Buckling of Bucket Foundations During Installation

There is a great politically will to expand the green energy market in these times. A proven green technology is wind turbines. Wind turbines have been installed in great numbers on land over the last decades. However, the current development in wind turbine design leads to larger turbine sizes in order to reduce the cost of energy. This limits the on land application due to transportation limitations and unwillingness from prospect neighbours. Thus, offshore wind energy started developing over the last couple of years. Although installing the wind turbines offshore resolves the before men tioned issues, it brings up the cost of energy mainly due to increased installation and maintenance costs. A very large part—up to 30–50% using current technology—of the installation cost origins from the expenses related to the installation of foundations. A new foundation concept—the bucket foundation—has been proposed to reduce the foundation costs. The bucket foundation promises to be cheaper as it is both faster to install compared to current technology and it does not require heavy installation equipment. Since the bucket foundation is essentially a relatively thin shell structure subjected to both axial and latteral pressure during the installation process buckling becomes a crucial design consideration. In 2005, a large wind turbine was to be installed nearshore the harbour in Wilhelmshaven, Germany. The windturbine was never erected since the skirt of the bucket foundation buckled during the suction assisted installation process. In this thesis, the phenomenon of buckling of the bucket foundation during installation is investigated by means of Finite Element Analysis. The influence of boundary conditions on the bucket foundation is adressed as well as the effect of including the surrounding soil and soil–structure interaction. Investigations are made regarding the buckling incident in Wilhelmshaven and the results are compared to current DNV and Eurocode standards. Finally, analysis of a new design of the cross section with a higher buckling capacity than an equivalent
circular cross seciton—denoted multi-shell—is presented. The main findings are listed below:
• The bucket lid shows an increase in the buckling load compared to a pinned end,
• Number of lid stiffeners does not substantially affect the elastic buckling load,
• The soil restraint increases the buckling load significantly during penetration,
• Considering the first mode shape from a linear buckling analysis as imperfect geometry
in a nonlinear buckling analysis is not sufficient for capturing the buckling incident in Wilhelmshaven; however, considering the first 21 mode shapes and introducing themost critical one as an imperfect geometry is,
• The multi-shell cross section is just as sensitive to imperfections as a circular cylinder,
• The new multi-shell design provides a significantly larger (75%) buckling load compared to the traditional design.