Section force calculation in flexible substructures modelled by wind turbine design tool Bladed

Abstract

Abstract Aeroelastic simulation tools are widely used to predict the coupled response for onshore, offshore and floating wind turbines. A key output of the analysis is the section forces in the flexible substructures, such as blades, towers, and floating foundations. Individual substructures are typically modelled as a single linear flexible body, which is a part of a multibody system used for modelling the complete wind turbine. However, in some floating foundation concepts, in particular those with pre-tensioned cables, geometric non-linearities in the substructure can have a significant effect. With the conventional method for calculating section forces, such non-linearities are not captured accurately. In this paper, a new equilibrium-based method for calculation of section forces is presented, as part of the wind turbine design tool Bladed. This method relies on equilibrium between the section forces and the applied loads. The section forces are determined in terms of Lagrange multipliers, which represent the internal constraint forces in the structural elements. This method is valid even when significant geometric non-linearities are present, with reduced order models, and for statically indeterminate substructures. A case study is presented demonstrating the self-consistency of the new method for the OC4 semi-submersible floating platform, modified to include pre-tensioned cables. A verification against ANSYS is also presented for some elementary cases.