become a critical source of stress intensification in the transition piece structure by increasing the stress concentration factors, creating singularity regions, and interfering with the local stress convergence. Finally, a discussion about manufacturing challenges related to the production of conical cans is introduced for the case of the proposed designs.