Coupled analyses of multi-unit floating offshore wind turbines in combined wave-current environments

Abstract

Deployments of offshore wind have focused primarily on shallow seas using bottom-fixed foundations. However, much of the world’s offshore wind resource lies in deeper waters where bottom-fixed foundations are not suitable, and floating platforms must be utilised. However, these Floating Offshore Wind Turbines (FOWTs) encounter economic and technical hurdles.

Economically, most of the developed floating foundations support a single Wind Turbine Generator (WTG), resulting in high Capital Expenditures (CapEx). Technically, these FOWTs are subject to environmental loads, and their dynamic responses are complex and not thoroughly understood.

Driven by these challenges, this research presented an innovative twin-turbine semi-submersible platform as an alternative solution for reducing the CapEx and Levelised Cost of Energy (LCoE), thereby facilitating the commercialisation of floating offshore wind farms. This platform is called W2Power, developed by EnerOcean S.L., and is engineered to support a pair of generic WTGs. The present work aims to investigate the implications of Wave-Current Interaction (WCI) on the dynamics of multi-unit FOWTs by means of coupled numerical analyses and physical model experiments.

Numerical analyses are performed at full-scale of the W2Power FOWT; herein, the floater model is first developed. A new mooring system is first designed, and a numerical model of the floater-mooring-towers-nacelles-turbines is established. Equipped with two 5 MW WTGs, the developed model incorporates structural hydrostatic and dynamics, hydrodynamic effects, mooring loads, and aerodynamics.

Based on Airy wave theory, two numerical WCI models are developed to analyse the effects of a uniform current interacting with regular and irregular waves. WCI models are integrated with the OrcaFlex programme for the coupled aero-hydro-servo-elastic analysis. Experiments are also conducted in the FloWave Ocean Energy Research Facility on a 1:40-scale physical model of W2Power under multi-directional waves combined with currents. Furthermore, the numerical simulations are validated against the tank trails.

The results revealed that the current’s presence significantly modified the wave profiles and the system’s static equilibrium. Wave directionality impacted the platform motions and mooring forces. Also, WCI influenced the dynamics of the platform motions and the mooring and nacelles of the FOWT. WCI particularly influences translational platform motions, mooring loads, and nacelles accelerations. Remarkably, WCI can lead to differences of up to ±22% and ±26%, respectively, in the line’s maximum tension and surge response, depending on the current direction and mooring layout. In turn, these variations have significant implications for fatigue loads, operational maintenance (O&M), and CapEx costs of the mooring systems.

In addition, comparative benchmarks of validations between measurements and simulations, with and without WCI, demonstrated the reliability of the WCI models in accurately capturing system dynamics. Thus, the frameworks established can serve as a foundation for other studies exploring the applications of WCI in the dynamic responses of FOWTs. This will contribute to the advancements of FOWT technologies, thereby facilitating the commercialisation of floating wind farms.