EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS Earthquake Engng Struct. Dyn. 2007; 36:2249–2271 Published online 20 August 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/eqe.737 Impedance of flexible suction caissons Morten Liingaard 1, ∗, † , Lars Andersen 2 and Lars Bo Ibsen 2 1 DONG Energy, Kraftvaerksvej 53, Fredericia DK-7000, Denmark 2 Department of Civil Engineering, Aalborg University, Aalborg DK-9000, Denmark SUMMARY The dynamic response of offshore wind turbines is affected by the properties of the foundation and the subsoil. The aim of this paper is to evaluate the dynamic soil–structure interaction of suction caissons for offshore wind turbines. The investigations include evaluation of the vertical and coupled sliding–rocking vibrations, influence of the foundation geometry and examination on the properties of the surrounding soil. The soil is simplified as a homogenous linear viscoelastic material and the dynamic stiffness of the suction caisson is expressed in terms of dimensionless frequency-dependent coefficients corresponding to different degrees of freedom. The dynamic stiffness coefficients for the skirted foundation are evaluated using a three-dimensional coupled boundary element/finite element model. Comparisons with known analytical and numerical solutions indicate that the static and dynamic behaviours of the foundation are predicted accurately using the applied model. The analysis has been carried out for different combinations of the skirt length, Poisson’s ratio of the subsoil and the ratio of the soil stiffness to the skirt stiffness. Copyright 2007 John Wiley & Sons, Ltd. Received 31 January 2006; Revised 4 June 2007; Accepted 18 June 2007 KEY WORDS: soil–structure interaction; boundary element method; dynamic stiffness; impedance; suction caisson; skirted foundation 1. INTRODUCTION Wind turbines have increased tremendously in both size and performance during the last 25 years. The general output of the wind turbines is improved by larger rotors and more powerful genera- tors. In order to reduce the costs, the overall weight of the wind turbine components is minimized, which means that the wind turbine structures become more flexible and thus more sensitive to dy- namic excitation at low frequencies. The foundation principles for the recent major offshore wind farm projects in Europe have been dominated by two types of foundation solutions: the gravitational ∗ Correspondence to: Morten Liingaard, DONG Energy, Kraftvaerksvej 53, Fredericia DK-7000, Denmark. † E-mail: molii@dongenergy.dk Copyright 2007 John Wiley & Sons, Ltd.