Impulse Based Substructuring for Coupling Offshore Structures and Wind Turbines in Aero-Elastic Simulations

In order to achieve the goal of 20% renewable energy in 2020, as set by the European Union, large offshore wind farms are either under construction or in development through-out Europe. As many of the "easy" locations are already under development, offshore wind farms are moving further offshore into deeper waters, which results in a set of new technical challenges. One of these challenges comes from the fact that the traditional solution of placing monopile-foundations is possibly no longer sufficient to anchor the latest generation of wind turbines to the seabed. This requires that different and more complex types of offshore structures, such as jackets, are used as foundations. Appropriate models for these more complex foundations, are often not available in aero-elastic simulation oftware. The current design practices, such as equivalent interface stiffness and mass matrices for the offshore structure, could lead to errors in the coupled simulations due to dynamics that are not modeled. In this paper an alternative method, Impulse Based Substructuring, is proposed to efficiently and accurately include the dynamic behavior of the support structure in the load simulations. The method is demonstrated using the NREL 5MW reference turbine and UpWind reference jacket. It is shown that the method is able to accurately compute the coupled dynamics and requires only a number of small augmentations to the standard Newmark time integration scheme for nonlinear finite element models.