Describing Six-Degree-of-Freedom Response of Foundations Supporting OWEC Jacket Structures

Thieken, Klaus (Leibniz University Hannover) | Achmus, Martin (Leibniz University Hannover) | Terceros, Mauricio (Leibniz University Hannover) | Dubois, Jan (Leibniz University Hannover) | Gerlach, Tim (Leibniz University Hannover)

OnePetro 

Abstract

A great potential for optimization of pile and bucket-based multi-footed foundations is the adequate incorporation of the non-linear foundation response in the overall structural model of the offshore wind energy converter (OWEC). Currently, this soil-structure interaction problem is treated by a costly and time-consuming iterative procedure between the geotechnical and the structural model. The paper presents a more sophisticated approach which makes the iterative procedure dispensable. Basic idea is to pre-define foundation responses for arbitrary loading conditions by less than twenty load-controlled numerical simulations. The foundation responses are implemented as non-linear macro-elements in the structural model enabling the incorporation of the soil-structure interaction in the equilibrium iteration of the structural model. The basic principle of the proposed approach can be used for all design purposes, i.e. ultimate limit state (ULS), serviceability limit state (SLS) and fatigue limit state (FLS) design.

Introduction

Economical pressure forces the installation of larger offshore wind energy converters (OWEC) located in deeper waters in a more cost effective manner. Due to the limitations of the currently dominating monopile foundations with regard to the economically feasible water depths, pile and bucket-based multi-foundations will probably become of increasing importance in the future offshore wind industry. A great potential for optimization of these structures is the adequate incorporation of the non-linear foundation response in the overall OWEC models. This regards likewise the consideration of the foundation response for the ultimate limit state (ULS), the serviceability limit state (SLS) and the fatigue limit state (FLS) design.

For pile-based multi-foundations (cf. Fig. 1, left) the soil-structure interaction may be treated by the integration of spring-bedded beam models, i.e. the p-y method for the lateral and moment response and the t-z method for the axial response. However, interaction effects between the lateral and vertical response are nevertheless disregarded by this approach. Anyhow, the soil-structure interaction problem becomes more serious for bucket-based multi-foundations (cf. Fig. 1, right) due to the missing of analytical spring approaches for the determination of the bucket behaviour. Due to the complex interaction mechanism between the single bucket components (lid, tip and skirt) the authors assume that such simplified approaches with sufficient accuracy will be hardly reached in near future even for buckets installed in homogenous soil conditions (cf. Achmus et al. 2013).