Design of grouted connections for monopile offshore structures

During the last years, offshore wind turbine structures were reported to settle on the monopile structure and the resulting force flow in the structures was different to that intended at the design stage. A joint industry project was therefore carried out by Det Norske Veritas (DNV) to investigate the structural capacity of these connections from autumn 2009 to January 2011. It was found that the axial capacity of the grouted connections is a more sensitive function to the diameter and surface tolerances than that accounted for in existing design standards. Based on this experience a design procedure with conical shaped connections was developed. In January 2011 another joint industry project on capacity of cylindrical shaped grouted connections with shear keys was initiated. In this project analytical design equations have been developed both for the Ultimate Limit State and the Fatigue Limit State. The recommended design methodology has been supported by a number of laboratory tests. Some of the main results and recommendations for design of grouted connections in monopiles from these projects are presented in this paper.