Keywords: offshore wind turbines, suction caisson, bucket foundation, buckling.

Using large suction caissons for offshore wind turbines is an upcoming technology also referred to as bucket foundations. The bucket foundation is a large cylindrical monopod foundation, typically made of steel. A bucket foundation is installed by suction forces and does not require heavy installation equipment. The initial part of the penetration of the foundation into the seabed is caused by the self-weight. Subsequently, suction is applied inside the bucket. The suction creates a pressure differential across the bucket lid effectively increasing the downward force on the bucket while the water flow reduces the skirt tip resistance.

The bucket foundation is constructed as a thin steel shell structure. As the water depth increases, the diameter of the suction caisson also increases to accommodate the larger forces on the structure. Hence, the aspect ratio between the caisson diameter and the wall thickness becomes very large. Thus instability in the form of buckling becomes a crucial issue during installation.

A crucial case of buckling occurred in Wilhelmshaven, Germany, in 2005 [1,2]. A large bucket foundation with a skirt length of 15 m and a diameter of 16 m was constructed to support a 6 MW offshore wind turbine. An attempt was made to install the bucket foundation offshore by using a combination of suction and skirt tip injection. However, as a result of a horizontal impact from the installation vessel as the skirt had penetrated 6.8 m into the soil, the bucket foundation buckled and the installation failed completely.

During installation, the bucket skirt becomes partly embedded and the degree of skirt fixation increases. The lateral restraints offered by the soil to the embedded skirt may increase the buckling loads significantly.

The authors have introduced an advanced three-dimensional, non-linear, finite element analysis by use of the commercial finite element package ABAQUS [3].

For the initial installation phase, the free height of the skirt is large and a low suction pressure is needed. For subsequent installation phases, a larger pressure is needed. Thus, a critical embedded depth is found in this paper. The main finding of this paper is that introducing an imperfect geometry based on the first linear pre-buckling mode shape can increase the buckling load significantly.

1

M. Liingaard, L.B. Ibsen, "Buckling Analysis - Wilhelmshaven", Aalborg University, Department of Civil Engineering, 2005.

2

C.L. Bakmar, L.B. Ibsen, M. Liingaard, "Buckling of Large Diameter Bucket Foundations during Installation in Sand", MBD Offshore Power A/S and Aalborg University, 2010.

3

ABAQUS, "Version 6.10: Documentation", Dassault Systèmes Simulia Corp., 2010.

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