Session: 09-01-06 Wind Energy: Aero-hydrodynamics 6

Paper Number: 130033

130033 - Influence of Frequency Dependent Wave Spreading on Offshore Wind Turbine Design 

Real ocean waves are directionally spread whereby the propagation of energy travels in different directions. Despite this, the use of 3-dimensional wave models has been limited in the offshore wind industry. This is partially due to the common assumption that unidirectional seastates are conservative within a design philosophy. This may not always be true given that in operating conditions the amount of aerodynamic damping is strongly dependent on direction.  

This paper aims to address this issue by providing the influence of frequency dependent wave spreading on various offshore wind turbine design scenarios. Specifically, the design of a monopile supported IEA 15MW turbine against fatigue and ultimate limit state failure has been considered. 

To conduct this study, we use our inhouse aero-hydro-servo-elastic code which has been verified against industry standard software such as OpenFAST and BLADED. Importantly, it captures the directional dependence of damping through explicitly modelling the aeroelastic behaviour. In addition, the foundation model is based on a multi surface plasticity method that captures the soil hysteresis whereby damping is inherently within the mechanics of the model. The wave structure interaction for extreme waves is inherently nonlinear and difficult to predict analytically. Therefore, an experimental based tool that includes higher harmonic nonlinear loading components has been used for extreme condition design and is based on machine learning. Finally, Ewans frequency-dependent spreading model has been adopted whereby a bimodal Gaussian distribution is assumed.

This study demonstrates that significant reductions in steel and geotechnical utilisation ratio are observed when frequency dependent spreading is included. However, the fatigue analysis also demonstrates that it is non-conservative in operating load cases to assume that seastates are unidirectional despite being a common design assumption. This latter deviation from common practice derives from the variation in damping with direction which cause resonant side to side responses.  

Presenting Author: Gerard V. Ryan University of Oxford

Presenting Author Biography: Gerry is a DPhil Student on the Wind and Marine Energy Systems and Structures EPSRC CDT.
He previously graduated with a BE in Civil Engineering from University College of Cork and a MSc in Engineering Fluid Mechanics for the Offshore, Coastal and Built Environments from Imperial College London. Following this Gerry spent two years in the offshore industry as a Structural Engineer working on various projects in the North Sea and further afield.

During his PhD Gerry has been involved in several R&D projects which include developing a multi surface foundation model to better capture soil-structure interaction and coupling this with a self-developed aero-hydro-servo-elastic tool. He has also worked in fluid structure interaction problems such as modelling nonlinear wave interactions with vertical columns based on the results of an extensive experimental campaign. In all of these R&D projects, Gerry has used his experience with steel design to quantify the practical implications of using these higher fidelity models within an offshore wind turbine design framework.

Authors:

Gerard V. Ryan University of Oxford
Ross A. McAdam Ørsted Wind Power
Thomas A. A. Adcock University of Oxford