Shell buckling of suction buckets for offshore wind turbines considering imperfection and soil parameter sensitivity

Abstract

Suction buckets or suction caissons in combination with jackets as substructures have become a prominent foundation solution for offshore wind turbines. These large shell structures with relatively thin walls are installed by pumping out the water inside the structure, which creates a pressure difference and thus a downward force. The circumferential compression on the cylindrical shell causes a risk of shell buckling. The prediction of the buckling capacity of such large cylindrical shells is challenging, since it depends significantly on the initial geometric imperfections resulting from the manufacturing process and on the boundary conditions imposed by the surrounding soil. Previous work on suction buckets revealed that the choice of a representative imperfection form and amplitude is very challenging and the buckling pressure is sensitive to soil modeling choices. In this work, various imperfection forms and amplitudes as well as different soil parameters are applied to geometrically and materially nonlinear finite element models and the buckling capacities are determined. The novelties in this contribution are in evaluating and comparing the effects of global and local imperfections in form of sinusoidal eigenmodes and weld depressions for an unstiffened cylindrical shell under hydrostatic compression. The results suggest that global imperfections are more detrimental than local imperfections and further, a new approach based on sorting eigenmode imperfections depending on the wave number to identify the most unfavorable eigenmode-affine imperfection pattern is developed. Additionally, this study is the first to show that the inclusion of soil plasticity and contact effects has a significant impact on the buckling pressure.