Impedance of flexible suction caissons

Abstract

SUMMARYThe 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.

Efficient Modelling of Wind Turbine Foundations

“…Alternatively, numerical analysis may be conducted using the finite-element method and the boundary-element method. See, for example, the work by Emperador & Domínguez (1989) and Liingaard et al (2007). For monopiles, analyses are usually performed by means of the Winkler approach in which the pile is continuously supported by springs.…”

“…The concept of the monopod bucket foundation has been described by Houlsby et al (2005;2006) as well as Ibsen (2008). Dynamic analysis of such foundations were performed by Liingaard et al (2007;2005) and Liingaard (2006) as well as Andersen et al (2009). The latter work will be further described by the end of this chapter.…”

“…www.intechopen.com Andersen & Clausen (2008) concluded that soil stratification has a significant impact on the dynamic stiffness, or impedance, of surface footings-even at the very low frequencies relevant to the first few modes of vibration of a wind turbine. Liingaard et al (2007) employed a coupled finite-element/boundary-element model for the analysis of a flexible bucket foundation, finding a similar variation of the dynamic stiffness in the frequency range relevant for wind turbines. This illustrated the necessity of implementing a model of the turbine foundation into the aeroelasic codes that are utilized for design and analysis of the structure.…”

Three-Dimensional Be-Fe Model of Bucket Foundations in Poroelastic Soils

“…Given its unbounded nature, the BEM is used to numerically treat it. A particular feature of the proposed model is that, unlike Liingaard et al [6], the problem can be handled directly without needing any artificial interfaces. This is achieved thanks to the usage of the Dual BEM.…”

“…They give an interesting discussion about the interface conditions between soil and foundation. Liingaard et al [6] studied the impedances of bucket foundations in elastic soils, including the variation of these under changes of geometry and soil properties.…”