Synopsis
In recent years, reports of offshore wind turbine topside structures settling on their monopile foundation structure have emerged, with the resulting force flow in the structures found to be different from that intended at the design stage. As a result, a joint industry project was carried out by Det Norske Veritas (DNV) between late 2009 and early 2011, to investigate the structural capacity of these connections. It was found that the axial capacity of the grouted connections is more sensitive to the diameter (capacity reducing more than predicted at larger diameters) and surface tolerances than had been accounted for in the existing design standards. This paper reviews the past and present industry practice relating to the design of grouted connections in monopile structures. The physical behaviour of the connections is explained and some of the most critical issues related to the design of large diameter grouted connections are assessed. Based on the experience with the settlements of cylindrical shaped grouted connections, a design procedure with conical shaped connections was developed. The recommended design methodology has been supported by a number of laboratory tests. The main findings from that joint industry project are presented in this paper.