Implementation of a non-linear foundation model for soil-structure interaction analysis of offshore wind turbines in FAST

Abstract

Bottom-fixed offshore wind turbines (OWTs) involve a wide range of engineering fields. Of these, modelling of foundation flexibility has been given little priority. This paper investigates the modelling of bottom-fixed OWTs in the non-linear aero-hydro-servo-elastic simulation tool FAST v7. The OWTs considered is supported on a monopile. The objective of this paper was to implement a non-linear foundation model in this software. The National Renewable Energy Laboratory's idealized 5MW reference turbine was used as a base for the analyses. Default modelling of foundation in FAST v7 is by means of a rigid foundation. This implies that soil stiffness and damping is disregarded. Damping may lead to lower design loads. A softer foundation, on the other hand, will increase the natural periods of the system, shifting them closer to the frequencies of the environmental loads. This may in turn lead to amplified moments at the mudline. Therefore, it is important to include soil stiffness and damping in analyses. In this paper, a non-linear foundation model is introduced in FAST v7 by means of uncoupled parallel springs. To verify that the implementation is successful, non-linear load-displacement curves of the foundation spring are presented. These show the typical hysteresis loops of an inelastic material, which confirms the implementation.