ABSTRACT:

Offshore wind turbines (OWT) are currently designed and certified based on loads calculated in so called aero-servo-hydro-elastic simulation tools. Large sets of time domain simulations are run, especially in the certification process. In ADCoS-Offshore - the software used in this study - branched support structures are modeled with finite element (FE) beam members. These structures usually consist of hollow steel tubes connected by joints. As the representation of the joints using FE beam members is not too precise, ADCoS-Offshore was extended with a substructuring feature to model the joints more accurately. In this paper, a model of a 5MWOWT that includes a "basic" support structure model, and an OWT model incorporating superelements to represent the support structure's joints are briefly described. Results are compared mainly in terms of post processed fatigue parameters and interpreted. Finally, conclusions are drawn indicating the necessary model fidelity for the fully-coupled simulations of this type of turbine under the given conditions.

INTRODUCTION

In the last few years offshore wind turbines (OWT) grew in size and moved to deeper waters further offshore. The decision on the type of support structure for an offshore wind project is mainly based on water depth, turbine size and "roughness" of metocean conditions; whereas soil conditions may also become important for some projects. Recent industrial developments and research results showed that the reasonable usability of the widely used gravity based and monopile solutions may be stretched to deeper waters and larger turbines. For the Thornton Bank project in Belgian waters for example, gravity based structures were used for 5MW turbines in water depths of up to 27m as described by Thomsen et al. (2007) and the feasibility of large monopiles for 5MW turbines in water depths of up to 30m is discussed by Seidel (2010).

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