Coupled Dynamics Response Analysis of a Hybrid Floating Platform Concept for Offshore Wind Turbines

Abstract

Deepwater Floating platforms for Offshore Wind Turbines (FOWT) are integrated systems that combine Wind Turbine Generators (WTG), floating platforms, and station keeping systems (moorings and/or tethers) responding to wind, wave, and current loadings in a complex way. During the design phase of a FOWT, a coupled dynamics simulation strategy is required to adequately predict the motion responses resulting from the interaction between the environmental loads and the assemblage of the FOWT. Since the introduction of FOWT technology, different coupling methods for numerical dynamic loads simulations have been used to assess and explore the aerodynamic and hydrodynamic response of a FOWT under various environmental conditions. However, an area that needs more attention involves the adequate modelling of coupled dynamics and intricate viscous effect resulting from fluid forces on the FOWT structure under complex environmental loading. This research introduces a streamlined but holistic coupled dynamics simulation strategy known as the SHARPE (Streamlined, Hydro-Aero-Rigid dynamics, Potentials, and viscous Effects) numerical modelling scheme. The SHARPE numerical modelling scheme provides a wholistic assessment of the motion response and loads assessment of a FOWT. In this approach, fluid viscous effects are adequately modelled by performing an unsteady state multi-physics Computational Fluids Dynamics (CFD) simulation. Here, viscous drag force coefficients and viscous damping coefficients are calculated by using a CFD program to solve the Reynolds Averaged Navier Stokes (RANS) equations near the walls of the FOWT assemblage. Thereafter, by introducing a Floating Rigid Body (FRB) model of the FOWT assemblage and a Finite Element (FE) model of the station keeping system, a global performance analysis of the FOWT is performed using an in-house Computer Aided Engineering (CAE) toolset. The FOWT assemblage adopted for this study is the National Renewable Energy Laboratory (NREL) 5-MW baseline wind turbine mounted on a newly developed inhouse hybrid floating platform identified as the TRIgon Hybrid Floater (TRIHF). The TRIHF concept for FOWTs is developed based on the design principles that cuts across a Spar-buoy, Tension Leg Platform (TLP), and Semi-Submersible. The SHARPE numerical scheme employed for this study enhances the capability of a potential-based hydrodynamic solver by implementing the viscous damping coefficients obtained from a free decay numerical test and the viscous drag force coefficients due to wind and ocean current loads. The site-specific environmental conditions employed for this research is based on the long-term hindcast data of a pre-assessed OWT Project Development Area (PDA) in New Zealand. Frequency Domain (FD) and Time Domain (TD) analysis are performed to simulate the aerodynamic responses, hydrodynamic responses, structure aerodynamic and hydrodynamic loads, and mooring line tensions.